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Thermal Topographical Rings as a New Tool for Laser Eye Surgery

Khalid A. Joudi, Somer M. Nacy*, Nebras H. Ghaeb

Biomedical Engineering Department, Al Khwarizmi College of Eng., University of Baghdad, Baghdad, Iraq

Abstract: Measurement of the corneal surface temperature during the laser surgery have been modified at the last few years, to be used as an extra useful monitoring tool during the dynamic ablation process. While the concentric Placido rings have been used before to measure the refractive errors, here, it have been modified to be used as a new suggested tool to study the thermal response upon the anterior corneal surface during laser eye surgeries. The thermal infrared camera was used to get an image captured at the end of the treatment, where contours with isotherms are derived and examined. The new contour lines introduce the temperature induced per location upon the corneal surface and reflect the biomechanical response behavior. Comparing the contour image with the image generated by the treatment system for the ablated depth showed a new indication for safety limits especially the effect of decentration and other irregular aberrations.

Keywords: LASIK, Placido disc, topographic map, corneal reshaping

Citation: Khalid A. Joudi, Somer M. Nacy, Nebras H. Ghaeb, Thermal Topographical Rings as a New Tool for Laser Eye Surgery. Journal of Biomedical Engineering and Technology, 5 (1). 1-5. January 2017. doi:10.12691/jbet-5-1-1

1. Introduction

Refractive errors (RE) are one of the most common abnormalities in ophthalmic field. They are either high or low order aberrations, according to the ability of correction with or without glass wear. A laser refractive surgery was developed to correct the low order aberration (LOA) refractive errors such as: myopia, hyperopia or astigmatism. Ultraviolet (UV) excimer laser generated from the argon fluoride (ArF) interaction is used to reshape the corneal surface to accumulate the refractive error, in which are widely known as Laser in situ keratomileusis (LASIK) or photorefractive keratectomy (PRK).

Measuring the RE started earlier in the 19th century with different physical suggestions. Placido disc was one of the inspecting test, suggested to check subjectively the frontal corneal surface, where black and white concentric rings are reflected back from the cornea. Such a tool gives a qualitative assessment of huge corneal astigmatism and is valuable in cases of irregular astigmatism and keratoconus [1].

Corneal topographer build upon the idea of Placido rings, used to evaluate patients’ pre and postocular surgery, to support surgical preparation, to assist in contact lens selection, and to evaluate patients with mysterious visual loss or visual complications from ocular disease [2,3].

Based on ring shaped (topographic maps) the laser ablates the anterior surface of the cornea as in PRK, or the internal stroma as per the LASIK, or in the laser epithelial keratomileusis, (LASEK), this procedure either alters the anterior curvature or adjusts the compliance of the structure [4,5].

2. Thermal Study of Human Eye

Many research studies measured the temperature of the anterior surface of the cornea and used these measurements as an indication for clinical consequences. Mapstone [6], used earlier in 1968 the bolometer to measure the temperature of the corneal surface to indicate clinical outcomes such as anterior segment inflammation and carotid artery diseases. Kitai et. al. [7], studied theoretically the thermal effects and the mechanical stresses, induced on the corneal surface during the excimer laser effect. Berjano et. al. [8], studied the effect of anisotropy of corneal thermal conductivity for the period of heating with currents in radiofrequency thermokeratoplasty. Berjano and Saiz [9], introduced a model to study the corneal heating with radiofrequency (RF) currents. They used the commercial program ANSYS V. 5.3 to estimate the temperature distribution at the cornea. Cvetkovic et. al. [10], developed Finite element method (FEM) for studying two dimensional (2D) steady state temperature distribution. Cvetkovic et. al. [11], used the FEM once more, to solve the time domain problems for the human eye, using energy density absorbed by the eye at the nth node with cylindrical coordinates. Ng and Ooi, [12,13], deliberated a model for the human eye, by considering the circulation of aqueous humor (AH) inside the anterior chamber of it.

Recently, more powerful numerical methods have been developed to solve heat transfer problems in the human eye. Chua et. al. [14], predicted the temperature of a human eye subjected to the laser source, where a Finite volume method (FVM) was used to discuss the impact of several phenomena, such as the aging of the human eye. Ooi et. al. [15,16,17], solved the 2D and 3D steady and transient heat distribution due to the laser effect on the eye, where they used the Boundary element method (BEM). Joudi et. al, [18] deliberated a 1D exact solution for bioheat model of the cornea for the LASIK procedure. Khanday et. al. [19], used the Laplace transform to solve the 1D FEM of multilayer human eye. They estimated temperature distribution for the corneal tissue based on direct environmental temperature.

Finally, the temperature distribution upon the corneal surface during the laser surgery attracted many manufacturers of the refractive surgery system like ALCON and SHWIND to create the smart pulse technology, in which the temperature are controlled and not exceed the approved limited [see for example references [20,21,22]].

