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, neck, shaft, and 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.

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.[6]

A weightbearing residuum is not the only advantage of TMA: Studies have shown that it is associated with a lower mortality than either AKA or BKA.[7, 8] In one study, TMA had a 30-day postoperative mortality of 3%,[9] whereas in another, BKA had a 30-day postoperative mortality of 6.3% and AKA had a 30-day postoperative mortality of 13.3%.[10]

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 days vs 20 days), fewer theater trips, and a longer time without toxicity (346 days vs 315 days).[11] 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.[12] 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.

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.[13] 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.

The choice of surgical instruments for transmetatarsal amputation (TMA) is left to the surgeon's discretion. Equipment that may be used includes the following:

• Scalpel with blades
• Dissection and cutting scissors
• Retractors and handheld clamps
• Needle holders, suture material (absorbable and nonabsorbable), and forceps (fine and toothed)
• Diathermy device
• Bone instruments (eg, saw, bone nibblers, osteotomes, mallet, and curettes)
• Irrigation

TMA is performed with general anesthesia or regional anesthesia, depending on the patient's level of anesthetic risk.

The patient is placed in a supine position on the operating table, with a tourniquet around the upper thigh. A considerable amount of wool should be placed under the tourniquet cuff, and a good amount of adhesive should be applied to keep the tourniquet in place.

Full sterile preparation is undertaken with antimicrobial wash and disinfectant. The ulcerated area is covered with a surgical glove. All necessary measures for protecting the supposed healthy tissue are taken before the surgical knife is applied to the skin.

The leg is elevated, and the blood is milked out of the limb and back up the vascular tree. The tourniquet cuff is inflated, usually to a pressure of 200-300 mm Hg.

Rehabilitation must be addressed. TMA results in the loss of some of the area normally used for weightbearing. Consequently, load distribution on the foot becomes unbalanced, and the remaining foot tissues are subjected to a higher-than-normal mechanical load. The healing foot is therefore under increased pressure, which is detrimental to skin healing, as well as to the biomechanics of motion. It should also be noted that neuropathic patients have a decreased level of stability and an increased incidence of falls.

Mechanical problems after TMA are due to the decrease in plantarflexion; thus, the patient typically experiences this instability when mobilizing. Decreased plantarflexion reduces the overall "support movement," which is defined as the sum of hip, knee, and ankle extensor movements and represents the total limb pattern involved in pushing away from the ground.[5]

The solutions to these mechanical problems are as follows:

• Regular sensory check for patients with diabetes, neuropathy, or both - Patients with reduced sensation need extensive education regarding care
• Use of orthotic devices - Temporary postoperative devices include plaster of Paris (if the foot is not weightbearing), walking splints or boots, definitive footwear (after the surgical wound has healed), custom-made footwear (eg, steel implants in the soles of shoes), and shortened shoes

The first step in a transmetatarsal amputation (TMA) is to make a curved fish-mouth incision just proximal to the infected tissue of the foot (see the image below). The incision runs from the midshaft of the fifth metatarsal laterally to the midshaft of the first metatarsal medially through a midplane axis. Alternatively, a dorsal incision is made transversely, with the residual plantar flap left longer to allow for closure. The residual flap should be slightly longer on the medial side than on the lateral side.

Fish-mouth incision on dorsal surface of foot.

The incision is started at the dorsal surface and extended downward to bone. As the incision is deepened, hemostasis is achieved by means of diathermy or vessel ligation and ties. The bony landmarks of the metatarsals are identified and presented by means of a periosteal elevator. The metatarsals are cut with a saw, and the bony ends are rounded with a bone nibbler (see the image below).

Metatarsal heads have been osteotomized.

Next, the plantar incision is made (see the image below). The assistant applies traction to allow tissue separation at the correct plane. A plantar flap is created. Plantar and dorsal tendons are stripped off their insertions and cut. Copious amounts of irrigant fluid are then used. Pulse lavage is sometimes used for more pressurized irrigation.

Top image shows fish-mouth incision on plantar surface; bottom image shows final appearance of stump.

Wound closure is the final step. The flaps are approximated with nonabsorbable sutures according to the clinical indication and the surgeon's preference for closure. Final hemostasis is achieved with ligation of superficial vessels.

Standard synthetic braided suture (eg, polyglactin) is used for closure of muscular and subcutaneous layers. Subcuticular skin closure with synthetic nonbraided suture (eg, poliglecaprone) results in low skin tension and good approximation. The ends may be bridged. Skin clips may be used for traumatic cases; this measure allows drainage of any underlying hematoma (if the need arises). A dressing is applied to the stump.

Boffeli and Waverly suggested that in cases where there is a substantial amount of compromised plantar tissue in association with neuropathic ulcers, forefoot gangrene, or infection, the plantar flap incision design can be modified so as to incorporate a medial or lateral plantar artery angiosome-based rotational flap.[14] A lateral plantar artery angiosome flap can be used to treat a plantar medial soft-tissue defect; a medial plantar artery angiosome flap can be used for a plantar lateral defect; and medial and lateral flaps can be combined to cover a central plantar wound defect.

Lumley et al reported good results with the use of free tissue transfers in 28 diabetic patients who had undergone TMAs that were not amenable to primary closure.[15]  They achieved a flap survival rate of 93% (26/28 flaps) and a limb salvage rate of 93% (25/27 limbs).

Holloway et al retrospectively reviewed 27 consecutive patients who underwent an open guillotine TMA without having a viable plantar flap, either  because of extensive tissue loss on initial presentation or because of secondary TMA flap necrosis.[16] After resolution of infection and debridement of nonviable tissue, negative pressure wound therapy (NPWT) was applied to the wound and continued until granulation tissue covered the exposed bone. The wound was then closed either by applying a split-thickness skin graft or by continuing NPWT.

After a TMA, it is important to perform a hemoglobin check with postoperative routine laboratory studies. Additional instructions include the following:

• Perform regular wound inspections
• Perform radiography, depending on clinical indications
• Perform an appropriate follow-up check when the patient is discharged, focusing on to stump and amputation viability

Rehabilitation will have to be addressed (see Monitoring & Follow-up). 

Although TMA has a lower mortality than higher-level amputations do, it is not without complications, including the possibility of further surgical intervention or even death. A study conducted by Sage et al reported that more than two fifths of patients who underwent TMA had complications.[17] Another study found that nearly one third of patients who underwent TMA had a further higher-level amputation at a later date.[5] The consensus is that proximal amputations have a greater probability of stump healing.

Although some studies have failed to show a statistically significant difference in healing rate between patients who have diabetes mellitus and those who do not, risk factors such as diabetes mellitus, ischemic heart disease, hypertension, and chronic renal disease are widely considered to increase the risk of delayed healing.

However, in a study of TMA outcomes that considered several independent variables (diabetes mellitus, hemoglobin A1c >8%, neuropathy, peripheral arterial disease, chronic kidney disease, active smoking status, previous surgery, and a clean margin metatarsal bone pathology specimen positive for osteomyelitis) as possible contributors to failure to heal, only neuropathy and a positive bone margin were found to be significant.[18]

Postoperative skin breakdown probably has multiple causes; however, studies have shown that most patients who have diabetes mellitus and a TMA experience sensory neuropathy. Hence, injury to the stump in a neuropathic patient could lead to skin loss or wound failure, thus eventually resulting in infection and the need for a higher-level amputation.

Heterotopic ossification is a fairly common sequela after partial foot amputation; perioperative pharmacologic or radiation-based prophylaxis to prevent this should be considered.[19, 20]