The Active Female: Health Issues Throughout the Lifespan 2008th Edition

3. Considerations of Sex Differences in Musculoskeletal Anatomy

Phillip S. Sizer  and C. Roger James 

(1)

SCD Program in Physical Therapy, Department of Rehabilitation Sciences, Texas Tech University Health Sciences Center, Lubbock, TX, USA

(2)

Department of Rehabilitation Sciences, Texas Tech University Health Sciences Center, 3601 4th Street STOP 6223, Lubbock, TX 79430-6223, USA

Phillip S. Sizer (Corresponding author)

Email: phil.sizer@ttuhsc.edu

C. Roger James

Email: roger.james@ttuhsc.edu

Abstract

The musculoskeletal anatomy of women and men is grossly similar yet individually distinctive. Sexual dimorphism in the human musculoskeletal system is apparent, but more subtle than in other species. Some musculoskeletal sex differences in humans are present at an early age, while others tend to appear later in life. Sex differences in gross skeletal geometry and specific tissue characteristics are common. Women tend to have different characteristics of specific bones and bony features than men which have been explained by both genetic and environmental factors. Women and men appear to have several differences in collagenous, cartilage, and bone tissues, which may predispose women to certain pathologies such as osteoarthritis and osteoporosis later in life. Sexual dimorphism can manifest itself in specific differences in each joint throughout the body, possibly resulting in sex differences in clinical pathology and symptomology such as differences in shoulder impingement; laxity and idiopathic capsulitis; elbow tendinosis; carpal tunnel syndrome; hip fracture and labral tears; anterior cruciate ligament injuries; ankle sprains and Achilles tendinopathy; cervical spine macrotrauma; thoracolumbar postural changes including kyphosis, lordosis, and/or scoliosis; and sacroiliac joint conditions. Consideration of the sex differences in musculoskeletal anatomy is important for both the general public and health care professionals in order to provide a basis for understanding normal and abnormal conditions that may exist. Moreover, a thorough appreciation that men and women have differences in musculoskeletal anatomy may help in the understanding that they have distinctive health care needs.

Keywords

Sexual dimorphismFemalePathoanatomyAnatomySex differencesStructure

3.1 Learning Objectives

After completing this chapter, you should understand:

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3.2 Introduction

The musculoskeletal anatomy of women and men is grossly similar yet individually distinctive. Structural differences exist between the sexes and these differences are due to both environmental and genetic factors. Sex differences in musculoskeletal anatomy can be described in terms of sexual dimorphism, which refers to physical differences in secondary sexual characteristics, such as size or color, between male and female individuals of the same species [12]. Sexual dimorphism is present in many species of birds, spiders, insects, reptiles, fish, and mammals. Examples include male pheasants which are larger and more brightly colored than female pheasants, female spiders which are usually larger than their male counterparts, male deer which grow antlers, and the males of most species of mammals which are larger than the females [2]. However, with a few well-recognized exceptions, such as body hair, muscle mass, and breast differentiation, sexual dimorphism in humans is more subtle as compared to other species [2]. Yet, most people recognize that men and women exhibit different physical characteristics that include differences in body height, weight, shape, size, and alignment of the extremities (e.g., pelvic width, body mass distribution, and ligament/tendon laxity) [24]. Some of these differences in body structure are widely recognized and ingrained in cultural beliefs and stereotypes. For example, an artist’s rendition of a typical man and woman was used to depict the sexes of the human species on the plaque of the Pioneer 10 spacecraft (Fig. 3.1) where the differences in gross structure are evident.

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Fig. 3.1

Symbolic representation of men and women as depicted on the plaque of the Pioneer 10 spacecraft in 1972. Source: NASA (www.​nasa.​gov/​centers/​ames/​images/​content/​72418main_​plaque.​jpg). Adapted with permission

The typical differences in physical characteristics of the sexes are further exemplified by population data. Data from standard growth charts [5] demonstrate typical sexual dimorphic differences, but the division between men and women is usually less than one standard deviation and is age-dependent [6]. For example, according to the clinical growth charts provided by the Centers for Disease Control (CDC), girls and boys at the 50th percentile are approximately the same height (usually within 1–2 cm) until puberty (Fig. 3.2). However, beginning at about the age of 14 years, the heights of girls and boys diverge at an increasing rate until growth slows in both sexes in the late teen years. At the age of 20 years, men are an average of approximately 14 cm taller than women (Fig. 3.2) [5]. Similar relationships are documented for body weight, with a relatively small sex difference observed before the age of 14 years and an approximately 12.5 kg difference (men greater than women) at the age of 20 years (Fig. 3.2) [5].

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Fig. 3.2

50th-percentile height and weight of girls and boys from 2 to 20 years. Sexual dimorphic differences in height and weight begin to emerge at about the age of 14 years. Values are rounded to the nearest 0.5 cm and 0.5 kg. Adapted from www.​cdc.​gov/​growthcharts (Anonymous 2006)

While sexual dimorphism is apparent in general body characteristics, other sexual dimorphic traits are less obvious. Reports of sex differences in skeletal and soft tissue components are prevalent in the literature, and these differences explain the differences in general appearance but also may influence movement patterns, injury risk, and the development and progression of musculoskeletal pathology. Consideration of the sex differences in musculoskeletal anatomy is important for both the general public and health care professionals in order to provide a basis for understanding normal and abnormal conditions that may exist. Moreover, a thorough appreciation that men and women have differences in musculoskeletal anatomy may indicate that they have distinctive health care needs. Therefore, the purposes of this chapter are to (1) examine sex differences in the anatomy of selected musculoskeletal components and (2) explore selected regional considerations in female functional pathoanatomy that are pertinent to women’s health issues.

3.3 Research Findings

3.3.1 Sex Differences in the Anatomy of Selected Musculoskeletal Components

3.3.1.1 Sex Differences in Skeletal Geometry

There are several differences in skeletal geometry between men and women. In the sports medicine literature, sex differences in musculoskeletal anatomy, including skeletal geometry, have been reported in context with common injuries that occur in active women and have mostly focused on lower extremity characteristics. For example, differences in size, shape, structure, or alignment of the pelvis, femur, tibia, tarsals, and toes all have been reported [3711]. Sex differences in the pelvis include a larger inlet and outlet [3], greater interacetabular distance [8], and a greater hip width normalized to femur length [47] in women as compared to men. Sex differences in the femur include increased femoral anteversion [4] and a narrower femoral intercondylar notch width [912] in women. However, other authors have reported no differences in notch width between the sexes [1314]. Greater genu recurvatum [311], more lateral patellar alignment [3], increased internal tibial torsion [11], greater slopes of the medial and lateral tibial plateaus [15], and more bunions and deformities of the lesser toes [3] in women have been reported, as well. Because of the mechanical linkage and interaction among structures of the lower extremity, skeletal differences in one or more interacting structures may result in differences in overall lower extremity alignment. For example, quadriceps angle (Q angle) is the angle formed by the intersection of a line connecting the anterior-superior iliac spine and the midpoint of the patella with a line connecting the midpoint of the patella and the tibial tuberosity. In the literature, it is generally reported that Q angle is greater in women than in men [710] and is a function of the structural and alignment characteristics of the involved bones (e.g., pelvic width, patella position, tibial torsion). Additionally, large deviations in Q angle have been suggested to contribute to selected knee and foot pathologies [316], although reports are equivocal.

