Comparative analysis of the biomechanical behavior of two different design metaphyseal-fitting short stems using digital image correlation

Abstract

Background: The progressive evolution in hip replacement research is directed to follow the principles of bone and soft tissue sparing surgery. Regarding hip implants, a renewed interest has been raised towards short uncemented femoral implants. A heterogeneous group of short stems have been designed with the aim to approximate initial, post-implantation bone strain to the preoperative levels in order to minimize the effects of stress shielding. This study aims to investigate the biomechanical properties of two distinctly designed femoral implants, the TRI-LOCK Bone Preservation Stem, a shortened conventional stem and the Minima S Femoral Stem, an even shorter and anatomically shaped stem, based on experiments and numerical simulations. Furthermore, finite element models of implant-bone constructs should be evaluated for their validity against mechanical tests wherever it is possible. In this work, the validation was performed via a direct comparison of the FE calculated strain fields with their experimental equivalents obtained using the digital image correlation technique.

Results: Design differences between Trilock BPS and Minima S femoral stems conditioned different strain pattern distributions. A distally shifting load distribution pattern as a result of implant insertion and also an obvious decrease of strain in the medial proximal aspect of the femur was noted for both stems. Strain changes induced after the implantation of the Trilock BPS stem at the lateral surface were greater compared to the non-implanted femur response, as opposed to those exhibited by the Minima S stem. Linear correlation analyses revealed a reasonable agreement between the numerical and experimental data in the majority of cases.

Conclusion: The study findings support the use of DIC technique as a preclinical evaluation tool of the biomechanical behavior induced by different implants and also identify its potential for experimental FE model validation. Furthermore, a proximal stress-shielding effect was noted after the implantation of both short-stem designs. Design-specific variations in short stems were sufficient to produce dissimilar biomechanical behaviors, although their clinical implication must be investigated through comparative clinical studies.

Keywords: Digital image correlation; Experimental validation; Finite element analysis; Short stem; Total hip arthroplasty.

Fig. 1

Femoral stems. a Trilock BPS stem. b Minima S stem

Fig. 2

DIC-measured and FE-predicted strains for each specimen in each of the two fields of view

Fig. 3

DIC-measured strain response to single-leg stance loading in a medial and b lateral measurement regions

Fig. 4

Medial field of view. a Best-fitting polynomial curves of the FE-predicted and DIC-measured strain data relative to the long axis of the bone for the intact and implanted bones with Minima S and Trilock BPS. b Linear regression analysis. Lateral field of view. c Best-fitting polynomial curves of the FE-predicted and DIC-measured strain data relative to the long axis of the bone for the intact and implanted bones with Minima S and Trilock BPS. d Linear regression analysis; R^2*: coefficient of determination in polynomial regression, R²: coefficient of determination in linear regression analyses

Fig. 5

a Mechanical test setup; b detailed view of the experimental test setup showing the implanted femur within the customized loading apparatus allowing proximal loading via a compressive joint reaction force at the femoral head paired with a tensile force applied through the abductor plate on the greater trochanter; c configuration of the data capturing cameras

Fig. 6

Comparison lines depicted on the DIC prepared surfaces, superimposed by their corresponding FE fields: a lateral surface; b medial surface. Key zones of interest at 2-cm increments along the long axis of the femur within each of the two fields of view; c Trilock BPS stem; d Minima S stem