Physics of Folding in Cataract Surgery
Narrative Responses:
Purpose
For thin elastic sheets, with stretching, the Föppl-Von Karman equation predicts that fold width is proportional to the square root of the product of thickness and length divided by the fourth root of the displacement .With compression, the width is predicted to be proportional to the thickness of the tissue.
Methods
Recorded cataract surgical videos were reviewed. Fold width and tissue thickness were estimated using the surgical aspiration port as reference. Two examples of folding were commonly noted during cataract surgery: the first was the folds commonly noted in the posterior capsule after implantation of the intraocular lens, the second was cortical folds related to compression on the soft cortical shell by posterior pressure. Theoretical estimates of folding were made using the thickness of the posterior capsule and the changes in the size of the capsular bag documented in the ophthalmic literature.
Results
Three cases with folds in the posterior capsule were magnified from the video. The distance between folds was about 60 microns. Using the reported size of the capsular bag with an IOL in place (about 10 by 12 mm) and a posterior capsule thickness of less than 7 microns, the estimated width of the folds is also approximately 60 microns. Three cases with cortical compression folds were noted. In all cases the width of the folds was approximately the same as the thickness of the cortex.
Conclusion
The observed folds in the capsule and cortex were consistent with the theoretically predicted width. By understanding the physics of folding, surgeons may better understand the stretching and compression forces at work during cataract surgery.