Understanding The Biomechanics Of The Transmetatarsal Amputation
Communication is of paramount importance between the surgeon and patient to avoid contrasting expectations. The surgeon must provide the patient with sufficient information of the entire process, particularly in cases when primary closure is not possible, and continued wound care and a second surgery may be required. The patient must be committed to strict adherence during the postoperative course as this can be a time-consuming process, taking weeks or months to achieve a satisfactory outcome. Additionally, the patient must recognize that a more proximal amputation may be necessary in the future.
Pertinent Biomechanical Considerations
The TMA has been successful in increasing limb salvage rates and decreasing the need for major limb amputation. However, a fundamental understanding of foot and ankle biomechanics becomes imperative for limb salvage sustainability, and to avoid preventable postoperative complications.
With respect to midfoot amputations, one of the goals is to maximize foot length in order to provide optimal biomechanics so that subsequent tissue breakdown does not occur. Potential techniques and modalities to help achieve this goal include skin grafts, pedicled and microsurgical free flaps, and Ilizarov frames.6 By maximizing foot length, one can shorten the lever arm for propulsion to a minimum and maximize the surface area of the plantar foot residuum to distribute pressures.
Researchers have shown that those with a TMA have a 40 to 48 percent shorter moment arm in comparison to the normal foot. This theoretically requires a 48 percent increase in the ground reaction force to generate a given plantarflexor moment, which would increase localized pressure at the distal aspect of the TMA stump, leading to increased risk of ulcerating.16 When taking this into consideration, the TMA is preferable over more proximal pedal amputations yet it is discouraged when one can confidently ensure forefoot salvage.
The most common biomechanical complication associated with the TMA is the equinovarus deformity with coinciding gait discrepancies resulting from the elimination of extrinsic musculature and intrinsic tendon alterations inherent to the pathophysiology of diabetes.
During the swing phase of gait, the anterior muscle group dorsiflexes the foot at the ankle joint and counteracts plantarflexory forces for ground clearance. The anterior muscle group also assists in deceleration of the forefoot at heel strike through eccentric contractions to prevent the forefoot from slapping onto the ground through midstance. However, with all long extensor tendons traversing the ankle joint one is transecting, the posterior muscle group, specifically the gastrocnemius-soleus complex, gains a mechanical advantage. This forces the foot into an equinus position at the ankle joint. Not only is it more difficult for the foot to now achieve ground clearance following propulsion without the help of the hip flexors but the contact phase can be significantly altered. The patient may now first touch ground on his midfoot or even the forefoot when the contraction is severe enough.
In the frontal plane, the supinatory forces of the tibialis posterior and triceps surae pull the foot into a varus orientation at the subtalar joint. The tibialis anterior, another subtalar joint supinator, pulls up on the medial ray and acts relatively unopposed as the extensor digitorum longus tendons have been transected and the peroneus brevis cannot alone counteract the overwhelming supinatory forces.17
The foot now assumes an equinovarus position. This will have many implications in terms of function but also comes into play when surgeons consider initial incision healing, ulcer occurrence or recurrence, infection and the need for a more proximal amputation. One can also attribute the pathogenesis of the equinus contracture to the non-enzymatic glycosylation of collagen.