3 months is too short period of follow up for the stability of visual acuity (VA) after this particular surgical procedure. Since you additionally had CXL and as it's known already during the first months after CXL K readings are increasing which also have influence on VA. There are a lot of studies published showing 3, 6, and the 12 month outcomes, and by the 12th month all the corneal parameters and VA as well are mostly stabilized.

Depend on your target and goals. Three months is enough time to report visual outcome improvements after TG-PRK/CXL treatment for Keratoconus, if you want to base your conclusions on the TG-PRK part of the therapy. For those even 1-month would be better.

For reporting stability, even 12 months are not long enough since there are reports on conitnuous flattening after CXL even after 2 years from the therapy.

I would suggest in any report for this:

1 month for the ablative contribution

longer than 12 months for the CXL

"To customize or not to customize in Keratoconus"

TG-PRK (in transepithelial mode) and the Athens protocol if combined with CXL are accepted methods and therapies for offering refractive treatments to KC patients in an attempt of improving CDVA and reducing refractive defects.

My good friend Yannis Aslanides has just published this:

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3821082/pdf/TOOPHTJ-7-63.pdf

Non topographic transepithelial PRK with simultaneous crosslinking improves vision, and may offer an alternative to keratoplasty in contact lens intolerant keratoconus.

"In this series we assessed a nontopographic protocol for the latter. The most notable finding of our series was a significant improvement in vision, both corrected and unaided. The proportion of eyes achieving a functional threshold of 0.1 LogMAR (Snellen 6/8.5) increased from 22 to 64%, a significant proportion who might otherwise have to consider keratoplasty. Notably, even in cases with residual refractive errors, these were easily correctable with glasses or soft contact lenses rather than rigid lenses, with benefits of wear time and tolerance. These visual acuity improvements compare favourably to those described in published series of

topographically guided excimer treatments. By comparison current studies of CXL alone demonstrate a modest and unpredictable improvement in visual function in just over half of cases, with the remainder remaining stable, but a proportion also worsening in acuity terms."

"Correction of topographic parameters was not directly targeted by the technique; however there was significant improvement in topographic parameters. This change in parameters was unpredictable and greater than expected given the ablation profiles, highlighting the unpredictable nature of topographic response in ectasia. A concern related to the use of PRK in keratoconus is the possibility of accelerating or inducing disease progression. However even without CXL, Alpins et al. followed patients up to 10 years after PRK for keratoconus with no evidence of progression, while Cennamo et al. found no progression at 2 years."

"We found no clinical suggestion of progressive ectasia in any eye, despite not excluding progressive disease from the initial study cohort. Although limited by the follow up period, there was some evidence of keratometric stability in Kmax over this period."

WHAT DO YOU THINK?

In my opinion, a transepithelial non-WFG approach in KC patients offers 3 main advantages:

1) the best attainable refractive predictability

2) likely a good tissue sparing approach

3) more topographic regularization than anticipated, since the epithelium is in general more regular than the underneath stroma, so etching the epithelium irregularities down to the stroma implies reducing stromal irregularities, which will be further improved by the new epithelium

on the contrary, a transepithelial corneal WFG approach in KC patients offers as main advantage:

1) the best attainable CDVA

2) the best attainable topographic regularization

Please share your thoughts.

A non TG non WFG is the best approach in mild keratoconus treatment. In fact it is more predictable and a tissue sparing approach.

Exactly, I think Ali this is a very smart approach.

However I can figure out more tissue sparing approaches (though less predictable) eventually providing even better CDVA but requiring some residual Rx.

We can talk about them more in detail.

In my opinion, a transepithelial non-WFG approach in KC patients offers 3 main advantages:

1) the best attainable refractive predictability

2) likely a good tissue sparing approach

3) more topographic regularization than anticipated, since the epithelium is in general more regular than the underneath stroma, so etching the epithelium irregularities down to the stroma implies reducing stromal irregularities, which will be further improved by the new epithelium

on the contrary, a transepithelial corneal WFG approach in KC patients offers as main advantage:

1) the best attainable CDVA

2) the best attainable topographic regularization

Using the standard TransPRK protocol and the aspheric ablation you will ablate some 15-25µm stromal tissue on the cone (depending on the cone progression and location). This is essentially the reason for the reported topographical regularisation despite of having used an aspheric profile.

It does not neccessarily mean overablation, but the 55µm assumed to the epithelium will be partly invested in stroma (and the total ablation will still be "accurate").

For that reason, I ask whether to customise or not since even in non-customised Transepithelial ablation you will get some degree of corneal regularization.

"Correction of topographic parameters was not directly targeted by the technique; however there was significant improvement in topographic parameters. This change in parameters was unpredictable and greater than expected given the ablation profiles, highlighting the unpredictable nature of topographic response in ectasia..."

Once again, the 50µm accepted (eg as per Athens protocol) refer ONLY to the refractive part. Also in the Athens protocol you remove prior (now after) refractive correction a PTK of 50-60µm!!! so about the same as our TransPRK profile (55µm).

Not knowing the individual the epithelial can be regarded as a limitation.

I would be interested in seeing those KC patients showing epithelial hyperplasia of around 150-160 microns at or close to the cone (post-microbial keratitis is a different story).

I agree that the degree of epithelial compensation is not predictable, however I cannot remember seeing any single cornea with an epithelial surface more irregular than the stromal surface (I do not mean they do not exist, but it seems to me a very unlikely case among refractive surgery candidates).

Now, if you accept this standpoint (the epithelial surface is always more regular than the underneath stromal surface), then it is clear that ablating a neutral profile transferring these same irregularities of the epithelial surface down to the stroma (and ideal flat PTK) implies reducing stromal irregularities, which will be further improved by the new epithelium.

My opinion is that it is sufficient to know the detailed epithelial thickness to make sure to ablate through the whole epithelium, and to be able to anticipate the refraction (low order components) produced by the epithelium. But for stromal irregularities, remember that you are measuring the anterior corneal surface in your topographies (i.e. the epithelial surface or even more the tear film interface, even more regular than the epithelium). So for transepithelial TG ablations is the epithelial surface (and its maximum thickness) the one of interest and not the underlying stromal surface.

The difference in ablation rate between epithelium and stroma, depends on the radiant exposure of the laser system in use. The higher the radiant exposure, the lesser the difference in ablation rate among both. Amaris and Wavelight are using the highest radiant exposures in the market, so they have the least difference between epithelial and stromal ablation rates (the difference lies between 5% and 15%, it further affects whether you consider Bowman's membrane as a part of the epithelium or of the stroma). iVIS and MEL80 (and MEL90) systems use moderate fluence levels, so for them the difference lies between 20% and 35%. Other systems with even less radiant exposure may get differences up to 50% more epithelial ablation.

For all cases epithelial ablation rate is higher than stromal one (epithelium has less water and higher refractive index).

For non-excimer systems (e.g. solid sate systems, as CustomVis Pulzar or Katana Lasersoft) working in 211-213nm and having less absorption in water, this difference will be less and eventually even the epithelial ablation might be slightly less than corneal ablation rate.

Also on my side, I remain very interested in further comments on this topic, and I encourage all members of this group (with big names out there) to join and contribute to this (and any other) discussion.

sam

Great Contributions. This is an awesome alternative especially for early stages of KC