Masked comparison of silicone hydrogel and hydrogel extended-wear bandage contact lenses after PRK

Engle AT, Laurent JM, Schallhorn SC, Toman SD, Newacheck JS, Tanzer DJ, Tidwell JL
Masked comparison of silicone hydrogel lotrafilcon A and etafilcon A extended-wear bandage contact lenses after photorefractive keratectomy. J Cataract Refract Surg. 2005 Apr;31(4):681-6.

Despite the popularity that LASIK has gained in the last years, photorefractive keratectomy (PRK) is still preferred as a refractive procedure for special cases when a surface refractive procedure is needed; e.g. insufficient corneal thickness and for the treatment of low or moderate refractive error. However, there are some potential drawbacks related with this surgical procedure such as:

• Reports of decreased vision for several days after PRK because of the time needed for the central corneal surface to re-epithelialize and then become smooth to provide clear vision (1).
• Wound healing
• Postoperative pain due to exposed corneal nerve endings
• Hyperopic shift (1)
• Corneal haze (2, 3)
• Regression (4), and
• Poor night vision, haloes and glare (5)

The paper published by Engle et al. in the April 2005 issue of the Journal of Cataract and Refractive Surgery (6) investigates the efficacy of silicone hydrogels and conventional soft contact lenses as bandage contact lenses after PRK. Therapeutic or bandage soft contact lenses are used as a mechanical protection to increase comfort, to assist wound healing, as a vehicle for drug delivery, to maintain ocular surface hydration and for vision enhancement in specific clinical conditions. In the particular case of PRK, bandage soft contact lenses primarily reduce postoperative pain and promote corneal re-epithelialization; and secondly, allow drug delivery such as antibiotics, steroids and artificial tears during the first days after surgery.

Engle et al. fitted an etafilcon A and lotrafilcon A contact lens in each eye of patients who underwent bilateral PRK. The variables they investigated were:

• Defect size (size of epithelium removed during the surgical procedure)
• Rate of re-epithelialization
• Discomfort, and
• Conjunctival injection

Overall, Engle and co-workers found that during the first two days after PRK, the size of the epithelial defect was significantly smaller, the rate of re-epithelialization was significantly faster and the level of discomfort was significantly lower in eyes wearing the lotrafilcon A lenses than in the eyes with the etafilcon A lenses. No differences were found between days 3 and 4 regarding the variables outlined above. It was reported that 2 eyes that wore etafilcon A, and that re-epithelialized on day 2, sloughed the healed epithelium sometime during the 24 hours after the lens was removed. No significant difference was found in conjunctival injection between the two lens types.

Immediately after PRK, many corneal changes that are related to the healing process take place. During this surgical procedure, the epithelium and the basement layer are lost. Initially, re-epithelialization occurs by migration and sliding of adjacent epithelial cells. Once this process ends, usually after 5 to 7 days, Basement membrane is layed down with hemidesmosomal attachment to Bowman’s layer. During this process, attachment is weak and easily broken. A key factor that may influence this complex cellular process is the availability of adequate levels of oxygen for corneal wound healing.

This paper by Engle et al. highlights the advantages of using lotrafilcon A as bandage contact lenses after PRK. It seems that the higher oxygen transmissibility that silicone hydrogel lenses offer to the cornea results in a faster rate of re-epithelialization, which should improve visual rehabilitation, contributes to pain relief and quicker restoration of a barrier against infection (6). These factors are important issues that need to be taken into consideration by the clinician during the recovery period of the PRK patient. From the patient’s perspective, the less discomfort experienced with silicone hydrogel lenses during recovery is an important finding when choosing bandage lenses after PRK.

An interesting finding was the sloughing of the healed epithelium in 2 eyes wearing the low-Dk soft contact lens. By six weeks, a full restoration of adhesion between epithelial cells is almost completed. However, it seems that low levels of oxygen reaching the cornea may have a negative impact during the corneal healing process. Earlier work with a cat model of corneal wound healing which shows that epithelial abrasions heal faster when high-Dk silicone elastomer lenses are worn compared to no lens wear or when a low-Dk soft lens is worn has also demonstrated the benefits of high-Dk lenses for therapeutic applications (7) and indicates that further investigation into the impact of hypoxia on corneal wound healing is warranted.

As the authors highlighted, the key issue of this study was the number of days required for re-epithelialization.  This is an important factor to reduce the possibility of corneal infections after PRK and to reduce postoperative pain. Based on Engle and co-workers' findings, it seems that silicone hydrogel lenses offer improved corneal physiology compared to low-Dk hydrogel lenses as bandage contact lenses after PRK. Practitioners should take the advantage of prescribing silicone hydrogel contact lenses not only after refractive surgery, but also for any other epithelial disorder where a bandage contact lens is needed, due to the high levels of corneal oxygenation they provide.

References

1. Rowsey JJ, Morley WA. Surgical correction of moderate myopia: which method should you choose? I. Radial keratotomy will always have a place. Surv Ophthalmol 1998;43(2):147-56.
2. Maldonado MJ, Arnau V, Navea A, Martinez-Costa R, Mico FM, Cisneros AL, et al. Direct objective quantification of corneal haze after excimer laser photorefractive keratectomy for high myopia. Ophthalmology 1996;103(11):1970-8.
3. Wang Z, Li S, Chen J, Yang B, Zheng H. The healing of excimer laser ablated cornea and the effects of corticosteroid. Zhonghua Yan Ke Za Zhi 1996;32(4):245-51.
4. Ramirez-Florez S, Maurice DM. Inflammatory cells, refractive regression, and haze after excimer laser PRK. J Refract Surg 1996;12(3):370-81.
5. Halliday BL. Refractive and visual results and patient satisfaction after excimer laser photorefractive keratectomy for myopia. Br J Ophthalmol 1995;79(10):881-7.
6. Engle AT, Laurent JM, Schallhorn SC, Toman SD, Newacheck JS, Tanzer DJ, et al. Masked comparison of silicone hydrogel lotrafilcon A and etafilcon A extended-wear bandage contact lenses after photorefractive keratectomy. J Cataract Refract Surg 2005;31(4):681-6.
7. Sweeney DF, Madigan MC and Holden BA. The effects of silicone elastomer lenses on the corneal epithelium [ARVO Abstract]. Invest Ophthalmol Vis Sci 2005;28(3):S163