Musculoskeletal Analysis of Spine with Kyphosis Due to Compression Fracture of an Osteoporotic Vertebra

Abstract

Mechanical stress analysis of vertebral bone is required to determine compressive strength of vertebra for osteoporosis patients in clinical situation of orthopedics. Stress occurred in vertebrae depend on condition of musculoskeletal system of spine. Spine structure is mainly constructed of many vertebrae and flexible intervertebral disks, and it is unstable by itself under standing condition. Supporting by erector muscles and ligaments keep the spine standing condition, so that loading condition of each vertebra depend on the muscle and ligament forces. Biomechanical simulation of musculoskeletal model taking into account of muscle and ligament forces and intervertebral joint forces is necessary to determine the loading condition for vertebral stress analysis. Commercial software to deal with the musculoskeletal system, e.g. AnyBody Modeling System (AnyBody Technology Inc.), is available nowadays, and it has been applied to clinical, ergonomic or sport biomechanics problem. Spine kyphosis is often caused by compression fracture of osteoporotic vertebra, because shape of the fractured vertebra is sphenoidal with sharp anterior side of vertebral body. Spine kyphosis makes gravity center of trunk shift to anterior side, and then intervertebral joint moment due to trunk weight increase. Additional compression fracture is concerned at adjoining vertebrae to the fractured one in the kyphotic spine. In this study, musculoskeletal simulation model with spine kyphosis was created by using AnyBody Modeling System considering compensation posture due to kyphosis. Kyphosis patient used to have compensation posture to recover body balance and face up. Intervertebral joint forces and muscle forces of the model were computed, and then influence of the kyphosis on the intervertebral joint force at adjoining joints was discussed comparing between kyphosis and intact model. Furthermore, influence of the compensation posture on muscle forces related to spine was considered.