Early Shear Failure of a 3-Peg Modified Dome Patellar Implant.

Patellar failure in total knee arthroplasty is a major source of complications postoperatively. Previous patellar failure reports commonly cited dissociations of modular and metal-backed patellar implants. However, mechanical breakage of monoblock all-polyethylene patellar implants is very rare. We present a case of an early shear failure of a 3-peg modified dome all-polyethylene patellar implant at 16 months. The patient underwent a revision procedure and at 1-year follow-up, the patient's patella and knee remained stable with no reported issues. Shear failure of polyethylene pegs requires excess cyclic shear stress imparted at the prosthetic-bone interface. Patellar implants with a cone design are more constrained and, if misaligned relative to the metallic trochlea, may impart excess shear force to the patella during flexion.