Abstract
Purpose :
(1) To use finite element (FE) analysis to estimate local iris stresses during mechanical pupil expansion, and to determine which commercial device (Malyugin ring, APX dilator and iris hooks) is most likely to cause iris damage; (2) To design and test a ‘smart’ shape memory pupil expander (PE) in ex vivo porcine eyes to address the problems of insufficient expanded dilation, high iris stresses and lack of protection to the pupil margin.
Methods :
We used a biofidelic iris FE model that consisted of the stroma, sphincter and dilator muscles, and simulated mechanical expansion of the pupil from 3 to 6 mm with the aforementioned PEs. FE-derived iris stresses were compared across devices. Based on these results, we developed a ‘smart’ shape memory PE (named Spiralis) to limit iris stresses. We engineered a flexible material that can be folded into a spiral shape for easy insertion into the eye. To expand the pupil, Spiralis can uncoil to its pre-programmed circular ring shape upon exposure to the temperature of the anterior chamber. Spiralis prototypes were molded with a c-shaped cross section, thickness of 300 µm and glass transition temperature (Tg) of 20°C. Insertion and pupil expansion were tested in 50 enucleated porcine eyes. A standard 2.65 mm cataract surgery incision was performed and a compact Spiralis was inserted into the pupillary aperture. Deployment and pupil expansion using Spiralis were assessed qualitatively.
Results :
Our FE models demonstrated that the APX dilator generated the highest stresses on the sphincter muscles, (max: 6.45 MPa; average: 5.11 MPa), followed by the iris hooks (5.68 & 5.22 MPa), and the Malyugin ring (2.14 & 1.58 MPa). Uniform expansion (used by Spiralis) generated the lowest stresses (0.44 & 0.38 MPa). Ex vivo validation showed considerable effectiveness of the shape memory material’s ability for pupil expansion. The device uncoiled spontaneously in a controlled fashion without any corrective surgical maneuverers, providing pupil expansion from 2 to 7 mm. Spiralis was easy to disengage and swift to remove via the incision without any noticeable macroscopic damage to the iris or other structures in the eye.
Conclusions :
Spiralis is able to deliver uniform and circular pupil expansion ex vivo, which is thought to limit high stresses in the iris as predicted by our FE models. In addition, Spiralis only required a single incision. Further validations in live eyes are warranted.
This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.