Section TWO Rigid gas-permeable lens fitting

myopia. Early techniques, such as the May–Grant and Tabb methods,9,10 used a series of progressively flatter lenses with a TD of about 10.00 mm and BOZD of about 8.50 mm. Results proved unpredictable and lens decentration frequently induced significant corneal distortion. Despite claims of a large reduction in myopia, results were limited to a decrease of approximately 1.25 D.

Orthokeratology has now achieved a degree of clinical reliability with the advent of:

•Reverse geometry lenses.

•Corneal topography measurement so that changes in corneal curvature can be carefully monitored.

•Rigid gas-permeable materials to improve corneal physiology and permit overnight wear.

14.2 Current approach

Modern orthokeratology attempts to use a single set of accurately fitted lenses worn on an overnight basis (see Section 14.6.2). A large percentage of the refractive change is achieved after one night of wear and the majority by the end of the first week.

Orthokeratology works by means of a squeeze film force beneath the lens acting tangentially across the corneal epithelium. This is thinned centrally and redistributed towards the mid-periphery.

Because there is thinning of the central cornea, there are some similarities with laser refractive surgery which uses the Munnerlyn formula to predict how much tissue needs to be removed. This is now also used in orthokeratology as the most reliable way of predicting the refractive change.

A = RD3 2

where A = ablation depth (or corneal thinning)

   R = refractive error

   D = diameter of the treatment zone

The maximum change in epithelial thickness has been suggested as 20 µm11 compared with the total thickness of the corneal epithelium of approximately 50 µm. Using this figure of 20 µm for treatment zones of 6.00 mm and 4.00 mm, the anticipated refractive changes would be respectively −1.75 D and −3.75 D.

The cornea is defined mathematically by its apical radius (Ro), eccentricity or e value (see Section 9.3) and chord diameter. The shape is in almost all cases that of a prolate elipse. As the myopia lessens, the e value reduces and the cornea becomes more spherical. The theoretical limit of myopia reduction is achieved when the eccentricity is reduced to zero, although in some cases the cornea may become slightly oblate to give a further small but unpredictable reduction in myopia.

In general, the success of orthokeratology depends on:

•Lid forces.

•Duration of lens wear.

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•Type of fitting.

•The corneal rheology of the individual patient.

Advantages of orthokeratology

•Good unaided vision for most of the day.

•It is not a surgical procedure.

•It is reversible.

•It is not painful.

•Treats both eyes at the same time.

•The technique uses established contact lens procedures with minimal risk of problems.

Compared with refractive surgery, orthokeratology does not:

•Involve postoperative pain.

•Incur the risk of corneal haze.

•Risk loss of visual acuity.

Disadvantages of orthokeratology

•It can treat reliably only low to medium degrees of myopia.

•Several visits are required over the first few months.

•Patients must use retainer lenses or the cornea will revert to its original shape.

•The precise reduction of myopia cannot be guaranteed.

•For the best results, careful patient compliance is necessary.

Indications

•Young, early myopes.

•Sport.

•Vocational use.

•Occasional spectacle wearers.

•Myopia up to −4.50 D.

•Low degrees of astigmatism.

•High corneal e values, over 0.50.

Contraindications

•Myopia greater than −4.50 D.

•With-the-rule astigmatism over −1.25 D.

•Where the astigmatism extends beyond the central region of the cornea.

•Against-the-rule astigmatism.

•Significant residual astigmatism.

•Low corneal e values.

•Large pupils where flare may be a problem.

•Where proper centration cannot be achieved.

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•Loose lids where there is insufficient force to mould the corneal shape.

•Keratoconus or other thin or irregular corneas.

•Unrealistic patient expectations.

14.3 Reverse geometry lenses

Reverse geometry lenses have been introduced since the advent of computercontrolled lathes where it is feasible to manufacture the secondary curve steeper than the BOZR. Reverse geometry lenses are used in orthokeratology to achieve good centration and optimum pressure distribution under the lens. They  also give a more rapid corneal response and the term accelerated orthokeratology is sometimes used.

The first reverse geometry lenses consisted simply of three curves, the central radius, the secondary steep or reverse curve and the periphery (Figure 14.1). The area beneath the second curve was called the tear reservoir. These lenses can still treat myopia up to about 3.00 D and the likely result is based on corneal eccentricity measured by means of topography. A change in myopia of 1.00 D is approximately equivalent to a reduction in e value of 0.21. It is therefore feasible to expect a cornea with Rx −2.50 D and e = 0.5 to reduce to emmetropia. If the Rx is −4.50 and e = 0.4, a reduction to only −2.50 D would be anticipated with three curve lenses.

