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Keywords:

  • cornea;
  • intra-corneal ring segments;
  • intraocular lenses;
  • keratoconus;
  • photorefractive keratectomy;
  • refractive surgery

Abstract

  1. Top of page
  2. Abstract
  3. Ablative Procedures
  4. Photorefractive Keratectomy for Keratoconus
  5. LASIK for Keratoconus
  6. Excimer Laser Treatment of Scars and Nebulae
  7. Additive Procedures
  8. Phakic Intraocular Lenses
  9. Refractive Lens Exchange
  10. Combined Procedures
  11. Conclusion
  12. References

Traditionally, keratoconus has been managed with glasses when mild, contact lenses when moderate and keratoplasty when severe. When cornea-based refractive surgery was first developed it appeared to be a useful option for keratoconus until reports of post-operative progressive ectasia emerged and thus keratoconus was considered a contraindication to refractive surgery. However, improvements in older techniques and the development of new techniques mean that there are now several viable options to avoid keratoplasty in contact lens-intolerant patients. This review discusses the risks and benefits of excimer laser refractive procedures, both with and without corneal collagen cross linking, as well as intra-corneal ring segments, phakic intraocular lenses and refractive lens exchange with toric intraocular lens implantation.

Keratoconus is a bilateral, asymmetrical, non-inflammatory, progressive thinning disease of the cornea. Protrusion of the cornea develops with a central or infero-central apex. The biomechanical properties of keratoconic corneas are different from those of normal corneas.[1] As the disease progresses, there is increasing myopia and irregular astigmatism and image quality is further reduced by higher-order ocular aberrations.[2] The goals of management of keratoconus are to prevent progression of ectasia, improve refractive error and aberrations, correct ectasia and restore the normal prolate shape of the cornea.

Prevention of progression of ectasia has only become a truly realistic goal with the relatively recent advent of corneal collagen cross-linking. A full discussion of corneal collagen cross-linking is beyond the scope of this particular review but in brief it uses UVA at 370 nm to activate riboflavin to create covalent bonds between collagen fibrils with resultant increase in the biomechanical strength of the cornea. Corneal collagen cross-linking is a safe and efficacious treatment for progressive ectasia with stabilisation or improvement of the maximum keratometric readings, visual acuity, uncorrected vision and spherical and cylindrical refractive measurements.[3-6]

Improvement of refractive error can be achieved with spectacles in only very mild cases of keratoconus, the vast majority of patients requiring rigid contact lenses to achieve satisfactory visual acuity. Once patients become intolerant of contact lenses, then historically the next step would have been to progress to corneal transplantation. There are now several refractive surgical options for these patients which are more conservative than corneal transplantation, including ablative procedures (excimer laser surgery) and additive procedures (intra-corneal ring segments [ICRS], phakic intraocular lenses and refractive lens exchange). There are pros and cons to each method and the variability of phenotypes in keratoconus often makes it difficult to establish which treatment option is the best for an individual patient.

Ablative Procedures

  1. Top of page
  2. Abstract
  3. Ablative Procedures
  4. Photorefractive Keratectomy for Keratoconus
  5. LASIK for Keratoconus
  6. Excimer Laser Treatment of Scars and Nebulae
  7. Additive Procedures
  8. Phakic Intraocular Lenses
  9. Refractive Lens Exchange
  10. Combined Procedures
  11. Conclusion
  12. References

Until recently the presence of keratoconus or forme fruste keratoconus (the milder non-progressive form of the disease) was an absolute contraindication to excimer laser refractive surgery, based on the observation of keratectasia post-operatively in susceptible corneas. Keratectasia is increasing myopia and astigmatism after excimer laser due to progressive corneal steepening centrally or inferiorly. It can occur as early as one week post-operatively and up to several years later. The incidence is greater after laser-assisted in situ keratomileusis (LASIK) but it can also occur after photorefractive keratectomy (PRK). The clinical presentation and topographic changes of keratectasia are very similar to keratoconus. Risk factors associated with keratectasia include thin pre-operative corneas, treatment of high myopia, thin residual stromal beds, pre-operative topographic changes of keratoconus or forme fruste keratoconus in the affected or contralateral eye or a family history of keratoconus.[7-10] The exact pathogenesis of keratectasia post-excimer laser refractive surgery is not clear but it is believed that the thinning and the biochemical stress induced by laser surgery trigger further biomechanical weakening of a pathologic cornea. Debate surrounds whether the actual process of ectasia in these cases is exactly the same as keratoconus or whether it differs. Studies with light and transmission electron microscopy of keratectatic corneas removed at the time of penetrating keratoplasty have yielded variable results, with some reporting that breaks in Bowman's layer typical of keratoconus are found in corneas with post-excimer laser keratectasia, whereas others have reported that this is not the case. Also, while keratectatic and keratoconic corneas both display collagen fibril thinning and decreased inter-fibril distance, immunohistochemical analysis demonstrates differences in protease inhibitor 1—proteinase inhibitor and transcription factor Sp1, suggesting the pathogenesis of the two conditions differs.[11, 12] Standard practice in refractive surgery now includes pre-operative topography, with six to eight per cent of patients requesting refractive surgery being declined treatment on the basis of pre-existing keratoconus and forme fruste keratoconus.[13, 14]

