Re-birth of PRK
Continuing refinements of technique and technology are bringing trans-epithelial PRK back to the surface
Dan Epstein MD,PhD
Transepithelial PRK (T-PRK) has several patient-friendly and surgeon-friendly advantages over other corneal surface refractive procedures in terms of efficiency, epithelial healing and haze, but whether that translates into better visual outcomes remains an open question, said Daniel Epstein MD, PhD, FARVO, Zurich, Switzerland.
“Transepithelial photorefractive keratectomy (T-PRK) was originally developed as an attempt to improve conventional PRK. I have reviewed the entire peer-reviewed literature to see if it has achieved that goal and the message is a bit confusing and contradictory,” Prof Epstein told the 23rd ESCRS Winter Meeting in Athens, Greece.
T-PRK originated as two-step procedure, consisting of phototherapeutic keratectomy (T-PTK) directly followed by PRK. Compared to conventional PRK, the two-step approach was more time-consuming and involved more corneal dehydration. The results were variable to the point that many concluded that the alcohol-assisted method was better, he noted.
However, a single-step T-PRK now exists, where the entire ablation is performed in a single continuous session that is shorter in duration than either conventional PRK or two-step T-PRK. It also has a built-in nomogram that takes epithelial thickness into account. The nomogram uses a population-based model of epithelial thickness. The laser algorithm aims for epithelial ablation depths of 55μm centrally and 65μm peripherally.
“The ablation rate of epithelium is different from that of stroma. The laser energy has to be adjusted to account of this in order to providing a smooth surface,” Prof Epstein said.
Some surgeons have been dissatisfied with the built-in nomograms and have developed their own modified nomograms, making comparison of techniques more difficult. Other variations on single-step T-PRK include aiming for over-correction to compensate for later regression when targeting refraction.
“Single-step T-PRK is still a work in progress,” Prof Epstein said.
Outcomes & comparisons
To provide an up-to-date perspective on the potential of T-PRK, Prof Epstein presented the results of a recent non-randomised study involving 146 eyes with a mean preoperative spherical equivalent (SE) of -3.9 and a mean astigmatism of 0.9 that underwent single-step procedure. It showed that at 18 months postoperatively the SE improved to -0.02D and astigmatism was reduced to 0.032D, and 98% were within 0.5D of target refraction.
The published peer-reviewed literature indicates that conventional PRK is better than two-step T-PRK in terms of efficacy, safety and complications. However, the studies suggest that single-step T-PRK outcomes are superior to conventional PRK in terms of safety indices and postoperative complications. But efficacy comparisons vary considerably.
Comparison studies also indicate that single-step T-PRK is easier to perform, causes less postoperative pain and provides faster re-epithelialisation than conventional PRK, Prof Epstein said. However, he pointed out that 15 years ago the same claims were made for laser-assisted sub-epithelial keratectomy (LASEK) compared to PRK and it took five years to correct that misconception.
The studies also suggest that T-PRK’s long-term efficacy is comparable with PRK, superior to alcohol-assisted PRK in high myopia but approximately equal to alcohol-assisted PRK in terms of refraction, visual acuity and haze. Compared to femto-LASIK, studies suggest that eyes undergoing single-step T-PRK have better visual acuity results but a slower postoperative recovery.
However, variations in T-PRK technique and study design make many of these comparisons inconclusive. For, example, some authors use mitomycin C whereas others do not, and there are wide variations in postoperative medication regimens. Moreover, as it is an evolving technique in the midst of evolving technology, T-PRK is undergoing continual modification and refinement.
The research shows that single-step T-PRK may induce less haze than alcohol-assisted PRK. That is most likely because the transepithelial ablation causes less keratocyte loss with a lower inflammatory response and a smoother stromal bed contour. A study attempting to quantify haze gave alcohol-assisted PRK a score of 0.4 and T-PRK a score of 0.2, he noted.
Research has also demonstrated a faster epithelial healing time for single-step T-PRK, with one study showing that the epithelium was completely healed by a mean of 2.5 days after T-PRK compared to 3.7 days after alcohol-assisted PRK. Re-epithelialisation is also faster after T-PRK than after LASEK and PRK with manual epithelium removal. However, that does not appear to improve visual acuity, the studies indicate.
As in all refractive surgery, over- and under-correction remains a problem. It is related to optical zone diameter. Ablations with smaller zones are more prone to under-correction, and those with larger optical zones are more prone to over-correction. However, the refractive outcome of T-PRK in the long term is similar to standard PRK.
Some authors have asserted that the risk of dry eye is less after T-PRK than after LASIK because fewer nerve-endings are severed and the period of stromal dehydration is shorter.
“To summarise, there is no question about it. T-PRK is an attractive procedure because it is a no-touch procedure that does show faster healing of the epithelium and it may cause less pain. But it is not clear that the bread-and-butter aim of the procedure, namely refraction and uncorrected visual acuity results, is so much superior to old-fashioned PRK,” he concluded.