Despite efforts to preserve limb length for rehabilitation purposes, the ratio of above-knee to below-knee amputations is roughly 1.0 and has not changed in several decades. Preservation of the knee joint reduces the oxygen consumption required to ambulate from 60% above baseline levels to 40%. Although young trauma patients undergoing an above-knee amputation may eventually be able to walk, an elderly patient after transfemoral amputation rarely does so. An attempt at performing a below-knee amputation is warranted in almost any patient who appears to be a potential candidate for rehabilitation.
The morbidity and mortality for major leg amputation is substantial, primarily due to the systemic disease burden of the patient population.
Difficulty in accurately predicting healing has led to a discrepancy in reported healing rates—ranging from 30% to 90%—for below-knee amputations. One-third of patients having below-knee amputations require reamputation. Approximately half of the below-knee amputations that do not heal will require conversion to the above-knee level. Patients undergoing amputation for acute severe infections have wound infections or breakdown of the amputation stump in up to two-thirds of cases. These patients should have primary guillotine amputation as a preliminary step to clear the infectious process before a definitive below-knee stump is created.
Absolute indications for primary above-knee major limb amputation include contracture at the knee joint (observed in debilitated patients with long-standing extremity pain who have been in a prolonged withdrawal posture with the knee flexed) and nonviable calf muscle or skin for creation of the below-knee flap. Primary above-knee amputation is relatively contraindicated in patients who are not candidates for rehabilitation. The frequent failure of healing of below-knee amputation, the higher perioperative morbidity and mortality in this population (making secondary operations more dangerous), and the modest functional benefit of preserving the knee joint in a nonambulatory patient are the major arguments in favor of primary above-knee procedures in such cases.
Amputations can be performed using proximal tourniquets. This offers the advantage of minimizing blood loss, particularly in an infected hyperemic limb. Regional anesthesia is frequently used, but there is scant evidence that this decreases perioperative mortality compared with general anesthesia. Blood transfusions are rarely required since the potential for significant hemorrhage is modest.
Guillotine amputations are simple circular incisions usually just above the ankle (which can be more proximal if necessary) that are carried through sharply to divide all structures at that level (hence the name). Absolute indications for guillotine amputation include severe foot infections that preclude limb salvage. Necrotizing diabetic foot infections with or without gross purulence are the most frequent indication. Removing the grossly infected foot allows the lymphatics to be cleared of infection and reduces the risk of below-knee stump infection. Most surgeons do not perform guillotine amputations prior to an above-knee amputation because the distance from the infection source is felt to be relatively protective. The calf muscles should be compressed to reveal purulence that may have tracked proximally. The major tibial vessels are oversewn at the end of the stump. Controlling the tibial vasculature can be difficult, particularly in young diabetic patients with normal circulation. The wound is then examined in 48 hours to determine if the infectious process has been controlled. If that is so, a formal below-knee amputation is then performed.
The most common procedure for below-knee amputation is the Burgess technique, which utilizes a long posterior flap.
The blood supply to a posterior flap is generally better than the supply to an anterior flap or to sagittal flaps, because the sural arteries (which supply the gastrocnemius and soleus muscles) arise high on the popliteal artery, an area not often diseased, whereas the more distal popliteal artery or tibial arteries are often diseased, especially in diabetics.
The use of rigid dressings and immediate postoperative prostheses has proved advantageous. Application of a rigid cast bandage has several potential advantages:
It controls postoperative edema, which may reduce pain;
it protects the stump from trauma, particularly when a patient falls during attempts at mobilization;
it allows the patient to be ambulatory with a temporary prosthesis much sooner.
The disadvantages include cost and personnel time to apply the rigid bandages and lack of patient compliance. A single randomized trial in vascular patients did not demonstrate a difference in wound complications with rigid dressings compared with standard bandages.
Use of immediate postoperative prostheses provides two advantages:
a rigid dressing
The rigid dressing controls edema, improves healing, prevents joint contractures, and protects from trauma. Early ambulation decreases hospital stay, increases rates of rehabilitation, decreases complications of prolonged bed rest (eg, decubitus ulcers, pneumonia, pulmonary emboli), and improves the patient’s psychologic outlook.
The skin flaps
The anterior incision is made approximately 10 cm below the tibial tuberosity and carried to the midpoint of the leg both medially and laterally. After the muscles of the anterior compartment have been transected, the fibula and tibia are divided, the tibia is beveled to avoid a sharp projection beneath the skin. The posterior flap is wedge-shaped and contains soleus muscle, gastrocnemius muscle, and skin; it is fashioned to avoid tension when the wound is closed and to provide a generous pad over the distal residual limb. Drains are generally not required for amputations performed for vascular disease, because bleeding is minimal, but they are often necessary for amputations performed for trauma or tumor.
Above-knee amputation may be performed at several levels, including knee disarticulation; the patient is left without a functional knee. Knee disarticulation amputation is technically more demanding than transfemoral amputation at a higher level. When fashioning an above-knee stump, one should preserve as long a lever arm as possible; amputation in the lower thigh is preferable to amputation in the mid or upper thigh.
The technique is straightforward. Short anterior and posterior flaps, sagittal flaps, or a circular incision may be used. The bone is divided slightly higher than the skin and soft tissue to avoid tension when the wound is closed. A simple dressing is then applied.