Disc prolapse: evidence of reversal with repeated extension
Numerous studies have investigated the loading mechanisms necessary to cause disc failure. Collectively, this work suggests that repeated forward bending causes stresses both in the nucleus and in the anulus resulting in prolapse and herniation. Repeated extension is a treatment used by manual therapists as it is thought to assist in returning the displaced portion of the nucleus back towards the center of the disc. This study was designed to enhance understanding of this possible mechanism.
McKenzie proposed that the direction of spine movement that centralizes radiating symptoms precisely corresponds with the direction in which a portion of the nucleus has abnormally migrated. Further, successful centralization is dependant on a hydrostatically intact nucleus that is contained within the outer anulus. Donelson et al reported that patients, who could not achieve centralization of symptoms as a result of repeated movements, did not respond well to conservative therapy and generally had a poor treatment outcome. Subsequently, Donelson et al investigated the theory that centralization is dependent on a competent anulus (the outer border not breached) by investigating the correlation of the McKenzie classification of the symptom response to movement, to whether or not the anulus was competent, as determined by discogram. Ninety-one percent of those that centralized had an intact anulus suggesting possible grounds for this component of the McKenzie theory.
From a biomechanical perspective, the McKenzie explanation seems possible. Flexion postures cause an increase in the hydraulic stress (flow-related) on the posterior anulus, and a large increase in the in vivo nuclear pressure (static). Supporting this argument, Aultman et al, repeatedly flexed specimens where the flexion axis was moved 30° to the left of the sagittal plane. Herniations were developed in the right postero-lateral portion of the disc. Thus, the site of the nucleus breach of the inner anulus was determined by the bending axis, and subsequent stress distribution, a finding also reported by Tsantrizos et al. Tampier et al further elucidated the herniation process by documenting the formation of small clefts in between the layers of the anulus through which the nucleus pulposus was pumped. In this way, the herniation progressed layer by layer as the anulus fibers delaminated to allow flow through small separations between anulus collagen fibers.
Adams investigated the effects of extension bending on healthy lumbar intervertebral disc and found that 2° of extension increased the maximum compressive stress within the posterior anulus by an average of 16%, compared with the neutral posture, in healthy intervertebral disc. However, in degenerate spines, the results were more variable. In 7 of 19 degenerated specimens extension caused a reduction, of up to 40%, in the maximum compression in the posterior anulus, whereas in the other degenerate discs, the compression was increased by 43% relative to the neutral posture. Adams suggested that the variability in the stress gradient across the disc created by extension may be a reasonable explanation for the variance in the success or failure of the McKenzie approach in discogenic patients.
The objective of this study was to create in vitro disc prolapse in the discs of spine motion segments and then evaluate the effects of extension movements on the position of the displaced portion of the nucleus in the anular layers. It was hypothesized that (1) repeated motion, opposite to the motion that caused the disc to prolapse, would reverse the position of the displaced portion of nucleus; and (2) discs that would not respond to reversal testing would have a portion of the nucleus displaced circumferentially in the anulus or have a full herniation, with the displaced portion of the nucleus breached through the outer anulus.
Repeated pure or combined extension after disc prolapse was found to redirect the displaced portions of the nucleus back to the central part in a number of discs. The fact that not all specimens responded seems to match clinical observation that the McKenzie approach can be effective with some patients with prolapsed discs but not with others. The disc height loss after the failure procedure distinguished between specimens that responded to reversal testing and those that did not respond.
This study showed that, in a number of spines, a displaced portion of nucleus could be directed back towards the center of the disc in response to particular active and passive movements or positions. This change in position of the tracked nucleus is the mechanism thought to underlie the success of the McKenzie derangement approach in specific individuals. The porcine cervical spine has been shown to be a good geometric, anatomic, and functional surrogate for human lumbar spines. This study sheds light on a theory that has been unsubstantiated in physiotherapy for over 20 years. We propose that the discs in our study that had greater disc height was more likely to reverse as greater extension of the segments could occur before the facet joints bring the range to a halt and also the stress in the posterior anulus was compressive rather than tensile. The increase in stiffness of the specimens, a measure of the damage of the intervertebral disc, did not correlate with the disc height loss nor did it distinguish between the categories of specimen behavior. The change in stiffness levels was similar to that reported by Callaghan and McGill and Drake et al both of which used comparable models and testing parameters. The increase in stiffness of in vitro specimens is usually explained to result from a change in the contact points across the segment associated with a decrease in disc height. However, the displaced portion of the nucleus within the posterior anulus may alter the torque required to rotate the specimen rather than change the contact points. The next questions are whether the returned nucleus material is able to form a plug in the clefts between layers, and splits in the collagen, of the anulus and also what are the optimal extension regimes, in terms of static postures or repeated dynamic motions that assist in returning tracked nucleus.
It is concluded that with repeated flexion, in porcine cervical spines, disc prolapse was initiated and that the displaced portion of nucleus can be directed back towards the center of the disc in response to particular active and passive movements or positions as illustrated with extension back pain exercises.