Introduction
The use of soft toric lenses (in preference to spherical soft lenses) is indicated when there is ocular astigmatism present, be it corneal or noncorneal, that warrants correction. Unlike rigid lenses, soft lenses do not mask corneal astigmatism but rather conform to the shape of the cornea. Consequently, correcting ocular astigmatism with soft lenses requires that cylinder be incorporated into the back vertex power (BVP) of the lens.
Numerous manufacturers of soft contact lenses have made extremely optimistic and unrealistic claims of their spherical lenses being able to correct satisfactorily astigmatism of between 1.00 and 2.00 D. Only rarely is this achieved. showed that there was no statistically significant masking of corneal cylinder with standard thickness soft spherical lenses. Indeed, the most helpful indication of the likely residual astigmatism found while wearing a spherical soft contact lens is the ocular astigmatism determined from an accurate subjective spectacle refraction.
For many years it was held that prospective contact lens wearers with clinically significant astigmatism could not be successfully fitted with soft lenses. Since the early 1980s, however, notable advances in soft toric lens technology have been made such that the correction of astigmatism with soft lenses is now a viable option for the majority of these patients. Indeed, annual contact lens fitting surveys over the last 15 years have consistently demonstrated a commensurate increase in toric lens fitting as a proportion of all soft lenses fitted. This is evident from Fig. 9.1 , which shows the extent of soft toric lens fitting as a percentage of all soft spherical and toric lenses prescribed in 17 nations between 2000 and 2020 ( ).
Based on the distribution of astigmatism in prospective contact lens wearers, it has been determined that, if all astigmatism of 0.75 D or more were corrected, 45% of lens wearers would need toric lenses ( ). This threshold is shown as a dotted line in Fig. 9.1 . It is evident from Fig. 9.1 that soft toric lens fitting has evolved to the point whereby, since about 2015, nearly all those with clinically significant astigmatism are being corrected with toric lenses in most countries.
Criteria for Use
When deciding whether or not to prescribe a soft toric lens, practitioners should avoid using criteria such as ‘all patients with cylinders greater than a certain amount should be fitted with soft toric lenses’. Instead, each patient should be assessed separately, taking into account the following factors.
Degree of Astigmatism
As a generalization, 1.00 D or more of astigmatism should be corrected, although there will be significant variability between patients. , in discussing the criteria for the prescribing of toric lenses, showed that 45% of the population required a cylindrical correction of up to 0.75 D and 25% of the population required a correction of 1.00 D or more. A more recent study by revealed that the prevalence of patients showing astigmatism of 0.75 and 1.00 D or greater in at least one eye was 47% and 32% respectively. As mentioned previously, the current soft toric lens prescribing rate suggests that almost all cases of astigmatism 0.75 D or more are now being fitted with soft toric lenses.
Note that soft toric lens misalignment – which is discussed in more detail later in the chapter – becomes more significant as the degree of cylinder is increased. For example, a patient with a soft toric lens incorporating a 1.25-D cylinder may be able to tolerate a 5-degree rotation from the expected lens location, whereas a toric lens patient with a 3.50-D cylinder will probably notice a significant drop in vision for the same degree of rotation off axis. Consequently – and not surprisingly – the success rate with soft toric lenses does decrease as the degree of astigmatism requiring correction is increased.
Degree of Spherical Ametropia
The amount of myopia or hyperopia associated with the astigmatism is an important consideration. Patients who are fitted with soft toric lenses that contain a low spherical component, for example +0.25/−1.50 × 180, will generally need their astigmatism to be corrected – and are often very critical of axis alignment – because the astigmatism is the most significant component of their refractive error. Conversely, there is often less need to prescribe soft toric lenses for patients with a high degree of spherical ametropia in addition to their astigmatism. For example, a contact lens patient with an ocular refraction of −6.00/−1.00 × 170 may be content with just wearing a soft spherical lens and not having the astigmatism corrected; on the other hand, a patient with an ocular refraction of −1.50/−1.00 × 170 is more likely to want their astigmatism to be corrected and so require a soft toric lens.
Cylinder Axis
The axis of the ocular cylinder is a critical factor. For example, an uncorrected cylinder with an oblique axis will cause greater degradation of visual image compared with an equivalent amount of uncorrected with-the-rule or against-the-rule astigmatism ( ). As will be discussed later in this chapter, soft toric lenses incorporating oblique cylinders may also show poorer stability due to complex lid lens interactions.