Further evidence supporting the existence of sex differences in skeletal geometry comes from the areas of forensic anthropology and archeology. Scientists from these areas have used knowledge of skeletal sexual dimorphism to determine the sex of deceased individuals from their skeletal remains. A large amount of literature exists that discusses the skeletal geometric characteristics commonly used and the ability of these characteristics to predict sex. The humerus [17], pelvis [1819], femur [172025], tibia [2628], talus [29], and calcaneus [2931] all have been used for this purpose. The ability to discriminate sex based on one or more skeletal geometric characteristics varies somewhat by which bone, as indicated by the percentage of individuals accurately categorized as male or female in the respective studies cited: tibia (96 %) [27], femur (95 %) [21], calcaneus (92 %) [30], pelvis (88 %) [19], and talus (81 %) [29]. Furthermore, different parameters from the same bone appear to be better discriminators than other parameters [181921242631]. For example, Mall et al. [24] reported that a differing percentage of individuals could be grouped correctly using a discriminant analysis when evaluating different characteristics of the femur. In this study, the femoral head transverse diameter (89.6 % of cases correctly categorized) was the best discriminator, followed by the head circumference (87.7 %), vertical head diameter (86.8 %), condylar width (81.4 %), maximum midshaft diameter (72.4 %), and maximum length (67.7 %) [24]. Using a combination of variables (midshaft diameter and head circumference), 91.7 % of the cases were classified correctly [24].

The presence and magnitude of sex differences in skeletal geometry appears to be dependent on a number of factors including skeletal maturity [1832], environmental stresses (i.e., loading) [17212328], and genetics (i.e., race) [19232533]. Consideration of these factors is important in establishing sexually dimorphic traits in skeletal geometry. Nevertheless, ample evidence exists to support the presence of sex differences in skeletal geometry.

3.3.2 Sex Differences in Musculoskeletal Tissues

In addition to sex differences in skeletal geometry, there are several reported sex differences in musculoskeletal constituents, including collagenous (e.g., tendon, ligament, skin), cartilage, and bone tissues [3437]. Collagen is a primary protein of connective tissues in mammals and constitutes more than 30 % of the protein in the human body [38]. It provides much of the strength of tendon, ligament, skin, and cartilage [3839]. Additionally, it is the main protein component of bone [3839].

In collagenous tissues, the collagen molecules align based on stress patterns and provide strength against tensile loads [3839]. Some of the reported sex differences in collagen include differences in thickness [34], orientation [34], content [40], diameter [35], volume [36], and metabolism [37] for the specific tissues examined. Some of these sex differences are associated with fiber strength. Moreover, sex differences in collagen degradation have been observed in subjects as young as 2 years of age [37], while other sex differences may not appear until after several decades of life [36]. Many disorders of collagenous tissues (e.g., lupus erythematosus, scleroderma, rheumatoid arthritis, dermatomyositis, Sjögren’s syndrome) have been associated with sex differences. Etiological factors associated with collagenous tissue diseases are thought to exist at many levels (e.g., genetic, cellular, organ, age, behavioral, environmental), but sex hormones are thought to influence the onset and course of these disorders [41].

Sex differences in articular cartilage are reported in the literature [4245] and have been associated with differences between men and women in the onset of osteoarthritis [434546]. Epidemiological evidence suggests that women are more likely (1.5–4 times at greater risk) to develop osteoarthritis than men [43]. Sex differences in articular cartilage morphology have been reported in children (age 9–18 years), which persist throughout adulthood, and increase during the postmenopausal years [424447]. Reported sex differences in articular cartilage morphology include greater cartilage volume, thickness, and surface area in male compared to female subjects [4245]. These differences appear to be partially related to other characteristics such as age, body mass index, bone region, physical activity, and the specific articulation involved [4347]. However, research suggests that sex differences remain at some cartilage sites even after adjustment has been made for these other factors [4243]. Furthermore, the sex differences in cartilage morphology have been associated with a faster cartilage tissue accrual rate in boys compared to girls (i.e., more cartilage tissue early in life) [43] and greater cartilage tissue degradation in older women compared to older men (i.e., greater loss of cartilage later in life) [48].

The presence of sex hormone receptors in cartilage tissue is thought to be an indicator that sex hormones influence these accrual and degradation processes [45464849]. Evidence for the role of sex hormones in cartilage morphology and metabolism has been demonstrated in both animal [4549] and human [48] models. In mice, sex hormones have been shown to influence the inflammatory induction of cartilage degradation via modulation of cytokine production and release in granulomatous tissue [49]. Additionally, male rats have demonstrated higher levels of proteoglycan and collagen, less glycosaminoglycan loss, and greater proteoglycan synthesis than female rats in vitro [45]. Furthermore, cartilage from female rats was shown to have greater susceptibility to degradation when implanted into female rats compared to male cartilage implanted in female rats or both male and female cartilage implanted into male rats [45]. In humans, urinary markers of cartilage degradation have provided evidence that cartilage loss is greater in women than in men [48]. Additionally, cartilage degradation was shown to be greater in postmenopausal women compared to age-matched premenopausal women and less in postmenopausal women undergoing hormone replacement therapy compared to postmenopausal women not undergoing the therapy [48]. Therefore, sex differences in cartilage morphology and metabolism that exist early in life appear to increase with advancing age and may be explained by the difference in sex hormones.

Sex differences in bone tissue also are reported in the literature [5053] and have been associated with differences between men and women in terms of fracture risk [5456]. Sex differences in bone tissue vary by skeletal site [3257], but are reflected by differences in both morphological [3251545660] and remodeling [505253576061] characteristics, particularly in osteoporotic individuals [39576263]. Sex differences in bone tissue are present at an early age [32505158], persist throughout adulthood [3952546061], and diverge even more in older age [39525355575961]. Bone mass accrues during childhood through adolescence and peaks at about the age of 30 years in both men and women [39]. A lower amount of peak bone mass has been associated with a greater risk for osteoporosis in later life; [50] therefore sex differences in the development of bone tissue during youth may partially explain some of the bone tissue differences between adult men and women. Several factors influence bone mass accrual, including nutrition (e.g., calcium, vitamin D), physical activity, lifestyle behaviors (e.g., smoking), genetic factors (race, sex), and hormonal factors (e.g., estrogen) [6264].

In children, serum markers of bone turnover have been shown to change significantly with pubescence in both boys and girls [50]. In girls, these markers were shown to peak at mid-puberty and decrease thereafter; in boys, the markers continued to increase through late puberty [50]. Moreover, the serum markers of bone turnover were overall higher in boys than girls even after adjustment for age, body weight, and pubertal stage [50]. These sex differences in bone turnover more than likely influence differences in peak bone mass [50], volumetric bone mineral density [51], selected measures of bone area [32], cortical thickness [32], plus ultimately compressive [51] and bending [58] strength. However, some authors have suggested that sex differences in some of these characteristics can be explained by differences in anthropometric dimensions (e.g., height and total lean body mass) [58].

