A COMPARISON OF HOMOGENOUS VERSUS PHYSIOLOGICAL TRABECULAR BONE PROPERTIES IN THE FINITE ELEMENT EVALUATION OF IMPLANT PERFORMANCE

Finite element (FE) methods of evaluating implant-bone behaviours have gained popularity due to their ability to estimate postoperative implant performance throughout the structure. However, although homogenous models for cancellous bone have been shown to satisfactorily replicate the biomechanics of human bone in compression test models, no studies have evaluated the efficacy of these models in a computational evaluation of the bone-implant construct. Accordingly, this finite element analysis investigated the effect of utilizing patient-specific trabecular bone material properties in comparison to using homogenous models mimicking commonly used polyurethane foam surrogates. We employed a stemless humeral component that relies primarily on cancellous fixation as used in total shoulder arthroplasty as our model. Five CT-derived (sex: male, age: 67 ± 20 years (mean ± standard deviation), mass: 72 ± 11 kg, height: 175 ± 3.1 cm) three-dimensional models of the proximal humerus were created from CT data using Mimics (Materialise, Leuven, Belgium). A generic implant was implanted into humeral models and assigned material properties using the Morgan et al [1] Young's modulus-bone density relationship using Mimics. Loads derived from previously published Orthoload [2] in-vivo telemetrized implant data, replicating a 30° abduction motion, were applied to each model. Each model was then reassigned new trabecular bone materials, representing the material properties of commonly used polyurethane foams [35PCF, 30PCF, 25PCF, 20PCF, 17PCF, 15PCF, 12PCF]. In addition to the CT-modelled homogenous bone models, a homogenous structure, representing a standard 13 cm x 18 cm x 4 cm 17 PCF foam block, was also tested under the same loading conditions as were applied to the humeral models. Maximum bone-implant distractions were the outcome variable employed. In all cases, increasing foam PCF decreased bone-implant relative distraction (p < 0.001) (Figure 1). When comparing foam material bone models to the CT-derived material models, bone-implant distractions after loading were most similar in the 17 PCF polyurethane foam homogenous models (CT-Derived Material: 18.78 ± 11.35 µm, 17 PCF Material: 18.10 ± 10.15 µm). Implant performance in the homogenous models was highly dependent on patient cortical bone morphology, with the 17 PCF CT-Models exhibiting higher average bone-implant relative distractions (18.10 µm) than their 17 PCF Foam Block counterpart (11.33 µm) (Figure 2). Simulated implant behaviour is dependent on the modelled trabecular bone material. When investigating macro-scale cancellous-base fixation behaviour, it is likely important to ensure that the selection of homogenous trabecular bone modulus approximates patient-specific bone properties and geometry. Moreover, employment of the outer cortical structure also has implications with regard to the response of the trabecular bed to loading of the implant. Further investigation into the effect of using synthetic bone models during experimental testing is suggested. For any figures or tables, please contact the authors directly.

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