Extraocular muscle expansion after deep lateral orbital wall decompression: influence on proptosis reduction and its predictive factors

To examine the relationship between extraocular muscle expansion and proptosis reduction (Δproptosis) in patients with thyroid eye disease who underwent deep lateral orbital wall decompression and to analyze the factors that contribute to extraocular muscle expansion. This retrospective, observational study included 133 sides from 77 patients with thyroid eye disease. The cross-sectional areas of the greater wing of the sphenoid bone (trigone), extraocular muscles, and superior ophthalmic vein were measured on computed tomographic images. Variables influencing Δproptosis were analyzed using multivariate linear regression analyses with stepwise variable selection. Predictive factors for the rate of postoperative increase in the cross-sectional extraocular muscle areas (Δextraocular muscle) were analyzed using the same statistical method. The amount of orbital fat removed (P < 0.001) and rate of Δlateral rectus muscle (P < 0.001) were positively and negatively correlated with Δproptosis, respectively (r = 0.425; adjusted r2 = 0.168; P < 0.001). The cross-sectional trigone area (P < 0.001) was positively correlated with the rate of Δlateral rectus muscle, whereas the preoperative cross-sectional lateral rectus muscle area (P < 0.001) and amount of orbital fat removed (P = 0.036) were negatively correlated with the rate of Δlateral rectus muscle (r = 0.551; adjusted r2 = 0.288; P < 0.001). Lateral rectus muscle expansion was negatively correlated with proptosis reduction and proved to be predictable before surgery. The results of this study will help predict proptosis reduction after deep lateral orbital wall decompression and to preoperatively plan additional orbital bony and fat decompression.