Functional Anatomy of the Disc and Lumbar Spine

Abstract

The lumbar spine is designed to provide axial rigidity to the lower trunk, to sustain axial compression loads exerted from the trunk and upper limbs, and to permit certain movements between the trunk and pelvis. The vertebral bodies and intervertebral discs are responsible for sustaining axial compression loads, and permit rotatory and translatory movements in all directions. The lumbar posterior elements are responsible for controlling these movements. The intervertebral disc sustains compression through the annulus fibrosus. The nucleus pulposus serves to brace the annulus, thereby preventing it from buckling under load. The disc accommodates rotatory movements by deforming under eccentric pressure. It resists translatory movements by tension developed in the annulus fibrosus. The annulus is weakest in resisting axial rotation. The disc is protected from excessive axial rotation by the zygapophysial joints, which also resist anterior shear. Muscles acting on the spinous and transverse process control the flexion and extension movements exerted on the lumbar spine. Prosthetic discs need to be designed and used with an appreciation not only of the compression functions of the disc but also of the often subtle movements that the disc accommodates in flexion and in axial rotation.