Orthopedic Surgery in Neuromuscular Disorders

Neuromuscular disorders (NMDs) in children include conditions that affect the spinal cord, peripheral nerves, neuromuscular junction, and muscles. Orthopedic treatment of children with NMD has been aimed at preventing worsening of deformities and providing stability to the skeletal system to improve the quality of life for these children. Patients with severe NMDs have listed their priorities as the ability to communicate with others, the ability to perform many of the activities of daily living, mobility, and ambulation ( ). The role of the orthopedic surgeon in achieving these goals includes prescribing orthoses for lower extremity control to facilitate transfer to and from wheelchairs, preventing or correcting joint contractures, and maintaining appropriate standing and sitting posture. Treatment must be individualized for each patient according to the particular disorder, the severity of involvement, and the ambulation status of the patient.

Fractures are common in children with NMDs because of disuse osteoporosis, steroid treatment, and frequent falls ( ). Fracture treatment with splinting, cast-bracing, or surgery emphasizes a quick return to walking if the patient is ambulatory. Spinal bracing may be necessary to assist with sitting balance, a knee-ankle-foot orthosis can provide stability for patients with proximal muscle weakness, and an ankle-foot orthosis (AFO) can help position the ankle and foot plantigrade to help prevent progressive deformities. Most children with NMD will eventually require the use of a wheelchair, and these chairs must be carefully contoured to accommodate the spinal deformities and pelvic obliquity that usually are present.

Muscular Dystrophy

The muscular dystrophies are a group of hereditary disorders of skeletal muscle that produce progressive muscle degeneration and associated weakness. The disorders differ in the distribution and severity of muscle weakness, the age of onset, the rate of progression, and the pattern of inheritance ( Fig. 9.1 ).

Fig. 9.1, Natural history of muscular dystrophies. The arrows show the average clinical course with level of disability and age.

Duchenne Muscular Dystrophy

Duchenne muscular dystrophy, a sex-linked recessive inherited trait, occurs in males and in females with Turner syndrome; carriers are female. It is reported to occur in 1 in 3500 live births ( ; ; ). There is a family history in 70% of patients, and the condition occurs as a spontaneous mutation in approximately 30% of patients ( ). In contrast, Becker muscular dystrophy, the second most common form, occurs in 1 in 30,000 live male births; other types of muscular dystrophy are rare. Children with Duchenne muscular dystrophy usually reach early motor milestones at appropriate times, but independent ambulation may be delayed; many are initially toe-walkers. The disease will usually become evident between ages 3 and 6 years. The ability to ambulate typically is lost by about age 10 years; however, with treatment with steroids ambulation can continue into the late teenage years.

Clinical features include large, firm calf muscles; the tendency to toe-walk; a widely based, lordotic stance; a waddling Trendelenburg gait; and a positive Gowers test indicative of proximal muscle weakness ( Fig. 9.2 ). The diagnosis usually is obvious by the time the child is 5 or 6 years old. Diagnosis is confirmed by a dramatically elevated level of creatine kinase (50 to 100 times normal) and DNA analysis of blood samples ( ; ). Muscle biopsy demonstrates variations in fiber size in internal nuclei, split fibers, degenerating or regenerating fibers, and fibrofatty tissue deposition. Dystrophin testing of the muscle biopsy will confirm the type of muscular dystrophy.

Fig. 9.2, Gowers sign. Child must use hands to rise from a sitting position.

Orthopedic Physical Examination

The degree of muscular weakness depends on the age of the patient and the type of dystrophy. Because the proximal musculature weakens before the distal muscles, examination of the lower extremities demonstrates an early weakness of gluteal muscle strength. The weakness in the proximal muscles of the lower extremity can be demonstrated by a decrease in the ability to rise from the floor without assistance of the upper extremities (Gowers sign). The calf pseudohypertrophy is caused by infiltration of the muscle by fat and fibrosis, giving the calves the feel of hard rubber ( Fig. 9.3 ). The extrinsic muscles of the foot and ankle retain their strength longer than the proximal muscles of the hip and knee. The posterior tibial muscle retains its strength for the longest time. This pattern of weakness causes an equinovarus deformity of the foot. Weakness of the shoulder girdle musculature can be demonstrated by the Meryon sign, which is elicited by lifting the child with one arm encircling the child’s chest. Most children contract the muscles about the shoulder to increase shoulder stability and facilitate lifting. In children with muscular dystrophy, however, the arms abduct because of the severe shoulder abductor muscle weakness, until they eventually slide through the examiner’s arms unless the chest is tightly encircled. Later in the disease process, the Thomas test demonstrates hip flexion contracture, and the Ober test demonstrates an abduction contracture of the hip.

