Patellofemoral Complications in TKA

Patellofemoral complications are among the most common sources of patient dissatisfaction and revision surgery after total knee arthroplasty (TKA). The patellofemoral joint is the most technically demanding aspect of TKA — achieving correct patellar tracking, avoiding component malrotation, and managing the decision to resurface or not resurface the patella all contribute to patellofemoral outcomes. Complications include patellar maltracking and dislocation, patellar fracture, component loosening, avascular necrosis of the patella, clunk syndrome, and persistent anterior knee pain. Recognition of the specific complication and its underlying mechanical cause is essential for successful management.

• Tibio-femoral rotational alignment and the patellofemoral joint: the single most important technical factor affecting patellar tracking is the rotational alignment of the femoral and tibial components; internal rotation of the femoral component (placing the femoral component internally rotated relative to the surgical transepicondylar axis — TEA) shifts the trochlear groove medially, increasing the effective Q angle and causing lateral patellar subluxation; internal rotation of the tibial component shifts the tibial tubercle laterally relative to the trochlear groove, also increasing lateral patellar tracking forces; combined internal rotation of both components is the most dangerous scenario — producing severe lateral patellar maltracking and potentially frank dislocation; femoral component rotation is measured on axial CT relative to the TEA (correct = parallel to the TEA); tibial component rotation is measured relative to the tibial AP axis (anterior tibial tubercle to the posterior cruciate ligament insertion)
• The tibial tubercle-to-trochlear groove (TT-TG) distance: the lateral offset of the tibial tubercle from the trochlear groove; normal <20 mm; >20 mm is associated with lateral patellar maltracking and instability; in TKA, the effective TT-TG is determined by the positions of the tibial component (which determines where the tibial tubercle lies relative to the tibial baseplate centre) and the femoral component (which determines where the trochlear groove lies); component internal rotation increases the effective TT-TG

• Intraoperative assessment — the no-thumb test: after all components are trialled and the knee reduced, the knee is taken through a full arc of motion without manually guiding the patella (the `no-thumb test`); the patella should track centrally in the trochlear groove throughout ROM; lateral subluxation or tilt on the no-thumb test = maltracking → investigate the cause: (1) component internal rotation (most common — femoral and/or tibial); (2) lateral soft tissue tightness (tight lateral retinaculum); (3) patellar component malposition (if resurfaced — placed too laterally); (4) overstuffing of the patellofemoral joint (femoral component too large anteriorly increases PF joint reaction force)
• Intraoperative correction options: lateral retinacular release (releases the tight lateral retinaculum — allows the patella to centrally track; however, aggressive lateral release devascularises the patella — see patellar AVN; the lateral superior geniculate artery must be preserved where possible); femoral component repositioning (if internal rotation is the cause — the component must be re-cut in external rotation); tibial tubercle medialization (moving the tubercle medially reduces the effective TT-TG distance and corrects lateral tracking)
• Post-operative patellar dislocation or persistent maltracking: first investigate the cause with CT (measure femoral and tibial component rotation relative to their respective reference axes); if component malrotation is confirmed — revision of the malrotated component is the definitive treatment (lateral retinacular release alone will not durably correct tracking if component malrotation is the cause); if rotation is acceptable — isolated lateral retinacular release (open or arthroscopic) may be effective; tibial tubercle medialization (anteromedialization osteotomy — Fulkerson procedure) can be performed when tibial component rotation is borderline or when other causes are excluded

