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Geometric Parameters of the In Vivo Tissues at the Lumbosacral Joint of Young Asian Adults

Study Design. Magnetic resonance images (MRIs) were used to determine the geometry of the tissues studied. Objective. To investigate the geometric parameters of tissues around the lumbosacral joint. Summary of Background Data. Cross-sectional area (CSA), moment arms, and line of action of the tissues located at lumbosacral joint (L5–S1), which are important in the study of spinal mechanics and models for the investigation of low back pain. It was insufficient information to describe the relative dimension of spinal tissues except muscles around the lumbar spine. Methods. MRIs from eight asymptomatic young Taiwanese male adults were collected to present CSA, moment arms, and line of action of the tissues located at lumbosacral joint (L5–S1). Four pairs of trunk muscles, erector spinae, rectus abdominis, abdominal oblique externus, and psoas muscles, ligamentum flavum, and facet joints at the bilateral sides were studied. Results. Spinal tissues at the level of lumbosacral joint were determined. The CSAs of the spinal elements at the L5–S1 normalized by the CSA of trunk ranged from 5.42% (the erector spinae) to 0.14% (the ligamentum flavum). The moment arm of the spinal elements relative to the trunk width and depth ranged from 40.91% (rectus abdominis at the y direction) to 0.38% (ligamentum flavum at the x direction). The profile of geometric elements of lumbosacral joint in the Asian male subjects was similar to the data collected from the white population. The data also showed that right–left symmetry in the aforementioned dimension. Conclusion. Normalized data of the CSA, moment arm, and line of action of the spinal tissues at the L5–S1 joint were reported in vivo through the MRI techniques. The profile of geometric elements of lumbosacral joint in the Asian male subjects did not differ from the white population. In addition to muscles, the geometry of facet joint and ligament was determined, which would be important to the calculation of force distribution on the lumbosacral joint.