Soft Toric Contact Lenses
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Soft Toric Contact Lenses
Today's technology allows you to offer the best contact lens options for patients who have astigmatism.
BY RONALD K. WATANABE, O.D., ANDOVER, MA.
Astigmatism: the bane of soft contact lens fitting. Although roughly 45% of vision-corrected people have at least 0.75 D of refractive astigmatism,1 only about 20% of the contact lens market share worldwide consists of soft toric lenses.2 Yet we have many great options for correcting astigmatism, and these include prescribing soft toric lenses. A scientific look at soft toric lens design and how corneal topography and eyelid anatomy affect the performance of these lenses will help our patients benefit from this technology.
The magnitude and axis of astigmatism are important factors when fitting patients with soft toric lenses. It has been said that it is acceptable to fit spherical soft lenses when a patient's cylinder power is less than 25% of the spherical power, but toric lenses provide superior acuity compared with spherical or aspheric lenses, even for low levels of astigmatism.3–6 A patient's sensitivity to cylinder power and axis may be more important to determining whether a patient is a good soft toric lens candidate. Simply rotating the cylinder axis in the phoropter or trial frame can tell you a great deal about your patient's sensitivity to lens rotation: A patient who doesn't notice blur until the axis is rotated 20° or more will not be sensitive to small lens rotations on the eye. Even high cylinders can be effectively corrected with soft toric lenses, although it is more challenging to deliver high visual quality in these cases.7 Against-the-rule (ATR) astigmats may be better candidates than those who have with-the-rule (WTR) or oblique astigmatism because of the geometry of lens design (see below). The pattern of corneal cylinder between these types of astigmats is significant. Central and limbus-to-limbus astigmatism (Fig. 1) have different degrees of success with soft toric lenses, the lowest success rates occurring when peripheral and central toricity do not match either in amount or axis.8,9
Figure 1. Comparison of central astigmatism (left) and limbus-to-limbus astigmatism (right) by corneal topography.
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The eyelids play a critical role in contact lens rotational position and stability. Tight lids exert greater force and torque on the surface of the lens than those that are not as tight; this may cause increased rotational instability and unpredictability. However, smaller palpebral apertures are associated with increased rotational stability, presumably because of a greater interaction between the lid and lens during all phases of the blink cycle. The angles of the upper and lower lids also appear to play a significant role in rotational position of contact lenses. When the outer canthus is higher than the inner canthus, the lens tends to rotate in such a way that the inferior edge rotates temporally and vice versa. However, this is not always predictable because the eyelid shape changes with each blink, and the lower lid tends to move nasally. This causes the lens to exhibit inferior nasal rotation, especially with thick, prism-ballasted designs.10,11
Today's soft toric lenses fall into two basic categories: prism-ballasted and dual thin-zone. Within each category, individual designs can vary considerably, and some lenses incorporate aspects of both designs.
Most currently available soft toric lenses are based on a prism ballast design. The prism creates a thickness differential from top to bottom such that the thin upper edge orients under the upper lid and the thicker, inferior portion is squeezed out inferiorly ("watermelon seed effect"). The thickness differential is acted on differently by different upper eyelid anatomies, as described above. Therefore, the same prism-ballasted lens may orient differently on different eyes. In addition, the variation in lens thickness from sphere power, cylinder power and cylinder axis may cause the lens to rotate differently when lens parameters are changed, even on the same eye. For example, WTR cylinder lenses have their thickest edge at 90°, while ATR lenses have their thickest edge at 180°. On a WTR lens, the upper lid may squeeze the thick part of the lens downward, inducing lens rotation. This also happens with oblique lenses, but ATR lenses already have the thinnest portion under the lid, so less rotation is induced.
Modern prism-ballasted designs incorporate a variety of design variations to minimize lens rotation and improve consistency across lens parameters. For example, eccentric lenticulation — an off-center lenticular cut on the front surface of the lens — decreases the variation in lens edge thickness resulting from lens power, and it decreases the overall mass of the lens. Several contact lens brands, such as Biomedics Toric (CooperVision) and PureVision Toric (Bausch & Lomb), incorporate designs with more consistent mid-peripheral lens thickness, so that the nasal and temporal halves of the lens are more symmetrical. This results in greater consistency in rotational orientation across lens powers and cylinder axes.
The basic thin-zone design consists of thin upper and lower portions that orient under the upper and lower eyelids and a thicker central portion that sits interpalpebrally. This design works best in high minus powers and ATR cylinders in which more material exists to be slabbed off. Clinically, this type of design tends to be less rotationally stable than prism-ballasted designs. However, thin zones are sometimes incorporated into prism ballast designs to add stability and comfort. The Acuvue Advance for Astigmatism (Vistakon) lens design is a unique variation of the thin-zone design. Its "Accelerated Stabilization Design" consists of thick mid-peripheral zones at 3:00 and 9:00 that interact with the upper and lower lids to keep the lens from rotating. Unlike a traditional thin-zone design, the central portion of the lens is very thin, to minimize interaction with the lids and add stability.
