Below-Knee Amputation

Anatomy and Physiology

Four fascial compartments in the lower leg contain muscles to the leg and foot and critical neurovascular structures. While a BKA divides all compartments, a thorough grasp of the relevant anatomy is vital in controlling blood loss intraoperatively and preventing known complications.

The anterior tibial compartment lies anterolateral 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 the 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 essential to understand the vascular anatomy of the leg as skin flaps for amputation are planned according to the blood supply.

The popliteal artery is the continuation of the superficial femoral artery and is the blood supply below the knee. It begins in the popliteal fossa, inferior to the popliteus muscle. The popliteal artery branches are the anterior tibial artery, posterior tibial artery, sural artery, medial superior genicular artery, lateral superior genicular artery, middle genicular artery, lateral inferior genicular artery, and medial inferior genicular artery. The arterial supply to the tibia is multifaceted. Branches of the anterior tibial artery supply the proximal metaphysis and epiphysis from the periphery via periosteal branches. The nutrient artery supplies the diaphysis. This artery penetrates the tibia posteriorly, distal to the soleal line near the center of the tibia, and sends branches toward the proximal and distal ends of the diaphysis.[9]

The penetrating branches of the posterior tibial artery supply the distal metaphysis and epiphysis from the periphery.[10] 

The arterial supply of the proximal epiphysis and metaphysis of the fibula is through branches of the anterior tibial artery and, more distally, by the fibular artery. This blood supply is noteworthy as graft reconstruction surgery of the mandible often uses the proximal fibula. Venous drainage of the tibia is via the anterior and posterior tibial veins, and fibula drainage is via the fibular vein. These veins drain into the popliteal vein.

The lymphatic drainage of the tibia and fibula is to the superficial and deep inguinal lymph nodes.[11]

Branches from the tibial nerve supply the knee joint and provide innervation to the proximal tibia. Distally, branches from nerves supplying the overlying muscle innervate the tibia below. These are branches of the deep femoral nerve and the tibial nerve. The sciatic nerve divides proximal to the popliteal fossa into the common peroneal (fibular) and tibial nerve—the common peroneal nerve winds around the fibular neck. The peroneal nerve innervates the posterior lateral lower leg. It allows for eversion and dorsiflexion. The superficial peroneal nerve is a cutaneous branch of the peroneal nerve and is responsible for sensation in the upper two-thirds of the posterior lateral leg.[12]

The superficial peroneal nerve branches terminate at the deep crural fascia, dividing into the medial and intermediate dorsal cutaneous nerves. These nerves supply the dorsal surface of the foot, except for the dorsal webbed space between the first and second toe.[13]

The deep peroneal nerve innervates the first- and second-toe webbed space. The tibial nerve is responsible for inversion and plantar flexion. The sural nerve is a cutaneous branch of the tibial nerve and provides sensory for the anteromedial lower leg. Cutaneous branches of the tibial nerve provide sensation to most of the plantar surface of the foot.[14]

Muscles demonstrating origin/insertion footprints on the tibia include:

• Tensor fasciae latae inserts on the lateral (Gerdy) tubercle of the tibia.[14]
• Quadriceps femoris inserts anteriorly on the tibial tuberosity.[14]
• Sartorius, gracilis, and semitendinosus insert anteromedially on the pes anserinus.[15]
• Horizontal head of the semimembranosus muscle inserts on the medial condyle.[16]
• Popliteus inserts on the soleal line of the posterior tibia.[17]
• Tibialis anterior originates at the upper two-thirds of the lateral tibia.[18]
• Extensor digitorum longus originates at the lateral condyle of the tibia.[19]
• The soleus and flexor digitorum longus originate at the posterior aspect of the tibia on the soleal line.[19]

• Muscles associated with the fibula:

  • The biceps femoris tendon inserts on the fibular head.
  • The fibularis longus and fibularis brevis tendons insert into the lateral fibula.
  • The extensor digitorum longus and extensor hallucis longus tendons insert on the medial fibula.
  • The fibularis tertius (FT) is a small muscle in the anterior compartment of the leg that inserts on the anterior surface of the distal fibula.[20]

Indications

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 sometimes necessitate bedside operation if there is insufficient time to reach the operative suite.

Definitive source control with debridement is critical if there is time to optimize a patient. Intravenous (IV) antibiotics are an important adjunct to operative treatment in these cases, limiting morbidity associated with a systemic bacterial infection.[7] Once the source of control has been optimized, evaluation for possible revascularization may be necessary to maintain the best outcome for healing and limb salvage.

