LAUSR

PURPOSE Severe entropion of the medial canthus results in ocular surface diseases and tear staining syndrome. However, detailed anatomical structures of the medial canthus and lacrimal ducts in dogs are poorly understood. We aimed to understand the anatomical structures of the medial canthus by analyzing the distances from the medial palpebral commissure to the superior lacrimal punctum (DSP) and to the inferior lacrimal punctum (DIP) and by histological examinations of the medial canthal anatomy. METHODS Dogs that underwent modified medial canthoplasty (MMC) between April 2017 and March 2021 were studied. As a reference, non-brachycephalic dogs that underwent other surgeries were also examined. DSP and DIP were measured preoperatively in all dogs in both the non-everted and everted positions. Histological examinations of the medial canthal anatomy were performed in four eyes isolated from beagles. RESULTS The ratios of DIP to DSP (mean ± SD) at the non-everted and everted positions in 242 MMC eyes of 126 dogs were 2.05 ± 0.46 and 1.05 ± 0.13, respectively (p < .01). The ratios of everted to non-everted positions for DIP and DSP were 0.98 ± 0.21 and 1.93 ± 0.49, respectively (p < .01). Histological findings indicated that the orbicularis oculi muscle (OOM) circumjacent lacrimal canaliculus transformed into collagen fibers and were attached to the lacrimal bone. CONCLUSIONS Histological studies revealed that the OOM circumjacent lacrimal canaliculus transformed into collagen fibers and these collagen fibers may be related to the difference between DSP and DIP.