Research Review By Dr. Jeff Muir©


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Date Posted:

April 2021

Study Title:

Potential mechanisms for lumbar spinal stiffness change following spinal manipulative therapy: a scoping review


Jun P, Pagé I, Vette A & Kawchuk G

Author's Affiliations:

Faculty of Rehabilitation Medicine, University of Alberta, Edmonton, Alberta, Canada; Department of Mechanical Engineering, University of Alberta; Department of Physical Therapy, University of Alberta

Publication Information:

Chiropractic & Manual Therapies 2020; 28: 15.

Background Information:

Low back pain (LBP) is a leading cause of musculoskeletal pain and disability, although research shows that in the majority of cases, the specific nociceptive source of LBP cannot be accurately identified (1, 2). Spinal manipulative therapy (SMT) is one of the most common treatment modalities for LBP (3-5), with recent studies observing alterations in spinal biomechanics in patients who received SMT for LBP and subsequently reported improvements in disability (6-8). These so-called “SMT responders” displayed several characteristics following treatment, including local biomechanical changes, rapid decreases in bulk stiffness, improvements in muscle contraction and increased disc diffusion (immediately) post-SMT.

These observations lead to questions about how to explain the effect of SMT on spinal stiffness and thus LBP in some, but not all, patients. In order to help explain this observation, the authors sought to review the literature for evidence that would offer mechanistic explanations for the changes in spinal stiffness noted following SMT. To do this, they conducted a scoping review to identify existing research and provide details for potential avenues for further research.

Pertinent Results:

Eligible Studies:
10 studies were eligible for full-text review, of which 3 recruited patients with LBP; 2 recruited healthy participants; 2 recruited participants both with and without LBP; and 3 employed animal models.

The application of SMT varied amongst studies and included: SMT or mobilization (SM) at the therapist’s discretion; SMT and SM at L4/5; lumbosacral SMT delivered by a clinician or a mechanical device; SM at L3 or L4; or SMT delivered to the most painful spinal area.

The authors were unable to identify descriptions of mechanisms that adequately explain the observed changes in spinal stiffness following SMT. They did, however, identify several themes, including:
  1. Change in muscle activity,
  2. Increase in mobility,
  3. Decrease in pain,
  4. Increase in pressure pain threshold,
  5. Change in spinal tissue behaviour,
  6. Central nervous system (CNS) changes, and
  7. Vertebral dysfunction correction.
1) Change in muscle activity:
6 studies suggested changes in muscle activity as an explanation for changes in spinal stiffness. While the specific changes in activity varied, one study (7) showed that SMT responders display both a decrease in spinal stiffness and an increase in lumbar multifidus (LM) thickness ratio following SMT and that these changes are negatively associated. Another study (6) found that increases in LM recruitment were common amongst SMT responders.

2) Increase in mobility:
6 studies tied changes in spinal stiffness to changes in mobility, 4 of which suggested that changes in spinal stiffness result in increased mobility, including one study (9) that observed a significant increase in lumbar ROM and a significant decrease in stiffness. Conversely, 2 other studies suggested instead that changes in mobility cause changes in spinal stiffness. One study (10) found no significant association between stiffness and mobility but suggested that mobility increases were a potential mechanism explaining the observed effect of SMT.

3) Decrease in pain:
5 studies reported a decrease in pain as a potential mechanism for change in stiffness, although 2 reported that changes in stiffness were responsible for decreased pain, while 3 reported the opposite: that decreased pain was responsible for decreased stiffness. Specifically, mechanoreceptor stimulation during PA mobilization was suggested as a possible mechanism (9). There was little consensus on the cause-and-effect relationship between pain and stiffness, however.

4) Increase in pressure pain threshold:
1 study (9) suggested that SMT induces an increase in pressure pain threshold (that is, reduced pain sensitivity), thus leading to a decrease in stiffness. Two animal studies observed that an increase in the applied force following SMT was indicative of increased tolerance to pressure, although they did not observe any change in stiffness post-treatment.

5) Changes in spinal tissue behaviour:
Various changes in spinal tissue behaviour were suggested as possible causal mechanisms for mitigating spinal stiffness. 1 study (11) suggested that changing viscoelastic properties caused by insufficient fluid recovery were responsible for stiffness, while another (12) suggested that repeated loading of the spine caused creep and relaxation of the spinal connective tissues. Others (7) suggested that intervertebral disc diffusion following SMT noted in SMT responders was possibly the causal mechanism.

6) Changes in CNS or reflex pathways:
Changes in sympathetic nervous system activation or activation of descending inhibitory mechanisms were suggested to lead to altered stiffness in 1 study (9), while another suggested that facilitation of postsynaptic alpha motoneuron and cortical motoneuron activity might play an integral role (6).

7) Correction of vertebral dysfunction:
1 study (13) suggested that increased spinal stiffness was an indicator of spinal dysfunction but did not discuss a potential causal mechanism in their study.

