Exploring cellular pathology after rotator cuff tears: implications of a shift towards fast fibers and increase in angiogenesis for repair and rehabilitation
Supraspinatus tears are common in orthopaedic clinical practice but there is a gap in the literature regarding the natural history of rotator cuff pathology which compromises our ability to predict the potential recovery success following surgical repair. Previous studies have examined fatty infiltration, atrophy, MRI occupancy ratio, tangent sign, and Goutallier scores as predictors for the functional recovery of muscle, but there are scarce data investigating the cellular and molecular pathology of rotator cuff tears. This study investigated the fiber-type composition and myosin heavy chain protein (MyHC) (I/slow, II/fast: IIa, IIx, IIb) and vascular endothelial growth factor (VEGF) of pathological supraspinatus via immunohistochemistry and western blot. Corresponding clinical and MRI data were used to identify factors that may influence recovery. Biopsies (n=27) from torn supraspinatus and control muscle (deltoid) were retrieved during arthroscopic rotator cuff repair surgery. Torn supraspinatus demonstrated 44 ± 1% slow fibers vs. 57 ± 1% in control muscle (p <0.01), while the proportion of intermediate or transitional fibers (mixed positive and negative staining) did not differ (3.4 ± 0.4% in supraspinatus, vs. 2.8 ± 0.3% in control). Differences in sex, smoking history, time from injury to surgery, and MRI data did not correlate with fiber type or myosin. VEGF was higher in pathological muscle (p <0.05). A shift towards fast fibers and greater VEGF suggests supraspinatus injury, consistent with disuse and/or denervation. These results together with muscle innervation status can provide insight into the overall health of the rotator cuff and allow us to better predict its ability to recover following injury.