TROUBLESHOOTING DIFFICULT TORIC FITS
Tackling tough toric fits is rewarding for both you and your patients
LONG D. TRAN, OD
I recently read a fantastic article about soft toric lenses in the August issue of Contact Lens Spectrum titled “Toric Lenses for Today and Tomorrow” by Dr. John Mark Jackson. In the article, Dr. Jackson encouraged practitioners to offer soft toric lens options to more astigmatic patients. While the toric lens category has seen steady growth during the last decade (Efron et al, 2011), data show that the percentage of toric lenses being utilized is still disproportionate to the percentage of astigmats in the general population (Morgan et al, 2016; Young et al, 2011).
Why is this so? There has never been a better time than now to fit toric contact lenses. Updated lens designs, combined with newer materials and extended parameters, all make it easier to prescribe the ideal lens for each patient.
There has been an ongoing discussion on the topic of troubleshooting toric lenses in eyecare journals such as this one. Most of the time, this discussion revolves around assessing on-eye lens rotation and compensating for axis misalignment. But what other factors can clinicians address to assure lens fitting success? Following are some factors that clinicians should consider for the most difficult toric lens fits.
Understanding each patient’s visual demands and goals for contact lens wear is essential. How patients use their eyes can influence the fitting strategy and the type of toric lens design selected to meet their visual needs. The approach to fitting an accountant who has 0.75D of with-the-rule astigmatism would be different compared to that for fitting a race car driver who has 2.50D of oblique astigmatism because their prescription and visual demands are not the same.
An understanding of a patient’s motivation can be helpful as well. As an example, is the patient motivated to tolerate the small amount of visual blur that comes with wearing a soft toric lens? Is the patient driven to adapt to GP lenses if that is his only viable option for clear and stable vision?
One simple test that can be used to assess a patient’s tolerance to the blur caused by cylinder axis misalignment is the Becherer Twist test. After manifest refraction has been performed, rotate the cylinder axis dial on the phoropter, one eye at a time, until the patient reports unacceptable blur. If the dial can be turned by greater than 20º in each direction, fitting is predicted to be successful more than 90% of the time using one diagnostic lens. If the dial can be turned by 15º, fitting success is predicted at about 90% of the time using two lenses. If the dial can be turned by 10º, fitting success is predicted at 70% of the time using three lenses. Finally, if the dial can only be rotated by 5º degrees or less, the patient is very sensitive to blur and may not be a good candidate for soft toric lenses (Becherer, 1990; Quinn, 1995) (Table 1). Knowing how tolerant or intolerant a patient is to visual blur allows you to select the appropriate lens design and prime the patient on any challenges that he might encounter in the future.
|DEGREES OF ROTATION TO BLUR||SUCCESS RATE WITH SOFT TORIC LENSES|
|±20º||90% with one diagnostic lens|
|±15º||90% with two diagnostic lenses|
|±10º||70% with three diagnostic lenses|
|±5º||Patient must accept variable vision|
TYPE OF ASTIGMATISM
When fitting contact lenses on a patient who has astigmatism, it is a good idea to consider the source of the patient’s astigmatism. Corneal astigmatism is defined as the astigmatism produced by the dioptric difference in curvature between the two principal meridians of the cornea. Refractive astigmatism, also known as total astigmatism, is the astigmatism determined by refraction. It is the sum of the corneal astigmatism and the internal astigmatism produced by the ocular components behind the cornea. Lastly, residual astigmatism is the amount of astigmatism remaining after correction with a spherical GP lens. This information can be useful when considering the best contact lens option for each patient.
If a patient’s refractive astigmatism is significantly greater compared to the corneal astigmatism, consider fitting a soft toric contact lens. Soft toric lenses are ideal when there is little or no corneal toricity. The front-surface toric design of a soft lens can correct the refractive astigmatism without the need for a complicated front-toric GP design.
If the source of the patient’s astigmatism is mostly from the cornea, pay close attention to lens stability and rotation when fitting a soft toric lens. These lenses will have a tendency to fit along the principal meridian of the cornea rather than along the patient’s refractive cylinder axis. These patients are good candidates for spherical or bitoric GP lens designs.
