Soft Toric Contact Lens Fitting Notes

Ulster University - Soft Toric Contact Lens Fitting

Learning Outcomes

  • Outline the prevalence of astigmatism in the UK and worldwide.
  • Discuss indications and contraindications for toric soft and GP lens fitting.
  • Describe differing toric lens designs.
  • Differentiate methods of soft lens stabilization.
  • Describe soft toric fitting assessment and characteristics.
  • Make adjustments for soft lens rotation.

Incidence of Astigmatism

  • The following data represents the power of cylinder in diopters (D) and the percentage of the sample:
    • 0 D: 32.0%
    • 0.25-0.50 D: 34.6%
    • 0.75-1.00 D: 17.7%
    • 1.25-2.00 D: 9.8%
    • 2.25-3.00 D: 3.8%
    • 3.25-4.00 D: 1.5%
    • Over 4.25 D: 0.6%
  • Source: Bennett & Rabbetts, Clinical Visual Optics

Worldwide Prevalence of Astigmatism

  • Referenced study: Zhang et al (2023) Epidemiology and Burden of Astigmatism: A Systematic Literature Review. Optom Vis Sci.100 (3):218-231

Contact Lens Correction Options

  • Soft Spherical Lens
    • May lead to suboptimal vision correction in the presence of astigmatism.
  • Thick Soft Lens (Decreased Dk/t)
    • Thicker lenses reduce oxygen transmissibility, potentially leading to corneal hypoxia.
  • High Modulus Soft Lens (Discomfort)
    • Higher modulus lenses can cause discomfort for some patients due to increased stiffness.
  • Toric Soft Lens
    • Specifically designed to correct astigmatism with soft lens comfort.
  • Spherical GP Lens (Small Diameter)
    • Suitable for regular corneas with minimal astigmatism.
  • Aspheric GP Lens
    • Can mask low amounts of corneal astigmatism.
  • Toric Rigid GP Lens
    • Ideal for correcting irregular astigmatism or higher amounts of regular astigmatism.

Proportion of Soft Lens Fits (UK)

  • Trends in UK Contact Lens Prescribing (2021):
    • Toric lenses account for a significant proportion of soft lens prescriptions.
    • Sphere: 38%
    • Toric: 43%
    • Multifocal: 13%
    • Myopia Control: 2%
    • Monovision: 3%

International Toric Lens Prescribing

  • Global contact lens prescribing data (2000-2020) indicates varying proportions of toric lens fits across different countries.
  • Morgan PB & Efron N (2022). Global contact lens prescribing 2000-2020. Clinical & Experimental Optometry, 105(3).298-312.

Toric Soft Lenses Indications

  • Prescribed when spherical lenses cannot provide adequate visual acuity.
  • Astigmatism is generally 0.75D or greater. Correction is particularly significant when spherical ametropia is low relative to cylinder power.
  • Suitable for patients with poor tolerance to toric GP lenses.
  • Considered when keratometry and optical assessments indicate that correcting with a rigid lens would necessitate a more complex bitoric design.

Toric Soft Lenses Contraindications

  • Irregular astigmatism.
  • Considerations for astigmats with high spherical ametropia.
  • Considerations for astigmatism in the non-dominant eye.
  • Existing GP lens wearers with good tolerance.
  • Astigmatism is purely corneal, and GP lens tolerance is good.

Toric Soft Lens Designs

  • Front Surface Toric vs. Back Surface Toric:
    • Capable of correcting both corneal and lenticular astigmatism up to about 4.50D.
    • Back surface may be spherical or aspheric.
    • The back surface of the lens is designed to neutralize the toric cornea, replacing it with a spherical front surface (similar concept to GP Back Surface Toric).
    • Cylinders as high as 6.00D may be corrected.

Choosing Parameters (BC/TD/Power)

  • Many toric soft lenses are available in a limited range of base curves (BC) and diameters (TD).
  • Always check the spherical/cylindrical parameter range for a particular lens.
  • Cylinder powers and axes availability may be limited.

Choosing Parameters - BVD

  • Remember to compensate for back vertex distance (BVD) when converting spectacle refraction to contact lens power.

  • Example:

    • Spectacle Rx: -1.50DS/-1.25 x 20, BVD = 12mm
    • Spectacle Rx: -6.00DS/-2.75 x 20, BVD = 12mm
    • Ocular Refraction: -5.59DS/-2.32 x 20
    • Contact Lens Rx: -5.50DS/-2.50 x 20 or -5.75DS/-2.25 x 20

Assessing Lens Fit

  • General soft lens fitting principles apply:
    • Centration
    • Coverage
    • Movement (Blink and Version)
    • Push-up Test
    • Comfort
  • Slightly reduced movement on blink and version may be acceptable compared to spherical lenses.