Clinical Application & Conclusions:

The authors found that, beyond their identification of 7 specific themes explaining the possible mechanism associated with changes in lumbar stiffness following SMT, they were not able to identify sufficient data to come to any substantial conclusions. The possible mechanisms discussed in this paper all warrant further study, as we attempt to scientifically unravel the mechanism(s) of action of spinal manipulation.

Study Methods:

The authors searched the following databases: MEDLINE, CINAHL, and PubMed. Relevant search terms included MeSH terms related to SMT (including spinal manipulation and mobilization), spinal stiffness, lumbar spine, and mechanism. The search period was the last 20 years (from January 1st, 2000 to June 10th, 2019), owing to the relative newness of the area of spinal mechanics and SMT responders.

Study Eligibility:
2 authors independently screened relevant articles, with a third reviewer independently screening citations where consensus could not be reached.

Specific eligibility criteria included:
  • Published in English between January 1st, 2000 and June 10th, 2019, and
  • Quantified lumbar spinal stiffness before and after SMT.
Studies were excluded if they did not use SMT as an intervention or if they only measured cervical spinal stiffness.

Data Extraction:
Specific sentences explaining or speculating about causal mechanisms were extracted from each eligible article. Relevant phrases included: “may be due / related / associated / explained / modified”, “is believed”, “was hypothesized”, “may permit / allow / result / modify / increase / decrease/ facilitate / inhibit / explained” or “have been reported / shown to”. Sentences also implying a causal effect were also extracted. Extracted sentences were pooled and categorized based on specific themes identified by the reviewers.

Study Strengths / Weaknesses:

  • The broad inclusion criteria provided a varied, general pool of information.
  • Their comprehensive review of broadly extracted sentences allowed for creation of several relevant and distinct themes.
  • Only 3 databases were searched for eligible articles.
  • Variations in methods for measuring spinal stiffness existed across included studies studies.
  • Variations with treatment (SMT vs. mobilization) and varying inter-practitioner reliability.
  • Small number of eligible studies and heterogeneity the studies that were incuded.

Additional References:

  1. Hartvigsen J, Hancock MJ, Kongsted A et al. What low back pain is and why we need to pay attention. Lancet 2018; 391: 2356–67.
  2. Haldeman S, Kopansky-Giles D, Hurwitz EL et al. Advancements in the management of spine disorders. Best Pract Res Clin Rheumatol 2012; 26: 263–80.
  3. Rubinstein SM, De Zoete A, Van Middelkoop M et al. Benefits and harms of spinal manipulative therapy for the treatment of chronic low back pain: systematic review and meta-analysis of randomised controlled trials. BMJ 2019; 364: l689.
  4. Foster NE, Anema JR, Cherkin D et al. Prevention and treatment of low back pain: evidence, challenges, and promising directions. Lancet 2018; 391: 2368–83.
  5. Oliveira CB, Maher CG, Pinto RZ et al. Clinical practice guidelines for the management of non-specific low back pain in primary care: an updated overview. Eur Spine J 2018; 27: 2791–803.
  6. Fritz JM, Koppenhaver SL, Kawchuk GN et al. Preliminary investigation of the mechanisms underlying the effects of manipulation: exploration of a multivariate model including spinal stiffness, multifidus recruitment, and clinical findings. Spine 2011; 36: 1772–81.
  7. Wong AYL, Parent EC, Dhillon SS, Prasad N, Kawchuk GN. Do participants with low back pain who respond to spinal manipulative therapy differ biomechanically from nonresponders, untreated controls or asymptomatic controls? Spine 2015; 40: 1329–37.
  8. Koppenhaver SL, Fritz JM, Hebert JJ et al. Association between changes in abdominal and lumbar Multifidus muscle thickness and clinical improvement after spinal manipulation. J Orthop Sport Phys Ther 2011; 41: 389–99.
  9. Shum GL, Tsung BY, Lee RY. The immediate effect of Posteroanterior mobilization on reducing Back pain and the stiffness of the lumbar spine. Arch Phys Med Rehabil 2013; 94: 673–9.
  10. Stamos-Papastamos N, Petty NJ, Williams JM. Changes in bending stiffness and lumbar spine range of movement following lumbar mobilization and manipulation. J Manip Physiol Ther 2011; 34: 46–53.
  11. Edgecombe TL, Kawchuk GN, Long CR, Pickar JG. The effect of application site of spinal manipulative therapy (SMT) on spinal stiffness. Spine J 2015; 15: 1332–8.
  12. Allison G, Edmonston S, Kiviniemi K et al. Influence of standardized mobilization on the posteroanterior stiffness of the lumbar spine in asymptomatic subjects. Physiother Res Int 2001; 6: 145–56.
  13. Haussler KK, Hill AE, Puttlitz CM, McIlwraith CW. Effects of vertebral mobilization and manipulation on kinematics of the thoracolumbar region. Am J Vet Res 2007; 68: 508–16.