Key Insights On Tendon Transfers For Drop Foot

Given the complexity of drop foot and its impact on gait biomechanics, these authors offer a primer on the diagnostic workup, and share their thoughts on the potential merits of tendon transfer procedures.

   Drop foot and foot drop are interchangeable terms that illustrate an abnormal neuromuscular disorder, which affects the patient’s capacity to lift up the foot at the ankle. Drop foot is further characterized by failure to dorsiflex the foot or move the foot inward or outward at the ankle. Pain, weakness and numbness may be associated with a loss of function.

   Typically, walking becomes a challenge due to the patient’s inability to control the foot at the ankle. These patients subsequently compensate with an exaggerated or high stepping walk referred to as steppage or foot drop gait.

   Notably, the patient may use a characteristic tiptoe walk on the opposite leg (raising the thigh excessively as if he or she were walking upstairs) while letting the opposite toe drop. This serves to raise the foot high enough to prevent the toe from dragging and prevents the slapping of the foot to the ground. Without compensation, however, the afflicted foot may appear floppy and typically drags or slaps down onto the floor.

   Other gaits such as a wide outward leg swing (to avoid lifting the thigh excessively or to turn corners in the opposite direction of the affected limb) may also be indicative of foot drop.

   Drop foot is not a disease but an indication of an underlying problem. Depending on the etiology, drop foot may be temporary or permanent. Often drop foot is caused by injury to the peroneal nerve stemming from the lumbar and sacral spine. Although other causes exist, lumbar disc herniation is a common cause. The peroneal nerve, a division of the sciatic nerve, innervates the muscle groups responsible for ankle, foot, toe movement and sensation.

   The peroneal nerve is susceptible to different types of injury. Some types of injury include nerve compression from lumbar disc herniation (e.g. L4, L5, S1), trauma to the sciatic nerve, spondylolisthesis, spinal stenosis, spinal cord injury, bone fractures (leg, vertebrae), stroke, tumor, diabetes, lacerations, gunshot wounds or crush-type injuries. Sometimes the peroneal nerve becomes injured when it is stretched during hip or knee replacement surgery, etc.

   Other causes of drop foot stem from a variety of etiologies such as: traumatic induced tumor; lower motor neuron lesions such as poliomyelitis and Charcot-Marie-Tooth disease; or diseases of the muscle fiber such as muscular dystrophy and polymyositis.

   With the classic drop foot, the common peroneal nerve is affected and leads to anterior and lateral muscle group weakness. Since the posterior group is unopposed, equinovarus deformity ensues from the pull of the posterior tibial muscle and triceps surae.

A Quick Overview Of Possible Treatment Options

   Drop foot is a complex problem. Determining the underlying cause of drop foot is one of the physician’s first endeavors and ultimately determines the course of treatment. Treatments include the use of an ankle foot orthosis (AFO), gait training, physical therapy and possible surgical reconstruction.

   For example, if drop foot is caused by nerve compression from a lumbar herniated disc, then a spinal procedure would possibly be indicated. If the drop foot has not progressed and is limited to a local level (the lower extremity), then surgical reconstruction associated with a tendon transfer is possible.

   The goal of a tendon transfer for drop foot is to create a stable, functioning plantigrade foot. In order to achieve this, surgeons frequently perform soft tissue releases, osteotomies and arthrodesis along with tendon transfers. Many combinations of such surgical procedures exist because of the wide gamut of neurological dysfunction and secondary deformity. If one treats drop foot early enough, soft tissue release and tendon transfers may help prevent or delay arthritic changes that require more extensive bony procedures.1

What You Should Know About Diagnosing Drop Foot

   The diagnostic process includes a comprehensive assessment of the patient’s symptoms, past and current medical histories, physical and neurological examinations, and imaging studies such as magnetic resonance imaging (MRI) and electromyography (EMG). One must consider the spine and make a final determination of the cause of drop foot prior to formulating a treatment plan.

   The physician often makes the diagnosis during the routine physical examination. Prior to the diagnostic workup, one can gain insight by simply assessing the patient’s gait. The patient will have difficulty clearing the floor during the swing phase of gait and will be prone to frequent tripping. If the patient can compensate, he or she avoids tripping via compensatory flexion of the knee and hip, which results in a steppage gait.2 Foot and ankle surgeons should assess resting stance to ascertain hindfoot to forefoot relationships and how they contribute to the deformity.

