A below-knee amputation (“BKA”) is a transtibial amputation that involves removing the foot, ankle joint, and distal tibia and fibula with related soft tissue structures. In general, a BKA is preferred over an above-knee amputation (AKA), as the former has better rehabilitation and functional outcomes. The rates of lower extremity amputation have declined in recent years, but still, 3500 trauma-related amputations are performed in the United States each year. This surgical operation carries significant morbidity, yet it remains a treatment modality with vital clinical and often life-saving significance given appropriate indications.
There are 4 fascial compartments in the lower leg, containing muscles to the leg and foot and important neurovascular structures. While a BKA divides all compartments, a thorough grasp of the relevant anatomy is vital to controlling blood loss intraoperatively and preventing known complications.
The anterior tibial compartment lies anteromedial to the spine of the tibia and anterior to the fibula. Within the fascia lie the tibialis anterior, extensor hallucis longus, extensor digitorum longus, and peroneus tertius. Also in the anterior compartment are the deep peroneal nerve and the anterior tibial artery and vein. The anterior tibial artery is the main blood supply to the anterior compartment of the leg with reinforcement by the perforating branch of the peroneal artery. The lateral compartment lies posterior to the anterior compartment and directly lateral to the fibula. This contains the peroneus longus and brevis and the superficial branch of the peroneal nerve for much of its course. It derives the arterial supply from the branches of the peroneal artery. The posterior leg holds both the superficial and deep compartments, the superficial containing the soleus, gastrocnemius, and plantaris muscles. The deep, muscular compartment contains tibialis posterior and the great and common toe flexors. The tibial neurovascular structures lie within the deep compartment. The posterior tibial artery is the main blood supply of this compartment. It is very important to understand the vascular anatomy of the leg as skin flaps for amputation are planned according to the blood supply.
There are three major categories of indications for proceeding with a BKA. These include urgent cases where source control of necrotizing infections or hemorrhagic injuries outweighs limb preservation. These operations are performed when death is imminent, and may at times, necessitate bedside operation if there is insufficient time to reach the operative suite.
Less acutely, urgent BKAs may be performed for chronic nonhealing ulcers or significant infections with the risk of impending systemic infection or sepsis. These patients are typically sick with multiple significant comorbidities and a chronic progressive or waxing/waning course of illness. Definitive source control/debridement is critical, yet there is typically the time to medically optimize a patient over a few hours or days. Intravenous (IV) antibiotics are an important adjunct to operative treatment in these cases, limiting morbidity associated with systemic bacterial infection.
In addition, relatively urgent BKAs may be performed where limb salvage has failed to preserve a mangled lower extremity. Adequate resuscitation and stabilization must always have occurred before such a decision, as judged by vital signs, lactate, base deficit, and the management of concomitant injuries. In cases of full-thickness burns to a majority of an extremity, serious or complete neurovascular compromise, or irreparable soft tissue defects, definitive BKA may be appropriate.
Finally, elective BKAs are appropriate in the non-septic or imminently sick population with problems such as venous stasis ulceration, multiple distal to mid-foot amputations with persistent infection or vascular insufficiency, or lack of distal foot/ankle function with refractory pain.
The most significant contraindication to performing a non-urgent BKA is vascular insufficiency at the planned amputation site or poor wound healing ability such that the surgical wound will not heal. This contraindicates elective or semi-elective procedures until the patient can be optimized. These patients should undergo a thorough preoperative workup including measurement of pulse volume recordings in bilateral distal extremities to determine adequate vascular flow. Doppler may be used to assess for gross blood flow, and ankle brachial indices can evaluate an individual and lower versus upper extremities. Oxygen pressures in the toes and transcutaneous oxygen pressure are useful for determining oxygenation on a microvascular level. In cases of profound vascular insufficiency, bypass grafting or the placement of stents may be necessary before performing a BKA.
In patients who are in extremis due to sepsis, blood loss, acute major organ failure, or other cause, every attempt should be made to stabilize the patient before starting a major surgical procedure. The one exception to this is the case of uncontrolled, spreading necrotizing infection, where source control is often life-saving. Conversely, in cases of acute hemorrhage, local tourniquets may be applied for several hours while resuscitation takes place.
