Closed Below-Knee Amputation
Jonathan D. Gates
Indications/ContraindicationsIndications
Amputation of the injured or diseased body part has been recognized as one of the oldest surgical procedures. The concept has been reasonably consistent but the technique for the transtibial, or better known below-knee amputation (BKA), has evolved over the centuries. The indications roughly remain the same in an effort to remove the source of unremitting pain, uncontrolled infection, or otherwise incurable injuries or tumors. As with many other surgical procedures, the advent of proper anesthesia has converted the procedure from one of rapid, often violent surgical expression to one of a more carefully conducted and suitably fashioned procedure to achieve limb length and wound coverage to provide the best patient outcomes. A carefully planned and meticulously performed amputation may serve as a life-saving maneuver and yet minimize the disability that follows, affording the patient a useful stump that maximizes mobility and dexterity.
No matter the level of amputation several general principles apply. Adequate blood supply is paramount and gentle handling of soft tissues during the procedure will minimize tissue damage and ischemia resulting in less residual necrotic tissue. In general, the more limb length the better in terms of rehabilitation potential. Removal of all nonviable or infected tissue is a required minimum, and supple, well-perfused myocutaneous skin flaps with adequate bone coverage is mandatory.
It is quite clear that the BKA requires an increased energy expenditure of from 10% to 40% above that required for intact ambulation. The increased energy expenditure for an above-knee amputee is upward of 50% to 70% more above baseline. The intact knee of the BKA allows for easier use of a prosthesis with up to 50% to 100% of unilateral below-knee amputees wearing a prosthesis. This is in sharp contrast to the above-knee amputee who has been reported to wear a prosthesis only 10% to 30% of the time. The number of patients who actually ambulate with the fitted prosthesis in each category is even lower. As one would expect, the patients with BKA have been shown to perform significantly better in standardized walking tests than those with an above-knee amputation.
Contraindications
Naturally, it goes without saying that if alternative methods to deal with the clinical problem exist and they do not put the patient’s life at risk then the thoughtful surgeon will choose less disfiguring operations. Interestingly enough, limb salvage following severe lower extremity trauma has become the standard practice in trauma centers across the world. In the civilian literature the 2-year outcomes comparing patients who underwent amputation to those who underwent reconstruction of their severe lower extremity injury had similar functional outcomes. In the more recent data from the military database, patients who had an amputation actually scored better in the Short Musculoskeletal Function Assessment (SMFA) questionnaire than did those who underwent lower extremity reconstruction. The SMFA is an accepted self-reporting measure of overall function.
Ischemia above the site of the proposed amputation level renders the stump ischemic and requires reamputation. Deep necrosis of the lower leg muscle groups that are so important for bone coverage and stump perfusion with successful closure might dictate a higher amputation level than the BKA.
An ipsilateral fixed knee contracture is considered a contraindication for a BKA as the angled stump would be permanently directed downward into the bed and damaged from pressure necrosis. Some surgeons feel that an ipsilateral hip contracture may also be a contraindication to a BKA as it would prohibit use of a prosthesis. Caution is advised against such a blanket statement in that some patients have such pain from an ischemic limb that the tendency is to contract the leg at both the hip and the knee. Following removal of the diseased lower leg with relief of the pain, the hip contracture may slowly release. A painful arthritic knee joint may also make it difficult for proper function of the prosthetic in the BKA.
Extensive infection or infected necrosis up to and at the site of proposed amputation level is a contraindication to the closed BKA. It is possible that this could be managed in specific cases with a two-staged procedure, the first of which is designed to remove the source of the infection and necrosis through a drainage procedure and the second being the definitive closed BKA.
Extensive edema of the lower leg especially proximal to the planned level of amputation should be considered a contraindication to BKA, and might warrant a guillotine amputation if the edema stems from uncontrolled infection in the foot. If this not done as an initial procedure, then the skin as well as deep muscles and fascia are firm, woody and noncompliant making the flaps difficult to maneuver into place and close without tension.
