Insertional anatomy of horizontal rectus extraocular muscles in patients with Esotropia and Exotropia

Strabismus, including esotropia and exotropia, significantly affects binocular vision and presents challenges in clinical management and surgical correction. Although previous studies have explored the anatomical characteristics of horizontal rectus extraocular muscles (EOMs) in strabismus, large-scale analyses of insertional anatomy remain limited. This study investigates anatomical variations in the insertions of horizontal rectus EOMs in patients with esotropia and exotropia, aiming to understand their impact on strabismus management.

A retrospective case-control study was conducted using clinical records from 4781 patients diagnosed with strabismus who underwent surgery at the Zhongshan Ophthalmic Center, Sun Yat-sen University, from June 2014 to August 2020. Anatomical measurements of horizontal rectus EOM insertions were taken using a Castroviejo caliper. Statistical analyses, including regression analysis, were performed to examine the relationship between EOM characteristics and clinical outcomes.

Significant anatomical differences were observed among esotropia, exotropia, and control groups. Esotropia patients exhibited shorter medial rectus (MR) insertion distances (mean: 4.81 mm) than both exotropia (5.09 mm) and controls (5.08 mm, p < 0.001), with moderate effect sizes. Exotropia subtypes showed wider MR and lateral rectus (LR) insertions, particularly in constant and pattern types (MR width p = 0.001, LR width p = 0.002). Regression analysis indicated that shorter MR insertion distance was significantly associated with greater esodeviation at both near (β = -13.60, p < 0.001) and distance (β = -12.76, p < 0.001), though with small effect sizes. Refractive error (SE) and age were also significant predictors of deviation magnitude and AC/A ratio.

Anatomical variations in the horizontal rectus EOMs are significantly associated with esotropia and exotropia. These findings enhance our understanding of strabismus pathophysiology and may help guide surgical strategies for severe cases, though further research is needed to refine clinical applications.