Despite the fact that accepted practice is to avoid excimer laser treatment in patients with existing keratoconus and forme fruste keratoconus, the argument has come full circle, with many surgeons now advocating the use of the excimer laser in these patients. In part this is due to the existence of more advanced excimer laser systems. Advances such as flying spot lasers, wavefront guidance and rapid eye tracking give greater flexibility than before and allow treatment of corneal irregularities as well as the overall refractive error. Furthermore, they allow reduced tissue ablation for a given correction. In keratoconus, with the associated high amounts of aberration, topography-guided systems are perhaps an even better option, potentially allowing correction of optical zone decentration and better irregular or asymmetric astigmatic correction.

Photorefractive Keratectomy for Keratoconus

  1. Top of page
  2. Abstract
  3. Ablative Procedures
  4. Photorefractive Keratectomy for Keratoconus
  5. LASIK for Keratoconus
  6. Excimer Laser Treatment of Scars and Nebulae
  7. Additive Procedures
  8. Phakic Intraocular Lenses
  9. Refractive Lens Exchange
  10. Combined Procedures
  11. Conclusion
  12. References

Photorefractive keratectomy has the benefit of leaving a thicker residual stromal bed after surgery than LASIK and has generally been considered to be a safer option in suspect or thin corneas. Success has been reported in two studies of patients with forme fruste keratoconus undergoing PRK for moderate myopic astigmatism,[15, 16] with post-operative refractive spherical equivalent within 0.50 D of the intended target in 41 to 95 per cent and uncorrected vision (UCV) improving to a mean of 6/7.5. There were no cases of acceleration of disease or development of keratoconus, although one study reported that 7.5 per cent of patients lost one line of visual acuity (VA) due to corneal haze. Based on their results, the authors concluded that photorefractive keratectomy appears to be safe in suspect keratoconic eyes with stable refraction but this may be different in eyes with early frank keratoconus. Cennamo and colleagues[17] reported on the treatment of mild to moderate keratoconus with PRK with topographically supported customised ablation. Post-operatively the mean uncorrected vision and visual acuity were significantly increased and the mean keratoconic topographic indices were all significantly lower than baseline data. There were no complications and the results were stable at 24 months, although the authors conceded that longer follow-up was required.

Despite these encouraging reports most surgeons remain very wary of photorefractive keratectomy in forme fruste or keratoconic corneas, together with the uncertainty of long-term stability, especially in the light of reports of keratectasia after PRK in normal, forme fruste and keratoconic corneas and patients with a family history of keratoconus.[9, 10, 18, 19] Kasparova and Kasparov[20] introduced the novel concept of combined photorefractive keratectomy and phototherapeutic keratectomy (PTK) in keratoconus, the idea being that the photorefractive keratectomy part of the treatment could influence the anterior corneal layers and block the pathological process, stimulating regenerative processes and preventing progression of keratoconus. This was based on their theory that after excimer laser treatment, the ablated layers are replaced by a new fibrocellular membrane with increased solidity and rigidity, which may then act like a ‘corset’ to the cornea, preventing progression. They reported on 70 eyes with keratoconus that underwent photorefractive keratectomy for spherical and cylindrical correction with an additional large zone phototherapeutic keratectomy. Initial results were good with respect to refractive and visual outcome but by six months post-operatively, progression of keratoconus was seen in 8.75 per cent and two eyes required penetrating keratoplasty.

The advent of corneal collagen cross-linking has opened the possibility of an alternative method of stabilising the cornea after excimer laser. Several authors have published studies of PRK combined with corneal collagen cross-linking for progressive keratoconus.[21-25] The collagen cross-linking can either be done at the same time as the PRK or on separate occasions. The technique used by most authors is to use minimal ablation, either limiting the total ablation to 50 μm or ensuring the residual corneal thickness post-operatively is not less than 400 μm. Refractive and visual results, as well as levels of patient self-reported quality of life, have been very promising, despite the fact that the limited ablation often means a full correction of refractive error is not possible. Kanellopoulos[23] compared sequential versus same-day simultaneous corneal collagen cross-linking and topography-guided PRK for the treatment of keratoconus. Three hundred and twenty-five eyes with keratoconus were divided into two groups to receive either collagen cross-linking with subsequent topography-guided PRK performed six months later or collagen cross-linking and PRK in a combined procedure on the same day. The refractive effect of the collagen cross-linking (cornea flattening) had to be anticipated and hence the authors elected to always attempt a significant under-correction of both the sphere and cylinder by at least 30 per cent. Follow-up ranged from 24 to 68 months. Statistically, the same-day simultaneous group did better in all fields evaluated, with greater improvement in uncorrected vision and VA and a greater mean reduction in spherical equivalent refraction and keratometric values. There was also less corneal haze in the same-day simultaneous group and it was theorised that collagen cross-linking at the time of the photorefractive keratectomy is likely to ‘kill’ keratocytes as deep as 300 μm that otherwise after photorefractive keratectomy may become activated as fibroblasts and cause scarring. Further arguments in favour of same-day simultaneous PRK/collagen cross-linking, rather than sequential treatments, include the fact that there may be better penetration of the riboflavin solution through the ablated stroma and the absence of Bowman's layer giving an enhanced collagen cross-linking effect and that when PRK is performed some time after the collagen cross-linking procedure, some of the stiffened cross-linked anterior cornea is removed, minimising the potential benefit of collagen cross-linking.