To give an even more rapid response and treat higher degrees of myopia, recent designs comprise four or five radii and include an additional reverse curve (Figure 14.2). The term double reverse geometry has been used. The portion of the lens between the central radius and alignment curve has been variously called the reverse, relief or return zone and creates the tear reservoir (TR).

14.3.1 Fitting three curve reverse geometry lenses

Lenses with only three curves have largely been superseded but are still occasionally used for low degrees of myopia, where only a small corneal response is required or for non-orthokeratology fitting (e.g. with grafts or post refractive surgery). Many of the fitting principles still apply to all reverse geometry lenses.

Unlike most rigid lens fitting, which is based on flattest ‘K’ and lens design, reverse geometry lenses are fitted according to:

BOZD

Figure 14.1  Diagram of reverse geometry lens

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The values of R0 and and e are obtained from corneal topography while y depends on the lens diameter.

The ideal lens has the same sag as the cornea with a compensation of 10 µm for the TLT. It gives minimum central clearance with touch at the outer edge of the steep reverse curve. Lenses are therefore calculated on the basis of the chord which represents the TD minus the width of the peripheral curve.

Sag of lens = sag of cornea + tear lens thickness

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In practice, the first fitting lens is generally determined using a computer programme which can rapidly calculate the radius required for different lens diameters.With average corneal eccentricity and a lens diameter of 10.60 mm, the correct radius is usually about 0.35–0.45 mm flatter than flattest ‘K’. Taking the Contex lens as an example:

BOZD

The most commonly used BOZDs are between 6.00 mm and 7.00 mm, although the range is from 6.00 to 8.00 mm in steps of 0.50 mm.

Second curve or tear reservoir

The tear reservoir (TR) is responsible for creating the negative pressure which, in turn, causes the corneal flattening. The curve is specified in dioptres, between 1.00 D and 9.00 D steeper than the BOZR. The tear reservoir is usually spherical but can be made aspheric for non-orthokeratology use.

•The most commonly used TRs are 4.00 D, usually optimum with a 7.00 mm BOZD and 3.00 D, usually optimum with a 6.00 mm BOZD.

•TRs of 5.00 D and over permit only small areas of central touch, less than 3.00 mm.

•TRs less than 3.00 D give too large an area of central touch.

Peripheral curve

The first choice of peripheral curve is usually tangential and 1.00 mm wide. This is designated a T edge. Sometimes either narrower or aspheric peripheries are used.

•The AEL is typically 0.12 mm, but can be from approximately 0.08 to 0.16 mm.

•Reducing the edge lift gives an overall tighter fitting.

•Increasing the edge lift gives an overall looser fitting.

Total diameter

Lenses are fitted with large TDs to achieve better comfort and centration.

•The first choice is 10.60 mm, but 11.20 mm lenses are sometimes more successful.

•Small corneas may require TDs of 9.80 mm.

•Lenses are sometimes fenestrated to improve tears flow and reduce adhesion.

Lens specification

Companies such as Contex in the USA produce a very wide range of lenses, but those most commonly used are clinical equivalents and designated the 704 and

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603 Series. They achieve the best balance between central touch and depth of TR. The 704 has a 30% greater TR volume than the 603 and is generally the design of first choice. A full specification is recorded as:

8.35:10.60 OK704T −0.75 where

14.3.2 Fitting four and five curve reverse geometry lenses

Four and five curve (double) reverse geometry lenses can be used to treat myopia of up to about 4.50 D. They also aim to achieve a rapid change of corneal curvature and power. Modern designs include the BE (NKL Contactlenzen), CRT (Paragon), Dreamlens (No. 7), E Series (Contex), Euclid (I-Go), Nocturnal (Scotlens), and Rinehart-Reeves (Nova).

There are several designs of double reverse geometry lenses similar in principle to those shown in Figure 14.2a and b.

There are four essential lens components:

1.The BOZR. This is the flatter than ‘K’ central curve which, for any given cornea, will give uniquely the required refractive result. In most cases, the BOZR is derived from the original Jessen formula which calculates the radius on the basis of flattest ‘K’ in dioptres plus the desired refractive change plus an additional flattening factor of 0.50 D or 0.75 D.

2.The reverse curve (or return zone or relief zone). This is the steep tears reservoir that joins the BOZR to the alignment curve. The reverse zone does not itself provide a practitioner fitting variable but, when combined with the flat BOZR, provides the negative pressure forces to redistribute the corneal epithelium. The reverse zone is usually 0.60 to 1.00 mm wide and may be divided into two curves to give overall a five curve lens (e.g. Rinehart-Reeves) or have a continuous sigmoid construction (e.g. Paragon).

