10
Future trends in refractive surgery
Shehzad A Naroo and W Neil Charman
Introduction
Instrumentation and techniques in refractive surgery are improving all the time. This is evident when we look at the two video clips (see Chapter 6 and the CD Rom) of a flap being cut in a LASIK procedure. Video clip 2 shows the Hansatome microkeratome from Bausch and Lomb, which is by far the most popular microkeratome in the UK today. It is a two-piece device and cuts a hinge in the superior position. If we compare this to video clip 3 of the Amadeus microkeratome from Advanced Medical Optics (AMO), it is clear that the cutting of the flap seems to be simpler with the one-piece Amadeus. The Amadeus cuts a flap in the nasal position, which some surgeons argue is better than a superior hinged flap, while others argue the opposite viewpoint.
Currently, there are a few very large manufacturers of refractive surgery equipment (excimer laser, microkeratomes, aberrometry equipment, etc.) and a handful of smaller companies. The past few years have seen many smaller companies being purchased by the larger ones to strengthen expertise in a particular area previously lacking in the larger company’s portfolio. This seems to benefit both sizes of company, as the larger players are able to support the research and development of the smaller ones and to merge their ideas with those that may exist in another area in which the larger company already has expertise.
When discussing future developments of refractive surgery it is possible to speculate on many things. A few of these are outlined in this chapter but first the likely growth in the industry is considered.
The UK market place
The past decade has seen an exponential growth in the refractive surgery market. In the UK the number of surgeons who perform a variety of techniques (laser and nonlaser) seems to be relatively steady, but an increasing number of surgeons are becoming involved with laser refractive surgery. The number of specialist refractive surgery clinics in the UK is interesting. These clinics offer only refractive surgery, for which some employ full-time refractive surgeons, while others rely on session surgeons, who may be involved in other areas of ophthalmology in a NHS hospital the rest of the time. A few years ago, photorefractive keratectomy (PRK) was gathering momentum in the UK, but then many clinics closed. More recently, we have seen an increase in clinics that specialize in laser in situ keratomileusis (LASIK), but some of these have closed also. One high-street clinic that relied heavily on co-management schemes with nation-wide optometrists was taken over recently by a high-street optical retailer. Some of the other group optical retailers have also chosen to become involved in refractive surgery, while some have decided not be involved at all. A few large-volume refractive surgery clinics appear to meet most of the nation’s refractive surgery needs and these clinics seem to be able to adapt to incorporate each new wave of popular techniques and hence seem to be here for the long term. Some of these clinics have grouped together to form the Eye Laser Association (ELA), which is able to promote refractive surgery and increase awareness of the area for interested patients and practitioners.
It is often suggested that the market place in the UK is a step behind that in the
USA. In the USA, the Food and Drugs Administration (FDA) began to approve excimer lasers for refractive surgery in 1996. Initially, the approval was granted to some excimer laser manufacturers for limited refractive errors. Gradually, further approvals were granted and the industry continued to grow. Meanwhile, in the UK excimer lasers were already being used for PRK and LASIK was beginning to emerge. The USA surgeons did not have the same early exposure to PRK as many of their global counterparts, which may partly explain some of the trends seen in the USA. For example, in the USA radial keratotomy remained popular, whereas in many other parts of the world it had a limited or short-lived popularity. Over the past few years there has been a steady growth in the refractive surgery industry in the USA (Table 10.1). However, the percentage growth year on year is declining steadily, which might suggest that growth in the industry is actually slowing down. Maybe this is to be expected, as the initial large group of patients who had waited to undergo this type of surgery have now explored this opportunity. There may now remain a slower flow of patients who reach the correct age or stability of ametropia to have refractive surgery. Perhaps when the next ‘big thing’ happens in refractive surgery, we will see another boom and then a gradual decline again.
Possible use of new types of laser
Over the first two decades of laser refractive surgery, the argon fluoride (ArF) excimer laser, which emits at 193nm, reigned supreme in terms of its ability to create
Table 10.1 Growth of the refractive surgery market in the USA
Year |
Number of treatments |
Growth (%) |
1997 |
182,000 |
|
1998 |
409,500 |
125 |
1999 |
980,000 |
140 |
2000 |
1,550,000 |
58 |
2001 |
2,250,000 |
45 |
effective corneal ablations safely and within an acceptable time span. Nevertheless, such gas-based lasers have their disadvantages in terms of cost, safety, beam stability and maintenance. For this reason, the search continues for both alternative laser sources and different approaches to laser refractive surgery.