LASIK for Keratoconus

  1. Top of page
  2. Abstract
  3. Ablative Procedures
  4. Photorefractive Keratectomy for Keratoconus
  5. LASIK for Keratoconus
  6. Excimer Laser Treatment of Scars and Nebulae
  7. Additive Procedures
  8. Phakic Intraocular Lenses
  9. Refractive Lens Exchange
  10. Combined Procedures
  11. Conclusion
  12. References

The rekindling of enthusiasm for PRK for forme fruste keratoconus and keratoconus has not been mirrored for LASIK. Aside from many reports of ectasia following LASIK in patients with pre-operative inferior steepening, forme fruste or keratoconus, as well as patients with a family history of keratoconus,[26-28] results from a study of LASIK used to specifically treat keratoconus demonstrated significant risks with regression of refractive outcome, progression of ectasia, loss of VA in 31 per cent and subsequent penetrating keratoplasty in 18 per cent.[29] On balance, based on literature reports, LASIK should not be considered for patients with forme fruste or keratoconus, as well as patients with a family history of keratoconus.

Excimer Laser Treatment of Scars and Nebulae

  1. Top of page
  2. Abstract
  3. Ablative Procedures
  4. Photorefractive Keratectomy for Keratoconus
  5. LASIK for Keratoconus
  6. Excimer Laser Treatment of Scars and Nebulae
  7. Additive Procedures
  8. Phakic Intraocular Lenses
  9. Refractive Lens Exchange
  10. Combined Procedures
  11. Conclusion
  12. References

The excimer laser has been reported to have a role in performing superficial keratectomy for nebulae on keratoconic corneas. Moodaley and colleagues[30] reported on 10 eyes in which the excimer laser was used to perform direct ablation to proud nebulae. Flattening was achieved in all cases and seven previously contact lens-intolerant patients were able to resume contact lens wear afterwards. The procedure is not without risk and rapid keratolysis and Desçemetocoele formation in a patient receiving excimer treatment for a raised subepithelial nodule has been reported.[31]

Additive Procedures

  1. Top of page
  2. Abstract
  3. Ablative Procedures
  4. Photorefractive Keratectomy for Keratoconus
  5. LASIK for Keratoconus
  6. Excimer Laser Treatment of Scars and Nebulae
  7. Additive Procedures
  8. Phakic Intraocular Lenses
  9. Refractive Lens Exchange
  10. Combined Procedures
  11. Conclusion
  12. References

Alternative approaches to the ablative procedures of the excimer laser are the additive procedures, which include ICRS and intraocular lenses (IOL). As these procedures do not involve the removal of corneal tissue, there is less risk of weakening the structural integrity of the cornea and triggering progression of ectasia, as well as offering some degree of potential reversibility of the procedure should there be any complications.

Intra-corneal ring segments

Intra-corneal ring segments are crescent-shaped half rings made of polymethyl methacrylate (PMMA) that are inserted into channels deep in the corneal stroma (Figure 1). They were initially developed and used to correct myopia, as an alternative to excimer laser refractive procedures but they did not reach the levels of accuracy and predictability of excimer laser treatments. They now play a significant role in the management of keratoconus when patients become contact lens intolerant and when the central cornea is still clear. The goal of insertion of intra-corneal ring segments is not to eliminate corneal disease but to delay or eliminate the need for a corneal transplant by decreasing corneal abnormality and restoring contact lens tolerance. They may also have a use for correction of low myopia in forme fruste keratoconus.

figure

Figure 1. Intacs intra-corneal ring segments, first day after insertion

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The three most commonly used types are Intacs, Kerarings and Ferrara rings. Intacs (Addition Technology Inc, Chicago, IL, USA) were approved for use in low myopia by the Federal Drug Administration in 1999 and for keratoconus in 2004. They are hexagonal in cross section and have an arc length of 150 degrees with an inner diameter of 6.8 mm giving a 6.8 mm optical zone. The thickness ranges from 0.25 to 0.45 mm in 0.05 mm increments. Intacs SK have been developed more recently to work more effectively in cases of moderate to severe keratoconus. They have an elliptical design, give an optical zone of 6.0 mm and are available in two thicknesses (0.40 mm and 0.45 mm). By virtue of their position closer to the visual axis and corneal centre, Intacs SK segments exert a greater flattening effect on the central cornea. The elliptical design was incorporated to lessen halos and glare and enhance central corneal flattening. If the spherical equivalent refraction is -6.00 D or greater or the mean simulated K value is 55.00 D or more, then Intacs SK is the recommended choice. Kerarings (Mediphacos Ltda, Belo Horizonte, Brazil) and Ferrara rings (Ferrara Ophthalmics, Madrid, Spain) are available in variable arc lengths from 90 to 210 degrees, variable thicknesses from 0.15 to 0.35 mm and diameters to give an optical zone of either 5.0 mm or 6.0 mm. The smaller optical zone of these rings means they have a greater effect on the central cornea but may cause problems with glare and halos in eyes with larger pupils. They have a triangular cross section that theoretically induces a prismatic effect to attempt to reduce these photic phenomena.

