Reasons for intrastromal corneal ring segment explantation.

Reasons for intrastromal corneal ring segment explantation In their recent article, Ferrer et al. report that 57 of 250 implanted intrastromal corneal ring segments (ICRS) had to be explantedda rate of 22.8%. The main cause of this very high explantation rate was extrusion (48.3%). There are 2 major causes of ICRS extrusion: (1) superficial implantation of a segment and (2) a segment that is placed too close to the incision. As a general rule, the thickness of the implanted ICRS should not be more than 50% of the corneal thickness in the ring track. Moreover, the incision depth should preferentially be set at 80% of the corneal thickness. Deeply located ICRS produce better results and also leave a greater amount of corneal stroma between the ICRS and the corneal epithelium, which could theoretically protect from extrusion related to progressive stromal thinning. Only rarely does an extrusion begin in the middle of the segment or far from the incision. An ICRS that is placed close to the incision, especially if implanted superficially, predisposes to adjacent corneal thinning and melting and subsequent extrusion. As our knowledge about the corneal response to ICRS implantation has evolved, we now implant thinner segments to achieve the same or better results than in the past. One of the most feared complications of ICRS implantation, ring extrusion, is now less common because we respect the pachymetry rule. We do not believe that the inflammatory cells and cell debris found in cases of extrusion are the cause but rather the consequence of the extrusion. The progressive epithelial and stromal thinning can lead to segment exposure, which in turn can lead to a local inflammatory reaction, triggering corneal melting around the segment, with consequent extrusion. It would be important to know the device used to measure the corneal pachymetry in Ferrer et al.’s study as there are differences among the most commonly used devices. The explanted ring thickness and ring track pachymetry should also be described to clarify the relationship between ring extrusion and corneal thickness.