Ocular Dominance
Uncorrected astigmatism is far more likely to be accepted by the patient if it is in the nondominant eye. For example, patients may tolerate uncorrected cylinder of up to 2.00 D in their nondominant eye, while at the same time requiring that cylinder as small as 0.50 D be corrected in their dominant eye. Related to this is the situation where a patient has unequal visual acuities. In this case, higher degrees of uncorrected astigmatism will usually be tolerated in the eye with the poorer acuity.
Viability of Other Alternatives
The practitioner also needs to consider whether soft toric lenses are the best option or if the patient would be better off with spectacles or rigid lenses. For example, a patient with high degrees (>5.00 D) of both corneal and spectacle astigmatism would most likely achieve better acuities with a rigid toric lens. In addition, no form of soft toric lens can correct irregular astigmatism. Patients with astigmatic errors of this nature, for example people who have keratoconus, are usually corrected with some form of rigid contact lens.
Visual Needs of the Patient
Usually, the less critical the visual task, the greater the amount of astigmatism that can be left uncorrected (and vice versa). For example, a musician may require that a cylinder as small as 0.50 D be corrected to enable music to be read. On the other hand, a person with no specific critical visual tasks may be happy with a cylinder as high as 2.00 D left uncorrected so long as the spherical component of their refractive error is corrected.
Design Considerations
Lens Optics
The optical considerations for soft toric lenses are different from those encountered when using rigid lenses. This is primarily because a soft toric lens will align with the anterior surface of the cornea such that a negligible tear lens forms between the back surface of the lens and the front surface of the cornea. Consequently, the optical principles of rigid toric lenses do not apply. There are no tear lens calculations to perform and all the ocular astigmatism will usually be corrected by incorporation of cylinder into the BVP of the soft toric lens.
Soft toric lenses can be manufactured with a toroidal back surface and a spherical front surface or, conversely, a spherical back surface with a toroidal front surface. Regardless of which of these optical configurations is prescribed, the end result on the eye will always be a bitoric lens form due to the wrapping of the front and back surface of the lens onto the cornea. The choice of design (i.e. toric back surface vs toric front surface) is generally based more on considerations relating to manufacture, lens stability and physiological performance.
Lens Stabilization
All forms of soft toric lenses need to be stabilized so that the toric optics of the lens can be maintained in the desired orientation so to correct the ocular astigmatism. The aim is to minimize rotation from the ideal in-eye orientation. The orientation of a soft toric lens on the eye must be predictable and consistent, otherwise suboptimal vision will result. The two major methods of lens stabilization for soft toric lenses are prism ballast and dynamic stabilization.
The theory of prism ballast is that base-down prism is incorporated into the lens so that the lens will be heavier at the prism base (due to excess lens mass). Gravity then acts to cause the prism base to locate inferiorly. Prism ballast has long been used as a technique for stabilizing toric forms of lenses, but it does have certain disadvantages when applied to soft lens designs. The additional thickness brought about by the prism can be a problem in that it reduces oxygen transmissibility in the thick prism zone and can also cause physical discomfort in patients with sensitive lids. In addition, soft toric lenses incorporating prism ballast often show excessive downward mislocation (sag) on the eye. The thicker edge in the region of the prism base can be thinned during the manufacturing stage to form a ‘comfort’ chamfer ( ), although this will slightly negate the intended thickness differential along the vertical axis of the lens.
One of the difficulties that arises with the use of prism ballast is that if it is going to be prescribed monocularly it may cause vertical prismatic effects that can make the patient uncomfortable. This then requires the use of a similar prism for the other eye, which can prove difficult if the other eye requires a spherical lens or is emmetropic. Fortunately, however, prism ballast in soft toric lenses does not often give rise to binocular problems ( ).
