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

Figure 4.22 Four-zone lens on a cornea of flat-K 43.500 (7.76 mm) and eccentricity of 0.50.

Figure 4.23 Same cornea as Figure 4.22 with the assumed alignment curve of 7.81 (Kr- 0.25 0).

ening, and conversely, flattening if the eccentricity is greater than the assumed 0.5.

Figure 4.24 shows the tear layer profile of an ideal-fitting four-zone lens. Note that in this example, the apical clearance is 5 I-Lm, and the alignment curve meets the corneal surface at the edge of the reverse curve and the edge lift curve.

How sensitive is this curve to change, and how do those changes affect the final fit?

In Figure 4.25 the effect of steepening or flattening the alignment curve by as small an amount as 0.05 mm (0.25 D) is shown. In effect, flattening the alignment curve by 0.05 mm results in a decrease of central clearance of 5 I-Lm, and

3.If the RC is flattened by 0.15 mm (0.750), the apical clearance is decreased by 10 I-Lm.

4.If the RC is steepened by 0.15 mm (0.75 0), the apical clearance is increased by 10 I-Lm.

These rules only apply if one curve is changed. If both curves are changed at the same time, the results are additive.

In the case of five-zone lenses, the second alignment curve acts as a fulcrum, and is less sensitive to change than the four-zone lens. For example Figure 4.26A shows an ideal fit, whilst Figure 4.26B shows that flattening the curve by as much as 0.20 mm (1.00 0) has no effect on the apical clearance. Steepening the second alignment curve by 0.05 mm (0.25 0) increases the apical

Figure 4.26 (A) Tear layer profile of an ideal-fitting five-zone lens. (8) Effect of flattening the second alignment curve by 0.20 mm (1.00 D). Note that the apical clearance remains unaltered, but the contact point is now counterformal. (C) Effect of steepening the second alignmentcurve by0.05 mm (0.25 D). The apical clearance is increased byapproximately 5 I-Lm.

clearance by approximately 5 I-Lm (Figure 4.26C). If the final curve is flattened too much, a counterformal periphery results.

The major problem with counterformal periphery lenses is that the control of centration is lost, and discomfort levels increase due to the heavy point of contact between the lens and the cornea.

The most important thing to note about these apparently minor changes to the alignment curves is that they all have a major effect on the apical clearance of the lens and are not detectable by fluorescein pattern analysis. As stated previously, the accepted minimum TLT at which fluorescein becomes visible is in the range of 15-20 I-Lm. All the changes used in the examples above have an effect of changing the clearance in the alignment

zone by approximately 10 urn at a maximum, and as a result, these changes would not be seen in the fluorescein pattern. The same changes would occur when aspheric alignment curves are used, as the corneal eccentricity controls the eccentricity of the alignment curve. If an aspheric alignment curve is too steep or flat, the point of contact between the lens and the cornea will move, resulting in a change in the apical clearance. A flat aspheric curve would decrease apical clearance as the point of contact would move further in towards the BOZO of the lens. A tight or steep aspheric alignment curve would increase apical clearance.

The use of a simple rule to determine the curvature of the alignment curve based on a relationship to the flat-K is shown to be highly inaccurate. However, errors also occur in using topography data, as different instruments will give different eccentricity values for the same cornea, resulting in different alignment curves depending on which instrument is used. The method of analyzing these inaccuracies and their correction is covered in greater detail in Chapter 6.

Other design variables

There are three approaches that can be used to design and fit reverse geometry lenses. The first takes the Jessen (1962)concept of using the liquid lens as the correcting factor. The original "orthofocus" technique basically consisted of choosing the BOZR of the lens such that the liquid lens provided the refractive correction, resulting in all lenses having a plano power. In effect, if the patient had a refractive error of 2.00 0 myopia, the BOZR was fitted 2.000 flatter than the Kf . This invariably led to unstable fitting, especially as the degree of myopia increased.

The efficacy of this approach to fitting has not been subjected to proper analysis, as Jessen never published the results of his studies. However, the concept has been applied to the fitting of reverse geometry lenses by EI Hage et al (1997), Reim (Oreimlens), Day (Fargo), Tabb (Nightmove), Stoyan (Conte x BB), Breece (Correctech), Gladys (Euclid Jade and Emerald), Blackburn (Orthofocus), Legerton (CRT), and Reinhart (R&R) such

that the refractive correction required is once again the liquid lens, plus an allowance for a compression factor of between 0.00 and 1.00 O. The second curve chosen is the alignment curve, with the RC being calculated to achieve the correct sagittal fitting relationship to the cornea. This can be summarized as follows:

1.Choose a BOZR based on the initial refractive error and modify the alignment curve and TR to adjust the fit.

The second concept is to use a constant TR curve and BOZO, as in the Contex OK and EZM, and modify the BOZR until the correct sag relationship between the lens and the cornea is achieved. This approach makes no predictions as to the refractive change that will be induced by the choice of the BOZR. This can be summarized as:

2.Keep the BOZO and TR constant, and modify the BOZR.

As has been shown previously, this concept is limited by the inability to control the variations in CF for different corneal shapes, with a resultant variability in results.

