Sports Medicine Research: In the Lab & In the Field: Overuse Activity Increases Pain Receptors in the Supraspinatus (Sports Med Res)
Wednesday, June 22, 2011

Overuse Activity Increases Pain Receptors in the Supraspinatus

Overuse of training increases mechanoreceptors in supraspinatus tendon of rats SHR

Pochini AD, Ejnisman B, Alves MT, Uyeda LF, Nouailhetas VL, Han SW, Cohen M, Albertoni WM. J Orthop Res. 2011 May 2. doi: 10.1002/jor.21320. [Epub ahead of print]

http://www.ncbi.nlm.nih.gov/pubmed/21538506

Overuse shoulder activity is very common among overhead athletes, recreational athletes, and manual labor workers. It has been suggested to cause rotator cuff tendonitis, tendinosis, and even tears. Previous animal research has identified degeneration (similar to a tendinosis condition) of the supraspinatus following an overuse treadmill program. In humans, these tendon injuries can cause substantial pain and disability, however inconsistencies among the severity of the injury and the amount of pain exist. For example, patients with full thickness rotator cuff tears may have no pain, while patients with small partial thickness cuff tears experience high levels of pain. Therefore, the purpose of this study was to determine the protein expression of S100 (a protein marker for mechanoreceptors; e.g., free nerve endings) within the supraspinatus tendon of overuse trained rats compared to normal control rats. Overuse rats were subjected to 1hr/day, 5 days/week of treadmill running at a speed of 17m/min for 4 months. Control rats assumed normal cage activity. After sacrifice supraspinatus tendons were harvested and treated for histology (a measure of tissue damage and cellular activity) and immunohistochemistry (a measure of protein expression in tissue; e.g., mechanoreceptors). For histology, the overuse group had increased cellularity (e.g., fibroblasts) and cells were more round in shape (represents active cell turnover and remodeling). For immunohistochemistry, the overuse group had increased expression of S100 (the protein marker for mechanoreceptors).

This study found that following overuse activity there is an increased presence of mechanoreceptors. Mechanoreceptors collectively describe Pacinian corpuscles (commonly in skin and responsible for sensitivity to vibration and pressure), Ruffini corpuscles (deep layers of skin and register joint motion), Golgi tendon organs (found in tendon and detect force), and free nerve endings (detect pain). However, the authors suggest that free nerve endings appeared to be majorly expressed compared to the other receptors. These results are very interesting and suggest that increased expression of mechanoreceptors may be a protective mechanism to decrease the overuse stress that is present. As with other injuries pain tells us that there is an injury present so that we can treat it and prevent injury progression. It would be interesting to see if rest or other treatments for overuse degeneration decrease the expression of mechanoreceptors. Although not discussed by the authors, I believe these results may shade light on the earlier situation of patients with partial rotator cuff tears having significant pain and patients with full tears not experiencing any pain. As we know from this study overuse activity increases pain receptors. When there is a partial tear in a tendon with increased pain receptors any activity of that muscle will cause tension in the portion of the tendon that is not torn, which will cause significant pain. However, in a patient with a full thickness cuff tear the tendon is no longer attached to the humerus, so when the muscle is activated tension is not created and therefore the pain receptors are not activated. This has been described clinically when evaluating Achilles or patellar tendon ruptures. If there is weakness and no pain a complete rupture is commonly suspected. However, this has not been described with rotator cuff tears. Future research is needed to confirm this theory but what are peoples thought with this? Have you seen similar inconsistencies in injury severity and pain as well? Do you feel treatment for overuse is reversing the expression of pain receptors or just allowing the tendon to recover physiologically from the degeneration?

Written by: Stephen Thomas

Reviewed by: Jeffrey Driban

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