Surface laser treatments: an alternative to LASIK?

the adjacent tissues. At wavelengths greater than 200nm the thermal effects become more marked locally. However,

even at 248nm the photons still cannot penetrate more than a few microns.10–13

Investigations of a range of excimer lasers have shown the ArF laser to produce the smoothest ablations of the corneal tissue, with minimal collateral damage from thermal diffusion. As an excimer laser, ArF has the advantage that it provides an exact, computer-controlled tissue removal, with a linear relationship between energy density and ablation depth. Dyer and Al-Dhair first noted this property in 1990.3 The major disadvantage of the excimer lasers for refractive surgery is their expense. This arises from the need for a sophisticated containment system for toxic gases, the daily need to replace the gas fills and the requirement for high-quality optical components to prevent irregular tissue removal that results from inconsistent beam energy. For these reasons, solid-state lasers are emerging in the market. However, the present-day excimer lasers are still 193nm ArF lasers.

Laser equipment

A wide variety of laser hardware and software is available. In broad terms, the types of lasers can be divided into broad-beam, scanning slit and flying spot lasers. Both secondand third-generation lasers (scanning slit and spot lasers) have the advantage of a smaller beam, which minimizes the effects of beam irregularities and results in a smoother ablation profile. Flying spot lasers have been in use in the UK since the late 1990s. Their small beam size makes them suitable for use in topographic feedback systems. Some systems utilize a combination of technologies.

Each type of laser has its own advantages and disadvantages. For example, broad-beam lasers are limited as to the maximum ablation diameter and have a high tendency to produce ‘central islands’ (which have a refractive power different to the rest of the ablation), but broad-beam lasers are able to achieve the required ablation profile quicker than are scanning lasers.

Software modifications have been made frequently to adjust the algorithms to provide optimum results. Recent developments have allowed ablation assisted by corneal topography to help customize the laser treatment for individual patients. The most recent developments are the wavefront analyzer (works on a variety of optical principles), and it is now possible to analyze higher order optical aberrations with a degree of accuracy. In theory, wave-

front analysis linked into an excimer laser would allow the correction of whole eye aberrations with the possibility of ‘supervision’. However, as for much of refractive surgery recently, media hype has far preceded clinical outcome data. In addition, it is possible that wavefront-guided treatments would need to be used either in ‘real time’ or with PRK rather than LASIK (because of the aberrations that LASIK causes by cutting the flap itself).

LASIK versus PRK

It is clear that each of these techniques has some advantages over the other. In the UK, PRK has been by far the more popular technique as it was developed earlier. LASIK was developed to overcome some of the problems encountered with PRK. These include retaining Bowman’s layer to give a low level of post-operative haze. An intact corneal epithelium helps to reduce the pain and the chances of surface infection. LASIK patients also enjoy a fast visual recovery. With healing after LASIK, the apoptosis is limited to the lamellar interface, and so less stromal wound healing, and usually the haze is limited to the flap margin only.14

PRK is an inherently safer operative procedure is needed as no cutting of the corneal stroma is involved, and hence the instrumentation is much simpler. The complication rate for PRK rises in direct proportion to the degree of treatment. The complication rate for LASIK is relatively constant, as the major complications are related to cutting the flap. For each individual surgeon, there is probably a point at which the risks of PRK outweigh the risks of LASIK. This point varies from surgeon to surgeon and also from patient to patient, depending on other factors. In the USA, consumer pressure has meant that the vast majority of patients receive LASIK.

The LASIK procedure requires sufficient corneal thickness to ensure the ablation does not approach within 250μm of the endothelium. The risk of significant endothelial damage is small – most studies quote a change in endothelial cell density similar to the physiological change with age,15 even for high myopic corrections. The risk of inducing a corneal ectasia is small if a minimum of 30% of the corneal thickness is left intact, and a thinner flap is cut for thin corneas. A central corneal thickness of <410μm usually contraindicates LASIK.16

A number of studies have shown that the long-term results for low-to-moderate

slit lamp after a few minutes. Post-opera- tive care for the LASIK patient is discussed in Chapter 5.

Complications of LASIK

Complications after LASIK can be classified into three broad groups, summarized in Table 8.1:

•Flap-related;

•Refractive;

•Miscellaneous.