3.The alignment curve (AC) (or fitting curve or landing zone) is designed to align with the mid-peripheral cornea and represents the key fitting curve of the lens by controlling centration and movement. It is approximately 0.80 to 1.50 mm wide and may be spherical, aspheric or a tangential straight line. It may also be divided into two curves.

4.The peripheral curve, approximately 0.30 mm wide, ensures adequate edge clearance.

Fitting these lenses is not simply a question of steepening or flattening the central radius as all parameters are inextricably linked to increase or decrease the lens sag in relation to the cornea. The alignment curve is the key to controlling lens behaviour and, for this reason, it is sometimes called the fitting curve.

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With many lenses, there are design features which laboratories regard as proprietary. These details are not made available to the practitioner who therefore does not have full knowledge of all lens curves fitted.

BOZR

Most lens designs select the BOZR on the basis of the original Jessen formula: Example: Rx −3.00/−0.25. Keratometry 8.03 mm/8.00 mm.

 42.00 D  (flattest ‘K’ in dioptres)

 −3.00 D  (required power change)

 −0.50 D  (additional flattening factor – the Jessen formula)

 38.50 D  = 8.76 mm

Required initial radius = 8.76 mm

Alternatively the BOZR can be determined by computer calulation using apical radius and eccentricity measurements derived from corneal topography (e.g. with the BE lens). The BOZR is not generally used as a fitting variable since a change of central radius has little effect on the overall sag of the lens.

BOZD

The BOZD is predetermined by the laboratory or calculated from the required reduction in myopia and treatment zone width (the Munnerlyn formula). It is generally about 6.00 mm to give an area of applanation or treatment zone of about 4.00 mm.

Power

The power of the prescription lens is selected to give optimum visual acuity while being worn. Because the lens is fitted flat, the BVP is usually quite low and can be calculated from the BOZR and the patient’s refractive error. For those lenses fitted with an additional flattening of −0.50 D or −0.75 D according to the Jessen formula, the BVP is always respectively +0.50 or +0.75 D.

Alignment curve

The alignment curve is assessed by fluorescein observation. It may be defined by a radius (e.g. Rinehart-Reeves) or by a tangential angle (e.g. Paragon). For most lenses, this is the key piece of information required by the laboratory. With the BE lens it is calculated by the laboratory as part of the overall lens design and assigned a cone angle.

Periphery

The periphery is an integral part of the lens design to ensure adequate edge lift and is usually predetermined by the laboratory.

Total diameter

The most common TD is in the region of 10.6 mm, although 11.0 mm lenses may be needed for large corneas or to improve centration.

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Typical specifications

BE

8.83 85651050 55.81 11.00 −0.50 Where:

8.83 mm = BOZR

85651050 = TRF number (a mathematically converted algorithm generated by the computer programme and unique for each lens)

11.0= TD

55.81= Cone angle

Paragon CRT

8.60 0.525 34 10.50 +0.50 Where:

8.60= BOZR

0.525 = Return zone depth (depth of tear reservoir)

34.00= Landing zone angle (tangential angle)

10.50= TD

+0.50 = BVP (constant, unless specified to the contrary)

Rinehart–Reeves

8.94:6.00/7.94:10.60 +0.75 Where:

8.94= BOZR

6.00= BOZD

7.94= Alignment curve

10.60= TD

+0.75 = BVP (constant, unless specified to the contrary)

14.3.3 Fitting lenses empirically

An empirical and simplified approach to orthokeratology is available from companies such as No. 7, I-Go and Scotlens using mathematical data derived from corneal topography. The practitioner carries out the usual preliminary assessment of the patient and provides details of refraction, keratometry and corneal diameter together with topography plots which are email to the laboratory.

With No. 7, for example, the correct lenses are calculated on the basis of the corneal data using its DreamLite software. Fitting and aftercare follow the routine outlined in Section 14.6. Although the actual calculation of the first lenses is delegated to the laboratory, topography plots, fluorescein assessment and slit lamp examination remain essential at all stages of aftercare to ensure that the corneal response is satisfactory. Where fitting changes are necessary, subsequent lenses are recalculated from the revised topography.

Scotlens software, on the other hand, requires greater practitioner input. Its Nocturnal system provides simulated fluorescein patterns which are assessed prior to ordering lenses and which are used in any later refinement of the fitting.

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