Mechanical Load-to-Shear Failure of Cemented Patellar Components in Well-Functioning Postmortem Total Knee Arthroplasties.

Background: Shear forces on the patella can lead to patellar loosening and failure in total knee arthroplasty (TKA). Our objectives were to: (1) evaluate the mechanical load-to-shear failure in a unique cohort of cemented patellar components in well-functioning postmortem TKAs and (2) determine the influence of clinical and radiographic factors on load-to-shear failure.

Methods: There were 22 patellae that were harvested from well-functioning postmortem TKAs (mean implantation duration 9.1 years [range, 1.7 to 19.6]). There were three all-polyethylene 3-peg onlay patellar designs evaluated. The patellae were evaluated for polyethylene damage, implant-to-bone size ratios, microarchitectural parameters, and implant-cement-bone interface. Patellar specimens were loaded with isolated shear stress using a servo-hydraulic test frame until failure. Univariate and multivariable linear regression models were used to analyze the influence of clinical and radiographic factors on load-to-shear failure.

Results: The mean load-to-shear failure was 1,881 +/- 621 N. There were nine patellae that failed at the bone-cement interface, seven that failed at the implant-cement interface, and six that involved both interfaces. There were no failures at the peg-implant junction. On multivariable analyses, load to failure was positively associated with bone volume fraction and negatively associated with body mass index, duration of implantation, and implant-to-bone surface area coverage. Load-to-shear failure was not associated with surface damage or radiographic parameters and did not differ across the three patellar implant designs.

Conclusion: In this novel cohort of cemented patellar buttons from well-functioning postmortem TKAs, load-to-shear failure was higher than previously reported in buttons cemented in cadaver native patellae. The amount of bony coverage by the patellar button was inversely associated with shear strength after accounting for other pertinent factors. When choosing between two patellar button sizes, surgeons should consider opting for the smaller size, which may improve the maximum shear strength of the patellar construct.