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HomemiophthalmologyTopography Guided Refractive Ablations

Topography Guided Refractive Ablations

Two refractive surgeons discuss Topography Guided Refractive Ablations and their experience with Drs. David Lin and Simon Holland at Pacific Laser Eye Centre, Vancouver.

WHAT IS TOPOGRAPHY GUIDED LASER?

GM: No longer considered experimental, topography guided (TG) treatments are steadily growing in popularity amongst refractive surgeons. Topography guided laser platforms have been available for almost 10 years now, but since more recent US Food and Drug Administration (FDA) approval of the technology on certain platforms (Wavelight Allegretto T-CAT (Alcon) and the Navex Quest EC-5000 (Nidek) there is an increasing interest in, and evidence base for, this modality of laser vision correction.

The underlying principle is simple – to ‘fix the shape’ as well as the refraction. Instead of correcting a patient’s refraction in isolation, or targeting more complicated wavefront measurements, topography guided treatments aim to correct the corneal anatomy, along with the refractive error. Put simply, if we first aim to make the cornea the best shape it can be, the best possible refraction can often be improved even further.

This ‘shape’ correction is achieved by a combination of targeted excimer laser ablations which elevate depressed areas, and smooth elevated areas to restore a more symmetrical, uniform corneal profile. This is attractive for standard refractive cases in which it now seems a higher level of best corrected vision may be achieved. It is of special interest to the corneal surgeon who is often confronted with highly irregular corneal shape, and increasingly, topography guided treatments are considered as an alternative to transplantation.

IS IT BETTER THAN PREVIOUS FORMS OF REFRACTIVE LASER?

AG: There is growing enthusiasm for topography guided treatments over wavefront guided or standard treatments based on recent studies. The application for TG treatments has primarily been in complex highly aberrated corneas. However, more recently, Alcon obtained FDA approval for TG treatment for primary LASIK patients with myopia and myopic astigmatism using the Allegreto Wavelight excimer laser (Alcon, Fort Worth, Texas). The outcomes from the FDA studies were very interesting. Of the 249 eyes studied, 93 per cent of patients achieved 6/6 or better; 65 per cent achieved 6/4.8 or better and 34 per cent achieved 6/3.8. More importantly, 31 per cent of patients actually saw between one and three lines better unaided than they had ever seen in glasses or contact lenses previously.

Additionally, statistical significance was achieved for improvement in light sensitivity, difficulty driving at night, reading difficulty and glare with the cohort of patients undergoing TG LASIK. These levels of improvement in best corrected vision were surprising and above those usually seen.1 There are several possible explanations for these results. Firstly, TG guided treatments correct the corneal shape, thereby addressing topographical higher order aberrations.

Secondly, TG treatments are also centered on the corneal vertex as opposed to the entrance pupil, which is a more consistent point of reference and induces fewer aberrations from the ablation.2 Thirdly, the treatment accounts for pupil centroid shift – the shift that occurs in pupil position under different lighting conditions. This is relevant to excimer laser corneal ablations since during the wavefront/topography measurement in patient workup, the lighting conditions used are mesopic, whereas during laser treatment, photopic lighting is typically used. The TG-treatment on both the Schwind Amaris and the Wavelight excimer uses limbal and iris vessel data, and the pupil size based on its position during acquisition of the topography maps. These are then matched with the eye of the patient at the time of surgery before proceeding with the ablation. These steps ensure treatment centration and orientation is accurate.

The underlying principle is simple – to ‘fix the shape’ as well as the refraction

HOW DO YOU ASSESS A PATIENT FOR THIS TYPE OF LASER?

GM: If TG treatments aim to optimise corneal shape and refraction, it follows that there are two critical sets of data required for treatment planning – the patient’s refraction of course, and their topographic map. In my practice, I use the Schwind Amaris SmartSurface laser (Schwind Eye-Tech Solutions). I rely primarily on the
SIRIUS topographer as a diagnostic device. It is a combination rotating Schleimpflug camera and Placido disc topographer, allowing high quality analysis of corneal thickness and surface shape.

