What is the optimum capsulorhexis size?

The optimum size of the capsulorhexis should be determined by the diameter of the intraocular lens (IOL) being implanted. Adverse effects may arise from the capsulorhexis being either too small or too large. At the most basic level, a capsulorhexis that is too small renders the extraction of the cataract more difficult, with a higher risk of anterior capsular damage from both trauma from the phacoemulsification probe and the smaller diameter for removal of lens fragments. Furthermore, opacification and fibrosis of smaller capsulorhexes can potentially infringe upon or even obliterate the visual axis in the post-operative period, impairing future fundal examination and detection of retinal pathology.
Nevertheless, an excessively large capsulorhexis can also be problematic, since it is preferable to maintain 360 degree apposition of the IOL against the anterior capsule. Perhaps the most important reason for this is to reduce the risk of developing posterior capsular opacification (PCO). A prospective study by Hollick et al 1 found that the average percentage area of PCO in patients with a small capsulorhexis (4.5-5mm) was 32.7%, compared with 66.2% in patients with a large capsulorhexis (6-7mm) lying completely off the lens optic. This overlapping of the anterior capsule may help prevent migration of lens epithelial cells and has been reported to be more important in the prevention of PCO than IOL edge design 2.
An overlapping capsulorhexis also appears to provide a more reliable refractive outcome, since larger capsulorhexes have been found to allow the IOL to move more anteriorly as the capsule fibroses, consequently inducing a post-operative myopic shift 3.
The size of the capsulorhexis should therefore be slightly less than the diameter of the optic; for an average 6mm optic, a capsulorhexis of 5.5mm would probably be optimal.

References
1. Hollick EJ, Spalton DJ, Meacock WR. The effect of capsulorhexis size on posterior capsular opacification: one year results of a randomized prospective trial. Am J Ophthalmol 1999: 128; 271-9
2. Smith SR, Daynes T, Hinckley M et al. The effect of lens edge design versus anterior capsule overlap on posterior capsule opacification. Am J Ophthalmol 2004: 138; 521-6
3. Cekic O, Batman C. The relationship between capsulorhexis size and anterior chamber depth relation. Ophthalmic Surg Lasers 1999: 30; 185-90

Mr Jonathan F B Goodfellow MBBS MRCOphth