What is a bitoric RGP lens?
Bitoric RGPs are rigid gas permeable contact lenses designed with two distinct curves on the front surface. These two curves cater to different meridians of the cornea, making them especially beneficial for individuals with astigmatism. Astigmatism is a common vision condition where the cornea or lens has an irregular shape, causing blurred or distorted vision.
Bitoric lens :
A rigid contact lens with astigmatic (toric or cylindrical) anterior and posterior surfaces.
What is Back toric lens
Back toric lens : A contact lens which has a back surface cylinder and spherical front surface for toric cornea fitting.
This lens design is rarer than the others and is used when there is a highly toric cornea (3.00D or more) with a spherical correction.
What is Front Toric lens
Front surface toric lens (FST) Contact lens with toric optics on only its front surface and a spherical base curve , intended to correct residual astigmatism.
This lens design is used when there is over 1.25D of prescription cylinder correction with a spherical or near spherical cornea. This lens is ordered with a cylinder power and axis. To prevent the lens from rotating, the lens has a prism base down to act as weighted ballast or stabilizer
In what circumstances is bitoric RGPs required?
Circumstances Requiring Bitoric RGPs
- High Astigmatism: Bitoric RGPs are often prescribed when individuals have a high degree of astigmatism that cannot be effectively corrected with regular spherical contact lenses. The dual-curve design of bitoric RGPs allows for precise alignment with the irregular corneal shape, resulting in clearer and more stable vision.
- Irregular Astigmatism: Some individuals may have irregular astigmatism, which means that the shape of their cornea is not symmetrical. Bitoric RGPs can be customized to match the specific irregularities, providing better visual correction than other lens types.
- Post-Surgical Correction: Patients who have undergone certain eye surgeries, such as corneal transplant or refractive surgeries like LASIK, may require bitoric RGPs to fine-tune their vision. These lenses can help manage any residual astigmatism that might persist after surgery.
- Keratoconus: Keratoconus is a progressive eye disorder characterized by the thinning and bulging of the cornea, leading to irregular astigmatism. Bitoric RGPs can be an effective option to improve vision in individuals with keratoconus by providing stable and consistent vision correction.
- Pellucid Marginal Degeneration: Similar to keratoconus, pellucid marginal degeneration is another condition that causes irregular astigmatism. Bitoric RGPs can help in managing this condition by providing customized correction for the irregular corneal shape.
- Post-Corneal Transplant: Individuals who have undergone corneal transplant surgery may require bitoric RGPs to achieve the best visual outcomes. These lenses can adapt to the new corneal shape and provide optimal vision correction.
- Visual Clarity and Comfort: Some individuals with astigmatism may find that bitoric RGPs offer superior visual clarity and comfort compared to other types of contact lenses. This can be a personal preference and may not necessarily be a medical necessity, but it highlights the versatility of bitoric RGPs in catering to different needs.
Are bitoric contact lenses a good choice?
Bitoric multifocal lenses offer a practical and convenient solution for individuals who have both astigmatism and presbyopia. They eliminate the need for reading glasses and provide clear vision at all distances, all while maintaining the comfort associated with soft contact lenses
- Elimination of Reading Glasses: Bitoric multifocal lenses provide a solution for individuals who previously had to decide between wearing reading glasses over their contacts or switching to bifocal rigid gas permeable (GP) lenses. These lenses allow wearers to see clearly at all distances, including up-close reading, without the need for additional reading glasses.
- Convenience: Unlike traditional bifocal GP lenses, which can be challenging to adapt to for individuals accustomed to soft lenses, bitoric multifocal lenses offer the comfort of soft contact lenses. This means wearers can enjoy the benefits of clear vision without sacrificing comfort.
- Correction for Astigmatism: Bitoric multifocal lenses also correct astigmatism. This addresses the specific vision needs of individuals with both astigmatism and presbyopia, providing clear vision at all distances.
- Flexible Wearing Schedule: Unlike GP lenses, which typically need to be worn every day for comfort, soft bitoric multifocal lenses can offer a more flexible wearing schedule. This flexibility can be appealing to those who prefer not to wear contact lenses continuously.
Toric hydrogel and silicone hydrogel lenses
Toric hydrogel and silicone hydrogel lenses, both available in stock (limited parameters) and custom prescriptions from numerous manufacturers, offer options for clinicians to achieve a good physical fit by selecting the appropriate base curve and OAD. To stabilize the refractive astigmatic axis, various methods such as prism, truncation, superior/inferior thin zones, or a combination of these techniques can be employed.
It’s important for the clinician to prescribe the astigmatic axis of the contact lens cylinder as closely as possible to match the patient’s own astigmatic axis, while also accounting for the estimated rotation of the lens on the eye. In some cases, undercorrecting the optical power of the patient’s astigmatism may not compromise visual acuity, reducing the visual disturbance caused by alignment variability or misrotation.
How do I determine base curve radii for a bitoric RGP lens?
Determining the base curve radii for a bitoric RGP (Rigid Gas Permeable) lens is a critical step in achieving optimal lens fit and visual performance for patients with significant corneal astigmatism. While there are varying opinions on the exact threshold for when a bitoric lens is necessary, several guidelines can help guide your decision:
- Assess Corneal Toricity: The first step in determining the base curve radii for a bitoric RGP lens is to assess the patient’s corneal toricity. This can be measured using corneal topography or keratometry. Corneal toricity represents the degree and orientation of astigmatism on the cornea.
- Consider Mandell’s Recommendation: Dr. Mandell suggests considering a bitoric lens when the corneal toricity reaches 2.00D. In other words, if the patient has 2.00D or more of astigmatism on their cornea, a toric back surface on the RGP lens is advisable for stability.
- Bennett’s Threshold: Dr. Bennett recommends using a bitoric lens when the patient has regular astigmatism of greater than or equal to 3.00D corneal toricity. This threshold is slightly higher than Mandell’s recommendation and may provide better visual outcomes for some patients.
- Calculate Lens Toricity: Once you have determined the corneal toricity, you’ll need to calculate the toricity of the lens base curve. This toricity should be in the opposite direction of the corneal toricity to correct the astigmatism. The goal is to have the lens align with the patient’s cornea to minimize rotation.
- Consider Lens-to-Cornea Toricity Ratio: According to Edrington, Stewart, and Woodfield, the ratio of lens toricity to corneal toricity is crucial for stability. For patients with with-the-rule astigmatism (where the astigmatism axis is vertical), a ratio between two-thirds and one is recommended. This means that if the patient has 3.00D of corneal toricity (with-the-rule), the most stable RGP lens should have between 2.00D and 3.00D of base curve toricity in the opposite direction (against-the-rule).
- Trial and Error: Fitting bitoric RGP lenses may require some trial and error. After selecting an initial base curve toricity, evaluate the lens fit and visual acuity. Adjustments may be necessary to fine-tune the fit and optimize vision.
- Consult with Specialists: If you are unsure about the appropriate base curve radii for a bitoric RGP lens, consider consulting with experienced contact lens specialists or fitting experts. They can provide valuable guidance and may have access to specialized fitting sets for bitoric lenses.
More resource: How to prescribe bitoric lens can be read here
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