   The Coleman block test can determine whether the heel is flexible or rigid, and the role of the first ray or forefoot upon the rearfoot.3 When it comes to performing a Coleman block test, the patient stands on a 1-inch block with the first metatarsal head dropped over the medial side of the block. Record any changes or lack thereof in the hindfoot.

How To Assess For Muscle Imbalances

   People with foot drop will have weakness in the anterior muscle compartment and have difficulty walking on their heels only. The peroneal nerve innervates the anterior muscles that dorsiflex the foot. The peroneal nerve also innervates the lateral muscle compartment and it is not uncommon to find weakness in this area too. The posterior tibial nerve innervates the posterior muscles that plantarflex the foot. These muscles often develop tightness and a contracture/equinus in the presence of foot drop. Numbness, tingling and paresthesias in the lower leg and the dorsal aspect of the foot and ankle may be associated with foot drop.

   Such muscle imbalances clinically result in dynamic deformities such as a plantarflexed first ray. This plantarflexed first ray stems from recruitment of the peroneus longus to compensate for a weak peroneus brevis as one may see with Charcot-Marie-Tooth disease. Peroneus longus overpull may not be associated with any disease as it can occur on its own.4

   To evaluate muscle imbalances, the examiner places one thumb underneath the first metatarsal head and the other underneath the lesser metatarsal heads, and then pushes upward. If the first metatarsal does not elevate, one can assume there is a fixed deformity. If there is elevation, the cavus deformity is flexible.

   The foot and ankle surgeon may note a cock-up hallux due to anterior muscle group weakness and the windlass mechanism. One may see lesser digital imbalance with hammertoe deformities. Symptomatic metatarsalgia is caused by distal migration of the fat pad underneath the metatarsal heads in association with claw toe deformity.5 The examiner should also document active and passive range of motion of all joints.

   A neurologic exam should include sensory, vibratory and deep tendon reflexes. Paresthesia or anesthesia over the course of the common peroneal nerve is a typical complaint after traumatic insult. Check reflexes at the ankle and knee to identify or rule out lumbar involvement. Manual muscle testing can determine the extent of involvement for the muscles involved. One would typically record the power and strength of each muscle by using the chart, “A Guide To Muscle Strength Testing,” on page 62.

Pertinent Points On Diagnostic Imaging

   One must take standard serial radiographs to determine biomechanics, joint adaptation, bone morphology, etc. Use long leg axial views to determine the calcaneal position relative to the tibia. Stress views of the ankle joint will illuminate ligamentous laxity or tear associated with ankle instability, and frequent ankle sprains subsequent to drop foot.6

   Advanced imaging such as MRI is indicated if one is suspicious of tendon attenuation, trauma or soft tissue pathology. Consider EMG if you are suspicious of more proximal lesions. The EMG is particularly useful in predicting the recovery and progression of symptoms if one repeats the testing.

Can Conservative Care Have An Impact?

   Physicians can employ lightweight orthoses to stabilize ankles in patients with foot drop. These orthoses are available in molded plastics as well as softer materials that use elastic properties to prevent foot drop. Additionally, one can fit shoes with traditional spring loaded braces to prevent foot drop while the patient is walking.

   While it is common to encourage regular exercise, long-term patient adherence is frequently challenging. Often, individuals with foot drop prefer to use a compensatory technique like steppage gait as opposed to using a brace or splint. Even though the mainstay of conservative care is an ankle-foot orthosis, many patients find this to be cumbersome.

   If the foot drop is the result of a peripheral nerve injury, physicians often advise a window for recovery of 18 months to two years prior to any surgical intervention. If it is apparent that no recovery has taken place, one can consider surgical intervention to repair or graft the nerve, although results from this type of intervention are mixed.

   Nonetheless, tendon transfers are a viable option for patients with adequate passive ankle joint dorsiflexion and stable skeletal alignment whose symptoms have neither improved nor become worse.

