Experimental shoulder pain models do not validly replicate the clinical experience of shoulder pain

Abstract

Background and aimsPeople with shoulder pain often present with abnormal shoulder muscle function. It is not known whether shoulder pain causes or is the result of muscle dysfunction. If pain leads to muscle dysfunction, therapeutic interventions that produce shoulder pain may be contraindicated. Experimentally induced nociception can be used to investigate a causal relationship between shoulder pain and muscle dysfunction. However, the validity of current experimental shoulder pain protocols has not been established. The aim of this study was to determine whether current experimental shoulder pain protocols validly replicate the clinical experience of shoulder pain with respect to pain distribution, quality and behaviour.MethodsNine pain free participants received two injections of hypertonic saline, one into the subacromial space and one into supraspinatus, in random order, at least 1 week apart. Investigators blind to the injection site assessed pain distribution, pain response to clinical tests which provoke shoulder pain and pain quality assessed using the McGill Pain Questionnaire.ResultsFollowing hypertonic saline injection into both the subacromial space and supraspinatus: pain was most commonly reported in the deltoid region and did not extend beyond the elbow; the most common response to clinical tests which provoke shoulder pain was a decrease in pain; and the highest rating of pain quality was in the sensory domain with very few responses in the affective domain.ConclusionsExperimental shoulder pain induced by injection of hypertonic saline into either the subacromial space or supraspinatus produced a pain distribution similar to that observed in clinical shoulder pain, but neither experimental pain protocol could reproduce the increases in pain intensity following shoulder provocation tests or the emotional distress commonly observed in people with clinical shoulder pain.ImplicationsPain induced by local shoulder nociception produced by hypertonic saline injection into shoulder structures has significant limitations as a model of clinical shoulder pain. While it is perhaps unsurprising that short duration, chemically-induced experimental pain does not replicate the quality of the clinical experience of shoulder pain, the validity of experimental shoulder pain models which produce the opposite response to provocation testing to clinical shoulder pain must be questioned.

Effect of movement‐evoked and tonic experimental pain on muscle force production

“…In a repeated measures design, participants took part in two experimental sessions, separated by at least 3 days (median [1st quartile, 3rd quartile]: 9 [7,16] days). Participants were asked to avoid any strenuous exercise before each session.…”

“…However, spontaneous and movement‐induced pain are defined as different features of musculoskeletal pain, 13 and therefore tonic pain models may not accurately replicate the motor adaptations induced by movement‐evoked pain, which is commonly observed in pathologies such as patellofemoral pain 7 and patellar tendinopathy 14 . In fact, it has been reported that pain induced with injection of hypertonic saline solution is often alleviated by movement or muscle contraction 12,15,16 . When investigating motor adaptations to pain, this paradoxical association between movement and pain may lead to a perception of movement as less threatening than what occurs when the movement elicits pain.…”

“… 14 In fact, it has been reported that pain induced with injection of hypertonic saline solution is often alleviated by movement or muscle contraction. 12 , 15 , 16 When investigating motor adaptations to pain, this paradoxical association between movement and pain may lead to a perception of movement as less threatening than what occurs when the movement elicits pain. Thus, it is possible that experimental approaches that induce movement‐evoked pain will result in larger motor adaptations to pain compared to tonic pain.…”