Understanding The Biomechanics Of The Transmetatarsal Amputation

Gabriel V. Gambardella, DPM, and Peter A. Blume, DPM, FACFAS

   One can perform the gastrocnemius recession endoscopically. Doing so minimizes the need for a long incision and avoids injuring an already pathologic tendon. There is less loss of plantarflexory muscle strength with the gastrocnemius recession than one would see with the TAL. Studies have shown that this surgery can decrease forefoot pressures and promote more efficient wound healing, and may reduce the risk of developing transfer ulcers postoperatively.32-34 Surgeons can also perform an Achilles tenotomy to correct the equinus deformity. However, we only consider this an option when performing a tibiotalocalcaneal, tibiocalcaneal or pantalar arthrodesis.

   Surgeons have advocated adjunctive tendon procedures for balancing the post-TMA foot. These procedures include a peroneus longus to brevis tendon transfer to augment pronatory forces; split tibialis anterior transfers to reduce inversion forces; and flexor hallucis longus and extensor digitorum longus transfers to normalize frontal plane deformity and reduce the risk of ulcer development at the plantar lateral aspect of the stump.35,36 All have demonstrated the ability to achieve deformity correction.

   However, one must take the need for an additional incision into account and avoid the aforementioned procedures in the dysvascular foot, an immunocompromised patient or when an active infection is present.36 When infection is present, we recommend eradicating the infection before performing any of the aforementioned tendon balancing procedures and posterior muscle group lengthenings in order to prevent bacterial seeding.

   Skeletal stabilization interventions can also effectively realign the foot and are probably longer lasting than tendon procedures, but are prone to more postoperative complications intrinsic to implanting hardware.

   Schweinberger and Roukis propose a method to correct the deformed TMA stump in the dysvascular foot when tendon transfers and additional incisions are contraindicated.37 Using a large diameter screw extending from the first metatarsal residuum into the talus and occasionally a lateral column stabilization screw, they have been able to achieve a stable, functional foot although they recognize that infection of hardware and screw migration pose concerns.

   Rearfoot arthrodeses, including tibiotalocalcaneal, tibiocalcaneal and pantalar arthrodesis, can also achieve an anatomically correct TMA stump posture. However, they require additional incisions and internal or external fixation. Therefore, surgeons should reserve these procedures for select patients and as a last resort to limb salvage.

Addressing Shoe Gear After A TMA

Although many surgical techniques have proven to anatomically and biomechanically normalize the deformed TMA stump, the astute podiatric surgeon must acknowledge possible extrinsic factors that can lead to complications and be prepared to address them postoperatively. Following a TMA and after one has eradicated any infection, obtained an acceptable foot/leg, and healed incisions and wounds, we must consider the impact shoe gear and orthoses may have on the diabetic foot. Keep in mind that we need to ensure the physiological distribution of plantar pressures without overloading any aspect of the foot. Other areas of concern include instability and imbalance during gait for those with a shortened foot length and peripheral neuropathy.

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