Excimer lasers revolutionized ophthalmology when they were introduced in the early 1990s for the surgical correction of myopia. Since that time, laser vision correction has become the preferred method of correcting myopia, hyperopia, and astigmatism. It is the most common corneal surgery with over 20 million patients having had LASIK worldwide, and approximately 1 million cases performed each year in the United States. In addition, it has an extremely high patient satisfaction rate of greater than 95%.
The laser devices have improved tremendously since the original broad beam machines that treated small central optical zones. With small spot technology, faster repetition rates, large optical and blend zones, eye tracking, iris registration, and wavefront treatments, laser vision correction is safer and provides better refractive outcomes as compared to two decades ago. While conventional and wavefront excimer laser procedures do an excellent job of treating regular, symmetric corneas, one of the major limitations is that they cannot adequately correct irregular astigmatism. Treating irregular corneas with customized ablation patterns guided by corneal topography has been an active area of research for years, and numerous studies from groups of international investigators have shown promising results.
Traditionally, corneal topography has been used as a diagnostic tool to measure corneal shape and patterns of irregular and induced astigmatism. It is now an indispensable instrument for anterior segment surgeons that is used routinely to screen potential refractive and cataract surgical candidates for corneal pathology and tear film abnormalities, and to monitor healing in postsurgical patients. The devices differ depending on the type of imaging technology used, such as placido-based (videokeratoscopy), elevation-based (rastersphotogrammetry, scanning slit), and interferometry-based (laser holography, Moiré fringes). The resulting data can be displayed in a variety of formats, most commonly as curvature (axial, instantaneous), power (refractive), elevation, difference, or relative maps. Topographers also include qualitative classification systems and quantitative measures, indices, and algorithms to assist in the analysis of the data, as well as keratoconus screening software to identify corneal maps suspicious for ectatic disorders.
Topography guided laser ablations rely on accurately measuring and registering the corneal data and then precisely delivering laser pulses to create a more symmetric corneal surface. Such procedures using both PRK and LASIK have been performed on regular corneas (myopia), corneas with naturally occurring irregularity (forme fruste keratoconus, keratoconus), and corneas with irregularity induced by previous refractive surgery (keratectasia, decentered laser ablations, small optical zones, irregular astigmatism and loss of best-corrected visual acuity). The results have consistently been positive, and topography-guided treatments produce less higher order aberrations than do wavefront-guided ablations. Systems that have the ability to deliver topography guided treatments include the WaveLight Allegretto Wave Eye-Q with Allegro Topolyzer (Alcon; CE mark 2003), EC-5000 with CATz (Nidek), MEL 80(Carl Zeiss Meditec), and LaserScan LSX (LaserSight).
While topography guided laser systems are available worldwide, none are FDA approved yet for treatment in the US. Hopefully the situation will change soon so that patients in the US can benefit from this advanced technology.
Results from some of the most recent literature are as follows:
- Kanellopoulos and Binder (JRS 2011): 32 eyes with corneal ectasia after LASIK were treated with combined topography guided PRK (WaveLight Allegretto) and corneal collagen cross-linking (CXL). 27 of the 32 eyes had improved vision and only 2 eyes experienced progression of the ectasia.
- Lin et al (JRS 2008): 67 eyes with decentered ablations and 48 eyes with small optical zones from previous LASIK underwent topography guided retreatment, and 16 keratoconus eyes underwent PRK as an alternative to penetrating keratoplasty (WaveLight Allegretto). All of the decentered eyes had an improvement of centration and 29% gained 1 or more lines of BSCVA. Similarly, all of the small optical zone eyes had an increase in optical zone size and 19% gained 1 or more lines of BSCVA. All of the keratoconus eyes had a reduction of astigmatism and 25% gained 1 line of BSCVA.
- Toda et al (JRS 2007): 32 eyes with decentered ablations, small optical zones, decreased BSCVA, and irregular astigmatism underwent topography guided PRK or LASIK (Nidek EC-5000 with CATz ablation). The majority of eyes had increased UCVA of greater than or equal to 2 lines and improved topographical maps, and 50% of eyes had a decrease in higher order aberrations.
- Jankov et al (JRS 2006): 16 eyes with small laser optical zones, decentered and irregular ablations after corneal graft, and corneal scars underwent topography guided LASIK (10 eyes) and PRK (6 eyes) (WaveLight Allegretto). All eyes in both groups had an improvement in UCVA and symptoms. Only 1 eye, in the PRK group, lost one line of BSCVA.
- Spadea and DiGregorio (JCRS 2005): 48 eyes with unsuccessful PRK and LASIK were retreated with topography guided PRK or LASIK (Zeiss MEL 80). All eyes had an improvement in UCVA.