Wavefront-guided laser vision correction

Wavefront-guided treatment helps avoid human error and is more precise

The Schwind Amaris 1050RS is widely regarded as the leading excimer laser platform for laser vision correction. Schwind, in partnership with CSO, have recently upgraded their offering for wavefront-guided treatment with incorporation of the Peramis pyramidal aberrometer. We recently published clinical results that are close to the theoretical limit for routine myopic laser vision correction using this system. But most Schwind users still default to wavefront-compensated (‘Aberration Free’) treatments rather than using wavefront-guidance. This is because Aberration Free treatments on the 1050RS platform already produce excellent results. So why change?

Wavefront-compensated treatments use mean figures for induced aberration from a population sample, and incorporate corrections for these induced aberrations into the excimer laser ablation profile. The dioptric equivalent value for total higher order aberrations for normal eyes is small – approximately 0.25D. So, any gains from customizing the correction of higher order aberrations (wavefront-guidance) in routine laser vision correction, as opposed to simply correcting for mean aberrations induced by treatment (wavefront-compensation), are likely to be small.

But there are other reasons for preferring wavefront-guided treatments.

First is safety. If you program enough laser treatments, with manual data transcription you will eventually make a transcription error. In wavefront-guided treatment, the refraction numbers are ported directly into the treatment planning software. It is not possible to type in the wrong sign, the wrong dioptric power or the wrong cylinder angle. Protection from human error is an important reason for preferring wavefront-guided treatment, particularly in a high-volume clinical practice.

Second, aberrometry is about twice as precise as manifest refraction in measuring lower order defocus. It is particularly strong on cylinder. But is this more accurate? Precision refers to measurement repeatability, whereas accuracy describes proximity to the true value. Aberrometry is more precise, but control over accommodation during measurement is poor in some patients, and this is a strong area for manifest refraction in the hands of an experience optometrist. In our work on nomogram optimization, no adjustments were required for astigmatism, but the correlation coefficient for the difference between manifest and aberrometry derived sphere has consistently been around 0.5, indicating that there is value in modulating the sphere target in wavefront-guided treatments using manifest refraction data.

When do we not use wavefront-guided treatment? We are looking for a consistent scan sequence (all measurements within 0.5D, cyl axis within 15° for 3 consecutive measures where cylinder ≥0.5D), and a wavefront acquisition diameter >5.5mm for a 6.5mm optical zone. Using these rules, 85% of our routine myopic LASIK treatments are wavefront-guided.

Take a moment to look at our results in more detail at the link below and remember the philosophy of success for the British Olympic Cycling team: it’s all about incremental gains. Contemporary laser vision correction modalities all produce very good results, but even small improvements are worth chasing.

Pyramidal Aberrometry