In adults, there are several reported sex differences in bone tissue. Differences between men and women have been reported in bone mineral density (men greater than women) [60], peak bone mass (men greater than women) [39], cortical thickness (men greater than women) [54], age-dependent hormonal responsiveness of osteoblasts (less responsiveness in older cells from women when compared to older cells from men) [52], bone turnover (men greater than women) [60], and bone strength (men greater than women) [3954]. In older adults, sex differences in bone mineral density [5557], cross-sectional area [55], cortical thickness [55], bone width [55], and strength [395559] remain considerable, with some differences (i.e., bone mineral density and strength) further diverging as compared to younger adult values [3955]. Furthermore, many of these sex differences do not disappear after adjusting for anthropometric factors such as height and weight [55].

The role of sex hormones in the loss of bone mass in older women has been explored widely in the literature. It is well known that a decrease in estrogen production following menopause is a primary contributing factor to the accelerated loss of bone mass in older women when compared to older men [6263]. However, older men also lose bone mass and are at greater risk for hip and vertebral fractures compared to younger men [53]. Even though men do not experience a physiological event comparable to menopause and therefore do not undergo a substantial decline in total serum testosterone or estrogen, some evidence suggests that a decline in the bioavailable estrogen (non-sex hormone-binding globulin bound) might explain the loss of bone mass in both women and men [53].

3.3.3 Selected Regional Considerations in Female Functional Pathoanatomy

3.3.3.1 Upper Extremity

Shoulder

Sex-based anatomical differences of the shoulder complex are tissue- and pathology-specific. For example, Pandley et al. [65] observed significant differences between men and women in the distribution of the articular branch of the axillary artery, which may influence decisions made during shoulder surgery. Differences have been associated with the incidence of external impingement of the shoulder. While women appear to have less prominent coracoid processes [66], no sex-based differences have been found in the role of the coracoid process in subscapularis impingement [67]. Furthermore, differences in the role of the acromion with external impingement have been observed. Historically, Bigliani [68] classified the acromial shape into three types. Type I processes are flat, Type II are curved, and Type III are hooked. Bigliani [68] suggested that these differences could lend to the incidence of impingement and any subsequent rotator cuff tearing. Later, Bigliani et al. [69] reported that 78 % of all full thickness rotator cuff tears were associated with a Type III acromion. More recent investigations have reported an increased incidence of full thickness rotator cuff tears in women versus men [70]. Selected investigators have suggested that acromial differences are acquired, resulting from altered tension loads imposed by the coracoacromial ligament and deltoid insertions [71]. Getz et al. [72] observed that Type III acromia were more common in female patients and discovered that Type II acromia were related to adaptive shortening of the glenohumeral joint posterior capsule.

Although external impingement has been associated with sex and age, the relationship between age, sex, and incidence of acromial type is controversial [7274]. More recently investigators have suggested that while the inferior surface of the acromion changes increased with age, they were not different between sexes [75]. However, investigators have observed limited sex-based differences in the acromiohumeral distance with the shoulder at rest, where women exhibited a reduced space compared to men [76]. The influence of gender-based differences in scapular position at rest on impingement pathology needs further exploration, where women appear to demonstrate less protraction versus their male counterparts [77].

While men appear to experience more frequent anterior dislocations of the glenohumeral joint [78], women appear to be more predisposed to glenohumeral instability [79]. This disparity appears to be related to the notion that not all joint instability results in dislocation, where grade I and II instability represents increased motion and possible humeral head perching on the anterior labrum, versus the frank dislocation of the head in grade III [80]. While glenoid fossa inclination appears to influence instability incidence [81], few sex differences in this architectural feature have been noted [82]. Recently investigators found that women demonstrate differences in glenoid fossa shape, by being more oval and exhibiting deeper anterior glenoid notches [83]. Although the woman’s predisposition appears to be more related to increased anterior capsular laxity and resultant hypermobility along with decreased joint stiffness [79], further research is merited for studying the relationship between these architectural findings and the onset and persistence of glenohumeral pathology.

Finally, women between the ages of 40 and 60 years are more predisposed to developing idiopathic capsulitis [84]. This condition is associated with increased thickening of the anterior-superior joint capsule at the coracohumeral ligament [85], along with a noninflammatory synovial reaction in the proximity of the subscapularis tendon [86]. These changes demonstrate active fibroblastic proliferation accompanied by tissue transformation into a smooth muscle-like phenotype that is similar to Dupuytren’s disease [87].

Elbow

Women appear to be at greater risk for developing tennis elbow due to tendinosis that emerges from mesenchymal changes in the collagenous constituents of tendons [88]. This condition, typically lasting greater than 12 months in duration, is more likely noninflammatory in nature [89] and affects one of four possible different regions of the tendinous insertions at the lateral elbow. The tendons that are at risk are specifically located about the lateral epicondyle of the distal humerus. The extensor carpi radialis longus (ECRL) originates on the distal 1/3 of anterior supracondylar ridge, possesses almost no tendon at the origin, and demonstrates an immediate transition into muscle. Extensor carpi radialis brevis (ECRB) starts from a 5 mm by 5 mm square area on the superior surface of the lateral epicondyle (10 % of origin) and collagen/fascial layers of intra-compartmental septa that share fascia with the extensor digitorum communis (EDC) coursing distally to the second and third metacarpals, especially fascia associated with third metacarpal. Thus, resistive wrist extension and resistive extension of the second and third metacarpophalangeal joints may be suggestive of tendinopathy at either EDC or ECRB. The ECRB tendon is juxtaposed between the muscle bellies of ECRL and EDC. This common physical finding merits palpatory discrimination between the two regions for a differential diagnosis. The ECRB can exhibit tendinopathy at its origin, along the tendon between ECRL and EDC, or at its musculotendinous junction more distally. Finally, the EDC can be found at the anterior surface of the lateral epicondyle. If involved in a lateral tendinopathy, resistive extension of the second through fifth metacarpophalangeal joints (MCPJ 4 and 5 differentiating this lesion from a lesion of the ECRB). This condition seldom occurs in isolation, but is typically discovered in combination with affliction to the ECRB [90].

The woman’s predisposition for lateral elbow tendinosis is increased when her estrogen decreases, especially after premature hysterectomy (at less than 35 years of age) and/or lowered estrogen levels from other causes [88]. While inflammatory tendinitis involves a chemically mediated inflammation due to tendon injury [88], the tendinosis to which women are more predisposed produces a non-chemically mediated degenerative change associated with long-term tendon stress [91], resulting in a condition that could persist long after 12–14 months. This process produces tissue necrosis that manifests as a “moth-eaten” appearance in the tendon [92]. As a consequence, the tendon becomes friable, along with possible bony exostosis at the lateral epicondyle [88]. This chronic condition can be accompanied by an imbalance between vasodilatory and vasoconstrictive variations [92], substance P, and CGRP proliferation in the vicinity of the affected tendon [92], accompanied by a high concentration of glutamate in the surrounding tissue [93].

Women also appear to be more predisposed to ulnar nerve lesions at the medial elbow [94]. The medial elbow anatomy affords three different predilection sites for ulnar nerve entrapment. The nerve first courses under the arcade of Struthers just dorsal to the medial intermuscular septum. Distally, the nerve courses through the cubital tunnel, whose boundaries are the medial collateral ligament complex (ceiling), medial epicondyle (medial wall), olecranon process (lateral wall), and cubital tunnel retinaculum (floor). In selected individuals, the retinaculum is dorsally bordered by the anconeus epitrochlearis muscle that is innervated by the radial nerve and is activated simultaneous with the triceps, lending to possible entrapment symptoms during resisted elbow extension. The retinaculum that courses from the medial epicondyle to the olecranon tightens with passive elbow flexion, creating increased nerve entrapment symptoms at end-range passive flexion. Finally, the nerve must course under the retinaculum between the two heads of the flexor carpi ulnaris. As a consequence, entrapment symptoms may increase during resistive wrist flexion-ulnar deviation as well.

