The use of rigid toric corneal lenses (in preference to rigid spherical corneal lenses) is indicated under the following circumstances:
To improve the vision in cases where a lens employing spherical front and back optic zone radii is unable to provide the adequate refractive correction.
To improve the physical fit in cases where a lens with a spherical back optic zone radius (BOZR) and spherical back peripheral zone radii fails to provide an adequate physical fit.
These two main uses of toroidal surfaces on contact lenses are not always distinct, such that occasionally a toric lens will be used for both physical and optical reasons. For example, when fitting an eye with both a high degree of residual astigmatism and a large amount of corneal toricity, a toric lens is required optically (to correct the residual astigmatism) as well as physically (to optimize the fit of the lens) ( ).
Forms of Toric Lens
There are many varieties of rigid toric corneal lens available to the practitioner. Most commonly, a lens will have both a toroidal back optic zone and peripheral zone. These lenses are generally used in attempting to obtain a good physical fit on a cornea that is too toroidal to allow a good fit with a lens having a spherical BOZR and spherical peripheral radii. Lenses with toroidal back optic and peripheral zones can be produced with or without a toroidal front optic surface. A lens that has a toroidal back optic zone and a toroidal front surface is said to have a bitoric construction. If the principal meridians are not parallel, then the lens is designated as having an oblique bitoric construction.
Occasionally, a rigid toric corneal lens may be prescribed, consisting of a spherical back optic zone and a toroidal peripheral zone. This type of lens can also be produced with or without a toroidal front surface, the latter usually being the preferred option. Lenses with spherical back optic zones and toroidal peripheral zones are used as a means of attempting to improve the physical fit of a lens on an astigmatic cornea without the optical complications inherent in the use of lenses with toroidal back optic zones.
Very rarely, a rigid toric corneal lens is produced with a toroidal back optic zone and a spherical peripheral zone, with the intention of improving the circulation of tears beneath the lens. However, when this is done, it is possible that the lens may become less stable with regard to resisting rotation. One limitation of employing a spherical periphery is that the peripheral radius must be greater than, or equal to, the flatter radius of the toroidal back central optic zone. Once again, this form of the lens can be made with or without a toroidal front surface.
The only other rigid toric corneal lens form consists of a spherical back optic zone and spherical peripheral zone combined with a toroidal front optic surface. This type of lens is required in the situation where there is significant residual (noncorneal) astigmatism but minimal corneal astigmatism. In this case, the residual astigmatism needs to be corrected by means of a toroidal front surface, with a spherical optic zone indicated for the back surface due to the negligible corneal astigmatism.
Criteria for Use
Since rigid corneal lenses with both spherical BOZR and peripheral radii are often used successfully on corneas with medium to high degrees of astigmatism, it is important to decide what degree of corneal astigmatism should indicate the use of toroidal back optic zones. In general, these lenses should only be used when a lens with a spherical BOZR cannot be made to fit successfully. It is rare to find that toroidal back optic zones are necessary unless the corneal astigmatism exceeds 2.50 D (i.e. the difference in the corneal radii, as measured with a videokeratoscope or keratometer, exceeds approximately 0.5 mm).
In cases of uncertainty (e.g. where the corneal astigmatism is between 2.00 and 3.00 D), a toroidal back optic zone would be used in preference to a spherical back surface curve in the following situations:
A spherical lens exhibits poor centration or excessive movement.
Excessive lens flexure is noted with a spherical lens.
Fluorescein patterns with a spherical lens reveal excessive bearing along the flatter corneal meridian, regardless of the BOZR that is fitted.
Significant 3 and 9 o’clock staining occurs with a spherical lens.
There is marked corneal distortion and spectacle blur upon removal of the spherical lens from the eye. This occurs as a result of poor alignment between the spherical lens and the toric cornea, with the spherical lens subsequently having a moulding effect on the toric cornea.
There is significant residual astigmatism. In this case, a spherical back surface may provide an adequate fit; however, a toric back surface is utilized to stabilize the lens and prevent rotation, owing to the presence of the correction for the residual astigmatism on the front surface of the lens.
A great deal depends on factors other than corneal astigmatism. Lid positions and tension are important. In a case of high with-the-rule corneal astigmatism – and a low, loose lower lid – a toroidal back optic zone may be needed to obtain a good physical fit and centration. But a similar eye with a firm, high lower lid may well be successfully fitted using a lens with spherical back-surface curves.
The majority of cases of corneal astigmatism are found with the steeper corneal curve in the vertical meridian (with-the-rule). If an attempt is made to fit such an eye with a spherical BOZR, the lens often exhibits harsh bearing along the flatter (horizontal) meridian of the cornea and poor centration, causing physical discomfort and/or poor vision. Such an example is illustrated in Fig. 16.1 . If the same eye is fitted using a lens with toroidal back optic and peripheral curves, then the physical fit and centration are usually much improved ( Fig. 16.2 ).
The presence of against-the-rule corneal astigmatism usually necessitates the use of a toroidal back optic zone earlier than would be required with an equivalent amount of with-the-rule corneal astigmatism. This is due to the tendency for rigid spherical corneal lenses to decentre laterally on corneas with even just moderate amounts (1.50–2.00 D) of against-the-rule astigmatism.
