A biomechanical investigation of the surface stress of a synthetic femur using infrared thermography validated by strain gauge measurements

As the North American population ages, there will be a massive increase in musculoskeletal impairments because these problems are most common in the elderly. A very common condition is osteoporosis, which can result in fractures. Therefore, the need for improved orthopaedic fracture repair implants is vital. Currently, the two main approaches in studying orthopaedic implants are strain gauge measurements and finite element modelling. This study introduces and validates a relatively new, non-destructive approach in analysing stress patterns in a biomechanics application. Lock-in infrared (IR) thermography calibrated with strain gauges was used to investigate the stress and strain patterns of a synthetic femur under dynamic loading. The femur was instrumented with strain gauges and tested using axial average forces of 1500N, 1800N, and 2100N at an adduction angle of 7 degrees to simulate the single-legged stance phase of walking. Three dimensional surface stress maps were obtained using an IR thermography versus strain gauge data with a Pearson correlation of R² = 0.99 and a slope ranging from 0.99 to 1.08, based on thermoelastic coefficient (Km) ranging from 1.067 x 10⁻⁵/MPa to 1.16 x 10⁻⁵/MPa, for the line of best fit. IR thermography detected bone peak stresses on the superior-posterior side of the femoral neck of 91.2MPa (at 1500 N), 96.0Mpa (at 1800 N), and 103.5MPa (at 2100 N). There was strong correlation between IR measured stresses and force along the anterior (R² = 0.87 to 0.99), posterior (R² = 0.81 to 0.99) and lateral (R² = 0.89 to 0.99) surface. This is the first study to provide an experimentally validated three dimensional stress map of a synthetic femur using IR thermography.