Katy L Townsend, Michael P Kowaleski, Kenneth A Johnson.
The Ohio State University, Columbus, Ohio, United States

ACVS Abstract 2007 

The goal of cementless design is to create an initially stable press fit that is conducive to bony in-growth. The objective of this study was to determine whether osteotomy level or stem size effects initial implant stability of the BFXTM cementless stem and to assess femoral hoop strain under axial load.

Our hypotheses were that resection level would not affect axial stability of the stem, and that larger implants would have greater initial stability. Twenty paired femurs were divided into two groups – differing in femoral neck resection level or stem size. Axial load was applied to the potted femurs, cortical strain was measured with strain gauges.

Pair-wise comparisons were made with paired t-tests with significance set at Po0.05. Interface stiffness did not differ in either group. The proximal osteotomy had a higher yield load, greater subsidence at failure, smaller canal fill and lesser subsidence stiffness, than the distal osteotomy. The larger implants had a higher yield load, greater subsidence stiffness, greater maximal load at failure, smaller subsidence at failure and greater canal fill, than the undersized implants. Tensile strains tended to be medially and compressive strains cranially for the stem size group.

Tensile strains were highest over fracture sites. A proximal osteotomy and a large implant results in increased yield load prior to subsidence, likely due to increased stability resulting from a press fit within the subtrochanteric cancellous bone block. Undersized implants and distal resection level results in lower subsidence stiffness and lower yield load respectively.