The effect of patella preparation for total knee arthroplasty on patellar strain: a comparison of resurfacing versus inset implants.

This study was conceived to quantify variables in surgical technique that may lead to patellar fracture after total knee arthroplasty. Anterior surface strain on load-bearing cadaveric patellae was measured before and after patellar resurfacing or inset prosthesis placement. Variables studied were i) the type of prosthesis (resurfacing vs inset), ii) the depth of reaming or osteotomy during surface preparation, and iii) the overall thickness of the polyethylene/patella composite after implantation. Comparison of measured patellar surface strain patterns for the different prosthesis styles, which were implanted at varying depths, provided statistically significant data from which the following clinically relevant conclusions can be made: i) Patellar resurfacing is superior to inset prosthesis placement when comparing postoperative patellar strain (22% vs 28% increase in strain compared with preoperative values). ii) Osteotomy for patellar resurfacing is more tolerant to error by excess cutting than is reaming for inset prosthesis placement (25% vs 42% increase in strain with a 2-mm error). iii) If the ideal depth of cut or reaming is surpassed, attempts to re-create the original patellar thickness by using a thicker prosthesis are mechanically detrimental. Key words: patella, strain, resurfacing, complications.