YAMOUT OPTICAL CENTER

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By Bilal Yamout 

Gone are the days when astigmatism, especially those who were difficult to fit because of their high-CYL requirements, were told that because of their astigmatism they could not wear soft lenses. Gone too is the frustration of the doctor and the patient when a lens needed to be replaced because not all lenses were created equal, and replacing the astigmatic soft lens was more a game of chance than a scientific procedure, notably when it came to high-CYL prescriptions.

It is now accepted that the more frequently contact lenses are replaced, the more healthful it is for the patient's eyes. Frequent replacement can greatly reduce such complications as giant papillary conjunctivitis and solution sensitivity problems. In optometric practices today it is commonplace to prescribe spherical soft contact lenses with replacement schedules ranging from 1 day to 3 months. Providing such a wide range of schedules, we have not only reduced the number of ocular complications, but we have also attracted new wearers into the contact lens market, and brought back many wearers who dropped out owing to solution reactions, allergies, and the inconvenience of contact lens care.

Patients enjoy the convenience of having a spare lens when traveling, as well as the ability to replace a torn lens without the added expense and inconvenience of a trip to the doctor's office. Patients particularly like being able to throw away a dirty lens and put on a fresh new one. Doctors enjoy prescribing the variety of wearing schedules, since they have thereby made their patients more successful and their practices grow.

As astigmatic soft lenses have grown in popularity through the years, a number of clinical fitting methods have been developed for predicting the patient's success prior to fitting. One of them is the Bickerer Twist, which allows the fitter to evaluate how much lens rotation a patient will tolerate before experiencing subjective blurring. Before a patient is trial-fitted with a contact lens, the subjective refraction in the Phoroptor is used with full refraction, and the axis dial is twisted clockwise and counterclockwise to the blur point reported by the patient. The range in degrees that the patient will tolerate is recorded. This test gives the fitter an idea of how visually critical the patient is, and demonstrates how stable a lens must be in order to provide adequate vision to the patient. Obviously, the tighter the range (i.e., less tolerance to blur) the more critical is the patient's visual perception and the less likely the patient will be willing to accept Toric lenses. For a patient having a tight range to be successful wearing Toric lenses, very little lens rotation could be present in the lens fit.

Another clinical fitting method that has proved useful in predicting potential patient success is the Off-Axis Dialing and Recovering Test, sometimes called the Bounce-Back Test. In this test a trial soft lens is placed on the eye and manually rotated off axis to determine how long (i.e., how many blinks) it takes to return to the correct orientation. A lens that fits well will return to the correct position within 30 to 60 seconds (10 to 15 blinks). If it takes longer, the chances of success with the lens may not be good. Steep fitting lenses tend to orient in off-axis positions and not return to the proper position; conversely, the on-eye orientation of flat-fitting lenses tends to fluctuate greatly.

The Remba Rule of Thumb is a useful benchmark that helps predict the success of a patient with astigmatic soft lenses. The Rule states that the highest rate of success in prescribing astigmatic soft lenses is achieved with patients whose spherical power is twice their cylinder power when written in the minus cylinder form (S > 2C). For example, if the spherical power is 4.00 D, success should be better when the cylindrical power is less than 2.00 D. Thus, according to this philosophy, small spherical ammetropia combined with large astigmatic corrections is generally contraindicated for soft Toric lenses. However, practitioners should remember that this rule is only a prediction, since in the case of new lens designs there are many successes that don't conform to it.

Patients must be able to see well with soft Toric lenses, and these quick, easy screening procedures effectively allow the practitioner to determine which patients have the greatest potential for success with soft Toric lenses. Remember that low ammetropia with small amounts of refractive cylinder and essentially spherical corneas offers the most challenging fitting opportunity regardless of contact lens design. Ultimately, proper lens powers and proper lens position are the two most critical factors that will determine the success of soft Toric lens fitting.

Figure 1. Typical rotational forces during blinking.

It is certainly critical to achieve the proper lens powers, but excessive on-eye lens rotation may still render that correction useless. Although most on-eye soft lens rotation occurs in a nasal direction, temporal rotation is often observed and actually occurs much more frequently than generally reported in the literature. This rotation, however, should not be a contraindication to fitting Toric lenses as long as the lens is stable in that location. Anatomical factors that influence the amount and direction of lens rotation include vector forces and tightness of the lids, the size of the palpebral aperture, the stability of the tear film after blinking, and dry eye considerations. (See Figure 1.). Mechanical and lens design factors that influence on-eye lens rotation include lenses that fit either too tightly or too loosely, the location of the lens cylinder (i.e.. back-surface vs. front-surface designs), and lens-lid interactions that are controlled by the thickness of both superior and inferior lens edges.

