Twente Spine Model: Development, validation, and application of a complete and coherent musculoskeletal model of the human spine

A comprehensive assessment of the spinal loads throughout the spine can advance our understanding of its functioning but is largely unavailable. Musculoskeletal modeling offers a non-invasive means to estimate in vivo spinal loads and can thus provide clinical insights into the spine’s functioning. The primary objective of this dissertation was to develop a validated, complete and coherent musculoskeletal model of the entire human spine for investigating the spinal loads. Firstly, an anatomical dataset (the Twente Spine Dataset) including necessary musculoskeletal parameters for creating this model was measured. For each muscle-tendon element, locations of the attachment sites at the origin, insertion, and via points were digitized, and morphological parameters consisting of the fiber length, tendon length, sarcomere length, optimal fiber length, pennation angle, mass, and physiological cross-sectional area were measured. Next, a complete and coherent musculoskeletal model of the entire human spine (the Twente Spine Model) was developed based on the previously acquired anatomical dataset. In this model, cervical, thoracic, and lumbar vertebrae, a flexible ribcage, and comprehensive muscular anatomy were incorporated. An inverse dynamics based static optimization routine minimizing muscle fatigue was used for calculating muscle and joint forces during basic neck and trunk movements. For validation of the predicted internal loads, quasi-static trunk tasks as measured in previous in vivo studies were simulated, and calculated intradiscal pressures at thoracic and lumbar discs and normalized resultant loads were compared. Subsequently, the sensitivity of muscle and intervertebral disc force computations against potential errors in modeling muscle attachment sites (muscle origin, insertion, and via points) were investigated. For this, every muscle attachment location was perturbed in the Twente Spine Model during upright standing, flexion, lateral bending, and axial rotation of the trunk. The changes in the T6/T7, T12/L1, and L4/L5 disc forces were analyzed, and an overall sensitivity index value was calculated for every perturbed muscle. Furthermore, electromyographic activities and trunk movements during isometric and dynamic trunk activities were simultaneously measured. Finally, musculoskeletal and patient-specific finite element models were used in combination to investigate if modeling more physiological load regimes can significantly affect the vertebral fracture risk prediction.