Wrist and Hand

Only a few afflictions of the wrist and hand appear to differ between men and women. Tendon pathologies, including tenosynovitis and tendinosis, seem to be more frequent in women [95], but data related to the pathoanatomical and physiological influences on these differences have not been explored. Similarly, carpal tunnel syndrome (CTS) is more common [9697] and prolonged [98] in women. However, multiple factors have been elucidated that may contribute to this difference. The etiology of carpal tunnel syndrome is multifactorial, resulting from anatomical, biomechanical, pathophysiological, neuropathological, and psychosocial influences. Anatomical factors, such as tunnel architecture and volume [99], lumbrical anatomy [100], and the shape of the hamate hook [101], have been associated with CTS. Specific anatomical and anthropometric factors appear to influence a woman’s greater predisposition to CTS [102103]. While carpal bone size and scaling do not appear to differ between men and women [104], hand-length ratios, space indices at the wrist, and digital features appear to differ between the sexes [102]. Along with these, differences in body mass index seem to predispose women to CTS [102].

The onset and progression of CTS appear to be related to an increase in intra-tunnel pressure [105]. Different factors appear to increase this pressure, including tunnel space narrowing associated with wrist movements [106107], carpal instability [108], increased muscle force production [109], and trauma that produces perineural edema and fibrosis [110]. Women may be more susceptible to these influences versus men, due to reduced available space for the median nerve within the tunnel. The median nerve appears to increase in cross-sectional diameter with sustained repetitive hand movement in women as compared to men [103], thus compromising the relative tunnel size and potentially increasing pressure within the tunnel in context with the previously discussed factors.

The individual suffering from CTS may experience sensory and/or motor changes, including paresthesias or true numbness that reflects deficits in neurophysiological function. Women have demonstrated greater deficits in neurophysiological function involving the median nerve when compared to men [111112]. However, controversy exists over the value of neurophysiological testing for the diagnosis of CTS [113]. Orthodromic median sensory latency is typically prolonged with CTS patients [114], and median nerve motor amplitudes are decreased in patients with CTS [112]. Yet, Glowacki [115] discovered a poor relationship between electrodiagnostic test outcomes and final symptom consternations. Differences between the median and ulnar motor latencies appear to be important for the diagnosis of CTS [111]. Padula [116] found that the difference between the median and ulnar motor latencies was greater in patients experiencing CTS versus controls.

The presence of autonomic disturbances appears to be associated with the woman’s predisposition to CTS [117]. This disturbance in neural function could be related to local sympathetic fiber stimulation and/or brachial plexus irritation associated with a double crush phenomenon, which has been observed in as many as 40 % of all patients suffering from CTS [118]. As result, a vasoconstrictive event could lead to decreased perineural microvascular flow and increased protein leakage from the vascular supply that produce epineural and perineural edema [110], as well as increased endoneural pressure and ischemia [119], contributing to the symptoms of CTS.

3.3.3.2 Lower Extremity

Hip Joint

Women are at greater risk for both microtraumatic stress fracture [120] and macrotraumatic frank fracture at the hip [121] especially involving the femoral neck [120]. This predilection appears to be influenced by differences in bony architecture about the hip and pelvis [122]. Acetabular depth and femoral head width appear to be less in women versus men [123]. The coxadiaphyseal angle has been reported to be wider in men versus women in selected races, thus potentially predisposing the women to higher incidence of stress reactions [124]. Women appear to have decreased femoral neck strength versus men, as evidenced by decreased femoral neck cross-sectional moment of inertia (CSMI) [125126]. Compressive stress (Cstress), defined as the stress in the femoral neck at its weakest cross section arising from a fall, is higher in women [126]. These features interact with women’s altered estrogen associated with menstrual irregularities [127] and menopause [121], thus enhancing their fracture risk predisposition. Over the past decade, postmenopausal women have relied upon the long-acting, bone density-maintaining effects of bisphosphonate administration for reducing the rate of fragility fractures in this population [128]. However, this benefit has been accompanied by an increase in atypical subtrochanteric fractures at a younger age in response to chronic use [128], especially witnessed in Asian females [129130]. The risk associated with bisphosphonate use continues to be small but controversial [131], where comorbidities and management strategies should be assessed when its usage is considered [128].

The outer margin of the hip acetabulum is completely lined with the cartilaginous labrum that serves to enlarge the articular surface [132133]. The labrum enhances the articular seal, fluid pressurization, load support, and joint lubrication of the hip joint [134]. The labrum possesses a variety of sensory endings important to proprioception and nociception [135]. The labrum is vascularized in a fashion similar to the meniscus of the knee, where the outer margins are well vascularized and the inner margin is lacking in blood vessels [136]. The labrum is at risk for traumatic vertical, as well horizontal, degenerative tears [133137]. The propensity for tears is increased by the deficiencies in the mechanical properties of the labral tissue, especially in women. Labra obtained from male patients have stronger tensile stress than those from female patients [138]. Moreover, labral degenerative changes may influence those same mechanical properties, adding to the risk of tearing [139140].

Labral tears appear to occur more frequently in the superior region of the acetabular structure, due to decreased mechanical properties accompanied by increased demand [139141]. The superior region of the labrum appears to be less well vascularized, lending to the susceptibility of that region to traumatic and degenerative tears [139141]. One significant mechanical contribution to this loading demand is the impact of the femoral neck against this region during full flexion of the hip [140]. Femoral neck architecture also appears to differ between men and women, where increased thickening and decreased coxadiaphyseal angulation of the neck and deformation/fullness of the neck diameter in older women predispose them to anterior acetabular labral trauma, especially when the hip is positioned in full flexion [142]. However, severity of such deformations and changes observed with imaging do not appear to correlate with the incidence of femoral-acetabular impingement and subsequent labral lesions [143], making the clinical examination paramount to diagnosis.

Knee Complex

Little evidence is available to describe sex-based differences in the patellofemoral complex of the knee. One might explain differences in terms of cartilage volume, where sex explains 33–42 % of the variation in knee cartilage volumes with women demonstrating decreased cartilage volume versus men [42]. However, T2 MRI examination of young, healthy volunteers did not reveal sex-based differences in the magnitude or spatial dependency of cartilage [144].

Investigators have attempted to describe sex-based differences in terms of patellofemoral contact areas at various positions of knee flexion. In males, Csintalan et al. [145] observed larger contact areas of posterior patellar surfaces with the knee flexed to 30°. In addition, they observed a greater change in the female’s contact pressures in response to varying vastus medialis activity with the knee positioned at 0°, 30°, and 60° flexion. While no differences were seen by Besier et al. [146] with the knee in full extension, they observed larger contact areas in male patellofemoral joints with the knee flexed to 30° and 60°. However, the contact areas were not different when the data were normalized by patellar dimensions of height and width. Investigators have turned their attention to the role of hip control deficits in landing in the development of anterior knee pain syndrome (AKPS), where decreases in eccentric control from the hip external rotators and abductors were shown to be associated with increased AKPS [147]. However, Cowan and Crossley [148] found no relationship between hip control deficits and gender in subjects suffering from AKPS.

