Welcome to Chiropractic OnLine Today's Journal Corner's Literature Review of
the Month. This month's topic centers on
New Concepts in Spinal Stability and Instability.
Welcome to Chiropractic OnLine Today's Journal Corner's Literature Review of
the Month.
2. Panjabi MM. The stabilizing system of the spine. Part II. Neutral zone and instability hypothesis. J Spinal Disorders 1992; 5(4):390-397.
3. Wilke HJ, Wolf S, Claes LE, Arand M, Wiesend A. Stability increase in the lumbar spine with different muscle groups. Spine 1995; 20(2):192-198.
4. Kaigle AM, Holm SH, Hansson TH. Experimental instability of the lumbar spine. Spine 1995; 20(4):421-430.
5. Cholewicki J, McGill SM. Mechanical stability of the in vivo lumbar spine: implications for injury and chronic low back pain. Clin Biomech 1996; 11:1-15.
The concept of spinal instability has changed in recent years. Previously, instability was described as ligamentous laxity that produced excessive movement in an intervertebral joint at end range. This was thought to be a relatively uncommon condition that would often require surgical fusion to correct.
However, thanks primarily to the work of Manohar Panjabi, PhD, it is now known that stabilization of the spine relates mainly to muscular factors rather than ligamentous and is probably far more common than the ligamentous type. In fact, instability may be the factor that leads to most cases of mechanical back pain. If this is the case, then its correction is essential to the appropriate management of the mechanical spine disorders. The papers reviewed here represent just a portion of the work that is being done in this area. This work has the potential to have a great impact on how we approach our patients.
This first paper by Panjabi (1) sets the stage for the introduction of the new concepts of spinal stabilization and the way that spinal pain may be generated. In it he conceptualizes three subsystems that all contribute to the stability of the spine. These are the passive - ligaments, bones, discs, etc., the active - muscles and tendons, and the nervous system. These all work on an intersegmental, providing stability of the apophyseal joints of the spine. When one subsytem becomes dysfunctional, one of three things can happen: 1) an immediate response from the other subsystems to adapt and compensate successfully, 2) a more long term successful compensatory adaptation, or 3) injury to any component of one or more subsystem.
Each subsystem has the ability to adapt to changes in the environment of the locomotor system as a whole. An important point that Panjabi makes is that a breakdown of the time to peak contraction, or reaction time, of the small intrinsic muscles can cause breakdown of the system and that improving time to peak contraction can enhance stability. This breakdown in the protection against instability is what may ultimately be the primary physical component of low back pain.
In Part II of his article (2) Panjabi talks about the neutral zone as being the essential component of the spine that is affected by instability. The neutral zone is defined as "a measure of spinal laxity in the vicinity of the neutral position." Abnormal increase in the size of the neural zone leads to pain by producing strain on the surrounding supportive passive tissues. The spinal stabilizing system may be signaled as a result of micromovements within the neutral zone. In traumatic situations producing instability, it has been shown that the increase in neutral zone is greater than the increase in gross range of motion.
The third paper in this group is by Wilke, et al (3) out of Germany and in it they showed that muscular factors were the essential components of maintaining the neutral zone within normal limits, thus providing stability for the intervertebral joint. They showed that it is the multifidis in particular that is the most important muscular factor in increasing stability of the motion segment. The multifidis provides greater than 2/3 of the stiffness increase in the L4-5 segment. Also showed that neutral zone is a better measure of stability that ROM.
The study by Kaigle, et al (4) out of Sweden induced instability in various ways in pig spines and analyzed the movement characteristics that resulted. It showed that when muscle contraction is induced through stimulation, the ROM of the unstable segment increases but there was an increase in the stability of the motion segment because the muscle contraction reduced the abrupt patterns of motion in the neutral zone.
They showed that in the neutral zone the joint is more susceptible to instability because the muscles are under reduced tension, so it is not with extreme ROM that one needs to be careful, but rather one needs to be sure that the muscles are prepared for sudden, unexpected movements in the neutral zone by being sure the muscles have a reaction time that prevents such events. Hypermobility as measured by increased ROM radiographically can mistakenly be interpreted as instability.
Finally, Cholewicki and McGill (5) measured muscle reactions with surface EMG in certain tasks and come up with a model for segmental and global spine stability. The muscles analyzed were: rectus abdominis, external and internal obliques, latissimus dorci, thoracic erector spinae, lumbar erector spinae and multifidis.
They showed that the stability of the lumbar spine increased during the most demanding tasks (those that produced the greatest joint compression forces) and diminished during periods of low muscular activity. This argues against the hypothesis that the spine maintains a constant stability safety margin when not involved in vigorous activities. It does show, however, that error in the CNS can lead to spine buckling at heavy loads, via a breakdown of the stabilization system. During very light tasks such as standing, the passive tissues are probably sufficient to maintain stability as this saves energy.
While the large, multisegmental muscles contribute the bulk of the stiffness to the spine, it is the small intersegmental muscles (multifidi and rotatores) that are primarily responsible for maintaining the stability of the spine as a whole. This model showed that the spine will buckle if the activity (stiffness) in the multifidis and lumbar erector spinae are 0, even with substantial forces being contributed by the large muscles. Increasing the activity of the small muscles increases spinal stability. They feel this highlights the importance of the motor control system to coordinate the muscle recruitment between the large and small muscles when handling small loads, which is when many injuries occur (e.g., someone who works at a heavy job all day and then injures their back picking up a pencil).
Cholewicki and McGill feel that this work helps explain why injury to the spine frequently occurs during activities that involve low loads, ie, that at high loads the stability of the increases proportionately, but at lower loads, a fine tuned motor control system is required to maintain stability levels. Without this, the momentary decrease in stability may lead to displacements or irritation to soft tissues that can lead to pain. With inappropriate activation of the intrinsic muscles, particularly the multifidi, there can be overcompensation by the motor control system that suddenly calls for a hard contraction of these muscles beyond their capability. This then leads to strain and injury to these muscles.
These papers point up to the importance of improving the stabilization mechanisms of the spine so that we can maximize the ability of the spine to protect itself from injury. As Cholewicki and McGill state, "Clinicians need to explore motor control training as an adjunct to the muscle strength improvement for reducing low back episodes." My next column will explore motor control training and what the literature is showing that we can do to help train the motor control system.