Several solid-state lasers have been developed with outputs at wavelengths around 200nm, similar to that of the ArF excimer laser. These include a flash-lamp pumped laser that employs the fifth harmonic of neodymium–yttrium aluminium garnet (Nd:YAG) and emits at 213nm, a diode-pumped fifth-harmonic neodymi- um–yttrium lithium fluoride (Nd:YLF) laser at 209nm and a fourth harmonic titanium–sapphire crystal laser that works at 208nm. None of these appears to have gained widespread acceptance to date, but undoubtedly the search for other alternatives will continue.
A more radical departure from current practice is offered by the use of pulses of infrared laser light of ultra-short duration to operate on the cornea.1 The Nd:YLF picosecond laser is used, which emits at 1053nm. The cornea is highly transparent at these wavelengths. When used to cut a corneal flap, the laser is focused at the required depth (e.g., 160μm) and the focused spots are moved in a spiral fashion outwards from the centre of the cornea. Since the pulses are of very short duration, each achieves a photodisruption effect mediated by plasma formation, stress waves and cavitation bubbles at the application site, rather than causing a thermal burn. High repetition-rate pulsing (1–2kHz) and the spiral movement mean that the ‘cut’ normally achieved by a microkeratome is created by a joining up of the ‘sheet’ of photodisruption sites, the flap being brought to the surface of the cornea by additional pulses anterior to the periphery of the spiral pattern. Once the flap is lifted, ablation of the underlying stroma can be undertaken with an excimer laser, as normal. A more ambitious procedure (picosecond laser keratomileusis) involves placing two overlying
spiral patterns at different and varying depths, so that an intrastromal lenticule is cut. This can be removed through a suitable aperture to correct the eye. As yet, only limited trials have been carried out, but doubtless further refinements, perhaps that employ different lasers, will follow.
Surgical restoration of accommodation
In normal eyes, the amplitude of accommodation declines steadily through the early and middle years of adulthood to reach zero at around the age of 50 years. Beyond this age, there is no true change in the power of the eye when objects at different distances are observed, although ocular depth-of-focus allows objects to be seen with reasonable clarity over a limited range of distances, which gives a subjective amplitude of accommodation of around 1D. Some form of near-vision correction is therefore normally required by most people older than about 40 years.
The loss in amplitude of accommodation is thought to be multifactorial in origin. As the lens ages, it increases in thickness and loses elasticity, as also does its capsule. There are changes in the geometry of the attachment of the zonule to the lens and the gap between the ciliary ring and the lens equator diminishes. The ciliary muscle is, however, thought to retain its power until much later in life.
This situation has led to two approaches to try to extend the period over which accommodation is possible into later life. In the first of these, the natural, relatively rigid lens is removed from the capsule and is replaced by a more elastic synthetic material with suitable characteristics.2,3 In the second approach, it is reasoned that the reduced gap between the ciliary ring and lens equator limits the possible extension of the zonular fibres and their associated changes in tension: hence, if the gap could be increased, accommodation would be restored. It is therefore suggested that the diameter of the ciliary ring be increased via ‘scleral
Future trends in refractive surgery ■ 77
expansion’, in which a series of cuts are made in the sclera around the cornea and plugs are inserted to expand the sclera and, with it, the ciliary body. At present neither technique has demonstrated real success, perhaps because each concentrates on only a single factor related to presbyopia. Objective measurements of patients who have undergone scleral expansion surgery have so far shown no evidence for restored accommodation,4,5 although claims have been made that subjective amplitudes are increased. With the development of better materials, the lens replacement method may eventually be at least partially effective, although it obviously cannot compensate for the loss in capsular elasticity or for other changes in the lens–ciliary body complex.