When selecting a soft toric lens brand, corneal curvature, corneal diameter (horizontal visible iris diameter, HVID), corneal toricity and refractive error must be considered. Eyelid architecture may also play a role, especially in patients with tight lid tension and narrow palpebral fissures. Keratometry readings and HVID have equally significant influences on the lens fit.12,13 Steeper and/or larger corneas have greater sagittal depths and therefore will require steeper base curves; flatter and/or smaller corneas will require flatter base curves. Since larger corneas tend to have flatter curvatures and vice versa, unusually large or small corneas may not follow fitting guide recommendations. For example, a smaller-than-average cornea with steep K readings may require a flatter base curve than initially expected. Also, consider the refractive error. Unusually high spherical and/or cylinder powers or oblique axes may not be available in all brands.
Lens material offerings are becoming broader each year. Currently, three silicone hydrogel toric lenses are available (PureVision Toric, Acuvue Advance for Astigmatism and Air Optix Toric [CIBA Vision]), with more due in the near future. PureVision Toric has approval for 30-day continuous wear. All three options are excellent choices for patients demonstrating hypoxic changes or those with high refractive errors that would result in thicker lens profiles. Two daily disposable options are also available — Focus DAILIES Toric (CIBA Vision) and ClearSight 1 Day Toric (CooperVision). Although their parameters are somewhat limited, these are wonderful options for patients with allergies, heavy depositors, part-time wearers or any other patients who would benefit from daily lens replacement. Proclear (omafilcon, CooperVision) and Extreme H2O (hioxifilcon, Hydrogel Vision) toric lenses are available for patients with dryness symptoms, and hioxifilcon can be manufactured into custom toric lenses by several soft lens laboratories.
When selecting lens power, keep in mind that the vertexed subjective refraction is usually the proper lens power. Certain lenses, such as the Hydrasoft Toric (CooperVision), recommend adding plus to the sphere and cylinder of the lens because of apparent lacrimal lens effects caused by the thicker lens profile. Using a fitting guide or proprietary calculator, or simply calling the company's order line will help you determine the correct lens power.
Settling time is critical for soft toric lenses. Since most brands are thicker than their spherical counterparts, they require more time to equilibrate with tear film. In addition, the action of the upper eyelid on the thickness profile of the lens will change over the first several minutes as the lens rotates to its preferred orientation. In most cases, 15 to 20 minutes should be sufficient to make an initial assessment, although final judgment should be made at a follow-up visit after the patient has been wearing the lenses for two or more hours. Thinner profile lenses may settle more quickly, but the longer the lenses are allowed to settle, the more accurate your evaluation will be.
Once the lens has settled, the lens should center well and provide full corneal coverage. Small decentrations are acceptable as long as full corneal coverage is maintained. The lens should lie flat on the ocular surface with no edge lift or limbal compression. A decentered lens with excessive movement, especially in conjunction with inferior edge lift, indicates a flat fitting relationship. A tight lens will center well but may not move sufficiently on the blink, and post-lens removal fluorescein evaluation will show a tell-tale lens imprint. A small amount of lens movement with the blink and eye movements is necessary for good corneal physiology.
Lens rotation should be measured while the patient looks in primary gaze with his or her head in a normal upright position. Virtually all soft toric lenses have one or more orientation markings that are visible through a slit lamp. Most have their markings at the 6:00 position on the lens, although several have markings at 3:00 and 9:00. If a 6:00 marking is covered by the lower lid, gently pull the lid down until the marking is just visible, so as not to disturb the rotation of the lens. Once the marking is visible, measure or estimate the amount of rotation from the expected position of the marking. Most experienced practitioners can estimate the amount of rotation to the nearest 5°. Some lens brands have three markings spaced 20° or 30° apart, which helps increase the accuracy of lens rotation estimation. For more accurate measurements, use a slit lamp reticule with internal protractor or a protractor on the slit lamp beam. Narrow the beam to a narrow slit and bring the illumination housing to center so that the beam is shining directly into the patient's eye. Center the beam on the center of the lens, then rotate the light housing until the beam goes through the lens center and the lens marking. The beam protractor will show the amount of lens rotation.
