Evaluation of a Fast-Solving Rigid Body Spine Model Inclusive of Intra-Abdominal Pressure.

Objective: Traditional spine biomechanical models often neglect the load-sharing effect of the intra-abdominal pressure (IAP) on the spine and can be computationally intensive. These limitations hinder their effectiveness in muscle recruitment simulations where iterative calculations are required. Thus, a need exists for validated fast-solving IAP-integrated musculoskeletal lumbar spine models, hence developed herein.

Methods: A rigid-body model consisting of the pelvis, lumbar vertebrae, a lumped thoracic spine and the ribcage, derived from MRI scans of a healthy adult male, was devised. The intervertebral discs were modeled as 3 degrees-of-freedom (DOF) gimbal joints using nonlinear moment-rotation relationships. Spinal ligaments were modeled as nonlinear tension-only springs. Two methods of modeling IAP were discussed and implemented. Model#1 represented IAP as normal force vectors on the diaphragm and the spine, while model#2 idealized the abdominal wall compliance using spring-damper elements inside the cavity. Level-by-level spinal stiffness was validated under pure moment loading up to 7.5Nm in flexion-extension, lateral bending and axial rotation.

Results: Model segmental stiffness profiles in all three bending modes were within one standard deviation of literature datasets. IAP model #1 revealed a linear increase in the spinal extensor torque about L3 with increase in IAP, consistent with literature, while model #2 suggested decreased spinal range of motion with increased abdominal cavity stiffness. The model consisted of 15 DOFs, compiled in 6sec and simulated in 1.4sec.

Conclusion: This MATLAB native model could be a useful tool to quickly and intuitively visualize physiological spine loading.

Significance: A novel fast-solving lumbar musculoskeletal model with IAP was presented in this research.