The mechanisms thorough which spinal manipulative therapy (SMT) exerts its alleged clinical effects are not well established. A new study investigated the effects of subject expectation on clinical outcomes.
Sixty healthy subjects underwent quantitative sensory testing to their legs and low backs. They were randomly assigned to receive a positive, negative, or neutral expectation instructional set regarding the effects of a spe cific SMT technique on pain perception. Following the instructional set, all subjects received SMT and underwent repeat sensory tests.
No inter-group differences in pain response were present in the lower extremity following SMT. However, a main effect for hypoalgesia was present. A significant interaction was present between change in pain perception and group assignment in the low back with participants receiving a negative expectation instructional set demonstrating significant hyperalgesia.
The authors concluded that this study provides preliminary evidence for the influence of a non- specific effect (expectation) on the hypoalgesia associated with a single session of SMT in normal subjects. We replicated our previous findings of hypoalgesia in the lower extremity associated with SMT to the low back. Additionally, the resultant hypoalgesia in the lower extremity was independent of an expectation instructional set directed at the low back. Conversely, participants receiving a negative expectation instructional set demonstrated hyperalgesia in the low back following SMT which was not observed in those receiving a positive or neutral instructional set.
More than 10 years ago, we addressed a similar issue by conducting a systematic review of all sham-controlled trials of SMT. Specifically, we wanted to summarize the evidence from sham-controlled clinical trials of SMT. Eight studies fulfilled our inclusion/exclusion criteria. Three trials (two on back pain and one on enuresis) were judged to be burdened with serious methodological flaws. The results of the three most rigorous studies (two on asthma and one on primary dysmenorrhea) did not suggest that SMT leads to therapeutic responses which differ from an inactive sham-treatment. We concluded that sham-controlled trials of SMT are sparse but feasible. The most rigorous of these studies suggest that SMT is not associated with clinically relevant specific therapeutic effects.
Taken together, these two articles provide intriguing evidence to suggest that SMT is little more than a theatrical placebo. Given the facts that SMT is neither cheap nor devoid of risks, the onus is now on those who promote SMT, e.g. chiropractors, osteopaths and physiotherapists, to show that this is not true.