EXTRAOCULAR-MUSCLE FIBER MORPHOMETRY FOLLOWING COMBINED RECESSION-RESECTION PROCEDURES IN RABBITS

Purpose: In earlier studies, we have reported extraocular muscle fiber atrophy following recession and fiber hypertrophy following resection of a horizontal rectus muscle. Changes seen in the operated muscle we re mirrored in the antagonist and were thought to be a compensatory re sponse to sustained changes in tension across the muscle pair caused b y the surgery and by changes in the rotational position of the globe. The purpose of this study is to examine the effects of combined recess ion-resection on extraocular muscle fiber diameter. Methods: In 16 ane sthetized rabbits, a 6-mm recession of the medial rectus was combined with a 6-mm resection of the lateral rectus in the left orbit. The hor izontal rectus muscles were removed from both orbits of four rabbits a t 2-, 4-, 8-, and 16-week postoperative intervals. Cross-sections were cut from the midbelly of each muscle, and muscle fiber diameters were measured with a computerized morphometry unit. Mean fiber diameters f rom the operated orbit of animals at each postoperative interval were pooled and compared with means from the unoperated orbit using the pai red-samples t test. Results: No statistically significant change in fi ber diameter was seen in either the global or orbital fiber layers at any postoperative interval examined.Conclusions: Because resection wou ld be expected to increase and recession to decrease the resting tensi on across an agonist-antagonist pair, our results suggest that a combi ned recession-resection yields no significant net change in resting te nsion, and minimizes compensatory changes in extraocular muscle fiber diameter.