Transmetatarsal Amputation

Transmetatarsal amputation (TMA) is a relatively common operation that is performed to safeguard limb viability. ^[1] Originally used for trench foot, TMA now has widespread uses in both orthopedic and vascular surgery because it treats patients with infection of the forefoot, necrosis, gangrene, and diabetic neuropathy, who commonly develop ulcerations. Bernard and Heute first described TMA in 1855, but it was McKittrick et al in 1949 who used it as an alternative to higher amputations in patients with the above signs and symptoms. ^[2]

The aims of TMA are as follows:

• To remove nonviable tissue so that the process of healing can take place
• To maintain limb functionality by preserving the maximum amount of midfoot distal to the ankle joint; this implies maintaining maximum length distally, allowing a larger surface area for weightbearing and mobility

Candidates for TMA are chosen on the basis of limited irremediable tissue loss, typically occurring as a result of infection or ischemic changes in the foot. ^[3] The essential factor that must be taken into consideration is the individual patient's vascular sufficiency, which directly affects healing after amputation.

The clinical indications for TMA are as follows:

• Chronic forefoot ulceration
• Forefoot gangrene (multiple digits)
• Combination of the above (potentially complicated by diabetes mellitus ^[4] )
• Severe crushed forefoot (not salvageable)

Contraindications for TMA include the following:

• Tracking proximal infection, such as cellulitis
• Lymphangitis

Anatomy

The metatarsal bones are numbered 1 through 5, from medial to lateral. Each metatarsal has a head, a neck, a shaft, and a base. The metatarsal bones are roughly cylindrical in form. The body tapers gradually from the proximal to the distal end. They are curved in the long axis and present a concave plantar surface and a convex dorsal surface.

The base at the proximal end is wedge-shaped, articulating proximally with the tarsal bones and by its sides with the contiguous metatarsal bones; its dorsal and plantar surfaces are rough for the attachment of ligaments.

The head at the distal end presents a convex articular surface, oblong from above downward, and extending farther backward plantar than dorsal. Its sides are flattened, and on each is a depression, surmounted by a tubercle, for ligamentous attachment. Its plantar surface is grooved anteroposteriorly and marked on either side by an articular eminence continuous with the terminal articular surface.

For more information about the relevant anatomy, see Foot Bone Anatomy.

Statistics from the 1990s indicated that approximately 10,000 TMAs were performed in the United States, compared with 32,000 above-knee amputations (AKAs) and 22,000 below-knee amputations (BKAs). ^[5] TMA, when feasible, is the logical preference because it is the only amputation procedure that allows for potential weightbearing. However, it is more likely to require revision surgery than BKA is. ^[6]

Rehabilitation from more proximal amputations (eg, AKAs and BKAs) for peripheral vascular disease (PVD) is seldom a success. Only 5% of amputees mobilize outside the confines of their home with a prosthesis, and most of those who do will become wheelchair-dependent within 5 years. In theory, TMAs should yield better mobilization percentages. In a study of 4965 nursing-home residents who underwent amputation, patients who underwent BKA (n = 1596) or AKA (n = 2879) recovered more slowly than those who underwent TMA (n = 490) and did not return to baseline function by 6 months. ^[7]

A weightbearing residuum is not the only advantage of TMA: Studies have shown that this procedure is associated with a lower mortality than either AKA or BKA. ^{[8, 9]} In one study, TMA had a 30-day postoperative mortality of 3%, ^[10] whereas in another, BKA had a 30-day postoperative mortality of 6.3% and AKA had a 30-day postoperative mortality of 13.3%. ^[11]

In a study comparing digital amputation (n = 77) with TMA (n = 70) in 147 diabetic patients with gangrenous toes, Elsherif et al found that TMA offered better outcomes, with a lower reintervention rate (15.7% vs 29.9%), a shorter median hospital stay (17 d vs 20 d), fewer theater trips, and a longer time without toxicity (346 d vs 315 d). ^[12] However, the differences did not reach statistical significance.

Tan et al retrospectively evaluated outcomes after TMA for peripheral arterial disease (PAD) limb salvage in 147 Asian patients and undertook to identify risk factors associated with TMA failure. ^[13] They reported a success rate of 63% for PAD limb salvage with TMA and noted that diabetes was an independent predictor of TMA failure. Patients in whom TMA failed were found to be at increased risk for nosocomial infections and 30-day readmissions.

In a retrospective cohort study aimed at defining risk factors for TMA failure (defined as BKA or AKA) in 341 patients with diabetes, Ron et al noted that the frequency of renal impairment (defined as a high creatinine level or a previous kidney transplant or need for dialysis) was higher in patients with failed TMA than in those with successful TMA. ^[14] They found that a Charlson Comorbidity Index (CCI) threshold value of 7.5 was the optimal predictive value for TMA failure; 71.8% of patients with a CCI higher than 7.5 had a failed TMA.

Adams et al assessed 3-year mortality and morbidity in 375 patients who underwent nontraumatic TMA, examining variations in TMA complication rates according to sex, age, race, and comorbid conditions. ^[15] After a nontraumatic TMA, 136 (36.3%) patients died within 3 years, 138 (36.8%) required a more proximal limb amputation, and 83 (22.1%) healed without complications. Patients with nonpalpable pedal pulses were three times as likely to require a proximal limb amputation, almost twice as likely to die within 3 years, and more than twice as likely not to heal after the TMA. Patients with end-stage renal disease (ESRD) were three times as likely to die within 3 years.