To address the problem of unwanted rotation, several stabilization features have been incorporated into soft Toric lenses, including prism ballasting, truncation, thin zones (double slab-off), eccentric lenticulation, and combination designs. Prism ballast in the form of 0.75 to 2.00 D of base-down prism effectively lessens the tendency to lens rotation by creating a differential lens edge thickness; the superior lens edge is thinner and the inferior lens edge is thicker. The use of this feature is based on what is referred to as the Watermelon Seed Principle. During the blink, when the upper lid squeezes the thin superior lens edge, the lens is directed away from its apex towards the base of the lens. The presence of a thicker inferior lens edge reduces the movement of the lens inferiorly and helps to stabilize lens orientation.

Although many patients and practitioners alike think gravity is the basis of this method of lens stabilization, it is really the differential in lens edge thickness. Although a truncation sometimes helps increase stability, it can make the lens less comfortable at the lower lid margin; and if the truncation is very large, the effect of the prism is actually reduced. Sometimes, this design is also used to minimize the inferior corneal desiccation (smile staining) associated with thin, low-water contact lenses that would otherwise rapidly dehydrate on the eye. The Wesley Jessen CSI® Toric, the Bausch & Lomb Gold Medalists™ Toric, the Sunsoft® 15.0 Toric, and the CIBA Focus® Toric are examples of lenses that use prism ballasting to stabilize on-eye lens rotation.

· Material Is Removed from Anterior Lenticular Surface · Upper and Lower Thin Zones Are of Equal Thickness · Edge Thickness Is Uniform Throughout Lens Circumference · Useful in High Cylinders and Oblique Axes
Figure 2. Eccentric lenticulation makes for on-eye stability

The thin-zone design is commonly referred to as a double slab-off, because material is removed from both the superior and inferior portions of the lens, making the middle of the lens the thickest portion. This design makes for a very comfortable lens, since it significantly minimizes lens-lid interaction and is basically thinner than a prism ballast lens. It works well for patients who have against-the-rule astigmatism, owing to an increase in horizontal lens thickness, and for high minus prescriptions. However, since the thicker portion of the lens edge is now oriented superiorly, increasing lens-lid interaction, it is somewhat less stable for low and with-the-rule prescriptions. The CIBA Vision TORISOFT ® is a very popular double slab-off design. Some practitioners use this form of lens stabilization when fitting monocular astigmatism to avoid the potential vertical imbalance that is theoretically created when using one prism ballast design with a spherical lens design.

A prism ballast design incorporating eccentric lenticulation provides greater on-eye stability owing to both the presence of prism ballasting and the control of the edge thickness profile. Like the slab-off design, this design can also improve on-eye lens comfort by producing thinner lenses. A uniform edge thickness around the entire lens circumference significantly reduces lens mass, especially in the inferior portion of the lens, minimizing lens contact with the lower lid. Actually, the prism ballasting is only present within the central two-thirds of a lens incorporating eccentric lenticulation. (See Figure 2.) Because of its minimal rotation, this design is extremely effective for both high and oblique astigmatic corrections. The Sunsoft Eclipse® and 15.0 Toric, the CooperVision Hydrasoft® Toric, and the Kontur 55 Toric use this design in the effort to control lens rotation. Recently, Sunsoft introduced Multiples™ a frequent-replacement Toric that also incorporates this sophisticated stabilization strategy.

The prescription can significantly affect the on-eye position of the contact lens. In fitting an against-the-rule cylinder prescription, it is critical to remember that the thicker portion of the lens edge is located horizontally. This positioning reduces the rotational influence the upper lid has on lens position, and offers a more consistent on-eye lens orientation, generally resulting in the highest possible success rate. Conversely, when fitting with-the-rule corneal astigmatism, the thicker edge of the lens is in direct apposition to the upper lid, causing greater variability in on-eye lens rotation. Although this type of correction has the second-highest success rate, it is often the mechanism responsible for temporal lens rotation owing to its highly variable rotational pattern. (See Figure 3.)

Figure 3. With-the-rule astigmatism showing a thick lens edge in the superior location, causing variable on-eye rotation.

When the orientation of the correcting cylinder is located obliquely (axis at either 45° or 135°), the thicker lens edges also align obliquely and create the greatest amount of lens-lid interaction and the least on-eye lens stability. For example, if the correcting cylinder axis is located at 45°, the thickest portion of the lens edge is located at the 10 o'clock position, generally resulting in the lens's rotating counterclockwise owing to the increased influence of the lower lid. With the correcting cylinder axis located at 135°, the thickest portion of the lens is oriented in the 2 o'clock position, generally resulting in the lens's rotating clockwise owing to the increased influence of the upper lid. Either of these two positions causes increasing instability and generally leads to the worst clinical success rates. Control of on-eye lens orientation is accomplished by understanding the effect the prescription will have on the dynamics of lens design, and choosing a stabilization strategy that will minimize lens-lid interactions.

