Laboratory evaluation of a novel technique for myopia correction: Continuous wave laser (CWL) cornea shrinkage coupled with CXL

Aug 01, 2011

Continuous wave laser (CWL) cornea shrinkage coupled with CXL

Herein, we report a novel thermal reshaping application where a continuous wave midIR laser is used to create collagen shrinkage midstromally, fully sparing the corneal epithelium (and endothelium) and to a large extent, the Bowmans membrane, secondarily applying high irradiance UV light with a custom (transepithelial) riboflavin formulation for rapid collagen crosslinking to stiffen collagen for durability/persistence of this refractive effect in a laboratory model.

Methods and technique

The TS-RXL delivery system consisted of a midIR (2013 nm) laser, fibre coupled to a PC-controlled scanner, delivering a focused spot (~600 µm diameter) onto the cornea through an applanating chilled sapphire lens. The chilled lens is housed in a cone assembly on the articulating arm. The temperature of the sapphire lens was settable and was cooled to ~8 °C. The TS-RXL arm was lowered to applanate the cornea. During this process, the cornea under illumination was directly visualized by the operator through the superior opening of the cone; and on a PC image fed from the video camera. Ring diameters, sequence of patterns, along with speed and power were settable on the PC screen by the surgeon. As a result of extensive flexibility in parameter selection (lesion depth/diameter/draw speed/sequence/opacification control), significant precision of outcomes (Dioptres of correction) was provided by the device.

Following an applanated cooling period of 30 seconds, the treatment was started under footswitch control. It should be noted that the key parameters of the laser application (power, speed, ring diameters, ring sequence) were surgeon specified at the start of each exposure. The surgeon could select one, two, three, up to seven ring applications in sequence. These rings can be placed anywhere on the applanated surface of the cornea (including selectively and precisely decentred) Intraring distance was set per surgeon's preference. Based on preliminary unpublished work that we have done with this device for correction of myopia, we decided to place the 3 sub-surface lesional rings at 3, 4 and 5 mm diameters, each ~80 µm deep, centred on the cornea.

The device was footswitch activated and each ring was created by the continuous wave laser, applying the laser energy in a circular fashion counter clockwise, to create a complete circular sub-surface annulur ring lesion. First, the 3 mm diameter ring was completed, then the 4 mm and finally the 5 mm ring was completed. The duration for laser application for each ring took about 4 seconds, and the total applanation duration of the lens on the cornea lasted about one minute (30 seconds chill+12 seconds Sx+15 seconds chill). The system is designed to provide constant but selectable linear draw speeds at each diameter thus inducing a uniform quality of lesion. Following IR laser delivery, the cooling continued for 15 seconds.