As mentioned in my previous article (Optician 10.01.14), there are some situations that are better suited to rigid lenses, particularly where parameters cannot be met for the desired prescription. We will now explore where rigid lenses are best used, and the basis of fitting them.
Rigid lenses began their life made from polymethylmethacrylate (PMMA) in the 1940s. Due to its non-permeability to oxygen, PMMA is now rarely used. In 1977, the material changed to cellulose acetate butyrate, or CAB. Although the permeability was still low to oxygen, at approximately 4x10-11 Fatt units it was still greater than that of PMMA.
Newer materials that are currently used are silicone acrylates and fluorosilicone acrylates. The former, which was introduced first, gave superior oxygen permeability to its predecessors because of the addition of silicon. This hydrophobic component required the help of a wetting agent (methacrylic acid) to ensure lens comfort. The latter, with the further addition of fluorine, improved the surface wettability, and helped with resistance to deposits while maintaining a high oxygen delivery to the cornea.
When to consider a rigid lens
Although soft lens parameters are forever increasing, encompassing more complex prescriptions, there are some situations where a rigid lens may be more appropriate.
For patients being fitted with lenses for the first time, once the preliminary examination (discussed later) has taken place, you are able to establish what lens will be most suitable. Those more suited to a rigid lens are those with:
? High levels of corneal astigmatism
? Significantly ‘flat/steep’ corneas
? An unstable tear film
? Significant pingueculae
? Significant limbal redness
? A history of allergy
? Small or large palpebral apertures
? Handling difficulties.
Rigid lenses should also be considered if a soft lens fitting has been unsuccessful, particularly if vision has not been sufficiently corrected.
As mentioned in the previous article, it is always important to have an open discussion with the patient, to ensure that they are happy with the clinical decisions being made, and that the choice suits their expectation.
Initial assessment
A full and thorough slit-lamp examination of the anterior structures is essential, as mentioned in the previous article, along with an up-to-date prescription. This may highlight underlying issues that may need to be dealt with prior to the lens fit, but also help with the indication that a rigid lens may be more suitable for the patient.
Keratometry
The keratometer plays an important role in rigid lens fitting. When fitting a soft lens, the readings are helpful when parameters vary and also to note a baseline of corneal curvature to note any change. ‘K’ readings can be read in millimetres or dioptres, the measurement in millimetres is relevant here. For a rigid lens, the ‘K’ measurements help to choose the appropriate lens, and are taken in two meridians at 90 degrees to each other. The larger the number the ‘flatter’ the cornea, the smaller the number the ‘steeper’ the cornea.
As we know, a keratometer measures the central 3-6mm of the cornea and establishes the amount of corneal astigmatism present. This is calculated by the difference between the ‘K’ values, where a difference of 0.1mm equates to 0.5DC. It is these measurements that will help to decide the back optic zone radius (BOZR) (discussed later) of the lens and also the design of the lens, if a spherical or toric version of the lens is required.
Horizontal visible iris diameter (HVID)
This is a measurement of the horizontal iris, taken using a ruler (Figure 1). The HVID corresponds to the size of the contact lens, which is the total diameter (TD). Whereby, the TD of the lens should be roughly 2mm smaller than the HVID.
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Vertical palpebral aperture
The vertical palpebral aperture is the distance between the top and bottom lid. The measurement also has some weighting on the decision of the TD of the lens. It would also be appropriate at this point to assess the lids themselves, deciding whether the patient’s lids are tight or loose. It is also important to notice the position of the lids themselves, as this will influence the habitual position of the lens when inserted.
Pupil diameter
This measurement should be taken in both ambient and low illumination and will influence the back optic zone diameter (BOZD) . This measurement is taken using a pupil gauge (Figure 2) and with the aid of a Burton lamp.
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Choosing the right lens
It is with the use of the above measurements, slit lamp examination of the anterior structures and recent prescription that a lens can be chosen.
BOZR
As mentioned above, the BOZR is influenced by the ‘K’ readings taken in the initial assessment. The initial lens that is chosen is usually fitted on the flattest ‘K’ reading taken or slightly flatter. Lenses are usually available in 0.1mm steps, but some are available in smaller increments of 0.5mm steps. Altering the BOZR of the lens has the greatest influence on the fluorescein pattern.
If the ‘K’ readings are indicating corneal astigmatism, a toric lens may be considered. Rigid lenses are able to cope with up to 1.50D of corneal astigmatism. Anything greater than this value, may require a toric rigid lens design.
