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Larger diameter capsulotomy strengthens capsular rim
Laboratory study and literature review look at ideal size and location of femtosecond laser capsulotomy in cataract procedures.
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Increasing the diameter of a capsulotomy may prevent posterior capsular opacification, maintain effective lens position and optimize capsular strength compared with a smaller diameter capsulotomy, according to one researcher.
Mark Packer, MD, and colleagues undertook a literature review of histology of human eyes and performed a laboratory study of biomechanics in porcine eyes to investigate the ideal size and location for capsulotomy in humans.
“The most important finding was the increase in capsular rim strength with diameter,” Packer said. “We found that the larger the diameter of the capsulotomy, the stronger it was — that is, the more force it took to tear and the greater the capsule stretched before it tore.”
Mark Packer
The research, published in British Journal of Ophthalmology, suggested an ideal capsulotomy diameter of 5.25 mm centered on the anterior pole of the lens capsule in humans because that is the thickest part of the anterior capsule, Packer said.
“So by making the capsulotomy bigger … basically it was a lot harder to damage compared to the smaller capsulotomy diameters,” he said. “That is the primary take-home message.”
Considerations to determine the ideal location and size of a capsulotomy included preventing posterior capsular opacification, maintaining effective lens position and maximizing mechanical strength, the study said.
To prevent posterior capsular opacification, the evidence to date supports a capsulotomy edge that adheres to the IOL optic with a 360° overlap, which applies to both manual capsulorrhexes and laser capsulotomies, according to the research.
Femtosecond laser assistance
The centration and circularity of femtosecond laser capsulotomies allow for construction of a 5.25 mm diameter while still ensuring a 360° overlap of the IOL optic, which is guided by the pupil safety margin and the method of capsulotomy centration, according to the research.
As a limitation of the technology, not all lasers are capable of centering the capsulotomy on the center of the lens capsule; most systems use the pupil center, which may not necessarily be the same as the center of the lens capsule, Packer said.
Laser imaging systems can be used to measure the anterior and posterior lens radii of the curvatures and the lens thickness from multiple angles in order to identify ideal centration of the capsulotomy.
Surgeons will need imaging capabilities to allow them to center the capsulotomy in the center of the lens capsule so there is less chance that the edge of the capsulotomy will run off of the edge of the IOL, he said.
After laboratory studies and literature review, the authors concluded that the ideal construction parameters are a capsulotomy centered on the lens axis for a complete 360° optic overlap by the anterior capsule margin and a diameter of 5.25 mm with the capsulotomy edge on the thickest part of the capsule.
“By following these guidelines, using the accuracy and precision of femtosecond laser capsulotomy, surgeons may reduce the incidence of capsule-related complications and posterior capsular opacification while providing the optimal foundation for lens extraction, stable IOL placement and predictable effective lens position,” the authors said.
Next step
The next step would be the initiation of a clinical trial with a large study comparing different capsule diameters and the rate of anterior capsule tears, “and there may be some variations within anterior chamber depths, corneal white-to-white dimensions or axial length,” Packer said.
“So, we really have to move into the clinic and look at the various factors that we suspect would be benefited by concentration on the lens capsule center,” he said. “Those being prevention of [posterior capsular opacification], more consistent effective lens position, refractive outcomes and, most importantly, a decreased incidence of anterior capsule tears.” – by Kristie L. Kahl
Reference:
Packer M, et al. Br J Ophthalmol. 2015;doi:10.1136/bjophthalmol-2014-306065.For more information:
Mark Packer, MD, can be reached at 1400 Bluebell Ave., Boulder, CO 80302; email: mark@markpackerconsulting.com.Disclosure: Packer reports he is a consultant for Alcon, Bausch + Lomb and Lensar.