A standard orthopedic operative set is essential. Also valuable to this operation is a tourniquet, fluoroscopy, large amputation blade, oscillating bone saw or manual saw, drill and bit set for performing myodesis of muscle to bone ends, silk hand ties, rongeur, and a suction drain.
The patient should be prepped and draped supine, with a bump placed under the ipsilateral hip to internally rotate the operative extremity such that the knee and ankle are vertically oriented. A thigh tourniquet may be placed in a nonsterile fashion as high as possible above the knee to prep and expose as much of the extremity as possible, or a sterile tourniquet may be applied after the patient is draped.
To safely perform a BKA, a standard surgical team should be assembled including the operating surgeon, anesthesiologist, scrub tech, and circulator. Given the difficulty of managing and operating circumferentially on a lower extremity, a first surgical assist and often a second are exceedingly helpful if available.
Nutritional status, directly impacting the ability of the postoperative site to heal, should be ascertained, including measurement of prealbumin, albumin, glycosylated hemoglobin, and total lymphocyte count. The presence and proximal extent of any neuropathy can be determined via physical exam and monofilament testing, impacting the appropriate operative level. A physical exam is also important to determine soft tissue viability; in cases of severe trauma, the wound may need to demarcate for several days until it is clear at what level the limb is salvageable.
In infectious cases, osteomyelitis is most effectively evaluated with MRI. This will also show the extent of osteomyelitis and associated soft tissue fluid collections if present. Ultrasound may likewise evaluate localized collections. Inflammatory labs, including ESR and CRP, are important in determining presence, degree, and acuity of infection. It may see a mild-to-moderate elevation in cases of chronic non-healing ulcers, while grossly elevated markers show an acute or abruptly worsening process.
Imaging is equally essential. Radiographic films must be taken to include an anterior-posterior and lateral view of the extremity including the foot, ankle, tibia/fibula, and knee to assess for concomitant fracture, subcutaneous air, intact proximal bone, etc. Similarly, an MRI may determine the adequacy of soft tissues. For instance, if a large degloving injury is present proximally, but poorly visualized on physical exam or other imaging, this may affect the operative site for a BKA.
There are several ways to perform a BKA, one of the most significant differences being guillotine versus completed amputation. A guillotine amputation is performed quickly with the goal of controlling infection or blood loss, or when completing a near-total amputation of a mangled extremity at bedside. This can be effective when tissue planes must demarcate over hours or days, with serial debridements taking place before closure can be performed. In contrast, where time and tissue allow, a completed amputation involves all steps as outlined below, resulting in a closed, sutured stump ready for application of a stump shrinker and prosthesis planning. The following outlines several of the basic steps commonly used to perform an uncomplicated BKA.
Once the patient is prepped and draped, the tibial tubercle and joint line are marked, with the BKA incision marked distally, typically 10 to 15 cm from the tibial tubercle. An anterior skin flap is drawn to include the anterior two-thirds of the leg, while the posterior flap is drawn 150% longer than the anterior flap to allow ample soft tissue for closure. The tourniquet is inflated. A skin incision is made down to the fascia circumferentially. The fascia incised and the muscles carefully divided down to the tibia and fibula.
The tibial nerve and deep and superficial peroneal nerves are identified within their respective neurovascular bundles. Each nerve is injected with 1% lidocaine (optional), placed under gentle traction, and sharply divided with a fresh scalpel blade. This allows for retraction of the nerve, avoiding development of a painful neuroma distally at the BKA stump. Each major artery, including the anterior and posterior tibial arteries, are identified and ligated with a silk tie.
The tibial and fibular shafts are cut with an oscillating saw, and the corners beveled with a rongeur or saw. A small hole is placed with a drill in the distal tibial shaft; the gastrocnemius aponeurosis is secured to this via a nonabsorbable suture.
The tourniquet is released and adequate hemostasis obtained through a cautery or ligation of major bleeding vessels. A drain is placed in the wound, and the fascia is then approximated, followed by the subcutaneous tissue and finally the skin. This may be sutured or stapled based on surgeon preference.