The flap-related complications occur mostly because of mechanical problems, but these are very rare with modern microkeratomes. Epithelial ingrowth results from the entrapment of corneal epithelial cells under the margin of the flap. Flap striae are caused by misalignment, slippage or tenting of the flap during the early postoperative period. Accidental eye rubbing could also lead to this complication. Flap striae can be reversed readily, if corrected during the early post-operative period. Total flap-related complications are quoted as 2–5%, although serious flap complications are about 0.1%.

Diffuse interstitial keratitis is uncommon and needs early aggressive therapy. It is treated with topical corticosteroids and possibly by lifting the flap and cleaning the surface. Retinal haemorrhage is thought to result from pre-existing pathology.19 Epithelial ingrowth, if sight threatening, may need the flap to be lifted and the surfaces treated with absolute ethanol.

achieved 6/6 unaided vision in one study.14 The predictability of achieving 6/12 or better is about 70–87% by 12 months, depending upon the initial level of myopia.17 The results of PRK and LASIK are very similar at 12 months postoperatively.

Accuracy

The percentage of eyes that achieve a residual refractive error within ±1.00D of emmetropia at 12 months is between 68 and 86% for <–12.00D of treatment and 40–50% for >–12.00D of treatment.8

Stability

The refraction has been shown to stabilize within 3–6 months after LASIK.20 Fiander and Tayfour showed a refractive change of

±0.50D between 1 and 6 months in 90.4% of high myopes and 81% of low myopes.21 This quick recovery has been attributed to the limited stromal healing required after LASIK.

Loss of best-corrected visual acuity

Most studies that involve low myopes (<–6.00D) show no eyes losing more than two lines of Snellen acuity. In the high myopia group, 0–9.5% of patients lost two or more lines of Snellen acuity after LASIK. This remarkable result with LASIK is because of the lack of haze and the limited stromal healing that is required.8

Contrast sensitivity

Results

Predictability

Of the patients with a pre-operative myopia of –2.00 to –6.00D, 94.8% achieve >6/12 unaided vision at 5.2 months post-LASIK. This percentage is 62.3% and 36.8% in the –6.00 to –12.00D range and the –12.00 to –20.00D range, respectively.16 Among the –1 to –3D group, up to 36% of the patients

Spatial contrast sensitivity is reduced at high and middle spatial frequencies at both 1 and 3 months post-operatively.22 In one study, contrast sensitivity was studied in 14 eyes (–6.00 to –19.50D) after LASIK. A reduction in contrast sensitivity was detected at 1 month, after which a rapid recovery was noticed, so that by the third month no statistically significant reduction in sensitivity was seen at any spatial

frequency (Catherine Chisholm, personal communication).

High myopia

The increase in IOP during surgery may cause problems in large myopic eyes with weak retinas. Arevalo et al. demonstrated that 20 of 29,916 eyes exhibited vitreoretinal diseases after LASIK (14 retinal detachment, four retinal tears and 2 miscellaneous conditions),19 although this may have been pre-existing.

LASIK for high myopia is possible, but full correction is not always feasible since about 250μm of the corneal thickness has to be maintained to prevent corneal ectasia. The greatest ablation that can be performed depends on the laser and the nomogram used.

Hyperopia

LASIK has been shown to produce slightly better results in correcting low hyperopes than PRK. Condon showed that 80% of the eyes achieved 6/12 or better. The stabilization period was about four times longer than that of myopia. About 7.3% of the patients lost two or more lines of best-corrected visual acuity (BCVA) in the >+4.00D group.7

LASEK

Anyone who investigates refractive surgery for themselves by reading advertisements, looking on the internet, etc., would be forgiven for thinking that LASIK is the latest thing and that PRK is ‘old hat’. Some providers in the UK do not offer any surface laser, even in its latest form of LASEK. They put forward LASIK as quicker, better and very safe, but is this really true?