AG: I use the Alcon Wavelight EX500 excimer laser system for performing all laser refractive treatments. It uses two systems to generate TG ablations, one is placido-based known as the Topolyser, the other is a Tomographer that uses a Scheimpflug camera, known as the Oculyser. The Topolyser is more tear film
dependent and has less effective data in the middle due to the scotoma from a centrally placed camera. Both systems produce quite similar ablation profiles and it’s useful to compare both for various treatment scenarios. Contoura uses the Topolyser based treatment profiles, which also provide the data for pupil and limbal vessel registration. There can be anatomical factors such as deep set eyes, that make acquiring good topography difficult and hence not suitable for TG guided treatment.

HOW DO YOU APPLY TOPOGRAPHIC TREATMENT PLANNING TO STANDARD LASER CASES?

AG: When performing topography guided treatment on ‘normal’ eyes one needs to look at the ablation profile of the higher order topographical aberrations in isolation and calculate the refractive effect it is going to have. This needs to done individually for both the spherical and the cylindrical components of the refractive error. Spherical adjustment: In Figure 1 we note a case where the topography guided ablation profile suggests correction for spherical aberration. It resembles the profile of a hypermetropic ablation, which would induce a myopic refractive effect. The difference between microns of average peripheral vs.central ablation would give us an idea of the myopic refractive effect induced, which then needs to be added to the overall refraction being treated.

Figure 1

Cylindrical adjustment: The topolyser provides a fourier analysis of the corneal shape to demonstrate aberrations like coma. In Figure 2 we see an example of someone with significant coma, measured as decentration. I compare the amount of decentration and its axis on the fourier analysis and modify the amount of refractive cylindrical refractive error being treated accordingly. As the topography guided treatment profile will address coma, in certain cases the total astigmatism treated would need to be reduced. Additionally, the measured topographic astigmatism and axis is taken into consideration when refining the cylindrical error being treated.3

Figure 2

Finally, once the refractive effect of the higher order aberrations has been compensated for, the patient’s refraction as measured by subjective refraction is added. As all laser platforms perform topography guided treatment differently, hence understanding the laser platform is extremely important in interpreting the information and what the treatment means for each patient so they achieve the best possible visual outcome.

TG Treatment For ‘Normal’ Eyes – Case Example

AG: One of my first cases of topography guided LASIK was a 29 year old IT consultant. He felt he had excellent vision in contact lenses but LASIK was a preferred lifestyle option for him as he enjoyed outdoor and underwater sports. His pre-operative best corrected visual acuity was 6/6 in each eye with a refraction of
-3.00/-0.50 x 93 OD and -2.50/-0.25 OS. The first day following topography guided treatment, he said he could see “further than he ever had in contact lenses”. His uncorrected visual acuity measured 6/3.8 in each eye. He is a good example of how topography guided treatment in ‘normal’ eyes can improve visual quality.

HOW DO YOU APPLY TG TREATMENTS TO ’ABNORMAL’ EYES?

AG: Outside of performing TG laser for patients with ‘normal’ eyes, the main indications for TG treatments are to regularise irregular corneas. This includes patients with:
• Keratoconus
• Highly aberrated corneas from scarring
• Decentered optical zones after previous refractive surgery
• Irregular or asymmetric astigmatism
• Post keratoplasty astigmatism and
• Post LASIK ectasia.

It is important to highlight that topography guided treatment in largely normal corneas with a mild degree of higher order aberrations is quite different to TG treatment of highly aberrated corneas. This is because the larger the amount of shape change in the cornea, the larger the induced refractive effect that needs to be compensated for. One such compensatory approach is the topographic neutralising technique or ‘TNT’.

CAN YOU EXPLAIN THE TOPOGRAPHIC NEUTRALISING TECHNIQUE (TNT)?

GM: An early principle established in planning topography guided treatments was that if the shape is altered, often the refraction can be too. The more extreme the shape that the planning software attempts to correct, the more profound this refractive surprise can be. In the early days of topography guided treatments this caught surgeons by surprise. The laser patterns applied to regularise shape are themselves like mini refractive treatments that need to be accounted for by the surgeon or, increasingly, by the software.

