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Results of Posterior Transfer of the Anterior Tibialis in Children with Myelomeningocele

Posted on: 10/29/2008
Spinal cord defects such as myelomenigocele can cause foot deformities as a result of muscle imbalance. Myelomeningocele is a protrusion of the meninges and spinal cord. Meninges is the covering around the spinal cord. In this condition, the meninges fail to close when the child is developing in the womb.

This type of defect is called a neural tube disorder. The neural tube is the protective sheath of bone and meninges that encase the entire spinal cord. It is formed early in utero (around the 19th day in the womb). The most common neural tube defect is called spina bifida occulta. This refers to an incomplete closure (incomplete fusion) of the arch of bone around the spinal cord.

Myelomeningocele is more severe than spina bifida. There is a failure of the bone to close around the spinal cord (like in spina bifida), but there is also a failure of the meninges to cover and protect the spinal cord. Generally these defects occur in the lumbosacral area. Children with an L4 myelomeningocele often have foot deformities requiring treatment.

One of those foot deformities is the topic of this study. The use of a posterior transfer of the anterior tibial tendon to rebalance the foot and ankle was investigated. The anterior tibial tendon normally pulls the ankle up toward the face. Without the opposing muscle, the foot is pulled up so far, the child is walking just on the calcaneus (heel bone). The patient can't point the toe or roll over the foot and push-off with the big toe when walking. In order to swing the foot forward without scuffing the heel every time, the affected individual exaggerates motion at the knee, hip, and pelvis.

Surgical treatment isn't always successful. The reason(s) for this aren't entirely clear. It was once thought that the problems were all related to muscle imbalances. Weakness from the neurologic impairment of one set of muscles in the leg can lead to the stronger muscles pulling the foot in one direction.

Over time, the constant uneven muscle pull can result in a foot and/or ankle deformity. More recent studies have also shown that external forces acting on the foot are important, too. Gravity and ground reaction forces are the most likely external factors to contribute to the problem.

Given this new information, surgeons are rethinking the best approach to the problem. Just releasing the overactive (unopposed) tendon and bracing the foot has not had good results for everyone. Studies show that there may be a specific subset of children who can benefit the most from this technique.

Likewise, tendon transfers (using one tendon in place of another to prevent or correct a deformity) are only successful in one-third to one-half of all cases. There are difficulties in studying these surgical approaches. Not all children with myelomeningocele have the spinal deformity at the same spinal level. The level of impairment determines which muscles are weak or imbalanced.

Moving the strong anterior tibialis muscle posteriorly allows it to function instead as a plantar flexor muscle (pulls toes down toward the floor). In this study, there were eighteen children with L5- or sacral-level myelomeningocele who had a posterior transfer of the anterior tibial tendon. They could all walk without the use of a walker or crutches. The posterior anterior tibial transfer was done by one surgeon. The surgical technique was described step-by-step.

Whenever necessary, the front part of the joint capsule was also cut to release the ankle. This procedure is called an anterior capsulotomy. In some cases, other muscles around the ankle had to be lengthened. This approach was used to keep the ankle from rolling inward causing a valgus deformity.

After the operation, the children were put in a long-leg cast for six weeks. This means the cast included the foot and ankle and went up over the knee. The ankle was held in a position of 20-degrees of plantar flexion (toe pointed downward) and 20-degrees of knee flexion.

When the cast came off, a special plastic brace called an ankle-foot orthosis was worn. The orthosis was designed to perform several functions. It helped with ankle plantar flexion, prevented the lower leg from moving forward too far when standing, and kept the child in an upright position by preventing the foot and ankle from moving too much into dorsiflexion (toes toward face).

Two different orthotics were available. The one selected for each child was chosen depending on whether there was a need to control upright posture and prevent a crouch position or to reduce ground-floor reaction.

After surgery, the children were further divided into two groups based on pre-operative testing. Group one had normal pelvic rotation when walking, especially during the single-limb stance phase (standing on one leg while swinging the other leg forward). Group two had increased pelvic rotation. The surgeons were interested in knowing if the procedure had any effect on pelvic motion.

Pressure distribution along the bottom of the foot was measured and compared to before the surgery. They also analyzed differences in the way each child walked, a process called gait analysis.

Kinematic studies using a motion analysis system with six infrared cameras were used to examine changes in the way the knee, hip, and pelvis moved from before to after surgery. In addition to the kinematic studies, X-rays were taken before and after surgery to measure bone and joint angles while standing.

They were also able to use a special platform that measures ground reaction forces. This is the force that comes up through the ground as the foot comes in contact with the floor. Ground reaction forces are important because there's more than just the foot and ankle affected when walking.

Any change caused by deformity in one joint will affect all the other joints in the kinetic chain. Any contact between the foot and the floor transfers the load to the knee, hip, pelvis, and spine. The change The change in where the contact occurs and with how much force is applied are both good before and after measures of the effect of the tendon transfer.

Using the Xrays, gait analysis, foot-pressure measurements, and changes in pelvic motion as measures of outcomes, here's what they found:

  • There were no significant differences in X-ray results between the two group after surgery
  • The surgery was able to redistribute the weight-bearing surface of the foot (more equal along the heel and forefoot)
  • More ankle motion was present in the stance phase (standing on one leg) while walking
  • Pressure on the heel was not less in the group with increased pelvic movement

    The tendon transfer, capsulotomy, and lengthening of other muscles in this sugery did reduce the ankle deformity. The transferred tendon helped (but does not completely replace) the weak calf muscle. The authors believe that if this type of surgery is done early enough in the child's life, it is possible to prevent foot and ankle deformities in children with low-level myelomeningocele.

    Long-term studies are still needed to see how well everything holds up over time. The authors advise using bracing to protect the transferred tendon and maintain balance in the foot. The proper use of the orthoses may be important, but assessing this was not part of this study.

  • References:
    Kun Bo Park, MD, et al. Surgical Treatment of Calcaneal Deformity in a Select Group of Patients with Myelomeningocele. In The Journal of Bone & Joint Surgery. October 2008. Vol. 90-A. No. 10. Pp. 2149-2159.

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