If the axes of the principal corneal meridians and the refractive cylinder do not coincide, a GP lens is preferred. For the same reason as above, a soft toric lens will fit along the principal meridians of the cornea rather than along the patient’s refractive axis, resulting in an induced cross-cylinder effect. A soft toric lens should not be completely ruled out, however, if there is good rotational stability (Epstein and Remba, 2010).
Lastly, compare the ratio between the manifest sphere and the manifest cylinder power to determine whether a toric lens is warranted. If the patient’s cylinder power is more than 25% of the sphere power, a toric lens is necessary. If the cylinder power is less than 25% of the sphere power, a spherical lens may be tolerated.
Toric contact lens fittings have been simplified by the availability of diagnostic lenses. It is best to select a diagnostic lens that matches the vertex-adjusted sphere power, the vertex-adjusted cylinder power, and the axis of the patient’s manifest refraction. In certain cases, the exact diagnostic lens may not be available. To save time and to prevent the need to order the exact diagnostic lens, select another lens that closely matches the patient’s prescription.
It is important to aim to match the cylinder axis first. Within the chosen axis, match cylinder power next. If the exact cylinder power is not available, choose the closest available power that is less than or equal to what the patient needs. For example, if the patient’s prescription calls for a cylinder power of –2.00D, select the diagnostic lens with a cylinder power of –1.75D rather than –2.25D (Becherer, 1990). Lastly, select the closest available sphere power. Again, err toward less minus power if the exact spherical power is not available. Additional sphere power can be added while performing the over-refraction.
In an ideal world, a toric lens would position where it is intended to fit, but factors such as cylinder axis, lens thickness, eyelid anatomy, and the torsional forces of the lids during blinking can contribute to lens rotation and cylinder axis misalignment (Young et al, 2002; Hanks, 1983). Most clinicians are familiar with compensating for axis misalignment using “LARS” (left add, right subtract). This method requires that the rotation be stable. For the LARS method to work, the subsequent lens needs to rotate in the same direction and by the same magnitude as the initial diagnostic lens. This simple method of compensation works considerably well for the majority of cases.
In addition to LARS, there are less popular diagnostic procedures that can be useful in assessing soft toric lens performance.
Lens oscillation is the rocking motion of the lens from its intended position after a blink. Lens oscillations can cause intermittent visual blur from unwanted astigmatism in patients wearing moderate-to-high cylinder (1.50D or higher) soft toric lenses. Look for lens oscillation by evaluating the lens on the patient’s eye using a thin slit beam on a biomicroscope.
Encourage the patient to blink hard and often, and observe the speed and distance of movement of the laser marking. Next, assess the quality of the patient’s vision after each blink. The patient’s visual quality is expected to be poor if there is a large amount of lens oscillation. If the lens has been given sufficient time to settle into its intended position but continues to oscillate more than what the patient can tolerate, consider switching to a steeper base curve or to a different lens brand.
Rotational velocity is the speed at which a poorly oriented toric lens returns to its intended position (Snyder, 1998). While the lens is on the eye, manually rotate the lens by approximately 45º. The lens should take no more than 15 seconds to return to its intended stable position. A tight-fitting lens will be slow to reorient, while a loose-fitting lens will exhibit variable and inconsistent positioning. Fast rotational velocity is especially important for patients who have demanding visual needs, such as competitive athletes.
A sphero-cylindrical over-refraction (SCOR) is a great troubleshooting tool that can yield useful information about soft toric lens fitting. When performed accurately, the following information can be determined:
• The spherical equivalent of the SCOR result should be plano if the powers of the soft toric lens are correct.
• If the SCOR axis is the same as the toric lens axis, the cylinder power in the lens is too low, and the patient is accepting more cylinder power.
• If the SCOR axis is 90º away from the toric lens axis, the cylinder power in the contact lens is too high, and the patient is rejecting the cylinder power.
• If the SCOR axis is at an oblique angle, the cylinder axis of the lens is misaligned.
• The SCOR result should improve patients’ visual acuity. Failure to improve the patients’ acuity indicates that decreased vision is likely due to the poor optical quality of the lens and not to the fit of the lens.
