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

1.The lens has standard curves, with the back optic zone radius (BOZR) fitted 1.50-2.50 0 flatter than the flat-K (Contex OK series).

2.The BOZR is based on the basis of the change in refraction required with an allowance for an extra "compression factor" of between 0.50 and 1.00 O. If the laboratory is only supplied with K readings, the eccentricity is assumed to be 0.50 (Oreimlens, CRT, Correctech, Emerald, Contex BB, Fargo).

3.The results of one topography reading are sent to the lab, and a lens design based on the data is made (Euclid Jade). Alternatively, the practitioner uses a specific computer program linked directly to the topographer as an aid to lens design (WAVE).

The sag-based fitting approach, as outlined in Chapter 4, as well as the work of Bara (2000) shows the inaccuracies that can occur if the first method is used. Mountford et al (2002) have shown that basing a lens design on the flat-K and an assumed eccentricity of 0.5 also leads to inaccuracies. The two values needed to design a lens based on the sag philosophy are the apical radius (Ro) and the eccentricity (e) or elevation at the common chord of contact between the lens and the cornea. The flat-K is not equal to the apical radius. The apical radius is the instantaneous radius of curvature at the corneal apex, and contains elements of both the steep and flat meridian. Depending on the degree of corneal astigmatism, and the eccentricity of the cornea, Ro can be significantly steeper than flat-K. Table 6.1 shows the difference between Ro and flat-K with varying eccentricities, assuming a nonastigmatic cornea. If astigmatism is present, the difference is greater for increasing degrees of astigmatism.

If the flat-K is used, and the eccentricity is known, the apical radius for nonastigmatic surfaces is calculated from the following formula (Oouthwaite 1995):

Ro = --JR/ + y2p _ y2

where Ro is apical radius, R, axial radius at the nominated chord, y is the half-chord (1.5 mm for the keratometer), and p is (1- e2 ) .

The accuracy of the assumptions used to generate an empirical lens design simply based on

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Table 6.1 The variation in apical radius value with changing eccentricity assuming a f1at-K value of

7.76 mm (43.50 D)

the flat-K value and an assumed eccentricity will be examined in detail in the following example.

A practitioner sends the laboratory the following data and asks for a lens to be made. The manufacturer designs the lens on the following basis. The BOZR of the lens is assumed to be equal to the refractive change required plus an extra 0.50 0 as a compression factor. The design is a four-curve lens, with a back optic zone diameter (BOZO) of 6.00 mm, a reverse curve width of 0.60 mm, an alignment curve that is 1.00 mm wide that meets the reverse curve with a tear layer thickness of 10 urn. The edge lift is a simple 10.00 mm curve 0.40 mm wide. The apical clearance of the lens is presumed to be 5 urn.

The data supplied to the lab are Flat-K 43.500, and the refractive change required -3.00 O.

Since the corneal eccentricity was not specified, the manufacturer automatically assumes that it is 0.50 for the sake of calculations. The lens would therefore have the following specifications:

BOZR = 43.50 0 - 3.00 0 - 0.50 0 = 40.00 0 = 8.43 mm

This equates to the flat-K minus the Rx change required (3.00 D) minus the compression factor of 0.50 0, leaving a final BOZR of 40.00 0 or 8.43 mm. The exact lens specifications and the tear layer profile of the lens are shown in Figure 6.1, and are based on an assumption that flat-K is equal to RD.

As can be seen from Table 6.1 Ro is only equal to flat-K if the eccentricity is zero. The actual value for Ro when flat-K is 7.76 mm and the eccentricity is 0.5 is 7.72 mm. If the same lens details are then fitted to the cornea with the Roset at 7.72 and an eccentricity of 0.5, the tear layer profile and the fit of the lens will vary. The tear layer profile of the original lens parameters on the actual corneal shape is shown in Figure 6.2. Note that the calculated lens parameters result in a lens that is too flat, with zero apical clearance and no peripheral touch in the alignment curve zone. This lens will commonly decenter superiorly until a tear layer under the lens is formed that causes the appropriate squeeze film and

surface tension forces required for a stable fit (Hayashi 1977, Guillon & Sammons 1994).

