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 7.5 Nm 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.