It was generally thought that all these rings act as spacing elements to shorten the arc length of the anterior corneal surface, thus flattening and stretching the corneal apex, although the exact method of action is now debated. If this assumption is correct, inserting the intra-corneal ring segments into smaller and tighter corneal channels should enhance their effect; however, this has not been found to be the case, with channel size shown to have no effect on refractive outcome.[32] Furthermore, if the apex flattening assumption is accepted, then it is likely that inserting two large intra-corneal ring segments would exert the largest flattening effect, but again this has not been found to be the case. In fact, significantly more improvements in uncorrected vision, visual acuity, K values and cylinder have been found with the use of a single inferior segment than double segments.[33]

Successful implantation of intra-corneal ring segments depends on several factors, including correct placement and accurate depth of implantation. The intrastromal tunnels can be created manually with a spiral mechanical channel dissector or with a femtosecond laser. Complications with mechanical dissectors include epithelial defects, anterior and posterior perforation, asymmetric segment placement and extension of the incision toward the central visual axis or the limbus.[34, 35] The use of a femtosecond laser, such as the Intralase (Abbott Medical Optics, Inc, Santa Ana, CA, USA), allows the creation of tunnels of varying widths and more accurate channel depth throughout the length of the channel compared to manual dissection, with fewer surgical complications.[34, 36, 37] The femtosecond method may also be faster, easier and more comfortable for the patient. The complications related to femtosecond laser-assisted tunnel creation include incomplete channel formation, system malfunction, endothelial perforation and segment migration.[38] When comparing the results of mechanical versus Intralase tunnels, there are no differences in visual or refractive results but there is a higher level of intra-operative complications with the mechanical method and possibly better results for primary spherical aberration, coma and other higher-order aberrations with the femtosecond laser method.[34, 39, 40]

The results of Intacs insertion are summarised in Tables 1 and 2. Most patients experience improvement in spherical and cylindrical refractive error, uncorrected vision, visual acuity and keratometric measurements. An important point to note is that significant refractive error may remain, and in fact insertion is not expected to correct more than 4.00 D of myopia in the best of circumstances. Results can be unpredictable and Intacs are unlikely to produce significant benefits when the maximum K values are over 55 D and with higher pre-operative sphere and cylinder.[41] Loss of two or more lines of visual acuity occurs in 3.5 to 6.1 per cent of eyes.[37, 42, 43] The complete effect may take more than six months[43, 44] and there is no difference in results for different age groups.[45]

Table 1. Summary of studies of visual outcome after insertion of Intacs
AuthorYearRing typeEyesDegree of keratoconus

Mean

pre-op VA

(logMAR)

Mean

post-op VA

(logMAR)

Mean

pre-op UCV

(logMAR)

Mean

post-op UCV

(logMAR)

  1. VA: visual acuity, UCV: uncorrected vision

Zare and colleagues[46]2007Intacs30

Mild/

moderate

0.250.130.600.29
Ertan and Kamburoglu[37]2008Intacs306

Moderate/

severe

0.480.281.100.64
Sansanayudh and colleagues[47]2010Intacs SK10

Moderate/

severe

0.510.251.190.66
Khan and colleagues[48]2012Intacs SK31

Moderate/

severe

0.440.291.400.88
Table 2. Summary of studies of refractive and keratometric outcomes after insertion of Intacs
AuthorYearRing typeEyesDegree of keratoconus

Mean

pre-op SE (D)

Mean post-op SE (D)Mean pre-op cyl (D)Mean post-op cyl (D)

Mean pre-op K

(D)

Mean post-op K

(D)

  1. SE: spherical equivalent, cyl: cylinder

Colin[43]2006Intacs57

Moderate/

severe

-4.60-2.10-4.40-1.4649.746.0
Zare and colleagues[46]2007Intacs30

Mild/

moderate

-6.93-3.23-4.65-3.9049.8447.90
Colin and Malet[42]2007Intacs100Mild to severe-6.93-3.80-4.62-3.3150.146.8
Ertan and Kamburoglu[37]2008Intacs306

Moderate/

severe

-6.04-3.09-4.11-3.8250.747.9
Sansanayudh and colleagues[47]2010Intacs SK10

Moderate/

severe

-8.08-5.03-5.05-3.9057.9450.07
Khan and colleagues[48]2012Intacs SK31

Moderate/

severe

-6.57-2.84  52.0746.15

Visual and refractive results are better when the inferior segment is thicker than the superior segment[49, 50] and under-correction can often be improved by exchanging the intra-corneal ring segments for thicker ones.[51] Furthermore, there is now evidence that a single inferior ring has a better effect than double rings in paracentral and peripheral cones, whereas symmetrical double segments work best for central cones.[33, 50] The theory is that in paracentral and peripheral cones, placement of a segment superiorly in the flat topographic area exacerbates the topographic power asymmetry by causing further flattening superiorly.

