Ординатура / Офтальмология / Учебные материалы / Orthokeratology Principles and Practice 2004

The inputs for this program are the apical radius and eccentricity values. The choice of lens diameter is fixed to either 10.60 or 11.20 mm. The input data are then used to determine the correct base curve (back optic zone radius or BOZR) of the lens required. The lens is based on the concept of a 7.00 mm optic zone (back optic zone diameter or BOZD) with a fixed 4.00 D steeper reverse curve (see Ch. 4), with the end result of the procedure occurring when the corneal eccentricity becomes zero. The program gives no information as to the trial lens that should be fitted, but the closest BOZR in the trial lens range should suffice for a trial fit (Fig. 8.13). The chord requested is the total diameter of the lens minus 2.20 mm.

The software for the WAVE fitting system was developed by Jim Edwards, and is an optional extension to the Keratron topography system. The software takes the raw elevation data from the Scout or Keratron topographer and creates a lens design and tear layer profile based on the data. The system uses very sophisticated mathematics to develop a lens design. The base curve (BOZR) is based on the refractive change required, with a compression factor of 1.25 D, and the rest of the lens design follows. The diameter is variable, with the optic zone (BOZD) and reverse curve (RC) widths being automatically recalculated. The optimal optic zone diameter is 1.00 mm greater than the pupil diameter.

The reverse zone is then initially calculated to be 12% of the BOZD. The diameter of the lens is 1.67 times greater then the BOZD. The program then generates a representative fluorescein pattern and tear layer graph of the lens (Fig. 8.14). The practitioner is then free to manipulate the tear layer profile to optimize the fit depending on clinical experience. The software does not output the exact curves of the lens as they are complex aspheres based on the raw elevation data and tear layer thickness. At varying points across the corneal surface, the surface area of the lens is matched to that of the cornea, assuring sagittal equivalency. This information is not shown in the outputs.

The beauty of the system is the ability to modify the tear layer profile in real time and assess the results. Also, a comprehensive problem-solving system allows the practitioner

to reinstall the original lens that was less than optimal and make the necessary modifications.

The data are then sent directly to the Optoform lathe at the laboratory for manufacture. WAVE also contains programs for the design of multifocals and bitorics, as well as nonsymmetrically rotational aspheres for complex cases such as keratoconus and postsurgical fitting.

The first-fit success rate is therefore highly dependent on the accuracy of the topography data. The reported repeatability of the Scout for elevation data is ± 12 urn for adults (Berrer et al 2002) and up to ± 0.25 mm for children (Chui &

Cho 2003).

The practitioner is advised to take at least six repeated readings of the eye and calculate the mean and standard deviation of error. The map closest to the mean value should then be used to

design the lens. The error values can then be used if refinement of the fit is necessary.

Ortho Tool 2000 (EyeDeal software and design)

The Ortho Tool program was written by Tom Geimer. It is based on a Microsoft Excel platform and is very easy to use. The opening statement in the instruction manual sets the tone: "It's a simple truth, everyone hates user's guides. Software developers hate writing them. Users hate reading them." Thus begins an enjoyable read through one of the best user's guides the author has ever read. The program will design standard alignment lenses, aspherics, bitorics, and RGLs for orthokeratology. There are two options for RGL design, either a practitioner pre-

ference design (controlled clearance) or the Reinhart R&R design. The input data are the keratometry values, the corneal eccentricity, and the refractive error. The program also includes a very useful section where the central and temporal K readings can be entered and the apical radius and eccentricity calculated. Alternatively, the apical radius can be calculated from K readings and eccentricity.

The lens design and fitting are based on sag calculations, with the resultant tear layer profiles graphically presented. The practitioner is free to manipulate the clearance values and the program calculates the required curves.

In this case, a cornea of Kf 7.77 mm, eccentricity 0.50, and a refractive error of -3.00 will be used to design a controlled-clearance fivezone RGL. The input data are shown in Figure

8.15. Note that the parameters of the lens are shown in the lower section of the sheet. The tear layer tab is then pushed, and the tear layer profile of the lens is shown (Fig. 8.16). The thickness of the tear layer at each zone of the lens is displayed, along with the calculated tear volume under the lens.

The lens design sheet allows the practitioner to change any of the variables like BOZO, RC width, and so on. In this case, the RC width has been changed to 0.50 mm from the original 1.00 mm (Fig. 8.17). The effect this has on the lens fit is shown in the tear layer sheet (Fig. 8.18). Note that the lens is now a poor fit, being too flat with no peripheral touch. If changes are made to the design section, all the lens parameters must be altered. The beauty of this program is that the simple act of manipulating the data is an excellent teaching tool for learning the interdependence of all the curves required for good contact lens design.

Figure 8.19 shows the effect of flattening the original BOZR from 8.50 to 8.55 mm. The change in tear layer thickness at the apex has been reduced to 2 IJ.m (Fig. 8.20). The Or tho Tools software can be used with either topography data or keratome try values. The most important thing to remember is that sag fitting is dependent on the eccentricity of the cornea. Those practitioners who do not use topography will therefore need to calculate the eccentricity values using the program that converts temporal K readings over specific chords to an eccentricity value.