PRK was superseded by LASIK, particularly in the USA, for complex reasons. The Food and Dug Administration (FDA) made every manufacturer of excimer lasers go through an initial accreditation process for PRK, which meant that the first American lasers on the market had a head start. These were VISX and Summit, both broad-beam lasers. One of the authors used an early Summit laser and the quality of the ablations were not nearly as good as those of today’s machines. The optical zones were only 5mm, there was no blend zone around the optic zone and the surface was rough with visible rings from the expanding diaphragm. Other laser manufacturers, such as Nidek and Technolas, rapidly brought out their own machines with flying spots, scanning slits, etc., which provided much smoother surfaces on the cornea. For surface lasers in par-

ticular, ‘smoother is better’, as there is less chance of haze developing. The standard way to remove the epithelium at the time was to scrape it off with a surgical blade. This leaves a rougher surface than taking it off with alcohol or a rotating brush and, again, tends to lead to more haze. Hence, many American surgeons were put off PRK by the development of bad haze in more patients than was comfortable. There was also the issue of post-operative pain, which can be quite severe and last for several days. When LASIK was introduced it offered a pain-free procedure, quick visual recovery and no risk of haze. This is because the corneal stroma is a relatively quiescent tissue in comparison with the very metabolically active epithelium. It is the interaction between the healing epithelium and the lasered stroma that causes the deposition of glycoaminoglycans (GAGs) and collagen IV, which causes the haze. Putting the laser ablation in the middle of the stroma meant that this was one complication, at least, that LASIK did not have, although a raft of new ones transpired rapidly, as mentioned above.

The public began to realize that there was an operation that was almost completely painless, and that you could see the next day, which gave what has been called the ‘WOW factor’. For a high myope to have clear vision the next day is a powerful selling point to encourage your friends to have the same surgery. Even if the complication rate was 5%, the noise from 95 very happy patients tends to drown out the unhappy five. The American psyche is also one that likes operations that are over quickly, easily and let one ‘get on with your life’. LASIK offered and still offers this. It is the nearest thing to what one might call ‘stealth surgery’ – an operation that even at one day post-operative is hard to detect, even using a slit lamp.

In Europe, the surgeons could use other lasers that were not yet available in the USA. Also, perhaps being of a more conservative nature, they tended to stick with PRK as well as to learn LASIK and begin to offer it to patients. Reports of the complications of LASIK began to filter across from America. This is not to say that LASIK was not taken up in Europe. LASIK is and remains a very successful operation with a very low complication rate. Quite a number of studies have compared LASEK and PRK to LASIK, some-

times with LASIK in one eye and PRK or LASEK in the other.23–25 All these articles

show that the final visual results are the same for both techniques, which is not surprising given that both operations use the same laser, albeit in a slightly differ-

However, this is the nature of progress in medicine and we learn from experience. Some patients, by their nature, are more ‘risk takers’ than others, which is fine as long as we make clear to patients what the risk factors in this surgery are in as much detail as is necessary. Doctors always have a prime duty to do the best they can for their individual patient, even if that patient is a fit –1.5D myope, and they should not be swayed by how much money can be made.

Commercial aspects

Excimer lasers are not inexpensive, with an average price of about £300,000. They are gas lasers and, as well as needing refills every few weeks, they also need regular servicing to clean the mirrors, check alignment of all the mechanical parts, and so on. Some need new laser chambers every year, which cost about £20,000. If they break down complete chaos can result for a clinic, as 20 patients may be waiting for laser treatment on any given day. Hence, one has to have emergency preparations to cover for this event, as it will certainly happen.

The microkeratomes for LASIK cost around £45,000 each and at least two headsets are needed, at an extra £12,000. The blades cost between £20 and £40 each and can be used for a maximum of one patient. Finally, there are the costs of marketing, staffing, billing and all the other accoutrements of a modern business. Optometrists will be aware of these costs, as they are similar in type, if not in degree, to those of any optometric practice. Hence, it is not surprising that commercial groups have set up many of the laser centres. Individual entrepreneurs, existing optometric chains, groups of businessmen and health care groups have all played their part. In the early-to-mid 1990s up to 40 different centres in the UK provided PRK. However, a sharp downturn occurred in the market and many of theses centres ceased to operate.34,35

However, doctors used to dealing with sick patients in the NHS, with perhaps a private practice on the side, are not used to the commercial world. Doctors used to be forbidden by the General Medical Council (GMC) to advertise at all, and even now there is a fine line between what is permitted and what is not. However, there is nothing to stop laser centres from advertising widely. Different centres put forward what they think is their best selling feature. This may be price, or the qualities of a particular laser system, or the backup of a large hospital, which are all legitimate selling tactics.

