Standardized Synthetic Femur Models - a Biomechanical Comparison

Introduction

Concerning biomechanical research, human specimens are preferred to achieve conditions that are close to the clinical situation. On the other hand, synthetic femurs are used for biomechanical testing instead of fresh-frozen human femurs, to create standardized and comparable conditions. A new generation of synthetic femurs is currently available aiming to substitute the validated traditional one. Structural femoral properties of the new generation have already been validated, yet a biomechanical validation is missing.

The aim of our study was to analyse potential differences in the biomechanical behaviour of two different synthetic femoral designs by measuring the primary rotational stability of a cementless femoral hip stem.

Methods

The cementless SL-PLUS® standard stem (size 6, Smith&Nephew Orthopaedics AG, Rotkreuz, Swizerland) was implanted in two groups of synthetic femurs. Group A consists of three 2^nd generation femurs and group B consists of three 4^th generation femurs (both: size large, composite bone, Sawbones® Europe, Malmö, Sweden).

Using an established method to analyse the rotational stability, a cyclic axial torque of ±7.0 Nm along the longitudinal stem axis was applied. Micromotions were measured at defined levels of the bone and the implant. The calculation of relative micromotions at the bone-implant interface allowed classifying the rotational implant stability.

Results

Lowest relative micromotions were located near the isthmus for both designs (2^nd 3.47 ± 1.43 mdeg/Nm and 4^th 5.97 ± 0.39 mdeg/Nm), whereas highest relative micromotions were located at distal tip for both designs (2^nd 8.42 ± 1.38 mdeg/Nm and 4^th 8.40 ± 0.39 mdeg/Nm). No statistically significant differences were found between 2^nd and 4^th generation femurs in the distal part (2^nd 8.42 ± 1.38 mdeg/Nm and 4^th 8.40 ± 0.39 mdeg/Nm; p>0.05), but proximally (2^nd 3.63 ± 1.10 mdeg/Nm vs. 4^th 6.55 ± 1.27 mdeg/Nm; p<0.05). Compared to the 2^nd generation, the 4^th generation femur resulted in less absolute micromotions and less standard deviation of micromotions.

Discussion

Compared to other implant designs, the SL-PLUS® stem resulted in low relative motions regardless the used synthetic femur. Within the two distal measuring levels, no significant differences could be observed. Proximally, at the level of the Trochanter minor, mean relative motions nearly doubled from group A to B. However, the anchorage principle of the SL-PLUS® stem was still similar in both synthetic femurs. Qualitatively, both synthetic femurs revealed a proximal fixation of the stem. Although the values of relative motion slightly differs, 4^th generation synthetic femurs are suitable to achieve similar results for measuring the primary stability of cementless femoral hip stems compared to 2^th generation synthetic femurs.

Future measurements with human specimens should validate weather one of the synthetic bone models is closer to the human situation.

Within this study we could show that the new 4^th generation synthetic femur designs could qualitatively give comparable results to older synthetic bone models regarding biomechanical tests, like primary stability measurements on cementless hip stems.