Early failure of the femoral component in unicompartmental knee arthroplasty

Unicompartmental knee arthroplasty has been proposed as treatment for unicompartmental arthritis. Overall, results have been satisfactory, with occasional failures of tibial component fixaton. Short-term loosening of the femoral components in unicompartmental knee arthroplasty that required revision to total knee arthroplasty is reported. All patients presented with pain and instability of the knee, with loosening confirmed radiographically and at revision surgery. The Porous-Coated Anatomic femoral component (Howmedica, Rutherford, NJ) was biomechanically tested after being implanted in cadaver limbs. Cyclic loading in flexion and extension under physiologic loads resulted in implant failure. The model of failure was rocking in the sagittal plane, which corresponds to the clinical findings. The femoral implant design consists of an angled bone-implant interface with straight arms, which causes shear at the bone-prosthesis interface and ultimate toggling and failure. A femoral component with a curved bone implant interface did not loosen under identical biomechanical testing in paired knees. It was concluded that the Porous-Coated Anatomic femoral component has an unacceptably high early clinical loosening rate, confirmed by comparative biomechanical testing.