Section TWO Rigid gas-permeable lens fitting

PRACTICAL ADVICE

•The cornea is actually just over 1 mm larger than the measured HVID.

•The maximum pupil size can be measured while using the blue light of the Burton lamp as the crystalline lens can be seen fluorescing through the pupil area.

•Most corneal topographers allow a fairly accurate measurement of the HVID (see Figures 14.4–14.6 where each square represents 1 mm).

10.2Back optic zone radius (BOZR)

•Diagnostic lenses are usually available in steps of 0.10 mm, although many prescription lenses can be ordered in 0.05 mm steps.

•The preferred fitting for most corneal lenses is alignment or very slightly flatter.

•Diagnostic lens selection is based on keratometry.

•For spherical corneas or where astigmatism is less than 1.00 D (≤0.2 mm with keratometry), the initial lens usually has a BOZR nearest to flattest ‘K’.

•Toric corneas with 1.00–3.00 D of astigmatism (≡ 0.2 to 0.6 mm with keratometry) should also be fitted on or near flattest ‘K’ by 0.05 to 0.10 mm to minimize flexure and achieve good acuity with a spherical lens.

•If the astigmatism is over 3.00 DC (≡0.6 mm with keratometry), a toric BOZR is recommended.

•The radius must be considered in relation to BOZD. Where a very large optic is required (e.g. 8.40 mm), the radius is usually flatter (e.g. ‘K’ + 0.10 mm) to achieve an alignment fitting.

•Additional factors such as lid tension, palpebral aperture, BVP and centre of gravity must also be considered (see Section 8.2).

•Right and left eyes generally require radii within 0.05 mm of each other, except in cases of anisometropia.

Example 1: (Alignment): ‘K’ 8.00 × 7.95

Initial radius selected: 8.00 mm Example 2: (Alignment): ‘K’ 7.97 × 7.83

Initial radius selected: 8.00 mm Example 3: (Toric cornea): ‘K’ 8.00 × 7.50 Initial radius selected: 7.95 mm

PRACTICAL ADVICE

•As keratometry measures only the central cornea, patients with the same corneal curvature do not necessarily give the same fluorescein pattern if fitted with the same BOZR. Peripheral topography may also differ.

•Other influences such as lid tension must also be taken into account to give the optimum fitting.

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10.3Total diameter (TD)

•TD is chosen on the basis of corneal size to be approximately 2.0 mm smaller than HVID.

•TD depends on pupil size, especially in low illumination.

•TD depends on the size of vertical palpebral aperture.

•The choice can be regarded as small (<9.20 mm), medium (9.20–9.70 mm) or large (>9.80 mm).

•Changing to a larger diameter generally stabilizes the fitting, although it does not always have a significant effect on the fluorescein pattern.

•Highly toric corneas fitted with spherical lenses need either small TDs to avoid excessive edge stand-off in the steep meridian or large TDs for lid attachment.

•The TD should be evaluated in relation to BVP. High powers require larger diameters for lens stability.

•The final choice of diameter also depends on the method of fitting (e.g. whether lid attachment or interpalpebral).

•Right and left eyes almost always require the same TD.

10.4Back optic zone diameter (BOZD)

•Often predetermined by the laboratory.

•Depends on pupil size, especially in low illumination.

•The pupil position must be assessed for aphakics.

•BOZD is chosen to be at least 1.50 mm larger than pupil size.

•The choice may be regarded as small (<7.30 mm), medium (7.30–7.90 mm) or large (>7.90 mm).

•A larger BOZD for a particular radius gives a greater sag and therefore a steeper fitting.

•A smaller BOZD is often chosen with a toric cornea to reduce the area of mismatch.

•A larger BOZD is often chosen to permit a flatter BOZR which gives less flexure on a toric cornea.

•An excessively large BOZD gives a periphery which is very narrow. The peripheral curves must therefore be much flatter than normal for adequate edge clearance. This results in a sharp transition.

•The BOZD should be considered in relation to BVP and lenticulation. High minus lenses frequently require very large BOZDs to avoid flare.

•If the lens is made larger and the BOZD increased accordingly, the sag will be greater. If the initial fluorescein picture was acceptable, then a clinical equivalent can be achieved by using the Rule of Thumb below.

