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

This result validates the use of the prefit corneal eccentricity value as a reasonably accurate predictor of the refractive change possible. The cornea starts off as an aspheric surface, but becomes spherical following orthokeratology treatment. The area of the spherical surface is dependent on complex interactions between aspheric and spherical surfaces, and will be dealt with in greater detail in Chapter 8.

The mean axial and tangential curves of preand postorthokeratology in 40 subjects are shown in Figure 7.10. The axial curves in the posttreatment cornea appear to steepen in the midperiphery and almost meet the prefit curvatures at approximately 3.50 mm from center. The tangential curves, however, intersect at approximately 2.25 mm from center. This is reflected in the topography plots as a difference in the apparent area of corneal flattening. In effect, the axial map tends to overestimate the area, whilst the tangential map underestimates the area of change. The same problem occurs when estimating the ablation zone diameter in refractive surgery (Roberts & Wu 1998). Both the axial and tangential radii give inaccurate data as to the actual area of corneal flattening due to the averaging nature of the algorithms used. The conclusion reached by Roberts & Wu was that the accurate estimate of the ablation zone following refractive surgery, and, by extension, the area of corneal flattening induced by orthokeratology (treatment zone TxZ diameter) was dependent on an analysis of the actual refractive power profiles.

Refractive power maps are based on Snell's law, and calculate the refractive power of the cornea at each point across the surface. A subtractive refractive power map is shown in Figure 7.10. The distance between the points of zero refractive change on the difference map represents the TxZ diameter as shown in Figure 7.10.

Lui & Edwards (2000) compared the effects of Contex OK704 lenses on a group of 14 orthokeratology subjects to those occurring in a control group (14 subjects) fitted with standard alignment lenses. As part of the analysis, the mean ring radius (MRR) changes from prefit to posttreatment were analyzed. Ring numbers 1 (0.6 mm diameter zone), 8 (3 mm diameter zone), 20 (7 mm diameter zone), and 26 (9.20 mm diameter zone) were used as these zone diameters related to the corneal apex (MRR1), the keratometer zone (MRR8), the maximum tear layer depth and BOZO of the lens (OK704) (MRR 20), and the contact chord in the periphery (MRR26).

Their results, over a lOa-day study, showed statistically significant changes in the MRR values preand postwear in the orthokeratology group, particularly with MRRl and MRR8, but not with MRR20 or MRR26. There was no statistically significant difference between MRRl and MRR8 following orthokeratology treatment, indicating a spherical surface over this zone. Further analysis of the topography data found central corneal flattening, mid-peripheral steepening, and little or no change in the extreme periphery. They postulated that the point of zero change in the peripheral area

(mean MRR23-24) should have some predictive value as far as refractive change is concerned, and found a reasonable correlation (r 2 = 0.34) between baseline MRRI and mean MRR23-25.

The refractive change achieved was also correlated with the mean ring power reduction over the central five-ring area:

y = 1.05x + 0.09

where y is refractive change and x is the mean ring power change. However, there was no correlation between refractive change and change in ACP change as measured by the TMS. This was thought to be due to the fact that the majority of lenses decentered to the superotemporal direction. By using the mean of the central five-ring power change, the effects of lens decentration were minimized.

Standard OK704 lenses were used in this study whereas tangential periphery lenses were used to control centration in Mountford's study. The difference in centration between the lenses in the two groups could account for the difference in relationship between apical corneal radius and refractive change. The other factor to consider would be the difference between instruments used, and the way apical radius was determined for each.

They also found a corresponding increase in corneal shape factor (p) towards sphericalization of the corneal surface as the procedure continued. Further analysis of the p-value changes compared to the axial curvature changes using Douthwaite's method (Douthwaite 1995) showed that once orthokeratology-induced changes occur in the cornea, the ellipsoidal model breaks down, meaning that instrument-generated values of either eccentricity or shape factor become invalid. The mean change in shape factor (p) was an increase of 0.20 associated with a mean refractive change of 1.50D. This is in agreement with the results shown in Figure 7.9, where topography-generated data were used to find the best-fit aspheric curve to the postwear shape. The lack of a correlation may be primarily due to the inability of the topographer to reconstruct nonaspheric surfaces.

