Methods for superficial excimer laser vision correction include transepithelial photorefractive keratectomy (TransFRK), laser epithelial keratomileusis (LASEK
) and in situ epi-laser keratomileusis (Epi-LASIK
). Stromal corneal ablation procedures include laser keratomileusis "in situ" with a flap (LASIK
, i.e. Laser-in-situ-Keratomileusis), created mechanically or with a femtosecond laser (Femto-LASIK
). Corneal lenticular extraction procedures have several commercial names depending on the manufacturer.
What is TransEpithelial PRK or TransPRK (also Trans-PRK) ?
Transepithelial photorefractive keratectomy is a simplified one-stage PRK (read more about PRK
). Here, the epithelium, as well as part of the stroma (the main layer under the epithelium), is removed by a laser without mechanical scraping of the epithelium with an instrument resembling a hockey stick or a rotating brush. Chemical agents such as dilute ethanol 5, 6 7
can be used to remove corneal epithelium. The total laser treatment usually takes less than 30 seconds, depending on the number of diopters. Removing the epithelium with a laser smoothes the cornea surface which can affect the clinical results 8
. Here you need to understand that the epithelium covering the cornea masks its surface with microroughness.
Features of TransPRK
Transepithelial photorefractive keratectomy (TransPRK) emerged as a modification of its predecessor PRK, and replaced mechanical and chemical cleaning of the corneal epithelium with a laser 9
. As in PRK, one of the main inconveniences after Trans-PRK compared to LASIK is postoperative pain until the epithelium is overgrown 10
. After TransPRK, similarly to PRK, soft contact lenses are widely used as a dressing to reduce pain and optimize the healing process of the epithelium 11
. Although the tear film between the lens and the cornea is only about 1–2 µm thick, this facilitates the penetration of postoperative drugs into the cornea (anti-inflammatory eye drops, topical steroids, etc.). For example, soft contact lenses impregnated with a non-steroidal anti-inflammatory drugs have long been shown to be effective in reducing pain without compromising visual results 12
Results after TransPRK
One of the first studies (2004) comparing epithelial healing, postoperative pain and visual results after epithelial-mechanical (conventional PRK), transepithelial PRK, and subepithelial keratomileusis (LASEK) after 6 months failed to find significant differences between the 3 methods. In other words, the results of corneal pain and the degree of corneal opacity were fairly similar 5
. Later, in 2012, transepithelial ablation was shown to be safer than mechanical scraping of the epithelium using chemical agents such as alcohol. It has been demonstrated that TransPRK provides faster epithelial healing, less postoperative pain and less corneal opacity at 1 week, 1, 3 and 6 months after surgery 6
. But it should be understood that other technologies are also not standing still and are being improved.
One large meta-analysis of the 16 most recent known studies of 1924 eyes after transepithelial PRK was published in 2020 9
. The included studies were published by authors from South Korea, Iran, Turkey, People's Republic of China, the Netherlands, Greece, Lebanon, Poland, and Saudi Arabia. The period of postoperative follow-up ranged from 3 to 18 months until 2019.
Here, characteristics such as clinical efficacy, safety and predictability of clinical results were tested. To understand these results, it is necessary to mention what these characteristics mean. Thus, efficacy
was defined as the proportion of treated eyes with postoperative distance visual acuity (UDVA) ≥ 20/20 (or 10 lines according to the Sivtsev’s table). Predictability
is the proportion of all eyes with a deviation from the target (planned) postoperative refraction of up to 0.5 diopters. Safety
was interpreted as the proportion of operated eyes that lost more than 2 lines of postoperative distance visual acuity even with glasses (BDVA) according to the Snellen chart compared to visual acuity with glasses before surgery.
The results of all studies had an efficacy of more than 76% with the exception of one lower value 13
. The small effectiveness of this study 13
can be explained by the fact that it also included patients with impaired distance visual acuity even with glasses before the surgery. In other words, this does not mean that this type of surgery in this study was not successful, but that the efficacy index took into account the preoperative distance visual acuity with glasses in all patients, even with complications. All 16 included studies confirmed the value of the TransPRK intervention. Thirteen studies reported 100% safety, while 13
and two others reported values between 93% - 98%. In terms of predictability, the obtained results showed an average estimate of 89% (95% of estimates in the 82% - 93% confidence interval).
Thus, it has been proven that TransEpithelial PRK is a useful technique of modern refractive surgery to avoid complications associated with flap, keratitis (inflammation), formation of macrostriae (micro stripes) or epithelial ingrowth 9
In conclusion, we can conclude that Trans-PRK is a tissue-preserving laser intervention with a low risk of developing ectasia - deformity and protrusion of the cornea (the tissue is thicker because there is no flap). However, do not forget that other superficial operations like PRK and LASEK are also improving. A recent analysis at the World Congress of the European Society for Cataract and Refractive Surgeons (Winter ESCRS 2021
) is an example of this 14
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