Jon R. Davids
DEFINITION
The gait pattern of children with cerebral palsy (CP) is frequently disrupted by dynamic overactivity and shortening of the medial hamstring muscles.
This disruption is characterized by increased knee flexion in stance phase, and decreased knee extension at the end of swing phase.
Surgical lengthening of the medial hamstrings is usually performed in conjunction with other surgical procedures selected to address all elements of soft tissue and skeletal dysfunction that compromise gait in children with CP.
This surgical strategy, termed single-event multilevel surgery (SEMLS), requires a comprehensive assessment of gait dysfunction using quantitative gait analysis.
Proper management (surgical, orthotic, and rehabilitative) in childhood can result in an improved gait pattern that will be sustainable in the adult years.
ANATOMY
The medial hamstrings consist of three muscles: the gracilis, the semimembranosus, and the semitendinosus. All three are considered biarticular muscles because they cross both the hip joint and knee joint.
The gracilis muscle is innervated by the obturator nerve and has its origin on the inferior pubic ramus and its insertion on the anteromedial aspect of the proximal tibia. It serves as a hip adductor and knee flexor. The gracilis muscle has a relatively small physiologic cross-sectional area and a relatively large ratio of tendon length to muscle fiber length, indicating that it is designed for maximal excursion and diminished force generation.7,9
The semimembranosus muscle is innervated by the sciatic nerve and has its origin on the inferolateral portion of the ischium and its insertion on the posteromedial aspect of the proximal tibia. It serves as a hip extensor and knee flexor. The semimembranosus muscle has a relatively large physiologic cross-sectional area and a relatively small ratio of tendon length to muscle fiber length, indicating that it is designed for minimal excursion and increased force generation.7,9
The semitendinosus muscle is innervated by the sciatic nerve and has its origin on the inferomedial portion of the ischium and its insertion on the anteromedial aspect of the proximal tibia. It serves as a hip extensor and knee flexor. The semimembranosus muscle has a relatively small physiologic cross-sectional area and a relatively large ratio of tendon length to muscle fiber length, indicating that it is designed for maximal excursion and diminished force generation.7,9
The lateral hamstrings consist of the biceps femoris muscle, which is innervated by the sciatic nerve. The muscle is considered to be uniarticular, crossing only the knee joint, with its origin on the posterior aspect of the middle third of the femur and its insertion on the fibular head. It serves as a knee flexor. The biceps femoris muscle has a relatively large physiologic cross-sectional area and a relatively small ratio of tendon length to muscle fiber length, indicating that it is designed for minimal excursion and increased force generation.7,9
PATHOGENESIS
CP is the consequence of an injury to the immature brain that may occur before, during, or shortly after birth. The nature and location of the injury to the central nervous system (CNS) determines the neuromuscular and cognitive impairments.
Common functional deficits are related to spasticity, impaired motor control, and disrupted balance and body position senses.
Although the injury to the CNS is not progressive, the clinical manifestations of CP change over time because of growth and development of the musculoskeletal system. The muscles typically exhibit a purely dynamic dysfunction during the first 6 years of life, characterized by a normal resting length and an exaggerated response to an applied load or stretch. With time, between 6 and 10 years of age, the muscles develop a fixed or myostatic shortening, resulting in a permanent contracture.
As such, it is best to consider CP not as a single specific disease process, but rather a clinical condition with multiple possible causes.14
NATURAL HISTORY
Ambulatory children with CP whose gait is disrupted by overactivity and shortening of the medial hamstring muscles typically walk with increased knee flexion in stance phase and diminished knee extension in swing phase.
This is usually associated with increased ankle plantarflexion in stance phase and is called a “jump” gait pattern.12,16
Children with a jump gait pattern will have overactivity of the medial hamstring muscle group; the lateral hamstring muscle group is rarely involved.
As the child gets older and heavier, ankle plantarflexor insufficiency (due to muscle weakness and foot segmental malalignment) will eventually occur. This will result in increasing ankle dorsiflexion and knee flexion in stance phase, which is called a “crouch” gait pattern.12,16
Teenagers with a severe crouch gait pattern will have involvement of both the medial and lateral hamstring muscle groups.
As the ground reaction force falls further and further behind the knee during the stance phase in crouch gait, the demands on the knee extensor muscles increase, eventually resulting in painful patellofemoral overload.
For this reason the crouch gait pattern is not sustainable, and by the late teenage or young adult years individuals with this gait pattern frequently lose the ability to ambulate.