More striking is the relationship between sex and knee ligament injury. Injury to the anterior cruciate ligament (ACL) can be a devastating event, and a woman’s increased risk for this injury over male counterparts is well documented [149150]. It has been reported that 70 % of all ACL injuries are a result of a non-contact mechanism [149151], where girls and women appear to tear their ACL two to eight times more frequently than men [152].

Since the reason for this increased ACL injury risk is unclear, investigators have explored many possible causes including anatomical, hormonal, and mechanical differences. One of the classic anatomical factors attributed to sex-based differences in ACL injury is the width of the femoral intercondylar notch or Grant’s notch. The intercondylar notch is found in the roof of the space between the femoral condyles, lending a point where the ACL could crimp or tear during forced rotational non-contact loading [150]. The female knee was once thought to possess a smaller notch versus men, lending them to greater vulnerability for traumatic tears [1214]. However, other investigators have suggested that increased female risk was based on differences in the ratio between the notch width and width of the femoral condyles (notch width index) [153].

The role of the notch width has remained controversial. Charlton et al. [154] have suggested that the narrower notch width in the female knee simply reflects the smaller diameter ACL within the notch, which still must constrain the same relative loads and stresses as the male ACL. This difference in diameter, along with an increase in creep deformation under sustained loading [155], subsequently renders the ligament to greater injury potential in female athletes. Murshed et al. [156] found no differences in notch width characteristics between the sexes, and Ireland et al. [13] suggested that any individual with a smaller notch width is at higher risk for injury, regardless of sex. More recently investigators have suggested that different regions of the notch may vary in width, where women appear to demonstrate greater narrowing at the base and middle of the notch versus their male counterparts [157]. In response, investigators turned to MRI three-dimensional (3-D) notch volume analysis to better describe differences. Van Eck et al. [157] found that males exhibited a larger 3-D notch volume versus females, furthering the disagreement regarding the role of the notch in female ACL injury risk.

Other morphological characteristics have been examined in terms of their contribution to increased ACL injury risk in females. Investigators have noted that the disparity between the intercondylar axis and joint motion axis of the femur at the knee to be greater in females and compounded in those females with ACL injury history, suggesting its role in ACL tear risk [158]. Investigators discovered that the strain of the anterior–medial bundle of the ACL was increased in females versus males and that this relative strain pattern was positively correlated with ACL cross-sectional area and lateral tibial slope [159]. Finally, Hohmann et al. [160] discovered that ACL-injured females demonstrate a significantly greater posterior tibial plateau slope versus an uninjured control group.

Static knee postural and alignment characteristics have been considered to be factors that could contribute to the woman’s greater risk for ACL injury [161]. The Q angle is a clinical measure used to determine the position of the knee in the frontal plane [161]. Livingston and Mandigo [162] compared Q-angles between male and female lower extremities and found no significant sex or right-to-left lower limb differences. Conversely, Tillman et al. [16] reported that women exceeded men in quadriceps angle (Q-angle) and thigh–foot angle (TF angle) [163]. Yet, the TF angle, which is a measurement of tibia external rotation (toeing out), is not clearly linked to ACL injury [164].

Biomechanical features have been linked to sex-based ACL injury predisposition [164]. Kinematically, differences in knee flexion angle at contact (<30°), tibial rotation in the coronal plane, and frontal plane motion have all been implicated [165]. Investigators have linked reduced contact and peak knee and hip flexion during selected load-bearing functional activities with female ACL injury [166167]. Similarly, investigators have observed decreased peak hip abduction in women when cutting during sports [166168].

Female athletes have exhibited increased valgus motion in the frontal plane during a landing or cutting maneuver, which may serve as a factor in female ACL injury predisposition [169]. Numerous investigators have observed this behavior [166167170], along with an increased variability in the valgus motion during the landing and/or cutting sequence [166]. Hewitt et al. [171] reported that these excessive lower limb motions could be reduced through appropriate jump training. Yet, sex-based sagittal [170] and frontal [172] plane movement differences have been disputed, where expected differences did not emerge, and the authors suggest that other factors are at play in producing the increased injury risk for the female ACL. Additionally, other authors have reported increased coronal plane excursion for the hip and knee in women versus men during drop-landing activities [173], producing increased internal rotation of the lower extremity during those activities. Finally, Joseph et al. [174] examined the timing of kinematic occurrences during a landing sequence in men and women. They found that maximal hip adduction, knee valgus, and ankle eversion occurred significantly earlier in women versus men. Moreover, maximal hip adduction and knee valgus occurred before maximal knee flexion in women versus after in men. Maximal ankle eversion occurred earlier in women than in men and women produced a significantly higher angular velocity of knee valgus versus men. The authors concluded that these differences predisposed the women to increased ACL injury.

Increased joint laxity and anterior tibial translation are associated with non-contact ACL injury [164175]. Trimble et al. [164] reported that sex and excessive subtalar joint pronation are the only predictors of knee joint laxity. Women exhibit increased anterior knee joint surface translation during extension [176]. This is accompanied by reduced protective hamstring activity during that translational movement [152176] that renders the female ACL less protected when exposed to anterior shear forces [177]. In a similar fashion, the female’s ACL injury predisposition may be related to the excessive subtalar joint pronation in the ankle [164166178], which appears to promote the previously discussed increase in tibial internal rotation [178] and tibial anterior translation [164]. This behavior does not appear to relate to genu recurvatum and the tibiofemoral angle [164]. In contrast, other authors have not observed the sex-based differences in subtalar pronation [16].

Differences in kinetic behaviors when cutting or landing have been attributed to increasing the female ACL injury risk [179]. Female athletes have been found to exhibit reduced peak knee flexor moments [180] and increased peak knee valgus moments [167180181] during cutting tasks. Similarly, Kernozek et al. [167] found increased greater peak vertical and posterior force than men during landing. These altered kinetic behaviors are accompanied by reduced leg stiffness during rapid load-bearing [182], which could translate into a reduced ability to diffuse stress from the ACL [183].

Load management appears to be related to appropriate co-contractive behavior between the quadriceps and hamstrings during cutting and landing. Thus, differences in muscle activation, timing, coordination, and force production may serve as a contributing factor to the female ACL injury predisposition [184185]. The woman’s difference in muscle activity may begin early, as girls developmentally increase the quadriceps strength disproportionately more than the hamstring strength [186]. While da Fonseca et al. [187] questioned the role of sex in co-contractive disturbances, several authors have suggested that female athletes exhibit greater quadriceps versus hamstring activity during landing and cutting [188190]. Similarly, women appear to demonstrate prolonged quadriceps recruitment and reduced hamstring activation during the post-contact phase of cutting versus male counterparts [191]. Increased soleus [188] and gastrocnemius [189] activity may contribute to the woman’s muscular recruitment differences, while decreased hamstring activity may reduce the woman’s ability to decelerate and control tibial translation, internal rotation, and anterior shearing [176192]. These differences may be exaggerated by prolonged exercise causing muscular fatigue. Stern et al. [193] found that after exercise, females exhibited significantly less quadriceps motor-evoked potential EMG amplitude compared to males, which may contribute to females’ increased risk for ACL injury in response to changes in central nervous system drive capacity.

