Calculation for LASIK Ablation

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Article initiated by:

Pedro Mario González Madrigal, MD

All contributors:

Vatinee Y. Bunya, MD, MSCE, Pedro Mario González Madrigal, MD, Koushik Tripathy, MD (AIIMS), Benjamin Buckner, MD, Vandana Reddy, MD, Sumayya Ahmad, MD, Brian Shafer, MD

Assigned editor:

Brian Shafer, MD, Sumayya Ahmad, MD

Review:

Assigned status Update Pending

 by Brian Shafer, MD on February 13, 2024.

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Contributing Editors:	add

Introduction

In 1949, José Ignacio Barraquer Moner described what we know today as the Barraquer´s thickness laws. The law states that “whenever tissue is either added to the periphery of the cornea or removed from its central part, a corresponding flattening is obtained and vice-versa; whenever tissue is either added to the center or removed from its periphery, a corresponding increase in curvature is obtained”. From this basic principle, the mathematics behind LASIK planning come to be. When planning a LASIK procedure, it is critical to adjust the following parameters to ensure safety, efficacy, and reproducibility.

Munnerlyn Formula

The Munnerlyn Formula estimates of the ablation depth in myopic corrections. t = S² D/3

• t  =thickness of the tissue ablated in microns
• S = diameter of the optical zone in millimeters
• D = dioptric correction (spherical equivalent)

While the Munnerlyn Formula serves as a guide, actual ablation depths vary between excimer laser platforms. Environment conditions such as humidity must be kept controlled as fluctuations can lead to unpredictable ablations. Higher levels of humidity can result in under-correction due to tissue swelling, while lower levels of humidity can lead to over-correction due to tissue desiccation.

Residual Stromal Bed

The residual stromal bed (RSB) is equal to the central corneal thickness (CCT) - ablation depth - flap thickness. Historically, a minimal of 250 microns was felt to be the standard, however most surgeons now feel that 300 microns is a safer thickness to avoid post-operative ectasia.

Percentage of Tissue Altered

In 2014, Santhiago et al,  described a new metric, percent tissue altered (PTA), to detect patients at risk of corneal ectasia despite normal topography.  PTA greater than 40% at the time of LASIK is associated with the development of ectasia in eyes with normal preoperative topography. PTA= (FT + AD) / CCT

• PTA=percent tissue altered
• FT=flap thickness,
• AD=ablation depth
• CCT=preoperative central corneal thickness.

The ablation depth for myopic LASIK on the VISX laser is between 12-14 microns, depending on the optical zone.

Flap thickness

Mechanical microkeratomes may have a wide variance from the intended actual flap thickness, with the average thickness being 120 microns. With the development of femtosecond lasers, flap creation is much more precise, and most surgeons use a flap thickness of between 100 - 120 microns. Thinner flaps can help preserve tissue in cases where the RSB or PTA is unfavorable.

Optical Zone

The conventional optical zone (OZ) is 6.5mm; decreasing the OZ to 6.0mm preserves tissue and may be used in cases with higher correction. Decreasing the optical zone may lead to greater photic phenomena such as glare, particularly in patients with larger pupils in scotopic conditions. With the more advanced wavefront-guided or wavefront-optimized ablations, there is a blend zone that typically extends at least 2mm beyond the optical zone to help minimize the induction of higher order aberrations.

Changes in Keratometry after LASIK

General keratometry changes are as follows:

• Flattens (reduces) by 0.8 D for correction of each diopter of myopia treated
• Steepens (increases) by 1 D for correction of each diopter of hyperopia treated.

There is a lack of consensus on the cut-offs for corneal steepening or flattening following LASIK.

Randleman Ectasia Risk Score

The Ectasia Risk Score System designed by Randleman et al is a screening tool developed by an evidence-based review of a large series of LASIK ectasia cases. The Ectasia Risk Score System scale may help to identify high-risk patients preoperatively.  It is a cumulative score system. Risk categories based on points are:

• 0-2 points=low risk
• 3 points=moderate risk
• 4 points=high risk.

May be summarized as :

• Abnormal topography, RSB <240 microns, corneal thickness less than 450 microns and Manifest refraction spherical equivalent (MRSE)> -14 D: each 4 points
• Inferior steepening pattern or skewed radial axis in topography, RSB between 240 to 259 microns, age between 18 to 21 years, corneal thickness between 451 to 480 microns, MRSE between -12 to -14 D: each 3 points
• RSB between 260 and 279 microns, age between 22 to 25 years, corneal thickness between 481 to 510 microns and MRSE between -10 to -12 D: each 2 points
• Asymmetric bowtie pattern in topography, RSB between 280 to 290 microns, age between 26 to 29 years, MRSE between -8 to -10 D: each one point
• Normal pattern or symmetric bowtie, RSB more than 300 microns, age more than 30 years, corneal thickness more than 510 microns, MRSE less than -8 D: each 0 point.

References

7. Robert S, Feder., Rapuano Christopher J. LASIK handbook, The Case-Based Approach, 1^st Edition. Lippincott Williams & Wilkins 2007.