Impact of sphenoid trigone size and extraocular muscle thickness on the outcome of lateral wall orbital decompression for thyroid eye disease

Background To retrospectively review lateral wall orbital decompression for thyroid eye disease (TED) and to evaluate pre-operative CT scans to analyse the variation in proptosis reduction. Methods Consecutive lateral wall orbital decompressions performed by a single surgeon were retrospectively reviewed. Pre-operative CT scan features and post-operative proptosis reduction were analysed. The sphenoid trigone cross-sectional areas were summed and multiplied by the slice thickness to yield bone volume. Cumulative extraocular muscle thickness was calculated by combining the maximum thickness of the four recti. “Trigone volume” and “cumulative muscle thickness” were correlated with proptosis reduction at 3 months post-surgery. Results Out of 73 consecutive lateral wall orbital decompressions, 17 orbits had prior endonasal medial wall orbital decompression. In the remaining 56 orbits, the mean pre-operative and post-operative proptosis were 24.3 ± 1.6 mm and 20.9 ± 2.3 mm respectively. The proptosis reduction ranged from 1 to 7 mm (mean of 3.5 mm ± 1.3 ( p < 0.001)). Mean sphenoid trigone volume was 895 ± 434.4 mm 3 . The mean cumulative muscle thickness was 20.4 ± 5 mm. The correlation coefficient between muscle thickness and proptosis reduction was − 0.3 and was statistically significant ( p = 0.043). The correlation coefficient between sphenoidal trigone volume and proptosis reduction was 0.2 ( p = 0.068). With a multivariate analysis, the coefficient of efficient of regression for muscle thickness was − 0.007 ( p = 0.42) and the coefficient of regression for trigone volume was 0.0 ( p = 0.046). Conclusion Proptosis reduction following lateral wall orbital decompression can be variable. Extraocular muscle thickness had a significant correlation with the outcome, with greater proptosis reduction in orbits with thin muscles. The sphenoidal trigone size had a weak correlation with decompression outcome.