On selling features, it is interesting that three of the authors’ consultant ophthalmologist peers have had LASIK, but chose their surgeon by reputation and not by the laser being used. It could be argued that modern excimer lasers are of good quality and the quality of the individual surgeon is the most important factor.

Ophthalmologists or optometrists employed by commercial laser centres have to maintain their clinical freedom to provide what they think is ethically the best treatment for each particular patient. This can lead to conflict with the business side of the enterprise, the main purpose of which is to make money for individuals or shareholders. Businessmen can apply pressure to treat unsuitable patients, and this must be resisted by any optometrist and/or ophthalmologist.

In surveys of patients who have had refractive surgery, always about 5% are ‘disappointed’. Sometimes the reason for this is clear to both doctor and patient, for example bad haze in PRK or flap striae in LASIK. However, in other cases it is not so clear why the patient is unhappy. The doctor or optometrist may think that a –8D myope who has a post-operative refraction of –0.75D should be happy and grateful. However, the patient’s appreciation is just that his or her uncorrected vision now is not quite as good as it was with contact lenses or glasses before surgery. Conversely, and perhaps fortunately for the surgeons, some patients are so pleased that their uncorrected vision is better that they do not notice they have lost some best-corrected spectacle vision, for whatever surgical reason. We think that they should be unhappy, but in fact they may be delighted.

Unilateral or bilateral treatment?

One question that often arises in the LASEK versus LASIK debate is whether to perform the procedure unilaterally or bilaterally. It was common in PRK surgery to separate the procedures for each eye by a few months, to allow the first eye to stabilize. LASEK has a quicker recovery period than PRK, and if applied to only low prescriptions this recovery time is very short. As a result, some surgeons advocate bilateral LASEK. In the case of LASIK, bilateral procedures have been the norm for a

while, so both eyes are treated while the patient is still on the operating table.36,37

There is obviously much financial sense in this for the various clinics or doctors, as it increases the throughput of eyes treated and hence the profits. However, what is

best for the individual patient? The arguments full into three groups: safety, accuracy and subjective.

Safety

Clearly, in a planned bilateral operation if there is any intra-operative complication in the first eye, treatment of the second should be abandoned. In practice, the complications are usually flap problems. The authors also include any marked epithelial loss or ‘slide’ caused by the passage of the microkeratome, because the incidence of the ‘sands of the Sahara’ syndrome is ten times higher in such cases and does not present until day 1 after the surgery if it occurs. Such epithelial problems occur more often in older patients. Any infection, inflammation, flap wrinkles, etc., almost always show up on day 1 post-operatively, so waiting 2–7 days between eyes should avoid a simultaneous bilateral problem.

LASIK and PRK are dependent on the technology of the excimer laser machine, which are gas lasers that are calibrated frequently. They are complicated machines, but are generally reliable and have many safety features. However, if a technical problem occurs and both eyes are treated, it could affect both eyes adversely without the surgeon being aware of it at the time of surgery. The most likely fault is a simple overor undertreatment, which could be corrected fairly easily in most cases. Worse is when a beam irregularity occurs. Checks are carried out on all the common machines to pick this up, but there has been a reported case in Canada of a mirror problem in a machine that resulted in irregular astigmatism in a group of bilateral patients. These patients were corrected with much angst, and it was realized that the fault in the machine should have been picked up before the surgery. However, it is a salutary lesson!

Accuracy

As a general rule it is fair to say the more extensive the treatment, the larger the spread of results (accuracy issues are dealt with in more detail below). If a patient falls within the normal spread of dioptres, then to treat both eyes at the same sitting does not give any significant increase in accuracy than to operate on them apart, which is the normal experience of most patients. However, if an eye behaves oddly, at the extremes of the statistical spread, operating on the eyes separately allows the surgeon to alter the laser settings for the second eye. Scientific articles disagree as to whether this really makes any difference, but the latest ones seem to indicate about a 20% improvement in accuracy.

Hence, for a –7D correction with a standard deviation of 0.93D, this leads to a mean improvement in accuracy of about 0.2D, which is small.