Nichols et al (2000) also found variability in the postwear Q-values generated by the Humphrey Atlas, primarily due to the effects of lens decentration. However, they reported a

mean change in ACP of 1.20 0 associated with a mean refractive change of 1.830, and a mean change of 0.11 in Q-value. The studies of EI Hage, Mountford, and Liu and Edwards found a relationship between change in refraction and change in corneal asphericity. The main differences between the studies is the higher correlation coefficient found by Mountford and EI Hage when compared to the other two studies. The probable cause for this is the control of centration of the lenses used. Liu & Edwards used the standard OK704, which tended to center superotemporally in the majority of subjects. Nichols et al used the 704C with a wide aspheric alignment zone, but were limited by the simple algorithm used to fit the lenses, and an inability (due to the experimental design) to modify the lens fit in order to control centration.

The EI Hage and Mountford studies were carried out in private practice, where lens design alterations were used to maximize centration. A lens that decenters superiorly will still give good high-contrast unaided VA, but the topography data will skew the results, resulting in a poorer correlation between the corneal asphericity changes and the refractive changes achieved. The only valid postwear topography outcome is a well-centered "bull's-eye" plot. Any lens-induced corneal distortion from lens decentration is considered to be clinically unacceptable, and the postwear refractive and shape changes basically invalid.

It is essential that the analysis of data from research into the corneal shape changes associated with orthokeratology is based on valid postwear information.

The conclusions that can be reached from the above analyses can be summarized as follows:

1.The predictive value of the differences in central and temporal keratometry is not valid.

2.The initial corneal eccentricity is a useful predictor of the refractive change possible.

3.There is a high correlation between refractive change and ACP change, making this a good objective method of measuring refractive change, as long as the lens centers correctly. The relationship breaks down if the lens decenters.

4.Once a reverse geometry lens starts effecting changes in corneal shape, the elliptical model

of corneal shape breaks down due to the inability of the videokeratoscope's algorithms to analyze the postwear shape correctly. It is therefore not possible to fit a reverse geometry lens using topographic data to the altered corneal shape with any accuracy.

5.Care must be taken when interpreting topography maps for the estimation of the area of central flattening. This is best performed by using the refractive power differences between the preand posttreatment surfaces.

6.Keratometry has severe shortcomings in monitoring the changes induced by orthokeratology when compared to corneal topography.

7.There are limitations as to the wider application of the results of these studies when using different topographers. Different topographers have different methods of calculating not only apical radius values, but also eccentricity.

However, the basic information found in the study is that there is a correlation between initial corneal asphericity and refractive change. In the case of the EyeSys topographer, the final change was found to be basically spherical, but this does not preclude the possibility of the final shape being oblate if the corneal topographer used has algorithms that can correctly calculate an oblate surface over the central 5.00-6.00 mm chord.

As will be shown in the later section on mathematical modeling of corneal shape change and refractive change (Ch. 8), there should be a correlation between these two factors even if the cornea becomes oblate.

THE EFFECT OF ORTHOKERATOL06Y ON ASTIGMATISM

One of the most problematic areas in orthokeratology is the effect of the procedure on corneal astigmatism. The earlier studies (Kerns 1976, Binder et al 1980) found an increase in unwanted with-the-rule (WTR) astigmatism, mainly due to the poor centration of the lenses used. Coon (1984) reported a decrease in WTR astigmatism using the Tabb fitting philosophy. However, the reports on the subject in the literature are mostly anecdotal, with little regard for the magnitude or direction of the astigmatic changes involved. The

"accepted" limit of the correction of astigmatism is given as 3.00 D (Wlodyga & Harris 1994). However, this limit appears to be set due to the fact that one patient had a reduction of 3.00 D in astigmatism.

