The Alcon/WaveLight refractive surgery laser platform comprises the EX500 excimer laser, and a series of diagnostic networked devices that constitute the Refractive Suite® (including Allegro Placido Topolyzer-Vario and Allegro Scheimpflug Oculyzer ІІ). (10)
This Refractive Suite applies ethernet networking, allowing diagnostic data import from networked screening devices into the laser planning software, including topography data from Allegro Placido Topolyzer-Vario and topometric data from Allegro Scheimpflug Oculyzer ІІ. (11) Multiple studies reported the application of topography-guided treatment with such platforms, whether in normal or irregular corneas. (12- 19)
The Oculyzer presents a Scheimpflug rotating camera, with a 360 degrees rotating light beam, scanning the cornea with high density of points from the corneal centre. Therefore, it should be the first choice when planning laser treatment in corneas with central irregularities or aberrations. However, it can be affected by ocular movement or corneal haze or cloudiness. (9)
In contrast, the Topolyzer, is a placido-disk based system with a camera imaging reflections from the surface of the cornea. Since the Topolyzer camera is located
at the centre of the topographer, a central scotoma is produced corresponding to the camera location, and hence a small area of central corneal data are extrapolated.
Therefore, it should be the first choice when planning laser treatment in corneas with more peripheral irregularities or aberrations. However, it lacks the provision of full surface information, with lower accuracy in cases of central corneal aberrations. (9)
A recent study by Cummings et al showed that both the Oculyzer and Topolyzer provide reliable and comparable results in pre-operative myopes and in subjects with mixed astigmatism. (9) This coincides with the present study, where statistically significant agreement between Allegro Oculyzer ІІ and Allegro Topolyzer-Vario was detected.
In a recent study by Kanellopoulos, clinical refractive data was modified with topographic data, considering cylinder axis and amount, introducing the new term "topography-modified refraction". This provided superior visual function regarding UCVA, residual refractive error, total high-order aberrations, coma aberration, and contrast sensitivity. He hypothesized that "topography-modified refraction" allows to bypass the bias induced by lenticular astigmatism, particularly active in young myopic patients. Such hypothesized lenticular astigmatism seems to be responsible for distortion of the objective cylinder axis and amount. (8) In the present study, 4 of 156 eyes (2.5 %) showed unexpected discrepancy between Allegro Oculyzer ІІ and Allegro Topolyzer-Vario cylinder axis. In these cases, the cylinder axis applied for laser treatment was the mean of the four measurements. 1 month postoperatively, these cases showed 0 cylinder with UCVA 1.
"These findings may change the current clinical paradigm of the optimal subjective refraction utilized in laser vision correction." (8)