Spinal loads as influenced by external loads: a combined in vivo and in silico investigation

Knowledge of in vivo spinal loads and muscle forces remains limited but is necessary for spinal biomechanical research. To assess the in vivo spinal loads, measurements with telemeterised vertebral body replacements were performed in four patients. The following postures were investigated: (a) standing with arms hanging down on sides, (b) holding dumbbells to subject the patient to a vertical load, and (c) the forward elevation of arms for creating an additional flexion moment. The same postures were simulated by an inverse static model for validation purposes, to predict muscle forces, and to assess the spinal loads in subjects without implants. Holding dumbbells on sides increased implant forces by the magnitude of the weight of the dumbbells. In contrast, elevating the arms yielded considerable implant forces with a high correlation between the external flexion moment and the implant force. Predictions agreed well with experimental findings, especially for forward elevation of arms. Flexion moments were mainly compensated by erector spinae muscles. The implant altered the kinematics and, thus, the spinal loads. Elevation of both arms in vivo increased spinal axial forces by approximately 100N; each additional kg of dumbbell weight held in the hands increased the spinal axial forces by 60N. Model predictions suggest that in the intact situation, the force increase is one-third greater for these loads. In vivo measurements are essential for the validation of analytical models, and the combination of both methods can reveal unquantifiable data such as the spinal loads in the intact non-instrumented situation.