A Clinical Comparison of Two Leading Toric Soft Contact Lenses
I.G. Cox, BOptom., PhD, T.L. Comstock, OD, MS, & W.T. Reindel, OD, MS
The need for an effective toric hydrogel contact lens has been recognized since the introduction and wide acceptance of the spherical hydrogel lens. Estimates of the incidence of significant astigmatism among the population of patients who are candidates for contact lenses range from 35 to 45 percent. Toric hydrogel lenses with acceptable clinical performance were introduced early in the 1980s, and with improvements in manufacturing technology in the 90s, have become an increasingly viable alternative to rigid gas permeable (RGP) lenses for a patient with -0.75D of astigmatism or more. Today, in many countries of the world, soft toric lenses are the preferred fitting option for patients with low to moderate amounts of corneal astigmatism.
The introduction of spherical disposable and planned replacement contact lenses in the late 80s significantly reduced the incidence of deposition-related adverse responses amongst spherical hydrogel lens wearers when worn on a daily wear basis. They also led to a rapid move in the marketplace from traditional to disposable or planned replacement modalities. A similar move to the more frequent replacement of toric hydrogel lenses amongst astigmatic patients has been hampered by the prohibitive cost of replacing soft toric lenses on a monthly or more frequent basis.
Advances in manufacturing technology now allow toric hydrogel contact lenses to be manufactured using low cost cast-molding technologies, producing lenses with consistent parameters at a cost that makes monthly, or even biweekly lens replacement an acceptable economic reality. While it's believed that this, in turn, will provide a reduction in the incidence of deposition-related adverse responses and improve comfort and visual acuity analogous to the reduction found in spherical soft lens wearers with increased frequency of lens replacement, not all cast-molded toric lenses will perform the same. Various toric lens manufacturers use different materials and designs, which could translate into differences in clinical performance. Table 1 illustrates market survey results from three countries, ranking the importance of benefits provided by soft toric lenses for practitioners, spectacle wearers and current soft toric wearers. Values represent the percentage of respondents who rated each statement a 9 or 10 using a 0-10 rating scale, where a rating of 10 indicates most desirable and 0 indicates least desirable. The market research survey results in Table 1 indicate that vision clarity is one of the most important attributes for soft toric lenses to clinicians, spectacle wearers and current toric wearers, suggesting concern about whether current soft toric lens designs can easily and consistently provide correction of astigmatism.
The following studies evaluated the performance of two cast-molded, planned replacement toric contact lenses: SofLens66 Toric (Bausch & Lomb) and the Focus Toric (CIBA Vision). The evaluation combined three multisite studies of similar protocols conducted worldwide and was specifically designed to compare the clinical, fitting and visual performance of these two lenses.
Materials and Methods
Two hundred and twelve habitual soft contact lens wearers completed one of three masked, daily wear crossover studies with periods of wear ranging from one week to one month (study protocols were identical except for crossover duration, which was site specific and determined by the most common lens replacement period in that region), to evaluate the clinical performance and patient acceptance of two currently available hydrogel toric contact lenses. Characteristics of the two types of lenses being compared are summarized in Table 2.
Patients were recruited from private practice populations at 23 different locations throughout North America and Northern Europe. Participating patients were given a preliminary examination including a spherocylinder refraction, a visual acuity exam, keratometry readings and a biomicroscopic examination and a diagnostic fitting with both lens types. Study lenses were ordered with powers and base curves based on the spectacle refraction and the fitting evaluation of the diagnostic lenses. Lens movement, centration and rotation, Snellen visual acuity with and without overrefraction, and anterior ocular physiology were monitored by the clinicians at dispensing and at the end of each crossover wearing period (one week to one month).
At the end of the wearing period for each lens type, patients were asked to report any symptoms they experienced while wearing the lenses. Patients then rated lens comfort, visual quality, dryness and handling using 100mm visual analog scales. At the end of the study, patients completed a questionnaire indicating their forced-choice lens preference under the following categories: comfort at insertion, comfort after all day wear, quality of vision, dryness, ease of insertion, overall handling, overall preference and purchase preference.
Patients were masked as to the identity of the study sponsor and the identity of the contact lenses they were wearing, and the order of lens wear was randomly assigned to each patient. Clinicians conducting the study were not masked, as a result of the lens markings used to identify the soft toric lenses.