For keratoconic corneas in particular, this problem required some thought. One of the early strategies devised to compensate for the refractive shift on the Allegretto platform was the topographic neutralising technique or TNT devised by Dr. David Lin at Pacific Laser Eye Centre in Vancouver. This is a four step algorithm with step one being to consider the effect of the software’s plano treatment – in other words, if we ask the software to correct the shape only, and assume the patient has no refractive error, what ablation profile is generated? Step two is to consider this profile and compensate for any astigmatism it induces. In the example shown (Figure 3), this generates a profile similar to a hyperopic ablation, therefore step 3 is to add a balancing myopic correction. Finally, step 4 is to add in the patient’s actual refractive error to be treated, generating the final ablation profile.4

Figure 3

TELL ME ABOUT TG TREATMENTS FOR ‘ABNORMAL’ EYES, THOSE WITH KERATOCONUS

Good Candidates
AG: Keratoconus patients with milder degrees of irregular astigmatism, with a closer correlation to refractive astigmatism, form the best candidates for topography guided treatment in combination with collagen cross-linking. One needs to be cautious about the degree of flattening that the planning software is trying to achieve as that invariably induces a hypermetropic effect. While this is useful in myopes, those with mixed astigmatism may be less ideal candidates.

Bad Candidates
GM: Advanced keratoconus cannot be treated with topography guided laser. Cases in which the calculated residual stromal depth is less than 300um must be approached with caution. Patients with steep keratometry values (>58) may be too difficult to regularise by methods of tissue removal and may be better candidates for ring segment implantation. Cases with corneal hydrops or scarring will likely always require transplantation. Additionally, I approach patients with unrealistic expectations with a high level of caution. In keratoconus this is a rehabilitative form of laser correction with longer recovery and often more modest goals – this must be understood by the surgical candidate. All cases are combined with simultaneous collagen cross linking. In those cases that are too thin or too steep for TG Photorefractive Keratectomy (PRK), other options such as ring segment implantation or a corneal transplant can be considered.

Keratoconus – Case Example

GM: A 29 year old nurse presented for review of keratoconus. She complained of migraines at work and was under the care of a neurologist for her headache. Her uncorrected vision was 6/60 OD and 6/6 OS with best corrected vision 6/24 OD and 6/5 OS. Her right refraction was –1.75/ –3.75 030 with her left eye close to plano. She underwent topography guided PRK in combination with collagen cross linking.

Three years post treatment, her uncorrected vision in the right eye remains 6/6. A surprising side effect of the treatment was a vast improvement in her headaches with cessation of migraine medication. Optometrists and ophthalmologists caring for patients with asymmetric high refractive error are familiar that this can be a cause of headache often overlooked by our medical colleagues (Figure 4).

Figure 4

AND WHAT ABOUT TREATING ‘ABNORMAL’ EYES WITH POST LASIK ECTASIA

GM: Post LASIK ectasia is a troubling diagnosis for the refractive surgeon. Fortunately, with modern practice and an understanding of risk factors, it is an increasingly rare complication. It does however, still occur and requires early diagnosis and prompt intervention. In the era of cross linking the ectatic cornea can be stabilised, but in moderate to advanced cases, the treatment algorithm becomes very similar to that for keratoconus. Topography guided surface treatments in combination with cross linking are an option for select cases of post LASIK ectasia.

Post LASIK Ectasia – Case Example

GM: A 59 year old man was referred 11 years after myopic LASIK complaining of left visual blur and difficulty night driving. Uncorrected vision was 6/6 OD and 6/60 OS, with best corrected vision 6/24. He was referred for corneal transplant, but TG PRK/cross linking was offered as a possible alternative. Three years post treatment, vision remains stable at 6/12 uncorrected and 6/9 best corrected. There is thus far no recurrence of ectasia (Figure 5).

Figure 5

Post LASIK Ectasia – Case Example

AG: A 57 year old man was referred with a history of LASIK 10 years ago. He presented with monocular diplopia and significant starbursting at nighttime for two months. His topography revealed post-LASIK ectasia. His uncorrected visual acuity in the affected eye was 6/24, with best corrected vision of 6/12 with
a refraction of -0.5/-2.75 x 120. He underwent TGPRK combined with CXL.

