Carla Baldrighi and Scott N. Oishi
DEFINITION
Multiple hereditary exostoses (MHE), first described by Boyer in 1814,2 is a familial disorder with an autosomal dominant mode of inheritance.8 It is also known as multiple osteochondromatosis, multiple osteochondromata, multiple cartilage exostoses, diaphyseal aclasis, or metaphyseal aclasis.5,18
MHE is a developmental disorder affecting numerous sites in the immature skeleton, except the skull. It is characterized by abnormal proliferation of epiphyseal chondroblasts that causes a subsequent defect in remodeling of the metaphysis. In the immature individual this leads to the two main characteristics of this condition: skeletal metaphyseal bony prominences capped with cartilage (exostoses) and retardation of longitudinal bone growth.
ANATOMY
Knowledge of the normal anatomy and biomechanics of the forearm in the immature individual is instrumental in understanding the pathogenesis of the deformity, and ultimately in planning appropriate treatment.
The ulna acts like a swivel hinge around which the radius rotates.
During forearm pronation–supination the relationship between the two forearm bones changes. This rotational movement requires perfect alignment of both radius and ulna as well as integrity of the ligamentous structures around the proximal and distal radioulnar joint and the interosseous membrane. Minimal axial or rotational bone deformity, asymmetric bone shortening, or ligament instability can hinder this function.
PATHOGENESIS
Osteochondromas are the most common benign bone tumor. Histologically, they resemble the epiphyseal growth plate and consist of a bony stalk covered by a cartilaginous cap. They arise from the peripheral aspect of the growth plate of bones that undergo endochondral ossification.10
About 15% of osteochondromas occur as MHE.22 In MHE, the exostoses vary greatly in number, location, size, and configuration. They tend to have a more irregular and bizarre shape than solitary osteochondromas. They also typically involve a significantly greater portion of the metaphysis or diaphysis. Progressively larger and more mature-appearing lesions with ossification are seen on the surface of the bone as the distance from the physis increases, so they appear to be migrating into the diaphysis of long bones.10
NATURAL HISTORY
The prevalence of MHE in the general population is estimated to be at least one in 50,000, with a median age of first diagnosis of 2 to 3 years of life (exostoses rarely develop before 2 years of age).17
In individuals with MHE an average of five or six exostoses, involving both upper and lower extremities, are found at the time of the first consultation.22
Over time the upper and lower extremities may appear disproportionately shorter compared to the trunk.
The phenotypic penetrance of this condition is age-related. By 2 to 3 years of age, 50% of the affected individuals show signs of the disease; the presence of exostoses is almost always evident by the age of 12. Once skeletal maturity is achieved most of the lesions will become quiescent and often will ossify.17 The overall penetrance of the isease in adult obligate heterozygotes ranges between 93% and 100%.7,8,28
Approximately 10% of individuals with documented manifestations of multiple exostoses have no family history of MHE.17
It is reported that 30% to 60% of the individuals with MHE show a deformity of the forearm.17 The natural history of forearm deformities is progressive and results in a variable and worsening amount of weakness, pain,4 functional problems (such as dislocation of the radial head, limited pronation–supination, ulnar deviation at the wrist, and compression on adjacent structures), and cosmetic deformity.
The deformities are almost always accompanied by discrepancy in length between the two bones. The asynchronous rate of longitudinal growth in an anatomic region where two bones are paired in close longitudinal relationship leads to a greater risk of anatomic distortion. Most of the longitudinal growth of the ulna occurs at the distal physis,16
which is also the more commonly affected physis (30% to 85% of the cases).17,18
A serious complication of MHE is the potential for malignant transformation of an exostosis into chondrosarcoma. This can occur at any age but it is exceedingly rare during childhood.17
The risk of malignant degeneration in adults with MHE is 0.57% to 5%.8,17,28
The clinical course of the chondrosarcoma is slow and the prognosis is in general favorable if the tumor is detected early. Metastases occur late, usually to the lung, via hematologic dissemination.
Patients affected by MHE have a normal life expectancy unless malignant degeneration and metastasis develop.22
HISTORY AND PHYSICAL FINDINGS
In the context of progressively enlarging, juxta-articular protuberances, the characteristic forearm deformities may develop.
