Efficacy Theory and Proposed Protocol for Presbyopia Correction using Scleral Softening by Non-invasive Infrared Diode Lasers

Purpose: To derive and provide analytic formulas and proposed protocol for accommodative gain of presbyopia eyes via laser scleral softening, which causes increased space between ciliary body and lens (SCL) and mobility of the posterior vitreal zonules (PVZ).

Study Design: To increase the accommodation of presbyopia by laser scleral heating/softening.

Place and Duration of Study: New Taipei City, Taiwan, between April 2022 and June 2022.

Purpose: To analyze the safety and efficacy of presbyopia treatment via scleral softening.

Methodology: The scleral softening efficacy is calculated based on the rate equation of scleral tissue with a rate coefficient given by an Arrhenius formula, Temperature spatial and temporal profiles are given by the numerical solutions of a heat diffusion equation with a volume heating source. Various effective depths including tissue damage depth, temperature penetration depth and conversion depth, governed by tissue absorption coefficient, light intensity and dose (or irradiation time), and the related threshold values, are introduced in replacing the conventional penetration depth based on a Beer's law.

Results: Given the the temperature spatial and temporal profiles, scleral softening efficacy can be calculated. Scleral surface damage can be prevented by cooling window. The suggested protocol for scleral softening treatments include: a diode laser at about 1.45 to 1.5 µm or about 1.86 to1.9 µm, or about 2.0 to 2.15 µm, wavelength (with absorption coefficient about 20 to 100 cm-1); laser power about 0.2 to 0.8 W per spot, having a total of 4 to 16 spots; and irradiation time of 100 to 600 ms. Results of corneal thermal shrinkage are demonstrated by the topography changes of pig eyes, in which the scleral softening does not affect the corneal shapes. The accommodative gain is proportional to the softening efficacy (Seff) of the scleral tissue after a thermal laser leading to the increase of PVZ mobility and SCL. However, the actual relation of Seff and the PVZ and SCL changes require measured data.

Conclusion: Safety and efficacy of scleral softening for presbyopia treatment depend upon the laser parameters (intensity, dose, spot size, wavelength) and the effective depths. By choosing the laser treated areas, a dual function treatment using scleral softening for presbyopia, and cornea stromal shrinkage for hyperopia is proposed and demonstrated by topography of pig eyes.