Limbal relaxing incisions transition to femtosecond laser-based technique

Laser-created incisions have the potential to improve results by increasing consistency in their placement, location, depth and configuration.

Astigmatism management at the time of lens-based surgery has finally gained wide-held popularity, in large part due to the recent rise in the availability and use of toric IOLs. It is hard to believe that the notion of correcting astigmatism concomitant with cataract surgery is now some 30 years old, as first suggested by Robert Osher and William Maloney.

The use of incisional astigmatic keratotomy, however, remained in the shadows despite efforts to refine the technique, instrumentation and nomograms by surgeons such as Spencer Thornton, Dick Lindstrom, Frank Price and Bruce Grene. It was not until these relaxing incisions were moved out toward the peripheral limbus, as suggested by Stephen Hollis and popularized by Jim Gills, Johnny Gayton and Dave Dillman, that their use began to gain some degree of acceptance. Despite their relatively forgiving nature and general benefit to patient’s uncorrected visual function, proponents and teachers of limbal relaxing incisions (LRIs), such as Bruce Wallace and myself, found that adoption of the technique remained frustratingly low. Slowly, with the availability of presbyopia-correcting implant technology and the emergence of the field of refractive cataract surgery, LRIs gradually became more mainstream as additional key opinion leaders such as Stephen Dell and Jonathan Rubenstein, among others, advocated for use. Still, creating incisions in virgin eyes undergoing lens implant surgery never significantly caught on with the general cataract surgeon, and upon their approval, toric IOLs quickly surpassed LRIs in popularity.

Success with LRIs

Over the years, my experience has shown that manual LRIs, although less powerful, can rival the results obtained with toric IOLs for mild to moderate levels of astigmatism (Figure 1). The keys to achieving a high level of success and avoiding what has been termed the shortcoming of LRIs — that is, undercorrection, regression and lack of predictability — are attention to detail and proper technique. Here are the main principles to achieving success with LRIs:

1. Proper centration. As with any astigmatic correction, choosing and locating the proper meridian for correction are vital. New guidance technology makes this step easier and more accurate.

2. Peripheral intralimbal placement. The term LRI is actually a misnomer. These relaxing incisions should be placed in peripheral yet clear corneal tissue.

3. Proper and consistent depth. To avoid undercorrection and regression, it is vital that these incisions be of adequate depth, preferably 85% to 90%, and consistent throughout their length. This is achieved by maintaining perpendicularity to the corneal surface with the footplates of a manual knife. A high-quality diamond blade, preferably adjusted to a depth determined by pachymetry, further heightens accuracy and improves outcomes.

With the advent and incorporation of femtosecond laser-assisted technology to our refractive cataract surgical armamentarium, astigmatic relaxing incisions are seeing a recrudescence of interest. Laser-created incisions have the potential to improve results by increasing consistency in their placement, location, depth and configuration (Figure 2). In fact, new opportunities and challenges to our traditional thinking are emerging: Perhaps incisions may not need to be perpendicular (or normal to the corneal surface) if they are of precise and consistent depth; most lasers currently create vertical, beveled incision architecture (Figure 3). Femtosecond lasers also permit incisions to be created only within the stroma, without breaking through the corneal epithelium. Although such intrastromal approaches generally have less effect, they afford the opportunity to titrate effect by selectively opening the incisions postoperatively.

Optical zone

Another new question is at what optical zone should laser relaxing incisions be placed? With traditional manual LRIs, the cuts are typically placed just inside of the true surgical limbus, using the latter as a sort of stencil. Manual LRIs would therefore have varying optical zones based upon the relative diameter of a given eye and the particular meridian in which the incision was being placed because the limbus is not perfectly circular in shape. Utilizing a laser, we are now capable of creating perfectly symmetric arcuate cuts at a highly consistent radius of curvature and optical zone, but it remains to be proven that such regularity will be better than varying these parameters based upon a given eye’s dimensions and anatomy. Furthermore, current imaging systems cannot accurately measure through an arcus senilis or other peripheral corneal opacity. As such, incisions are typically placed at an empiric optical zone of 9 mm or adjusted based upon an individual patient’s findings. These latter variables will require further study in this new age of laser-assisted relaxing incisions.

Utilizing sophisticated computer modeling along with our early clinical results, a femtosecond laser-assisted nomogram has been derived based upon my previous NAPA nomogram used for manual LRIs (Table). Because of limbal anatomic variation and technical issues when imaging and measuring the peripheral limbal structures, most laser nomograms are currently utilizing a specified optical zone. The NAPA manual nomogram assumed that bladed incisions were to be placed at the peripheral most extent of true clear corneal tissue, typically approximately 1 mm inside of the true surgical limbus. The modified laser nomogram depicted in the Table specifies use of a 9-mm optical zone, which was chosen based upon an average corneal diameter of approximately 11.5 mm. The optical zone can be reduced for smaller eyes less than 11 mm or increased for larger eyes greater than 12 mm. Similarly, unusually dense or extensive peripheral corneal opacities may require adjustment to a smaller optical zone. Traditionally, manual incisions were optimally set for 90% depth. Due to some question as to the accuracy of current laser-based incisions, most laser manufacturers are currently recommending a laser depth setting of 80% to 85% to avoid penetration. Future studies will be needed, on specific laser platforms, to validate these recommendations. Lastly, this nomogram presumes epithelial penetration along the entire arc length of the incision. If the incisions are created intrastromally without breaking through the epithelium, their arc length will need to be increased.

Future of astigmatic correction

Although the excitement currently surrounding the development of laser-assisted relaxing incisions is much deserved, there are a number of factors and new variables that will require study and further refinement in order for this more automated technique to fully take hold and compete with toric IOLs. For that matter, I do not see the use of manual blade-created relaxing incisions disappearing in the near future; in fact, their use will likely increase as more surgeons begin to accept that patients judge the quality of their surgery by their refractive outcome, and not all surgeons will have access to a femtosecond laser within the immediate future. The beauty is that refractive cataract surgeons now have multiple options to address pre-existing astigmatism, and the accuracy of our results will continue to improve.

To that end, one final statement should be made as to the important role that emerging automated reference and guidance systems will play in all forms of astigmatic correction. Accurate identification of the desired steep meridian over which the correction needs to be centered is pivotal for all such treatment modalities. New reference and guidance systems along with methods for intraoperative monitoring will further serve to improve our lens-based surgical outcomes. It is hoped that in the near future the refractive results obtained through implant surgery will rival that of LASIK and modern keratorefractive surgery.