Changes in Sagging Extraocular Muscle Following Surgical Recession of the Superior Rectus Muscle in Rabbit Eyes

PURPOSE: To verify whether central muscle sag actually induces excessive recession after recession of the rectus muscle in rabbit models. METHODS: Twenty-four eyes of 12 rabbits were divided into two groups. In the right eyes (the muscle sagging group), half of the width of the original insertion was used to create a scleral suture during a 4-mm superior rectus muscle (SRM) recession and a central muscle sag of 2 mm was generated. Conventional 4-mm SRM recessions were performed in the left eyes (control group). At postoperative weeks 1, 2, and 4, the distances between the locations of the new insertion and the original insertion were measured at the center and nasal and temporal poles. RESULTS: At postoperative week 4, the difference between the two groups at center was 1.38 ± 1.17 mm (P = .008). The mean extent of the recession at the nasal edge of the insertion was 5.21 ± 0.94 mm in the muscle sagging group, which showed posterior migration and slippage relative to the mean extent of the recession in the control group (4.29 ± 0.50 mm; P = .014). The mean extent of the recession at the temporal edge of the insertion was 5.50 ± 0.74 mm in the muscle sagging group and 4.50 ± 0.77 mm in the control group (P = .005). CONCLUSIONS: Overcorrection that occurred in cases of central muscle sag in the rabbit models resulted from posterior migration of recessed muscle that occurred in conjunction with slippage of its nasal and temporal edges.