Features

Many articles are published for refractive surgeons under the umbrella heading of 'clinical pearls'. Naturally, such articles deal with small nuances of technique or procedures on the basis of the author's experience.

The purpose of this article is to examine equivalent areas of the subject relevant to the optometrist's involvement with refractive surgery, whether or not they are actively involved in a laser eye surgery co-management programme. Key to the optometrist's interaction with laser eye surgery patients is the delivery of easy-to-understand word pictures to allow the non-expert to fully visualise what can be perceived to be a daunting conceptual area.

Laser eye surgery has been part of the vision correction scene for almost 20 years, and as such it has become an established option for a large proportion of the patients seen by optometrists. There cannot be many practitioners in general practice who are not regularly asked for their thoughts and opinions on the subject. As with any aspect of the world of vision, patients expect their primary eye care practitioner to be able to provide informed and objective guidance in response to such enquiries on laser surgery.

The aim of this article is to suggest ways to better inform the patient enquiring about laser vision correction, based on up-to-date information. The promise of a simple, relatively economical and quick procedure that will correct errors of vision at one visit offers such allure that it cannot fail to capture the public's imagination and also not a little fear and opposition from those advocating other methods of vision correction.

Laser vision correction is a fast-changing field where it seems that very quickly after a potential problem is identified, a solution is proposed. There are a number of common misunderstandings, myths and hangovers from the early days of refractive surgery that are still being told quite regularly to patients these days.

Long-term stability of outcome

There have been so many advances in the field that it simply is not acceptable to hark back to articles or conversations from years ago or to resort to catch-all statements about 'more research being needed' or 'it has not been properly proved yet'.

The truth is that there have been a number of studies that have been published in peer-reviewed journals or presented at international refractive surgery conferences that have addressed the issue of long-term (more than five years) follow-up of the outcomes of laser eye treatment.

A particular study of note was carried out by the highly respected Spanish academic and surgeon Professor Jorge Alio, which was a retrospective analysis of 300 eyes which had a full 10 years of follow-up history. Alio found good levels of stability, safety and efficacy. Some modest regression of the prescription was evident, but of course since the initial surgery was carried out 10 years ago, the hardware and techniques used did not benefit from modern-day refinement. In particular, smaller treatment diameters used in very early treatments were associated with high levels of regression of the prescription, due to epithelial hyperplasia filling in the treated zone and changing the resultant curve worked on to the cornea.

The fast-changing world of refractive surgery has adopted measures over the years to adapt to any inaccuracies, thus ensuring that modern treatment will not be subject to even the same small levels of undesired outcomes seen in these studies.

Refinement of techniques

The 'k' in the terms Lasik and Lasek stands for 'keratomileusis', the surgical procedure of reshaping or 'sculpting' the cornea (Figures 1 and 2). The term predates the use of laser, with the original use of the term describing a surgical lathe being used to reshape a trephined cylinder of stromal tissue.

The later use of laser revolutionised the accuracy with which the treatment could be delivered. Increased treatment diameters were possible which meant that the regression that had been evident in early treatments could be avoided.

Also linked to treatment diameters were night vision problems experienced by patients. Small diameter treatments were sometimes associated with postoperative complaints of halos and starburst phenomena around lights at night, which were sometimes disabling. The emerging laser surgery field had to face a difficult issue was it acceptable to treat one problem at the cost of inducing another, which may be equally as disabling to that patient in its own way as the first?

It was for this very reason that the laser surgery industry started searching for methods of improving outcomes for patients, starting with increasing the optical zone size of the treatments and eventually culminating in the development of wavefront-guided treatments.

Wavefront

To deliver the requirements of refractive surgery patients, it is essential to address more than a post-operative visual acuity equal to or surpassing that of their spectacles. Every practitioner will be conversant with the Seidel aberrations that detract from the quality of an image, but the wider field of optics can deal quantitatively with these aberrations using wavefront analysis.