Dynamic stabilization was initially developed by and with this technique the dominant lens orientation effect is achieved by pressure from the upper lid (primarily) and the lower lid. used the analogy of the ‘watermelon seed’ to illustrate how dynamic stabilization works. Simply put, pressure applied to the thin end of a watermelon seed by the fingers (i.e. the pressure exerted on a thin zone of a lens between the upper lid and globe) causes the watermelon seed to move away from the fingers (i.e. causes the lens to orientate away from the squeezing force of the eyelid and globe). demonstrated that the effect of gravity is insignificant and that the effect of the thickness profile interaction with the upper lid as described above is the dominant stabilizing component.
With dynamic stabilization, the contact lens toricity is confined to the central portion of the lens. The superior and inferior (‘dynamic stabilization’) zones of the lens incorporate a thickness differential. The action of the lids on these superior and inferior lens chamfers serves to stabilize the lens in the correct orientation. Such a design is shown in Fig. 9.2 . Many similar approaches, referred to as ‘double slab-off’, ‘thin-zone’ or ‘reverse prism’ designs, are manufactured throughout the world.
Oxygen transmissibility is not reduced as much with dynamic stabilization when compared to prism ballast, as with the former additional thickness is generally not incorporated into the lens. Indeed, the excessive thickness of prism ballast lenses can be avoided and, by producing toroidal back surfaces, the average lens thickness is only slightly greater than that of equivalent spherical designs. The main disadvantage of dynamic stabilization is that the thickness differential which can be achieved at the edge of the lens is dependent on the spherical power of the lens. Lower powered lenses will have a reduced thickness differential and, for this reason, a design incorporating prism ballast is often more effective in stabilizing a soft toric lens that has a low spherical power component ( ).
Other forms of lens stabilization, which are less effective and so not commonly used today, are toroidal back surface, peri-ballast and truncation. Clinical experience has shown that a toroidal back surface alone is insufficient to achieve lens stabilization. Peri-ballast features a lens with a minus carrier (peripheral zone), with the carrier being thicker inferiorly. In other words, the prismatic thickness profile changes are confined to the lens carrier outside the region of the optic zone, where the carrier is thicker inferiorly (prism base down). Peri-ballast is generally considered less effective than prism ballast for lens stabilization due to the reduced amount prism in the lens. Truncation refers to the technique of slicing off the bottom of the lens, so as to form a ‘shelf’ that will rest upon – and therefore align with – the lower lid. In the early days of soft toric lenses, this was a reasonably successful method of stabilizing lenses with thick edges, especially when combined with prism ballast. Unfortunately, the truncated edge can make the soft lens uncomfortable to wear and measurement of the lid angle can be difficult and imprecise. Quite often the truncation does not work, with the lid angle appearing to have no effect on the positioning or location of the truncated lens.
Mode of Replacement
Presently, nearly all soft toric contact lenses are prescribed on a disposable basis, with a recent survey revealing that less than 1% of new soft toric lens fits did not involve any planned lens replacement ( ). Most disposable soft toric lenses are mass produced by a process of cast moulding and prism ballast is the method of stabilization that is used predominantly in disposable toric lens designs.
The majority of disposable lenses are replaced at monthly, two-weekly or daily intervals and disposable soft toric lenses are available in these three modalities, as well as in both conventional hydrogel and silicone hydrogel materials. As noted in Part 4, the rationale for the planned replacement of soft contact lenses is based on the tenet that cleaner lenses should produce fewer adverse ocular effects. Disposable soft toric lenses also make it possible to undertake a lens-wearing trial on a prospective patient using a lens with the appropriate BVP. This allows the practitioner to more accurately ascertain if the cylinder axis of the soft toric lens in situ will adopt the correct orientation for the ocular correction.
Virtually all disposable soft toric lenses are produced as a stock range of lenses encompassing a certain number of cylindrical powers (such as −0.75, −1.25, −1.75 and −2.25 D), a set choice of spherical powers (e.g. from +6.00 to −9.00) and cylinder axes in 5-degree or 10-degree steps – usually the latter – most often covering the complete spectrum from 0 to 180 degrees. The choice of back optic zone radius (BOZR) and total lens diameter (TD) for these lenses is also usually limited; hence – given that the contact lens practitioner has chosen to use a particular type of disposable soft toric lens – the main decision in fitting and prescribing these lenses generally relates to the specification of BVP.