An accurate physical examination of the upper extremity, including evaluation of the comparative length of the forearms as well as range of motion of the elbow, wrist, and forearm (flexion and extension, radial and ulnar deviation of the wrist, varus and valgus angle of the elbow, and pronation–supination of the forearm), is instrumental to assess the progression of the condition.
The classic clinical description is a bowed, short, and knobby-appearing forearm with the wrist in an ulnarly deviated position, which limits radial deviation.
FIG 1 • A. Significant ulnar deviation of the wrist, which can also be present in these patients. B,C. The patient has limited pronation and supination. D. Obvious radial head dislocation, reported. E. Patient with severe involvement of the left forearm.
During growth the affected ulna typically remains relatively shortened and curved, and this often leads to significant bowing of the radius. When the ulna is shorter the ulnar collateral ligament acts as a tether, causing bowing of the radius. In addition, the local presence of the exostosis itself causes radial bowing by disturbing hemiepiphyseal growth.12
Cubitus varus deformity, radial head dislocation, or both may also occur. Radial head dislocation is reported to occur in 22% of the affected forearms.18 Symptoms of this can be varus deformity of the elbow, limitation of elbow motion, and pain.
A mild flexion deformity of the elbow is often present.
At the wrist level an increased ulnar tilt of the radial epiphysis, ulnar deviation of the hand, and progressive ulnar translocation of the carpus are often present. These deformities lead to a loss of radial deviation of the hand and loss of pronation–supination of the forearm (FIG 1).
The loss of forearm pronation–supination may develop early and become progressively more severe as the child ages.21
Despite their impressive appearance these exostoses are surprisingly well tolerated and frequently result in minimal loss of function.21
A recent study evaluating the forearm in untreated adults with MHE demonstrated that the function subjectively reported by the patients is greater than the one objectively measured.14
IMAGING AND OTHER DIAGNOSTIC STUDIES
The diagnosis of MHE can be easily made by clinical inspection in a child with a positive family history.
Plain radiographic evaluation is usually sufficient to confirm the diagnosis and to determine the number, location, and morphology of the exostoses (FIG 2).
The earliest radiologic sign is an asymmetric or beaked overgrowth of the cortex next to the growth plate.3
Like solitary osteochondromas, the exostoses may be described as sessile, pedunculated, or cauliflower-like; they nearly always point away from the physis. In MHE, the lesions tend to be bigger and have a more bizarre shape. Characteristically in the distal ulna they are often narrow, with a pointed end (FIG 2).
FIG 2 • Radiograph showing large osteochondroma of distal ulna affecting the epiphysis and causing significant tethering of the radius. Characteristically the distal ulna is narrow with a pointed end.
FIG 3 • The radial articular angle (RAA) and carpal slip (CS). A. The RAA is defined as the angle between a line running along the articular surface of the radius and another line that is perpendicular to a line joining the center of the radial head to the radial border of the distal radial epiphysis (the radial styloid in skeletally mature individuals). The normal range of the angle is 15 to 30 degrees. B. CS is measured by determining the percentage of the lunate that is in contact with the radius. First, a line is drawn from the center of the olecranon through the ulnar border of the radial epiphysis (the radial articular surface in skeletally mature individuals).1 This line normally bisects the lunate. An abnormal CS is defined as being present when ulnar displacement of the lunate exceeds 50%. (Adapted from Akita S, Mursae T, Yonenobu K, et al. Long-term results of surgery for forearm deformities in patients with multiple cartilaginous exostoses. J Bone Joint Surg Am 2007;89A:1993–1999.)
In older children and teenagers, irregular areas of calcification of the cartilaginous cap may be present, particularly in the more voluminous lesions. Extensive calcification with changes in the shape and thickness of the cartilaginous cap should raise suspicion of a possible chondrosarcomatous transformation.