Many soft lenses for astigmatism are custom-designed. Since custom-designed lenses do not generally rotate as much with the blink, they may often develop protein spots inferiorly; and because they are generally expensive, patients may tend to wear them too long, compromising their ocular health. Although a great effort is made to instruct these wearers on the importance of cleaning their lenses, they often slack off, because they find compliance to be inconvenient. In response to the reduced compliance rates associated with disposable spherical lenses, frequent-replacement soft Toric lenses were introduced as a solution to the astigmatism problems. Soft Torics with 1-month and 3-month replacement cycles are currently available, and several manufacturers are investigating a disposable astigmatic soft lens that may soon be introduced in the market.

Like conventional disposable lenses, a soft Toric frequent-replacement lens must have good optics, provide a mechanism to stabilize on-eye lens rotation, and be made of a material that will be durable enough to survive the length of time for which the lens is prescribed. More importantly, frequent-replacement soft Torics must be absolutely identically reproducible. Variations from lens to lens increase chair time and defeat the purpose of frequent replacement. One reason frequent-replacement spherical lenses have sold so well has been the manufacturers' success in producing lenses that are identical. If the lenses used in a frequent-replacement program are not identical, the program will soon turn out to be a practice nightmare. Finally, the lenses must be produced at a cost that allows the practitioner to realize a profit, and be offered to the patient at a price that is perceived to be commensurate with the promised advantages of ocular health and convenience.

Initially, frequent-replacement Toric lenses were introduced for either monthly or quarterly replacement. However, owing to the magnitude of possible prescription combinations, the manufacturers often limit the parameters within which they can be prescribed. Bausch & Lomb's Gold Medalist Toric. CIBA Vision's Focus Toric, and CooperVision's Preference® Toric™ are examples of the currently available frequent-replacement soft Toric designs. Both the Bausch & Lomb and the Preference Toric designs are offered as 3-month replacement products. Although the Focus Toric was initially offered with the same quarterly replacement cycle, it is currently available for monthly replacement. The Gold Medalist Toric is lathe-cut with a front-surface cylindrical design, whereas both of the other products are back-surface cylinders. All three designs incorporate prism ballasting as their primary means of lens stabilization.

Bausch & Lomb's Gold Medalist Toric is offered in two base curves (8.3 and 8.6 mm) and three astigmatic powers (-0.75 D, -1.25 D, and -1.75 D). Axis orientation is limited to ±20° either side of 90°, or 180° in 10° increments, with spherical powers available from +4.00 D to -6.00 D. This 57% hefilcon A material is limited to daily wear. It works well with spherical corneas owing to its front-surface cylinder design.

The Focus Toric lens is also offered in two base curves (8.9 and 9.2 mm), but only two astigmatic powers (-1.00 D and -1.75 D). Axis orientation is available in 10° increments around the clock, and spherical lens powers are available from +4.00 D to -6.00 D. This 55% vifilcon A material has approval for daily, flexible, and extended wear. It provides excellent patient comfort, but owing to its slick surface may give rise to complaints from patients who experience difficulty removing the lens or develop frequent lens tears. Anecdotally, this lens has been used with moderate success in dry-eyed patients.

Finally, the Preference Toric is limited to a quarterly replacement schedule, and offers two base curves (8.4 mm and 8.7 mm) and four astigmatic power options (-0.75 D, -1.25 D, -1.75 D, and -2.25 D). Axis availability is around the clock in 5° increments, with a spherical power range from Plano to -8.00 D. This 42.5% water lathe-cut material is only approved for daily wear; however, it has a reputation for being extremely deposit-resistant, and has proved to be very useful for those patients who demonstrate excessive lens deposits in a short time span. While all of these lenses have their inherent advantages, obvious compromises are involved when it comes to prescribing for the low to moderate off-axis astigmatism; and there are limited options for the higher astigmatic. Unfortunately, although frequent-replacement Toric designs offer the benefits of disposability, they are somewhat limited in the prescriptions they cover.

Recently, Sunsoft introduced their new frequent-replacement Toric soft lens, called Multiples. Like the Bausch & Lomb and Preference Torics, it is designed for a 3-month replacement cycle and is provided to the practitioner in one package of four lenses. Although only one base curve is offered (8.9 mm), astigmatic powers can be custom-ordered from -0.50 D to -6.00 D in 0.25 D steps. Axis orientation is also available around the clock, but in 5° increments instead of the normally prescribed 10° increments, and spherical lens powers range from +3.00 D to -6.00 D.

