Abstract
The rabbit supraspinatus is a useful translational model for rotator cuff (RC) repair because it recapitulates muscle atrophy and fat accumulation observed in humans after a chronic tear (the “first hit”). However, a timeline of RC tissue response after repair, especially with regard to recent evidence of muscle degeneration and lack of regeneration, is currently unavailable. Thus, the purpose of this study was to characterize the progression of muscle and fat changes over time after the repair of a chronic RC tear in the rabbit model. Two rounds of experiments were conducted in 2017–2018 and 2019–2020 with N = 18 and 16 skeletally mature New Zealand White rabbits, respectively. Animals underwent left supraspinatus tenotomy with repair 8 weeks later. The unoperated right shoulder served as control. The rabbits were sacrificed at 1-, 2-, 4-, and 8-weeks post-repair for histological and biochemical analysis. Atrophy, measured by fiber cross-sectional area and muscle mass, was greatest around 2 weeks after repair. Active muscle degeneration peaked at the same time, involving 8% of slide areas. There was no significant regeneration at any timepoint. Fat accumulation and fibrosis were significantly increased across all time points compared to contralateral. Statement of Clinical Significance: These results demonstrate model reproducibility and a “second hit” phenomenon of repair-induced muscle atrophy and degeneration which partially recovers after a short time, while increased fat and fibrosis persist.
Highlights
Rotator cuff (RC) repair is one of the most common orthopedic procedures, yet it suffers from a non-healing or retear rate of roughly 25% for chronic small-to-medium tears, (Cho and Rhee, 2009; McElvany et al, 2015; Kwon et al, 2019) and up to 90% in large or massive tears (Galatz et al, 2004)
The univariate ANOVAs showed that, of all variables tested, only the percentage of centralized nuclei was significantly different between groups (0.7 ± 0.4% in 2018 vs. 2.4 ± 1.7% in 2020) (p < 0.001)
This study provides a detailed timeline of gross and histological changes in the rabbit supraspinatus model of chronic tear repair, which successfully replicates several key aspects of human rotator cuff (RC) repair
Summary
Rotator cuff (RC) repair is one of the most common orthopedic procedures, yet it suffers from a non-healing or retear rate of roughly 25% for chronic small-to-medium tears, (Cho and Rhee, 2009; McElvany et al, 2015; Kwon et al, 2019) and up to 90% in large or massive tears (Galatz et al, 2004). Studies have associated repair failure with a number of factors, including chronicity, patient characteristics, surgical technique, and/or the nature of the RC tear itself (Cho and Rhee, 2009; McElvany et al, 2015; Park et al, 2015; Kwon et al, 2019). Tear size, retraction, and fatty infiltration are some of the most consistently observed risk factors for retear (Cho and Rhee, 2009; McElvany et al, 2015; Park et al, 2015; Kwon et al, 2019). These tissue changes after tear can be considered the “first hit” to the muscle and tendon. It has been shown that even successful repairs do not reverse muscle atrophy and fat accumulation, nor prevent disease progression (Gladstone et al, 2007; Cho and Rhee, 2009; Kwon et al, 2019). Limited data are available to determine if RC repair surgery generates a similar, secondary injury – a “second hit.”
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