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

Figure 6.23 The mean and standard deviations of the range of corneal data values from the Medmont topographer. Note that the chord over which the measurements are taken is variable, as is the axis.

the order of ± 0.01 mm or ± 2 urn. Depending on the topographer used and the experience of the practitioner, the common range of error when measuring the elevation of human corneas can be from ± 2 urn to ± 15 urn (Cho et al 2002). The important question, then, is how many repeated readings are required in order to achieve a repeatability standard deviation of ± 2 urn?

Hough & Edwards (1999) give the following relationship that is used to determine the number of repeated readings that would fulfill the requirements set out above:

N =(S,/sy

where N is the number of readings required,S, is the reproducibility standard deviation, and S, is the repeatability standard deviation.

Cho et al (2002) measured the repeatability and reproducibility of four different topographers on a group of Asian subjects, and determined the number of readings required from each instrument with the S, limit set to 2 urn. The Medmont E300 had the best performance with only two repeat readings required. The Humphrey Atlas required 12, the Dicon 64, and the Orbscan 552.

However, this degree of accuracy in fitting is only required to determine the final lens parameters, and not the trial lens. Since the difference in trial lens sag between successive lenses is in

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the range of 10 urn, the tolerance for topographical elevation should be in the range of ± 5 urn.

HOW TO USE THE TOPOGRAPHY DATA

There are now two known facts that the practitioner must understand. Firstly, the accuracy of the topography data is unknown, but the standard deviation of error is known. Secondly, the accuracy of the sag height of the contact lens is known to a tolerance of ± 2 urn.

The concept with topography fitting is to compare that which is known (the sag of the trial lens) to that which is not known (the sag of the cornea) and use the corneal response to the trial lens to refine the fit.

For example, if the topographer gives a corneal elevation value of 1500 urn with a standard deviation of ± 10 urn, then the range of possible corneal elevations to the 95% confidence level is 1480-1520 urn, which is based on the 95% confidence level being equal to twice the standard deviation.

The sag philosophy states that lens sag is equal to corneal sag (elevation) plus the TLT. Taking the mean elevation value and adding 5 urn for TL1~ the required sag of the trial lens is 1505 urn. In other words, the mean value is always used as the basis for determining the initial trial lens.

Once the mean and standard deviation of error of the instrument for the individual patient are recorded, the practitioner will have a good idea of the range of trial lenses that may be required to achieve an optimal fit. For example, in the above case where the corneal elevation range is from 1480 to 1520 urn, the number of possible trial lenses, assuming a sag difference of 10 urn between lenses, is four.

The important fact to remember is that the topographer may have a standard deviation of error of ± 10 urn, but the lens has an error of ±2fLm.

The trial lens based on the mean value is then inserted and the fluorescein pattern assessed mainly for lens centration. If the fluorescein pattern is either grossly too steep or flat, then the initial indications are that the topography data are hopelessly inaccurate, and should be repeated. In most cases, however, the fluorescein pattern will show the typical reverse geometry appearance. The

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patient is then taught insertion and removal and advised to sleep in the lenses that night and return in the morning with the lenses in situ for assessment. A full description of the postwear clinical assessment is given in Chapter 9.

ASSESSING THE POST-TRIAL TOPOGRAPHY

The next step is to perform corneal topography measurements. This is probably the most valuable tool in assessing the accuracy of the corneal response to the lens, and provides invaluable information for refining the fit of the final lens design. The plot is taken and then a subtractive plot of the prefit and postwear generated.

Topographical analysis shows that there are four common outcomes from an overnight trial: a bull's-eye pattern, smiley face, smiley face with fake central island, and central islands.

Bull'seye

If the corneal data from the topographer were accurate, the trial lens will be an ideal fit, and will result in a bull's-eye postwear plot (Fig. 6.24). The

diagnosis of the bull's eye is always done using the axial, tangential, and refractive power subtractive maps. The axial difference map shows the refractive change achieved at the corneal apex, and has a high correlation with the change in subjective refraction (Mountford 1997, Soni & Nguyen 2002). The refractive change achieved after the first night's wear is dependent on the number of hours the lens has been worn (Sridharan 2001) and the actual apical clearance of the lens (Mountford & Noack 2001). If the fit is ideal, and the apical clearance correct, the refractive change achieved in the first night of wear will be approximately 70% of the total change required (Swarbrick & Alharbi unpublished). However, the assessment of a successful overnight trial is not based on refractive change, but on the centration of the treatment zone.

The tangential power map is ideal for assessing the accuracy of centration of the treatment zone (Fig. 6.24B). Note that the "red ring" is perfectly centered around the pupil zone.

Refractive power maps also playa part in the postwear assessment of the changes. The treatment zone diameter is overestimated with the

Figure 6.24 The axial (Al. tangential (B), and refractive power (e) maps of a bull's-eyetopography plot taken after an overnight-wear period. The axial mapshows the refractive change, the tangential the centration of the effect, and the refractive map the treatment zone diameter. Note the excellent centration.

c

axial map and underestimated with the tangential map. The refractive power map gives a more accurate measure of the actual zone (Fig. 6.24C).

The cursor is moved from the center of the map nasally and temporally until the point of zero change between the preand postwear maps is

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reached. The addition of the two values gives the treatment zone diameter. Also, in cases where it is difficult to make a final decision as to small degrees of decentration due to the corneal apex being decentered, the preand postwear refractive power maps can be used to see if the refractive change is central with respect to the initial corneal power distribution.

