Biomechanical stress maps of an artificial femur obtained using a new infrared thermography technique validated by strain gages

Femurs are the heaviest, longest, and strongest long bones in the human body and are routinely subjected to cyclic forces. Strain gages are commonly employed to experimentally validate finite element models of the femur in order to generate 3D stresses, yet there is little information on a relatively new infrared (IR) thermography technique now available for biomechanics applications. In this study, IR thermography validated with strain gages was used to measure the principal stresses in the artificial femur model from Sawbones (Vashon, WA, USA) increasingly being used for biomechanical research. The femur was instrumented with rosette strain gages and mechanically tested using average axial cyclic forces of 1500 N, 1800 N, and 2100 N, representing 3 times body weight for a 50 kg, 60 kg, and 70 kg person. The femur was oriented at 7° of adduction to simulate the single-legged stance phase of walking. Stress maps were also obtained using an IR thermography camera. Results showed good agreement of IR thermography vs. strain gage data with a correlation of R2 = 0.99 and a slope = 1.08 for the straight line of best fit. IR thermography detected the highest principal stresses on the superior–posterior side of the neck, which yielded compressive values of −91.2 MPa (at 1500 N), −96.0 MPa (at 1800 N), and −103.5 MPa (at 2100 N). There was excellent correlation between IR thermography principal stress vs. axial cyclic force at 6 locations on the femur on the lateral (R2 = 0.89–0.99), anterior (R2 = 0.87–0.99), and posterior (R2 = 0.81–0.99) sides. This study shows IR thermography's potential for future biomechanical applications.