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 important to determine the following variables, noting that some of these reference LASIK utillizing a microkeratome, which continues to be replaced by femtosecond lasers:

Flap Size

Flap size is typically between 8.0mm and 10.0mm, but may be tailored for individual circumstances depending on the femtosecond laser used. A smaller flap diameter has the advantage of altering less corneal tissue, preserving more of the native innervation and in theory decreasing 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 and high corrections, in which cases the blended zone may extend farther out on the cornea.

Ring Size (microkeratome)

  • 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

With the development of femtosecond lasers, flap creation is more precise, and most surgeons use a flap thickness near 100 microns. With the microkeratome, the most commonly used thickenss is 120 microns. Mechanical microkeratomes may have a wide variance from the intended actual flap thickness. Thinner flaps usually occur in thinner corneas and when the same blade is used for the second eye. Some surgeons measure a central pachymetry during surgery, before and after constructing the flap to determine its thickness. It is important to abort surgery if the flap thickness seems to be thicker than planned, this would leave a thinner stromal bed for ablation.

Treatment Zone

Typically, an ablation zone of 6.5mm is used. In special circumstances, an ablation zone of 6.0mm may be used. For example, a thinner cornea with a higher correction may use a 6.0mm treatment zone to conserve tissue. Additionally, if a patient has a small pupil size in scotopic light, a 6.0 ablation may be used.

Ablation depth

Ablation depth per diopter with vary between systems. Nomograms for each laser exist and it is recommended that every surgeon who is new to the equipment ask for this information.

The Munnerlyn Formula will help the refractive surgeon with an estimate 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


The ablation depth does not include the transition (blend) zone of the surgery which usually adds extra 8 microns. Many factors must be taken into account at the time of the surgery; barometric pressure and ambient humidity will change slightly the required ablation. High humidity will induce “undercorrection” and viceversa.

It is recommended to leave at least 300 microns of stromal bed but opinions in this matter differ.

Percentage of Tissue Altered

In 2014, Santhiago et al, [4] described a new metric to detect patients, with normal topography, at risk of corneal ectasia.  

There is a relationship between preoperative corneal thickness, ablation depth, and flap thickness in determining the relative amount of biomechanical change that has occurred after a LASIK procedure.

PTA= (FT + AD) / CCT

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


Percent tissue altered greater than 40% at the time of LASIK is associated with the development of ectasia in eyes with normal preoperative topography. [4]

Changes in keratometry after LASIK

It is estimated that keratometry

  • 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

Published in 2008, 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.  Although, controversy exists around its usefulness, it certainly should be taken into consideration.

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.