Choroidal thickening drives surgical failure in RRD with choroidal detachment: a geometric model for predicting silicone oil underfill and guiding steroid use

Rhegmatogenous retinal detachment with choroidal detachment (RRD-CD) poses persistent surgical challenges, and the role of perioperative steroids remains controversial. This study developed a geometric model quantifying the mechanical impact of choroidal thickening on vitreous cavity dimensions, and to provide a mechanistic rationale for silicone oil (SO) underfill and retinal re-detachment in RRD-CD. A spherical geometric model quantified the effects of choroidal thickness changes (Δr) on vitreous cavity volume (ΔV) and the surface area of the inner choroidal boundary (RPE–Bruch’s membrane complex) (ΔS). We retrospectively reviewed clinical data from 23 patients with RRD-CD and selected three representative cases to illustrate the model’s predictions as proof-of-concept. The model predicts that a 1-mm uniform increase in choroidal thickness—assuming global, uniform thickening as a theoretical upper-bound scenario—reduces vitreous cavity volume by approximately 1.65 mL (23%) and the RPE-Bruch’s membrane complex surface area by 289 mm² (16%). Postoperative resolution of choroidal edema causes re-expansion of the vitreous cavity, which may convert an intraoperatively complete SO fill into an effective underfill. The model further suggests that the expanded RPE-Bruch’s membrane area may become mismatched with the fibrotic, non-compliant neuroretina, potentially creating a mechanical substrate for recurrent detachment. These theoretical predictions offer a mechanistic explanation for why surgery relying solely on subretinal fluid drainage and silicone oil tamponade may fail when substantial choroidal thickening is present. Observed postoperative silicone oil underfill in the clinical cases supports the model’s predictions. Choroidal thickening alters intraocular geometry, potentially influencing RRD-CD surgical outcomes. This geometric model offers a conceptual framework that reframes the rationale for preoperative steroid use from empirical anti-inflammatory therapy to a geometry-correcting intervention, though this hypothesis requires prospective validation. The model supports the consideration of incorporating preoperative choroidal morphometry into surgical planning for RRD-CD.