Shear Fatigue Testing of All-Polyethylene Patellar Components

Many total knee arthroplasty patellar implants are designed as all-polyethylene components with either 1 or 3 pegs for cement fixation. Failure of patellar components due to peg fracture is rare, but has been reported [1–4]. Excessive shear forces due to high patient weight and/or activity level have been implicated as potential causes of these rare failures. In order to assist with the design of all-polyethylene patellar components, a test method was developed to replicate the peg shear failure mode observed clinically. Peak patellofemoral forces have been predicted during mid-flexion activities such as stair climbing or sitting [5], therefore testing utilized patellofemoral contact load magnitudes consistent with these activities. The Natural-Knee^® II System (N-K II) all poly patellar components manufactured from Durasul™ Highly Crosslinked Polyethylene were chosen for testing as a representative patella component design.