As a consequence of the woman’s differences in kinematic, kinetic, and neuromuscular control strategies, the ACL potentially sustains greater loads with athletic activity. Unfortunately, the female ACL demonstrates different mechanical behaviors amidst these altered strategies. During passive cyclic loading, the female ACL appears to exhibit greater creep versus the male ligament [155]. This difference in ligament creep could compound the previously discussed deleterious mechanical effects, as quadriceps electromyographic activity may increase after ACL creep, while hamstring co-activation is not likely to change [194].

Another factor that appears to interact with the anatomical and biomechanical influences contributing to female ACL injury predisposition revolves around changes in sex hormones. Ovarian sex hormone fluctuations have been related to increased non-contact ACL injury [177195]. Estrogen and progesterone receptors have been identified within the substance of the ACL [196], likely responsible for the relationship between peaks in estrogen levels and increased laxity [197]. Exposure to estrogen appears to increase metallomatrix protease activity and decrease fibroblastic activity within the ligament, lending to increased tissue laxity [197]. More recent findings suggest that increased estrogen levels negatively correlate with hamstring rate of force production, suggesting a reduced protective response from that important muscle group during upswings of estrogen [198]. However, correlation between injury risk and specific menstrual phases is controversial at best, since other authors have reported increased ACL injury during the follicular [177199] and luteal [199] phases.

Ankle and Foot

Women appear to be more predisposed to ankle and foot injuries than men in selected populations. Heir [200] found that women in military physical training were at greater risk for developing Achilles tendinopathy and ankle sprains than their male counterparts. Moreover, Knobloc et al. [201] found that symptomatic females suffering Achilles tendinopathy do not benefit as much as symptomatic males from 12 weeks of eccentric training in terms of pain reduction or improvement in functional scores. Structural differences have been noted in the female foot, which demonstrate smaller width and length, as well as specific shape [202]. However, sex differences are not observed in terms of medial longitudinal arch measurements [203] or overall arch height [204].

Women appear to be at greater risk for ankle inversion trauma. Hosea et al. [205] found that female athletes are at greater risk for grade I inversion trauma, where there is disruption of the anterior talofibular ligament. The same authors found no sex-based differences in grade II (anterior talofibular and calcaneofibular ligament involvement) or grade III (the same ligaments plus the posterior talofibular ligament). This predisposition is related not only to the sports in which female athletes participate [206] but also by selected structural differences in the lower extremity. Female athletes’ risk of ankle inversion trauma is increased by increased tibia varum and rearfoot eversion while weight bearing [206]. Neuromuscular responses may contribute to the female’s predisposition to ankle inversion trauma. Investigators reported that while males demonstrate decreased peroneus longus reflex amplitude following neuromuscular fatigue, the same reflex in females increased, suggesting the female’s reduced protective response during a sudden inversion perturbation [207].

Other structural differences have been noted that add to the female’s predisposition to ankle and foot affliction. Women demonstrate decreased cartilage thickness over the talar dome, which is at risk for developing osteochondritis and necrotic changes [208]. Women also appear to demonstrate increased obliquity of the first metatarsal base, resulting in increased metatarsus primus varus and potential increased incidence of clinical hallux valgus [209]. Additionally, women demonstrate increased incidence of hallux rigidus in the first metatarsophalangeal joint, with the vast majority of the subjects demonstrating a flat joint configuration.

Achilles tendinopathy has been attributed to numerous factors, including histochemical, pathomechanical, and neurophysiological influences. In addition, sex has been touted as a factor lending to the development of Achilles tendinopathy, possibly interacting with other factors. Marked deficiencies have been noted for tissue histochemical responsiveness in female rabbit tendons [210]. Sex-based factors may contribute to differences in tendon pathology and response. For example, a female’s tendon may experience increased load in response to footwear with hard soles and insufficient rearfoot control or high heels [211], all of which have been associated with increased incidence of tendinopathy. However, this sex-based predilection for Achilles tendinopathy is controversial, where more recent studies have questioned differential female predisposition [212]. Sex-based differences in Achilles tendon properties and pathology may be related to muscle and tendon strength differences, rather than other sex-specific tissue characteristics [213].

3.3.3.3 Spine

Cervical

Sex-related anatomical differences in the cervical spine can be observed in the vertebral structure [214215], lending to clinically relevant differences in bony processes and the joints they form, as well as the foraminal spaces through which important neurovascular components course. For example, investigators have observed sex differences in dimensions of lower cervical vertebral laminae and pedicles. Rezcallah et al. [214] found that women demonstrated smaller pedicular widths, lengths, and transverse angles at C3 through C7 when compared to men. Similarly, Xu et al. [215] observed smaller laminar height, width, thickness, and angulation in women at levels C2 through C5. The role of sex-based cervical vertebral structure differences is not clear [216]. Yet, men appear to have a smaller vertebral canal-to-body anterior–posterior diameter ratio versus women, potentially predisposing them to a smaller canal in proportion to their overall axial skeletal morphism and decreased incidence of cervical myelopathy [217]. Conversely, women appear to demonstrate greater spinal canal narrowing after whiplash-type injury versus their male counterparts [218], possibly contributing to their increased incidence of whiplash-related disorders [219], as well as the latent clinical sequelae and delayed recovery status post-whiplash trauma [220221].

Female cervical zygapophyseal (facet) joints may be at greater risk for injury during a whiplash trauma versus their male counterparts. Excessive segmental translation has been shown to be a potential cause of injury. Simulated rear-impact vehicular accidents using human volunteer subjects showed greater degrees of cervical retraction in women that were unaware at time of rear-end impact [222].

Yoganandan [223] found that the facet articular surfaces in female cadavers were less adequately covered by cartilage than similar specimens in men. In addition, these joints exhibited a greater distance from the dorsal-most region of the facet joint to the location where the cartilage began to appear (called a cartilage gap), potentially lending these joints to greater translation during unanticipated loads. Stemper et al. [224] found that female cadaveric specimens exhibited increased compression in the dorsal region of the facet joint during the early phase of whiplash. These biomechanical behaviors could predispose the female facets to injury in the subchondral bone during normal physiological and traumatic loads, especially when accompanied by endplate perforations and older age.

Female cervical discs may be at greater risk for failure when exposed to unexpected abnormal loads. Truumees et al. [225] examined the geometric characteristics and loading response of the cartilaginous endplates found in cadaveric cervical discs. They found that the female sex was associated with significantly lower endplate fracture loads when exposed to compression.

Thoracic

The primary sex-based differences that have been observed in the thoracic spine appear to center around differences in postural alignment. Fundamental sex-based differences have been observed in children and adolescents regarding the extent of the thoracic kyphosis in the sagittal plane. The presence and severity of kyphosis is especially more marked in women [226]. Thoracic kyphosis changes as children age, where the rate of change is greater in women versus men [227]. This change in the kyphotic curve seems to progress in a fashion similar to the lumbar spine lordotic curve during childhood. However, the relationship between the change in kyphosis and lordosis decreases in girls by the age of 15, but not in boys [228].

