The obvious biomechanical consequence associated with a TMA is shortening of the lever arm of the foot which impacts balance, gait, and posterior muscle group function. Care is taken to recreate a natural metatarsal parabola while attempting to preserve a functional metatarsal length when possible. Many patients undergoing TMA have pathologic mechanics due to prior surgery, digital deformity, degenerative arthritis, or neuropathic fracture which is often the reason for recurrent wounds and persistent infection. The TMA procedure is intended to improve the mechanics of this pathologic foot type which goes against the commonly held belief that partial foot amputation makes the foot more prone to problems. The ideal zone of resection is at the metatarsal neck just proximal to the metaphyseal flare. Leaving the distal metaphyseal flare predisposes the foot to recurrent tissue breakdown, since the flare is pronounced on the plantar aspect. This diaphyseal level of resection is also preferred since metaphyseal bone resection may encourage heterotopic ossification (HO) (Fig. 19.2) as described in Chap. 17 [8]. Excessive metatarsal shortening should be avoided although the extent and location of open wounds or gangrene ultimately dictate location of metatarsal resection in an effort to transect the metatarsals short enough to achieve soft tissue coverage. Rotational flaps are beneficial in this regard, which are incorporated to provide soft tissue coverage without excessive bone resection. The anticipated extent of osteomyelitis based on preoperative imaging and direct surgical inspection also influences the ideal level of bone resection. A TMA is not expected to produce a clean bone margin in cases of metatarsal osteomyelitis. A metatarsal preservation approach is preferred which combines selective bone debridement with medical treatment of residual osteomyelitis in an effort to maintain optimal postoperative foot function. Advanced surgical techniques including medullary reaming, staged resection, and insertion of antibiotic cement also supports achievement of a clinical cure of metatarsal osteomyelitis without excessive surgical resection.

The outlook for gangrene-related TMA is not as optimistic when compared to osteomyelitis associated with neuropathic ulceration, due to compromised healing potential associated with vascular disease. There is less concern for osteomyelitis with gangrene although dead tissue frequently becomes infected while waiting for demarcation of gangrene. There is a fine line between allowing forefoot gangrene to demarcate and letting dry gangrene turn into infected wet gangrene. A close “wait and watch” approach is reasonable, yet waiting too long is counterproductive as vascular compromise and infection are a dangerous combination. Vascular intervention is common during this waiting period which is ideally performed prior to TMA. Antibiotics are only needed if infection is suspected or confirmed. Once healed, the gangrene-related TMA stump tends to remain durable without the tendency for recurrent ulceration unless there is concomitant peripheral neuropathy or residual deformity.

Posterior muscle group contracture or ankle equinus is a common cause of recurrent ulceration after TMA. Moore and Jolly noted that the gastrocnemius and soleus have little opposition once the extensor hallucis longus and extensor digitorum longus have been eliminated and only the tibialis anterior remains to provide dorsiflexion [9]. Many patients with neuropathic ulcers leading to TMA have preexisting equinus, which will need to be addressed if TMA is to be successful. Tendo-Achilles lengthening (TAL) or gastrocnemius recession may be performed concomitantly or after complete healing of the TMA site depending on the clinical situation (Fig. 19.3). There is risk of cross-contamination from the infected or contaminated amputation site when clean and dirty procedures are performed in the same setting; however, this is rare due to the effectiveness of isolating the surgical sites. The authors’ preference is to delay the Achilles lengthening procedure to see if it is necessary once the patient is ambulatory. One benefit of delayed posterior lengthening is the ability to rehabilitate and ambulate during the initial 6 weeks of tendon healing without risk of harm to the healing amputation site. Trying to maintain ankle dorsiflexion after concomitant TMA and TAL procedures is difficult as posterior splints can cause pressure sores on the posterior heel or plantar foot. Profound equinus contracture after TMA is more common when open guillotine TMA procedures are left to heal secondarily over the course of many months. Prolonged non-weight bearing with this approach almost assures equinus contracture. Primary closure and flap coverage of the amputation site allows prompt healing and timely return to full ambulation at about 6 weeks postoperative which is the best method to avoid equinus contracture.

Compared to TMA, Lisfranc amputation is a less desirable level of amputation with regard to weight bearing function, resistance to recurrent wounds, and longevity of the stump (Fig. 19.12) [3, 4]. Lisfranc amputation is generally indicated for midfoot wounds with associated osteomyelitis in the proximal metatarsals, extensive forefoot gangrene, frostbite, and mangling injuries of the forefoot (Fig. 19.13). Extensive heterotopic ossification (HO) from prior metatarsal resection may also warrant Lisfranc disarticulation in an attempt to limit HO formation after revision surgery which is discussed in Chap. 17. Caution should be taken before proceeding with this procedure because the native biomechanics of the foot are greatly altered which may need to be addressed at the time of surgery. Disarticulation of the first metatarsal from the medial cuneiform joint may result in complete loss of tibialis anterior (TA) and peroneus longus (PL) tendon insertions depending on the need to remodel the plantar aspect of the medial cuneiform. Disarticulation of the fifth metatarsal from the cuboid results in complete loss of the peroneus brevis (PB) tendon insertion. Loss of TA function leads to foot drop and equinus contracture, while loss of PL and PB function leads to unopposed inversion through the tibialis posterior tendon. An attempt should be made to preserve or transfer these tendons at the time of amputation when possible.