Deformation and initial stability in hip arthroplasty. Effect of neck geometry and fixation – an experimental cadaver study
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Introduction: The search for and development of the optimal joint implant include preclinical testing. Restoration of the individual and natural biomechanics in the hip joint is a central goal in hip arthroplasty, and can be achieved by varying neck length, version and angle. Modular necks are one way to achieve these adjustments despite a growing concern regarding their outcome. In hip arthroplasty, the implants can be attached to the bone with or without cement. Both methods have achieved good clinical results. In this thesis, the effect of varying the femoral neck angle and length was tested in an experimental setup simulating everyday activities. Further, a cemented and an uncemented femoral stem with similar geometrical shape were compared in a preclinical setup.
Methods: All implants were tested in human cadaver femurs by loading in a hip simulator in single leg stance and stair climbing activity. Changes in deformation pattern of the proximal femur were measured by strain gauges. Initial stability of the femoral stems was investigated using a micromotion jig. The effect on the deformation pattern and initial stability was studied when the neck version, angle and length were varied, due to either an eccentric femoral head or a modular neck. The deformation pattern and initial stability of a cemented and an uncemented stem of similar geometry were compared.
Results: Strain was reduced in the proximal femur for all implants tested, especially proximally on the medial side, compared to the intact femur. Increased offset combined with retroversion or reduced neck–shaft angle in an eccentric femoral head gave significantly increased strain values compared to the standard situation. All three eccentric femoral head configurations gave overall small micromotion of the femoral stem; up to 40 μm. When testing the modular necks, the varus neck increased the micromotion up to 60 μm. Micromotion was significantly higher during stair climbing compared to single leg loading, and for distal level compared to proximal level in all modular necks. The short neck had higher loss of strain in distal position on the lateral side, and the retroverted neck retained more strain proximal medially. The cemented stem had slightly higher strains than the uncemented stem on the medial side, while uncemented stem had higher strains on the lateral side of the proximal femur. The differences were small, but statistically significant.
Conclusion: Varying the femoral neck version, angle and length by either an eccentric femoral head or a modular neck gave some variations in cortical strains in the proximal femur compared to a standard design. However, the differences might be too small to have any clinical significance. The initial stability was acceptable for the tested implants when varying the femoral neck angle and length. The cemented stem was more stable than the uncemented stem, as expected. However, both stems had small micromotions at the bone-implant interface, and in a range, that is not expected to have a negative impact on long-term stability.