['The purpose of this investigation is to document quantitatively and qualitatively the changes that occur over time in the rotator cuff muscle after surgical detachment, simulating a chronic, unrepaired rotator cuff tear.', 'The supraspinatus muscle was unilaterally detached from the greater tuberosity in 20 New Zealand white rabbits. All tendons were tagged and retracted from the insertion on the greater tuberosity. Five rabbits were killed at each designated time interval of 6 weeks, 3 months, 6 months, or 1 year after surgery. All animals underwent whole-body perfusion at the time of death for tissue preservation. Gross and histologic evaluations were performed to quantify the progression of fatty infiltration over time.', 'Loss of muscle and fatty infiltration were evident 6 weeks after detachment of the supraspinatus tendon. The fatty infiltration increased over time from 6 weeks to 1 year (P = .002, analysis of variance). The fatty infiltration was most pronounced near the supraspinatus insertion, and it progressed from the musculotendinous junction toward the muscle origin (Pearson correlation, r = -0.51; P < .0001).'] Does fatty infiltration of the torn rotator cuff worsen over time in a rabbit model?