Histological Evidence of Muscle Degeneration in Advanced Human Rotator Cuff Disease.
BACKGROUND
Cellular remodeling in rotator cuff muscles following a massive rotator cuff tear is poorly understood. The aim of the current study was to provide histological evidence to elucidate the mode of muscle loss in advanced human rotator cuff disease and to assess tissue-level changes in relation to findings on noninvasive imaging.


METHODS
Rotator cuff muscle biopsy samples were taken from the scapular fossae from 23 consecutive patients undergoing reverse total shoulder arthroplasty in order to evaluate muscle composition in severe rotator cuff disease. Markers of vascularity; inflammation; fat distribution; and muscle atrophy, degeneration, and regeneration were quantified.


RESULTS
The samples primarily consisted of dense, organized connective tissue (48.2% ± 19.1%) and disorganized, loose connective tissue (36.9% ± 15.9%), with substantially smaller fractions of muscle (10.4% ± 22.0%) and fat (6.5% ± 11.6%). Only 25.8% of the biopsy pool contained any muscle fibers at all. Increased inflammatory cell counts (111.3 ± 81.5 macrophages/mm) and increased vascularization (66.6 ± 38.0 vessels/mm) were observed across biopsies. Muscle fiber degeneration was observed in 90.0% ± 15.6% of observable muscle fascicles, and the percentage of centrally nucleated muscle fibers was pathologically elevated (11.3% ± 6.3%). Fat accumulation was noted in both perifascicular (60.7% ± 41.4%) and intrafascicular (42.2% ± 33.6%) spaces, with evidence that lipid may replace contractile elements without altering muscle organization.


CONCLUSIONS
Dramatic degeneration and inflammation of the rotator cuff muscles are characteristics of the most chronic and severe rotator cuff disease states, suggesting that muscle loss is more complicated than, and distinct from, the simple atrophy found in less severe cases.


CONCLUSIONS
In order to address degenerative muscle loss, alternative therapeutic approaches directed at muscle regeneration must be considered if muscle function is to be restored in late-stage rotator cuff disease.
1Departments of Bioengineering (M.C.G. and A.J.E.), Orthopaedic Surgery (T.C., M.P., S.S., and S.R.W.), and Radiology (S.R.W.), University of California San Diego, San Diego, California 2Kaiser Permanente, San Diego, California.
Michael C
MC
Gibbons
Anshu
A
Singh
Oke
O
Anakwenze
Timothy
T
Cheng
Maxwill
M
Pomerantz
Simon
S
Schenk
Adam J
AJ
Engler
Samuel R
SR
Ward
D000368:Aged; D000072228:Arthroplasty, Replacement, Shoulder; D001706:Biopsy; D005260:Female; D006801:Humans; D008297:Male; D018482:Muscle, Skeletal / Q000473:pathology*; D009133:Muscular Atrophy / Q000473:pathology*; D000070636:Rotator Cuff Injuries / Q000473:pathology* / Q000601:surgery*