Subjective

Sometimes, especially in very high myopes, full recovery of vision can take up to 2 weeks. Thus, if both eyes are lasered on the same day, the patient may struggle to cope for this period. Similarly, the patient may find that the quality of vision after LASIK in one eye is not to his or her liking. For example, there may be night-vision problems, and so the patient may not want to go ahead with the second eye. For these and similar reasons patients may prefer one eye to be operated on at a time. However, unilateral treatment does lead to a feeling of anisometropia, which for some patients may be too uncomfortable to tolerate with a spectacle correction. This can, of course, be overcome by contact lens use in the non-treated eye, unless contact lens intolerance was the reason behind the patient’s initial option for surgery.

LASIK on both eyes on the same day is more convenient, but is a slightly greater risk than operating on separate days. It is up to each patient to decide what he or she wants to do, after consultation with the individual surgeon. In most cases the risk is the same whether both eyes are operated on 2 minutes apart or several hours apart on the same day, and in some cases the risks may not be reduced significantly by separating treatment to each eye by a few days.

Photorefractive keratectomy

With PRK the situation is a little different. The only real complication of PRK is haze, which is not an immediate post-operative complication, as most LASIK complications are. Haze is maximal at around 6 weeks post-operatively, although there are cases of haze developing late at 6 months or occasionally even later.38,39 Hence, when PRK began, the eyes were operated on separately, at least 3 months apart, to reduce the risk of bilateral bad haze. The risk of haze is greater with higher myopic prescriptions and also with hyperopia, because of the more extensive shape changes made on the cornea with such prescriptions. Also, the speed of recovery of useful vision after PRK for high myopia is slower than that for low myopia, as the epithelium ‘models’ its new shape. Most refraction occurs at the first air–fluid interface of the cornea, which is disturbed much more by PRK than by LASIK.

PRK has improved incrementally over the years. Newer lasers leave a smoother surface, and blend or transition zones also

ent capacity often manage their own complications or may refer privately to a more experienced colleague. However, in law there is nothing to prevent a refractive patient from being referred to the NHS for a second opinion by his or her GP.

Generally, such problems are well managed through mutual respect among professionals, with a sense of proportion and goodwill on both sides. In fact, with common sense, relationships between public and private sectors can be managed to everyone’s satisfaction, including that of the most important individual, the patient.

Accuracy issues

At present there is no way to measure what an excimer laser is doing ‘in real time’ (i.e., while a cornea is under the laser). Hence, if one person’s cornea is more or less dense than the average, the laser will take off more or less tissue with each pulse. Surgical technique is particularly important in this respect. The longer the surgeon takes doing the surgery, the more the cornea dries out and compacts, which means that the laser tends to take off more tissue and overtreat the patient. Hence, it is very important for each surgeon to develop a standard technique. The excimer laser does not penetrate water. Some surgeons have a ‘dry’ technique, in which the corneal surface is wiped a lot, whereas others have a relatively ‘wet’ technique. Hence, each surgeon has to develop his or her own personal algorithms. Even using the same laser in the same clinic, two surgeons may use slightly different

laser settings. Although the room in which a laser is situated is air-conditioned, the geography can make a difference. Hence, a laser in, for example, Calgary, which is at high altitude and has very dry air, will tend to overtreat more than a laser in Houston, with a high humidity in the operating room despite the best efforts of the air conditioning.

Corneal healing also plays a part. The epithelium in both PRK and LASIK may hypertrophy and alter the result. In the final analysis, refractive surgery is about biological systems and not electromechanical ones,

Attempts to try and measure laser ablation during surgery are ongoing. The main problem is that the smooth air–fluid interface is highly disturbed by the act or surgery. Topography machines of various sorts are thus not useful. Research is underway to try and establish real-time measurements by using interference patterns. Haag–Streit now supply a non-con- tact optical coherence pachymeter, which is claimed to measure corneal thickness to an accuracy of 1 or 2μm with great speed. Some laser manufacturers are testing whether to fit this to their systems to establish a feedback loop. Corneal thickness can be measured just before surgery and then as the ablation proceeds. When the required amount of tissue has been removed, the laser is turned off.

We can be certain of one thing – that refractive technology will advance and improve with time. Professionals in the field should keep up with these advances, try to discern the reality from the hype, and so offer the best solution to their patients.

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