Abstract
BackgroundSpinal manipulation (SM) has been shown in a systematic review to have a statistically significant effect on the pressure pain threshold (PPT) in asymptomatic subjects, when SM is compared to a sham intervention. The magnitude and duration of this effect is unclear.ObjectivesTo determine the effect-size of SM in asymptomatic subjects and its duration.MethodThis is a secondary analysis of data from a previous review. We sought to compare the effect-sizes in the various articles but had to calculate them ourselves, at different follow-up time measurements. Effect-sizes (Cohen’s d or Hedge’s g coefficient) were considered low, medium, and large, at the cut points of 0.2, 0.5, and 0.8, respectively.ResultsEffect-sizes were reported in 6/8 studies, but all had calculated ‘within-group’ changes, not ‘between-group’ differences. Immediately after SM, only one study of four (with four measurements) had a statistically significant ‘medium’ effect size (d = 0.56; 95% CI: 00.4–1.08 to d = 0.70; 95% CI:0.18–1.22). Five minutes after SM, 4/5 studies found a statistically significant ‘medium to large’ effect-size (d = 0.51; 95% CI: 0.04–0.98 to d = 1.24; 95% CI: 0.28–2.20). Ten minutes after SM, two studies reported a ‘medium’ effect-size with statistical significance (d = 0.58; 95% CI: 0.11–1.05 to d = 0.80; 95% CI: 0.12–1.48). We drew no conclusions for the effect-sizes at one minute and thirty minutes after SM, as no between-group statistical difference was found.ConclusionAuthors need to revise their approach to ‘effect size’. Our calculations showed that the effect-size of SM on PPT may go from ‘medium’ to ‘large’ within the first five minutes but appears to diminish again within ten minutes. Research of this type should collect information for longer periods and compare results to other interventions to put results into perspective.
Highlights
The effect of treatment In order to study the validity of a treatment, it is necessary to compare its outcome with the outcome of another treatment
Our calculations showed that the effect-size of Spinal manipulation (SM) on pressure pain threshold (PPT) may go from ‘medium’ to ‘large’ within the first five minutes but appears to diminish again within ten minutes
Design This work consists of a secondary analysis of data from our previous systematic literature review, using data from eight randomized controlled trials that reported the regional effect of spinal manipulation on PPT in asymptomatic subjects compared to a sham procedure [8]
Summary
The effect of treatment In order to study the validity of a treatment, it is necessary to compare its outcome with the outcome of another treatment. In order to test the specific effect of this treatment, it would be necessary to compare it with a well masked placebo. If the baseline measurements in the two groups is more or less identical, there is no need to take the baseline measurements into account, and it would be sufficient to test only if the difference between the outcome measurements is statistically significant. The clinical significance can be judged by comparing the estimates of the treatment and placebo groups and by calculating, for example, the numbers needed to treat, in clinical studies. Spinal manipulation (SM) has been shown in a systematic review to have a statistically significant effect on the pressure pain threshold (PPT) in asymptomatic subjects, when SM is compared to a sham intervention.
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