PATIENT HISTORY AND PHYSICAL FINDINGS
The clinical history, as provided by the child and the parents, usually contains complaints of inability to stand up straight, inability to keep up with peers in play and sports, inability to walk distances (such as at the grocery store or the mall), and anterior knee pain after walking activities or at the end of the day.
FIG 1 • Standing lateral radiograph of the knee in a child with cerebral palsy and a crouch gait pattern. There is a patella alta, with fragmentation of the inferior pole of the patella, indicating chronic overload of the knee extensor mechanism.
A straight-leg raise of 60 degrees or less is indicative of shortening of the medial hamstrings.
A popliteal angle of 45 degrees or greater is indicative of shortening of the medial hamstrings.
IMAGING AND OTHER DIAGNOSTIC STUDIES
Radiographic imaging is not required when determining the need for lengthening of the medial hamstrings to improve gait in children with CP.
If lateral radiographs of the knee are obtained, however, they will frequently show patella alta, with fragmentation of the inferior pole of the patella; these are the sequelae of chronic and progressive patellofemoral overload (FIG 1).
Relevant data from quantitative gait analysis include sagittal-plane kinematics and kinetics at the knee, and dynamic electromyography (EMG) of the medial hamstrings.
Sagittal-plane knee kinematics will show increased knee flexion of greater than 20 degrees at initial contact in loading response at the beginning of stance phase, and diminished knee extension at terminal swing at the end of swing phase (FIG 2A).4 Midstance alignment of the knee may be variable.
In jump gait pattern, full knee extension in midstance may occur. This is not a contraindication to lengthening of the medial hamstrings.
In crouch gait pattern, increased knee flexion will be of greater magnitude and is present throughout the stance phase.
Sagittal-plane knee kinetics will show an increased internal knee extension moment in stance phase (FIG 2B)10.
FIG 2 • Sagittal-plane knee kinematic plots of a child with a jump gait pattern (A) and kinetic plots of a child with crouch gait pattern (B). The age-matched normal motion (mean ± 2 standard deviations) appears as a light purple band and the subject's data are indicated by a blue line. A. The gait cycle is on the horizontal axis, the direction of motion on the vertical axis. Kinematic indicators for lengthening of the medial hamstrings are increased flexion at initial contact and diminished extension at terminal swing (arrows). B. The gait cycle is on the horizontal axis, the internal moment on the vertical axis. Crouch gait pattern is characterized by an increased internal extension moment at the knee throughout stance phase (red arrows). C. Dynamic electromyography (EMG) of the medial hamstrings in a child with cerebral palsy. Three gait cycles are shown, separated by the solid vertical lines. The stance and swing phases of each cycle are separated by the dashed vertical lines. The normal timing of activation of the muscle is noted by the horizontal red lines at the bottom of the strip. The actual timing of activation of the muscle for the subject is shown by the oscillating red line at the middle of the strip. The dynamic EMG indicator for lengthening of the medial hamstring muscles is prolonged activity in midstance (indicated by the circles in each gait cycle).
In jump gait pattern, the increased knee moment will occur in the loading response and terminal stance subphases of stance phase.
In crouch gait pattern, the increased knee moment will be of greater magnitude and will occur throughout the stance phase.
Dynamic EMG of the medial hamstrings will show prolonged activity of the muscle group into the midand terminal stance subphases of stance phase (FIG 2C).
DIFFERENTIAL DIAGNOSIS
Increased knee flexion in stance phase may be the consequence of:
Overactivity or shortening of the medial hamstring muscles
Surgical lengthening of the medial hamstring muscles is appropriate.
Ankle plantarflexor insufficiency, with disruption of the ankle plantarflexion– knee extension couple. This may be a consequence of muscle weakness or foot skeletal segmental malalignment, resulting in lever arm deficiency.6,8,15
The ankle plantarflexor insufficiency must be addressed directly to improve the gait deviation at the knee.
Dyskinetic CP, with disrupted balance and body position sense. In such cases, mildly increased knee flexion in stance phase gives a sense of stability to the child and is habitual.
Surgical lengthening of the medial hamstring muscles will not correct the gait deviation in this situation.
Increased knee flexion at the end of swing phase may be a consequence of:
Overactivity or shortening of the medial hamstring muscles
Surgical lengthening of the medial hamstring muscles is appropriate.
Increased ankle plantarflexion at the end of swing phase. Increased knee flexion will occur in swing phase to promote limb clearance and improve the foot position in the transition from swing to stance phase.
