Below-Knee Amputation (Transtibial Amputation - BKA)

Indications

1. Vascular / Ischemic (Most Common)

• Chronic limb-threatening ischemia (CLTI) - arterial occlusive disease causing rest pain or tissue loss with no viable revascularization option; chronic ischemia and infection remain the most common indications for amputation among hospital inpatients
• Peripheral arterial disease (PAD) with inadequate arterial flow into the foot and no revascularization options
• Acute limb ischemia that is irreversible (profound sensory/motor loss, inaudible Doppler signals) - immediate amputation may be required

2. Infectious

• Major infections of the foot (necrotizing fasciitis, wet gangrene, severe diabetic foot infection)
• Osteomyelitis not amenable to conservative measures
• Complications of diabetes mellitus - diabetic foot complications constitute a major proportion of BKA indications

3. Trauma

• Mangled lower extremity with non-reconstructable injury (evaluated using LEAP - Lower Extremity Assessment Project criteria)
• Devascularized limb with non-viable tissue

4. Oncological

• Soft tissue sarcoma or bone tumor with direct and extensive involvement of major neurovascular structures, or tumor size encompassing too much of the limb to allow a functional remnant (now rare - only 5-10% of sarcomas require amputation)
• Advanced melanoma in extreme cases (intractable pain, sepsis source control)

5. Other

• Wet or dry gangrene not reversible with revascularization
• Severe non-reconstructable trauma with tissue loss
• Failed limb salvage attempts

Contraindications to BKA (Favoring AKA Instead)

• Fixed knee flexion contracture - a BKA is not an option if the patient has this
• Insufficient viable tissue in the anterior or posterior lower leg for flap coverage
• Inadequate arterial perfusion - ankle-brachial index (ABI) > 0.5 or angiographic evidence of patent iliac flow to a patent profunda artery (even with occluded superficial femoral artery) is sufficient for healing; below this threshold, BKA healing is unlikely

Why BKA is Preferred Over AKA

• Retention of the knee joint results in far better prosthetic function
• Ambulation rates with or without assistance are seen in up to 65% of BKA patients
• Energy cost of walking is significantly lower than with an above-knee amputation
• 30-day mortality for BKA is ~8.9% vs. ~27.7% for above-knee amputation in PAD patients

Surgical Technique (BKA)

Anesthesia

Staged vs. One-Stage Approach

• Septic/grossly infected foot: a two-stage approach is preferred - first a guillotine amputation as distal as possible to healthy tissue (wound left open), then formal BKA once infection resolves (typically within a week)
• Clean cases: single-stage BKA

Level of Bone Transection

• Tibia divided 10-15 cm distal to the tibial tuberosity (approximately the level of greatest calf circumference)
• The optimum bone length is at least 12 cm below the knee joint
• The fibula is transected 2 cm proximal to the level of tibial transection

Flap Technique

• The anterior incision length = two-thirds of the circumference of the calf at the level of tibial transection, placed 1 cm distal to the planned bone cut
• The longitudinal medial and lateral limbs = one-third of the circumference each
• The posterior flap length = one-third of the circumference; err on the side of making it too long and trim at closure

Step-by-Step Procedure

1. Skin incision down to fascia, with ligation of the greater and lesser saphenous veins
2. Anterior compartment: deepen anteriorly through periosteum; divide anterior compartment muscles at the skin incision level; identify and suture-ligate the anterior tibial neurovascular bundle
3. Bone cuts:
   • Tibia cleared circumferentially of periosteum and transected; the anterior edge of the tibia is beveled at 45 degrees to prevent pressure necrosis under the prosthesis
   • Fibula transected 2 cm proximal to the tibia with bone cutters
4. Posterior compartment: retract tibia anteriorly; identify and suture-ligate the posterior tibial and peroneal neurovascular bundles; divide posterior musculature with an amputation knife or electrocautery (caution to protect the posterior flap skin)
5. Nerve handling: the posterior tibial nerve is retracted distally, highly ligated, sharply divided, and allowed to retract (prevents painful neuroma at weight-bearing surface)
6. Hemostasis: manual compression, ligate remaining vessels; bone wax is NOT used
7. Flap rotation: posterior flap rotated anteriorly; debulk posterior musculature (gastrocnemius) as needed for tension-free closure
8. Closure: bone end covered and stabilized with deep posterior musculature (myodesis); fascia approximated with absorbable sutures; subcutaneous layer closed to minimize skin tension; skin closed with monofilament suture or staples; optional closed-system drain

Alternative Flap Techniques

• Equal anteroposterior (sagittal) flaps - equal-length medial and lateral myocutaneous flaps to cover the tibia
• Skew flap - equal anteromedial and posterolateral flaps
• These have lower primary healing rates than the long posterior flap

Special Variant: Ertl (Osteomyoplastic) Amputation

Postoperative Management

• Rigid dressing preferred in early postoperative period to control edema and support wound healing
• Early prosthetic fitting may begin 5-21 days after surgery if the residual limb can transfer load and patient has adequate physical reserve
• Dressings ideally left undisturbed for the first 4-5 days unless there is clinical concern

• Mulholland and Greenfield's Surgery, 7e - Chapter 92
• Sabiston Textbook of Surgery - Chapter 104
• Fischer's Mastery of Surgery, 8e - Chapter 245
• Miller's Review of Orthopaedics, 9e - Chapter 10
• Pye's Surgical Handicraft, 22e - Chapter 25

• Frame the case around the WIFI score (Wound-Ischemia-Foot Infection) - examiners love this classification
• Always justify why BKA was chosen over AKA (knee preservation, energy cost, ambulation rate)
• Address the two-stage vs. one-stage decision if the patient had active infection
• For complications, lead with stump failure rate (~15-25% in vascular) and phantom pain (50-80%)
• End with functional outcome - the 65% ambulation rate shows BKA is a reconstructive, not purely ablative, procedure