The fitting philosophies also vary depending on the particular school of thought used regarding the treatment of astigmatism (Patterson 1975, Potts 1996). However, the commonly accepted method (Wlodyga & Harris 1994, Potts 1996) consists of fitting the initial lens one-third of the difference between the flat and steep meridians steeper than the flat meridian. This essentially has the effect of reducing the astigmatism prior to the active flattening of the cornea. There have been no reported studies as to the success or otherwise of this procedure, only scattered case reports. Also, the main accent is on the reduction of WTR astigmatism, as against-the-rule (ATR) astigmatism has proven almost impossible to reduce with orthokeratology. The exact reason for this is simply not known, although one study (Paige 1981) reported on the reduction of ATR astigmatism in one subject using up to 3000 transverse ocular movements per day!

There have been few reports on the effects of reverse geometry lenses and astigmatism in the literature. Soni & Horner (1993)found a 60% retention of the original astigmatism in their cohort. Liu & Edwards, on the other hand, reported no change in astigmatism in their orthokeratology subjects.

The gold standard assessment of the effect of orthokeratology on corneal astigmatism requires a vector analysis approach in order to analyze correctly the magnitude and axis changes that occur. Mountford & Pesudovs (2002) performed a retrospective analysis of 23 patients with between 0.50 and 2.00 0 prefit corneal astigmatism using two different vector analysis techniques and corneal topographical analysis.

The two vector analysis techniques used in the study were the Bailey-Carney and Alpins methods. The Bailey-Carney technique (Bailey & Carney 1970) was primarily developed to measure the effects of rigid contact lens wear on corneal curvature, whereas the Alpins technique (1997) was developed in order to evaluate refractive surgery outcomes with respect to astigmatism.

Vector analysis involves the resolution of two variables so that the line joining the variables

186 ORTHOKERATOLOGY

(the vector) demonstrates both magnitude and direction, which makes it an ideal method for the analysis of astigmatic changes. The results for the group of 23 subjects analyzed using the Bailey-Carney method yielded a mean reduction in astigmatism of 50.2%.

The Alpins method

Noel Alpins, a Melbourne ophthalmologist, developed this technique of vector analysis in order to assess properly the outcomes of refractive surgery, penetrating keratoplasty, and cataract surgery with respect to astigmatic changes. It is, in a lot of ways, a marked advance on the Bailey-Carney method in that the data obtained can be represented as an index of success or failure that can be subjected to statistical analysis. The subjects were divided into two subgroups for analysis: group 1 was 0.50 D or greater and group 2 0.75 D or greater.

For a total correction of 0.50 D astigmatism and greater, orthokeratology has an efficacy of 0.62 or 62%, and in order for the 100% goal to be attained, the treatment would need to be 60% more effective. Similarly, for the 0.75 D group, orthokeratology has an efficacy of 56%, and an increase in effectiveness of 80% would be

required in order to correct preexisting astigmatism totally.

If a more realistic aim of a 50% reduction in preexisting astigmatism is the target, then orthokeratology is highly effective, with treatment efficacy slightly greater (7%) than that required for the 0.50 D group, and 2% less than that required for the 0.75 D group.

However, standard deviation of the angle of error values shows a markedly high degree of variability. Alpins considers a good outcome to show little deviation from the original astigmatic axis, with a small standard deviation. The comparatively large standard deviations in the angle of error caused by orthokeratology indicate a high degree of unpredictability in the axis of the posttreatment astigmatism. However, this was not reflected in the results of the 23 subjects, where the relative change in astigmatic axis from the original axis was within 10°. In fact, the deviation from sphericity locus as measured for the Bailey-Carney analysis showed a maximum deviation of 11 0. The probable cause for the difference between the calculated standard deviation using the Alpins method could be partly due to the relatively small degrees of astigmatism involved. None of the cases reviewed had oblique astigmatism induced.

Figure 7.11 A subtractive refractive power mapshowing the results of an orthokeratology treatment.The line shows the treatment zone diameter.

Figure 7.12 The preand postorthokeratology astigmatism. The majorchange occurs within the central 2.00 mm chord. At the keratometer zone (3.00 mm)the changes are not statisticallysignificant.