Of greater interest are the sex-based differences in the development and progression of adolescent idiopathic scoliosis (AIS), where the individual develops a rotatory 3-D deformation in the thoracolumbar spine, especially in the frontal plane. A single thoracic curve is the most common in selected populations, followed by other configurations of single and double curves in the thoracolumbar spine [229]. Investigators have reported increased incidence of AIS in female adolescents [229231], where female prevalence appears to be genetically coded [232]. Girls appear to be at greater risk for developmental curve progression versus boys, especially in the age prior to the onset of menses [231]. Similarly, girls with scoliosis generally grow faster than girls without the same condition [233]. Yet while an age of more than 15 years, skeletal maturity, postmenarchal status, and a history of spine injury are all associated with the prevalence of back pain in people with AIS, sex, family history of scoliosis, leg length discrepancy, curve magnitude, and spinal alignment are not [234].

Investigators have looked at not only the interactions of physical changes with sex in scoliotic patients but also the role of sex affecting psychosocial factors associated with the condition. Girls with scoliosis seem to be at greater risk for psychosocial stresses, including feelings about poor body development, troubled peer interactions, and health compromising behaviors [230]. Yet, while investigators have evaluated the impact of scoliosis on health-related quality of life (HRQoL), the role of sex in that evaluation is controversial. While Ugwonali et al. [235] discovered that male adolescents scored higher on validated self-report instruments that measured HRQoL, Bunge et al. [236] found no effect of sex, curve type, and curve size on a similar battery of measures. Additionally, adolescents undergoing brace-based management did not appear to score on HRQoL instruments differently from age-adjusted norms.

Lumbar

Premenopausal women demonstrate decreased bone density in the lumbar vertebrae versus men [237]. Ebbesen et al. [238] found that adult women demonstrate lower vertebral bone mass than age-matched men. Additionally, women exhibited decreased compressive load tolerances, accompanied by increased mechanical stress [238239]. However, bony differences may not be limited only to adults. Gilsanz et al. [240] found similar bone mass differences in preadolescent and adolescent girls, who demonstrate lower vertebral bone mass than age-matched boys. However, these investigators reported that these differences were likely related to differences in bone size versus bone density [239240]. Naganathan and Sambrook [241] went further to report that volumetric bone density of the third lumbar vertebra did not differ between the sexes, whereas observed differences in areal bone density were likely related to differences in bone size.

An account of sex-based differences in low back pain (LBP) is controversial. However, Korovessis et al. [242] has reported a higher incidence of LBP and dorsal pain (DP) in female youth, especially with those girls involved in sports. However, the structural etiology of sex-based differences in back symptoms has not been fully elucidated. Evidence of differences in intervertebral disc structure or function is scarce. Gruber et al. [243] found that the female sex was a contribution to the cellular proliferation potential within the annulus fibrosis surrounding the disc nucleus pulposus, along with a contribution from increased age, degree of degeneration, and surgical modification. Investigators have found differences in lumbar zygapophyseal facet size, pedicle facet angle, and facet shape [244245]. These differences appear to be related to a greater incidence of degenerative anterolisthesis at L4 in women versus their male counterparts [244].

Sex differences are observable in the posture and postural control of the lumbar spine. Norton et al. [246] found a greater lumbar spine lordotic curve in women versus men. O’Sullivan et al. [247] examined the impact of unstable versus stable sitting surfaces on recruitment and control of the superficial lumbar multifidus, transverse fibers of internal oblique, and iliocostalis lumborum pars thoracis. While these investigators found no sex-based electromyographic (EMG) differences, they did observe that women exhibited greater medial–lateral postural sway versus men on an unstable surface. While the role of posture and postural control in the development of LBP is inconclusive, future studies could examine the role of these neuromuscular factors on the development of lumbar pathology.

Investigators have observed sex-based differences in lumbar muscle cross-sectional area and muscle geometry [248]. These differences, coupled with differences in trunk motor control strategies, could have an influence on biomechanical behaviors of the lumbar spine. Women exhibit decreased type II fiber diameter versus men, lending to decreased strength and increased endurance of the lumbar muscle groups [249]. Moreover, women appear to experience greater compressive and anterior–posterior shear loading at the lower lumbar spine [250]. These loading differences appear to be related to altered coactivation of the muscles surrounding the lumbar segments, where women produce greater flexor antagonistic coactivation than men [251]. It is likely that these altered behaviors result in distorted strategies for controlling dynamic spinal loading conditions. For example, Granata et al. [252] examined the effect of sustained flexion postures on protective paraspinal muscle reflexes. These investigators not only observed a detrimental alteration in the reflexive activity after sustained trunk flexion but also found that women demonstrated greater detriments in the protective reflexive response. Moreover, they appear to have decreased stiffness and increased segmental motion in the lumbar spine versus men [253]. These factors added together could lend the female lumbar segments to the development of clinical lumbar instability [254].

Sacroiliac and Pelvis

Women appear to suffer from pain associated with the sacroiliac joint (SIJ) more frequently than men, most likely associated with anatomical differences and hormonal fluctuations. The incidence of SIJ clinical hypermobility in the joint is greatest between the ages of 18 to 35 years. However, this prevalence appears to be sex-specific, where the SIJ mobility begins to decline at 35 years old in men and 45 years old in women. Thus, SIJ-related pain that is associated with clinical instability could persist in women after 45 years, especially when the individual is on estrogen replacement.

Anatomical changes are seen in the SIJ throughout the course of life, and those changes appear to be different between men and women [255257]. By the second decade, differences between the sexes are observable [258]. While the male synovial capsule thickens and the joint architecture visibly adapts, the female SIJ soft tissues become more pliable as hormones fluctuate with the onset of menses.

Although the sacral vertebrae start ossifying in the third decade, the mobility of the female SIJ continues to increase, producing a ratio of mobility of approximately 5:1 compared to men. Pregnancy can increase mobility of the sacroiliac joint 2.5 times, increasing the dynamic movement disparity between women and men [258]. Movement persists in the female SIJ through the fourth and fifth decades, whereas the male SIJ demonstrates a further decline of motion in the same time frame [259260]. While complementary ridges and depressions form on the iliac and sacral cartilages and the synovial membranes thicken in both men and women, men appear to be more prone to the development of periarticular osteophytes and sacroiliac bridging, further lending the male SIJ to decreased mobility [258261263].

The external contours of the SI joint articular surfaces are generally a C-shape in men and an L-shape in women, lending the female articulation to greater translation during select situations such as pregnancy and delivery. The joint surfaces at the S1 level are the largest compared to the smaller surfaces at S2 and S3. Each SIJ surface is approximately 17.5 cm [2] in surface area, well suited for absorption and transfer of large forces [264]. The sacroiliac joint itself is found deep within the sacrum and ilium. The iliac cartilage is thin (0.5 mm), bluish, dull, and rough, compared to the sacral cartilage which is thick (3 mm), white, shiny, and smooth [261]. The iliac cartilage is the same relative thickness in both sexes, in contrast to the sacral cartilage which is thicker in women [265].

Women present with SIJ-related pain in the last trimester of pregnancy [266267], in response to increased relaxin that changes the stiffness of the elaborate ligament system and produces a hypermobile state in the joint [268269]. The ligamentous system of the SIJ enhances stability by increasing the friction in the SI joint and contributes to a self-locking mechanism [270272]. In addition, the system offers proprioceptive feedback in response to activity due to a rich plexus of articular receptors.