At least two full views of the forearm (true anteroposterior and lateral containing both the elbow and the wrist) are necessary to properly assess the ulna variance, the radial articular angle (RAA), the carpal slip, and the relative radial bow. These radiographic measurements are useful in the surgical planning phase (FIG 3).1
Alterations of the radial head on radiographs must always be assessed. They range from hypertrophy and flattening to a complete radial head dislocation.
The Taniguchii classification correlates the regional involvement of the forearm with the generalized severity of the disease.23
Masada morphologically classified the involvement of the forearm in MHE into three types (FIG 4).12 This classification was also used for treatment planning.
Computed tomography (CT), magnetic resonance imaging (MRI), and magnetic resonance angiography are performed at times for specific and symptomatic lesions. These can be especially helpful to detail the anatomic position relative to soft tissue structures, or when malignant transformation is suspected.24
DIFFERENTIAL DIAGNOSIS
Langer–Giedion syndrome
Madelung deformity
Chondrosarcoma
FIG 4 • Masada classification of the involvement of the forearm in multiple hereditary exostoses. (Adapted from Masada K, Tsuyuguchi Y, Kawai H, et al. Operations for forearm deformity caused by multiple osteochondromas. J Bone Joint Surg Br 1989;71B:24–29.)
NONOPERATIVE MANAGEMENT
The only treatment for MHE is surgery, but the mere presence of an exostosis is not, by itself, an indication for surgical removal.
The conspicuous number of lesions and the fact that they are mostly asymptomatic warrant a cautious surgical approach.
SURGICAL MANAGEMENT
Surgical treatment of forearm deformities in MHE remains controversial. A number of operative techniques have been proposed.5,16,12,19
The main surgical indications are.
To improve forearm function (pronation–supination)1
To relieve pain from external trauma or irritation of the surrounding soft tissue4
To improve appearance
To rule out malignancy when there is a rapid increase in size of a lesion15
When evaluating the surgical indications in an individual patient, it is important to distinguish between the functional deficit and the cosmetic appearance.
The postoperative appearance of the forearm has been shown to be unrelated to the functional outcome.14
Despite maintaining good function even without treatment,1,14 a percentage of patients find the arm cosmetically unpleasant because of the shortening, angulations, and deformities.14 If surgery is being undertaken for cosmetic rather than functional purposes, the hopes, concerns, and expectations of patient and parents must be thoroughly discussed and accurately outlined.
If function is the main concern, the goal of surgery is to maintain or improve function until reaching skeletal maturity and not to prevent the deformities.
Some authors5,12,15 advocate an aggressive approach based on the rationale that forearm deformities are equal to functional impairment. Their surgical treatment employs procedures such as excision of the exostoses and ulnar lengthening, associated at times with radial hemiepiphyseal stapling or osteotomy. They feel this is the only way to prevent the development or progression of deformity in the upper extremity.
According to these authors, the surgical indications include relative ulna shortening (with or without bowing) of more than 1.5 cm, RAA of greater than 30 degrees, carpal slip of more than 60 degrees, and bowing of the radius or the ulna (or both).5
However, we and others believe that the mere presence of forearm deformities alone is relatively unrelated to functional impairment,1,14,21 and therefore we do not recommend surgical correction of the deformities only to prevent a possible, but not predictable, future functional impairment.
Symptomatic dislocation of the head of the radius is defined as interfering with joint motion or causing significant pain.
Procedures
Exostosis excision alone is indicated when a lesion becomes symptomatic or when it alone causes limitation of forearm pronation–supination. This procedure alone does not correct the forearm deformities that may be present.
If significant forearm deformity is present, exostosis excision is combined with ulnar tether release with or without radial osteotomy.
Radial osteotomy is performed in the skeletally mature or nearly skeletally mature patient, as significant remodeling of the radius is unlikely.
If the patient has significant growth potential remaining, ulna-tether release alone can lead to impressive correction.
Ulna lengthening with or without radius epiphyseal stapling remains a common procedure reported in the literature, but we do not use this regularly. We think it can lead to a shorter forearm, as well as bearing the incumbent risks associated with lengthening procedures.
The treatment for symptomatic radial head dislocation is usually surgical excision once the patient is skeletally mature. Excision before this time may lead to instability, growth disturbances, and possible worsening of the wrist or elbow deformity. In rare instances, however, exostosis excision with ulnar osteotomy may be effective in relocating the radial head.