Post-operatively white/yellow channel deposits are often seen near the implants. These deposits primarily consist of intrastromal lipid accumulations and keratocytes and are thought to arise in response to corneal injury. They are non-progressive and do not require intra-corneal ring segments explantation.[37, 42, 43, 51, 52] Haze may also be seen in the area of the incision.[43] More serious complications include migration toward the wound, incision melt, dislocation into the anterior chamber, stromal thinning, epithelial breakdown, extrusion of the segments, vascularisation and infection, although these complications do not occur frequently.[36, 37, 46, 48, 53-58] Adverse post-operative visual symptoms may include fluctuating vision, glare, diplopia and halos.[43] These symptoms improve over time and in a study of patient post-operative satisfaction, 76 per cent of patients reported an improvement in their quality of vision after the Intacs procedure.[43] If complications or patient dissatisfaction require removal of the Intacs, it is a relatively straightforward procedure and the cornea returns to its original shape. Rates of removal vary from one to 19 per cent and histological changes appear to be entirely reversible.[37, 42, 43, 48, 59]

While the aim of implantation of intra-corneal ring segments is to defer corneal transplantation and improve both spectacle-corrected visual acuity as well as contact lens tolerance, the presence of the segments can sometimes complicate the contact lens fitting procedure by altering the natural shape of the cornea. Most authors report improved contact lens fit and tolerance post-operatively in most patients but not in all patients.[42, 48] Intra-corneal ring segments create an abnormal corneal profile with the flattest area in the mid-periphery and abnormal relative steepening of the peripheral cornea. As a result, rigid contact lenses may tend to centre over the intra-corneal segments and steepened periphery rather than the corneal apex.[60] Success has been reported in post-operative lens fitting[61] and approaches to managing this have included increasing the lens diameter to obtain adequate corneal coverage and improve centration,[62] mini-scleral design contact lenses, which minimise poor lens centration by vaulting over the entire irregular corneal surface[60] and piggyback lenses.[63]

When comparing Intacs, Kerarings and Ferrara rings, no one type is overall significantly better than any other. A study comparing Intacs SK to Kerarings found no statistical difference between the two in any post-operative outcome, including uncorrected vision, visual acuity, refractive results and keratometry.[64] In contrast, another study found that patients implanted with Kerarings had greater improvement in visual acuity and greater decrease in maximum keratometric values compared with Intacs at one year post-operatively.[39] Kaya and colleagues[65] compared the results of Intacs and Ferrara rings and found no significant difference between the two groups with regard to uncorrected vision, visual acuity and keratometry; however, mean higher-order aberrations decreased in the Intacs group and increased in the Ferrara rings group. In both groups, post-operative scotopic contrast sensitivity decreased significantly when glare was introduced but this decrease was greater with Ferrara rings than Intacs. The decrease was also worse with larger pupils suggesting that in eyes with larger pupils, intra-corneal ring segments with a larger inner diameter are preferable. Conversely, Intacs have a more limited effect in correcting astigmatism than Ferrara rings.[66]

There is debate about whether Intacs halt the progression of keratoconus. Bedi and colleagues[67] reported on 105 eyes with Intacs over five years and found that 93 per cent of eyes with pre-operative progressive keratoconus showed no post-operative progression. Potential explanations may lie in keratocyte activation and new collagen formation, which are known to manifest after Intacs implantation or alternatively, at least in some of the patients, natural cross-linking with increasing age. On the other hand, Colin[43] in a study of 100 eyes found that around 20 per cent of eyes had a reduction in uncorrected vision and visual acuity between one year and two years after insertion of Intacs, as well as a reduction in corneal thickness, all thought to be due to progression of the keratoconus and changes in irregular astigmatism.

The answer to post-operative stability may lie in corneal collagen cross-linking, which may be done simultaneously with insertion of ring segments or at a later date and success has been reported with both approaches. It is not yet clear which approach is better. A study comparing the results in eyes receiving an inferior Intacs segment alone, compared to eyes that received an inferior Intacs segment with simultaneous corneal collagen cross-linking showed that the eyes with Intacs and collagen cross-linking had a significantly greater reduction in cylinder and keratometric values than the Intacs only group.[68] This may be explained by a simple additive effect of the procedures or alternatively, the collagen cross-linking induced increase in biomechanical rigidity in the cornea may lead to increased rigidity locally or across the Intacs segment, producing further flattening. Ertan, Karacal and Kamburoglu[69] confirmed that corneal collagen cross-linking may have an additive effect when performed subsequent to the insertion of ring segments. Kilic, Kamburoglu and Akinci[70] evaluated simultaneous collagen cross-linking and intra-corneal ring segment implantation in 131 eyes. After creation of the Intacs channels with the Intralase, riboflavin solution was injected into the channels prior to insertion of the Intacs, as well as standard topical application of riboflavin prior to exposure to UVA light. The spherical and cylindrical refraction improved, there was significant improvement in uncorrected vision and visual acuity and there were no complications.

