Prioridad No. 4 - Seguridad Alimentaria y Nutricional (ODS 2)

INDICATIONS/CONTRAINDICATIONS

Hamstring lengthening is indicated for children with excessive knee flexion in terminal swing and stance and with popliteal angles greater than 40 degees. In general, various nonoperative options will have been tried prior to surgery, such as knee immobilizers at night, a home stretching program, and/or botulinus toxin A injections into the hamstrings.

SURGICAL PROCEDURE

A single midline incision (usually 4–5 cm) is made at the junction of the middle and distal thirds of the posterior thigh. The medial hamstrings are almost universally tight in those with hamstring contractures and are approached first. It is extremely rare to find a child with isolated lateral hamstring tightness in the absence of previous

hamstring surgery.

The dissection is carried medially and the semitendinosus is isolated. (The

semitendinosus is the most superficial posteromedial structure and is tendinous in nature.) Isolation of the semitendinosus is most easily accomplished by dissecting directly toward the tendon with tenotomy scissors or electrocautery. The deep fascia is incised with dissecting (tenotomy) scissors. Once the tendon is isolated, a right-angle clamp is placed around the semitendinosus tendon from lateral to medial to minimize the risk of neurovascular damage (Fig. 9-3A). In most cases, the

semitendinosus is transected with electrocautery at approximately the level of the musculotendinous junction. If a concomitant distal rectus femoris transfer (DRFT) is to be performed, the semitendinosus is transected proximal to the musculotendinous junction to allow for sufficient length for the DRFT.

FIGURE 9-3 A. Anatomy of the posterior distal thigh and knee. The semitendinosus is the most superficial medial hamstring. Because the

neurovascular structures are relatively close during the surgery, the right-angle clamp should be placed from lateral to medial when isolating the semitendinosus to minimize the potential risk of neurovascular injury. These structures are at highest risk during posterior capsulotomy. B. Aponeurotic lengthening of the semimembranosus is typically performed at two levels. When the biceps femoris requires lengthening, a comparable aponeurotic lengthening is performed, also typically at two levels. C. Typical appearance following aponeurotic lengthening of the semimembranosus. This photograph demonstrates the lengthening at one of the two levels typically performed. The arrow points to the underlying muscle, which is left intact. Note the white aponeurosis, which has separated, allowing additional knee extension despite preservation of the underlying muscle.

The fascia over the semimembranosus is incised with tenotomy scissors, and the muscle is isolated. The discrete aponeurosis is then cut with a knife. A #15 blade provides finer control and feel than does electrocautery (Figs. 9-3B and 9-3C). The aponeurosis is cut transversely at two levels, with the underlying muscle left

undisturbed. The proximal cut should be made first so that the tissue is still on tension when the distal cut is made. It is not necessary to lengthen the gracilis; it does not appear to be a significant cause of medial hamstring tightness, and routine

lengthening will cause excessive knee flexor and hip extensor weakness.

P.89 P.90 In the past, rechecking a popliteal angle with each sequential hamstring lengthening was thought to be necessary. However, it now appears that rechecking a popliteal angle may put undue tension on the nerve (with a theoretical increase in the risk of peroneal nerve palsy), so it is no longer necessary to check the popliteal angle intraoperatively (Fig. 9-4). Instead, the knee is brought into full extension, with the ipsilateral hip extended. If the knee easily falls into extension following the medial hamstring lengthening, the lateral hamstrings are not lengthened.

FIGURE 9-4 The popliteal angle test should not be performed intraoperatively, as this can cause undue tension on the sciatic nerve, with potential deleterious effects on the nerve (particularly the peroneal branch) postoperatively.

If the lateral hamstrings are quite tight following medial hamstring lengthening, the biceps femoris should be lengthened. The biceps femoris can be accessed easily through the midline incision that was used for the medial hamstring lengthening.

Dissection is aimed directly toward the tight biceps femoris with dissecting scissors or electrocautery. The fascia over the biceps is incised, and the discrete aponeurotic band, located laterally in the biceps, is identified and isolated. This band should be transected at two levels using a #15 blade (as with the semimembranosus), leaving the underlying muscle intact.

