Abstract
Purpose
Primary uncemented total hip arthroplasty is the fixation of choice for most patients undergoing hip replacement in North America. Initial stem stability will be detected by the fit and fill of the implant to the bone as well as the overall stem geometry as it relates to its mechanical stability. This study aims to determine the geometric and structural parameters of common cementless tapered femoral stems to better understand their early clinical performance with respect to bony fixation.
Method
Templates for six proximally coated tapered uncemented femoral stems: Accolade (Stryker), Profemur (Wright), Taperloc (Biomet), M/L Taper (Zimmer), Anthology (Smith-Nephew) and Trilock (Depuy) were scanned and saved as images on a PC. Stem sizes were assigned according to AP plane width measurements taken at the most lateral implant shoulder, and images analyzed using ImageJ (US National Institutes of Health). Measurements were made of stem length, and at various locations, the width and thickness of the stem. Coronal plane stiffness was calculated according to beam theory, assuming an elastic modulus of 114 GPa for the Profemur, Taperloc M/L Taper, Anthology and Trilock stems. The Accolade stem has a modulus of 85.5 GPa. due to TMZF alloy which is 25% more flexible. Stem thickness and taper ratios were calculated as a measure of lateral vs. AP fill of the femur. All measurements were plotted as manufacturer-specific curves relative to stem width.
Results
Stiffness-size curves between different stem designs were similar, although overall flexibility varied markedly between manufacturers. Stem stiffness varied over a range of up to 30% (3.1 to 4.3 kNm2) depending on width, with the greatest range in larger stem sizes. The most flexible stems, in order of increasing stiffness, were: Accolade, Trilock, and Anthology. The Taperloc, M/L Taper, and Profemur designs were stiffest and approximately equivalent at all widths. The greatest stem flexibility variation occurred at the implants lateral shoulder, where the Accolade design exhibited a 22% lower stiffness than the next closest design.
Conclusion
Stem geometry differs significantly amongst commercially available tapered blade type stems designs which may have significant implications for overall stem stiffness. The Accolade stem exhibits greater relative flexibility in its larger sizes. These findings provide some insight into the clinical reports of a high percentage of stem subsidence with the larger size Accolade. Our data suggest this may be due to lower structural stiffness and greater micromotion inherent to this stem design, which in turn may inhibit early implant osteointegration.
