How sharp is sharp?

October 1, 2005

Major advances have been made in the understanding of the pathogenesis of posterior capsule opacification (PCO) post-cataract surgery, and improvements in surgical technique and lens biocompatibility and design have reduced the rates of opacification in recent years. Nonetheless, opacification remains an important complication in patients undergoing phacoemulsification or extracapsular cataract extraction combined with intraocular lens (IOL) placement, with approximately up to 25% of patients developing visually significant PCO during the first five years post surgery.

Many studies have claimed that various factors influence epithelial cell growth in PCO, including mechanical, immunological, pharmacological and surgical. However, studies have also shown the importance of IOL material and design in the prevention or impediment of PCO development.

Previously, Nishi and co-authors have demonstrated that IOLs with sharp edges decrease the rate of PCO significantly, however, the extent of this sharpness required to effectively prevent lens epithelial cells from growing into the visual axis has yet to be defined.

Standardized scanning electron microscopic pictures were taken to evaluate the optic edges and the programme EPCO 2000 was then used to evaluate the area of the edges. By calculating the area above the edge as a partial area of a defined circle, the investigators were able to state an exact size of the edge for each lens.

In order to fix the position of the cell in a way that it overcame the floating force of optics but did not influence the growth of the lens epithelial cells, various weights and materials were tested by the study investigators. This weight would correspond, though not imitate, the pressure that the lens capsule exerts on the IOL in vivo. Silver at a weight of 0.7 g was found to cause no reaction with the cells or the culture medium and effectively stabilized the whole system.

The investigators evaluated many characteristics during the 33-day observation period, including the position of the lens and weight, lens epithelial cell density, changes in cell structure, and more. Further, the density of lens epithelial cells found around the edge of the lenses was determined by a predefined category system, with cell density falling into one of four categories.

The categories were defined as follows:

Category 1: no lens epithelial cell has reached the edge of the lens
Category 2: only the cytopodiae, not the cell bodies, have reached the optic edge
Category 3: the cell formation around the edge is still of low density; the cell bodies do not touch each other
Category 4: the cells have reached the highest density; the cell bodies are in contact with each other

A simple "yes" or "no" result was recorded when evaluating cell growth underneath the optic of the lenses: either cells were present or the edge acted as a barrier for prevention of cell growth in the optic area. Qualitative analysis of cell density around the lens edge was performed at the exact point when cells grew underneath the lens for the first time.