Estimation of force and extensor moment contributions of the disc and ligaments at L4-L5.

An anatomic-mathematical representation of the individual ligaments and disc at L4-L5 was used in a model to examine their role in joint behavior. Geometry of the anatomic components was obtained from cadaveric dissection and the anatomic literature; mechanical properties were obtained from various literature sources. Resistance to flexion of the L4-L5 joint was simulated using various ligament cross-sectional areas, and combined loading modes of compression coupled with bending in an attempt to understand physiologic conditions that may occur during activities such as lifting. Results suggest that the interspinous ligament sustains the greatest stress during flexion at L4-L5, is the dominant component in resistance to flexion, and also creates large shear components that must be resisted by the facet complex. Simulated compressive loads of 6300 N were noted to allow an additional 2 degrees of flexion at the joint.