Vertex-To-Limbus Distance (Vtld) As A Parameter For Contact Glass Selection In Hyperopic Smile, And Its Relationship With Shift-On-Suction (Sos)

To define a new parameter for contact glass size selection, by measuring the vertex-to-limbus distance (VTLD), and its relationship with the instantaneous displacement of the contact glass on the cornea which occurs at the moment of suction (shift-on-suction) (SOS) in a cohort of eyes undergoing hyperopic SMILE.

London Vision Clinic, London, UK 

Retrospective analysis of 61 eyes undergoing hyperopic SMILE. VTLD was calculated using preliminary software on the MS-39 (CSO Italia), and is defined as the geometrical distance between the corneal vertex and the limbus. White-to-white (WTW) was also measured using the MS-39. The surgical videos were retrospectively analysed, and the width of the contact glass was calculated, calibrated by the cap thickness. The size of the contact glass was then used as a reference to determine the x,y displacement of the treatment relative to the pupil centre immediately before and  immediately after suction. The SOS was calculated as the difference. Linear regression analysis was used to determine the relationship between VLTD and SOS.

The mean WTW was 12.22 mm ± 0.38 (11.10 to 13.10), and VTLD was 5.62 mm ± 0.22 (4.89 to 6.19). The mean vertical SOS was a superior displacement of 0.03 mm ± 0.09 (0.40 (S) to 0.15 (I)), and horizontal was a temporal displacement of 0.02 mm ± 0.13 (0.30 (N) to 0.45 (T)). The magnitude of SOS was of 0.14 mm ± 0.09 (0.00 to 0.46 mm). In this cohort, 34% of eyes had a suction shift of less than 0.10 mm, 73% less than 0.20 mm, and 93% less than 0.30 mm. Linear regression analysis showed a statistically significant decrease in SOS with a larger VTLD (P<0.001), however, there was a large amount of scatter (R² = 0.02).

Given SMILE is centred on the corneal vertex, vertex-to-limbus distance may act as a more appropriate parameter for contact glass selection and suitability determination in hyperopic cases. Shift-on-suction is inversely correlated to VTLD, although there is a large amount of scatter. Given the importance of centration in hyperopic treatments, further research is required to determine the cause and parameters associated with SOS.