Endurance testing and finite element simulation of a modified hip stem for integration of an energy harvesting system

Abstract

Instrumented implants are a promising approach to further improve the clinical outcome of total hip arthroplasties. For the integrated sensors or active functions, an electrical power supply is required. Energy harvesting concepts can provide autonomous power with unlimited lifetime and are independent from external equipment. However, those systems occupy space within the mechanically loaded total hip replacement and can decrease the life span due to fatigue failure in the altered implant. We previously presented a piezoelectric energy harvesting system for an energy-autonomous instrumented total hip stem that notably changes the original implant geometry. The aim of this study was to investigate the remaining structural fatigue failure strength of the metallic femoral implant component in a worst-case scenario. Therefore, the modified hip stem was tested under load conditions based on ISO 7206-4:2010. The required five million cycles were completed twice by all samples (n = 3). Additionally applied cycles with incrementally increased load levels up to 4.7 kN did not induce implant failure. In total, 18 million cycles were endured, outperforming the requirements of the ISO standard. Supplementary finite element analysis was conducted to determine stress distribution within the implant. A high stress concentration was found in the region of modification. The stress level showed an increase compared to the previously evaluated physiological loading situation and was close to the fatigue data from the literature. The stress concentration factor compared to the original geometry amounted to 2.56. The assessed stress level in accordance with the experimental fatigue testing can serve as a maximum reference value for further implant design modifications and optimisations.

Performance of a Piezoelectric Energy Harvesting System for an Energy-Autonomous Instrumented Total Hip Replacement: Experimental and Numerical Evaluation

“…The validation of the FEA model by the quasi-static experiment showed excellent correlation regarding the coefficients of determination. The experimentally measured values of the strain gauge strain ε SG and the displacement of point C in vertical direction d C,v were in the range of the magnitudes predicted by the FEA and are plausible concerning other studies [42,63]. In contrast to the strain, the deviation for the displacement d C,v between the numerical data and the experiment was not negligible.…”

“…), the sensitivity analysis also sheds light on the mechanical properties and safety of the system. Regarding the metallic implant component, the stress σ imp was for all configurations far below the literature fatigue data of 470 MPa for the same material at equal loading (maximum stress at 10 7 cycles, 10 Hz, R = 0.1, in Eagle's medium at 37 • C) [62] and below the maximum stress value of 417 MPa from previous simulations accompanying our endurance testing [42]. The maximum occurring stress was only 10.7 MPa higher than the overall maximum for the unmodified reference geometry for the physiologically based boundary conditions [41].…”

“…However, we cannot rule out compensating errors. For example, our previous studies showed smaller experimental strain values than for the FE model [42]. In the latter work, we also determined the malpositioning of the strain gauge as a relevant cause of deviations.…”

“…In this study, we did not extend our sensitivity analysis to the positioning of the strain gauge. From previous analyses, we know that malalignment can result in deviations in the lower single-digit percentage range [42].…”

“…In a previous study, the endurance performance of the mechanically loaded modified metallic total hip stem was tested experimentally [42], since the fatigue properties may have been impaired by the introduced cavity. A standard endurance test based on ISO 7206-4:2010 [43] was conducted and an additional finite element analysis (FEA) of the same setup provided further information on the stress and strain distribution and local stress concentration in the cavity [42,44]. Our results in [42] showed sufficient remaining safety margin, since the test was passed by all the samples twice and the implant even endured additional loading cycles with increased force levels.…”