Fig. 9.3, Pseudohypertrophy of the calf in a patient with muscular dystrophy.

Orthopedic Treatment

The major goal of early treatment is to maintain functional ambulation as long as possible. Between ages 8 and 14 years (with a median of 10 years), children with Duchenne muscular dystrophy typically have a sensation of locking of the joints. Contractures of the lower extremity may require early treatment to prolong the child’s ability to ambulate, if even for 1 to 2 years. This requires prevention or retardation of the development of contractures of the lower extremity, which would eventually prohibit ambulation. It is easier to keep patients walking than to induce them to resume walking once they have stopped. When children with Duchenne muscular dystrophy stop walking, they also become more susceptible to the development of scoliosis and severe contractures of the lower extremities. Scoliosis will develop in nearly all children with Duchenne muscular dystrophy, usually when they require aided mobility or shortly after becoming wheelchair bound. The use of steroids has been shown to decrease the occurrence of scoliosis in these patients ( ; ; ; ).

Bone Health—Osteoporosis and Fracture Management

Because of disuse- and steroid-induced osteoporosis and frequent falls, fractures are common in children with neuromuscular disease ( ). Patients with Duchene muscular dystrophy who are on glucocorticoid therapy often develop osteoporosis ( ). Studies have shown a significant decrease in bone mineral density on dual-energy x-ray absorptiometry scans in boys with Duchenne muscular dystrophy, with 44% sustaining fractures. One study found that 33% of patients with Duchenne or Becker muscular dystrophy had sustained at least one fracture; full-time wheelchair use was a significant risk factor for fracture ( ). Up to 30% of Duchene muscular dystrophy patients develop symptomatic vertebral fractures ( ). Most fractures are nondisplaced metaphyseal fractures that heal rapidly. Minimally displaced metaphyseal fractures of the lower limbs should be splinted so that walking can be resumed quickly. If braces are being used, they can be enlarged to accommodate the fractured limb and allow progressive weight bearing. Displaced diaphyseal fractures often require surgical stabilization to allow mobilization during fracture healing. McAdam et al. reported fat embolism syndrome following minor trauma in five Duchenne muscular dystrophy patients, four of whom died ( ). Medical treatment of disuse and steroid induced osteopenia may decrease the frequency of fractures in this patient population. An algorithm for osteoporosis monitoring, diagnosis, and treatment of patients with Duchenne muscular dystrophy has been developed ( Fig. 9.4 ) ( ).

$Fig. 9.4, Osteoporosis monitoring, diagnosis, and treatment algorithm for patients with Duchenne muscular dystrophy. ∗Signs of clinically significant bone fragility are low-trauma fractures of long bones or vertebra. † Clinical stability refers to the absence of nonvertebral fractures, stable healed vertebral fractures, absence of new VFs in previously normal vertebral bodies, absence of bone and back pain, and a BMD score appropriate for height or >2 SDs. BMD , Bone mineral density; DXA , dual-energy X-ray absorptiometry.$

Correction of Lower Extremity Contractures

Three approaches have been used for surgical correction of lower extremity contractures:

1.
Ambulatory approach. The goal of surgery during the ambulatory period is to correct any contractures in the lower extremity while the patient is still ambulatory. Early aggressive surgery may be indicated for the first appearance of contractures in the lower extremities; a plateau in muscle strength, usually around age 5 to 6 years; and difficulty maintaining an upright posture with the feet together. Some surgeons suggest that surgery should be performed before deterioration of the Gowers maneuver time or time to rise from the floor ( ; ; ; ); others have recommended surgery later in the ambulatory phase, just before the cessation of ambulation ( ; ).
2.
Rehabilitative approach. Surgery is done after the patient has lost the ability to walk but with the intention that walking will resume ( ). Surgery during this stage usually only allows for minimal ambulation with braces.
3.
Palliative approach. The palliative approach treats only contractures that interfere with shoe wear and comfortable positioning in a wheelchair.

A comparison of ambulation and foot position in three groups of patients with Duchenne muscular dystrophy (those who had surgery to maintain ambulation, those who had surgery to correct and maintain foot position, and those who had no surgery) found that the mean age at cessation of ambulation for those who had surgery was 11.2 years, compared to 10.3 years in those who did not have surgery ( ). Foot position was neutral in 94% of those who had surgery, and none had toe flexion deformities; 96% of those who had surgery reported being able to wear any type of shoes, compared to only 60% of those who had no surgery. In contrast, another study of full-time wheelchair users with Duchenne muscular dystrophy found no significant differences between patients who did and did not have foot surgery with respect to shoe wear, hypersensitivity, or cosmesis ( ). Hindfoot motion was significantly better but equinus contracture was significantly worse in those who had not had surgery.