In cases where the initial corneal eccentricity is high, but only a low refractive change is required, the appearance of the bull's eye will not be as distinctive. Also, since the treatment zone diameter increases in diameter over the first week of wear, it is not uncommon to see incomplete bull's-eye pattern formation after the first overnight trial (Fig. 6.25). The single most important factor is the centration of the effect. Bull's-eye patterns are the only acceptable result of a trial lens fitting. The resulting corneal shape changes are wellcentered and even in appearance, with little or no distortion in the central zone, resulting in good unaided visual acuity.

Smiley-face pattern

Smiley-face patterns (Fig. 6.26) are indicative of a flat-fitting lens that has decentered superiorly, typically due to instrument underestimation of the corneal elevation or sag. Underestimation of the elevation is equivalent to an overestimation of the eccentricity. If the eccentricity is overestimated, the alignment curve or tangent of the lens will be too flat. When the rest of the lens construction is based on this value, the result is a flat-fitting lens. Also, the apical clearance will theoretically be less than zero, and the lens back surface will come into direct contact with the corneal apex. The force of the lid, combined with a redistribution of the tear film squeeze forces behind the lens will make the lens move superiortemporally in order for equilibrium to occur.

When the lens decenters in this manner, the topography shows an area of flattening superior to the pupil, with a crescent of inferior steepening in the pupil zone. This is the classical smiley-face pattern. Figure 6.26A shows the axial power change. Refraction in theses cases usually yields a moderate decrease in myopia, but an increase in with-the-rule astigmatism. Unaided high-contrast

acuity is also usually good, but associated with symptoms of ghosting and flare. Confirmation of the decentration is best done with the tangential power map (Fig. 6.26B). Note that the "red ring" is decentered superiorly. The degree of decentration is related to the underestimation of the corneal elevation, in that the greater the decentration, the greater the underestimation of the elevation.

Smiley-face patterns are an unacceptable outcome for an overnight trial, as the resulting topography has induced corneal distortion. The patient should return for a further overnight trial with a steeper lens until a bull's-eye plot is achieved.

Smiley face with fake central island

Smiley faces with fake central islands (Fig. 6.27) represent an even greater underestimation of the corneal sag than a normal smiley face. In these cases, the lens sag is much less than the corneal sag, leading to heavy central touch that results in epithelial damage. The disruption of the surface causes distortion and linkage of the reflected mires, which the instrument then reconstructs as an area of steepening. The key diagnosis is made by the tangential power difference map (Fig. 6.278) and the appearance of central corneal staining. Further confirmation can be made by deleting the color map from the image and inspecting the central mires for distortion (Fig. 6.28). Once again, this is a totally unacceptable outcome, and the patient must be scheduled for a further overnight trial with a steeper lens until a bull's-eye plot is achieved.

Central island pattern

Central islands (Fig. 6.29) are usually caused by a steep or tight lens as a result of instrument overestimation of the corneal sag or elevation. Since the development of the dual RGLs such as the Dreimlens and the BE, or lenses of larger total diameter, central islands have become the more common postwear plot, as compared to smiley faces, which are more common with the threecurve RGLs. The classic hallmarks of a central island are an area of relative steepening centrally, surrounded by an annulus of marked corneal flattening. The central island itself can be either steeper than the original cornea, or simply steeper than the surrounding annulus of corneal flattening.

power is steeper than the original value, will not resolve. The difference is that a small island indicates a small degree of inaccuracy in the

initial fit, whereas a steep island indicates a large degree of initial fitting error. The BOZR of the lens is usually not at fault in these cases. An

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with central islands is always perfect. When a central island occurs, unaided visual acuity is usually worse than the prefit values. Also, overrefraction leads to no clear endpoint, and the best corrected visual acuity is usually two lines or more less than normal. The refraction data from a central island are of no clinical importance.

It should be appreciated that some topographers, particularly the Keratron, can give false central island maps due to the reconstruction algorithm used. The Keratron does not apply smoothing to the central data, so as the arc-step method approaches the tangent normal to the optic axis, the value of the apical corneal power becomes unpredictable. Keratron users are advised to compare the refractive power maps before and after, and not subtractive, to determine if a true central island is present.

Figure 6.28 If the color mapis removed, the distortion of the Placido mires becomes visible. This leads to invalid central data and the appearance of a central divot (see Figure 6.31).

be decentered either inferiorly or laterally. The cause is the same in either case: overestimation of elevation leading to a tight alignment curve, or a lens diameter that is too small. The second is termed a "central divot" (Fig. 6.31), and is caused by apical touch leading to epithelial disruption centrally. The mires of the placido become linked, and the reconstruction algorithm represents the data as a small area of gross central flattening. They can also occur if the tear layer is disrupted from a bound lens. Simply instilling a drop of lens lubricant and repeating the capture can resolve the differences. If the divot is due to tear layer instability, it will not be present on the second plot. However, if epithelial compromise did occur, the divot will still be present. Other types of abnormal topography outcomes also occur, and are discussed in greater detail in Chapter 9.

OTHER POSTWEAR RESULTS

There are two other possible outcomes from an overnight trial that occur rarely. The first is a frowny face (Fig. 6.30), where the lens appears to

REFINING THE FIT USING POSTWEAR TOPOGRAPHY DATA

The major force responsible for the corneal shape changes seen in orthokeratology is the squeeze film

force generated by the postlens tear layer (see Ch. 10).The squeeze film force is dependent on the variation in TLT between the apex and the periph-

ery of the lens. The accuracy of the lens fit is therefore dependent on the actual TLT under the trial lens compared to the calculated TLT from the