The range of powers (spherical and cylindrical) and cylinder axes available with planned replacement lenses has increased significantly over the last 10 years. In many cases, this choice of parameters is quite extensive. Consider a lens such as the Biofinity Toric XR monthly disposable toric lens from CooperVision which has a spherical power range from +8.00 to −10.00 D, cylinder powers from −0.75 to −5.75 D in 0.50 D steps and cylinder axes in 5-degree steps from 0 to 180 degrees. There are other similar disposable toric lenses on the market that also have a wide selection of parameters.
Given the increased popularity of daily disposable lenses ( ) it is no surprise that there are even daily disposable toric lenses – which have usually had a limited scope of powers and axes – now available with a wide range of spherical and cylinder powers and a complete range of cylinder axes in 10-degree steps from 0 to 180 degrees. With such a broad scope of parameters being available to the practitioner when fitting disposable toric lenses, it is no wonder that the need for custom made soft toric lenses has decreased markedly over the last two decades to less than 1% of new soft toric lens fits ( ).
Principles of Correction
It is clear that to produce a stable ocular correction for the astigmatic eye, the lens must align closely over the central cornea in front of the pupil. It must provide the correct power while it is in situ and must stabilize effectively to prevent the rotation of the meridional powers away from their intended orientation.
Fitting
The fitting principles for soft toric lenses are very similar to those for soft spherical lenses, as outlined in Chapter 8 . A well-fitting lens is comfortable in all directions of gaze, gives complete corneal coverage and appears properly centred. On blinking there should be about 0.25–0.5 mm of vertical movement when the eye is in the primary position. On upwards gaze or lateral movements of the eye, the lens should lag by no more than 0.5 mm. The total diameter of the lens is very important because this parameter will influence both lens centration and lens stability. Generally, when specifying the lens diameter, the practitioner should err on the large side, as a larger diameter means that more area is available for the stabilization zones to take effect in the periphery of the lens.
While it has been noted previously that nearly all soft toric lenses are prescribed on a disposable basis – and the choice of BOZR and TD for these lenses is usually limited – there is still a reasonable range of BOZR (from 8.4 to 8.9 mm) and TD (from 14.0 to 14.5 mm) when comparing the various brands of disposable toric lenses that are available on the market which can provide the practitioner with some flexibility in regards to fitting.
Some practitioners advocate fitting soft toric lenses very steep (tight) with minimal lens movement, on the assumption that this will aid stability and reduce lens rotation. With the designs available today, however, this is not necessary. If a lens is too tightly adherent to the eye, it will not be affected by the locating forces designed to stabilize lens orientation ( ). Consequently, a steeply fitting lens may decrease stability and lead to undesirable factors such as limbal indentation and fluctuating vision – the latter being caused by the soft lens vaulting the corneal apex. A well-fitting lens will reveal stable lens orientation with a quick return to axis if mislocated. A tight-fitting lens will show stable lens orientation but a slow return to axis if mislocated. A loose-fitting lens will demonstrate an unstable and inconsistent lens orientation ( ).
Determination of Back Vertex Power
As noted previously, due to the absence of a tear lens, the BVP for a soft toric lens should be similar to the spectacle refraction (or ocular refraction if the vertex distance effect is significant). The BVP of the lens can either be determined empirically or by performing a sphero-cylindrical over-refraction (SCO) over a spherical or toric diagnostic lens. With empirical prescribing, the BVP ordered for the soft toric lens will be equal to the ocular refraction of the patient, based on the assumption of an afocal tear layer under the soft toric lens. For the latter method, use of a spherical trial lens is generally preferred, as an SCO with a toric trial lens may require complex calculations involving oblique cylinders in order to determine the required lens power. When using a spherical trial lens, the resultant toric lens power is simply calculated by adding the SCO to the BVP of the trial lens. With both methods, some arbitrary allowance for lens rotation may have to be incorporated into the final lens prescription.
Effect of Lens Rotation
A considerable degree of cylindrical error can be induced when a lens does not stabilize satisfactorily and rotates away from the intended orientation ( ); this phenomenon is demonstrated in Appendix I . For example, if the contact lens correction incorporates a power of plano/−2.00 DC × 180, Appendix I reveals that a mislocation of the axis by 10 degrees results in a sphero-cylindrical error of +0.35/−0.69 DC × 40. A useful rule-of-thumb here is that a lens made to specification but mislocating on the eye will produce an over-refraction with a spherical equivalent equal to zero. Where the sphere or cylinder power is incorrect, the spherical equivalent of the over-refraction will not equal zero ( ).
