Understanding Inaccuracy and Low Repeatability in Bone Torsional Mechanical Testing

This study aims to raise awareness about the widespread use of flawed torsion test setups for measuring the torsional mechanical properties of bones, prevalent in both past research and current practices. It addresses critical inaccuracies in bone torsion testing that impact the understanding of key properties such as torsional rigidity and shear modulus. The widely adopted machine-mounted angle transducer (MMAT) often leads to incorrect measurements by combining specimen, machine and parts’ rotations, introducing errors from slippage, mechanical tolerances and other resources. To correct this, a specimen-mounted angle transducer (SMAT) was developed using standard laboratory equipment, including linear variable differential transformers (LVDTs) and metal pins. The SMAT focuses solely on the central bone specimen's rotation, offering more precise measurements. Torsion tests were conducted on eight porcine femur samples, and finite-element twin (FE-twin) models were developed through 3D scanning of bone geometry. These models were calibrated using MMAT and SMAT data to evaluate differences in torsional rigidity and shear modulus. The results revealed significant discrepancies between MMAT-based data and literature values, while SMAT-based tests aligned more accurately with theoretical predictions and strain gauge measurements. Additionally, scanning electron microscopy (FE-SEM) and X-ray diffraction (XRD) analyses highlighted micro structural differences between fractured and unfractured bones. The findings underscore the importance of accurate torsion testing for clinical and research applications, offering improved methods for designing orthopaedic devices and assessing bone health.