Free Choice OK

The Free Choice program was written by Don Noack. The RGL section is shown here, but it also contains design data for aspherics and standard lens designs. The inputs are topography-based, with apical radius and eccentricity used to calculate the lens based on sag fitting.

The first page gives the practitioner the choice of lens design for a specific refractive change, with three-, four-, and five-zone variations (Fig. 8.21). A standard compression factor of 0.500 is used for all designs. In this case, the standard cornea of Ro 7.80 mm, eccentricity 0.50, and a 3.00 0 refractive

change will be used as an example. The data are entered and the program shows the various curves and diameters for the lens assuming a 5 IJ.m apical clearance (Fig. 8.22). In order to alter any of the lens parameters, the practitioner must choose the tear graph option. This then shows all the same data again (Fig. 8.23).

However, the program is intended to be used as a problem-solving device, so the tear graph page allows for alteration of all the curves in order to refine the fit. For example, if the original lens, designed for a corneal eccentricity of 0.50, is worn and results in a central island, then the likely tear layer profile of the lens on the cornea can be visualized by changing the eccentricity value to 0.55. This is the original 0.50 with the assumed standard deviation in determination of eccentricity of 0.05 added. Thus the lens was effectively too steep due to the eccentricity being underestimated. The standard deviation of the eccentricity measurement is added to the original value. The lens now appears too steep (Fig. 8.24).

The practitioner then applies the rules outlined in Chapter 4 for refining the fit.

1.If the alignment curve is flattened by 0.05 mm, the apical clearance decreases by 5 IJ.m. Steepening the alignment curve by 0.05 mm increases the apical clearance by 5 IJ.m.

2.If the RC is flattened by 0.15 mm, the apical clearance is decreased by 10 IJ.m. Steepening the RC by the same amount increases apical clearance by 10 IJ.m.

In the above example, the apical clearance is 16 IJ.m, so the first step taken is to flatten the RC by 0.15 mm from 7.062 to 7.212 mm (Fig. 8.25).

The resultant tear layer profile shows a lens with tight alignment curves. The apical clearance is approximately 8 IJ.m, so flattening the alignment curve by 0.10 would lead to a drop of 10 IJ.m in apical clearance. Alterations to the second alignment curve do not effect as great a change in apical clearance, so in this case, both alignment curves will be flattened by 0.05 mm, which will reduce the apical clearance to approximately zero. The final alteration is then to steepen the RC by 0.05 mm, which will increase the apical clearance to the 5 IJ.m level required. The result is

shown in Figure 8.26, with the alignment curves flattened by 0.05 and the RC steepened by 0.05 from those shown in Figure 8.25.

The program also allows for alterations in the width of the curves; the practitioner is then able to alter the radii of the zones until the ideal tear layer profile is achieved. Like the Ortho Tool program, this is an excellent teaching aid for practitioners to understand the interrelationships of curves and diameters for sag fitting.

1.calculate the elevation or sag values from Ro and eccentricity for those topographers that do not give elevation data. Several repeated measurements on the same eye can then be entered into a spreadsheet and the mean and standard deviations calculated. This allows the practitioner to predict the likely variations in topographic data

2.determine the optimal trial lens based on the corneal data

3.refine the final lens prescription by using a postwear analysis system.

BE software

The BE lens and fitting software was developed by Don Noack and John Mountford. It is a purely topography-based system, and requires inputs of apical radius, eccentricity, or elevation in order to calculate the lens. It is used to:

The first page (Fig. 8.27) gives the practitioner the choice of calculating the sag of the aspheric cornea, or the choice of tangent contact points for the prescription lens. These are preset to

or 1/ 2 tangents. The most commonly used tangent is the with the wider tangents being

used for smaller lens diameters. Figure 8.28 shows the aspheric sag calculation page. The apical radius and eccentricity values of the repeated topography plots are calculated in turn and the mean and standard deviation calculated by transferring the results into a spreadsheet. The sag is calculated over the chord of contact between the lens and the corneal surface, which, for an 11.00 mm TD lens with a 1/ 4 tangent, is

9.35 mm.

Once the tangent option is chosen the program moves to page 2, where the relevant apical radius, eccentricity, or elevation values are entered. The refractive change possible with corneal sphericalization as well as the final Ro and TxZ diameter are shown (Fig. 8.29).

The "extra refractive change" box has two functions. Firstly, if the refractive change required

is less than that calculated for corneal sphericalization, the difference can be typed in and taken into account for later calculations. For example, the calculated refractive change in this case is 3.00 D. If the patient had a refraction of -1.50, the lens would cause overcorrection. Allowing for an initial overcorrection of 0.50 D, the actual required refractive change is 2.00 D, not 3.00 D. The "extra" +1.00 D would therefore be placed in the "extra refractive change" box.

Secondly, assuming that the Rx change required is 3.00 D, an extra -0.50 D is added to take regression into account. The limit of the exercise is an extra 1.00 D over that value predicted. If an extra 2.00 D were to be placed into the box, the practitioner would be warned that the TxZ diameter would be too small. Also, there are absolute limits as to the refractive change possible

Figure 8.27 The first page of the BE software. The practitioner can choose between the aspheric sag calculator and the tangent width required for the lens.

Figure 8.28 The aspheric sag calculator. The inputs are the apical radius, eccentricity, and chord. The sag is then calculated.