The mean corneal eccentricity in the population is approximately 0.50, with a standard deviation of 0.15, so as a comparison in the above example, it will be assumed that the patient did not have an eccentricity of 0.5, but either 0.35 or 0.65, one standard deviation either side of the norm. The tear layer profile of the initial calculated lens on the 0.35 eccentricity cornea is shown in Figure 6.3. As with the example above, the lens design arrived at by the laboratory is too flat for the corneal shape and will decenter. In both of these cases, a smiley-face topography plot will result from wearing the lens (see later), with

induced with-the-rule astigmatism and poor unaided acuity.

Finally, the eccentricity is assumed to be 0.65, with a resultant change of apical radius to 7.69 mm. The ordered lens on this cornea will be far too steep, with approximately 14 urn of apical clearance and a steep alignment curve (Fig. 6.4). Steep or tight lenses result in central island postwear topography plots, with poor unaided visual acuity that is usually worse than the initial unaided visual acuity. The differences between the reverse curve and alignment curve values and those supplied by the manufacturer are shown in Table 6.2. Note that in all cases there are significant differences in the curves, and that none of the lenses are interchangeable.

If the practitioner is faced with these less than ideal responses to the lens that was sent, the only course of action is to contact the laboratory with the results and ask them to redesign the lens. The usual method of rectification with most of the empirically designed lenses is to alter the alignment curve. The alignment curve with most fourcurve designs is based on the axial radius of curvature of the cornea at a chord of 4.60 mm, or the start of the alignment curve zone. The tear layer of such a lens is shown in Figure 6.5. Clinical experience showed that this curve was too flat, so the value was changed to allow for a tear layer thickness (TLT)of 10 urn at the junction of the reverse and alignment curves (Reim 2000). This, in effect, results in the alignment curve

144 ORTHOKERATOLOGY

Table 6.2 The effect of the actual corneal shape on the values for the reverse and alignment curves. Note that in all cases the values differ significantly

BOZR, back opticzone radius.

being approximately 0.05 mm or 0.25 0 steeper than the calculated Ra• The effect this has on the tear layer profile is shown in Figure 6.6.

Luk et al (2001) cite various manufacturers' data and give the following description of the alignment curve construction. These data have already been presented in Chapter 4, but warrant repeating due to the importance in lens fitting.

•If the corneal eccentricity is between 0 and 0.3, the alignment curve should be equal to the central flat-K value.

•If the corneal eccentricity is between 0.31 and 0.55, the alignment curve should be 0.25 0 flatter than the flat-K value.

•If the corneal eccentricity is between 0.56 and 0.80, the alignment curve should be 0.50 0 flatter than the flat-K.

These generic values lead to gross errors, as shown in Table 6.3. Assuming a flat-K of 43.50 0, and varying eccentricities, the Ra value can be calculated at a chord of 4.60 mm. The value is then steepened by a further 0.250, as shown above. The resultant calculated values of the alignment curve vary dramatically with changing eccentricity.

So, what actuallyhappens when the laboratory alters the alignment curve? In the case of the cornea with an eccentricity of 0.35 (Fig. 6.3), the advice would be that the lens was too flat, so the alignment curve would be steepened. !~e closest alteration is shown in Figure 6.7. ThIS IS still not an ideal situation as the depth of the tear layer at the reverse curve / alignment curve intersection is 20 urn, instead of the required 10 urn, and the alignment curve is in effect too steep. The practitioner would then try this lens, which may result in edge compression and binding, or a central island. The laboratory would be requested to alter the fit again. This time, since the alterations to the alignment curve didn't work, the reverse curve and the alignment curve would be altered. The result is shown in Figure 6.8. At this

Figure 6.5 Tear layer profile of the lens when the alignment curve is equal to the apical radius at a chord of 9.20 mm. Note that the apical clearance is 0.6 urn, and that the tear layer thickness at the chord of

9.20 mm is approximately 5 urn,

Table 6.3 The flat-K and calculated alignment curve +0.25 Dare shown. The difference between the two is shown in the third column. The values calculated from corneal data differ markedly from the generic values of the alignmentcurves

stage, the tear layer profile is almost exactly what is required, and the final lens curves are very similar to those arrived at by calculation (Table 6.3). However, it has taken three different lenses to arrive at a lens with similar parameters to the lens based on actual corneal data.