The five familiar Seidel aberrations can be expanded to some 21 Zernike aberrations, arranged in five 'orders'. An optimum outcome for a refractive surgery patient would have to include some evaluation of quality of vision, and to this end the wavefront-guided treatments address higher-order Zernike aberrations, defined as those Zernike aberrations occupying the third, fourth and fifth orders (Figure 3). Conventional standard laser eye surgery, like spectacles and contact lenses, is only capable of treating up to second-order Zernike aberration, being the familiar sphere and cylinder of the spectacle prescription.

Importantly, however, although conventional laser eye surgery is not capable of correcting higher-order aberrations, and there are numerous reports of conventional treatments inducing greater amounts of higher-order aberration.

Wavefront analysis provides a mechanism to describe the aberrations of the eye's optical system in terms of not just spherocylindrical refractive error, as would be evident from a spectacles or contact lenses model, or indeed from conventional laser eye surgery, but also to include the higher-order Zernike wavefront aberrations responsible for degradation of the image in low illumination to the detriment of the visual comfort of the individual.

This technology provides the clinician with information regarding the entire refractive status of an eye, information that can then be further utilised in the construction of an algorithm by which the eye can be treated and the total error addressed. There have been a number of comparative studies between wavefront-guided and non-wavefront guided treatments, with the result that wavefront-guided treatment is now considered to be the gold standard worldwide (Figure 4).

A common misconception is that wavefront surgery is capable of producing a significantly better level of visual acuity than other correction methods. However, to compare wavefront treatment and non-wavefront treatment in this way is to look at wavefront on its least favourable playing field.

Initial responses to the advent of customised wavefront-guided laser surgery asked why if all of the aberrations have been addressed, does the visual acuity only improve relative to the preoperative figure in a minority of patients.

The answer to this question lies in the theoretical limitations to visual acuity, and the factors involved in the determination of such.

Snellen determined normal functional visual acuity in 1869 as that resulting from a minimum angle of resolution of one minute of arc. This translates mathematically into a Snellen chart acuity of 6/6 or 20/20. It has been stated that in a diffraction-limited model, taking into account chromatism error and the Stiles-Crawford effect, the theoretical limit of foveal visual acuity is between 6/4 and 6/1.5. While this limitation seems to represent a visual acuity considerably in excess of that exhibited by current treated patients, even those having wavefront-guided treatment, the bottom-line limiting factors are the richness of the receptor mosaic and the possibility of surgery-induced aberrations. The latter can be adjusted for by ongoing algorithm development and predictive compensation for flap-induced aberration, but the former is an example of an absolute biological factor.

Certainly, in the quest for enhanced human visual ability, customised wavefront-guided laser eye treatment offers a promising tool, continually under refinement and development.

At the present time, however, the main goal of wavefront is to deliver better quality of vision by reducing the effect of higher-order aberrations which may detract from the image. A good analogy between wavefront and non-wavefront treatment for optical professionals is to compare the flat spectacle lenses that would have been dispensed as a matter of course in the 1920s with the highly refined best form designs of today.

Sitting in a testing room and looking through the correct prescription of flat lenses at a test chart would give very similar acuity to viewing a test chart in testing room conditions with the most modern best form lenses. It is when the total performance of the lenses is examined, including off-axis performance, that the benefits of the design can be seen.

So it is with wavefront-guided laser eye treatment. The actual curve worked on to the stroma by the laser has been determined to deliver superior quality of vision by eliminating aberration.

Femtosecond laser flap creation

The improvements seen in the quality of laser surgery outcomes with increasing diameters and wavefront-guided treatment have been further augmented by the advent of the femtosecond laser to create the Lasik flap. It has already been established that the laser surgery effect is brought about by reshaping the stroma, and that presents the problem of how to bypass the epithelium to expose the stroma. The whole area of 'the flap' is one that patients can often be somewhat uncomfortable with, but a simple analogy can be used to explain why this is necessary. Ask the patient to consider a ream of white printer paper, which is discovered upon opening to have the top 100 sheets as pink. How would you reach enough white sheets to print your letter? You'd lift the pink sheet section out of the way, take out your white sheets, then you could replace the pink sheets afterwards.

