Summary of results

This study aimed to investigate immediate changes in PPT following a clinical chiropractic consultation that included SMT provided on the basis of clinical findings. In contrast to prior research, this study was conducted in a real-world clinical setup, with actual patients already scheduled for a chiropractic consultation. We assumed that most participants would receive SMT during their consultation [35], and in fact, all participants did, often at several vertebrae, not necessarily limited to their region of pain. This allowed us to investigate changes in PPT in relation to several a-priori selected factors previously shown to affect PPT changes following SMT [8, 9, 36].

Although PPTs were observed to change substantially for some patients, the mean group difference was minimal, and the changes were not associated with any of our a-priori or post-hoc selected independent factors. Overall, immediate changes in PPT did appear not to be related to clinical outcomes in this population of Danish chiropractic patients. Subgroups may exist in relation to changes in PPT after SMT, but if so, they do not seem to be characterized by the clinical response to chiropractic care. While we did see a slightly higher mean increase of PPT at the SMT site, the association between the proximity of the PPT test site and the SMT site was minimal, and noteworthy increases were also observed multiple vertebrae away from the test site. However, we did see a small increase in PPT in those who received myofascial treatment in addition to SMT.

Spinal manipulation and changes in pressure pain thresholds

These findings do not align with previous publications as we expected to find a systematic increase in PPT independent of all other factors [4,5,6,7, 34]. One explanation could be that important differences between a clinical encounter and an experimental setup influence PPT changes following SMT. This raises questions about the generalizability of much previous laboratory-based work in this area.

The clinical encounter and therapeutic alliance between patients and chiropractors, including the trust and reassurance it entails, might influence pain sensitivity [17, 37]. However, a post-hoc analysis of the current data did not indicate any difference in pain sensitivity between new patients and patients attending follow-up visits before or after treatment. Arguably, trust and the therapeutic alliance are built and strengthened over time. Pain testing in a laboratory setup is a very different experience from a clinical encounter, whether as a new patient or a repeat visit, and this could potentially explain why changes are observed with treatment in the laboratory but not in the clinic.

In addition to the different settings (laboratory versus real-world), other factors may explain the lack of a significant PPT increase observed in this study. In QST research, it is well known that the initial pain test often introduces more variance in pain sensitivity than subsequent tests [38,39,40]. This is likely due to an initial degree of apprehension about unfamiliar painful tests on behalf of study participants. If that is not taken into account, a significant increase in PPT will likely be erroneously attributed to an intervention when in fact, it stems from such non-specific effects. Our study considered this aspect by providing three practice PPT attempts. In a recent randomized controlled trial employing a convincing SMT sham procedure, such considerations were indeed taken into account and employed several practice PPT tests to familiarize participants with the procedure before assessing pre-SMT pain thresholds. The authors found no effect of SMT on PPT measured up to 30 min after treatment [10], and our results echo that conclusion. However, the addition of myofascial treatment may result in small increases in PPT compared to only receiving SMT. While an interplay between these two treatments is anecdotally reported in chiropractic practice, the synergetic effect on SMT-related PPT changes is unknown. PPT changes following myofascial treatment have previously been shown [41], supporting the possibility of an additive effect on PPT when SMT and myofascial techniques are used together.

Topographic mapping

Our results provide further evidence in the field of topographic mapping in spine pain patients. While our approach was more straightforward than a previous study conducted at our laboratory [23], the results were similar in that PPT values increased the more caudally on the spine we tested. However, there was a negligible difference between adjacent or nearby vertebrae, similar to what we demonstrated earlier in persistent low back pain patients [42]. Also, the region of clinical pain had minimal impact on pre-test PPT, and while the participants with multiple pain sites had the lowest PPT values, it did not reach statistical significance.

Methodological considerations

While we considered the clinical design an overall strength, especially in contrast to the large body of experimental laboratory research, it could be argued that a clinical design comes at the cost of internal validity [43]. Each clinical encounter is unique by its very nature [44], and each participant will have received individualized treatment, adding variability to the data. Obviously, individual study participants are unique, whether enrolled in a clinical or laboratory study. However, in the present study, we included a mixed population of different pain regions, pain duration, the number of previous visits, etc. Our inclusion criteria were broad, and the participants are likely also heterogeneous on other important aspects such as their psychological state (e.g., mood and anxiety) [45, 46].

We considered our study to reflect real-world chiropractic practice, but we have to recognize that our QST procedure did follow the same stringent prescriptive procedures as other QST studies. However, our study differs in that the SMT was applied pragmatically in an actual clinical setting on patients during the normal course of their care, not volunteer “pain” patients or healthy individuals. Therefore, despite the prescriptive nature of the QST procedures, the study did allow for whatever impact the clinical context may have had on PPT, although we did not quantify these factors. Additionally, using this design, we cannot estimate whether a patient’s experience or knowledge of being involved in a research study impacted the results.

Due to time constraints and multiple testing sites, we limited the PPT assessment to one test per test site. Potentially, our study would have higher reproducibility if two or three trials were used instead. The first PPT trial is likely different from the second, but it is not clear in what direction [38,39,40]. Some studies report it as higher, while others as lower. We attempted to limit this by using multiple practice attempts. Further, in a post-hoc assessment, we could not find any trend suggesting that the PPT values go in one direction depending on the test sequence. The results of the reliability studies are challenging to translate to our study, as they were all completed on healthy participants. Even if we accept a variance error in our PPT estimates, it would be a systematic error, thereby not impacting our research aims. Also, due to time constraints, we only asked clinicians to report on whether non-SMT treatments were provided but not where. Perhaps myofascial therapy directly at a point related to the PPT test site would result in higher PPT increases, which we cannot explore with our data.

We attempted to assess PPT immediately following their consultation, but we do not know the time lapse between the final SMT and PPT data collection. We attempted to limit this by re-measuring participants (post-session) as soon as possible. The majority of participants received additional care before and/or after SMT, which may have included a combination of other manual treatments, rehabilitation and education, potentially prolonging the time delay between SMT and PPT measurement. The implications of this are unknown as it is uncertain how long a change in PPT following SMT, if present, would last [47]. However, we are undoubtedly not measuring PPT in the refractory SMT period.

We did not use a commercial algometer to capture the PPT, like Somedic or Wagner products. However, the equipment has undergone rigorous validation at our laboratory, with results to be published soon. Further, we have previously published studies using the same type of algometer [24]. We opted to use a spherical probe instead of a circular flat probe to ensure deep-tissue stimulation as opposed to skin irritation theoretically. However, this limits our ability to compare our results with prior publications directly.

Implications for future research

Considering the existing body of literature, contrasted with our findings and those of a previous methodologically rigorous randomized trial [10], we find that the effect of SMT on pain sensitivity remains unclear. There may be important differences between experimental and clinical settings that impact PPT and the extrapolation of experimental data to clinical settings. In order to move this area forward, more studies are needed which take into account potential sources of error, such as those discussed in the preceding discussion. Also, we have a limited understanding of PPT as a clinically relevant outcome measure—it is not evident that an increase in PPT should be correlated with a clinical improvement [9, 48]. Researchers and clinicians should be cautious about assuming such a relationship exists.

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