A similar course of events would occur if the initial lens was fitted to the 0.65 eccentricity cornea. The laboratory would be advised that the lens was too tight, so the first alteration would be made by flattening the alignment curve. The result of this is shown in Figure 6.9. In this case, the alignment curve has been flattened from

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Figure 6.6 The effect of steepening the alignment curve by0.05 mm is to increase the apical clearance to 5 urn, as well as increasing the tear layer thickness at the 7.20 mm chord to 10 um,

7.936 mm to 8.05 mm. The usual change in alignment curve is 0.25 0 at a time, so once again, three lenses would be required to arrive at the correct result that could have been achieved if the original corneal data were used. Note that in none of the cases outlined has the BOZR been altered. The entire fitting philosophy of the lens is based on a relationship between the BOZR and the refractive change required, so in effect the BOZR remains constant.

The preceding examples illustrate that empirical fitting that is based solely on keratometry data is inherently inaccurate due to the false assumptions made with respect to flat-K being equal to apical radius and the assumption of a mean eccentricity value of 0.50. Those practitioners who prefer empirical fitting ought to make certain that they follow the suggestions of Bennett & Sorbara (2000), in that the laboratory must be supplied with accurate topography data on which to base the calculations. The practitioner must also ensure that the manufacturer uses the data to design the lens, and not simply the flat-K values and a nominal eccentricity. Even if the lens design and fitting are based on topography (WAVE, Euclid Jade), errors are still inherent due to two main factors. Firstly, the lens fit is usually based on the data from a single topography reading, and secondly, errors due to the reconstruction algorithms used by the topographer are not taken into account. These factors will be examined in greater detail in the section on topography-based fitting (see later).

To date, there has been only one report in the literature comparing the outcomes of empirical versus trial lens fitting. Fedders et al (1999) measured the changes in refraction, corneal shape, and fluorescein pattern appearance in a small group of subjects (3) following 4 h wear of OK-3 lenses. The first set of lenses were fitted by supplying the manufacturer with the K readings and the spectacle refraction. The second set were trial-lens-fitted and the fit altered until an optimal fluorescein pattern was achieved. The empirical lenses tended to be flatter in BOZR than the trial-fitted lenses, and caused less refractive change. The fluorescein patterns between the lens fits were very similar, leading to the conclusion that between fitting methods subtle differences occur that are not apparent with fluorescein pattern analysis. Further research into the differences between the first-fit success rate and corneal response to the different fitting methods is required.

Empirical fitting is potentially inefficient, timeconsuming, and expensive when compared to trial lens fitting with standard spherical RGP fitting (Bennett et al 1989). It is even less efficient when applied to reverse geometry fitting.

TRIAL LENS FITTING

The basics of trial lens fitting with RGLs for orthokeratology is essentially the same as for normal lens fitting. The practitioner is given a fitting nomogram by the manufacturer in order to select the BOZR (base curve) of the initial trial lens. The nomogram is usually based on a predetermined relationship between the BOZR of the lens and the flat-K reading. The trial lens is then inserted and the usual tests of fluorescein pattern analysis and lens movement and centration carried out. From these data, the practitioner is then able to prescribe a lens for the patient.

RGLs exhibit a totally different fluorescein pattern to that of a conventional lens. All current orthokeratology lenses show essentially the same fluorescein pattern, an area of "central touch" that is approximately 3-4 mm in diameter, a narrow and deep ring of fluorescein (the tear reservoir) approximately 0.5-1.00 mm wide, a wide alignment curve (1.00 mrn), and an edge

clearance. It is the appearance of the fluorescein pattern that has led to the totally false assumption of heavy central bearing with the lenses. Central bearing is always associated with epithelial insult and the formation of corneal scarring (Korb et al 1982), or lens decentration. RGLs are fitted with apical clearance. It is the limitations of fluorescein pattern analysis that give the appearance of touch.

