Purpose:
To evaluate the potential for a photodisruption laser to rupture the crystalline lens capsule during a laser surgical treatment that does not strike the capsule directly.
Methods:
A finite element analysis simulation using LS-Dyna was performed on a model of a lens capsule where the variation in capsule thickness, Young’s modulus of elasticity and particularly the failure stress and failure strain, as published by Krag (1996) for porcine lenses and Krag (2003) for human lens capsule. A unique attribute of the non-linear finite element analysis program LS-Dyna, which is the automobile industry standard for crash safety evaluation, is the inflatable air bag model. We created models of lens capsules that can be inflated to the point of capsule failure for several animal species to simulate the effect of a photodisruption laser which creates gas bubbles which increase lens volume. We inflated the lens capsule until rupture and evaluated the change in thickness and volume at the time of rupture and compared that to laboratory measurements of expansion due to laser treatment. Mechanical strength of four rabbit lenses were measured using a custom device, with separate measurements for anterior and posterior capsules.
Results:
The posterior capsule is the first to rupture during inflation in all models. Human lens capsule are relatively strong and do not show capsule rupture for simulated moderate laser treatment. The measured failure stress for the rabbit posterior capsule averaged 0.45 N/mm2 which is approximately 8 times weaker than Krag (2003) had shown for 60 year old human posterior capsules.
Conclusions:
Human lens capsule models do not rupture for moderate photodisruption laser treatment while similar levels of laser treatment in rabbit lenses may cause rupture of the posterior capsule.
Keywords: laser • presbyopia