Physical Medicine and Rehabilitation: State of the Art Reviews
Vol. 13, No. 3, October 1999. Philadelphia, Hanley & Belfus, Inc.
ANATOMIC AND BIOMECHANICAL PRINCIPLES OF THE LUMBAR SPINE
by Dennis M. Lox, MD
A thorough comprehension of the pathophysiology of the lumbar spine anatomy and biomechanical functions is necessary to completely evaluate complaints of lower back pain. Potential nociceptor-generating sites include the anterior and lateral annulus fibrosus (excluding the nucleus pulposus), the anterior and posterior longitudinal ligaments, the facet joint capsule, the suprasspinous and infraspinous ligaments, the sacroiliac joint, and the periosteum.1 Other potential pain generator sites include muscle, fascia. Ligaments, tendons, nerves, and dural membranes.2 Understanding the effect of the degenerative cascade,3,4 inflammatory and chemical responses,5-17 and interrelationship between psychosocial factors and the importance of the psyche in the perpetuation of chronic lower back pain is essential in the diagnosis and elucidation of patients’ complaints of pain.18-24
The Three-Joint Complex
Historically, the subcomponents of the spine have been poorly understood. It was not until 1911 that Goldthwait indentified the lumbar facet joints as a potential cause for low back pain. 25 Mixter and Barr uncovered the intervertebral disc as a source of pathology in 1934. 26 Schmorl and Junghanns first introduced the concept of the motor segment, which later was referred to as the functional spinal segment of motion segment. 27
The intervertebral joint is made up of three separate components, hence the reference to the three-joint complex. The motion segment is the smallest segment that contains all the subcomponents of the spine that the rest of the spine contains. It also demonstrates the same characteristics of adjacent level lumbar vertebral segments.
Figure 1. The pathologic changes in the posterior joints (left column) and in the disc (right column) that lead to the changes in the three-joint complex (center column). (Adapted from Kirkaldy-Willis WH: Managing Low Back Pain, New York, Churchill Livingstone, 1988.)
The intervertebral disc acts through a compressive or hydraulic-like mechanism. It is a circumferential joint that reacts to anterior and posterior forces through compression and translation and is maintained by the stability of the posterior elements (zygaphphyseal) or facet joints. The posterior facet joints function as hinge-type joints to maintain stability in a circumferential fashion. Factors that affect the intervertebral disc will ultimately affect the posterior elements, and likewise, factors influencing the posterior elements will have an effect on the intervertebral disc. Rotational and shear forces on the intervertebral disc will result in a long-term impact on the resultant counterpart.28,29 This may result in a slow degenerative process and ultimately lead to a degenerative cascade. 30-32
As the normal process of aging occurs, or with traumatic injury, the nucleus pulposus becomes desiccated or dehydrated, which may result in circumferential tears of the annulus fibrosus.33 The intervertebral discs may begin to bulge as the annulus becomes more lax. If complete disruption of the annular fibers occurs, herniation of the nucleus pulposus can occur; however, if the annulus remains intact, then the nucleus pulposus may continue to degenerate,
leading to a loss in intervertebral disc space height. The progressive collapsing of the intervertebral disc alters the structural arrangement of the posterior elements of zygapophyseal joints resulting in increased stress and weight bearing on the synovial of the zygapophyseal joints. This results in synovitis, and ultimately, degeneration of the joint capsule may occur.34 As this process continues, osteophytes may develop posteriorly along the vertebral and plates associated with the disc along the posterior elements with an associated hypertrophy of the ligamentum flavum (Fig. 1). This process may progress along may pathways, including a benign stable course, or progress to osteophytosis leading to narrowing of the lateral recesses or central can ( Figs. 2 and 3).
Figure 2. The spine cut in the midsagittal plane and viewed from the middle showing dynamic lateral stenosis at the unstable L4-L5 three-joint complex. In the unstressed state, there is no narrowing (A), but with rotation, there is narrowing of the lateral recess and pinching of the fifth spinal nerve (B). (From Kirkaldy-Willis WH: Managing Low Back Pain. New York, Churchill Livingstone, 1988; with permission.)
If significant dysfunction occurs at the particular segment, abnormal motion may occur because of ligamentous laxity, and joint disruption and segmental instability may follow (Fig. 4). If significant joint disruption occurs, degenerative spondylolithesis can result as laxity allows anterior or posterior slippage to occur (Figs.5 and 6). Multilevel involvement arises as the process continues on a degenerative cascade (Fig.7).4 The associated soft tissues structures surrounding the three-joint complex including capsular joints, ligament, and muscles all function interactively to aid in motion and strength of each segment. Studies evaluation individual joint stiffness, load failure, and response to varied surgical procedures have been reported. 35
Figure 3. Fixed lateral stenosis results from osteophytic enlargement of the superior facets, and nerve compression does not change with alterations of position. (From Kirkaldy-Willis WH: Managing Low Back Pain. New York, Churchill Livingstone, 1988; with permission.)
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