Urgent BKAs may be performed where limb salvage has failed to preserve a mangled lower extremity.[15] 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.[16]

Finally, elective BKAs may be appropriate in non-septic patients suffering from extensive nonhealing tissue loss, multiple distal to mid-foot amputations with persistent infection, unreconstructable vascular insufficiency with a nonhealing ulcer, or lack of distal foot/ankle function with refractory pain.[17]

Contraindications

Vascular insufficiency at the planned amputation site is the most significant contraindication to performing a non-urgent BKA. This contraindicates elective or semi-elective procedures until the condition 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 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. Some researchers used indocyanine green near-infrared (ICG NIR) fluorescence imaging to predict postoperative skin flap necrosis.[18][19]

In patients in extremis due to sepsis, blood loss, acute major organ failure, or other causes, 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 the source control is often life-saving. Conversely, in cases of acute hemorrhage, local tourniquets may be applied for several hours while resuscitation occurs.

Equipment

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.

Personnel

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.

Preparation

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 severe trauma cases, the wound may need to demarcate for several days until it is clear at what level the limb is salvageable.[20][21]

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 the presence, degree, and acuteness 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.[22][23]

Imaging is equally essential. Radiographic films must 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 sizeable degloving injury is present proximally but poorly visualized on physical exam or other imaging, this may affect the decision for a BKA.

Technique or Treatment

There are several ways to perform a BKA, one of the most significant differences being guillotine versus completed amputation. Moreover, several designs for skin flaps have been introduced to cover the amputation stump. A long posterior skin flap and unequal (skewed) anterior and posterior muscle and skin (myocutaneous) flaps have been widely used.[24]The latter technique has been primarily introduced by Ernest M. Burgess. The Burgess technique was later modified by Lutz Brückner to address the specific limitations of occlusive arterial disease.[25] 

Guillotine amputation is performed quickly to control infection or blood loss or when completing a near-total amputation of a mangled extremity at the 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 outlined below, resulting in a closed, sutured stump ready to apply a stump shrinker and prosthesis planning.[26] The following outlines several basic steps commonly used to perform an uncomplicated BKA.[24]

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 is incised, and the muscles are carefully divided into the tibia and fibula.

The tibial 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 nerve retraction, avoiding the development of a painful neuroma distally at the BKA stump. Each major artery, including the anterior and posterior tibial, is identified and ligated with a silk tie.

The tibial and fibular shafts are cut with an oscillating saw, and the corners are beveled with a rongeur or saw. A small hole is placed with a drill in the distal tibial shaft; the gastrocnemius aponeurosis is secured via a nonabsorbable suture.

In the "Ertl" technique, a subperiosteal exposure of the bony portion of the tibia and fibula should be performed, with the canal opened to ensure continuity of the medullary canal. The periosteal sleeve with chips of attached cortical bones should be fashioned to enclose the distal bone ends and filled with bone graft. Synostosis is created between the distal tibia and fibula to create a solid weight-bearing surface for improved prosthetic function. The tibiofibular bone bridge provides a degree of weight bearing.[27][28]

The tourniquet is released, and adequate hemostasis is obtained through a cautery or ligation of major bleeding vessels. A drain is placed in the wound, and the fascia is 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 protect healing soft tissue and prevent the development of early flexion contracture at the knee, limiting 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, and the provisional prosthetic should be 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.

Complications

As with all surgical procedures, there are possible acute complications of uncontrolled bleeding, infection, 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.[29]

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 various 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.[30]

Clinical Significance

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. For instance, many patients with severe non-healing foot ulcers have difficulty ambulating and can regain function by removing 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 partly on the postoperative maintenance of lower extremity muscle strength.[31]

Therefore, for frail or elderly patients, this procedure 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 specific populations; patients with end-stage diabetes mellitus who receive a BKA for foot ulcers have been shown to have an average postoperative life expectancy of around three years.[32]

Enhancing Healthcare Team Outcomes

In the peri-surgical environment surrounding a BKA, close communication across all healthcare disciplines forming the interprofessional team 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 are critical. The emergency clinician must recognize which patients require emergent versus urgent versus planned surgical care. This is determined 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 the 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 is 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, vascular insufficiency at the BKA site, 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.[33] 

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 to follow up closely after hospital discharge.[30] Interprofessional care coordination before, during, and after these procedures will result in better patient outcomes. [Level 5]