Introduction Rotator cuff tendinopathy is a significant source of musculoskeletal disability. Accompanying muscle changes may be important determinants of the prognosis. The aim of this study was to compare histological muscle changes in partial-thickness to full-thickness tears of the supraspinatus tendon.

Methods Muscle biopsies of nine partially torn and 15 fully torn supraspinatus tendons were compared histologically. H&E staining was performed to assess degenerative changes. Immunohistochemistry was used to assess slow and fast myosin heavy chains (type I and II fibres; Anti-slow skeletal myosin heavy chain antibody and Anti-fast skeletal myosin heavy chain antibody), satellite cells (CD56), proliferation (Ki67) and apoptotic cells (activated caspase-3; Asp175).

Results Full-thickness tears demonstrated significant atrophy of both slow (type I) and fast (type II) myosin heavy chains compared to partial tears. The area ratio of type II to type I was doubled in full-thickness tears. Partial thickness tears revealed significantly more satellite cells and proliferative activity than full-thickness tears. There was no detectable apoptosis using an antibody recognising active caspase-3.

Discussion Progression of rotator cuff tendinopathy is accompanied by a change in muscle fibre phenotype from endurance type I fibres to type II fibres more prone to fatigue, illustrating an aspect of muscle disuse. The rotator cuff muscle's ability to regenerate appears to be reduced when a full-thickness tear is established. Apoptosis does not appear to be of importance in muscle changes accompanying rotator cuff tendinopathy.