Predicting Lens Rotation
showed that, on average, soft toric lenses will tend to rotate nasally by about 5–10 degrees, where nasal rotation is designated as rotation of the inferior aspect of the lens towards the nose. They also showed, however, that there was significant variability between soft toric lens wearers in the actual amount and direction of lens rotation; variations also occur between different lens designs.
The nature of both contact lens materials and their associated designs is continually changing; performance characteristics that were typical for soft toric lenses one or two decades ago may be of questionable relevance today. have examined the performance of several lenses with differing methods of stabilization and have developed recommended performance standards for soft toric contact lenses ( Table 9.1 ).
| Excellent | Acceptable | Poor | |
|---|---|---|---|
| Comfort (1–100) | ≥90 | 80–89 | <80 |
| Mean lens mislocation (°) | ≤±6 | ±7 to ±10 | >10 |
| SD of lens location (°) | <±6 | ±6 to ±10 | >±10 |
| Rotational recovery/10 blinks (°) | >10 | 6–10 | <6 |
| % lenses within ±10 degrees | ≥90 | 70–89 | <70 |
The variability in performance of soft toric lenses can be due to the following factors.
Lid Anatomy
Variation in lid tension (tightness), lid location, lid angle, lid symmetry and palpebral aperture size can all have a significant effect on the location and stability of a toric lens on the eye. Tight lids are more likely to affect lens movement and location than loose lids ( ) and it has been shown that – for prism-stabilized lenses – a smaller palpebral aperture size is associated with a more stable lens orientation ( ).
Lens–Eye Relationship
The optimal fitting relationship between the lens and the eye may vary from one patient to the next. The type of fit (steep, alignment or flat) will, in turn, have a significant bearing on lens position. For example, the degree of adherence between lens and eye is a very important factor ( ). If a lens is too tightly adherent to the eye, it will not be affected by the locating forces designed to stabilize lens orientation. Conversely, have shown that less lens movement is associated with more stable lens orientation; however, increased tightness is associated with slower reorientation speed.
Lens Thickness Profile
Although most soft toric lenses manufactured today have the contact lens toricity confined to the central portion of the lens, a thickness differential due to the astigmatic correction can still have a significant effect on lens location. The lens thickness profile is determined by the power of the lens – in particular, the axis and magnitude of the astigmatic correction. For soft toric lenses incorporating dynamic stabilization, showed that rotational influence is greatest for lenses with cylinders at oblique axes (either between 30 and 60 degrees or 120 and 150 degrees), followed by lenses incorporating correction for with-the-rule astigmatism (150–30 degrees) and is least for lenses with against-the-rule axes (60–120 degrees).
postulated that the principal factor affecting lens rotation is the initial point of contact between the upper lid and the thicker meridian of the lens. For toric lenses with oblique axes, the implication is that there will be notable rotational effects as contact from the upper lid will always affect one edge of the thicker meridian before the other. As the upper lid comes down, it will force the lens down at this first point of contact, causing it to rotate in a certain direction. This principle is illustrated by the example shown in Fig. 9.3 . A mislocating effect also occurs with lenses correcting for with-the-rule astigmatism as the lid contraction angle will usually be at a slight angle to the thickest axis of the lens ( ). For toric lenses incorporating a correction for against-the-rule astigmatism, upper lid contact with the thicker (horizontal) meridian will be fairly symmetrical and so the rotational effect is minimal. However, the influence of the lower lid can override that of the upper lid depending on its position, tightness and amount of lateral movement ( ).
Allowing for Lens Rotation
If it is expected that the soft toric lens to be ordered will rotate when placed on the eye of the patient, then an allowance must be made for this rotation, otherwise the cylinder axis of the lens in situ will not adopt the correct orientation for the ocular correction. When allowing for nasal rotation in the right eye, the amount of rotation should be subtracted from the required cylinder axis and vice versa for the left eye. When allowing for temporal rotation in the right eye, the amount of rotation should be added to the required cylinder axis and vice versa for the left eye. Hence:
- •
If left eye and nasal rotation – add
- •
If left eye and temporal rotation – subtract
- •
If right eye and nasal rotation – subtract
- •
If right eye and temporal rotation – add
The acronym ‘LARS’ (left add, right subtract) – relating to nasal rotation of the inferior aspect of the lens – can be quite useful.
