Efficacy of osteopathic manipulative treatment
The overall results clearly demonstrate a statistically significant reduction in low back pain with OMT (Figure 2). Further, the meta-analysis results are quite robust as indicated by the comprehensive sensitivity analyses (Figure 3). Stratified meta-analyses to control for moderator variables demonstrated that OMT significantly reduced low back pain vs active treatment or placebo control and vs no treatment control. If it is assumed, as shown in a review , that the effect size is -0.27 for placebo control vs no treatment in trials involving continuous measures for pain, then the results of our study are highly congruent (ie, effect size for OMT vs no treatment [-0.53] = effect size for OMT vs active treatment or placebo control [-0.26] + effect size for placebo control vs no treatment [-0.27]).
It has been suggested that the therapeutic benefits of spinal manipulation are largely due to placebo effects . A preponderance of results from our sensitivity analyses supports the efficacy of OMT vs active treatment or placebo control and therefore indicates that low back pain reduction with OMT is attributable to the manipulation techniques, not merely placebo effects. Also, as indicated above, OMT vs no treatment control demonstrated pain reductions twice as great as previously observed in clinical trials of placebo vs no treatment control . Thus, OMT may eliminate or reduce the need for drugs that can have serious adverse effects .
Because osteopathic physicians provide OMT to complement conventional treatment for low back pain, they tend to avoid substantial additional costs that would otherwise be incurred by referring patients to chiropractors or other practitioners . With respect to back pain, osteopathic physicians make fewer referrals to other physicians and admit a lower percentage of patients to hospitals than allopathic physicians , while also treating back pain episodes with substantially fewer visits than chiropractors . Although osteopathic family physicians are less likely to order radiographs or prescribe nonsteroidal anti-inflammatory drugs, aspirin, muscle relaxants, sedatives, and narcotic analgesics for low back pain than their allopathic counterparts, osteopathic physicians have a substantially higher proportion of patients returning for follow-up back care than allopathic physicians . In the United Kingdom, where general practitioners may refer patients with spinal pain to osteopaths for manipulation, it has been shown that OMT improved physical and psychological outcomes at little extra cost .
In our study, the effect sizes for OMT in the United Kingdom, where osteopaths are not licensed physicians, were generally comparable to those in the United States, where OMT is provided by licensed physicians. This consistency suggests that the results truly reflect the effects of OMT itself, and not other elements of low back care. It is not surprising that osteopaths in the United Kingdom achieved pain reduction with OMT similar to that of their physician counterparts in the United States. The training of osteopaths in the United Kingdom is highly focused on OMT, whereas osteopathic physicians undertake a medical curriculum that necessarily relegates OMT to one of many therapeutic approaches, albeit a fundamental one for osteopathic practitioners. Regardless of the career training path of the provider, it appears that OMT achieves clinically important reductions in low back pain.
There are several potential limitations of this study that should be addressed. First, as with any meta-analysis, the individual trials varied somewhat with respect to methodology, including trial setting, subject characteristics, OMT and control treatment interventions, and pain measures (Table 1). Such heterogeneity has been commonly observed in previous meta-analyses of spinal manipulation, including a recent meta-analysis performed in collaboration with the Cochrane Back Review Group . The latter study addressed potential heterogeneity by presenting stratified results according to chronicity of low back pain, type of control group, and duration of follow-up. This approach is analogous to the methods used in our study. Further, it should be noted that the assumption of homogeneity among trials was not rejected statistically in any of our eight overall or stratified median contrasts meta-analyses.
Second, because five trials each included repeated pain measures and two trials each included two control treatments, there was no unique set of independent outcomes for meta-analysis. Such interdependencies were noted to be a problem in an early meta-analysis of spinal manipulation . We used the median contrasts method to address this problem because the median outcome represents an observed outcome that is easy to compute and is less vulnerable to extreme observations than other measures of central tendency. Further, sensitivity analysis was used to assess the range of possible combinations of outcomes. Thus, for the overall meta-analysis, there were 729 potential contrast combinations. Of these, both the best-case and worst-case scenarios demonstrated statistically significant results favoring OMT, thereby providing unequivocal evidence for the efficacy of OMT. Robust findings were also observed for trials performed in the United States and for intermediate-term outcomes.
Third, because there were a relatively small number of eligible trials, there were not sufficient contrasts for certain analyses and some results were imprecise. The latter phenomenon likely obviated the statistical significance of some results. Nevertheless, it is important to note that the direction of results favored OMT in each of the 43 meta-analyses and sensitivity analyses presented herein (Figure 3).
Fourth, there exists the possibility that the results of unpublished trials of OMT for low back pain may have altered significantly the conclusions of this study. To address this issue, we performed file drawer analysis by computing the fail-safe N . This represents the number of unpublished trials of OMT for low back pain that would have met our inclusion criteria, and that also would have demonstrated an effect size averaging ≥ -0.10, which is assumed to reflect clinically insignificant levels of pain reduction. A total of 16 unpublished trials (assuming one control group per trial) with, in the aggregate, clinically insignificant pain reduction outcomes would have been needed to obviate the significance of our results. Only recently has government funding for research in the area of complementary and alternative medicine become more widely available, in response to the public's interest in such treatments. Historically, it is highly unlikely that 16 trials of OMT for low back pain would have been sponsored, conducted, and subsequently not published.
Finally, this study focused only on the efficacy of OMT with respect to pain outcomes. Generic health status, back-specific function, work disability, and back-specific patient satisfaction are other recommended outcome domains  that were not assessed because the included OMT trials did not consistently report these data.