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The majority of rotator cuff tears is asymptomatic {1}. What makes a rotator cuff tear symptomatic or asymptomatic in humans?

References:

  • {1} Minagawa, Hiroshi, Nobuyuki Yamamoto, Hidekazu Abe, Masashi Fukuda, Nobutoshi Seki, Kazuma Kikuchi, Hiroaki Kijima, and Eiji Itoi. "Prevalence of symptomatic and asymptomatic rotator cuff tears in the general population: from mass-screening in one village." Journal of orthopaedics 10, no. 1 (2013): 8-12. https://scholar.google.com/scholar?cluster=6848408356619886538&hl=en&as_sdt=0,22 ; https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3768248/

    In conclusion, our survey of 664 residents in one village demonstrated that the prevalence of rotator cuff tear was 22.1% in the general population, which increased with age. Asymptomatic tear was twice as common as symptomatic tear.

    […]

    In this study, “asymptomatic” was defined as having neither pain nor any other symptoms related to the shoulder when the survey was conducted including the past history of shoulder pain, whereas “symptomatic” was defined as having pain or any other symptoms related to the shoulder when the survey was conducted.

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    Caption: Prevalence of symptomatic and asymptomatic tears. Asymptomatic tear accounted for 50% of all tears in the 50s. However, in the 60s and over, the percentage of asymptomatic tear was significantly greater than that of symptomatic tear.

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The underlying reason is difference in the rate of firing of torned and intact muscle in movement of shoulder joint.

Asymptomatic patients had minimal pain (<3 on the visual analog scale and no loss of active range of motion compared with the contralateral side); symptomatic patients had pain greater than 3 on the visual analog scale and decreased range of motion compared with the contralateral side (>10 degrees of motion loss). Electromyographic activity from 12 muscles and kinematic data were collected simultaneously during 10 functional tasks. Both symptomatic and asymptomatic cuff subjects demonstrated a trend toward increased muscle activation during all tasks compared with normal subjects. During the internal rotation tasks, asymptomatic patients had significantly greater (P<.05) subscapularis activity than symptomatic patients (65% maximal voluntary contraction [MVC] vs 42% MVC). During the carrying task, asymptomatic patients demonstrated significantly less (P<.03) upper trapezius muscle activation than symptomatic patients (16% MVC vs 50% MVC). During shoulder elevation tasks, symptomatic patients had significantly greater supraspinatus (52% MVC vs 28% MVC, P<.03), infraspinatus (32% MVC vs 16% MVC, P<.05), and upper trapezius (39% MVC vs 20% MVC, P<.04) muscle activation compared with asymptomatic patients. During heavy elevation (8 lb), asymptomatic patients showed a trend toward increased activation (P<.06) of the subscapularis compared with symptomatic patients (34% MVC vs 21% MVC). Differential shoulder muscle firing patterns in patients with rotator cuff pathology may play a role in the presence or absence of symptoms. Asymptomatic subjects demonstrated increased firing of the intact subscapularis, whereas symptomatic subjects continued to rely on torn rotator cuff tendons and periscapular muscle substitution, resulting in compromised function.

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Source: Kelly, Bryan T., Riley J. Williams, Frank A. Cordasco, Sherry I. Backus, James C. Otis, Daniel E. Weiland, David W. Altchek, Edward V. Craig, Thomas L. Wickiewicz, and Russell F. Warren. "Differential patterns of muscle activation in patients with symptomatic and asymptomatic rotator cuff tears." Journal of shoulder and elbow surgery 14, no. 2 (2005): 165-171. https://www.ncbi.nlm.nih.gov/pubmed/15789010

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