The ages of the 212 patients who completed the studies ranged from 18 to 52 years old. Nineteen patients failed to complete the studies, sixteen due to non-lens related reasons, and three due to poor visual acuity or unacceptable lens fit. Patient K-readings ranged from 8.65mm-6.82mm. Ninety-five of the completed eyes (22%) had central K-readings ¾7.85mm, 193 of the completed eyes (46%) had central K-readings between 7.80mm-7.54mm, and 136 of the completed eyes (32%) had central K-readings >=7.50mm. The spherical refractive error ranged from plano to -8.00D, while refractive astigmatism ranged from -0.50 to -2.75D. Fifty-seven percent of the eyes had with-the-rule (WTR) astigmatism and 30 percent had against-the-rule (ATR) astigmatism, while 13 percent had oblique axes.
All patients had previous soft toric lens wearing experience. Twenty-eight percent of the patients were currently wearing CIBA Vision Focus Toric lenses, 31 percent were currently wearing Bausch & Lomb Gold Medalist Toric lenses and the rest were wearing a variety of other brands.
There were statistically significant differences (z-approximation) between the two lens types in the frequency of reported symptoms, problems and complaints (Table 3). Significantly fewer patients reported at least one occurrence of blurred vision (11% vs. 18%) or problems with handling (11% vs. 20%) while using the SofLens66 Toric lenses than while using the Focus Toric lens.
Contact Lens Centration and Movement -- Clinicians detected no significant differences in centration and movement between the Bausch & Lomb SofLens66 Toric lens and the CIBA Vision Focus Toric lens. With grading choices of excellent, fair or poor, combined results from both the initial and final visits revealed excellent centration for 88 percent of the eyes wearing the Bausch & Lomb SofLens66 Toric lens and for 87 percent of the eyes wearing the CIBA Vision Focus Toric lens. Centration was judged to be fair for all remaining eyes. Using a scale of excessive, adequate, insufficient or adherent, combined results from both the initial and final visits revealed that movement of the SofLens66 Toric lens was judged to be adequate significantly more often (98% of eyes) than compared with eyes wearing Focus Toric lens (91 % of eyes). Excessive movement and insufficient movement were significantly lower for eyes wearing SofLens66 Toric lenses. No lenses were judged to be adherent.
Lens Orientation -- At the dispensing study visit, the SofLens66 Toric demonstrated a mean absolute rotation of 3.9±4.9, while the Focus Toric demonstrated a mean absolute rotation of 5.6±6.4. At the end of the wearing periods, these values were 4.9±7.5 and 7.6±9.1 for the SofLens66 Toric and Focus Toric, respectively. Taking 5 rotation as a clinically acceptable deviation of a soft toric lens in situ, Table 4 shows that 77 percent of the eyes wearing the SofLens66 Toric had acceptable lens rotation at the dispensing visit compared with 73 percent at the end of the study. For the Focus Toric lens, these values were 66 percent and 58 percent, respectively.
Anterior ocular physiology -- There were no clinically significant incidences of findings or differences in slit lamp exams (0 - 4 grading scales) for the SofLens66 Toric and the Focus Toric lenses during the studies.
Visual Acuity -- Snellen visual acuity measurements pooled across both initial and final visits was 6/6 or better significantly more often (71%, z-approximation, p¾0.001) with the SofLens66 Toric contact lens than compared with eyes wearing the Focus Toric contact lens (61%).
Patient Assessment -- Visual analog scales were used to quantify subjective patient responses for comfort, visual quality, handling and lens dryness. Scales were 100mm long and anchored at either end by opposing descriptives. Patients quantified their assessment of the response in question by marking the scale with a pen. Their assessment was then measured and recorded as a value from 0 to 100.
Patient visual analog assessments of the performance of the SofLens66 Toric lens and the Focus Toric lens at the conclusion of wear for each lens type are shown in Table 5. Patients rated the SofLens66 Toric lens significantly higher than the Focus Toric lens for comfort, visual quality and lens handling. Patients rated the two lens types equivalent for lens dryness.
Patient Preference -- At the conclusion of the wearing periods for both lens types, patients completed a forced-choice preference questionnaire. Patient preference results are presented in Table 6. Patients significantly preferred the SofLens66 Toric lens in terms of comfort both at insertion and after all-day wear, quality of vision, less dryness, overall handling, overall preference and purchase preference (z-approximation). Patients showed no preference for either lens type for ease of insertion.