CORNEAL TOPOGRAPHICAL ANALYSIS IN ASTIGMATISM

Topographical analysis of the preand posttreatment astigmatism was performed on 16 subjects who exhibited 0.75 D or greater corneal astigmatism. The preand postorthokeratology astigmatism over the central 8.00 mm chord is shown in Figure 7.12.

Figure 7.13 The change in astigmatism from the center to the peripheral cornea.

Note that the major change in astigmatism appears to occur within the central 2.00 mm chord. This change in astigmatism is shown in Figure 7.13. Paired Student's t-tests were performed on the preand postwear values, with the differences over the central 3.00 mm chord being statistically significantly different. The P-values

P = 0.01. There was no statistically significant difference for the 2.00-5.00 mm half-chords.

Figure 7.14 A subtractive axial power map showing the flattening of the central 2.00 mm chord with astigmatism. Note the greater change in the vertical meridian (blue bowtie).

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As shown in Figure 7.11, the major change in astigmatism occurs within the central 3.00 mm chord, with the change at the 2.00 mm chord being approximately twice that occurring at the 3.00 mm (keratometer) chord. The percentage change in astigmatism for the different chords was: 1.00 mm 55.6%, 2.00 mm 49%, and 3.00 mm 29.7%. In other words, the keratometry values, if used to assess astigmatic change, would only record approximately 50% of the change occurring closer to the corneal apex.

Figure 7.14 shows a subtractive difference map of the change in astigmatism with orthokeratology. Note that the major change in the astigmatism occurs within the central 2.00 mm chord, and that the simulated keratometry values do not correspond to the reduction in astigmatism achieved. The relationship between the preand postcorneal astigmatism over the central 1.00 mm chord is shown in Figure 7.15. Although statistically significant, the correlation is very poor (r 2 = 0.11, P = 0.04, df 14), indicating a poor predictability between initial and final astigmatism.

All three methods of analysis point to a reduction of approximately 50% in preexisting astigmatism of up to 1.75 D if reverse geometry lenses are used. This would therefore indicate that the degree of astigmatic reduction possible with the procedure is limited to approximately 1.00 D, so

Pre- vs.posl-orthokeralology astigmatism (1.00 mmchord)

Figure 7.15 The relationship between the preand postorthokeratology astigmatism overthe central 1.00 mmchord. Although statisticallysignificant, the correlation is poor.

the level of preexisting astigmatism is of major importance when assessing a patient's suitability for the procedure. From a clinical viewpoint, it could be argued that a residual astigmatic error of 0.50-0.75 D would have little negative impact on unaided vision, especially if it is WTR in direction. This means that the maximum indicated error that could be treated and expected to yield acceptable results would be in the order of 1.50 D.

OTHER CONSIDERATIONS

The above analysis is based on the use of spherical reverse geometry lenses and their effect on astigmatic change, but does not take into consideration the effect, if any, of more novel lens designs and fitting philosophies. There are also some unanswered questions with respect to the treatment of astigmatism that should be addressed, namely:

1.Corneal eccentricity has been shown to be a good indicator of the refractive change possible with orthokeratology. Therefore, could a difference in eccentricity between the steep and flat meridian also playa role in determining the astigmatic change possible?

2.The shape changes induced by the lens are, in some way, related to the tear film squeeze forces developed under the lens. Could the difference in tear layer profile and thickness between the steep and flat meridian also play a role in astigmatic change?

3.Why does ATR astigmatism increase with the use of reverse geometry lenses, and what lens design variations can be used to try and control this effect?

4.Why does limbus-to-limbus astigmatism respond so poorly to orthokeratology when compared to central astigmatism?

The problem of astigmatism and orthokeratology is quite complex. Hopefully, as the sophistication of corneal topographical analysis increases, our understanding of the nature and shape of the astigmatic cornea will enable the development of specific fitting philosophies and lens designs in order to improve not only the predictability of treatment, but also the efficacy of treatment for astigmatism.