3. Eye Thermography

The Infrared Thermography Imaging is a biomedical imaging method that depends on high-resolution infrared imaging. It is noninvasive, and not a static imaging investigation technique [23]. Merla et. al, [24] reviewed the progress in measurement of ocular temperature throughout the last century. They concluded that existing instrumentation offers the potential to measure ocular surface temperature with more accuracy and speed than previously possible. In 2005, Singh and Bhinder [25], presented their work comparing between the non-contact infrared and remote sensor thermometer in normal and dry eye patients, and they reached to a point that IR Thermometry is better at recording the absolute temperature from any point on the eye than the remote sensor thermometer. Tan et. al. [26], investigated the exactness and sensitivity of the thermo-tracer camera in observing the anterior eye surface temperature, and found the regular ocular surface temperature (OST) for young Chinese. Acharya et. al. [27,28], used image processing methods on the IR eye images for the analysis of the OST of normal subjects of three groups (young, adult and elder ages). Brunsmann et. al. [29], used the IR to estimate the mean temperature as a measure of the thermal load throughout corneal refractive surgery. Tan et. al. [30], assessed topographical variation in the OST between young, elderly and subjects wearing contact lens with the thermographic method. Su et. al. [31], developed IR thermal image with spatial and temporal variation of the OST over a 6-second eye open thus estimating the dry eye syndrome. Mosquera et. al. [32], used porcine eyes with 100 different refractive errors to check the corneal temperature using IR thermography camera.

4. Material and Method

The LASIK system used here is ALCON ALLEGRETTO WAVE EYE–Q 400 Hz manufactured by Erlangen in Germany, it is a scanning spot laser beam system. This laser beam set agreeing to clinical factors place in the LASIK system of the interface computer. The additional set of the arrangement is the (PMMA) disk.

More than 25 settings were prepared and measured during the refractive ablation. Table 1. Shows the treated refractive aberrations.

Table 1. Refractive Errors

An infrared camera (FLIR T–335; FLIR Systems Pty Ltd., Victoria, Australia) was used to determine the corneal surface temperature. The camera is interfaced to a computer to record the whole process through an interface card, with the ability to read each frame from the recorded video. The image provided by the camera is with a resolution of 320 X 240 pixels and measures the OST within the thermal sensitivity of 0.05°C @ +30°C. The IR camera is placed at a distance of 45 cm with an angle of about 45° as shown in Figure 1.

Figure 1. IR camera setup with respect the LASIK system

The emissivity of the cornea is taken to be 0.96 [26] while the optical zone of the treatment is 6.5 mm. During the entire measurement process and for every image, the maximum, minimum and mean values were evaluated over the region of interest.

5. Results and Discussions

Calculating the volume of a single spot for the cornea, and dividing it by the volume of a single impact on PMMA, presenting a ratio called Cornea to PMMA ratio (CTPR) [33]. CTPR has been calculated previously by Joudi et al. [33], to be 1.05 ± 0.03 at the central location and about 1.15 ± 0.05 at the peripheral. The discrepancy between the two values is mainly due to the surface shapes of a flat PMMA and a spherical cornea, and material properties.

The real image is recorded by the IR camera then processed by the interface program supplied by the IR camera manufacturer to build the temperature matrix with the dimension of 100 X 100 pixels. The MATLAB program is used then to process the results. Figure 2. Shows the ablation profile produced by the LASIK system and the IR image produced by the IR camera for -1.00 D.

Figure 2. Ablation profile and IR image of -1.00 D

Figure 3. Represents the new contours built after processing using MATLAB, and the mesh grid in three-dimensional form for -1.00 D.

Figure 3. Temperature Contour and Mesh grid of -1.00 D

The contours clearly showed the maximum at the central and fine gradient towards the peripheral with Gaussian behavior temperature profile just like the beam profile produced by the LASIK system. Figure 4. Shows the same images for +1.00 D.

Figure 4. Temperature Contour and Mesh grid of +1.00 D

The main difference between the two treatments is that the myopic is a central correction while the hyperopic is a peripheral correction. Evaluation of temperatures for each treatment is listed in Table 2. All measurements displayed a slight temperature increase during laser ablation. In contrast, the temperature rise did not depend on the amount of the refraction correction or the treatment duration. The time for correction of -1.00D is about 2 sec with 15.53 µm ablation depth, while the time for correction of the +1.00D is about 4 sec with 15.91 µm ablation depth, so for the same ablation depth temperatures were 32°C and 35.8°C respectively. While the time for -6.0D is 14 sec with ablation depth of 87.96 µm but for +6.0 D with ablation depth of 101.60 µm is 40 sec with temperatures of 53.0°C and 40.2°C, respectively.

Table 2. Ablation temperature for different refraction errors

Studying new temperature contour with the ability to represent the mesh grid for it is very useful, especially for decentered (treatment or cornea) because the decentration will cause a high-temperature gradient during the treatment of high degrees of error (long treatment time). Activating the denaturation of tissues or, at least, change the thermal and optical properties of the same tissue. Using the CTPR number, the PMMA disk results are simulated to the real cornea, see Table 3. The irregularities in the corneal surface are checked thermally as it cause temperature disturbance, wave distributed upon the surface of the cornea during the treatment.

Table 3. Expected Ablation temperature for different refraction errors for the real corneal surface

The above table shows the expected temperature at the central position during the laser surgery for the real corneal surface. The interchanging between the mechanism of ablation, radiation, convection and conduction are mainly affecting the distribution of the thermal wave at the contour style on the cornea surface. Measuring temperature at the central location (thinnest location) is important as it is critically optical point.

List of Abbreviations

1D, 2D and 3D: One, Two and Three Dimensional.

AH: Aqueous Humor.

ArF: Argon Fluoride gas.

BEM: Boundary Element Method.

FEM: Finite Element method.

FVM: Finite Volume Method.

IR: Infrared.

LASEK: Laser in epithelial keratomileusis.

LASIK: Laser in situ keratomileusis.

LOA: Low Order Aberration.

PRK: Photorefractive Keratectomy.

RE: Refractive Error.

RF: Radio Frequency.

UV: Ultra – Violate.

References

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