Estimating the Benefit of a Collar on Uncemented Press-Fit Stems

A collar has been shown to be beneficial in reducing peri-prosthetic fractures (PPF) and revisions in uncemented total hip arthroplasty using tapered press-fit stems (revision rate at 15 years 4,8% compared to 6.1% for collarless)1. The purpose of this study was to develop a numerical model to estimate the benefit of adding a collar to an uncemented stem. Materials and Methods Two human cadaveric femurs were implanted with a collared re. collarless CORAIL stem (DePuy Synthes) until failure using a material testing machine (constant velocity 0.1 mm/s). Shear, axial compressive and tangential tensile strains as well as fracture load were recorded using Digital Image correlation and compared between with vs. without collar. A finite element model (FEM) based on a CT scan of one femur was used to numerically estimate the same parameters for three different stem types with vs. without collar (CORAIL, EMPHASIS, ACTIS all DePuy Synthes). Results The collar increased axial compressive strains instead of causing critical excessive tangential tensile strains (hoop strains) that can result in PPF during loading. This results in substantially higher loads required for a fracture. In the experimental part of the study the collar increased the fracture load for the CORAIL stem by nearly 100% (6824N vs. 3465N, ratio 1.96). The results of the FEM model yielded a similar tendency for the CORAIL stem but with a higher benefit ratio (3.37). The EMPHASYS stem showed very similar results (ratio: 3.47), whereas the ACTIS stem showed a smaller benefit (ratio: 1.69). Discussion The ACTIS stem benefits less from the collar due to its fit-and-fill design in the metaphyseal region associated with a larger width compared to the two other stem designs investigated. This approach can be used for any uncemented stem design. Acknowledgement. The study was financially supported by DePuy-Synthes