Knee Arthrodesis

• Incidence: 1-2% of failed TKA cases
• Increasing due to rising TKA revision rates
• Male predominance (PJI more common in males)
• Age: Variable (younger patients may prefer limb salvage)

Knee fusion was common before arthroplasty era. Modern indications are almost exclusively salvage after failed reconstruction. The procedure permanently sacrifices knee motion in exchange for a stable, pain-free limb.

• Bone stock: Often severely deficient after failed TKA/revisions

• Femoral canal: May be damaged, sclerotic, or have cement

• Tibial canal: Similar concerns; may have previous stem tracts

• Soft tissue: Often compromised, previous incisions, extensor mechanism damage

  Careful assessment of these anatomical factors is critical for successful fusion.

• Optimal position:

  • Flexion: 0-15° (10-15° preferred for sitting)
  • Valgus: 5-7° (matches mechanical axis)
  • Rotation: Neutral (0-5° external rotation acceptable)

• Limb shortening:

  • Average 3-5cm after fusion
  • Required for foot clearance (stiff knee cannot flex)
  • Compensated with shoe raise

• Energy expenditure:

  • Walking requires 50-100% more energy
  • Patients have significantly altered gait
  • Hip and ankle compensate for lost knee motion

• A long single-piece (through-knee) nail is a full-length nail passed from the piriformis fossa or greater trochanter down across the fused knee into the tibia (as in Puranen's trochanteric-entry series). It gives one continuous column but must violate the hip on insertion, cannot generate compression at the fusion interface, and cannot independently match a mismatched femoral and tibial canal diameter.
• A modular fusion nail is a two-component construct - a femoral segment and a tibial segment joined at a central coupling at the level of the old joint line. Each segment is sized to its own canal, inserted through the knee wound only, and the coupling is then engaged to actively compress the resected femoral and tibial surfaces together. The Wichita Fusion Nail is the archetypal example; other purpose-built systems use a Morse-taper or clamp coupling.

• Compression at the interface improves bone-to-bone contact - the single strongest predictor of union emphasised across the evidence base (Puranen; Conway).
• Independent sizing accommodates the widened, often mismatched canals left after implant and cement removal.
• The hip and ankle are spared - insertion is through the knee, so the nail does not have to transit the hip, and the construct does not create a knee-to-hip stress column.
• Easier exchange/revision than a single long nail if one segment fails.
• Trade-offs: the coupling is a potential mechanical weak point that can loosen, and a modular nail does not bridge a large segmental defect on its own - a gap of more than a few centimetres still needs bone graft, an allograft-prosthetic composite, or a segmental (endoprosthetic) fusion construct.

• Biological - inadequate bone contact or a residual gap, and a poor host (smoking, malnutrition, low vitamin D, diabetes, immunosuppression).
• Mechanical - insufficient stability or compression (a common reason compression external frames fail), or a broken nail signalling an established nonunion.

• Optimise the host - stop smoking, correct nutrition and vitamin D, and control diabetes before any revision.
• Improve mechanics and biology together - for a failed external-fixation fusion, convert to an intramedullary nail (higher union rate) or re-apply compression; for a failed nail, consider exchange nailing, added compression, or dual-plate augmentation, always combined with bone grafting (autogenous iliac crest, or structural allograft for a defect).
• Biological adjuncts - graft the interface circumferentially; a vascularised fibular graft is an option for a large segmental defect.
• Endpoint - when the limb cannot be salvaged (uncontrolled infection, repeated failure, or unreconstructable bone), above-knee amputation is the definitive alternative (see the amputation topics).

• IM nail: 85-95%
• External fixation: 65-75%
• Plate fixation: 75-85%

• Walking with aid: 75-80%
• Community ambulator: 60-70%
• Return to work (sedentary): 50-60%
• High satisfaction: 40-50%

• Bone stock quality
• Infection control
• Patient compliance
• Fixation method
• Comorbidities (diabetes, vascular disease)

• High-resource settings: MDT care (arthroplasty surgeon, infectious diseases, plastic surgery, microbiology); modular fusion nails and segmental endoprostheses available; microprocessor-controlled prosthetic knees make amputation a competitive functional alternative.
• Limited-resource settings: External fixation (compression frames, Ilizarov) remains a mainstay where implant cost or supply limits modular nails; longer treatment in-frame is accepted in exchange for low hardware cost and the ability to manage active infection.

• Document the permanent loss of knee motion, 3-5 cm shortening with shoe raise, altered high-energy gait, and only moderate satisfaction.
• Record that amputation and continued non-operative management were discussed as genuine alternatives, with realistic union and reinfection figures.