Rotational stability should be good with each blink and in various positions of gaze. A small amount of movement is expected on secondary gazes, but excessive rotation will cause blurry vision. This is important for patients who read in downgaze for large portions of the day or those who frequently change fixation. Head and eye movements cause varying amounts of lens rotation with different lens designs,14 and prism ballasted lenses may rotate up to 30° to 60° when patients lie on their sides.10 Test the lens' ability to return to resting position after extreme rotation by turning the bottom of the lens 45° in either direction and measuring the time it takes to return to its resting orientation. If it takes longer than 15 seconds for the lens to return to its original orientation, it may not recover well from rotation caused by extreme eye movements or forceful blinking. In most cases, steepening base curve, increasing lens diameter, or increasing prism ballast will improve rotational stability. However, if the lens appears to be too tight, a flatter base curve will actually improve rotational stability.
Anecdotally, soft toric lenses do not provide optimal visual quality. However, studies show that soft toric vision is superior to spherical and aspheric soft lenses, and that it even approaches that of spectacles and rigid gas permeable lenses.3–6,15 Nevertheless, there are times when soft toric vision needs improvement.
Consistent blurry vision is usually resolved with a simple sphere or cylinder power change. Simply add to the existing lens power a spherical over-refraction (SOR) or spherocylindrical over-refraction (SCOR) with cylinder axis equal to or 90° away from the axis of the existing lens. Remember that thicker and/or stiffer lens designs may create a lacrimal lens that decreases the amount of minus power you need in the lens. If the axis of the SCOR is oblique to that of the lens, then a crossed cylinder calculation is required. This usually results from lens rotation, so if you get this type of OR, re-check the position of the markings.
Figure 2. Example of LARS to compensate for toric lens rotation.
An oblique SCOR can be resolved in two ways. A simple method is to place the lens power and the SCOR into a trial frame, then measure the entire stack of lenses in a lensometer. When doing this, be sure to account for any contact lens rotation when placing the cylinder lens in the trial frame. Also, when calculating the final lens power, be sure to adjust for lens rotation again, assuming that the new lens will rotate to the same location as the first. Example: diagnostic lens power -3.00 -1.75 × 180; SCOR +0.50 -1.25 × 145; lens is rotated 20° counterclockwise; place into a trial frame the following trial lenses: -3.00 and +0.50 (or one - 2.50 lens), -1.75 × 20, -1.25 × 145. The resultant measures -3.00 -1.75 × 180 in the lensometer. Since the new lens should also rotate 20° counterclockwise, you must use LARS to compensate for this rotation by subtracting 20° to the resultant to give a final lens power of -3.00 -1.75 × 160 (Fig. 2).
Another method of resolving an oblique SCOR is to use a calculator. Several Web sites, including CooperVision.com and EyeDock.com, have online calculators that help resolve the SCOR for you. CooperVision's program, "ToriTrack," calculates the assumed lens rotation based on the patient's subjective refraction, diagnostic lens power and SCOR.16 Therefore, you do not have to enter the amount of rotation into the calculator, which eliminates one possible source of error when troubleshooting lenses. Other calculators, such as the Eyedock calculator, require you to enter the amount of lens rotation, so be as precise as possible when taking this measurement.
Fluctuating vision even with an OR is usually caused by lenses that are too loose-fitting or too steep. Loose-fitting lenses do not stabilize well on the eye, resulting in rotation with each blink and with eye movements. Loose-fitting lenses also tend to decenter, which can affect vision. If the lens is significantly decentered, moves more than 0.5 mm with each blink or has unstable rotational position, then a tighter fitting is needed. Sometimes, custom lenses with more extensive parameters can sufficiently tighten the fit. If the lens fit is not making sense to you, re-examine the relationship between the keratometry readings and the HVID or perform corneal topography to rule out an irregular cornea.
Steep-fitting lenses may also cause fluctuating vision because of variations in lens draping. Because a steep-fitting lens does not evenly drape over the ocular surface, particularly with thicker toric lens designs, the vision will change as the lens surface shape changes before, during and after each blink. It is common for a patient to report that vision clears immediately after the blink but becomes blurrier as the eye stays open.
Additionally, it is not unusual for a steep-fitting lens to rotate off-axis and then stay at that incorrect axis longer than expected. This results in blurry vision that does not clear unless the lens is manually rotated to re-align it again. In such cases, refitting with a flatter base curve and/or smaller diameter should solve the problem.
A well-fit soft toric lens will provide clear, stable vision throughout the day, with minimal adverse corneal physiology changes. On average, soft toric lenses take more time and expertise to be fit successfully. Inform patients up front and set fees to reflect this expertise.
Successful fitting of more specialized lenses, like torics, can help you build your practice through patient referrals. Careful assessment of the patient's ocular characteristics and lens fit will lead you to the most appropriate lens design—and ultimately to contact lens success.
Dr. Watanabe has no financial interest in any of the products mentioned in this article. References available upon request.
Optometric Management, Issue: April 2008