The stump is dressed with a sterile dressing and placed into a well-padded splint or knee immobilizer. This will both protect healing soft tissue and prevent the development of early flexion contracture at the knee which would limit postoperative mobility with a prosthesis.
In the immediate postoperative setting, the limb stump should be serially examined every 24 to 48 hours for necrosis of the skin edges, bleeding, and signs of infection. Any drains should be removed once there is sufficiently minimal drainage according to surgeon preference. Once the wound is healing well, a stump shrinker may be placed, providing circumferential compression around the stump and distal extremity. A prosthetics company should be contacted, with a formal patient evaluation performed and the provisional prosthetic is chosen. Ultimately, there are many forms of prosthetics for lower limbs, and patient preference, condition, and insurance, among other factors, will dictate which prosthetic is the best long-term option.
As with all surgical procedures, there are possible acute complications of uncontrolled bleeding, infection, and acute postoperative pain, and broader medical complications including acute blood loss anemia and stress-induced cardiac ischemia. With a BKA performed for infection or acute soft tissue trauma, a second operation may be necessary if distal skin edges further demarcate or if the area of infection was not adequately resected.
Chronic complications of BKAs include the development of painful neuromas from transected nerves, highlighting the importance of proper intraoperative technique as described above. Phantom limb pain, or the perception of pain or troubling sensation in the missing limb, is a common complaint. They address this with a mirror box, local injections, adjustment to the prosthesis, or a variety of other modalities. The psychiatric and psychosomatic effects of a BKA should not be overlooked in postoperative patients as this cohort has been shown to have higher rates of depression and suicide.
The timely performance of a BKA can be both life-saving in severe illness or trauma, and restorative through improving function with an appropriate prosthesis. Many patients with severe non-healing foot ulcers, for instance, have difficulty ambulating and can regain function with the removal of the infected limb and fitting for a prosthesis. Similarly, patients with chronic pain from lower extremity trauma may undergo a BKA as a palliative or similarly functional measure, often with satisfactory results. It is important to note that, an individual's metabolic demands with ambulation will rise significantly after a BKA, although this depends in part on the postoperative maintenance of lower extremity muscle strength. Hence, for the frail or elderly patient, this is a procedure which must be undertaken in conjunction with nutritional guidance and an overall discussion of patient health and mobility. Additionally, the long-term clinical survival of BKA patients is notably poor in certain populations; patients with end-stage diabetes mellitus who receive a BKA for foot ulcers have been shown to have an average postoperative life expectancy around 3 years.
In the peri-surgical environment surrounding a BKA, close communication across all healthcare disciplines is paramount. The initial workup of ischemic and infectious limb-compromising diagnoses often begins in the emergency department or local clinic, where prompt assessment, triage, and workup is critical. The emergency physician must recognize which patients require emergent versus urgent versus planned surgical care. This is determined both via the clinical picture, including vital signs and exam, and through an assessment of infectious labs, CBC, BMP, lactic acid, base deficit, blood cultures, and radiographic imaging.
Once surgical services, typically orthopedics, general surgery, or vascular surgery, are consulted, preparation may be made for operative management of critical lower limb disease. At this point, the healthcare team includes the surgeon, the patient (who must provide informed consent for a BKA, either personally or via proxy), anesthesia providers in the operating room, operating room management and staff, and the bedside nurses either in the emergency department or on the floor. In all urgent cases, close communication between all team members is critical. For instance, a delay in an operating room being available due to inadequate anesthesia coverage can significantly affect a patient's outcome.
After BKA, floor nursing plays a significant role in managing pain, recording drain outputs, and informing the covering physicians of any change in vital signs or overall status. Surgical and hospitalist services should closely monitor the postoperative patient for the potential necessity of reoperation, of vascular insufficiency at the BKA site, or of systemic electrolyte disturbances, sepsis, or other medical problems requiring management. Discussion may now be initiated regarding prosthetic devices and a postoperative plan, starting with devices such as a stump shrinker, limb protector, and knee immobilizer.
Finally, attention should be given to the postoperative patient's mental status, including the potential need for psychiatric evaluation and care. Every postoperative patient should have an attentive primary healthcare provider with whom to follow up with closely after hospital discharge.