• Patellar AVN: the blood supply to the patella enters from the inferior pole (via the infrapatellar branches of the geniculate arteries) and the medial and lateral sides; the lateral blood supply is disrupted by lateral retinacular release; the medial blood supply is disrupted by the medial parapatellar arthrotomy; if BOTH lateral retinacular release AND patellar resurfacing are performed, the patella may be significantly devascularised; AVN of the patella presents as progressive bone loss and collapse on serial X-rays; prevention — preserve the medial patellar blood supply during arthrotomy and minimise the extent of lateral release
• Patellar fracture: see patellar resurfacing article; minimum 12–15 mm residual thickness; AVN → structural failure → fracture; management by fracture pattern and implant stability (Ortiguera-Berry classification)
• Patellar component loosening: aseptic loosening of the cemented patellar button; more common with dome-shaped all-polyethylene cemented patellar components; associated with malalignment, inadequate cement technique, and poor bone quality; presents with anterior knee pain, crepitus, and a visible `halo` radiolucent line around the patellar component on X-ray; management — revision of the patellar component if bone stock allows; patellectomy or trabecular metal patellar augment if bone is insufficient
• Overstuffing the patellofemoral joint: if the femoral component is oversized anteriorly (too large AP dimension) or the combined thickness of the resurfaced patella plus the femoral trochlear surface is excessive, the PF joint is `overstuffed` — increased PF joint reaction force → anterior knee pain, limited flexion, component wear; intraoperative check — compare the resected patellar thickness to the patellar button thickness; the reconstructed patella should not be thicker than the native patella

• Clunk syndrome (posterior-stabilised TKA): a fibrous nodule forms on the quadriceps tendon at the level of the superior patellar pole or on the anterior surface of the femoral component cam; during knee extension from a flexed position (typically from approximately 60–90° of flexion to full extension), this nodule engages the intercondylar box or the cam of the PS femoral component, producing an audible and palpable clunk; patients describe a sudden jerk at the knee during standing from a chair; the clunk is reproduced on examination; diagnosis is clinical and confirmed by arthroscopy; treatment — arthroscopic excision of the fibrous nodule (effective in most cases); modern PS TKA designs have modified cam geometry to reduce clunk syndrome incidence
• Patellar clunk vs crepitus: true patellar clunk (fibrous nodule engaging the PS cam) is distinct from generalised crepitus (grating sensation from roughness or polyethylene debris) or patellofemoral crepitus (unresurfaced native cartilage on a metal trochlear groove); accurate diagnosis requires clinical correlation and arthroscopy if needed

• Most important factor for patellar tracking: rotational alignment of femoral and tibial components; femoral internal rotation shifts trochlear groove medially; tibial internal rotation shifts tubercle laterally; combined internal rotation = worst outcome → lateral dislocation
• No-thumb test: mandatory intraoperative assessment; knee through full ROM without manually guiding patella; lateral subluxation/tilt = maltracking → investigate and correct before wound closure
• Causes of intraoperative maltracking: (1) femoral/tibial internal rotation (most common); (2) tight lateral retinaculum; (3) patellar component lateral malposition; (4) PF overstuffing; correct by lateral release, component repositioning, or tubercle medialization
• Post-op maltracking: CT first (measure component rotation vs TEA for femur, AP axis for tibia); if malrotation confirmed → revision of malrotated component (definitive); if rotation acceptable → lateral retinacular release ± Fulkerson osteotomy
• Patellar AVN: lateral retinacular release + resurfacing = devascularisation; preserve medial blood supply; avoid aggressive lateral release; if established AVN → observation if stable; patellectomy if collapse/fracture
• Patellar component loosening: dome all-poly button; radiolucent `halo` on X-ray; anterior knee pain + crepitus; revision if bone allows; trabecular metal augment or patellectomy if insufficient bone
• PF overstuffing: oversized femoral component; reconstructed patella thicker than native; → anterior knee pain + restricted flexion + wear; intraoperative check: reconstructed thickness = native thickness
• Clunk syndrome (PS TKA): fibrous nodule on quadriceps tendon/anterior femoral cam; engages PS intercondylar box during extension from flexion (~60–90°); audible/palpable clunk; arthroscopic nodule excision; modern PS designs reduce incidence
• TT-TG distance: normal <20 mm; >20 mm → lateral maltracking; increased by component internal rotation; Fulkerson medialization osteotomy reduces TT-TG