Figure 6

I recall him sitting up post PRK in the laser room and saying that the multiple images he saw from his right eye had disappeared. His uncorrected post-op vision at six weeks was 6/9, BCVA -0.75/-1.00×50 = 6/9 and he remains stable six months post treatment (Figure 6).

HOW IS TG USED TO TREAT ‘ABNORMAL EYES’ WITH DECENTERED ABLATION?

AG: Since the advent of advanced laser systems with features of pupil tracking and limbal vessel registration, decentred ablations are seen less frequently.

I looked after a 43 year old patient recently who was referred with poor vision post PRK due to a decentred ablation and some post PRK corneal haze in his right eye. He was frustrated with his visual quality, which was poorer than what he had with his contact lenses.

He underwent transepithelial topography guided PRK (Figures 7 and 8) with application of mitomycin C and did very well with his uncorrected vision improving from 6/12 to 6/6 unaided.

Figure 7


Figure 8

IS TREATING ECTASIA WITH AN EXCIMER LASER AND CROSS LINKING SAFE?

AG: One of the early concerns with laser treatment for keratoconus and post LASIK ectasia was that the laser removal of already thinned tissue would weaken an already weak cornea. Studies presented by Professor John Marshall5,6 have shown that PRK’s effect on corneal tensile strength is predominantly isolated to the surface layers of the cornea. Therefore, with proper treatment planning, the weakening effect is limited to less than 10 per cent of corneal tensile strength. Furthermore, the ablation profile of most TG systems provides more treatment in the superior thick cornea rather than the thinnest point and allows a redistribution of the corneal tension in fibres and improvement in shape.7 Finally, 10 years of clinical experience have now demonstrated that the far more profound change to the cornea occurs as a result of cross linking. In effect, the cornea is weakened by 10 per cent by the laser but then strengthened by a factor of 300 per cent by the crosslinking procedure. After the combined procedure, that patient’s cornea is much stronger with a better and more stable shape than before, which provides the patient with better visual quality. This is reflected in the fact that recurrent ectasia after this procedure is surpassingly rare, but hyperopic surprise due to corneal flattening occurs in five to ten per cent of cases.

CAN TG TREATMENTS BE COMBINED WITH OTHER PROCEDURES?

AG: Specifically for patients with keratoconus and post-LASIK ectasia, the TG treatment is performed as a PRK and combined with corneal collagen cross linking to stabilise the keratoconus. In select cases of keratoconus, I consider implantation of intracorneal rings, such as Keraring, prior to performing TGPRK. My criteria for considering keraring first are:
• Kmax higher than 55D
• High degree of refractive myopia
• Thin corneas where residual stromal bed thickness would be insufficient for CXL if performing PRK alone to achieve the desired refractive outcome.

Combined Keraring and Topography Guided PRK with CXL – Case Example

AG: ME was a 20 year old male who loved football and was highly intolerant of his rigid contact lenses. His UCVA was 6/48 OD and 6/30 OS. He underwent combined keraring and topography guided PRK and cross linking. He now enjoys uncorrected visual acuity of 6/9 OD and 6/6 OS and is stable two years post treatment.

There always needs to be a detailed clinical discussion with the patient of their expectations and desired refractive outcome as well as the limitations of the treatment in very complex eyes. The aim is primarily to regularise thecornea with a simultaneous aim to minimise residual refractive error.

In this case, the patient who had combined keraring implantation and topography-guided PRK and cross-linking, was intolerant of contact lenses has been able to function well without glasses or contact lenses (Figure 9).

Figure 9

GM: TG PRK is necessarily combined with cross linking in cases of ectasia, and as Dr. Gupta discusses, in cases of keraring implantation. We have also had success in performing this procedure as a late treatment for eyes that have previously been cross linked or undergone ring segment implantation.

IS TG TREATMENT PLANNING NOW THE BEST WAY TO APPROACH THE IRREGULAR CORNEA?

