Introduction: Patellar complications after total knee arthroplasty (TKA) remain a common reason for failure. Fully congruent patellar components, with larger contact areas and a polyethylene articular surface that is free to rotate in the frontal plane (LCS design), were designed to accommodate patellar mechanics and decrease wear. However, it remains to be determined whether the perceived advantages of a mobile-bearing, fully congruent patella design are realized in-vivo. The purpose of this study is correlate wear patterns on congruent mobile-bearing patellar components with patellar mechanics that existed after TKA.
Methods: Uncemented metal-backed patellar components were retrieved at revision surgery from 26 knees with meniscal bearing (23 knees) and rotating platform (3 knees) LCS mobile bearing prostheses (DePuy Orthopaedics). Mean patient age and time in-situ was 75+11 years and 11+4 years, respectively. Revision reasons included bearing wear (11), patella wear (7), instability (2), pain (3), loosening (1), osteolysis (1), and unknown (1). Polyethylene damage was assessed using optical microscopy. Articular wear area was measured using digital image analysis and the % area, location and incidence of each damage mode was calculated.
Results: The average damage area on the retrieved patella occupied 69%+15% of the surface. Burnishing, delamination and scratching modes occupied the largest areas. Delamination was noted on 58% of the retrieved patellae, predominantly located in the superior-medial quadrant. Nine (35%) patellae were fractured, with the fracture plane typically oriented in the medial-lateral direction or along the lateral edge. Twently one (81%) patellae had subsurface cracks oriented along the superior-inferior axis on the extreme lateral edge and along the medial-lateral axis. None of the patellae had embedded third body debris, but the embedded superior metal pin was exposed due to extreme damage in 4 patellas. The original femoral and tibial components were left in-situ in all knees at the time of revision, such that only the polyethylene tibial and patellar articulations were exchanged.
Discussion: Despite severe wear of the components, there was only a 5% incidence of osteolysis noted intra-operatively. Cyclic compressive and tensile forces during knee flexion likely caused initiation and propagation of cracks resulting in patellar bearing fracture. The delami-nation patterns on the retrieved patellae are consistent with bearing rotation into an incongruent bearing position during knee flexion, with presumably high contact stresses occurring in the delaminated superior-medial quadrant. Fully congruent mobile-bearing patella components must maintain mobility between the articular surface and metal back so that areas of incongruent contact, and associated high contact stresses and delamination, do not occur during in-vivo function.