Decreased hip flexion in swing phase. Increased knee flexion will occur in swing phase to promote limb clearance.
NONOPERATIVE MANAGEMENT
In children less than 6 years of age, with primarily dynamic deformity of the medial hamstrings, a communityor homebased stretching program may be effective at improving knee extension during gait for a limited period of time.
Injection of botulinum toxin into the medial hamstrings, which decreases muscle spasticity via a reversible neuromuscular blockade, may also be effective for dynamic deformity in younger children 3.
Serial stretch casting of the knee has been shown to be effective for the treatment of mild myostatic deformity of the medial hamstrings, particularly after surgical lengthening.18
Use of an ankle–foot orthosis may be effective treatment for increased knee flexion in stance phase that is the consequence of ankle plantarflexor insufficiency and disruption of the ankle plantarflexion–knee extension couple.5
SURGICAL MANAGEMENT
Achieving optimal outcome after lengthening of the medial hamstring muscles requires careful patient selection, proper surgical technique, appropriate postoperative orthotic management, and adequate rehabilitation resources (primarily physical therapy for conditioning and gait training) in the months after the surgery.
Preoperative Planning
Optimal clinical decision making and preoperative planning require the integration of data from five fields—clinical history, physical examination, diagnostic imaging, quantitative gait analysis, and examination under anesthesia—in a process described as a diagnostic matrix.4
When considering lengthening of the medial hamstrings, the examination under anesthesia should include repeating the straight-leg raise and popliteal angle measurements as described above.
In children with CP that includes significant spasticity, it is frequently difficult to determine the relative contributions of dynamic overactivity and myostatic contracture to muscle deformity and dysfunction. When the child is under anesthesia, the spastic component is effectively removed, allowing the physician to perform a clinical examination to determine the presence or absence of myostatic or fixed muscle shortening.
Surgical lengthening of the muscle is most appropriate when significant myostatic or fixed shortening is present.
Lengthening of the lateral hamstring muscle is not necessary for children with either jump or crouch gait patterns.
Lateral hamstring lengthening is indicated only for teenagers with a severe crouch gait pattern, whose popliteal angle measurement fails to improve adequately (as described below) after lengthening of the medial hamstring muscles.
Positioning
The child is placed on the operating table in the supine position.
A tourniquet may be placed about the most proximal portion of the thigh. The extremity is carefully cleaned and draped to allow adequate exposure for the surgical approach to the medial hamstring muscles. Use of a tourniquet to minimize blood loss and maximize the operative exposure is favored when performing SEMLS that includes both soft tissue and skeletal surgeries.
Approach
The medial hamstring muscles are usually exposed via a posteromedial approach at the distal third of the thigh.
This approach is particularly appropriate when lengthening of the medial hamstring muscles and transfer of the rectus femoris muscle are to be performed at the same time as part of SEMLS.
TECHNIQUES
LENGTHENING OF THE MEDIAL HAMSTRING MUSCLES (IN CONJUNCTION WITH TRANSFER OF THE RECTUS FEMORIS MUSCLE)
A 6to 10-cm incision is made over the posteromedial margin of the distal third of the thigh, ending two or three fingerbreadths proximal to the posterior skin crease of the knee joint (TECH FIG 1A).
The incision is carried down through the subcutaneous tissues, past the saphenous vein and the sartorius muscle, exposing the three muscles of the medial hamstring muscle group (TECH FIG 1B).
The gracilis and semimembranosus muscles are exposed at the level of their myotendinous junction, and the semitendinosus is exposed at the level of its distal tendon.
A proximal tenodesis is performed between the gracilis (at its myotendinous junction) and the semitendinosus (at its proximal tendon), using two throws of a nonabsorbable suture (TECH FIG 1C).
The semitendinosus muscle is then completely transected 1 cm distal to the tenodesis with the gracilis muscle (TECH FIG 1D).
A fractional lengthening of the semimembranosus muscle is performed, using two transverse incisions, separated by 2 cm, through the broad and thin tendon overlying the muscle at the musculotendinous junction (TECH FIG 1E).
Care should be taken not to cut the muscle tissue underlying the tendon at this level.
A fractional lengthening of the gracilis muscle is performed, using a single transverse incision located 1 cm distal to the tenodesis with the semitendinosus (TECH FIG 1F).
Care should be taken not to cut the muscle tissue underlying the tendon at this level.