Phakic Intraocular Lenses

  1. Top of page
  2. Abstract
  3. Ablative Procedures
  4. Photorefractive Keratectomy for Keratoconus
  5. LASIK for Keratoconus
  6. Excimer Laser Treatment of Scars and Nebulae
  7. Additive Procedures
  8. Phakic Intraocular Lenses
  9. Refractive Lens Exchange
  10. Combined Procedures
  11. Conclusion
  12. References

Alternatives to intra-corneal implants are intraocular lenses. They offer correction of higher spherical and astigmatic errors than intra-corneal ring segments and have a much faster rehabilitation than corneal transplantation. As with all surgical options for keratoconus, they should only be considered when the patient is contact lens intolerant. Furthermore, the patient should have a stable refraction as on-going progression can affect the long-term outcome, the central cornea should be clear and the anterior chamber needs to be of sufficient depth. Disadvantages include the risk of endothelial damage and difficulties with intraocular lens power calculations due to low repeatability of keratometric measurements. They are also less suitable for eyes with advanced keratoconus or highly irregular astigmatism.

Anterior chamber and posterior chamber options exist. Anterior chamber phakic intraocular lenses include the Artisan and Verisyse phakic intraocular lenses (Figure 2). These are iris-supported lenses developed by Ophtec BV (Groningen, Netherlands), which were initially marketed as the Artisan lens in Europe and subsequently marketed as the Verisyse in the USA. The FDA clinical trial published in 2008[71] reported excellent refractive outcomes in 662 eyes with axial myopia from -4.50 D to -22.00 D. Standard (Artisan, Verisyse), toric (Artisan Toric), foldable (Artiflex) and foldable toric (Artiflex Toric) versions are available. Optic diameters vary from 5.0 to 6.0 mm and toric versions have a maximum cylindrical component of 5.00 D. The lens power used is based on the Van der Heijde formula, which depends on central corneal keratometry, anterior chamber depth and pre-operative spherical equivalent. Results in eyes with keratoconus are encouraging. Large amounts of spherical error (up to -22.00 D) can successfully be corrected, as well as highly significant improvement in uncorrected vision. Many patients also achieve increased visual acuity. Cylindrical correction is good at lower levels of astigmatism but limited to a degree by the range of available cylindrical powers in intraocular lenses. Complication rates are low but include halos and glare in low light, endothelial cell loss (up to 10 to 11 per cent at up to five years after intraocular lens insertion), cataract formation, dislocation of the intraocular lens and pigment dispersion.[72-79]

figure

Figure 2. Verisyse phakic intraocular lens. Photograph courtesy of Professor Charles McGhee.

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Endothelial cell loss may be less in keratoconic patients due to a deep anterior chamber, although eye rubbing may make it worse. Baikoff and colleagues[80] introduced the concept of crystalline lens rise, which is defined by the distance between the anterior pole of the crystalline lens and the horizontal plane joining the opposite iridocorneal recesses. Using anterior chamber optical coherence tomography, they looked at 87 eyes with an Artisan intraocular lens (nine with pigment dispersion) and found that the higher the crystalline lens rise, the greater the risk of pigment dispersion. The Artisan and Verisyse intraocular lenses are fixed to the iris by the process of enclavation (entrapment of iris tissue in the claw-like haptics of the lens), which, assuming the surgeon fixes the intraocular lens in the correct axis at the time of surgery, offers a benefit for toric versions over toric intraocular lenses that are not fixed to the iris and hence, may rotate out of the ideal axis post-operatively.

Posterior chamber phakic intraocular lenses are placed between the iris and crystalline lens. Options include the Visian Implantable Collamer Lens (ICL) and the Visian Toric Collamer ICL (TICL) (Staar Surgical Co, Monrovia, CA, USA) (Figure 3). They are made from hydrophilic porcine scleral tissue, a collagen-based biocompatible material and have a plate haptic form designed to be inserted through a 3.2 mm clear corneal tunnel. Potential complications include cataract in up to 33 per cent of eyes, pigment dispersion syndrome, pupil block glaucoma, chronic uveitis and iris ovalisation.[81-84] A laser peripheral iridotomy needs to be performed one week pre-operatively to avoid pupil block. As with anterior chamber intraocular lenses, the visual and refractive results are excellent, with studies showing 96 to 100 per cent of eyes achieving ±1.00 D of the intended spherical target, post-operative residual cylinder less than 1.00 D in 87 per cent and significant improvements in uncorrected vision and visual acuity.[85, 86]

figure

Figure 3. Visian Implantable Collamer Lens (ICL). The arrows indicate the posterior surface of the ICL, and the anterior surface of the crystalline lens. Note the space in between the two lenses. Photograph courtesy of Professor Charles McGhee.