Total diameter
The TD is roughly 2mm smaller than the HVID measurement taken. It is also influenced by the pupil size, as it must be larger than the pupil in low illumination.
Increasing the TD of the lens can help to stabilise the lens. Higher powered lenses are better suited to a larger TD so they are more stable. Altering the TD, does not influence the fluorescein pattern to a great degree.
BOZD
The BOZD should be larger than the pupil size in low illumination, but is often decided by the production lab. It should be 1.50mm larger than the pupil at its largest.
Back vertex power
The initial lens chosen should be as close to the refraction as possible, considering the back vertex distance for high prescriptions.
The lens assessment
The slit lamp and its various illuminations and filters should be used to assess the lens.
White light
White light should be used initially with low magnification, low illumination and a wide beam.
? The lens position should be noted habitually, and any sign of lid attachment. The lens should be situated within the limbus, ideally centrally on the cornea
? Vertical lens movement (VLM) is assessed by asking the patient to blink and noting the amount the lens moves. This should be between 1mm and 1.5mm. This ensures there is sufficient tear exchange behind the lens
? The lids should now be held apart, and the lens observed to drop down slightly from its central position
? If the lens is not sitting centrally, the fit should be observed, by manipulating the lids to place the lens centrally
? The patient should then be asked to look to the right and to the left, and the movement of the lens observed. The lens should not pass the limbus, and the expected movement is 0.8-1.00mm.
Blue light
Blue light is used with the instillation of fluorescein to assess the fit of the contact lens. Three main areas are assessed at this point, the centre, the mid-periphery and the edge.
Alignment fit (Figure 3)
? The centre of the lens should be on alignment or with slight clearance
? There should be mid peripheral alignment
? There should be edge clearance of 0.5mm.
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Flat fit (Figure 4)
? There will be a central area of touch, appearing dark blue due to the lack of fluorescein
? The area of touch is surrounded by a large band of fluorescein, in the area of the mid-periphery, running in to the edge of the lens
? There is excessive movement with this lens due to its instability, which will cause discomfort for the patient
? The fit of the lens creates a negative tear lens power, so during the over-refraction more plus power is accepted.
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How to fix a flat fit
The BOZR has the greatest influence on the fluorescein pattern, by steepening the BOZR, the lens will centre better and give an alignment fitting.
The TD can also be increased to stabilise the lens, but this has little effect on the fluorescein pattern.
Steep fit (Figure 5)
? There is pooling of fluorescein in the centre of the lens. This is sometimes accompanied with an air bubble
? The smaller the area of central pooling, the steeper the lens
? This is surrounded by a dark blue area of mid-peripheral touch
? There is a very thin edge clearance
? There is little to no movement of the lens
? The fit of the lens creates a positive tear lens power, so during the over-refraction more negative power is accepted.
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How to fix a steep fit
Flattening the BOZR will improve the fluorescein pattern and encourage movement of the lens.
The TD can be increased to increase lens movement, as above, will not change the fluorescein pattern.
Once an appropriate lens has been fitted, the patient should be instructed on ‘insertion and removal’ of the contact lens, and hygiene during this process. For rigid lenses, it is important to instruct the patient to ‘wet’ the lens prior to insertion using a wetting solution. This helps with lens comfort on insertion. This tuition should also include how to care for the lens.
The lens must be cleaned with a specific rigid lens cleaner and thoroughly rinsed through with saline, before being left to soak in soaking solution overnight. There are also multipurpose solutions which can clean, rinse and wet the lens. Tap water must not be used to clean the lens.
The patient should be advised to gradually increase their wear time, especially if they are new contact lens wearers.
It is always important to give patients emergency advice, even if they are existing wearers, so they are sure of the action needed to be taken if things do not go to plan.
A leaflet can be a helpful way of summarising the key points for the patient, including the number for the practice in case any queries arise.
The patient should then be asked to return for a follow-up after a short period of experiencing the lens in their own environments. This gives the patient the opportunity to discuss how they have got on with the lens, but also an opportunity for the practitioner to assess the lens and the health of the eye. As for most fitted with a gas-permeable lens, the prescription may be complex; the practitioner must ensure that a sufficient level of acuity has been achieved and the patient is satisfied with it. As always when fitting a contact lens, patient expectations should be fully understood by the practitioner. If they cannot be met this should be explained to the patient, to ensure they are aware of a compromise if needed.
? Simrit Sandhu works in the contact lens clinic at City University and in practice in Cheapside, London