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THE EFFECT OF INTERFERENCE FIT ON PRIMARY HIP STEM STABILITY AND FEMORAL FRACTURE
Poster P-70
Implant and Joint Biomechanics
S443
THE EFFECT OF INTERFERENCE FIT ON PRIMARY HIP STEM STABILITY AND FEMORAL FRACTURE Ulrich Hansen, Mohammed Rafiq Abdul-Kadir, Duncan Lucas, Andrew Amis Imperial College London, UK.
Introduction The most commonly reported complications related to cementless hip stems are loosening and thigh pain; both of these have been attributed to high levels of relative micromotion at the bone-implant interface due to insufficient primary fixation. Primary fixation is generally believed to rely on sufficient interference-fit between the implant and the bone. However, attempting to achieve a high interference-fit often leads to intraoperative femoral fracture (Meek et al., 2004). The optimal range of interference-fit is not well understood.
Figure 2, which includes a 100 microns interference fit, shows hoop stresses on the order of 50MPa. Comparing this to the transverse tensile strength of cortical bone (~50MPa) 100microns represents the critical level of interference fit above which the femoral canal will fracture. From Figure 3, which
Methods A 3D finite element model was constructed to predict micromotion and therefore instability of femoral stems implanted in the femur.
Figure 1: Model of implant inserted in the femur. Material properties for the bone were based on the grey-scale value of the CT images. The models were loaded with physiological stair-climbing loads including all relevant muscle forces (Bergmann et al., 1998). The objective of this study was to estimate the effect of interference fit. Therefore, we varied the interference fit in the finite element models noting the effect on bone stresses and implant micromotion.
Results
Figure 3: Contour plots of micromotion over the surface of the stem with interference fits of (from left to right): 0, 5, 25, and 50 microns, respectively. shows predicted micromotion for four levels of interference fit, it is clear that the interference-fit had a very large effect on micromotion predictions. In the case of no interference fit, and using a critical level of micromotion in the range of 50-150 microns, the entire surface area is unlikely to osseointegrate. In contrast, with 50 microns of interference all but the most proximal part of the implant was predicted to osseointegrate. The effect of the interference fit was most dramatic at low levels of interference.
Discussion It was confirmed that interference fit has a very significant effect on micromotion and ignoring this parameter in an analysis of primary stability is likely to underestimate the osseointegration of the stem. Furthermore, it was predicted that the optimal level of interference fit is around 50 microns as this is sufficient to achieve good primary fixation while having a safety factor of two against femoral canal fracture. This result is of clinical relevance as it indicates a recommendation for the surgeon to err on the side of a low interference-fit rather then risking femoral fracture.
References Figure 2: Hoop stresses in femoral bone caused by an interference fit of 100 microns.
16th ESB Congress, Posters
Bergmann, G. (Ed.), HIP98 - Compact Disc, ISBN 3980784800, 2001. Meek et al, JBJS, 86-A(3): 480-485, 2004. Journal of Biomechanics 41(S1)
Implant and Joint Biomechanics
S443
THE EFFECT OF INTERFERENCE FIT ON PRIMARY HIP STEM STABILITY AND FEMORAL FRACTURE Ulrich Hansen, Mohammed Rafiq Abdul-Kadir, Duncan Lucas, Andrew Amis Imperial College London, UK.
Introduction The most commonly reported complications related to cementless hip stems are loosening and thigh pain; both of these have been attributed to high levels of relative micromotion at the bone-implant interface due to insufficient primary fixation. Primary fixation is generally believed to rely on sufficient interference-fit between the implant and the bone. However, attempting to achieve a high interference-fit often leads to intraoperative femoral fracture (Meek et al., 2004). The optimal range of interference-fit is not well understood.
Figure 2, which includes a 100 microns interference fit, shows hoop stresses on the order of 50MPa. Comparing this to the transverse tensile strength of cortical bone (~50MPa) 100microns represents the critical level of interference fit above which the femoral canal will fracture. From Figure 3, which
Methods A 3D finite element model was constructed to predict micromotion and therefore instability of femoral stems implanted in the femur.
Figure 1: Model of implant inserted in the femur. Material properties for the bone were based on the grey-scale value of the CT images. The models were loaded with physiological stair-climbing loads including all relevant muscle forces (Bergmann et al., 1998). The objective of this study was to estimate the effect of interference fit. Therefore, we varied the interference fit in the finite element models noting the effect on bone stresses and implant micromotion.
Results
Figure 3: Contour plots of micromotion over the surface of the stem with interference fits of (from left to right): 0, 5, 25, and 50 microns, respectively. shows predicted micromotion for four levels of interference fit, it is clear that the interference-fit had a very large effect on micromotion predictions. In the case of no interference fit, and using a critical level of micromotion in the range of 50-150 microns, the entire surface area is unlikely to osseointegrate. In contrast, with 50 microns of interference all but the most proximal part of the implant was predicted to osseointegrate. The effect of the interference fit was most dramatic at low levels of interference.
Discussion It was confirmed that interference fit has a very significant effect on micromotion and ignoring this parameter in an analysis of primary stability is likely to underestimate the osseointegration of the stem. Furthermore, it was predicted that the optimal level of interference fit is around 50 microns as this is sufficient to achieve good primary fixation while having a safety factor of two against femoral canal fracture. This result is of clinical relevance as it indicates a recommendation for the surgeon to err on the side of a low interference-fit rather then risking femoral fracture.
References Figure 2: Hoop stresses in femoral bone caused by an interference fit of 100 microns.
16th ESB Congress, Posters
Bergmann, G. (Ed.), HIP98 - Compact Disc, ISBN 3980784800, 2001. Meek et al, JBJS, 86-A(3): 480-485, 2004. Journal of Biomechanics 41(S1)