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A disadvantage with any type of phakic intraocular lens is that, unlike corneal-based treatments that aim to normalise the shape of the keratoconic cornea, the intraocular lens can correct only spherical and cylindrical error. It is known that eyes with keratoconus have significant aberrations that affect visual quality, with higher levels of vertical coma, primary coma and coma-like aberrations in keratoconic eyes than in normal eyes.[2, 87] Kurian and colleagues[88] reported on a study of stable keratoconic eyes implanted with the Visian ICL. They analysed the optical performance described in terms of the modulation transfer function (MTF), Strehl ratio and objective scatter index (OSI). While the refractive results were excellent, the mean post-treatment MTF was 18.96 cycles per degree, indicating poor visual quality of the eyes. The Strehl ratio showed that these eyes performed at approximately 10 per cent of the maximum visual quality and the mean OSI was 4.45 indicating significant scattering. They concluded the aberrations associated with the refractive error in keratoconus that are uncorrected by the phakic intraocular lens have an effect on ultimate visual quality.

On balance, phakic intraocular lenses represent a viable option in the treatment of stable mild keratoconus with relatively low irregular astigmatism, a clear central cornea and relatively good pre-operative spectacle-corrected visual acuity.

Refractive Lens Exchange

  1. Top of page
  2. Abstract
  3. Ablative Procedures
  4. Photorefractive Keratectomy for Keratoconus
  5. LASIK for Keratoconus
  6. Excimer Laser Treatment of Scars and Nebulae
  7. Additive Procedures
  8. Phakic Intraocular Lenses
  9. Refractive Lens Exchange
  10. Combined Procedures
  11. Conclusion
  12. References

Once patients have reached the age of presbyopia or in patients with detectable cataract, lens removal with insertion of an intraocular lens is a better option than a phakic intraocular lens. Visually significant cataract has been observed to occur in patients with keratoconus at a younger age (mean 55.3 years) than in the general population (mean 69.6 years).[89] There is an increasing number of toric intraocular lenses available, including ‘off the shelf’ lenses such as the Acrysof Toric intraocular lenses (Alcon Laboratories Inc, Frenchs Forest, NSW, Australia) with cylindrical powers up to 6.00 D and spherical powers from +6.00 to +30.00 D and more customised lenses, such as the Rayner toric intraocular lenses (Rayner Intraocular Lenses Ltd, Hove, UK) with cylindrical powers up to 11.00 D and spherical powers from -10 to +35 D. This enables correction of the high refractive errors associated with keratoconus.

Cataract extraction with implantation of these toric intraocular lenses has been reported to have good results, with significant improvement of uncorrected vision and spherical and cylindrical refractive error,[90-92] although it has also been noted that patients with moderate to severe keratoconus often still require rigid gas-permeable (RGP) lenses after surgery.[89] As with phakic intraocular lenses, irregular astigmatism is a challenge. If the pre-operative difference between RGP contact lens corrected vision and spectacle corrected vision is high, a toric intraocular lens may not be suitable.

Selecting the correct power and axis of the toric lens can be challenging. Intra-operative autorefractometry has been recommended because intraocular lens power calculation by ultrasonic biometry can be inaccurate in almost one-third of eyes.[92] Debate surrounds the issue of which method should be used to determine the axis and power of pre-operative astigmatism. One study found no difference between standard and topography-derived keratometry in terms of the desired spherical equivalent refractive outcome and that achieved[89] but others have suggested that videokeratography-derived keratometric values might be more accurate than standard keratometry and manual keratometry might be the best method to obtain accurate determination of the axis of corneal astigmatism.[91, 93] Furthermore it is important to ensure that keratoconic patients cease wearing rigid gas-permeable contact lenses for sufficient time pre-keratometry to allow the cornea to return to its natural shape. This may take months in patients who have worn RGP lenses for many years. The best lens formula to use in keratoconic eyes has yet to be established. A study comparing three formulae (SRK, SRKII and SRKT) in keratoconic eyes found that in mild keratoconus the most accurate intraocular lens power was found by using the SRKII formula. In moderate to severe cases no valid conclusion could be made.[89]

Refractive outcomes may be further compromised by post-operative intraocular lens rotation. Approximately one-third of the astigmatic correction is lost, if the lens is rotated 10 degrees off axis, two-thirds of the effect is lost with 20 degrees of rotation and a net increase in astigmatism will result if the lens is rotated more than 30 degrees off axis. Haptic style and intraocular lens length play a part in rotational stability, as well as surgical technique with regards to capsulorrhexis size and shape and complete removal of viscoelasticity from the anterior chamber.[94, 95] It is important to monitor the vision and refraction in the post-operative period, for if the toric intraocular lens rotates out of the ideal axis then surgery to correct it is easier before the capsular bags fuse.

If the desired refractive outcome is not achieved or if the keratoconus progresses post-operatively then correcting vision can be very difficult. Because of the irregular corneal shape most of these patients will require RGP contact lenses which will correct the corneal astigmatism but not the cylinder in the intraocular lens. If the implanted intraocular lens is spherical then fitting a contact lens is relatively straightforward but with a toric intraocular lens, the iatrogenic astigmatism must be dealt with on the front surface of the contact lens. In some cases, if successful contact lens fitting is not possible because of the cyclinder in the intraocular lens, the toric intraocular lens may need to be exchanged for a spherical intraocular lens. For this reason, toric intraocular lenses ideally should only be considered in patients with mild keratoconus and low levels of relatively regular astigmatism.