Salient Anatomical Insights On Tendon Transfers

   The basics of tendon transfers should start with knowing the anatomy and function of tendons.7 The tibialis anterior, tibialis posterior, Achilles and peroneal longus tendons are the primary effectors surrounding the ankle and subtalar joints. When considering tendon transfer, you should estimate the strength and excursion of the muscle you are trying to replace. Based on the work by Silver, et al., obviously one should not expect, for example, the flexor digitorum longus to replace the triceps surae.8

   Another consideration is whether a tendon is “phasic” or “non-phasic.” When one transfers a tendon, it is realistic to expect the muscle to have a loss of strength. When it comes to tendons that are in phase with the tendon they are replacing, they retain more strength, and muscle reeducation and rehabilitation are faster.

   Phasic tendon transfers in drop foot may not be practical. Therefore, relying on non-phasic tendon transfers is often necessary. For example, one would reroute the posterior tibial tendon anteriorly in cases of drop foot. Making a conscious effort to dorsiflex the foot while it is in the swing phase will bring about foot clearance. This reeducation of the posterior tibial tendon can take a great deal of time before it becomes an unconscious endeavor.

   The greater the distance a tendon is from a joint axis, the greater the force that it exerts across the joint as a result of a longer lever arm. This is important when determining whether to pass a tendon beneath the retinaculum or subcutaneously. When one pursues a transfer beneath the retinaculum, the tendon is closer to the joint. This diminishes the lever arm and muscle strength, but increases tendon range of motion. The opposite holds true for subcutaneous placement. The Blix curve describes the relationship of muscle length and contractile force.9

   Ideally, the surgeon should place the tendon that he or she is transferring at zero tension (physiological tension), midway between completely relaxed and maximum length. You will have achieved zero tension when the foot is in the desired position without any slack. Surgeons frequently place tendons in slightly greater tension because there is a degree of muscle stretching.

Other Essential Considerations For Facilitating Optimal Outcomes

   Regardless of the tendon one seeks to transfer, adhering to a few key principles and weighing the necessity of adjunct procedures will provide better, more predictable results.10

   The tendon must have adequate strength. Without adequate strength of a grade 4 or better on a five-grade scale, the tendon that one is transferring functions mostly as a tenodesis/suspension. The line of pull should be direct with no tendon angulations. This maximizes the value of the muscle’s strength. Acute angulations will limit the gliding mechanism of the tendon and result in scarring and binding.

   One should fixate tendons to bone. Providing fixation to stable structures allows for better reliability. Be advised that soft tissue, such as tendon-tendon attachments, may undergo further atrophy, resulting in a loss of correction. It is essential to have adequate range of motion of the joint so the tendon transfer will cross over. With longstanding contracture, tendon transfers alone will not conquer the pull of stronger muscles. In drop foot, Achilles tendon lengthening or gastroc recession may be required to address longstanding plantarflexed contracture.

   Surgeons should subsequently determine whether the drop foot is in a fixed or flexible state. One cannot correct rigid deformities via tendon transfers alone.5 You must correct the fixed deformity prior to considering a tendon transfer.

   Consider early surgical intervention to correct muscle imbalances in order to prevent development of a fixed deformity like one may see with pes planovalgus. Arthrodesis is only indicated if there are degenerative changes.5 Osteotomies may be preferable to arthrodesis if the deformity is semi-rigid and the joints are not arthritic. This will place the transferred tendons in a more optimal position.

   Tissue handling and incision planning are important factors. Once you have detached tendons from their insertions, keep them in moistened saline gauze until you are ready for rerouting.11 Also try to maintain as much of the musculotendinous junction as possible to preserve the muscle and decrease the incidence of scarring.11

A Closer Look At The Different Tendon Transfer Procedures

   Posterior tibial tendon transfer has long been the mainstay to combating drop foot. Watkins et al., first reported on this tendon transfer in 1954.12 Classically, the surgeon transfers the posterior tibial tendon through the interosseous membrane and inserts the tendon to the third cuneiform.

   There have been many modifications since the induction of this procedure. Perhaps most common is the split posterior tibial tendon transfer. In this procedure, one splits the tendon longitudinally to the musculotendinous junction and inserts the lateral half into the peroneus brevis just proximal to its insertion. This technique balances the hindfoot and restores dorsiflexory power without weakening posterior tibial tendon function. When the surgeon cannot sacrifice the posterior tibial tendon, he or she can utilize the peroneal longus tendon in a similar fashion.