The third and final method of lens design is to standardize the fitting by making the tear layer profile constant for all eye shapes. This can be summarized as follows:

3.The lens is specified in terms of TLT at the corneal apex and the edge of the BOZO. A computer program then calculates the BOZR/TR combination that will fulfill the specification.

This is the basic concept behind the BE design.

A common question is: why use a constant tear layer profile? The answer can be best described in a single word: standardization. The development of the sag fitting philosophy was based on the simple concept that different lens designs behaved differently on corneas of varying shapes, and that the "ideal" contact lens fit should have a fluorescein pattern, movement, and centration properties that could be duplicated irrespective of corneal shape.

As a result, the shape of the tear layer profile became the method of standardizing the design

Figure 4.27 The construction of a four-zone lens based on the Jessen factor. The back optic zone radius (BOZR) is determined first, depending on the refractive change required. This is shown in (A). The second curve chosen is the alignment curve (AC), shown in (B). The reverse curve

(C) is the final curve chosen to bring aboutsagittal equivalency and join the BOZR to the AC. Courtesy of Patrick Caroline.

and the fit of the lens. With the BE design, the TLT at the BOZO is constant irrespective of corneal shape. The refractive change is controlled by manipulating the apical clearance. The BOZR is not based on the Jessen concept, but still has an effect on the refractive change due to the change in BOZR with apical clearance.

If a lens is constructed using the first philosophy stated above, the choice of curves is as follows.

A

1.The BOZR is the first curve chosen due to the required refractive change (Fig. 4.27A).

2.The second curve chosen is the alignment curve, as the overall sag of the lens must equal the corneal sag at the point of contact with the alignment curve (Fig. 4.27B).

3.The final curve chosen is the RC. Note that the RC is responsible for joining the BOZR to the alignment curve in order that the requirements of sag fitting are fulfilled (Fig. 4.27C).

In the case of three-curve lenses, the BOZR can still be fitted according to the Jessen factor, but the RC will then be dependent on the corneal eccentricity. If the two are linked, as with the Contex lenses, then the sag of both curves needs to be equal to the corneal sag at the edge of the RC.

When a lens is constructed using the constant tear layer profile technique, the order of curve choice is reversed. The construction is as follows:

1.The alignment curve or tangent is derived from the axial radius of curvature of the cornea at the point of contact with the surface (Fig. 4.28A).

2.The RC is chosen to give the required TLT at the edge of the BOZO (Fig. 4.28A).

3.The BOZR is then calculated so that the curve meets the requirements of the apical clearance and the depth of the tear layer at the BOZO (Fig.4.28B).

At first glance, these different techniques may appear to have little in common, except that they are all based on sag fitting and must obey the same rules in order for the fit to be correct.

This is the simple point that needs to be remembered. All reverse geometry lenses, irre-

B

Figure 4.29 Relationship between flatness of fit of back optic zone radius (BOZRj to the clearance factor of OK 704T lenses. The corneal data is Ro 7.80, eccentricity 0.50.

spective of fitting philosophy or design variations, have the same basic construction and must be fitted according to sag philosophy if the fit of the lens is to be optimal. Where much of the variation in fitting success with the lenses occurs is in the assumptions made by various manufacturers as to the underlying corneal shape. If the lenses are empirically fitted from K readings and refraction, then the assumed corneal shape will be a prime area for inaccuracy when compared to the actual corneal shape. These variations are covered in Chapter 6.

As shown above, all reverse geometry lenses follow the same pattern of construction that is primarily based on the lens having a sagittal relationship to the cornea. The difference in TLT at the corneal apex and the edge of the BOZO (the CF) can therefore be used to examine further the similarities and differences in designs.

The definition of lens design based on the CF concept can be used to find the relationship between the fit of the lens with respect to the corneal curve and the associated steepening of the RC that is required to maintain sagittal equivalency with the cornea. In the case of the Contex lenses, if the BOZO of the lens is kept constant and the TO varied, the width of the TR curve is altered, as the peripheral curve width of all the OK series is constant depending on the design used. The Oreimlens has a constant TR width of 0.60 mrn, and the El Hage TR curve varies with the TO of the lens and the BOZO

Figure 4.30 Relationship between flatness of fit and reverse curve steepening. TR, tear reservoir; BOZR, back optic zone radius.

chosen. In the case of a cornea of Ro 7.80 mm and e 0.50 fitted with a 10.60 mm diameter Contex OK704T, the relationship between the CF and the difference between the BOZR and corneal curve (Ro) is shown in Figure 4.29. For the sake of simplicity, the difference between the BOZR and Ro will be described in terms of "flatness of fit" and converted to diopters. As the CF is increased, the BOZR is flattened in a linear relationship to the clearance factor.

Figure 4.30 shows the same relationship, except the steepening of the RC with respect to the BOZR (in diopters) is used instead of the CF. Once again, there is a linear relationship between the two in that, if a specific BOZR is chosen for an individual eye, it must have a specific RC in order to maintain sagittal equivalency of fit.