Posterior knee capsulotomy is only very rarely indicated in children with CP, though it has been advocated by previous authors. In almost all children who are potential candidates for posterior capsulotomy, the capsule itself is not as tight as the neurovascular bundle following hamstring lengthening. Therefore, the posterior capsulotomy is rarely of significant benefit in children with CP (unlike those with significant knee contractures associated with other maladies, such as arthrogryposis).

When necessary, posterior knee capsulotomy may be performed with the patient supine or prone. Although it is certainly easier to perform concomitant surgeries at other levels with the child supine, posterior capsulotomy is more difficult with the child supine if the knee flexion contractures exceed 20 degrees. If knee capsulotomy is requisite, it may be worth positioning the child prone, even if other surgeries may require reprepping the child supine. The posterior capsulotomy may be performed through a midline incision (usually 6–8 cm in the distal third of the thigh). The

dissection is deepened between the semimembranosus medially and the biceps femoris laterally. The peroneal and tibial nerves, as well as the popliteal artery and vein, are identified, isolated, and protected in this direct, posterior approach (Fig. 9-3A). Vessel loops may be placed around these important structures if desired. With the

neurovascular structures protected, dissection is carried directly toward the femur until the medial and lateral heads of the gastrocnemius are identified. An elevator is then used to sweep these off the posterior knee capsule, which is then divided with either a #15 blade or electrocautery. When the knee is extended, the capsule is seen to spread.

In children with marked crouch and patella alta, some physicians advocate patellar tendon shortening and extension osteotomy of the femur in conjunction with

hamstring lengthening. When performed, this operation should be reserved for

children with severe crouch in adolescence. However, these procedures are often not requisite, as severe crouch can often be attained without the addition of patellar tendon shortening and femoral extension osteotomy.

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POSTOPERATIVE MANAGEMENT

Hamstring lengthening is often combined with other procedures. In general, a long-leg walking cast can be used for 3 to 4 weeks following hamstring lengthening. Most

children with CP will stand, but not walk, with the cast(s), due to the children's limited balance, strength, and motor control. Following cast removal, knee immobilizers are used at night for at least 6 to 12 months.

For children with significant knee flexion contractures (≥20 degrees) and marked crouch (usually ≥45 degrees) preoperatively, a knee-ankle-foot orthotic (KAFO) is often helpful to enhance gait for at least the first 3 to 6 months following cast

removal. In such children, the initial postoperative casts may be changed at 3 weeks and measurement for KAFOs may be done. The long-leg casts are continued until the KAFOs are ready for delivery.

COMPLICATIONS TO AVOID

Though rarely discussed, genu recurvatum is relatively common following hamstring lengthening. The risk is significantly greater following combined medial and lateral hamstring lengthening than is that following isolated medial hamstring lengthening.8 Because of this risk of recurvatum, as well as risks of increased anterior pelvic tilt and weak hip extension, lateral hamstring lengthening should not routinely be performed in all cases.

Unfortunately, numerous cases of extreme recurvatum also occur after some isolated medial hamstring surgeries. In almost every one of these cases, the medial hamstring surgery consists of a simple transection of each medial hamstring (semitendonosus, semimembranosus, and gracilis). Such routine transection of all the medial hamstrings should never be performed in ambulatory children with CP, because it leads to

significant hip extensor weakness, genu recurvatum, anterior pelvic tilt, and the inability to achieve adequate knee flexion in swing phase.

Neuropraxia is well known following hamstring lengthening surgery. The risk appears to

be less if the popliteal angle is not checked intraoperatively.

Recurrent hamstring contracture is possible as well. Postoperative casting followed by the use of knee immobilizers for at least 6 months postoperatively seems to decrease this risk.

PEARLS AND PITFALLS

Lateral hamstring lengthening is not needed in many cases. Medial hamstring lengthening suffices in most cases to allow sufficient knee extension.

Combined medial and lateral hamstring lengthening increases the risk of genu recurvatum postoperatively.

Posterior knee capsulotomy is rarely indicated in children with cerebral palsy.

The neurovascular bundle is often under sufficient tension following combined medial and lateral hamstring lengthening that knee capsulotomy will usually not safely allow additional knee extension.

Avoid extending the knee (to assess or lengthen hamstrings) with the hip flexed 90 degrees intraoperatively, as this is associated with stretch injury of the peroneal nerve.