Abstract
Purpose :
Magnetic resonance imaging shows that the ON lacks enough length to maintain slackness beyond 26° adduction and so must stretch and/or retract the eye for additional adduction. This study was performed to quantify the posterior scleral deformation caused by this adduction tethering, and finite element model (FEM) determined if it is consistent with individually measured tensile properties of the sclera, ON, and ON sheath.
Methods :
A benchtop setup was built to during image using multiple cameras the posterior sclera of 12 postmortem human eyes (age 74 ± 9 years) with long attached ONs during incremental adduction from 26-32° under normal IOP, and elevated IOP to 30 mmHg without adduction. Deformation of speckle-patterned posterior sclera was quantified by 3D-DIC as local relative surface area changes. Local sclera, ON, and ON sheath specimens of the same eyes were subjected to tensile testing to obtain properties for FEM. FEMs with 5% pre-stretch of the ON and its sheath were compared to FEMs without pre-stretch.
Results :
During 6° adduction tethering, sclera within 1 mm on the nasal border of the ON sheath was compressed, and the surface area ratio was 0.976 ± 0.007. At the same time, and temporal sclera within 1 mm from the ON sheath border was stretched to 1.003 ± 0.010. However, the surface area ratios in the region within 2 mm from the ON sheath margin were not distinguishable from the unity (P=0.338). There was little scleral deformation superior to the ON sheath. Tensile testing suggested strain stiffening uniquely occurred in the ON sheath above 3.4% strain, but not in the ON or sclera. Individualized FEMs incorporating ON sheath pre-stretch demonstrated scleral deformation during adduction consistent with experimental results. IOP elevation from 15 to 30 mmHg did not deformation the posterior sclera, and 45 mmHg did so only minimally.
Conclusions :
ON tethering in adduction locally deforms the posterior sclera at the ON junction with the globe, more so than marked IOP elevation. This deformation may be increased by strain stiffening of the ON sheath.
This abstract was presented at the 2024 ARVO Annual Meeting, held in Seattle, WA, May 5-9, 2024.