Combined Procedures

  1. Top of page
  2. Abstract
  3. Ablative Procedures
  4. Photorefractive Keratectomy for Keratoconus
  5. LASIK for Keratoconus
  6. Excimer Laser Treatment of Scars and Nebulae
  7. Additive Procedures
  8. Phakic Intraocular Lenses
  9. Refractive Lens Exchange
  10. Combined Procedures
  11. Conclusion
  12. References

Each of these treatments has pros and cons, no one treatment offering the perfect solution for the correction of vision. Therefore, a combination of methods may be considered.

One of the key issues for any intraocular lens procedure is the stability of keratoconus post-operatively. Combining corneal collagen cross-linking with intraocular lens insertion may offer the possibility of improved outcomes. In general, it is believed that the best outcomes are achieved when collagen cross-linking is performed sometime prior to intraocular lens implantation, particularly as corneal collagen cross-linking produces statistically significant reductions in keratometric values and refractive errors over time.[96] Two studies of corneal collagen cross-linking followed subsequently by Artiflex toric phakic intraocular lenses revealed similar results to other studies with the Artiflex intraocular lens, as well as stability of the results, although longer follow-up is required to establish true stability.[97, 98] If collagen cross-linking has not been done and keratoconus is seen to progress after intraocular lens insertion, then the collagen cross-linking can be safely done in the presence of an intraocular lens. Hafezi, Majo and Danasoury[99] studied the effect of corneal collagen cross-linking with riboflavin and UVA on the optical and material characteristics of the Visian ICL and found that the technique did not affect the optical or material characteristics of the intraocular lens, even when the UVA irradiation dose was increased by a factor of ten.

Combining intra-corneal ring segments with intraocular lenses offers the possibility of improved outcomes by combining the effects of improving corneal shape with the intra-corneal ring segments with the ability of the intraocular lens to correct high spherical and cylindrical refractive error. Any type of intra-corneal ring segments can be combined with any type of phakic intraocular lens. Several studies, from single case reports to larger studies,[100-104] report on results of combining Intacs or Kerarings with either an Artisan/Verisyse lens or a Visian ICL, with good visual and refractive outcomes. The insertion of the intra-corneal ring segments can be done simultaneously with the intraocular lens or at a date earlier to the intraocular lens insertion. Benefits of simultaneous surgery include a better view for intraocular lens insertion before the rings are inserted. Benefits of sequential surgery include the option to allow the patient to undergo a contact lens trial after insertion of intra-corneal ring segments before committing to additional intraocular lens insertion and keratometric readings can be taken after insertion of the ring segments, which theoretically allows for better prediction of the intraocular lens power before implantation.

Iovieno and colleagues[105] evaluated the efficacy of Intacs implantation, followed six months later by same-day photorefractive keratectomy and corneal collagen cross-linking in five eyes of four patients with mild to moderate progressive keratoconus and contact lens intolerance. They limited the photorefractive keratectomy to a maximum of 50 μm of stromal ablation to minimise tissue ablation and reduce the risk of ectasia after photorefractive keratectomy. The aim of the photorefractive keratectomy was to treat part of the refractive error and regularise the cornea rather than fully correct the ametropia. Six months after the PRK/CXL significant improvements were noted for uncorrected vision, visual acuity, spherical equivalent refraction, keratometry and total aberrations. No patient lost lines of visual acuity or developed haze. They concluded the combination of implantation of intra-corneal ring segments followed by sequential same-day PRK/collagen cross-linking might be a good option for improving visual acuity in select patients with keratoconus.

Conclusion

  1. Top of page
  2. Abstract
  3. Ablative Procedures
  4. Photorefractive Keratectomy for Keratoconus
  5. LASIK for Keratoconus
  6. Excimer Laser Treatment of Scars and Nebulae
  7. Additive Procedures
  8. Phakic Intraocular Lenses
  9. Refractive Lens Exchange
  10. Combined Procedures
  11. Conclusion
  12. References

Although keratoconus was once considered a contraindication to corneal refractive surgery, many contemporary refractive procedures are now relatively safe alternatives to keratoplasty in contact lens-intolerant patients with keratoconus. Photorefractive keratectomy has become a viable option now that corneal collagen cross-linking can significantly protect against the risk of progressive ectasia post-operatively, although uncertainty around long-term stability means that only patients where ablation can be kept to a minimum should be considered. Intra-corneal ring segments can improve corneal shape and hence visual acuity and refractive error while at the same time having the benefit of being reversible in the case of an adverse outcome. They represent a good option for mild to moderate keratoconic corneas. Phakic intraocular lenses in pre-presbyopic patients and refractive lens exchange with toric intraocular lens implantation in presbyopic patients can correct the high refractive errors associated with keratoconus and can be combined with corneal techniques for improved results; however, caution must still be exerted with all of these techniques as the follow-up period of the relevant studies is still relatively short.

References

  1. Top of page
  2. Abstract
  3. Ablative Procedures
  4. Photorefractive Keratectomy for Keratoconus
  5. LASIK for Keratoconus
  6. Excimer Laser Treatment of Scars and Nebulae
  7. Additive Procedures
  8. Phakic Intraocular Lenses
  9. Refractive Lens Exchange
  10. Combined Procedures
  11. Conclusion
  12. References
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