Four patients with a single-design metal-backed patellar component underwent revision of patellar components of both knees. In each case, a markedly symptomatic side and a mildly symptomatic side were present. Implant retrieval analysis allowed the authors to confirm a mechanism of implant failure we have postulated previously. This type of failure was characterized as a Type II metal-backed patellar failure mechanism, as it is specifically related to failure of the ultrahigh-molecular-weight polyethylene (UHMWPE) and not to fixation of the implant. This failure pattern can result in massive intraarticular wear debris deposition.
The defined mechanism of failure is (1) UHMWPE deformation or wear, leading to (2) UHMWPE split or fracture at the implant periphery, (3) widening/deformation of the capture mechanism with loss of UHMWPE capture, and subsequent (4) metal-to-metal abrasion—the final component of debris generation.
The design parameters resulting in this failure mechanism include (1) UHMWPE that is too thin, (2) a capture mechanism dependent upon UHMWPE integrity, (3) a patellofemoral geometry that allows the possibility of edge loading.
As patellar fixation does not appear to be a clinically significant problem with cemented all-UHMWPE components, the use of metal-backed patellar components, by any technique or with any design, appears unwarranted.