GM: Despite all its advantages, TG treatment planning is not the best modality for corneal rehabilitation in every case. Highly irregular scarring from infection or trauma can often be better treated with transepithelial phototherapeutic keratectomy (PTK), with topographic systems not yet refined enough to handle the often-geographic shaped irregularities in these complex scars. Corneal scarring that is confined to the pre-Bowman’s layer is better treated by direct peeling as part of a manual superficial keratectomy procedure. In highly advanced cases, the corneal surgeon may still have to resort to that time honoured intervention – a corneal transplant.

Corneal Scarring – Case Example

GM: A 25 year old man suffered from recurrent left herpes simplex keratitis in his teens, leaving highly irregular corneal stromal scarring. Stability had been demonstrated for several years with the patient referred for consideration of visual rehabilitation in the affected eye. In the affected eye pre-op uncorrected visual acuity was 3/36, with best corrected vision of 6/18+ with -5.0/ +3.0 010.

Eleven months post left trans epithelial PTK, uncorrected vision remains stable at 6/12, with best corrected vision of 6/6+4 with -2.25/+2.0 090. Figure 8 demonstrates pre- and post-treatment topography (Figure 10).

Figure 10

CONCLUSION

Topography-guided laser has become an extremely useful tool in refractive surgery, not only for aberrated but also for normal eyes. It provides another tier of refinement to an excellent procedure, giving refractive surgeons an opportunity to make best better, and corneal surgeons another alternative to transplantation.

Dr. Greg Moloney MBBS (Hons), MMed, FRANZCO, FRCSC was Australia’s first surgeon to perform a Boston keratoprosthesis implantation (artificial cornea) and osteo-odonto keratoprosthesis procedure (tooth in eye). He is the convenor and lecturer at the Sydney Eye Hospital DMEK course and has published numerous peer
reviewed articles and presented at national and international meetings. In 2012 he was awarded a Canadian national research award for his work on new techniques in corneal transplantation. For three years in a row (2014, 15 and 16) he won first prize in the American Society of Cataract and Refractive Surgery film festival (corneal and refractive section).

Dr. Aanchal Gupta MBBS, M.Med. (Ophth. Sci.), Grad. Dip. (Refract. Surg.), FRANZCO is a cataract, corneal and refractive surgeon. She was the first ophthalmologist to perform DMEK and intracorneal ring segment implantation in South Australia. She has particular expertise in microincision cataract surgery, corneal transplantation, collagen-cross linking and topography-guided laser for management of patients with progressive keratoconus. In addition to this, Dr. Gupta is involved in all forms of laser vision correction including blade-free LASIK and advanced laser surface ablation. Dr. Gupta has authored numerous peer-reviewed international research publications and is involved in ongoing research in corneal transplantation and keratoconus. She is a clinical lecturer at the University of Adelaide.

References
1. Results from FDA T-CAT-001 clinical study for topographyguided vision correction (with the 400HZ ALLEGRETO WAVE® Eye-Q excimer laser) 2012
2. Arbelaez MC, Vidal C, Arba-Mosquera S. Clinical outcomes of corneal vertex versus central pupil references with aberration-free ablation strategies and LASIK. Invest Ophthalmol Vis Sci.2008;49(12):5287–94
3. Kanellopoulas J. Topography-modified refraction (TMR): adjustment of treated cylinder amount and axis to the topography versus standard clinical refraction in myopic topography-guided LASIK. Clinical Ophthalmology 2016:10 2213–2221
4. Lin DT, Holland S, Tan JC, Moloney G. Clinical results of topography-based customised ablations in highly aberrated eyes and keratoconus/ectasia with cross linking. J Refract Surg, 2012.
5. Marshall J, Angunawela R, Tengroth J. Wound healing and biomechanics of corneal flap creation. Keynote address. XXIV Congress of the European Society of Cataract and Refractive Surgeons. London: Sept. 9-11, 2006.
6. Marshall J. Sub-Bowman’s keratomileusis (SBK) vs. PRK. 11th European Society of Cataract & Refractive Surgeons Winter Refractive Meeting. Athens, Greece: Feb. 2-4, 2007.
7. Cummings, SimLC for Keratoconus, EyeWorld Ophthalmology News July 2012.

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