Many practitioners work on the principle that clockwise rotation necessitates adding the allowance for rotation to the required cylinder axis, and counterclockwise rotation requires subtracting the allowance for rotation to determine the final cylinder axis. Hence:
- •
If clockwise rotation – add
- •
If counterclockwise rotation – subtract
If, at the dispensing or aftercare visit, the lens rotation is not what was expected (but the lens location is stable), simply reorder the lens with the revised allowance for lens rotation. Generally, rotational stability is a more important factor than the degree of rotation. Lenses that give suboptimal, but stable acuity are more likely to be acceptable than those that give moments of clear vision followed by moments of poor vision as the lens rotates.
Measurement of Lens Rotation
Soft toric lenses will usually have markings on the lens at a specific reference point so the degree of rotation can be assessed when the lens is on the eye. The markings may be in the form of laser trace, scribe lines ( Fig. 9.4 ), engraved dots or ink dots ( Fig. 9.5 ). The lens markings do not represent the cylinder axis; they are simply a point of reference with regard to which the rotation of the lens can be assessed. They may either be at the 6 o’clock position of the lens or in the horizontal lens meridian at the 3 and 9 o’clock positions. The latter situation is preferable as the markings can then be observed without having to retract the lower eyelid (which would interfere with the dynamic stabilizing forces that normally act to orient the lens). In addition, having two widely spaced markings about 14 mm apart at the 3 and 9 o’clock positions, as opposed to one mark or a set or marks at the 6 o’clock position, makes it easier to quantify the angle of rotation. Many laboratories that opt for the 6 o’clock indication provide three lines on their lenses, each separated by the same known angle, thus also facilitating a determination of lens rotation.
Estimation is a straightforward and reasonable technique for assessing the degree of lens rotation made simpler if the practitioner remembers that there is 30 degrees between each hour on a clock face. Clinical experience has shown that this is a satisfactory method of assessing lens rotation, with errors more likely to occur when evaluating higher amounts of lens rotation ( ).
When assessing lens rotation, it is important to realize that it is the angular orientation of the marker on the lens that is significant and not the position of the marker on the cornea. Fig. 9.6 shows a soft toric lens on a left eye with the marker indicating that the lens is rotating nasally by about 20 degrees (given that the reference point for the marker is the 6 o’clock position). However, a closer look at the marker reveals that it is vertically orientated, the expected orientation if the lens was not rotating. In this case, the apparent nasal rotation is due to a nasal decentration of the contact lens ( Fig. 9.6 ).
Determining Lens Misalignment
The usual method of determining lens misalignment is to simply estimate the degree of lens rotation by observing the location of the soft toric lens on the eye. This value is then compared with the expected lens rotation that has been incorporated into the BVP of the contact lens. The difference between the actual and expected values represents the degree of lens misalignment.
An SCO can also be used to determine the actual degree of lens misalignment. The lens misalignment is deduced by calculating the effective BVP of the lens on the eye (BVP in situ ). The SCO obtained over the mislocating soft toric lens is subtracted from the ocular refraction (Oc Rx) of the patient ( ). That is:
BVPin situ=OcRx−SCO
Calculating the BVP in situ will require the resolving of obliquely crossed cylinders and this is best done by matrix optics ( ) using the following method:
- 1.
Express both the sphero-cylindrical ocular refraction and the SCO in dioptric power matrix form ( F ), whereby:
<SPAN role=presentation tabIndex=0 id=MathJax-Element-2-Frame class=MathJax style="POSITION: relative" data-mathml='F=|S+Csin2θ−Csinθcosθ−CsinθcosθS+Ccos2θ|’>𝐹=∣∣∣∣𝑆+𝐶sin2𝜃−𝐶sin𝜃cos𝜃−𝐶sin𝜃cos𝜃𝑆+𝐶cos2𝜃∣∣∣∣F=|S+Csin2θ−Csinθcosθ−CsinθcosθS+Ccos2θ|
F = | S + C sin 2 θ − C sin θ cos θ − C sin θ cos θ S + C cos 2 θ |
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