Repeat assessment of the popliteal angle is made after lengthening of the medial hamstring muscles.
The angle should be improved (ie, decreased) by 30 to 40 degrees.
TECH FIG 1 • Medial views of the right knee. A. The posteromedial skin incision used for exposure of the medial hamstring muscles. B. The three muscles of the medial hamstrings group—the gracilis (solid arrow), the semimembranosus (dashed arrow), and the semitendinosus (dotted arrow). Each has distinct myoarchitecture. C. Proximal tenodesis of the gracilis and the semitendinosus muscles (circle). D. The tendon of the semitendinosus has been transected (arrow) distal to the tenodesis (red circle). E. A two-level fractional lengthening (arrows) of the semimembranosus muscle has been performed. F. A fractional lengthening (arrow) of the gracilis muscle has been performed distal to the tenodesis of the gracilis and semitendinosus muscles (circle).
LENGTHENING OF THE LATERAL HAMSTRING MUSCLE
A 3- to 5-cm incision is made over the posterolateral margin of the distal third of the thigh, posterior to the posterior margin of the iliotibial band, ending three to five fingerbreadths proximal to the head of the fibula (TECH FIG 2A).
The incision is carried down through the subcutaneous tissues, past the posterior margin of the iliotibial band, exposing the biceps femoris at the level of its myotendinous junction (TECH FIG 2B).
The common peroneal nerve, which is located adjacent to the posteromedial margin of the biceps femoris muscle, should be identified and gently retracted away from the muscle before lengthening.
A fractional lengthening of the biceps femoris muscle is performed, using a single transverse incision (TECH FIG 2C).
Care should be taken not to cut the muscle tissue underlying the tendon at this level.
TECH FIG 2 • Lateral views of the right knee. A. The posterolateral skin incision used for exposure of the lateral hamstring muscle. B. The lateral hamstring muscle is exposed at the myotendinous junction, which is relatively wide and long. The myoarchitecture of the biceps femoris muscle is similar to the semimembranosus muscle. C. A single-level fractional lengthening of the biceps femoris muscle has been performed.
FRACTIONAL LENGTHENING OF THE MEDIAL HAMSTRING MUSCLES (WITHOUT CONCOMITANT TRANSFER OF THE RECTUS FEMORIS MUSCLE)
A 6- to 10-mm incision is made over the posteromedial margin of the thigh, at the juncture between the middle and proximal thirds (TECH FIG 3A).
The incision is carried down through the subcutaneous tissues, exposing the three muscles of the medial hamstring group.
The gracilis, semimembranosus, and semitendinosus muscles are all exposed at the level of their myotendinous junction (TECH FIG 3B).
A fractional lengthening of each of the three muscles is performed, using two transverse incisions, separated by 2 cm, for the semimembranosus muscle, and a single incision each for the gracilis and semitendinosus muscles, through the broad and thin tendon overlying the muscle at the musculotendinous junction.
TECH FIG 3 • A. Medial view of the right knee, showing the posteromedial skin incision used for exposure of the medial hamstring muscles. The incision is more proximal than that used for performing medial hamstring lengthening in conjunction with transfer of the rectus femoris muscle. B. Medial view of the incision in the right thigh, showing the three muscles of the medial hamstrings group—the gracilis (solid arrow), the semimembranosus (dashed arrow), and the semitendinosus (dotted arrow). All three muscles have been exposed at the level of their myotendinous junctions. C. Fractional lengthening has now been performed on each of the three muscles. Single-level fractional lengthening is sufficient for the gracilis (solid arrow) and semitendinosus (dotted arrow) muscles. A double-level fractional lengthening is usually necessary for the semimembranosus muscle (dashed arrow).
Care should be taken not to cut the muscle tissue underlying the tendon at this level (TECH FIG 3C).
Repeat assessment of the popliteal angle is made after fractional lengthening of the medial hamstring muscles.
The angle should be improved (ie, decreased) by 15 to 30 degrees.
COMBINED FRACTIONAL LENGTHENING AND TRANSFER OF MEDIAL HAMSTRING MUSCLES (WITHOUT CONCOMITANT TRANSFER OF THE RECTUS FEMORIS MUSCLE)
A 6- to 10-cm incision is made over the posteromedial margin of the distal third of the thigh, ending two to three fingerbreadths proximal to the posterior skin crease of the knee joint (TECH FIG 4A).
The incision is carried down through the subcutaneous tissues, past the saphenous vein and the sartorius muscle, exposing the three muscles of the medial hamstrings group.