•Conversely, steepening the BOZR by 0.05 mm and reducing the BOZD by 0.50 mm gives an equivalent fitting.

•If the BOZR is too flat but it is also necessary to increase both the TD and BOZD, increasing the two diameters may well give the desired fitting.

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Figure 10.1  Tear layer profile/BOZD

Figure 10.2  Tear layer profile/BOZD

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PRACTICAL ADVICE

•Remember that if the BOZR is altered, the BVP must also be adjusted.

•If the TD is changed to improve corneal coverage or lens stability, it is usually necessary to alter the BOZD. This has a greater influence on the lens sag and therefore gives a more noticeable change in the fluorescein pattern than altering the TD alone.

RULE OF THUMB

If a given BOZR over a stated BOZD gives satisfactory central alignment, add 0.05 mm to the BOZR for each 0.5 mm increase in the BOZD in order to maintain an equivalent fit.

Typical fittings

Example 1: 7.80:7.50/8.60:8.30/10.50:9.30 (Figure 10.1)

Example 2: 7.80:8.20/8.70:8.90/10.50:9.80 (Figure 10.2)

10.5Peripheral curves

•The first peripheral curve for corneas between 7.50 mm and 8.20 mm should be at least 0.70 mm flatter than the BOZR.

•For corneas over 8.20 mm, the first peripheral curve may need to be   0.80–1.00 mm flatter than the BOZR; conversely, for corneas less than 7.50 mm, the periphery may be 0.50–0.60 mm flatter than the BZOR (see Section 8.3).

•For a tricurve, the radius of the final peripheral curve is typically chosen between 10.00 mm and 12.00 mm with a width of 0.50–1.00 mm.

•The final peripheral or edge curve assists lens removal and promotes good tears exchange.

•A flat peripheral curve gives less corneal irritation but greater lid sensation.

•Too little peripheral clearance gives unsatisfactory tears exchange because of increased capillary attraction. It can also cause arcuate staining and lens adhesion (see Section 8.3).

Excessive clearance results in an unstable fitting because of an inverted tears

meniscus and reduced lens adhesion (see Section 8.3). It can also cause peripheral dimpling and 3 and 9 o’clock staining.

Examples:

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Figure 10.3  Tear layer profile/tight periphery

Figure 10.4  Tear layer profile/average periphery

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Figure 10.5  Tear layer profile/loose periphery

10.6Back vertex power (BVP) and over-refraction

•Diagnostic lenses used for over-refraction should be as close as possible to the anticipated BVP.

•Minus lenses for hypermetropes should be avoided and vice versa as the over-refraction is likely to be innacurate. The centre of gravity and edge design are different and therefore also influence the fitting.

•The BVP of the contact lens and the over-refraction in the trial frame or phoropter (after allowing for back vertex distance) are added together to give the required BVP for the prescription lens.

•If a lens is fitted steeper than ‘K’, a positive liquid lens is created, requiring more negative power in the over-refraction to neutralize the effect   (Figure 10.6A).

•If a lens is fitted flatter than ‘K’, a negative liquid lens is created, requiring more positive power in the over-refraction to neutralize the effect   (Figure 10.6B).

•Different BVPs may well be found for the same degree of withand   against-the-rule astigmatism.

•The final BVP should correlate with the spectacle Rx after taking into account the back vertex distance. If they differ by more than 0.50 D, the over-refraction should be repeated with a different diagnostic lens.

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Figure 10.6  (A) Steep contact lens – positive lens power; (B) flat contact lens – negative lens power

PRACTICAL ADVICE

•The BVP found after over-refraction provides a good indication of how close the fitting is to alignment and whether a different BOZR is required.

•The tear lens power increases by +0.25 D for each 0.05 mm that the BOZR is steeper than the cornea. The over-refraction will therefore need to be −0.25 D greater.

•The tear lens power decreases by −0.25 D for each 0.05 mm that the BOZR is flatter than the cornea. The over-refraction will therefore need to be +0.25 D greater.

•It is useful to over-refract with the initial diagnostic lens in case a subsequent fitting proves less satisfactory.

RULE OF THUMB

A change in radius of 0.05 mm ≡ 0.25 D change in power (if the radius is in the region of 7.80 mm).

If it is decided to alter the BOZR, this rule of thumb allows the calculation of the BVP for the new lens without having to check it on the eye.

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