In conclusion, analysis of the data shows that orthokeratology is effective in reducing preexisting astigmatism by approximately 50%. This would therefore set the prefit level of astigmatism to an upper limit of 1.50 0 with the rule.

STABILITY AND RETENTION OF INDUCED CORNEAL SHAPE CHANGES

As stated at the beginning of this chapter, one of the important factors to consider when assessing the suitability of a patient for orthokeratology is the question of the stability of the shape changes induced and how they impact on the quality of unaided vision following lens removal.

There is very little information in the literature concerning the stability of the effects of accelerated orthokeratology; however the common understanding is that those who respond faster to the treatment will lose the effect at a faster rate (Wlodyga & Harris 1994). However, some doubt was cast on this observation by Horner et al (1993), who found that the corneal response to the OK-3 lens in terms of refractive change was timedependent, in that the longer the lens was worn, the greater the effect appeared to be. Following wearing times of 1, 2, and 4 h, the ranges of refractive change were 0.10-2.370 (1 h), 1.01-1.810 (2 h), and 1.43-2.560 (4 h). The trend appears to

Relationship between change in apical corneal power, retention and regression Inorlhokeratology withtime

Figure 7.16 The change in apical corneal power (ACPI. retention (Ret) and regression (Reg) is shown. Note that the error bars for the regression decrease with time.

be increased refractive change with increased wearing time, but the other obvious factor is the variations in response, as shown by the spread of the range. Similarly, the regression of the effect to 95% of baseline levels was shown to vary with wearing time, in that the I-h group recovered at the rate of 50.9% per hour, compared to 36.6% for the 2-h group and 30.5% for the 4-h group.

There was no correlation between the degree of induced change and the rate of recovery, in that a greater change in refraction was not related to a faster rate of recovery or regression. This was in distinct variance with the findings of Poise et al (1983a), who found that the greater the induced refractive change, the greater the "instability index" and the faster the regression.

The effect of regression and retention in a group of successful orthokeratology patients was studied by Mountford (1998). From a group of 364 subjects, 48 were isolated who fulfilled the requirements of regular reviews at 7, 30, and 90 days postfitting, with a maximum variance of

± 1 day from the required time interval. The choice of eye to be used for analysis was randomized, and the changes in apical corneal power as measured by the EyeSys topographer (version 3.20) measured at time zero (lens removal following overnight wear) and again at a mean of 8.52 ± 0.57 h postwear. Apical corneal power was chosen as the means of assessing refractive change due to the high correlation with refractive change and the objective values obtained. The end-of-day ACP values were subtracted from the immediate postwear values recorded on lens removal in the morning to yield the regression of the effect. The change in refraction from the prefit values was recorded as the change in ACP from prefit to postovernight wear. The retention of the effect was the difference between the change in ACP and the regression at the end of the day.

The mean and standard deviation values for changes in ACp, retention, and regression over the treatment period are shown in Figure 7.16. Note that there appears to be a slight increase in the change in ACP over the period, and a decrease in the overall regression with time. The ACP and retention results were analyzed with paired Student's t-tests, whilst chi-square analysis was performed on the regression data due to the non-

Relationship between change in ACP and regression (7days)

A

Relationship between change in ACP and regression (30 days)

B

Relationship between change in ACP and regression (90 days)

c

Figure 7.17 (A) The relationship between change in apical corneal power (ACP) and regression at 1 week. There is a large scatter, and the correlation is not statistically significant. (B) The same relationship at 30 days. The scatter of the results issmaller, but there isstill no correlation.

(C) Byday90, the mean regression is0.38 D, but there is still no correlation between refractive change and regression.

normal distribution of the results. The change in ACP from the prefit values yielded the following results: 7 vs 30, P = 0.014; 7 vs 90, P = 0.0002; and 30 vs 90, P = 0.06 at the 95% confidence level. The

P = 0.06 at the 95% confidence level.

