Bitoric Contact Lens Fit Spherical Power Effect: Bruce W. Anderson, OD
A 54-year-old female who was a previous contact lens wearer presented for a contact lens refit from her current contact lenses. She stated that she had been previously fit with new contact lenses; however, she found these to be very uncomfortable and unwearable and had since reverted back to an older, more comfortable, set. She had been a contact lens wearer for approximately 25 years.
Test Procedures, Fitting/Refitting, Design and Ordering
Visual Acuity (with Contact Lens):
OD: +0.25DS 20/20
OS: +0.25DS 20/20
OD: Power: -4.75D
Base Curve Radius (BCR): 42.75D (7.89mm)
Overall Diameter (OAD):9.5mm
OS: Power: -4.50D
Base Curve Radius (BCR): 41.75D (8.08mm)
Overall Diameter: 9.50mm
Slit Lamp Evaluation:
Slit lamp evaluation of her current contact lenses revealed that they were both riding superiorly. There was significant edge clearance at the inferior periphery of both lenses. Upon blinking, the lens would push down and “bump” into the lower lid. The fluorescein pattern was alignment to slight apical clearance.
Diagnostic Data at Fitting Visit:
OD: -4.50 – 2.25 x 014 20/20
OS: -4.25 -2.50 x 015 20/20
OD: 42.00 @ 010; 44.50 @ 100
OS: 41.37 @ 180; 44.37 @ 090
I proceeded with the contact lens fitting using the following diagnostic lenses:
OD: Power: -3.00D
Base Curve Radius (BCR): 44.50D (7.58mm)
Overall Diameter (OAD): 9.2mm
OS: Power: -3.00D
Base Curve Radius (BCR): 44.00D (7.67mm)
Overall Diameter: 9.2mm
OD: -3.50DS 20/20
OS: -3.50DS 20/20
Slit Lamp Evaluation:
Slit lamp evaluation of the diagnostic lenses revealed that both lenses exhibited apical clearance with fluorescein application. They were both positioning slightly high on the cornea.
Contact Lenses Ordered:
Bitoric (Spherical Power Effect)
Material: Boston XO
OD: Power: -4.00/-6.25D
BCR: 42.00 (8.04mm)/44.25 (7.63mm)
Overall Diameter (OAD): 9.5mm
OS: Power: -4.00/-6.50D
BCR: 41.50 (8.13mm)/44.00 (7.67mm)
Overall Diameter: 9.5mm
The base curve radii are often selected “On K” to 0.25D flatter “K” in the flat meridian and 0.50D to 0.75D flatter than steep “K” in the steeper meridian. The attached Mandell-Moore Guide (also available on the gpli.info website) uses a similar – and very simple – method to determine their powers and base curve radii via use of their “Fit Factor.” The base curve radii are often selected flatter than the corresponding keratometry values which, in fact, should result in an alignment fluorescein pattern due to the asphericity of the cornea.
Varying the base curve radius to keratometry value per meridian induces a slight amount of toricity to assist in good centration and tear exchange (i.e., similar to fitting a spherical GP lens on a cornea with 0.50 -0.75D toricity, often considered an ideal patient for an alignment GP lens-to-cornea fitting relationship). In this case it was decided to fit the flat meridian “On K” and fit the steeper meridian slightly flatter than “K”.
The contact lenses were dispensed resulting in a visual acuity in both eyes equal to 20/20-. There was a plano over refraction in each eye with unchanged visual acuity. Both contact lenses were positioning slightly superior. For both lenses, there was an alignment lens-to-cornea fitting relationship in the horizontal meridian and slight apical bearing in the vertical meridian.
Follow-Up Care/Final Outcome
On her two follow-up evaluations, the visual acuity was a solid 20/20 in each eye. There was a plano over refraction OU. Both lenses again, exhibited an alignment fitting horizontally and slight apical bearing vertically. She was scheduled for a one year routine evaluation.
Discussion/Alternative Management Options
The purpose of a bitoric GP spherical power effect lens design is to create a well-centered alignment fitting relationship on a toric cornea or does not exhibit good alignment (i.e., often showing bearing along the flatter meridian and excessive clearance peripherally in the steeper meridian. This patient initially presented with spherical contact lenses that were fit on a moderately toric cornea. Because of the spherical lens design on the toric cornea, it created an inferior edge clearance. This presents a problem in terms of comfort, as well as potential drying of the inferior cornea.
To alleviate the fit problem, a bitoric design was provided. A spherical power effect lens design is used in that there was no residual astigmatism. This design will correct the vision the same as a GP spherical lens in terms of the vision correction. Diagnostic lenses were used to determine the prescription needed and to evaluate the fit and fluorescein pattern of a known spherical lens on the toric cornea. From this, it was noted that the initial diagnostic lens was too steep (as would be expected). Using the difference in the keratometry readings as a reference, 80 to 90 percent of the total corneal curvature difference was incorporated within the bitoric contact lens design. The flatter base curve in this design is set to be equivalent to the flatter keratometry reading. This is calculated based on keratometry readings or corneal topography-based simulated “K” readings.
The goal of fitting the bitoric lens on a toric eye is to achieve a close alignment of the contact lens on the eye and create an appearance in the fluorescein pattern as if the fit were a spherical lens on a slightly toric eye. The lenses being slightly flatter in the vertical meridian will encourage more vertical lens movement and tear flow under the lens. This will likely provide an upper lid adherence of the lens, which will usually be a more comfortable fit. This will overall create a more comfortable lens design. The diameter used for these contact lenses was made in an adequate size to ensure complete pupil coverage in both photopic and scotopic illumination. Most lens designs will be 9.2 to 9.8mm in diameter, assuming a slight superior fit.
The condition of the cornea in this case needs to be considered, because this patient was wearing spherical contact lenses. There is always concern that the cornea may be somewhat flattened and sphericalized through the wearing of a spherical lens on a moderately toric cornea. This should be reevaluated at the follow-up visit after the bitoric lenses are dispensed and worn for a period of time. This will ensure that the proper fluorescein pattern and centration of the contact lenses is obtained. If the fit was not optimal at the follow-up visit, the lenses should be redesigned.
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