This lens design has a molded front surface combined with a lathe-cut posterior surface. Daily wear, extended wear, and flexible wear are all approved wearing schedules for this 55% water methafilcon A material. This design has the potential to expand the frequent-replacement soft Toric market by offering corrections to patients who previously could not be fitted. Multiples provides practitioners with their first opportunity to prescribe a frequent-replacement lens for patients who require up to -6.00 D of cylinder correction.

It is estimated that about 45% of all optometric patients are potential wearers of Toric soft lenses. One indication that soft Toric lens use is required is a spherical lens that demonstrates a one- or two-line drop in visual acuity as compared to the acuity of the patient's best spectacle correction. Such diminished visual acuity is often exacerbated by visual fatigue experienced by patients who work at computers or who drive at night. As visual tasks become more exacting, the patient notices a decrease in visual acuity due to residual astigmatism. Another indication of the need for Toric lens use is a patient manifesting -0.75 D or greater residual astigmatism with an over-refraction of a spherical soft contact lens. The following symptoms experienced by spherical lens wearers are also possible indicators of Toric lens use:

 

Figure 4. The axis moves away from the stable position after blink.

 

Figure 5. When the lens is too steep, it may not move freely when the patient gazes upwards or sideways.

STEEP LENS SYMPTOMS

• Fluctuating or blurring VA after the blink (See Figure 4.)
• Up-gaze movement and lag minimal to none (See Figure 5.)
• Slow lens reorientation after mislocation
• Tugging and excessive movement of bulbar 
  conjunctiva vessels on post-blink (See Figure 6.)

Figure 6. When the lens is too steep, it may exhibit bubbling and cause sclera indentation, conjunctiva drag, and capillary branching.

Figure 7. Excessive movement after blink.

FLAT LENS SYMPTOMS

• Decent ration, especially on post-blink (See Figure 7.)
• Edge lift inferiorly (See Figure 8.)
• Rotation greater than 5° per blink, 
  and/or unstable orientation between blinks (See Figure 9.)
• Drop or lag greater than 0.5 mm on up-gaze or post-blink
• Discomfort and irritation
• Fluctuation in vision

 

Figure 8. Edge flexing at the lower lid margin.

Frequent-replacement Toric lenses should orient to the desired 6 o'clock position within 1 or 2 minutes of insertion. If this does not happen, the lens should be repositioned and observed again after about 5 minutes to see if it maintains that off-axis orientation. Consistent off-axis orientation can be compensated for with the LARS formula. This relationship specifies that if the axis mislocation is rotated left on either eye, the amount of that mislocation is added to the spectacle axis so that the new lens ordered should then place the cylindrical correction at the proper on-eye position. (For example, SRx = --2.00-2.00cx 180°, axis mislocation 15° left rotation in the left eye. If the lens axis is ordered at axis 15°, when the lens rotates 15° left (i.e., nasally) on the left eye, it should stabilize at 180° and properly orient with the spectacle axis.)

 

Figure 9. Erratic lens rotation.

Conversely, if the axis mislocation is rotated right on either eye, the amount of that mislocation is subtracted from the spectacle axis. Excessive initial lens movement indicates that the lens does not fit and requires the evaluation of an alternative base curve or a different lens design. However, as long as the patient manifests reasonably good distance and near visual acuity (20/20 to 20/25), the lens may be dispensed and reevaluated in several days. Often the lens fit becomes more stable as the patient adapts to the presence of the lens on the eye. The fitter should make sure that the lens exhibits some movement upon a blink, and does not bind at the edge, causing vascular compression in the conjunctiva.

If visual acuity is not adequate even though the lens orients well, keratometric over the contact lens surface while it is on the eye should be performed. A lens that is properly draping the cornea exhibits clear, stable mires. If visual acuity is still not adequate, a spherocylindrical over-refraction should be performed. Several sophisticated crossed cylinder analysis programs are available from various manufacturers that rapidly provide a mathematical solution offering alternative spherical and/or cylindrical powers, as well as axis orientation to resolve the problem. However, before a new lens is ordered this analysis should be viewed with reference to the actual on-eye orientation and stability. If an improvement in visual acuity is obtained with a spherocylindrical over-refraction, the lens may have been defective or mislabeled, the original baseline refraction might have been incorrect, or the lens may be rotating on the eye. Regardless, a new lens must be ordered to resolve this difficulty.

Table 1 provides a quick look at the profitability of prescribing conventional astigmatic Toric soft lenses versus frequent-replacement soft Torics. It is my belief that in optometric practices today, the greatest profit potential lies within the specialty area of the contact lens market. I am convinced that practitioners will find that frequent-replacement Torics will not only gratify their wearers, but will also supplement the growth of the practices.

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