A case of bilateral patellar component failure due to fatigue fracture of the all-polyethylene fixation pegs in a highly cross-linked ultra–high-molecular-weight polyethylene design is presented. To recreate this failure mode, a novel test method was developed to investigate the effects of peg orientation and cement technique on patella fatigue strength under cyclic compression and shear loading. Patellar peg orientation had a minor effect on shear strength, whereas lack of cement in the backside patellar groove had a substantial effect. The shear fatigue strength exceeded in vivo force estimates when the patellar groove was fully cemented. The test results and retrieval analysis suggest that high activity level and inadequate cement fixation of the patellar component may contribute to all-polyethylene patellar component peg fractures.