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Are safety concerns regarding MICS lenses and PCO justified?
Lens design is not optimized for PCO prevention
 Georgia Cleary |
In recent years, cataract surgeons have seen a trend toward smaller incisions during phacoemulsification. Smaller incision sizes have driven the development and commercialization of IOLs that can be inserted into the eye without the need for wound enlargement. In order to maximize the benefits of microincision cataract surgery (MICS), microincision IOLs must perform as well as conventional IOLs in terms of refractive outcomes, visual performance and posterior capsule opacification (PCO).
The IOL design features that minimize PCO have been well-established by rigorous clinical trials. A square profile to the posterior optic edge is critical. Non-hydrogel biomaterials and a narrow optic-haptic junction are also important. These features help to prevent the migration of lens epithelial cells (LECs) from the equatorial capsule to the posterior capsule behind the IOL optic. However, most MICS IOLs do not possess these characteristics. The need for insertion through a small incision means that MICS IOLs are typically composed of soft, flexible hydrophilic acrylic materials. Hydrogel IOLs are less likely to possess a true square posterior edge profile due to blunting of the optic edge that occurs after rehydration during the manufacturing process. Further, because MICS IOLs are thinner than conventional IOLs, plate-haptic and complex closed loop haptic designs are often utilized. While these designs help to improve IOL stability and resistance to capsular bag contraction, they provide an Achilles’ heel for LEC migration.
Given these design features, it is no surprise that higher rates of PCO have been observed with MICS IOLs in comparison with conventional IOLs. Reducing the overall IOL size and volume to facilitate insertion through a smaller wound has resulted in compromised PCO performance. Further improvements in design must occur before MICS IOLs achieve the same high standards as conventional IOLs.
Georgia Cleary, MBBS, MRCOphth, can be reached at St. Thomas’s Hospital, London. Disclosure: Dr. Clearly has no direct financial interests in the products discussed in this article.
Volume-related differences are not significant
 Ehud Assia |
Numerous factors regarding lens design and configuration were found to be associated with a lower rate of posterior capsule opacification (PCO), such as posterior convexity, haptic angulation, IOL material, 360· square edge and symmetrical in-the-bag fixation. This is in addition to surgical factors not related to the IOL design, such as meticulous cleaning, hydrodissection and proper sizing of the capsulorrhexis. In contrast, IOL volume was never proven to be a significant factor to affect PCO.
Conventional IOLs occupy only a small portion of the volume of the evacuated capsular bag. The content of the capsular bag, 9.5 mm in diameter and 4.5 mm thick, is replaced by an optical disc, 6 mm in diameter and about 1 mm thick. The IOL haptic occupies only a small portion of the lens equator. In practice, three-piece IOLs with thin PMMA loops exhibit similar PCO rates as compared to one-piece acrylic lenses with much thicker loops. The volume of aspheric lenses is reduced by about 25% compared with spheric lenses of the same design. Nevertheless, aspheric lenses were not shown to be associated with increased rate of PCO.
Because microincision IOLs that are approximately 1.5 mm share the same basic characteristics of mini-incision IOLs that are 2 mm to 2.5 mm or small-incision IOLs that are 2.8 mm to 3.2 mm, their PCO rate is expected to be the same. The “no space, no cells” concept does not apply to thin-optic lenses.
Full-size lenses, made of expandable or injectable materials that can be inserted through a microincision, have the theoretical potential to further reduce PCO rate by refilling the entire volume of the capsular bag. Unfortunately, this is still only in theory.
Ehud Assia, MD, is an OSN Europe Edition Editorial Board Member and head of the ophthalmology department, Tel Aviv University, Israel. Disclosure: Dr. Assia has no direct financial interests in the products discussed in this article.