Currently the most common approach is to correct contractures just before the patient has a significant decline in ambulation and before the patient has to use a wheelchair (i.e., the ambulatory approach) ( Fig. 9.5 ) ( ). This is only after a thorough discussion with the family and patient about the goals of surgery.

Fig. 9.5, Natural course of Duchenne muscular dystrophy: age-related stages. Correction of contractures generally is done during the ambulatory phase.

Mild equinus contractures of the feet can help force the knee into extension, which in turn helps prevent the knee buckling caused by severe weakness of the quadriceps. Stretching exercises and nightly bracing can be used to prevent the contractures from becoming severe. Flexion and abduction contractures of the hip, however, impede ambulation and should be minimized. Exercises to stretch the hip muscles and lower extremity braces worn at night to prevent the child’s sleeping in a frog position are helpful initially. If surgery is indicated, the foot and hip contractures can be released simultaneously, usually through small incisions. Ambulation should be resumed immediately after surgery if possible. Polypropylene braces are preferred to long-term casting. Prolonged immobilization must be avoided to prevent or limit the progressive muscle weakness caused by disuse.

Percutaneous Release of Hip Flexion and Abduction Contractures and Achilles Tendon Contractures

A small (no. 15) blade is inserted percutaneously just medial and distal to the anterior superior iliac spine ( Fig. 9.6 ) to release first the sartorius muscle, then the tensor fasciae femoris muscle, taking care to avoid the neurovascular structures of the anterior thigh. Then, through another small incision approximately 3 to 4 cm proximal to the upper pole of the patella, the fascia lata is released, as is the deeper lateral intermuscular septum. The Achilles tendon is released through a small posterolateral incision.

Fig. 9.6, Tenotomy sites for release of hip flexors ( 1 ), tensor fasciae latae and fascia lata ( 2, 3 ), and Achilles tendon ( 4 ).

Open Lengthening of the Achilles Tendon

A posteromedial incision ( Fig. 9.7A ) is used to expose the Achilles tendon from its insertion to approximately 10 cm proximally, preserving the tendon sheath. The posteromedial two thirds of the tendon is divided near its insertion. With moderate dorsiflexion force applied to the foot, the medial two thirds of the tendon is divided approximately 5 to 8 cm proximal to the site of the distal division. The foot is then dorsiflexed so that the tendon lengthens to the desired length ( Fig. 9.7B ). The tendon can be sutured in a side-to-side fashion with absorbable suture. The tendon sheath and subcutaneous tissues are carefully closed to prevent adherence of the tendon to the overlying skin, and a short-leg cast is applied with the ankle in maximal dorsiflexion.

Fig. 9.7, Sliding lengthening of the Achilles tendon. (A) Posteromedial incision. (B) Two cuts are made through one half of the tendon in opposite directions. Rotation of the fibers must be followed accurately. Placing the foot in dorsiflexion causes the tendon fibers to separate.

Transfer of Posterior Tibial Tendon to Dorsum of Foot

In patients with marked overpull of the posterior tibial muscle, a posterior tibial tendon transfer of the posterior tibial tendon to the dorsum of the foot ( Fig. 9.8 ), combined with other tenotomies or tendon lengthening, has been found to give better results than posterior tibial tendon lengthening alone. Although transfer of the posterior tibial tendon is technically more demanding and has a higher perioperative complication rate, most patients retain the plantigrade posture of their feet, even after walking ceases. Despite the more extensive surgical procedure, early ambulation of the patient usually is not impeded.

Fig. 9.8, Posterior tibial tendon transfer. (A) First ( 1 ) and second ( 2 ) incisions. (B) Third ( 3 ) and fourth ( 4 ) incisions and clamp placement for pulling posterior tibial tendon from posterior to anterior compartment of the leg. (C) Position of the transplanted tendon and suture tied over a felt pad and button the plantar aspect of the foot.

Transfer of Posterior Tibial Tendon to Dorsum of Base of Second Metatarsal

Transfer of the posterior tibial tendon to the dorsum of the base of the second metatarsal rather than the dorsum of the foot has as a cited advantage the more distal placement of the posterior tibial tendon, which increases the lever arm in dorsiflexion of the ankle. This technique also allows easier plantar flexion and dorsiflexion balancing of the ankle at the time of surgery. Lengthening of the posterior tibial tendon is required to have sufficient length for the tendon to be transferred to the second metatarsal with this technique.

Equinus contractures can be corrected by either a percutaneous or open Achilles tendon lengthening. If an open procedure is needed because of severe contractures, lengthening or release of the posterior tibial, flexor digitorum, and flexor hallicus longus tendons also may be needed. Once these lengthening procedures or releases are done, the child will need an AFO to continue to stand or ambulate.

