BiomechanicsDegenerative mechanics of the lumbar spine
Introduction
Degeneration of the spine is a prevalent problem that generally advances with age, although its occurrence is not restricted to the elderly. Degeneration has been linked to low back pain; however, the exact relationship between the two remains unknown [1]. Mechanical property changes resulting from degeneration are also likely contributors to lumbar spine instability that lead to other pathologies and can be accelerated by injuries or deformities [2].
Each level of the spine is a three-joint complex consisting of one intervertebral disc and two facet joints, with complex load sharing between the three joints [3]. Degenerative changes of the spine can include disc degeneration, facet joint osteoarthritis, vertebral body degeneration and ligament degeneration [4]. Initial degenerative changes in the lumbar spine most commonly occur within the intervertebral disc and, more specifically, within the nucleus pulposus [5]. Disc degeneration and facet joint osteoarthritis are usually concomitant, but studies have found that disc degeneration generally precedes facet joint osteoarthritis [6], [7].
The goal of this review is to discuss the effect of disc degeneration on the biomechanical behavior of the spine, specifically, the load-bearing and kinematic behavior after degeneration. The other components of spinal degeneration are not addressed in this paper. To fully investigate the effects of disc degeneration on load-bearing and kinematics, one must understand the process of disc degeneration as well as possess knowledge of the normal intervertebral disc behavior. The following two sections provide a brief background on degeneration and the succeeding sections focus on load-bearing and kinematic behavior of the degenerated spine compared with that of the normal healthy spine.
Section snippets
Background
The three functional roles of the spine are load-bearing, provision of movement, and protection of neural elements [3]. First, the spine must support the weight and resulting bending moments of the head and upper torso and transfer the load to the pelvis. Second, the spine must allow motion between the torso and pelvis. Finally, the spine must protect the spinal cord and nerve roots from damage. Disc degeneration directly affects the first two roles of the spine. Because these three fundamental
Intervertebral disc degeneration
The exact cause leading to degenerative changes within the disc is not clearly understood, but one important mechanism is a decrease in the nutrition to the disc [5]. This mechanism largely depends on the vertebral end plates, which provide the primary source of nutrition for the intervertebral disc [8], [9]. As degeneration progresses, fissures, fractures and clefts form in the end plate, and the overall result is a thinner end plate [10], [11], [12]. A loss of water volume within the nucleus
Load bearing
The lumbar spine transmits loads between each intervertebral level through the intervertebral disc and two facet joints. Studies show that in compression, the load is transmitted primarily through the anterior column, whereas the facet joints support about 18% of the compressive load in the segment [10]. The percentage of load transferred through the posterior elements is highly dependent on spinal posture [17] and likely also the degree of disc degeneration. The magnitude of load, in general,
Kinematics
Kinematic behavior of the spine is nonlinear and can be characterized by two distinct phases. The first, coined the neutral zone (NZ), occurs within small motions from a neutral position and where the spine deforms easily with little resistance when a small load is applied. The second phase, termed the elastic zone, is characterized by an increase in stiffness as the load increases [3]. Because of the geometry of the adjoining vertebrae and presence of connecting ligaments, the motion of the
Summary
Degenerative changes in the intervertebral disc significantly affect load-bearing and kinematic patterns on the lumbar spine. In compression, disc degeneration decreases the load transferred through the nucleus and increases the load transferred through the annulus fibrosus, whereas in shear there is no significant change in loading patterns. The proportion of load that is transmitted through the posterior elements also increases with advancing degeneration. The overall changes in motion
Acknowledgment
We thank Mr. Juay Seng Tan for his suggestions and valuable insight.
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