Calculation for LASIK Ablation

From EyeWiki


Introduction

LASIK has become the standard approach for corneal refractive surgery for most ophthalmologists. Time has taught the refractive surgeon the importance of creating a good surgical plan.

In 1949 José Ignacio Barraquer Moner described what we know today as the Barraquer´s thickness laws. They state 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”. [1] [2]

When planning a LASIK procedure, it is critical to adjust each of the following parameters to ensure safety, efficacy, and reproducibility.

Flap Size

Flap size is typically between 8.0mm and 10.0mm, but may be tailored for individual circumstances such as prior corneal transplant, peripheral neovascularization, or scarring. A smaller flap diameter has the advantage of altering less corneal tissue, preserving more of the native innervation, which in theory may decrease symptoms of dryness. A larger flap avails more of the corneal tissue to ablation, which is especially important in cases of a large scotopic pupil, significant astigmatism, hyperopia, and high corrections where the blended zone may extend further into the periphery.

Microkeratome Ring Size

While microkeratome flap creation is less common, there remain surgeons who prefer manual flap creation.

  • For steep corneas, ( K´s >45D ) a 8.5mm ring is recommended to avoid a buttonhole creation. This ring may be also used in cases of corneal neovascularization to avoid hitting a vessel.
  • For flat corneas, ( K’s <45D ) a 9.5mm ring is recommended to avoid a free cap.

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 110 microns. Creation of flaps as thin as 90 microns can preserve tissue in cases of higher ablation, and thicker flaps can be created to avoid superficial scarring.

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 optical zone may lead to greater photic phenomena such as glare, particularly in patients with larger pupils in scotopic conditions.

Ablation depth

The Munnerlyn Formula estimates of the ablation depth in myopic corrections. [3]

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 systems, and therefore it is critical to utilize nomograms specific to the surgeon's excimer laser. 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. The residual stromal bed (RSB) should be greater than 300 microns to avoid post-operative ectasia.

Percentage of Tissue Altered

In 2014, Santhiago et al, [4] 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. [4]

PTA= (FT + AD) / CCT

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

Changes in Keratometry after LASIK

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

Randleman Ectasia Risk Score

The Ectasia Risk Score System designed by Randleman et al[5] 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 [5]:

  • 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

  1. Barraquer JI. Queratoplastia Refractiva. Estudios e Informaciones. Oftalmologicas. 1949;2:10-30.
  2. Barraquer JI. Modification of refraction by means of intracorneal inclusions. Int Ophthalmol Clin. 1966;6(1):53-78.
  3. Munnerlyn, CR, Koons, SJ, Marshall, J. Photorefractive keratecomy: a technique for laser refractive surgery, J. Refratc. Surg., 1988, 14, 46-52
  4. 4.0 4.1 Santhiago, M., Smadja, D., Gomes, B., Mello, G., Monteiro, M., Wilson, S., & Randleman, J. (2014). Association Between the Percent Tissue Altered and Post–Laser In Situ Keratomileusis Ectasia in Eyes With Normal Preoperative Topography. American Journal of Ophthalmology, 158, 87-95.
  5. 5.0 5.1 Randleman JB, Woodward M, Lynn MJ, Stulting RD. Risk Assessment for Ectasia after Corneal Refractive Surgery. Ophthalmology. 2008; 115:37–50.
  1. Robert S, Feder., Rapuano Christopher J. LASIK handbook, The Case-Based Approach, 1st Edition. Lippincott Williams & Wilkins 2007.
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