The authors of a clinical commentary 1 stress the necessity of a unifying framework in which to explore issues of spinal stability—one that broadens the current definition to include nonmechanical sources of pain and functionality. Seeking to expand the understanding of spine stability as it relates to lumbar back pain (and, by extension, pelvic girdle and sacroiliac joint pain), the authors stressed the relative lack of clinical knowledge as it pertains to the delicate interactions between neurofunctions and biomechanical behaviors of the lumbar spine.
What’s at Stake
The definition of spinal stability (or instability) has traditionally been (and is currently) framed in terms of static mechanical behaviors and observations. But recent advancements in diagnostic technology reveal the complexity of spinal functionality, which includes neural and mechanical interactions between the central nervous system and spine that appear to be designed to regulate segmental stability in response to activity involving spinal motion.
The Evolution of the Concept of Spinal Stability
The historical framework on the issue of spinal stability posits that the body’s weight works as a destabilizing influence on the spinal column, while the relative stiffness of the muscles and tissues surrounding the spine provide a stabilizing influence. The traversing muscles of the upright spine increase their levels of stiffness in response to trunk activity. It is assumed that approximately 2 percent activation is required to support the upright spine.
An observation of degenerative spine changes may be inefficient on its own in the diagnosis of lower back pain and stability because it does not consider the quality of trunk musculature and control, which could counterbalance (or contribute to) spinal deficits. This is also true regarding join laxity or hypermobility, which should be considered in relation to muscle control.
Comprehensive analysis of the spine system is therefore necessary to accurately diagnose spinal deficiencies, which is why diagnosticians expanded the historical spinal conceptual framework to include loading events and how delayed trunk muscle reflexes could contribute to back injuries or pain. In this framework, the timing of muscle activation was an important element in maintaining spinal stability and avoiding back injuries.
Newer spinal models included the components of the spine/trunk, as well as neurological or sensorimotor pathways that influence spinal stability and motor behaviors, and a new emphasis was placed on the importance of neural control in avoiding spinal instability.
The Role of Spinal Stability in Back Pain
Research suggests the central nervous system may have an integral role in monitoring spinal stability, but the underlying mechanisms at play in this relationship are unclear. There is an evident relationship between the varying activation levels of the trunk muscles and spinal stability during loading, fatigue, lack of sensory input, a reduction in spinal stiffness post-flexion, exertion, lifting, and managing unstable loads. Conscious or unconscious neuromuscular controls appear to be modulating the rate of muscular activation to avoid spinal instability. The trunk muscles, while capable of activating independently, may be interconnected through thickly layered fascia across multiple spinal segments and extra muscular fascia. This creates biomechanical coupling that helps increase spinal control by dispersing applied spinal forces.
While the neural and mechanical coupling can support and protect a healthy spine, they may contribute to a lack of control in a damaged spine, as the joints could become lax across many levels with degenerative disc disease, injury, or spondylolisthesis. This can create segmental hypermobility that affects muscle control, which could contribute to further instability. In cases of extreme back pain and degenerative lumbar spine problems with fatty infiltration and fibrosis, for example, some deeply segmented muscles, including the multifidus, may atrophy, creating reflex or neural inhibition that can further undermine spinal stability. The timing and ratio of muscle activation across the spectrum of deep-to-superficial levels is integral to spinal stability, meaning a combination of joint hypermobility, reflex inhibition, and muscle wasting could undermine spinal control and stability in lower back patients. Therefore, increasing trunk muscle mechanical stiffness through coactivation is a commonly used strategy to help protect the spine from instability in healthy and lower back pain afflicted individuals.
While many back-pain patients coactivate trunk muscles in an attempt to avoid spinal instability, contractions of 2-5 percent above normal may contribute to muscle pain and spinal fatigue, setting the stage for further instability. In fact, lower back pain patients appear to have more fatigable paraspinal muscle fiber types than healthy individuals, which reduce maximum muscular capacity and contribute to spinal instability. It is possible that these patients might resist fatigue by strengthening their paraspinal muscles.
Advancements Lead to Improved Spinal Assessments
Spinal Imaging advancements are helping diagnosticians in the assessment of spinal kinematics and a systems-based approach to the examination of spinal stability. Other clinical measures include radiographies, ultrasounds, manual testing, and questionnaires, though no single method provides a framework in which to consider spinal stability from a multidimensional perspective. While single-plane digital videoflouroscopy and 2 digital videoflouroscopic systems provide a limited view of spinal kinetics, the authors of this commentary hope for improved clinical diagnosis through better spatial resolution that allows the examination of spinal movement with lateral flexion/extension radiographs.
The issue and understanding of spinal stability continue to evolve. Studies indicate the central nervous system is integral in monitoring and responding to spinal stability. While neural and mechanical coupling help keep healthy spines in check, they can contribute to segmental instability in injured or degenerating spine segments by inducing joint laxity, neural inhibition, muscle weakness, and a reduction in the capacity to generate adequate muscular force. They may also contribute to fatigue, increased tissue loading, and back pain.
Future research should make use of recent scientific and technological diagnostic advancements to expand upon the current understanding and framework of spinal stability. To be effective, this forward-thinking strategy might include reconsidering the very term, “spinal instability” to incorporate the matrix of neuronal, musculature, and biomechanical influences that provide a more complete framework into understanding the issue.