Ideally, patients with refractive astigmatism of -0.75D or more requiring a soft contact lens should be fit with a toric hydrogel lens. Maintaining lens orientation and axis location has traditionally been the chief difficulty in designing and fitting toric contact lenses. During the blink, the palpebral fissure closes from the lateral canthus to the medial canthus. The resulting force has vectors both horizontally and vertically generating the potential to rotate the bottom of the contact lens inferiorly and nasally. The ideal soft toric lens will combine several physical characteristics (lens thickness profile, design and material properties) to maximize the interaction of inferior directed lid force and the lens, while minimizing the interaction of the nasal direction lid force and the lens, providing correct orientation and minimal rotation of the lens during and between blinks. Improved designs and materials of toric hydrogel lenses have increased to a level that has enabled soft toric contact lenses to become the primary means for the contact lens correction of refractive astigmatism for many clinicians.
More recently, however, it has become overwhelmingly accepted that frequent replacement of any soft contact lens will improve comfort and the visual performance of the lens, while reducing the incidence of adverse responses. Recent advances in manufacturing techniques have allowed toric hydrogel lenses to be manufactured using low-cost cast-molding technologies, producing lenses at a cost that makes monthly, or even bi-weekly lens replacement, an economic reality. The concern is whether these lower cost, planned replacement lenses provide acceptable clinical performance on astigmatic patients.
The results of these studies demonstrate that both the SofLens66 Toric lens and Focus Toric lens are viable alternatives for correcting astigmatic patients with a frequent replacement soft contact lens. Ultimately, however, astigmatic patients will be more satisfied with a soft toric lens that provides the highest, most stable visual acuity, while maintaining other clinical attributes such as comfort and lens handling. In this study, the SofLens66 Toric demonstrated superior visual performance over the Focus Toric lens as measured by visual acuity, reported symptoms of blurred vision, and patient assessment and preference of visual quality. Visual performance in a soft toric lens relies primarily on the magnitude and stability of rotation the lens design provides.
A lens that rotates inconsistently with blinking will provide reduced subjective visual quality, even if a reasonable visual acuity can be achieved. The SofLens66 Toric demonstrated excellent orientation and rotational stability, with approximately 75 percent of patient fittings showing no more than ±5 of rotation at both the dispensing and final study visits, and 88 to 94 percent of patient fittings showing no more than ±10 of rotation at the dispensing and final study visits. This excellent axis stabilization may be attributed to the particular geometry of the SofLens66 Toric contact lens.
Although prism ballasting with increased thickness from the apex to the base of the lens is not unique to the SofLens66 Toric lens, the prism ballasting in this design is controlled to maintain the same vertical thickness profile of the lens across all spherical powers. Anterior and posterior optic zone diameters are also adjusted to minimize variations in thickness resulting from differing cylinder powers. The 360 degree comfort chamfer reduces differential lens mass in the periphery of the lens, particularly in prescriptions with oblique axes. This combination of controlled design elements provides a similar lens thickness profile regardless of the lens sphere, cylinder and axis parameters, providing excellent axis stabilization and consistent fitting performance for all patients.
It should also be noted that the excellent axis rotational stability and corresponding visual quality demonstrated by the SofLens66 Toric lens wasn't achieved by sacrificing other measures of clinical performance. Lens centration and movement were rated excellent by the investigators, and patients rated the Soflens66 Toric superior in both comfort and handling. In terms of preference, patients chose the SofLens66 Toric contact lens more frequently for lens comfort, both at insertion and after all-day wear, quality of vision, less dryness, overall handling, overall preference and purchase preference, demonstrating the excellent clinical performance of the SofLens66 Toric contact lens.
It's clear from the results of this study that soft toric contact lenses that are produced using low cost cast-molding technologies in an effort to provide a lens at a cost that makes monthly, or even bi-weekly lens replacement, an economic reality, do not compromise lens quality, reproducibility, clinical performance or patient satisfaction. Clinicians can now provide the benefits of more frequent replacement to their astigmatic soft contact lens patients without concerns of reduced patient satisfaction or reduced clinical performance. CLS
References are available upon request to the editors at Contact Lens Spectrum. To receive references via fax, call (800) 239-4684 and request document #57. (Be sure to have a fax number ready.)
Dr. Cox is the optometric research fellow at B & L Headquarters in Rochester, NY.
Dr. Comstock is the director of Global Clinical Research for B & L Vision Care, Rochester, NY.
Dr. Reindel is the director of Global Professional Marketing for B & L Vision Care, Rochester NY.