TECHNIQUES
EXOSTOSIS EXCISION AND ULNAR TETHERING RELEASE
The location of the incision in the distal forearm varies depending on where the osteochondroma is located. Planning of this is important, as the ability to access the distal ulna is imperative whether the osteochondroma is located on the distal ulna or radius.
If the patient has ulnar involvement only, the incision can be placed on the subcutaneous border of the ulna between the flexor carpi ulnaris and the extensor carpi ulnaris. Care must be taken to identify and preserve the dorsal branch of the ulnar nerve.
If the patient has osteochondroma of both the radius and ulna, the incision has to be modified to allow exposure of both bones as well as the distal ulna.
A tourniquet of appropriate size is used.
Once the initial incision is made, the osteochondroma is carefully exposed and excised (TECH FIG 1A–C). Care must be taken to preserve satisfactory bony cortex for stability.
Next, the distal ulna is exposed and the ulna-tethering force is released.
This is usually done by transecting the distal ulna through the epiphyseal area, leaving the triangular fibrocartilage complex attached to the distal fragment.
In the skeletally mature patient a radial osteotomy can be performed if the forearm bowing and deformity is severe (TECH FIG 1D–F).
Wire fixation is usually adequate for the osteotomy.
As stated earlier, in the skeletally immature patient, release of the tether alone is usually adequate.
TECH FIG 1 • A. Exposure of large osteochondroma of the distal ulna. B. Dissection and exposure of the osteochondroma. Significant tethering is present distally. C. After excision of osteochondroma and release of ulnar tethering. D. In a skeletally mature patient, a radial osteotomy is performed after exostoses excision and ulnar-tether release. E. After excision of exostoses, ulnartether release, and radial osteotomy. F. Radiograph taken 30 months after surgery shows improved forearm–wrist alignment.
RADIAL HEAD EXCISION
The radial head is then exposed and excised (TECH FIG 2).
Layered closure is then performed and the extremity is immobilized for 2 weeks, followed by institution of range-of-motion exercises.
An incision is made over the prominent radial head with the forearm in pronation to protect the posterior interosseous nerve.
Dissection is then carried down in the interval between the anconeus muscle and extensor carpi ulnaris.
TECH FIG 2 • A. Patient with painful radial head dislocation. B. Exposure of the radial head. Forearm is in a pronated position. C. Radial head exposed before excision. D. Excised radial head. Significant degenerative changes are present.
POSTOPERATIVE CARE
Exercises to maintain the range of motion of the fingers are encouraged immediately after surgery regardless of the technique used.
In case of exostosis excision and ulna-tethering release, casting is performed for 4 weeks, followed by range-of-motion exercises and splinting.
If an osteotomy was performed, casting is continued until radiographic evidence of healing is seen.
OUTCOMES
Many MHE patients do not need surgery. In patients who require surgery, we feel that ulnar-tether release, with or without exostoses excision, with or without radial osteotomy, provides the most reliable result with the fewest complications. In selected patients this can greatly improve function, in addition to the improved cosmesis of the extremity.
For symptomatic radial head dislocations we prefer excision, as this usually leads to a consistent, reproducible result with little risk.
COMPLICATIONS
The ulnar lengthening is fraught with complications, reported in the literature to range from 0% to 100%.1 Recurrence of the deformity in the skeletal immature patient is commonly reported.1,5,12,13,15,16 This calls into question the indications for this procedure.
The main complications in progressive distraction lengthening are nerve damage, fractures at the lengthening site, and pin tract infection. When only the ulna is lengthened, the distracting tension is not exerted directly on the neurovascular bundle, with minimal risk of nerve dysfunction.
When performing radial head excision, the surgeon has to be careful in the initial dissection to avoid injury to the posterior interosseous nerve as well as to the stabilizing structures of the elbow.
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· Masada K, Tsuyuguchi Y, Kawai H, et al. Operations for forearm deformity caused by multiple osteochondromas. J Bone Joint Surg Br 1989;71B:24–29.
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