Why are bone torsional rigidity and stiffness measurements often inaccurate?

• Torsion tests using only machine-mounted angle transducers yield flawed stiffness measurements. • A new setup and guidelines enhance torsion measurement accuracy. • FE-twin modelling and microstructural analysis validate the new method and findings. This study aims to raise awareness about the widespread use of flawed torsion test setups for measuring the stiffness-related torsional mechanical properties of bone, both in previous studies and in current practice. The widely adopted machine-mounted angle transducer (MMAT) introduces significant measurement errors by capturing combined rotations from the specimen, machine components, and fixtures, including slippage and mechanical tolerances, rather than the true rotation of the bone specimen itself. This results in distorted estimations of torsional rigidity and stiffness, undermining the accuracy of biomechanical analyses and the design of orthopaedic devices. 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 microstructural 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. This paper delivers an important message: bone torsional rigidity and stiffness measurements are unreliable unless the torsion test setup includes direct measurement sensors, either contact or non-contact, mounted on or focused directly on the specimen.