   The Bridle procedure is a combination of the posterior tibial and peroneal longus tendon transfers.13 One would insert the posterior tibial tendon into the second cuneiform and anastomose the peroneal longus to the peroneal brevis. Proceed to suture together the aforementioned tendons proximally to the anterior tibial tendon.

   The stirrup procedure is a salvage procedure. While it does not restore ankle range of motion, the procedure does maintain a dorsiflexed position, which allows for ground clearance upon the swing phase of gait. With this procedure, the surgeon transects the extensor tendons proximal to the ankle joint and then sutures them to the anterior aspect of the tibia. When one can sacrifice no other tendon for transfer, the stirrup procedure may be a viable choice.

In Conclusion

   The goal of surgery for drop foot is a stable plantigrade foot that is shoeable, brace free and prevents tripping. Tendon transfers are excellent options for appropriate patients.

   With these patients, surgery is a balancing act to enact a tripod effect. The first and fifth metatarsal heads, and heel should form a triangle in a stable, plantigrade plane. Deviation from any one of these points can cause an imbalance. Proper patient selection, surgical planning and postoperative rehabilitation are key ingredients to success.

Dr. DiDomenico is affiliated with the Forum Health/Western Reserve Care System in Youngstown, Ohio. He is the Director of the Reconstructive Rearfoot and Ankle Surgical Fellowship within the Ankle and Foot Care Centers and the Ohio College of Podiatric Medicine. Dr. DiDomenico is a Fellow of the American College of Foot and Ankle Surgeons.

Dr. Cane is affiliated with the Forum Health/Western Reserve Care System in Youngstown, Ohio. He is a Fellow of the Reconstructive Rearfoot and Ankle Surgical Fellowship within the Ankle and Foot Care Centers and the Ohio College of Podiatric Medicine. Dr. Cane is an Associate of the American College of Foot and Ankle Surgeons.

For further reading, see “Understanding The Impact Of Diabetic Neuropathy On Gait” in the June 2008 issue, “A Guide To Conservative Stabilization Of The Neuromuscular Foot” in the November 2008 issue or “Essential Insights On Flexor Tendon Transfers” in the April 2007 issue.

To check out the archives or get information on reprints, visit www.podiatrytoday.com.

References:

1. Holmes JR, Hansen ST. Foot and ankle manifestation of Charcot-Marie-Tooth disease. Foot & Ankle 1993 Oct; 14(8):476-86. 2. Stewart JD. Foot drop: where, why, and what to do? Pract Neurol 2008; 8(3):158-69. 3. Krause FG, Wing KJ, Younger AS. Neuromuscular issues in cavovarus foot. Foot Ankle Clin. 2008 Jun;13(2):243-58. 4. Hansen ST. Functional reconstruction of the foot and ankle. Lippincott, Williams & Wilkins, Philadelphia, 2000, pp. 190-91. 5. Younger AS, Hansen ST Jr. Adult cavovarus foot. J Am Acad Orthop Surg 2005; 13(5):302-15. 6. Manoli A 2nd, Graham B. The subtle cavus foot, “the underpronator.” Foot Ankle Int 2005 Mar; 26(3):256-63. 7. Jeng C, Myerson M. The uses of tendon transfers to correct paralytic deformity of the foot and ankle. Foot Ankle Clin 2004; 9(2):319-37. 8. Silver RL, de la Garza J, Rang M. The myth of muscle balance. A study of relative strengths and excursions of normal muscles about the foot and ankle. J Bone Joint Surg Br 1985; 67(3):432-7. 9. Liu SH, Panossian V, al-Shaikh R, Tomin E, Shepherd E, Finerman GA, Lane JM. Morphology and matrix composition during early tendon to bone healing. Clin Orthop Relat Res 1997 June; 339:253-60. 10. Rodriguez RP. Surgical reconstruction of the paralytic flaccid foot. Foot Ankle Clin 2000; 5(2):349-79. 11. Myerson M. Reconstructive foot and ankle surgery. Elsevier, Philadelphia, 2005, pp. 169-181. 12. Watkins MB, Jones JB, Ryder CT Jr, Brown TH Jr. Transplantation of the posterior tibial tendon. J Bone Joint Surg Am 1954 Dec;36-A(6):1181-9. 13. Rodriguez RP. The Bridle procedure in the treatment of paralysis of the foot. Foot Ankle 1992 Feb;13(2):63-9.