Understanding The Biomechanics Of The Transmetatarsal Amputation
The patient with diabetes who requires surgical intervention typically presents with one or more of the following clinical exam findings:
• a full-thickness wound and concomitant osteomyelitis suggested by a fairly reliable positive probe-to-bone test, radiographic evidence of osteolysis and cortical destruction, or with positive MRI and/or bone scintigraphy findings when radiographs are equivocal;
• gross tissue changes consistent with skin necrosis and critical limb ischemia with a superimposed infection (wet gangrene);
• dry gangrene with unrelenting ischemic forefoot pain;
• crepitus in the soft tissue and radiographs indicating subcutaneous emphysema (gas gangrene); or
• fulminant infection, abscess formation and extensive soft tissue necrosis.
When bone is involved, we maintain that surgical debridement of the infected bone and necrotic tissue supplemented with long-term antibiotic therapy provides the best results in terms of sustainable limb salvage rates. This is consistent with published results.11
In some instances, such as with a global forefoot infection when extensive tissue loss renders a non-salvageable forefoot or when vascular reconstruction cannot sustain adequate forefoot hemodynamics, a primary TMA becomes the surgery of choice. Surgeons also commonly perform the TMA as a secondary or tertiary procedure. Examples of this include performing a TMA after a local forefoot amputation (digit, metatarsal or ray) when infection has recurred; when insufficient perfusion for healing warrants a more proximal amputation; when ulcers develop after partial ray resections secondary to overload; or as a staged procedure following radical debridement of a diabetic foot infection.
Prior to performing the TMA, a comprehensive vascular evaluation including ultrasound Doppler is critical to assess perfusion to the six pedal angiosomes to ensure initial wound healing and the ability to eradicate infection, increasing the probability of a successful amputation.12-15 One should carefully plan the incision, keeping in mind the principles of angiosomes and ensuring viable skin availability needed for primary closure. Alternately, surgeons also need to consider the possible need for secondary means of closure, such as skin grafting, for postoperative management.
Ideally, one should preserve a long plantar musculocutaneous flap for bone coverage and to optimize healing. Surgeons should bevel the osteotomies and make them as distal as possible in a manner that respects the metatarsal parabola to preserve physiologic plantar pressure distribution during gait, and avoids overload and subsequent tissue breakdown. If one knows preoperatively that the extent of tissue necrosis or infection will prohibit primary closure, make the patient aware that he or she will require continued local wound care, or negative pressure wound therapy followed by eventual skin grafting or flap closure.