The amount of cylinder power present in the SCOR can also be used to approximate how much cylinder axis misalignment is present, and vice versa (Table 2). If the cylinder power from the SCOR is approximately one-half of the toric lens cylinder, there is approximately 15º of axis misalignment. If the cylinder power from the SCOR is the same as the toric lens cylinder, there is approximately 30º of axis misalignment. Conversely, the opposite is also true. If there is 15º of lens axis misalignment, the cylinder power in the SCOR is expected to be one-half of the toric lens cylinder; and if there is 30º of lens axis misalignment, the cylinder power in the SCOR will be 100% of the toric lens cylinder (Snyder, 1989; Jackson, 2016). The following equation shows the relationship between the SCOR cylinder and degrees of axis misalignment:
SCOR cylinder = (degrees of axis misalignment/30) x (cylinder power of soft lens)
|CYLINDER POWER OF CONTACT LENS||DEGREES OF AXIS MISALIGNMENT|
Evaluating the retinoscopic reflex and performing over-keratometry can provide information about the optical quality and fit of the lens. Distortion of the retinoscopic reflex signals poor lens optics or a steep lens fit. Distortion of the keratometry mires immediately after a blink indicates a steep-fitting lens, whereas distorted mires that clear immediately after a blink indicates a flat-fitting lens (Bennett et al, 2014).
“Chasing the axis” occurs when a diagnostic lens doesn’t rotate in the same direction or magnitude as the previous diagnostic lens of the same brand (Hom, 2000). This problem can be frustrating and time-consuming for clinicians as well as for patients, and the only resolution is to fit a steeper base curve lens or to change lens brands. Fortunately, this problem is uncommon as today’s soft toric lenses are manufactured with computer-controlled precision.
Custom soft toric lenses and extended range soft toric lenses are available for patients whose prescriptions fall outside of the typical diagnostic lens parameter range. These lenses can also be used for patients who are GP lens intolerant. Many independent contact lens laboratories manufacture custom soft toric lenses in a wide range of sphere powers, cylinder powers, and axes. When ordered in high refractive powers, these custom lenses may have a thicker profile compared to mass-produced lenses, so be sure to select a material with the highest possible oxygen transmission available.
Despite the best attempts, certain patients may not be able to successfully wear soft toric lenses. In these cases, GP lenses can be a viable option. As previously mentioned, patients who have high amounts of corneal astigmatism (>1.50D) can achieve excellent visual acuity through GP lenses. A GP lens with a toric back surface can better align with the corneal toricity, resulting in enhanced lens centration and stable vision.
Patients whose refractive cylinder axis does not coincide with the principal meridians of the cornea also benefit from a GP lens. A soft toric lens will settle along the principal meridians of the cornea rather than along the refractive cylinder axis. However, a GP lens with a bitoric design will align with the cornea, and any residual prescription can be placed on the front surface of the lens. This results in an enhanced fit and stable vision, while avoiding the induced cross-cylinder effect of a misaligned soft toric lens.
Patients who have irregular astigmatism typically do not do well in soft toric lenses. The fit of the soft lens is unpredictable due to the irregular surface of the cornea, and the resulting vision is often unacceptable. In some cases, the thickness profile of a custom soft toric lens can mask a small amount of corneal surface irregularity. Sometimes fitting the soft lens steep can cause it to vault the cornea just enough to create a thin tear lens. This may provide additional masking of the corneal surface irregularities.
Patients who have irregular astigmatism benefit from GP wear. Studies show that GP lenses do well in masking corneal surface irregularities, providing stable vision and maintaining a healthy physiological environment for the cornea when fitted properly. Unlike soft contact lenses, certain GP lens parameters—such as base curve radius, overall diameter, power, and peripheral curve systems—can be customized to meet patients’ visual needs and practitioners’ fitting needs. In the event that a patient is unable to wear a conventional GP lens design, large-overall-diameter GP lenses and scleral lenses can also be great options.
Helping patients who have astigmatism can be a rewarding experience. Improvements in lens materials, stabilization designs, and parameter range have made fitting soft toric contact lenses more efficient and predictable.
For challenging prescriptions and demanding personalities, toric fitting success goes beyond LARS. An understanding of both patients’ visual demands and diagnostic test results can help you achieve better fitting success. Take the time to tackle the tough toric fits; your patients will appreciate it. CLS
To obtain references, please visit www.clspectrum.com/references and click on document #252.
Dr. Tran is the senior associate at St. Johns Eye Associates in St. Augustine, FL. He is also a lecturer for the STAPLE Program. (His spouse is an employee of Johnson and Johnson Vision Care, Inc.)