INTRODUCTION: Following Total Knee Arthroplasties, patellofemoral complications have shown to be responsible for approximately 50% of re-operations. Contemporary patellar designs employ both “onlay” and “inset” configurations. The latter promotes ease of placement, reduced bone removal and a heralded theoretic advantage of increased strength at the fixation interface. However, to date, no reports have compared the disassociation strengths of these two patellar component modes of fixation. The purpose of this study is to quantify the shear disassociation strength for both onlay and inset patellar fixation techniques.

METHODS: Two sets of synthetic solid foam patellae were prepared using standard milling techniques for symmetrical, three-peg onlay and inset polyethylene cylinders of identical dimension. The use of synthetic bones in mechanical testing was validated in the past. The cylinders were cemented to the synthetic patellae, using standard cementing techniques. The fixation resistance of both groups was measured using an Instron Testing Machine. A compressive joint force simulating chair rise was applied perpendicular to the anterior surface of the patellar component model. A shearing displacement was then applied to the composite until patellar component disassociation.

RESULTS: The mean shear strength of the onlay group was 2540 N SD 236 N, (n=7) and 3180 N SD 186 N, (n=6) for the inset group. The inset patellae was 25% (640 N) stronger than the onlay patellae, (p=0.0002, two-tailed student t-test).

DISCUSSION/CONCLUSION: The results of the study demonstrated a significantly higher resistance of inset patellar fixation to shear stress compared to onlay patellar fixation. Although further in-vivo studies are indicated, the data suggests that the use of inset patella in total knee replacements may offer stronger fixation and consequently decreasing morbidity associated with patella implant loosening.