The overuse theory for musculoskeletal joint pain cannot explain adequately the occurrence of shoulder pain in those who do not engage in activities that involve repeated and stressful use of the shoulder since the percentage of the painful right shoulders usually does not match the percentage of dominant right arms in such individuals. An alternative hypothesis is presented to propose that shoulder pain is caused by postural immobility in the decubitus or side position during sleep. Prolonged pressure on the shoulder caused by the weight of the thorax can produce enough damage to cause subsequent shoulder pain. In order to test this hypothesis, a preliminary study was carried out to compare the laterality of shoulder pain with the laterality of sleep position. The calculated laterality ratios for sleep position and shoulder pain were found to be strikingly similar, suggesting a causal relationship between the two phenomena. However, the prevalence of shoulder pain in the general population was found to be smaller than the percentage of the time people would spend sleeping in the decubitus position. This discrepancy could be explained by the idea that in order for shoulder pain to develop subjects may have to spend longer times in the same decubitus position before changing to another position than the average person would. Additional evidence from published clinical studies also supports the postural theory of shoulder pain. More studies can be done to test this hypothesis by focusing on the sleep habits of patients with shoulder pain. According to the present hypothesis shoulder pain should for the most part occur on the side that the patient preferred to sleep on before the onset of shoulder pain. The postural theory of shoulder pain provides the possibility for a new and noninvasive method to treat shoulder pain by the modification of posture during sleep.
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