Murat V. Kalayoglu, M.D., Ph.D.
Contributing Editor
Capsular opacification remains an important complication after cataract surgery. Posterior capsular opacification used to be an extremely common complication for patients undergoing extracapsular cataract extraction, where up to 30-50% of patients undergoing surgery for senile cataracts developed opacification at some point. Improvements in surgical technique and development of posterior chamber intraocular lenses have reduced the rates of opacification to single digits; however, posterior opacification of the capsule following cataract extraction remains one of the most frequent complications of phacoemulsification surgery.
Pathogenesis of posterior capsular opacification post – cataract surgery is well understood. In the normal lens, lens epithelial cells are confined to the anterior capsule and do not proliferate and grow into the posterior capsule. In addition, germinal “E” – type epithelial cells of the lens equator undergo active mitosis throughout life to continually produce lens fibers. In the normal lens, no lens epithelial cells grow into the posterior capsule. However, in some patients undergoing cataract surgery, opacification results from proliferation of lens epithelial cells into the posterior capsule. This is likely due to anterior or equatorial epithelial cells that remain following removal of the cataract, some of which become activated. These lenticular epithelial cells proliferate posteriorly and form an opaque membrane on the posterior capsule over time. In the process, these cells acquire fibroblast–type properties with contractile capacity, thereby leading to contraction and wrinkling of the posterior capsule in some cases. Indeed, inhibitors of mitosis have been shown to inhibit opacification when placed into the anterior chamber after extracapsular cataract removal. Therefore, posterior capsular opacification is somewhat of a misnomer, since the capsule itself never opacifies; instead, it is the posterior migration of activated epithelial cells that leads to the formation of the opaque membrane on the posterior capsule.
Two separate approaches are used to prevent the formation of posterior capsular opacification after cataract surgery. The first is optimizing “clean-up” of cortical material during the procedure itself. Theoretically, since activated anterior capsular and equatorial lens epithelial cells are responsible for the opacification, then removing them at the time of surgery should lead to lower rates of opacification. Towards this end, most cataract surgeons now emphasize the importance of adequate hydrodissection followed by slight rotation of the lens. The hydrodissection procedure itself has been shown to reduce the number of lens epithelial cells in pathological studies. The second approach depends on forming an optic/haptic – bag barrier, and requires in-the-bag capsular fixation. This allows direct contact of the intraocular lens with the posterior capsule, thereby preventing any activated epithelial cells access to the central posterior capsule.
In addition to these surgical and mechanical effects, advances in lens design and biocompatibility have greatly reduced rates of posterior capsular opacification. One piece PMMA lenses are not as effective in preventing posterior capsular opacification as the silicone lenses, which have relatively good anti-proliferative properties. In general, compared to PMMA lenses, hydrophobic acrylic intraocular lenses have demonstrated even lower rates of capsular opacification. A recent study (Ophthalmology 2005 112 : 67-72) comparing the rates of capsular opacification following surgery between silicone and acrylic intraocular lenses found no statistical difference in opacification or Nd:YAG capsulotomy rates; other studies have shown that acrylic lenses have the lowest opacification rates when compared with PMMA and silicone lenses. Importantly, the above study used lenses constructed in a particular geometrical pattern: rectangular optic with sharp edges. In fact, optic geometry likely plays a very important role; studies have suggested that square, truncated edges may decrease the rates of capsular opacification by virtue of the geometry itself. Furthermore, the sharp posterior edge - not anterior edge - of the optic seems to be the important factor in reducing opacification.
Even though a relatively simple procedure – Nd:YAG capsulotomy – is available in developed countries to re-open the visual axis after posterior capsular opacification, many developing countries have little or no access to an Nd:YAG laser. Therefore, capsular opacification remains an important complication following cataract extraction for international ophthalmology. Lens biocompatibility and design have improved to the point that opacification rates have been reduced significantly; however, none of the currently available lenses guarantee a complete absence of opacification. Additional improvements in lens polymer and design will further contribute to reducing opacification rates while maintaining excellent visual acuity.