Phorcides Guided Topographic Ptk Combined With Cross Linking For Keratoconus

Combination of transepithelial topography guided Phototherapeutic Keratectomy (PTK) with Corneal Cross-linking (CXL) has become an accepted treatment for keratoconous over the last 15 years. However, estimation of the refractive effect of the topographic component of the PTK has been unpredictable. The Phorcides Analytic Engine uses vector analysis and lens theory to predict the effect of the topographic treatment profile giving a modified refractive correction. This software has shown impressive results in virgin eyes undergoing topography guided LASIK. This study reports 3-6 month safety and efficacy outcomes of applying this treatment to keratoconus.  

Private Ophthalmology clinic, Brisbane, Australia.

Retrospective single centre consecutive case series of all Phorcides treatments performed by 2 surgeons DG and BC. Alcon WaveLight Contoura treatment plans, Pentacam anterior and posterior surface keratometry and subjective refraction were input to the Phorcides planning software to calculate modified refractive corrections. Transepithelial topography guided PTK was performed and immediately followed by refractively neutral 45-50 um PTK on the Wavelight EX500 excimer laser. An accelerated corneal cross linking protocol was performed on the same day. Postoperative assessments were performed at 1 week, 1 month, and 3 months.

48 eyes of 46 patients were included.  Mean SER reduced from -1.86 D (range: -11.13 to +3.25 D) to -1.57 D (-7.38 to 2.00 D) at 3 months. Refractive cylinder reduced from -3.49 D (-0.50 to -9.75 D) to -2.08 D (0.00 to -8.25 D). 9 eyes (18%) had a reduction of 3.00 D cylinder or more. Mean KMax reduced from 54.5 D (46.8-66.2 D) to 50.4 D (41.4-59.6 D). 16 eyes (33%) had a reduction in KMax of 5D or more. In 42% of eyes (n=20) CDVA improved by 2 or more lines, 29% (n=14) by 1 line, 21% (n=10) were unchanged. 4 eyes lost 1 line of CDVA (with none worse than 0.2) and no eyes lost more.  Mean UDVA improved from 0.5 logMAR (0.1 to CF) by a mean of 0.3 logMAR, with 1 eye improving by 1.0. Mean thinnest corneal thickness reduced from 482 to 435 um.

Phorcides guidance is a safe and effective modification to the established treatment protocol of transepithelial PTK combined with cross linking for keratoconus. It provides excellent early improvements in both corrected and uncorrected vision compared to cross linking alone and removes the “rule of thumb” estimations often required in refractive topographic treatments. Good refractive predictability was found at three months follow up. Further long term stability data is required.