Correction of Spinal Deformities

Approximately 75% to 90% of boys with Duchenne muscular dystrophy develop scoliosis ( ; ). The age of onset of scoliosis is generally about the same as the age at which they lose the ability to walk, between 10 and 14 years ( ). A significant association has been shown between prolonged ambulation and a reduced risk of scoliosis development ( ), and the use of steroids has been shown to slow the progression of scoliosis and delay the need for surgery ( ). Before steroids were used in Duchenne patients, scoliosis developed in more than 90%; since steroid use began, the rate has fallen to only 10% to 20%. A review of the Nationwide Inpatient Sample from 2001 to 2012 demonstrated a significant decrease in the rate of scoliosis surgery in patients with Duchenne muscular dystrophy. reported that only 20% of their patients receiving steroids required spinal fusion, but 90% of patients not receiving steroids did.

Weakness in the trunk and paraspinal muscles leads to collapse of the developing spine into what is usually a long C-shaped curve with the apex in the thoracolumbar region. Over time, the curve progresses and involves the entire thoracic and lumbar spine, resulting in pelvic obliquity ( Fig. 9.9 ). This scoliosis does not respond to nonoperative treatment such as bracing and adaptive seating, and it is almost inevitably progressive, often increasing by 8 to 12 degrees per year ( ). Surgery generally is recommended when the scoliotic curve measures 20 to 30 degrees ( ; ; ), although some recommend spinal fusion at the onset of the deformity in patients who use a wheelchair full-time even when the curve is less than 20 degrees ( ; ; ). A delay in surgery allows the curve to progress further and pulmonary and cardiac function to worsen, adding to the risks of spinal surgery in these patients.

Fig. 9.9, (A) Spinal deformity in a 13-year-old boy with Duchenne muscular dystrophy; his forced vital capacity was 34%. (B) After posterior spinal fusion with pedicle screw construct and iliac screws for pelvic fixation.

The patient’s pulmonary function probably is more important than the size of the curve in decision making. Ideally, vital capacity should be 40% to 50% of normal. Patients who have a forced vital capacity of less than 35% are at high risk for pulmonary complications, whereas those with a forced vital capacity of greater than 50% are likely to have few postoperative pulmonary problems and usually can be weaned from the ventilator the night of surgery or the day after ( ; ). A few small studies have indicated that spinal surgery can be safely done on patients with a vital capacity of less than 30% ( ; ; ).

The goals of spinal surgery in patients with Duchenne muscular dystrophy are to obtain and maintain sitting balance and to correct pelvic obliquity so that the patient is able to use a wheelchair for the remainder of his life. The effect of spinal fusion on pulmonary function remains a matter of debate, with some studies finding no effect on the rate of pulmonary decline ( ; ; , ; ) and others finding reduced rates of decline ( ; ; ; ). Most authors agree that pulmonary function does not improve after surgical fusion of scoliosis ( ; , ); however, patients who have had spinal stabilization do have a substantially enhanced quality of life compared with patients who have not ( ; ). Suggested benefits of spinal fusion in patients with Duchenne muscular dystrophy include preservation of sitting balance, prevention of back pain, improvement in spinal decompensation, freeing the arms of the necessity of trunk support, improvement in body image, and possible slowing of the deterioration of pulmonary function ( ).

Segmental spinal instrumentation is recommended in patients with Duchenne muscular dystrophy because of the neuromuscular etiology and because the bone is relatively osteopenic due to nonambulation and chronic steroid use. Instrumentation with sublaminar wires and cross-link rods ( Fig. 9.10 ) or a unit rod or segmental instrumentation with pedicle screws ( Fig. 9.11 ) has been a useful technique for these patients. In patients with smaller curves and fixed pelvic obliquity, the fusion and instrumentation can end at L5. If fixed pelvic obliquity is more than 15 degrees, fusion to the pelvis with iliac screws, Galveston technique ( Fig. 9.12 ), or S-rod pelvic fixation is recommended. The fusion should extend to the upper thoracic spine, to T2 or T3. The sagittal contours of the spine, especially lumbar lordosis, should be maintained for sitting balance and pressure distribution.

Fig. 9.10, (A) Thoracolumbar curve of 77 degrees. (B) After correction with hooks and sublaminar cables, the curve is 22 degrees.

Fig. 9.11, Unit rod for neuromuscular scoliosis. Single, continuous ¼-inch stainless steel rod has a U bend at the top and bullet-shaped ends for insertion into the pelvis.

Fig. 9.12, Stabilization of the pelvis with the Galveston technique. Segment of rod is driven into each ilium.

Because of the pulmonary compromise in these patients, rapid postoperative mobilization is important. We generally use no orthoses after surgery.

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