The gracilis and semimembranosus muscles are exposed at the level of their myotendinous junction, and the semitendinosus is exposed at the level of its distal tendon.
A clamp is placed on the proximal portion of the tendon of the semitendinosus muscle, and the tendon is transected distally (TECH FIG 4B).
The gracilis muscle is transected proximally at the myotendinous junction, and the proximal portion of the muscle is released (TECH FIG 4C).
The gracilis and semitendinosus muscles are retracted to expose the myotendinous junction of the semimembranosus muscle, where a two-level fractional lengthening is performed (TECH FIG 4D).
The proximal portion of the semitendinosus muscle–tendon unit is transferred to the distal portion of the gracilis tendon and is secured with two throws of a nonabsorbable suture (TECH FIG 4E).
Repeat assessment of the popliteal angle is made after combined fractional lengthening and transfer of the medial hamstring muscles.
The angle should be improved (ie, decreased) by 30 to 40 degrees.
TECH FIG 4 • Medial view of the right knee. A. The posteromedial skin incision used for exposure of the medial hamstring muscles. B. A clamp is placed on the proximal portion of the tendon of the semitendinosus muscle (solid circle), and the tendon (dotted line) is released distally (dotted circle). C. The tendon of the gracilis muscle (arrow) is released proximally at the muscle's myotendinous junction (circle). D. A two-level fractional lengthening (arrows) of the semimembranosus muscle has been performed. E. The proximal portion of the semitendinosus muscle–tendon unit (dotted arrow) is transferred to the distal portion of the tendon of the gracilis muscle (solid arrow). The transfer is secured with two throws of a nonabsorbable suture (circle).
POSTOPERATIVE CARE
Lengthening of the hamstring muscles is rarely performed in isolation for children with CP; rather, it is usually a component of SEMLS.
If full knee extension against gravity can be achieved immediately after surgery, use of a knee immobilizer is appropriate.
Passive knee range of motion is initiated at 1 to 2 weeks after surgery.
Weight bearing and gait training are begun 2 to 6 weeks after surgery, depending on which other surgeries have been performed as a part of SEMLS.
If full knee extension cannot be achieved, a long-leg fiberglass cast is applied with the knee positioned in extension against gravity alone.
The cast is univalved in the operating room to accommodate postoperative swelling and facilitate spreading of the cast in the first few days after surgery.
Gradual serial stretch casting, correcting the residual knee flexion deformity at a rate of 5 degrees per week, is instituted.18
OUTCOMES
The early goals of surgical lengthening of the hamstring muscles are to improve knee extension at initial contact in stance phase and at terminal swing in swing phase.
Improved dynamic alignment during gait (kinematics) should result in improved loading of the knee (kinetics) in stance phase.1,8,11,13,17
Intermediate follow-up at 10 years after SEMLS has shown that kinematic and kinetic improvements after surgery are maintained despite deterioration in the static findings on physical examination.2
COMPLICATIONS
The most common and significant early complication after lengthening of the hamstrings is neuropraxia of the sciatic nerve, which is due to excessive stretching of the nerve after correction of the dynamic and static knee flexion deformities.
Neuropraxia of the sciatic nerve is characterized by pain and hypersensitivity about the foot. When this problem is encountered acutely in the immediate postoperative period, the position of immobilization of the knee should be adjusted toward increased flexion to relax the nerve.
If a neuropraxia develops during the period of serial stretch casting of a residual knee flexion contracture, the stretch casting should be terminated for 1 to 2 weeks, and then resumed at a slow rate.
If a neuropraxia is first appreciated during the rehabilitation period, management with medications such as gabapentin (mechanism of effect unknown) and physical therapy modalities for desensitization are appropriate.
Recurrence of knee flexion deformity and increased knee flexion during the stance phase of gait may occur in the years after surgery due to a variety of factors.
As children grow, they pass through finite periods of accelerated rates of growth (growth spurts), where the longitudinal growth of the bones is greater than that of the muscles.
Muscle shortening may recur and is usually effectively treated with a period of homeor community-based stretching exercises.
As children grow, they get heavier, and as a result greater muscle forces are required to balance external forces during gait. Weakness of the ankle plantarflexor, knee extensor, and hip extensor muscles is common and may result in development of a crouch gait pattern in the years after SEMLS.
Avoidance of obesity and ongoing muscle strength training and cardiovascular conditioning are important elements for maintaining optimal gait function in children with CP.
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