Transmetatarsal amputation

Indications

The indications and technique for performing a transmetatarsal amputation have changed very little since McKittrick and Warren initially described them in the 1940s and 1950s. They proposed three basic criteria, and I have added a fourth:

• Gangrene of one or more toes, without entering on to the foot

• Stabilized infection or open wound involving the distal portion of the foot

• An infected lesion in a neuropathic foot

• Moderate to severe forefoot deformity.

Careful preoperative preparation is necessary with drainage of infection, culture-directed antibiotics and daily wound care. Successful amputation requires attention to detail, careful planning of skin flaps, atraumatic operative technique and, whenever possible, primary closure of the wound. The use of an ankle or thigh tourniquet is desirable: however, this is at the discretion of the surgeon. Relative contraindications to the use of a tourniquet include ischaemia or recent lower extremity revascular-

Fig. 8.26 Transmetatarsal and mid- Image Not Available foot amputations. Three levels of amputation. From Sanders (1997) with permission from Elsevier Science.

ization. A bloodless field enables the surgeon to work more efficiently and saves operating time. However, prior to closing the wound, the tourniquet must be released and all bleeders ligated or coagulated. Meticulous haemostasis is required to prevent blood loss and haematoma formation. If ankle equinus is noted, it should be corrected at the same time as the transmetatarsal amputation.

Technique

The patient is placed in a supine position with the foot and lower half of the leg prepped and draped in the usual manner. Bony landmarks are identified for the 1 st and 5th metatarsal heads and bases. The desired level of bone resection is determined, e.g. mid-shaft level, and then using a skin marker a line is drawn across the dorsum of the foot from mid-shaft of the 1st metatarsal to mid-shaft of the 5th metatarsal. Lines are then extended distally, along the 1st and 5th metatarsal shafts, to the bases of the hallux and 5th toe, and then curved across the plantar skin just proximal to the sulcus of the toes. This approach will create a short dorsal and long plantar flap.

Starting anteromedially at the 1st metatarsal shaft, the knife is held perpendicular to the skin and an incision is made through the skin, across the dorsum of the foot, ending at the previously determined level on the 5th metatarsal shaft. The dorsal incision is deepened to expose the long extensor tendons. Vessels are identified, ligated or electrocoagulated. The incision is then carried down to bone. Prior to transection of the metatarsals, an osteotome or key elevator is directed distally away from the dorsal skin incision, to reflect the soft tissues and periosteum. The dorsal flap should not be undermined.

Incisions are then carried distally toward the toes and then across the plantar aspect of the foot, developing a long thick myocutaneous flap. The plantar flap is retracted using rake retractors. It is important to keep the dissection close to the metatarsal shafts, thereby creating a thick viable plantar flap. The plantar flap is reflected proximally to the intended level of bone resection. The metatarsal bones are then cut transversely, with a power saw, at the level of the dorsal skin flap. The cuts are angled slightiy from dorsal-distal to plantar-proximal. The 1st and 5th metatarsals are bevelled medially and laterally to prevent focal points of pressure. The distal foot is grasped securely with a small bone clamp, and then removed by sharp dissection.

The plantar flap is inspected, and debrided as necessary. Exposed flexor tendons should be grasped without tension and excised. Antibiotic solution is used to irrigate the wound copiously. The plantar flap is then brought up over the resected metatarsals and approximated with the dorsal flap. If the plantar flap is too long, redundant skin should be remodelled. The flap should be carefully marked with a skin marker and excess skin removed. Accurate trimming of the skin is accomplished by placing several Allis tissue forceps on the edge of the skin to be excised. The surgical assistant holds the forceps with gentle tension, and a fresh blade is used to trim the excess skin. Placing a malleable retractor beneath the plantar flap, while it rests on the dorsum of the foot, provides a firm supporting surface to cut on.

The tourniquet is deflated prior to wound closure, and bleeders are ligated or coagulated. Some oozing of blood from the transected bone marrow and from muscle is to be expected. The skin flaps are approximated without tension and secured with a few simple interrupted, subcutaneous 3-0 absorbable sutures. A TLS drain is placed in the wound, exiting the skin on the dorsolateral aspect of the foot. Skin flaps are carefully positioned and secured with 4-0 nylon sutures in a simple interrupted fashion, or with stainless steel staples. The technique for performing a transmetatarsal amputation is illustrated in Fig. 8.27a-f.

Dressings and postoperative care

Dressings consist of non-adherent fine mesh gauze (petrolatum, 3% Xeroform™ or Adaptic™), placed on the suture line, wide gauze sponges (4 x 8s, with a long dorsal and plantar tail, secured around the stump) and padding for protection of the heel and lateral border of the foot. Dressings are held in place by 1.2-m gauze bandage. A well-padded plaster splint is applied to immobilize the foot and ankle. Drains are generally left in place for 48 h. Normal sterile saline moist-to-dry dressings are applied when there is persistent drainage from the wound or

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