This has been succinctly stated by UK practitioner Basil Bloom as follows:

At no time during the orthokeratology process should a properly fitting lens come into direct contact with the corneal epithelium at the apex. Such contact will result in epithelial insult (Bloom unpublished).

TRIAL LENS SETS

There are many different designs of RGL available, but, as shown in Chapter 4, the basic mathematical relationship between all the designs means that the fluorescein patterns will all appear similar. The important factors to take into account before ordering a trial lens set are:

1.the design

2.the material from which the trial lens set is made. Is the oxygen transmissibility (Ok/f) high enough to use for a safe overnight trial?

3.the dimensional stability of the material. How often will the lenses need to be checked for warpage and steepening, necessitating replacement?

4.the cost of the lenses and replacements

5.quality and accuracy of manufacture

6.engraving of the lens or color differences so the BOZRs are not mixed up

7.Does the set come with complete fitting instructions and / or a computer program to aid in lens selection and design variables?

8.the sagittal difference between successive lenses in the set.

Various manufacturers offer different combinations of the above variables, but of prime importance are the quality and accuracy of the trial lenses, and the dimensional stability of the material. For example, trial lenses made from Boston XO have a higher Dk and appear to be more

148 ORTHOKERATOlOGY

dimensionally stable than those made from Equalens 2. Mid-Dk materials can produce unacceptable degrees of corneal swelling on overnight trials and should be avoided for use as trial lenses.

The other important factor to consider is the logic and thought that have gone into the trial lens set design. As shown previously, the alignment curve or tangent is the major means of controlling lens centration, and varies considerably with corneal eccentricity. Since the trial lens will be required to fit a large range of different corneal shapes, the thought that has gone into varying the alignment curves or tangents in the set is of major importance. It is of little value having a trial lens set where the inaccuracies induced by too great a difference in relative sag between one lens and the next steepest or flattest lens renders the trial wear period invalid. Finally, the trial lens set should come with a complete set of instructions outlining lens design variations in order to modify the fit, and preferably a computer program that the practitioner can use to help choose the initial trial lens and then modify as necessary.

The corneal response to the lens is totally dependent on the accuracy of the lens fit, as this is what determines the tear layer squeeze film pressure and forces that initiate and control the response (see Ch. 10). If the trial lens set parameter

variables limit the change in tear layer profile to a minimum of 20 urn (or 0.10 mm BOZR steps), the results can be - and will be - quite variable.

However, designing a trial lens set for RGLs is a major task. As stated previously, the common corneal shape varies in apical radius and eccentricity. Most eyes have an eccentricity in the range of 0.30-0.70, so the problem with designing a trial lens set begins with having enough lenses in the set to fit the range of eccentricities adequately, let alone the variations in apical radius. Figure 6.10 shows the total combinations of corneal sags at a chord of 9.20 mm with eccentricities ranging from 0.30 to 0.70 and a range of steep to flat apical radius values. The colored bands represent differences in sag of 10 um. Note that for any given combination of Ro and e there is a range of sagittal equivalent eye shapes. In order to cover this range of corneal shapes, a 49-piece trial lens set with a sagittal difference of 10 urn between lenses would be required.

However, matters are complicated by the addition of the alignment curve. If an alignment curve that is 1.00 mm wide and contacts the cornea at a chord of 9.20 mm with a TLT of 10 urn at the junction of the reverse curve is required, the range of lenses increases. The variation in alignment curve radius for the range of corneal shapes is shown in Figure 6.11. Note that there are

EqUIvalent sagsfO(eoandeccentrlCity combinations ale ChOrd of9.20 mm

Figure 6.10 The range of equivalent sags for combinations of Ro and eccentricityfrom 7.00 to 8.50 mm. The bands of equal color are sagittal equivalents.

