Noncontact-laser penetrating keratoplasty: In vivo comparative evaluation in rabbit and cat

Background: With their inherent precision and avoidance of tissue deformation, non-contact laser trephines may minimize graft postoperative astigmatism. Laser-cut corneal button geometry surpassed handheld and equaled Hanna and Krumeich vacuum held trephines, without significant endothelium or wound healing differences for linear cuts between laser and metal blades. Materials and Methods: To compare the laser with metal trephines, we performed 8mm diameter grafts on 12 rabbits and 12 cats. A new laser system, using an advanced pulsed HF laser coupled to a computer controlled optical delivery system and equiped for quasi-instantaneous simultaneous 8-point corneal marking (200 ns) for precise suture placement at the 5.5 to 10.5 mm diameter and rapid corneal trephination (π6 sec), or a new disposable sterile vacuum-assisted Hessburg-Barron (HB) trephine was used in each procedure. Each animal (12 laser and 12 metal), had surgery with 1 instrument, unilaterally, followed by 16 interrupted sutures. These were removed at POD # 14 (rabbit) and at POD #28 (cat). We studied two other groups (4 rabbits and 4 cats histopathologically using vital stains, LM, TEM and SEM. Digital keratometry and corneal topography was performed using the SK-1 and TMS at selected intervals. Two animals of each series were followed for >6 months. Results: Laser corneal marking of the button and recipient tissues improved ease of suture placement and uniformity. Laser trephination was more rapid (π 6sec) than with the vacuum-assisted Hessburg-Barron and other manual trephines (-30-60sec). Laser cut buttons were geometrically more accurate. Wound healing, assessed biomicroscopically, and wound strength, assessed by raising the IOP at a constant rate of 60 mmHg/sec, revealed no statistical differences and equal healing speed in rabbits. Acute endothelial damage was found with the H-B trephine but not with the HF laser trephination system. Although Bahn introduced the cat as a good model for graft studies, we found otherwise. Surgery, keratometry and topography were difficult to perform due to the restricted access to the eye, the slit-shaped iris and the spherical cornea, 3 factors that prevented accurate centering. Postoperative infections were frequent and difficult to treat. Severe inflammatory response to sutures required steroid treatment. Woundhealing was inconsistant and the 11-0 and 10-0 sutures cut (cheeze-wire) the soft corneal tissue and had to be replaced several time during the 1st month. Intense corneal neovasularization occurred in an eyes. We operated 25 animals, but achieved acceptable results only when the graft was secured using eight 10-0 and eight 9-0 radial sutures. More cats operated with the H-B trephine required resuturing. Biomicroscopy wound apparence was equal, but the laser group seemed to heal faster. No statistical difference was found by keratometry (SK-1) or by topography (TMS). At 6 months, the average postop astigmatism was slightly lower in the laser group (4.3 vs 6.1 D). Conclusions: Circumferential keratotomies were more accurately and more easily performed with the laser. No statistical differences were found in wound strength and healing. The laser produced a slighly lower astigmatism. These initial results suggest the safety of HF laser corneal marking-trephination and its potential for PK procedures in humans.