The origins of the uncemented tapered wedge hip stem design currently offered by several orthopaedic device companies can be linked back to the cemented Straight Mueller type stem design first used in 1977. The design, a wedge shape with a taper angle of 6 degrees, maintains a single medial curvature for all sizes and increases laterally in the width to accommodate different size femurs. Although evolutionary improvements have been made over the years the basic body geometry of the stem has stayed mainly unchanged with excellent clinical survivorship. Over the past decade, the demographics of hip replacement have changed, with a large increase in younger male patients in the age range of 40 to 60 years. In this study the femoral fit of a novel tapered stem, designed to fit a wide array of patient types, is compared to a standard predicate tapered stem design. A bone morphology study was performed on a patient population of 556 patients using three dimensional digital data from CT-scans. To characterize the fit of the stem designs we analyzed the ratio of a distal (60mm below lesser trochanter) and a proximal (10mm above lesser trochanter) cross section. The same measurements were taken with the standard tapered stem design and the novel tapered stem design, with a given constant implantation height of 20mm above the lesser trochanter. The fit of the stems was classified as Type 1, where there was both proximal and distal engagement, Type 2, proximal engagement only, Type 3, distal engagement only. The distal and proximal engagement, Type 1, was specified with a maximum engagement difference of 2mm proximal to distal.Introduction
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