Relationship of alignment curve radius to apical radius for different eccentricities

Figure 6.11 The alignment curve radius is dependent on the apical radius and eccentricity. A line drawn parallel to

the x-axis at any alignment curve valueshows the R and o

eccentricity valuesthat it will fit.

common alignment curves for various eccentricities. The design of a trial set can be simplified if the alignment curve radius is based on the mean eccentricity value of 0.5. The BOZR can be preset to a nominal 3.50 0 flatter than Ro, and the reverse curves calculated for sagittal equivalency. The parameters of a trial lens set based on these assumptions are shown in Table 6.4. Note that there is an approximate sag difference of 12 urn between each lens in the series.

In order to fit the closest correct trial lens, the practitioner simply needs either to extract the corneal elevation value at a chord of 9.20 mm from the topographer, or alternatively calculate the value from the apical radius and eccentricity values. Once the corneal sag is known, the lens with the closest matching sag in the set is found. The trial lens sag should always be greater than the corneal sag to allow for apical clearance.

For example, assuming an apical radius of 7.43 mm and an eccentricity of 0.62, the corneal sag over the 9.20 mm chord is 1.4524 mm. The closest lens in the set is the 8.60 BOZR lens with a

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Table 6.4 The parameters of a trial lens set designed on the alignment curve radius based on the mean eccentricity value of 0.5. The back optical zone radius (BOZRj can be preset to a nominal 3.50 D

sag of 1.4605 mm that will give an apical clearance of 4.8 urn. The reverse curve is 6.85 mm and the alignment curve 8.09 mm. The tear layer profile of the trial lens is shown in Figure 6.12.

The analysis of the fit of the lens can then be refined by following the steps outlined in the section on topography-based fitting.

The Dreimlens, R&R, CRT, EZM, and BE trial lens sets are designed on this type of basis, with sag differences between lenses of approximately 10 11m. Trial lens sets that have a BOZR difference of 0.10 mm (20 11m) between lenses will have a limited range of applications.

The Dreimlens trial set

The Dreimlens is not usually fitted by means of trial lenses in the USA, but this is the preferred method of fitting in Asia. The trial set was originally designed for use in Hong Kong. This is a large (56-lens) set. It is a four-zone design with a

6.00mm BOZD, 0.60 mm wide reverse curve,

1.00mm wide alignment curve, and a 0.40 mm wide peripheral curve. The BOZR is set at a target of 3.00 D refractive change, and the alignment curves are standard, +10 11m, -10 11m, and -20 11m.

As shown in Chapter 4, variations of 10 11m in alignment curve mean that the TLT at the reverse curve / alignment curve intersection is initially 10 11m (standard lens), and is then varied in 0.05 mm radius steps either steeper (+10 11m) or flatter (-10 and -20 11m). The BOZR of the trial lens is 3.75 D flatter than flat-K, but is labeled as the flat-K value. The initial trial lens selected is based on the flat-K value and the standard alignment curve. If the lens is too steep or flat, a lens with a looser or tighter alignment curve is used.

Alterations to the reverse curve are made by changing the BOZR. A change of 0.10 mm in BOZR alters the sag of the reverse curve by 4 11m and the alignment curve by 10 11m. The assessment of the trial lens is based on the fluorescein pattern and lens movement and centration. Lenses that decenter superiorly are considered to be too loose, and those that decenter inferiorly too tight. Lateral decentration is resolved by increasing the diameter to 10.6 mm and using a five-zone lens. Once the preferred fit is achieved, overrefraction is performed and the lens ordered. Overnight trial wear periods are not performed.

Paragon CRT trial set

This is also a large set, consisting of over 100 lenses. There are 20 base curves at 0.1 mm intervals and four return or reverse zone depths with three different tangential peripheries. Further custom lenses are available for patients with corneal topographies that fall outside the 80% of corneas that can be fitted from the inventory. The available return zone depths range from 350 to 750 microns in steps of 25 microns. These values represent the sag difference between the BOZD and the tangent. The cone angles range from 27° to 37° in 1° intervals. All trial lenses in the set have one of three preset cone angles. The BOZD is a constant 6.00 mm and the reverse zone and tangent are each