Abstract
BACKGROUND
In vitro tests have shown that when a force is applied to the proximal femur within the range of directions spanned during physiological activities, the direction of principal strain vary by a very narrow angle (Cristofolini et al, 2009, J. Engng. Med.). This shows that the anatomy and the distribution of inhomogeneous and anisotropic material properties of the bone tissue make the structure of the proximal femur optimized to withstand a wide range of loading directions.
The increasing use of hip resurfacing is associated with early neck fractures of the implanted femur. The aim of this study was to elucidate if such fractures could be caused by a non-physiological state of stress/strain post-implantation. While the possible role of notching at the neck-implant interface has already been elucidated, it is not know whether a resurfacing implant could make the principal strain vary in magnitude and direction in a way that could compromise integrity of the proximal femur.
METHODS
The aim of this study was to measure if the direction of the principal strain in the proximal femur was affected by the presence of a resurfacing prosthesis. Seven human cadaver femurs were instrumented with 12 triaxial strain gauges to measure the magnitude and alignment of principal strains in the head-neck region. Each femur was implanted with a typical resurfacing prosthesis (BHR). All femurs were tested in vitro before and after implantation with a range of loading conditions to explore the range of loading directions during daily activity (Fig. 1).
FINDINGS
Comparison of the strain distribution before and after implantation showed that:
In the natural conditions the principal tensile strain was significantly larger where the cortical bone was thinner; the compressive strain was larger where the cortical bone was thicker. This should be considered when designing a resurfacing prosthesis.
The strain magnitude varied greatly between loading configurations both in the intact and implanted condition: this suggests that different loading configurations must be simulated for the preclinical validation of a resurfacing prosthesis.
In the natural conditions, the direction of the principal strain varied significantly between measurement locations, but varied little between loading configurations (less than 10° when the hip force spanned a 21° cone, Fig. 2). This confirms that the anatomy and the distribution of anisotropic material properties enable the proximal femur to respond adequately to the changing direction of daily loading.
In the resurfaced femurs, when the force spanned the same 21° cone, the direction of principal strain at each measurement location varied by less than 10° (Fig. 3), similar to the natural condition.
In the resurfaced femurs, the direction of principal strain lied within less than 10° from the direction in the natural conditions.
INTERPRETATION
Our results show that resurfacing does not disturb the alignment of principal strain in the proximal femur. In other words, the most critical directions of stress/strain after implantation stay aligned with the same direction as in the intact femur, which is the direction for which the inhomogeneous and anisotropic structure of the proximal femur is optimized.
