Amputation is one of the oldest surgical procedures known to humans. The earliest forms of below-knee amputation (BKA) occurred on or near battlefields, where wounded warriors had few alternatives other than to face the surgeon’s knife. Amputations for lower extremity trauma are still performed when the leg is so mangled that salvage is precluded. However, the most common indications for limb amputation today is not trauma, but rather critical limb ischemia (CLI) and diabetes with infectious gangrene.

In the United States, between 1988 and 1996, an average of 133,735 hospital discharges/year were for amputations, and the predominant cause was vascular disease in 82% (Figure 1). The rate of amputations occurring in dysvascular patients increased from 38.3/100,000 people in 1988 to 46.19/100,000 people in 1996.

With the numerous techniques available to the vascular surgeon to accomplish limb salvage, it is surprising that any patient would ever eventually require a major lower extremity amputation for irreversible lower extremity ischemia. However, some patients with chronic lower extremity ischemia simply do not offer a suitable anatomic alternative for limb-salvage procedures, either open or endovascular. Furthermore, some patients who come to the hospital with acute lower extremity ischemia, despite aggressive mechanical thrombectomy and/or intraarterial thrombolysis, sustain irreversible tissue loss. After repeated failures of infrainguinal arterial reconstructions for limb salvage, a BKA may be the only reasonable alternative left.

Determining Amputation Level

It is very important to have some idea of the healing potential of the skin at the level of amputation that is selected before surgery. Because the ability of an amputation stump to heal depends on skin blood flow, if the skin at a particular level will heal, then an amputation at that level will also heal.

In general, a longer lever arm offers an improved mechanical advantage that will be of benefit to the patient during rehabilitation. Therefore, it requires significantly more energy to ambulate with an above-knee prosthesis (50%–70%) than it does with a below-knee prosthesis (10%–40%). This fact can help to explain the lower percentage of prosthetic use in above-knee (10%–30%) amputees compared to below-knee amputees (50%–100%). However, if a patient is unable to walk before the amputation or has another disability apart from a leg with infection and/or critical ischemia, such as a neuromuscular disorder, stroke, or hip osteoarthritis, then amputation at the transfemoral (above-knee) level should be considered. Before amputation, the longer the interval between a patient’s most recent independent ambulation and operation, the less likely rehabilitation will result in autonomous locomotion even after successful healing of the amputation stump.

Initially, a thorough physical examination with palpation of the pulses must be performed. If the femoral pulse is absent, a BKA stump is unlikely to heal primarily. O’Dwyer found that less than 25% of BKAs healed when a femoral pulse was not present. If the femoral pulse is not normal, an ipsilateral iliac endovascular intervention or even an open groin arterial reconstruction should be considered to ensure that the planned BKA will have enough perfusion to heal primarily. Special attention should be directed to the skin where the proposed skin incision is planned. The skin must be healthy, without signs of infection, breakdown, or ischemia.

Finally, both hip and knee joints should be examined for range of motion. Patients who have been at bed rest for some time develop joint stiffness and/or flexion contractures. These joint abnormalities are very serious and pose a significant threat to successful rehabilitation even if the amputation heals uneventfully. If amputation is planned electively, it is beneficial to begin physical therapy exercises well in advance of the intended procedure to improve knee and hip joint mobility and enhance upper body strength.

Unfortunately, no known objective test can perfectly predict BKA stump healing. However, some standard testing metrics deserve mention. Absolute pressure at the ankle of greater than 60 mm Hg as determined by Doppler insonation has predicted healing BKAs 50% to 90% of the time. Inability to compress calcified vessels with a blood pressure cuff, however, can be problematic and is one reason this objective test is not always reliable.

The transcutaneous partial pressure of oxygen (tcPO₂) diffusing through the skin can be determined directly over the proposed surgical incision site. A value of greater than 40 mm Hg correlates very well with primary amputation site healing, and values less than 20 mm Hg correlate well with failure to heal. The accuracy of transcutaneous oximetry has been reported to be between 87% and 100% in predicting amputation healing. The examination should be performed in a room with constant warm temperature, and the technician must avoid testing skin areas that are involved with edema and/or infection. These latter technical pitfalls can produce false-positive and false-negative results. Unfortunately, there is no absolute threshold of tcPO₂ that accurately predicts healing 100% of the time.

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