The SIJ ligament system can be divided into four different layers, the most superficial layer being the thoracolumbar fascia to which numerous muscles attach and impose control, including the latissimus dorsi, gluteus maximus, transverse abdominis, and serratus posterior inferior [272]. The next layer associated with the SIJ includes the sacrospinous and the sacrotuberous ligaments that constrain sacral nutation (or anterior sacral rotation about its internal transverse axis of rotation at S2) and control movement of the pubic symphysis on the anterior aspect of the pelvic ring [273274].

The long dorsal SI ligament (also known as the longissimus ligament) courses from the posterior superior iliac spine to the inferior lateral sacrum outside the coccyx. This ligament is approximately 2 cm wide and 6 cm long. It is the only ligament that maximally tightens during counter-nutation, lending it to strain and clinical symptoms during a woman’s third trimester of pregnancy after the fetus descends [275]. The iliolumbar ligament constrains both SIJ movements and movement of the lower lumbar segments with respect to the sacrum [276277]. Along with the dorsal SIJ ligaments that are less developed in females [278], the self-locking mechanism is further enhanced through the constraints imposed by the deep interosseus ligaments, especially during nutation [270272]. These ligaments are found to be thicker in females [278], and their stiffness decreases under the influence of hormonal changes in the final stages of pregnancy, so that the birthing process can be enabled. Moreover, the SIJ tends to counter-nutate during these stages, where the sacral base tips posterior and opens the pelvic inlet for fetal decent. Counter-nutation reduces ligament constraint and promotes joint hypermobility that can contribute to postpartum pelvic pain [268279280], which can persist several years after birth [281].

3.4 Contemporary Understanding of the Issues

Sexual dimorphism in the human musculoskeletal system is apparent, but more subtle than sex differences often observed in other species. Some musculoskeletal sex differences in humans are present at an early age, while others tend to appear later in life, especially at puberty and menopause. Sex differences in gross skeletal geometry and specific tissue characteristics are common. For example, women are generally shorter, have less body mass, and have a different general morphological appearance than men. Women tend to have different characteristics of specific bones and skeletal features than men, which have been explained by both genetic and environmental factors. In the pelvis, women tend to have a larger inlet and outlet, greater interacetabular distance, and greater hip width normalized to femur length. In the femur, women tend to have greater femoral anteversion and narrower intercondylar notch features. Additionally, there are several differences in specific characteristics of the femur, such as head diameter and circumference that are relatively strong predictors of sex. In the knee, tibia, and foot, women tend to exhibit greater genu recurvatum, greater quadriceps angle, more lateral patellar alignment, increased tibial torsion, greater tibial slope angles, and more bunions and deformities of the toes. Additionally, women and men appear to have several differences in collagenous, cartilage, and bone tissues, which may predispose women to certain pathologies such as osteoarthritis and osteoporosis later in life. In other collagenous tissues, there are sex differences in collagen thickness, orientation, content, diameter, volume, and metabolism. In cartilage, women tend to have less cartilage volume, thickness, and surface area at specific sites. Additionally, prepubescent girls tend to have slower cartilage accrual rate, and postmenopausal women tend to have greater cartilage degradation than their male counterparts at either age, respectively. In bone tissue, women tend to have a slower accrual rate in youth, less peak bone mass, and slower bone turnover in adulthood as compared to men. Additionally, women tend to have decreased volumetric bone mineral density, less bone area, decreased cortical thickness, plus less compressive and bending strength at some bony sites compared to men, even after correction for anthropometric differences such as height and weight.

Sexual dimorphism can manifest itself by specific differences in each joint system throughout the body, possibly resulting in differences in clinical pathology and symptomology. While differences in subacromial space have been attributed to sex-based differences in clinical impingement at the shoulder, the role of those variations remains controversial. More trustworthy are the female glenohumeral capsular responses that appear to contribute to sex-based differences in the incidence of joint laxity and/or idiopathic capsulitis. Hormonal differences appear to affect tissue changes related to the woman’s higher incidence of tendinosis in the lateral elbow tendon structures, while the female predisposition for increased incidence of carpal tunnel syndrome seems to relate to differences in architectural shapes in the wrist and hand especially found around the tunnel. Similarly, architectural differences are at the root of the female predilection for fracture responses at the hip joint, while tissue biomechanical differences accompany architectural distinctions in contributing to the female incidence of acetabular labral tears.

The woman’s increased risk for anterior cruciate ligament injury has received special attention in the literature, which has suggested that several factors are responsible for this elevated incidence. Anatomical, hormonal, mechanical, and neurophysiological differences have all been examined, and multiple mechanisms have been proposed. While the femoral intercondylar notch has been examined, its role remains controversial. Similarly, the role of static measures including Q-angle and thigh–foot angle has remained questionable, while differences in joint movement at both the knee and hip during cutting and landing have been deemed partially responsible for sex-based differences in ACL injury. Joint laxity and tibial translation behaviors appear to contribute to this disparity, along with altered joint motion responses in the subtalar joint. Moreover, locomotor control strategy differences are exhibited by female athletes, lending to their heightened predisposition. Finally, the woman’s hormonal fluctuations affect not only changes in ACL architecture but also the biomechanical response of the tissue to stress, along with neuromuscular control of the extremities.

Selected sex-based differences have been observed in the structural and mechanical features of the ankle and foot. These differences appear to contribute to the woman’s increased risk for both ankle sprains and Achilles tendinopathy, especially in those who are athletically inclined. The role of cervical spinal architectural differences in female musculoskeletal health is unclear. However, the woman’s cervical spine structures that include the facets and intervertebral discs appear to respond more poorly to macrotrauma, such as whiplash. The preadolescent female is more susceptible to developing thoracolumbar postural changes that include excessive kyphosis, lordosis, and/or scoliosis. These differences not only are influenced by physical differences in the vertebrae, articular structures, intervertebral discs, and/or attached musculature but are apparently influenced by psychomotor control and psychobehavioral variations as well. Finally, sacroiliac joint differences between men and women are influenced not only by architectural disparities but additionally by the influence that hormones have on the integrity of the complex capsuloligamentous structures surrounding the joint itself.

3.5 Future Directions

Sex differences in musculoskeletal anatomy are evident in gross body structure, regionally and at the tissue level. Much is known about sex-based differences in musculoskeletal anatomy and how these differences manifest in functional and health-related disparities. However, a great deal of information remains unknown. Future research should continue to investigate the relationships among structure, function, and health, especially in relation to sex-based differences. A few specific recommendations for future directions include the need to better understand:

·               The influence of sex-based differences on scapular position at rest and during elevation with external impingement of the shoulder

·               The influence of sex differences on the relationships among glenohumeral structure, hypermobility, and pathology

·               The influence of pathoanatomical and pathophysiological mechanisms on sex-based differences in tendon pathologies such as tenosynovitis and tendinosis

·               The comorbid fracture risks associated with chronic bisphosphonate administration used for treating osteoporosis

·               The role and influence of sex-based differences in the development of hip neuromuscular control deficits in the development of knee disorders such as anterior knee pain syndrome and ACL injury

·               The influence of sex-based differences on posture and postural control in the development of low back pain and lumbar pathology

3.6 Conclusion

There exists an abundance of literature that documents sex differences in general body characteristics, skeletal geometry, musculoskeletal tissue characteristics, and joint-specific functional anatomy and pathomechanics. While the musculoskeletal anatomy of men and women is grossly similar, important differences exist that may influence the way in which the general public views and health care professionals respond to women’s musculoskeletal health issues.

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