Although rigid corneal lenses may be successfully fitted with either apical clearance or apical contact, it is generally more satisfactory to fit lenses with toroidal back optic zones in or near alignment. The physical fit, as denoted by the fluorescein pattern, will be similar to that seen with a well-fitted spherical lens in alignment with a cornea devoid of clinically significant astigmatism.
A toric corneal lens aligning too closely to the cornea can lead to poor tear interchange. Consequently, it is advisable to use a toroidal back optic zone with the steeper radius fitted slightly flatter (longer radius) than the corresponding corneal radius so as to assist the interchange of tears. The flatter radius will generally be fitted ‘on K’ or else a little steeper than its corresponding corneal radius.
Consider the type of physical fitting which might be derived from keratometer readings:
The back optic zone radii should always be chosen such that there is at least a 0.3 mm meridional difference in radii (or 1.50 D difference if the BOZR are specified in dioptres). Otherwise, the toroidal BOZR may not position properly on the toroidal cornea, leading to lens rotation and possible visual disturbance depending on the type of toric lens design. (Note that BOZR indicates BOZR for a spherical surface and back optic zone radii for a toroidal surface.)
The peripheral radii are usually chosen to reflect the type of peripheral fit preferred by the practitioner concerned. Each meridian is considered separately, and the peripheral fittings in the two principal meridians are selected to provide the same difference between the back optic and peripheral radii most commonly used by the practitioner in fitting spherical corneas. In addition, the peripheral curves will usually have the same degree of toricity as the BOZR. For example, if a practitioner usually specifies the secondary curve 0.9 mm flatter than the BOZR for a spherical lens, then for a lens with toroidal BOZR of 7.90/7.40, the secondary curve ordered would be 8.80/8.30.
For lenses with a spherical back optic zone and a toroidal peripheral zone, the peripheral curve region should be as large as possible to increase the likelihood of alignment with the toric cornea. These lenses are usually fitted fairly small to minimize meridional sag differences and slightly steeper centrally than the flatter corneal meridian to achieve a compromise fit. The meridional difference in the peripheral curves should be at least 0.6 mm to help minimize lens rotation ( ).
A typical case might be as follows:
Although this type of lens can be very useful in certain cases where a fully spherical lens is not adequate, the toroidal peripheral zones are, at best, only an attempt at compromise. They usually rotate more than lenses with all toroidal back-surface curves, and the steeper peripheral radii occasionally end up in close proximity to the flatter corneal meridian, thus causing slight corneal abuse.
The calculations involved in determining the necessary radii and power of these lenses are quite straightforward and the complexity of this topic is often exaggerated. It is important, however, that the fundamentals of the optics of contact lenses are understood if some of the complications of toroidal optic surfaces on corneal lenses are to be appreciated.
To help understand and perform some of the calculations needed in toric lens work, the reader is referred to Chapter 13 and also to .
Calculating the back vertex powers (BVPs) for a rigid lens with a toroidal back optic zone is undoubtedly a more complex task than determining the BVP for a spherical lens, and yet the two processes involve the same basic principles. For spherical lenses, the power of the contact lens in the air plus the power of the tear lens in the air should add up to the ocular refraction. With toric lenses, the same rule applies, but here the two separate meridians must be considered.
Example 1: Calculating the BVP for a rigid lens with a toroidal back optic zone Spectacle refraction (vertex distance ignored): +2.50/−3.00 × 180
(Note that the effect of the vertex distance must be taken into account if this distance is great or if the refractive power in either meridian exceeds 4.00 D.)
A rigid spherical corneal trial lens with BOZR 7.95 mm and BVP +1.00 D is placed on the cornea. Refraction with this lens in situ gives +1.00 DS (no residual astigmatism) and 6/6 acuity. Note that the overrefraction (OR) is usually best performed over a spherical trial contact lens aligned along the flattest meridian of the cornea and only one OR is required to calculate both BVPs.
Based on the keratometry readings, BOZR of 8.00 and 7.55 mm are chosen to fit the horizontal and vertical meridians, respectively.
The power determination is now performed in the same way as with a rigid spherical lens, except that two meridians need to be considered instead of one. The BVP that needs to be ordered (BVP CL ) is calculated by taking into account the BVP of the trial lens (BVP trial ) and the OR and then using the formula:
For a spherical lens, if the BOZR of the trial lens is different from the BOZR to be ordered, then, when determining the BVP CL , it is necessary to take into account the change in tear lens power that will occur as a result of changing the BOZR.
Based on a tear lens refractive index of 1.336, this change in tear lens power is given by the formula:
where BOZR final is the BOZR that has been chosen for the lens to be ordered and BOZR trial is the BOZR of the trial lens.
It can be approximated that for every 0.05 mm decrease in BOZR, −0.25 D must be added to the BVP of the contact lens. Likewise, +0.25 D must be added to the BVP of the contact lens for every 0.05 mm increase in BOZR. This approximation only holds for relatively small differences in BOZR and, if in doubt, it is safer to use the above formula.
Given that the BOZR is being changed from that used for the fitting, the BVP that needs to be ordered (BVP CL ) is given by: