Ciliary muscle traction during accommodation is able to induce optic nerve head deformation

To use finite element (FE) modeling and in vivo optical coherence tomography (OCT) imaging to explore the effect of ciliary muscle traction on optic nerve head (ONH) deformation during accommodation.

We developed a FE model to mimic the ciliary muscle traction during accommodation, and varied the stiffness of the sclera, choroid, Bruch's membrane (BM), prelaminar neural tissue and lamina cribrosa (LC) to assess their effects on accommodation-induced ONH strains. To validate the FE model, OCT images of the right eyes' ONHs from 20 subjects (25 ± 1.5 years, range: 24-28 years) were taken for an accommodative stimulus of 6-diopters (6D) and a relaxed state of 0D. Based on OCT images, the accommodation-induced ONH effective strains, choroidal thickness and BM opening (BMO) changes were measured.

Our FE model predicted that the ciliary muscle pulling force could be transmitted to the ONH through the choroid and BM. The LC strain caused by the ciliary muscle pulling (0.0177) is comparable to that induced by an IOP elevation of 45 mmHg (0.0173). The accommodation-induced LC strain increased with stiffer choroid and BM. Three-dimensional strain tracking showed that the LC strains under 6D and 0D accommodation were 0.057 ± 0.033 and 0.023 ± 0.014 (p < 0.05). The reduction in choroidal thickness and the enlargement in BMO were observed during accommodation.

Our study showed that during accommodation, ciliary muscle traction could induce large ONH deformations and that stiffer choroid and BM increased the pulling force transmitted to the ONH, thereby leading to higher LC strains.