The Change of Extraocular Muscle Layers After Tenotomy: Histologic and Immunohistochemical study

ABSTRACT

PURPOSE: Extraocular muscle (EOM) consists of two layers, orbital and global layers which are readily distinguished by their histology. This study was conducted to investigate the changes of histology, myosin heavy chain (MHC), and MHC isoforms of the global and orbital layers of EOM after tenotomy. METHODS: Forty-two New Zealand white rabbits were used. The rectus muscles were harvested at day 3, week 1, week 2, week 4, and week 8 after EOM tenotomy. EOM mass change was measured. The EOM were serially sectioned in coronal plane and stained with Masson's trichrome. The diameters of muscle fibers and cross sectional areas of two layers were measured in the middle of the muscles. Changes of MHC isoforms were also measured using immunohistochemistry. RESULTS: The EOM mass was decreased at all periods of surgery especially at day 3 and week 1, increased maximally at week 8. The diameters of EOM fibers in global layer were decreased at day 3, week 1, and week 2 after EOM tenotomy and increased maximally at week 8. The immunohistochemical stains of fast and slow MHC were weakened in the global layer at week 1 after tenotomy. CONCLUSIONS: EOM changes due to atrophy appeared at day 3, week 1, and week 2 after EOM tenotomy while EOM atrophy was recovered at week 4 and week 8 after the surgery. These changes did not appear on the EOM orbital layer but was shown in EOM global layer. These results were due to the histological and functional differences between the EOM global and orbital layers.