Biomechanical Testing of the Proximal Femoral Epiphysis: Intact and Implanted Condition

Abstract

There is renewed interest in resurfacing hip prostheses. While stemmed prostheses have been extensively studied in the past, little is known about the biomechanics of epiphyseal prostheses. Our aim was to develop a combined experimental-numerical tool to study the intact and operated epiphysis. Bone and implant stress, relative micromotion and failure mode in the intact and implanted bone were investigated. Twelve pairs of cadaver human femurs were studied intact, to fully characterize the proximal epiphysis. Four were then implanted with a commercial resurfacing prosthesis. They were tested in the elastic range, while strains were measured with 15 rosettes. Implant micromotions were measured in the operated condition. A total of 7 loading scenarios were simulated to cover the range of typical motor tasks. Additionally, Finite Element (FE) models were built using a validated procedure for assigning inhomogeneous material properties based on CT data. To allow extensive validation of the FE model, additional measurements were taken in vitro: bone deflection in various points, indirect measurement of load application point, digitizing of the bone surface and gauge locations. The FE models were also used to identify the most critical load scenario to recreate in vitro spontaneous head-neck fractures. Strain measurements were successfully obtained in intact and implanted femurs, providing the natural strain pattern, and indicating moderate stress-shielding in the operated condition. Results on the 6 femurs that were modeled showed that FE can predict overall displacements with an accuracy of 0.4mm, and principal stress with an accuracy of 10% (Root Mean Squared, RMSE). In vitro failure tests were successful: all specimens fractured, with a variety of failures ranging from sub-capital to trans-trochanteric. This confirms the suitability of this test model to replicate spontaneous fractures in elderly subjects. In conclusion, an experimentally validated FE method was developed, that run in parallel with an optimized in vitro simulation. These tools can successfully predict the stress distribution and the failure mode in the proximal femur both in its natural condition and with a resurfacing prosthesis.

A Method to Improve Experimental Validation of Finite‐Element Models of Long Bones

“…An experimental set‐up used for investigating the strain distribution and the strength of the proximal femur [10, 15, 18] was modified to provide more accurate validation for FE models. A sketch of the set‐up in exercise, when testing a femoral bone, is shown in Figure 1.…”

“…The set‐up consists of the load cell forming the base of the testing machine, with the FP‐transducer mounted on top of the load cell. The bone specimen is potted on top of the FP‐transducer; to allow application of the force in the desired directions [15, 16], interchangeable wedges are used to tilt the bone by given angles. A vertical force is applied to the proximal end of the specimen by the actuator of the testing machine, through a system of low‐friction cross‐rails that eliminated any horizontal force component.…”

“…For instance, when the hip joint force is applied to the femoral head in vitro [6, 10, 12–16], it is not possible a priori to determine accurately the position of the resultant force, as: (i) the contact area between the bone surface and the loading device is difficult to measure accurately due to the large deformation of the bone surface; and (ii) even if the contact area was accurately measured, the distribution of the contact pressure (and its resultant) cannot be easily measured experimentally. Additionally, long bones undergo significant deflection when loaded (of the order of 10 mm [15, 17, 18]). It is not possible to predict a priori the changing position of the applied force while the bone specimen is deflecting due to the material deformation as load is applied.…”

Multiscale investigation of the functional properties of the human femur

“…Ferré et al 1990;Katz et al 1992;Spirakis et al 1992;Gajda et al 1995;Tyrer et al 1995). To validate the FE modelling technique adopted by our group, a total of eight human femurs were tested in vitro (Cristofolini et al 2006): bone strains were measured at a large number of points, while a set of loading scenarios that covered the range spanned in daily activities was applied (Bergmann 2001;Bergmann et al 2001). Dedicated transducers were built to enable accurate control of the boundary conditions (Juszczyk et al in press), to ensure that the applied loads in vitro replicated the external loading applied at the body level (see §5).…”

Mechanical testing of bones: the positive synergy of finite–element models andin vitroexperiments

“…For instance, when the hip joint force is applied to the femoral head in vitro (Cristofolini et al 1994(Cristofolini et al , 2007aCody et al 1999;Ota et al 1999;Keyak et al 2005;Taddei et al 2006), it is not possible a priori to determine accurately the position of the resultant force, as: (i) the contact area between the bone surface and the loading device is difficult to measure accurately owing to the large deformation of the bone surface and (ii) even if the contact area was accurately measured, the distribution of the contact pressure (and its resultant) cannot be easily measured experimentally. Additionally, long bones undergo significant deflection when loaded, of the order of some millimetres (Cristofolini & Viceconti 1999a,b;Cristofolini et al 2006). Consequently, it is not possible to predict, a priori, the changing position of the applied force while the bone specimen is deflecting under the applied load.…”