However, the relationship between these two factors also varies depending on the width of the RC that forms the TR. Figure 4.31 shows the variation in steepening of the TR curve with three different RC widths, 0.30, 0.60, and 0.90 mm. If the BOZR is predetermined, then the steepening of the TR curve is also linked linearly to the width of the TR curve.

The effect of variations in CF on corneal shape and refractive change has been studied in a controlled experiment by fitting a group of six subjects with lenses of varying CFs (Bara 2000). Each of the subjects wore a lens with a specific change in CF for a period of 6 h, with a J-week interval

Relationship between flatness of fit (FoBOZRI and TR curve (TR·BOZRj fordifferent TR curve widths

Figure 4.31 Relationship between back optic zone radius and reverse curve (RC) for varying RC band widths. TR, tear reservoir, BOZR, back optic zone radius.

between episodes. A control group was used for comparison. The CF variations were 30, 50, and 80 urn, with a BOZO of 7.00 mm and a tangential peripheral curve. The TO was varied during the trial wear periods to maximize centration. The changes in apical radius, eccentricity, visual acuity (log minimum angle of resolution or logMAR) and apical corneal power were measured at 'l-h intervals during the 6-h wear period.

The change in apical radius for the treatment and control groups for the different CF lenses is shown in Figure 4.32A. The relationships between the lens design and change in eccentricity and visual acuity are shown in Figure 4.32B and C.

The lens with the deepest CF (6.000 reverse curve) showed the greater and faster effect when compared to the other designs and the control group. The interesting fact to note with the results is that the refractive change appears to be related

Figure 4.32 The change in (A) apical radius R ' (B) o

eccentricity, and (C) log minimum angle of resolution (log MAR) vision for a group of subjects fitted with lenses where the RC is 2.00, 4.00, and 6.00D steeper than the BOZR. All lenses arefitted according to sag philosophy. The lens that caused the greatest change whencompared

to the controlsis the 6.00 D lens. The 2.00 D lens changes were not statistically different to the controls. Courtesy of Christa Sara.

to the BOZR, in that greater CF values are associated with a flatter BOZR.

This is the single factor that all reverse geometry designs have in common, irrespective of the fitting philosophy used. The EI Hage, Reim,

Figure 4.33 Relationship between back optic zone radius (0) and reverse curve (0) for Oreimlens. The corneal data are Ro 7.80, eccentricity 0.50. TR, tear reservoir; FTK flatter than K.

Figure 4.34 Relationship between lens fit (back optic zone radius) and clearance factor.

Relationship between BOZR and clearance factor for EI Hage lens (10.00 mm TO)

and other philosophies are based on the selection of the BOZR as determined by the initial refractive error. The only difference between them is the amount of extra flattening used as a compression factor. However, most designs are also based on a peripheral alignment curve that contacts the peripheral cornea and are based on corneal shape, indicating constancy in the sag relationship between the lens and cornea.

In the following examples, the standard cornea of Ro 7.80 mm and e 0.50 is fitted using both philosophies. The refractive error range is from 1.00 0 to 4.00 0 in 0.50 0 steps. Both lenses are assumed to touch the cornea centrally, with a TLT of zero.

In the case of the Oreimlens, the BOZR is equal to (Fo - Rx) - 0.75 0, with a 6.00 mm BOZO, 0.60 mm TR width, and 10.00 mm TO. The relationship between the BOZR (in effect, flatness of fit in diopters) and TR curves for the different refractive error corrections is shown in Figure 4.33. Note that, once again, there is a linear relationship between the BOZR and the TR curves. Similarly, there is a linear relationship between the BOZR and the clearance factor (Fig. 4.34).

El Hage's BOZR is equal to (Ro - 0.2) x Rx, with the CF at the edge of the TR calculated by adding the difference in sag between the initial prefit corneal shape and the BOZR of the lens over the same chord. The CF is determined by adding an allow-

Figure 4.35 Relationship between back optic zone radius (BOZR) and clearance factor for EI Hage'slens.

ance of 12 urn per diopter of refractive change at the 7.00 mm zone. Figure 4.35 shows the relationship between flatness of fit and the CF for a 10.0 mm diameter lens. As with the other reverse geometry lenses, the relationship is linear.

In effect, the main difference between the two designs is that the clearance factor per diopter of flattening is 13 j.Lm/1.00 D for the Oreimlens, and 21 j.Lm/1.00 0 for the El Hage design. The disparity in the two is due to the difference in TR curve widths and the flatter BOZR used by Reim, but regression analysis shows a linear relationship between the two (Fig. 4.36).

Relationship between change Inapical corneal power and lens fit (EI Hage)

Relationship between change in apical corneal power and lens fit (Drelmlens)

lium, the regression line would be at a 45° angle between the x- and y-axes. The correlation for the fitting philosophies based on a predetermined BOZR is much poorer: Reim (r2 = 0.19, P = 0.0004, df 59) and El Hage (r2 = 0.24, P = 0.Q1, df 59). Once again, there appears to be no direct molding effect.

The results of fitting a group of subjects with the aim of a 4.00 0 reduction in myopia is shown in Figure 4.38. As can be seen from the results, there is a wide variation in outcome with respect to the final refraction, indicating that there is some unpredictability in the approach.