Understanding The Biomechanics Of The Transmetatarsal Amputation
Restoring Anatomical Alignment
Studies have shown that up to 44 percent of patients will fail to heal or require a more proximal amputation following a midfoot amputation while many others may require additional debridements or stump revision to obtain skin closure.26,27 Although the reason for re-amputation may not always be biomechanically induced, there are a variety of methods, both conservative and surgical, the podiatric surgeon can implement to obviate the complications associated with the biomechanically flawed foot. The primary objective is to achieve a functional, stable foot residuum that can accommodate weightbearing and ambulation, and avoid future tissue breakdown and major limb amputation. Surgeons have proposed both tendon balancing procedures and skeletal stabilization operations to meet these goals.
Various authors have described the tendo-Achilles lengthening (TAL), tenotomy and gastrocnemius recession in the literature for patients with an equinus ankle deformity. The Silfverskiold test can help differentiate between the muscle responsible for the contracture. When one supplements this test with clinical findings as to the degree of contracture, he or she can choose an appropriate surgical technique. In doing so, the podiatric surgeon can help reduce the risk of ulcer recurrence, re-infection and additional amputations.
Through a minimally invasive approach, it is reasonable that many podiatric surgeons will perform a TAL. This will somewhat restore normal ankle joint kinematics, maintain an anatomically acceptable foot-ankle relationship that will hopefully translate into a more acceptable gait pattern, and reduce the risk of tissue breakdown although this procedure does not come without risk. Researchers have proven that percutaneous TAL dramatically reduces peak plantar pressures by approximately 27 percent while also providing a significant increase in ankle joint dorsiflexion and facilitating healing of the TMA incision.15,28
However, Maluf and co-workers demonstrated that the decrease in plantarflexory power and plantar forefoot pressures is transient, and both variables increase significantly eight months postoperatively. This renders forefoot ulceration an imminent concern that warrants vigilant postoperative surveillance by both the surgeon and patient.29 More than half of patients undergoing percutaneous TAL may develop new or recurrent foot ulcers, either in the forefoot or heel, substantiating the need to address other factors that may be pivotal in tissue breakdown.30 These factors could be unaddressed osseous prominences that one sees with Charcot deformity or inappropriate shoe gear.
Accordingly, surgeons must perform the TAL with great caution and take care not to completely rupture or over-lengthen the tendon. Extreme dorsiflexion (>15 degrees) can generate a calcaneal gait, increasing the risk of developing limb threatening heel ulcers. One should perhaps avoid extreme dorsiflexion in patients with an insensate heel pad as up to 47 percent of patients may develop acute transfer ulcers to the heel.31
The gastrocnemius recession is also an option for patients with TMA-induced equinus and has reported advantages over a TAL. The lengthening with this surgery is more controlled. Therefore, there are reduced risks of over-lengthening and iatrogenic tendon rupture. As a result, there is less of a chance of creating a calcaneal gait and the risk of a transfer heel ulcer remains low.