Two values, 0.500 and 0.75 0, were used to determine the effects of regression using chisquare analysis. A regression of 0.50 0/ day was felt to be of little clinical significance (see later) whereas a regression of 0.75 0 could be considered to be more clinically significant. The results for 0.50 0 regression were: 7 vs 30, P = 0.12; 7 vs 90, P = 0.0012; and 30 vs 90, P = 0.06. For regression of 0.750 and less the results were: 7 vs 30, P = 0.08; 7 vs 90, P = 0.003; and 30 vs 90, P = 0.026.

The relationship between change in ACP and regression over the time intervals was assessed by regression analysis (Fig. 7.17).

There was a poor correlation between change

90 days, r 2 = 0.02, P = 0.26), and there was no statistical significance in the relationships. This is in agreement with the findings of Soni & Horner (1993), in that, although the degree of refractive change appears to be time-dependent, the regression is not related to the magnitude of change induced.

The major change in refraction occurred in the first 7 days, with only a small gain of approximately 0.26 0 at day 30, and little if no gain up to day 90. Regression, however, appears to

Figure 7.18 The change in refraction overtime with overnight orthokeratology. Notethat the majorchange occurs in the first night. The regression by day 10 was not statisticallysignificant. Courtesy of Helen Swarbrick and Ahmed Alharbi.

decrease with time. The interesting point to note in Figure 7.17 is the gradual contraction of the data points over time, to the stage where the mean regression at day 90 was 0.38 D.

Lui & Edwards (2000)studied refractive change over a 100-day period of mainly daily wear of OK704 lenses. The major refractive change occurred in the first day, with approximately 80% of the total change occurring within the first 10 days. Little or no further change occurred after day 40. They did not report on the retention of the effect.

Swarbrick & Alharbi (2001) performed morning and evening reviews on a group of overnight orthokeratology subjects using BE lenses. They found a 70% reduction in refractive error on the first night, with the maximum change and stability being reached by day 10. Interestingly, they found the regression to be not statistically significant when compared to the morning results by day 10, and that the effect remained stable for the remaining 90 days of the study (Fig. 7.18).

Nichols et al (2000) found a similar result in their study using the OK704C, in that the mean regression was minimal (approximately 0.25 D) by day 60. Nguyen et al (2002) also found virtual stabilization of the refractive changes by day 10 when studying the effects of the Contex BBseries. The results of both the Nguyen and Swarbrick studies differ from the Nichols and Mountford

studies in that the stability of the changes occurred at a much faster rate. Both Swarbrick & Alharbi and Nguyen et al used the newer design four-curve lenses (BE and Contex BB) whereas Mountford and Nichols et al used the older three-curve designs. There have been numerous anecdotal reports (Day et al 1997, Optcom Orthokeratology Forum) that the newer designs cause faster refractive changes than the older designs, and that the effects are better retained in a shorter period of time. The refractive and VA changes were similar for the newer designs, indicating that there really is no "superior design," irrespective of the claims made by manufacturers. However, it appears that the major difference between the original three-zone lenses and the newer fourzone designs is that the changes not only occur more rapidly, but that the regression appears to be less.

Further controlled research is required in order to determine if there are any real differences in outcome with different designs.

In general, the studies all show that, with a correctly fitted lens, the major refractive change will occur in the first night of wear, reaching a maximum change by approximately day 10. Also, by the same time period, the stability of the changes is well maintained, leading to a stable refraction and unaided VA. From a clinical viewpoint, a deliberate plan of mild overcorrection of the order of 0.50 D in order to compensate for any mild regression effects, and also as an attempt to reduce the number of nights the lenses are worn, would be a worthwhile aim for practitioners to consider.

From a purely anecdotal and clinical viewpoint, some practitioners have noted that the number of nights per week the lenses need to be worn to maintain the refractive and visual acuity gains decreases with the length of time the lenses are worn. If the patient is mildly overcorrected, it is not uncommon for the majority (approximately 70%) to be dropped back to every second night's wear after 3-4 weeks. By 6 months, some patients are down to as little as two nights' wear per week, and in rare cases of up to 3 years of orthokeratology lens usage, one night per week is all that is required to maintain the effect. These clinical observations raise some interesting questions as to the underlying mechanism that controls the