Lens Rotation

  • Eyelids tend to rotate soft toric lenses off-axis.
  • More common with oblique cylinders.
  • Factors influencing rotation:
    • Lid position and tension
    • Visual Point of Apposition (VPA)
    • Force of blink
    • Normal action is to rotate lens to bring the thickest point to the horizontal meridian
    • Water content, modulus, thickness

Lens Stabilization Methods

  • The ideal lens should move on blink but not rotate.
  • Stabilization is needed to maintain adequate visual acuity.
  • Most soft toric lenses are slightly larger than their spherical equivalents to improve stability and centration.
  • Methods:
    • Truncation
    • Prism Ballast
    • Peri Ballast
    • Dynamic Stabilization

Truncation

  • A section of the lens is cut off, usually at the bottom, to interact with the lower lid and stabilize the lens.

Prism Ballast

  • Uses prism, typically base-down, to create a differential thickness profile that stabilizes the lens.

Peri Ballast

  • Similar to prism ballast but utilizes eccentric lenticulation.
  • Thinned superior lens edge.
  • Prism is restricted more to the lens periphery.

Dynamic Stabilization

  • Utilizes thicker and thinner zones in the lens design to promote stability through lid interaction.

Soft Toric Lens Stabilization

  • Examples of stabilization methods:
    • Optimized prism ballast
    • Dual thin zones
    • Prism ballast
    • Dynamic stabilization
    • Peri-ballast

Prismatic Effect

Thickness (um)

  • Figure shows thickness variations in lens designs.

Toric Lens Thickness

  • Dependent on Design - Impact on Dk/t
  • Examples:
    • Acuvue Oasys for Astigmatism -2.75 / -2.25 x 20
    • Air Optix for Astigmatism -2.75 -2.25 x 20
    • Biofinity Toric -2.75/ -2.25 x 20
    • PureVision Toric -2.75 / -2.25 x 20

Toric Lens Thickness - Impact on Dk/t

  • Lens power impacts Dk/t.
    • 1-DAY ACUVUE® MOIST
    • DAILIES AquaComfort PLUS® Toric
    • claritiⓇ 1 day toric
    • MyDayⓇ toric

Toric Lens Oxygen Transmission

  • Oxygen transmission (Dk/t) within the central 7.50 mm of a range of soft toric lenses.
  • Holden-Mertz criteria 24.1 Dk/t.

Lens Rotation

  • Lenses may still rotate slightly despite stabilization methods.
  • Acceptable, as long as it rotates the same amount each time.
  • Compensate for lens rotation using a mnemonic:
    • LARS – Left ADD Right SUBTRACT
    • CAAS – Clockwise ADD Anticlockwise SUBTRACT

Assessing Lens Rotation

  • Use reference points on the lens, NOT the astigmatic axis, to assess rotation.

Assessing Lens Rotation - Slit Lamp Method

  • Use slit lamp controls to estimate rotation.
  • LARS – Left: Add, Right: Subtract.
  • CAAS – Clockwise: Add, Anticlockwise: Subtract.

Assessing Lens Rotation - Clock-face Method

  • Use a clock-face diagram to estimate the degree of rotation.

LARS

  • Left Add, Right Subtract

CASS

  • Clockwise Add, Anticlockwise Subtract

Assessing Lens Rotation Example

  • Fitted with Acuvue Oasys 1 Day For Astigmatism – 8.5 / 14.3 / -3.00 /-1.75 x 180.
  • The toric lens is observed to rotate 10 degrees anticlockwise on slit lamp evaluation.

Assessing Lens Rotation

  • Fitted with Alcon Total 1 For Astigmatism – 8.5 / 14.5 / +2.00 /-2.25 x 70.
  • The lens is observed to rotate 15 degrees clockwise on slit lamp evaluation.
  • The question is: What lens would you try next?

Soft Toric Fitting - Problem Solving

  • Flowchart for problem-solving:
    • Is the lens orientating correctly?
      • Yes: Is the lens rotationally stable?
        • Yes: Is the lens fitting correctly with respect to all other aspects?
          • Yes: Check contact lens & spectacle prescription
          • No: Alter lens fit
        • No: Change stabilization design of lens
      • No: Which direction is the lens rotating?
        • Clockwise: Subtract degree of rotation from cyl axis and apply new lens
        • Anticlockwise: Add degree of rotation to cyl axis and apply new lens

Soft Toric Fitting Summary

  • A large range of lenses is available; most work very well.
  • Daily disposables have a more limited parameter range.
  • Fitting and assessment are relatively straightforward.
  • Assess as per soft lens fit and correct for lens rotation.
  • If one design/stabilization method is not effective, try an alternative.
  • Consider lens thickness and reduced Dk/t with hydrogel materials.
  • Dk/t is not a concern with SiHy materials.

Further Reading

  • Veys J, Meyler J & Davies I (2002). Essential Contact Lens Practice (Chapter 7). Butterworth Heinemann Optician
  • Evan K & Hiscox R. REVISED Essential Contact Lens Practice 8: Soft toric contact lens fitting. https://www.opticianonline.net/cpd-archive/5975/
  • Gasson A & Morris J (2010). The Contact Lens Manual (Chapters 23 & 24). Butterworth Heinemann
  • Efron N (2024) Contact Lens Practice. 4th Edition. (Chapter 10: Soft Toric Lens Design & Fitting & Chapter 16: Rigid Toric Corneal Lens Design & Fitting). Elsevier.