Accommodation is obviously also lost after cataract or clear lens extraction and intraocular lens (IOL) insertion. Bifocal, multifocal and varifocal IOLs can offer patients reasonable distance and near vision, but with the penalty of a generally reduced image contrast at all object distances. For some patients, ‘pseudo-accommodation’ – actually enhanced depth-of-focus caused by small pupil diameters, and small amounts of astigmatism and, perhaps, higher-order aberration – can also give a reasonable range of clear vision. Recently, however, several ‘accommodative’ single-vision IOLs have been produced and show considerable promise. The concept of these is that, although the IOL itself does not change power, it moves forward within the eye for near vision, so that the combined power of the eye and cornea increases. Several designs of lens are being evaluated currently. All are designed so that pressure on the lens supports (haptics) flexes them in such a way that the desired movement of the lens optic can be achieved. Different designs use changes related to near-vision in the ciliary body, capsular and vitreous pressures to produce the required movement. Optical and physiological constraints limit the achievable objective accommodation to about 1.0–1.50D, but (unlike the multifocal IOLs) no compromise in retinal image quality is involved. Although the longer-term stability and performance of these lenses have yet to be explored, the initial results are encouraging.6
Finally, it is clear that, in principle, PRK or LASIK-type ablations could be used to produce many of the types of ‘static’ corrections for presbyopia achieved by contact lenses or IOLs. Thus, for example, one eye could be corrected for distance vision and one for near, to give a monovision correction. With scanning-spot lasers and suitable controlling algorithms, bifocal,
78 ■ Refractive surgery: a guide to assessment and management
multifocal or varifocal correction to an eye should be possible. Attempts along these lines have so far not been particularly successful, however, possibly because the scanning-spot sizes were too large to achieve the desired corneal topography. Replacement of the flap in LASIK will obviously tend to smooth out the abrupt transitions in power across the pupil required for true bifocal or multifocal geometries.
Intraocular contact lenses
The main role of IOL implants has been to replace the power of the crystalline lens after cataract removal, but manipulation of the lens parameters can offer more possibilities for refractive purposes.
Calculations of the required lens parameters from ocular dimensions and powers for intraocular insertion are widely known. There are now very useful systems for accurate measurements of these ocular parameters, which enable better visual outcomes from the insertion.
An example is the IOLMaster (Carl Zeiss, Jena, Germany), a recent non-inva- sive device that provides a complete set of ocular dimension measurements. It uses partial coherence interferometry to measure anterior chamber depth and axial length. The measurements obtained have been proved to be as accurate as those of ultrasound biometry,7 and are highly repeatable.8 It has the benefits of its noncontact character, and its corneal power measurements, obtained using image analysis, show a high correlation with those obtained with conventional keratometry7 and videokeratoscopy.8
The use of phakic IOLs for refractive correction, without crystalline lens removal, is discussed in Chapter 4, but here it is worth
mentioning these devices when used in conjunction with laser surgery. In cases of high ametropia, some surgeons are beginning to advocate the use of anterior or posterior chamber phakic IOLs – often referred to as implantable contact lenses9
– in conjunction with a partial correction by excimer laser surgery. This combined technique of a phakic IOL and laser epithelial keratomileusis (LASEK), PRK or LASIK is known as bioptics.
It can be expected that for patients with high ametropia, future wavefront analysis systems and lasers will allow an IOL to be inserted to correct the majority of the ametropia. The cornea would be reshaped by excimer laser to yield total wavefront aberration correction, if this could not be achieved with the IOL itself. This may increase the chances of creating a visual acuity better than would be expected with other refractive surgery procedures in some patients (e.g., patients with over 10D of myopia). A single laser procedure on a patient with this degree of refractive error may require the removal of so much corneal tissue that the patient must have quite a thick cornea originally: at this level of laser correction the chances of reaching a satisfactory post-operative refractive error may be slim. The bioptics technique may give the patient a better chance of reaching emmetropia, as the laser part of the surgery would only top up the change of refractive error achieved with the phakic IOL. Of course, one immediate problem that comes to mind is that the patient would have potential risks from both procedures: that is, not only the complications of the laser refractive surgery, but also the risk of complications of cataract, chronic iritis or
Figure 10.1 |
Figure 10.2 |
Bausch and Lomb Orbscan IIz |
The Pentacam. Courtesy of Birmingham |
|
Optical Group |
endothelial cell loss with the phakic lenses commercially available at the moment.
Another implantable contact lens device that is worth noting is the corneal inlay. This has actually been around for some time in various forms. In the early days, synthetic or human corneal implants were placed underneath corneal buttons created in keratophakia.10,11 More recently, with the development of different synthetic materials and more precise methods to create a hinged corneal flap with a microkeratome, this technique has appeared again. It may prove to be useful for certain groups of refractive errors.
Advances in measurement devices
Equipment used in preand post-operative assessment in refractive surgery has improved greatly over the past few years. One example of this is the corneal topography devices. Although interest in assessing corneal topography has been around for many years, the real turning point was probably the introduction of colour-coded maps, initially by Klyce12 and later by Maguire et al.13 The revolution in corneal topography was probably aided by the growth in other areas of ophthalmic work, such as contact lenses, but probably the major driving force was the commercial nature of the growing refractive surgery market. Nowadays, nearly all manufacturers of excimer lasers have a compatible corneal topography device.