Preoperative PlanningThe goals of a BKA are to remove all necrotic, infected, painful, damaged or cancerous tissue, to create a wound that heals without issue, and to provide a stump that favors prosthetic fitting that will maximize the chance of future ambulation. The challenge then becomes to achieve this in the setting of a patient who often has significant comorbidities. By definition, the dysvascular patient requiring a BKA will have a degree of coronary disease. The operative surgeon often will follow the American Heart Association guidelines for preoperative optimization of the vascular patient. The patient with chronic peripheral vascular disease and no viable options for reconstruction may be nonambulatory, making it difficult to determine their physical capability or rule out active coronary syndromes. Nonetheless, evidence for congestive heart failure, acute or recent myocardial infarction, and unstable angina elevate the risk of any surgical procedure. Preoperative optimization with the help of the vascular medicine specialist or cardiology will help manage the use of antiplatelet agents, statins, antihypertensives, and beta blockade.
It is important that the surgeon stress to the patient that all options have been exhausted and the amputation is the next definitive step in the direction of rehabilitation. If time permits, a preoperative discussion with a physiatrist and even a current amputee serve to provide a positive outlook for the patient.
The choice of amputation level takes into consideration many factors. However, often the level is determined by the adequacy of blood flow at the tissue level that favors healing. Palpable pulses at the popliteal level will almost guarantee healing of the BKA in the absence of any infection or suture line tension. The converse is not true in that the absent palpable pulse at the popliteal is not necessarily a guarantee for failure.
The noninvasive tests of ankle-brachial indices (ABIs) and pulse volume recordings (PVRs) have been explored as a method to help decide proper amputation level. An absolute ankle pressure of 60 mm Hg has been shown to be a reliable measure of healing of the BKA in the wide range of 50% to 90% of individuals. The variability and less than optimal results are thought to be secondary to the incidence of noncompressible vessels in the dysvascular patient. The more invasive angiographic evaluation has been examined as a predictor of healing but unfortunately angiographic scores do not correlate well with healing of amputations.
There are a variety of radioisotope scans, scintigraphy, and skin perfusion pressure tests that are available but have had variable results in terms of guaranteed healing of an amputation. They are limited by the experience of the observer and the availability of the equipment, hence have not enjoyed widespread clinical acceptance.
Transcutaneous oxygen measurements are more readily available and completely noninvasive. The sensor is placed on the region of interest and the transcutaneous partial pressure of oxygen (tcPO2) is recorded in mm Hg while the patient is breathing room air and in the supine position. The exact number for a tcPO2 to confirm healing of a planned amputation at the desired level has been reported over a wide span of measurements from 16 mm Hg to 40 mm Hg.
The combination of a good physical examination with attention to quality of pulses, evaluation of skin rubor, ankle perfusion pressures, and a tcPO2 measurement of at least 30 to 40 mm Hg may be helpful in the armamentarium to determine the most appropriate amputation level.
SurgeryPositioning
The choice of anesthesia may include general, epidural, or spinal anesthesia. The patient is positioned in the supine position; antibiotics and deep venous thrombosis prophylaxis are administered well before the incision is made.
Many of the patients who undergo BKA have open wounds. It is critical to learn to combat infection at the surgical site in every way available. An infection in the surgical wound of the BKA may be sufficient for the patient to end up with a reamputation at a higher level. This starts well before the incision is made and requires an appropriate choice of broadspectrum antibiotics to treat any active infection and surgical drainage to drain and control foot sepsis. Preoperative skin cleansing the night before with chlorhexidine begins to establish the barrier to bacterial infection.
On the day of operation, all wounds on the extremity of interest are completely covered with dry dressings and sealed with clear or iodine impregnated drapes. The actual operative skin preparation therefore does not include the preparation of this bacteria-laden region with cross contamination of the site of the intended amputation. The foot is sealed in a Lahey or intestinal bag and tied tightly to prevent movement once the bag covers the necrotic or infected areas. The skin preparation and the draping are similar to a circumferential preparation for any lower limb revascularization so as to allow for complete freedom of movement of the limb to visualize the entirety of the incision.
No tourniquet is used in the dysvascular patient so as to avoid any further damage to an already tenuous blood supply. This may be even more important to avoid in the presence of a previously placed above-knee bypass graft that may help to maximize healing of the BKA. The use of a proximal tourniquet may be more appropriate for
the traumatic amputation as the blood supply proximal to the amputation site is often adequate.
the traumatic amputation as the blood supply proximal to the amputation site is often adequate.
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