Muscle tissue atrophy, extramuscular and intramuscular fat accumulation, and fat gradient after delayed repair of the supraspinatus tendon: A comparative study in the rabbit.

Abstract

To investigate the atrophy of supraspinatus (SSP) muscle tissue and accumulation of extramuscular fat (e-fat) and intramuscular fat (i-fat) after delayed repair of the SSP tendon, and to correlate CT findings with histology. One SSP tendon of 36 rabbits was transected, then repaired in groups of 12 at 4, 8, or 12 weeks and then followed for 12 weeks. Thirty-six normal shoulders served as controls. We compared the SSP muscle, e-fat weights and volumes, muscle tissue and i-fat areas on histology, e-fat and attenuation values on CT between the experimental and control shoulders. CT-to-histology correlations were run. SSP muscle tissue atrophy, e-fat and i-fat accumulation were present after tendon repair delayed by 4, 8, or 12 weeks (all p < 0.05). Both e-fat and i-fat accumulation presented increasing proximal-to-distal gradients (both p < 0.05). CT depicted the muscle tissue atrophy, e-fat and i-fat accumulation, and increasing proximal-to-distal gradients (all p < 0.05). We found strong CT-to-histology correlations (p < 0.05). SSP muscles showed tissue atrophy, e-fat and i-fat accumulation after successful SSP tendon repairs delayed by 4, 8, or 12 weeks. Subtracting i-fat from the muscle area determined the SSP muscle tissue atrophy. Increasing proximal-to-distal fat gradients were diagnostic if not pathognomonic of an initial SSP tendon disruption.