We have also seen a move from the traditional Placido devices (as described in Chapter 2) to instruments like the Orbscan Corneal Analysis unit from Bausch and Lomb (Figure 10.1). Although the Orbscan has been around for a few years it has remained almost exclusively the only instrument able to calculate the anterior and posterior corneal shape, corneal pachymetry and anterior chamber. One version of the Orbscan incorporates a Placido disc to enable more accurate data to be obtained from the anterior cornea. The most recent adaptation of the Orbscan uses software that links directly into the Bausch and Lomb Zyoptix custom-abla- tion system, which includes an excimer laser and wavefront aberrometer. Other manufacturers have made similar systems to create custom ablations, linking their excimer lasers with either corneal topographers or aberrometers, or both. A recent instrument from Oculus, the Pentacam (Figure 10.2), uses a rotating Scheimpflug camera and is able to show the shape of the anterior and posterior cornea (Figures
10.3 and 10.4), the corneal thickness and the anterior chamber depth, as well as the shape of the crystalline lens. If the patient’s pupil is dilated, the Pentacam can map the anterior and posterior lens curvatures. This is a very useful device for looking at IOLs, as well as the results of corneal surgery.
Conclusion
Although challenging problems remain to be solved completely, some millions of patients have already received substantial benefits from refractive surgery. The prospects are good for further developments and refinements that will reduce complications from their already low level and give standards of vision that will consistently equal, or even surpass, those provided by other methods of refractive correction. Eyecare professionals will, undoubtedly, want their patients to have full and informed access to the new opportunities for improved quality of life brought about by these advances in refractive surgery.
REFERENCES
1Krueger RR (1998). The picosecond laser for nonmechanical laser in situ keratomileusis. J Refract Surg. 14, 467–469.
2Nishi O, Nakai Y, Yamada Y and Mizumoto Y (1993). Amplitudes of accommodation of primate lenses refilled with two types of inflatable endocapsular balloons. Arch Ophthalmol. 111, 1677–1684.
3Haefliger E and Parel JM (1994). Accommodation of an endocapsular silicone lens (Phaco-Ersatz) in the aging rhesus monkey. J Refract Corneal Surg.
10, 550–555.
4Elander R (1999). Scleral expansion surgery does not restore accommodation in human presbyopia. J Refract Surg. 15, 604.
5Mathews S. (1999). Scleral expansion surgery does not restore accommodation
in human presbyopia. Ophthalmology 106, 873–877.
6Kuchle M, Nguyen NX, Langenbucher A, Gusek-Schneider GC, Seitz B and Hanna KD (2002). Implantation of a new accommodative posterior chamber intraocular lens. J Refract Surg. 18, 208–216.
7Nemeth J, Fekete O and Pesztenlehrer N (2003). Optical and ultrasound measurement of axial length and anterior chamber depth for intraocular lens power calculation. J Cataract Refract Surg. 29, 85–88.
8Santodomingo-Rubido J, Mallen EAH, Gilmartin B and Wolffsohn JS (2002). A new non-contact optical device for ocular biometry. Br J Ophthalmol. 86, 458–462.
9Zaldivar R, Davidorf JM and Oscherow S (1999). Combined posterior chamber
Future trends in refractive surgery ■ 79
Figure 10.3
Image of the anterior eye cornea viewed by the Oculus Pentacam. The anterior and posterior corneal surfaces are visible, as well as the anterior crystalline lens surface. Early opacities can be seen in the crystalline lens. (Courtesy of Birmingham Optical Group)
Figure 10.4
In this Oculus Pentacam image of the anterior eye the brightness has been adjusted so that the anterior and posterior corneal surfacesare less clear, but there is a clear image of an IOL resting in the lens capsule after phacoemulsification cataract extraction and IOL insertion. (Courtesy of Birmingham Optical Group)
phakic intraocular lens and laser in situ keratomileusis: Bioptics for extreme myopia. J Refract Surg. 15, 299–308.
10Barraquer JI and Gomez ML (1997). Permalens hydrogel intracorneal lenses for spherical ametropia. J Refract Surg. 13, 342–348.
11Werblin TP, Patel AS and Barraquer J (1992). Initial human experiments with Permalens: Myopic hydrogel intracorneal lens implants. Refract Corneal Surg. 8, 23–32.
12Klyce SD (1984). High resolution graphic presentation and analysis of keratoscopy. Invest Ophthalmol Vis Sci.
25, 1426–1435.
13Maguire LJ, Singer DE and Klyce SD (1987). Graphic presentation of computer-analyzed keratoscope photographs. Arch Ophthalmol. 105, 223–230.