This explains the need for the flap, of course, but there often still remains a feeling of disquiet in the patient's mind about the whole concept of creating the flap. This has been greatly reduced by the emergence of femtosecond laser flap creation. After all, the patient is often happy with the concept of a laser working on their cornea to correct their sight, and it is only a short journey from this to becoming happy with the idea of laser flap creation.

The femtosecond laser utilises the simple principle that if power is equal to energy divided by time, the application of a relatively modest amount of energy for an extremely short period of time produces a very powerful effect. A pulse of energy delivered for a few hundred femtoseconds produces a cavitation effect within the stroma, creating a bubble a couple of microns across, made of carbon dioxide and water from the vaporised stromal tissue. The bubble size and position is controlled by a computer, and in approximately 30-45 seconds a cleavage plane is created in the cornea some 9mm across and generally around 100 microns below the epithelial surface. At the flap edge, the bubbles are stacked up to the surface at an angle, thereby allowing the Lasik flap to be lifted, exposing the stromal tissue for treatment.

Again, there has been a wealth of research showing that femtosecond laser flap creation is associated with superior outcomes when compared to alternative methods of flap creation. This has included studies carried out to assess suitability of post-laser patients for work in the most challenging of environments, namely aerial combat and astronautics.

A study for the US Naval Aviation Medical Center, San Diego shows that pilots undergoing laser eye surgery with femtosecond laser flap creation regain their flight status faster and have better outcomes than when their flaps are made by a blade. This study involved some 300 US Navy pilots undergoing wavefront-guided laser eye surgery. One day after treatment, as many as 70 per cent of those in the femtosecond group achieved better then 20/20 vision, compared with 47 per cent of those treated using a blade. Even one month after surgery, there was still a clear difference, with 83 per cent of the femtosecond sample having better than 20/20 vision, compared with 80 per cent of those with a conventional mechanical flap.

Dry eye

It was true in the early days of Lasik that patients were too frequently left with symptomatic dry eye lasting a number of weeks, but this is not a common occurrence with modern Lasik techniques thanks to the thinner flaps created with the femtosecond laser, more refined laser beam profiles and better patient selection. Lasik dry eye was most clearly associated with the ablation of corneal nerves.

When microkeratomes were the most common means of flap creation, flaps of between 150 and 200 microns were usual and this meant that the nerve plexus was involved in the majority of treatments. Thinner modern flaps are around 90 to 120 microns thick, meaning less ablation of nerve tissue and subsequently significantly less post-operative dry eye is seen in femtosecond laser patients. Patients who appear to be in a risk group for developing dry eye symptoms are frequently diverted from Lasik to surface ablation procedures like Lasek. This reduces the chances of symptomatic dry eye still further.

Further reading

Jorge Alio, 10 years after LASIK and PRK: is the cornea stable? Presentation at European Society of Cataract and Refractive Surgeons, London 2006 JL Alió, G. Castro De Luna, D. Ortiz, MJ Garcia Vissum-Instituto Oftalmológico De Alicante, Spain.

Schallhorn S, Tanzer, D, Comparison of Visual Outcomes with Femtosecond & Mechanical Microkeratomes for Wavefront guided LASIK, U.S. Naval Medical Center, San Diego, CA, presentation to the European Society of Cataract and Refractive Surgeons, Lisbon, Portugal, Sep 2005.

Dermott J - All-Laser LASIK in a High-Volume Laser Vision Chain Cataract & Refractive Surgery Today Europe, Januaru 2008, www.crseurope.com/pages/whicharticle.php?id=299.

● Jay Dermott is training and clinical research manager for Ultralase

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