Contact Lens Power Calculation, Selection, and Fitting in Optometry

Contact Lens Fitting Method

Contact lenses are a widely used alternative to spectacles, offering better aesthetics, wider fields of vision, and convenience in certain cases. The process of determining the correct contact lens power, selecting the appropriate type, and fitting them properly requires precision and expertise. Below is a detailed explanation of these processes.

1. Contact Lens Power Calculation

Since contact lenses sit directly on the cornea, unlike spectacles that rest a few millimeters away from the eye, the power required for contact lenses is often different from spectacle prescription. The following steps are followed:

A. Convert Spectacle Prescription to Contact Lens Power

Step 1: Identify the Spectacle Prescription

• The spectacle prescription (Rx) is measured in diopters (D) and given in the form:

  • Spherical (SPH)
  • Cylinder (CYL)
  • Axis (if astigmatism is present)

Step 2: Apply the Vertex Distance Correction

• Spectacle lenses sit about 12-14 mm away from the cornea.

• If the spectacle power is greater than ±4.00 D, a vertex distance calculation is necessary.

• The formula to convert spectacle power to contact lens power is:

  $$CLP = \frac{SPH}{1 - (Vertex Distance × SPH)}$$

  Example:

  • Spectacle Rx: -6.00 D
  • Vertex Distance: 12 mm (0.012 m)
  • Contact Lens Power:
    
    $$CLP = \frac{-6.00}{1 - (0.012 \times -6.00)}$$
    $$CLP\approx -5.50D$$
    CLP ≈ -5.50 D

• If the power is below ±4.00 D, the spectacle power is usually prescribed as it is.

Step 3: Cylinder Power Adjustment (If Astigmatism is Present)

• If CYL ≤ -0.75 D, spherical soft contact lenses (SCL) may work.
• If CYL > -0.75 D, toric lenses are required, and their power is adjusted based on the nearest available toric lens power.

2. Contact Lens Selection

Choosing the right contact lens depends on several factors:

A. Type of Contact Lens

1. Soft Contact Lenses

   • Made of hydrogel or silicone hydrogel.
   • Provide comfort and are easy to adapt to.
   • Suitable for daily, bi-weekly, or monthly use.

2. Rigid Gas Permeable (RGP) Lenses

   • Smaller in diameter, made of oxygen-permeable material.
   • Offer sharper vision, especially for irregular corneas or high astigmatism.
   • Require adaptation but are long-lasting.

3. Hybrid Lenses

   • Combine a soft lens periphery with an RGP center.
   • Used for irregular corneas (e.g., keratoconus).

4. Scleral Lenses

   • Cover the entire cornea and part of the sclera.
   • Used in cases of severe dry eyes, keratoconus, and corneal irregularities.

B. Base Curve (BC) and Diameter (DIA) Selection

• Base Curve (BC): Determines how well the lens fits on the cornea.

  • A flatter cornea requires a higher BC (e.g., 8.6 mm).

  • A steeper cornea requires a lower BC (e.g., 8.4 mm).

• Diameter (DIA): Affects lens stability.

  • Usually ranges between 13.8 mm to 14.5 mm for soft lenses.

C. Oxygen Permeability (Dk Value)

• Higher Dk lenses allow more oxygen to reach the cornea, reducing hypoxia risks.

• Silicone hydrogel lenses have high Dk values.

D. Wear Schedule

• Daily Disposable (Single-use)

• Bi-weekly or Monthly Replacement

• Extended Wear (Approved for Overnight Use)

3. Contact Lens Fitting Process

Proper fitting is essential to ensure comfort, good vision, and corneal health.

Step 1: Pre-Fitting Examination

• Refraction Test: To determine refractive error.

• Keratometry/Topography: Measures corneal curvature.

• Tear Film Assessment: Determines suitability for contact lenses.

• Slit Lamp Examination: Checks for any eye abnormalities.

Step 2: Trial Lens Fitting

• A trial lens is applied to assess:

  • Movement (0.5 - 1.0 mm is ideal).

  • Centration on the cornea.

  • Comfort level.

  • Vision clarity.

Step 3: Over-Refraction

• Determines if any power adjustments are needed over the trial lens.

Step 4: Finalizing the Prescription

• Once the best-fitting lens is determined, a prescription is given with:

• Lens brand.
• Power (SPH, CYL, AXIS if needed).
• BC and DIA values.

Relationship Between Keratometer Reading in D (Diopters) and mm (Millimeters)

Keratometer readings are measured in diopters (D) and millimeters (mm). The relationship between these two is given by the formula:

$$r (mm) = \frac{337.5}{D}$$

Where,

• r (mm) = Corneal radius of curvature
• D = Keratometer reading (in diopters)

Keratometer Reading and Appropriate Contact Lens Base Curve (BC)

How to Choose the Correct Base Curve?

1. For Soft Contact Lenses (SCL):

   • The base curve selected is typically 0.7 mm flatter than the keratometer reading.

   • Example: If the keratometer reading is 7.85 mm (43D), the base curve should be around 8.5 mm – 8.9 mm.

2. For Rigid Gas Permeable (RGP) Lenses:

   • The base curve is selected close to or slightly flatter than the keratometer reading.

   • Example: If the keratometer reading is 7.85 mm (43D), the base curve should be 8.0 mm – 8.3 mm.

The keratometer reading helps in selecting the correct base curve (BC) for contact lenses, ensuring a proper fit and providing clear, comfortable vision.

4. How to Insert and Remove Contact Lenses

A. How to Insert Contact Lenses

1. Wash Hands with soap and dry them.

2. Inspect the Lens for damage and ensure it is not inside out.

   • If the edges flare out, it's inside out.

3. Hold the Upper and Lower Eyelids Open with your fingers.

4. Place the Lens on the Cornea using your index finger.

5. Blink Gently to center the lens.

B. How to Remove Contact Lenses

1. Look Up and Pull Down the Lower Eyelid.

2. Pinch the Lens Gently with thumb and index finger.

3. Store the Lens in Solution (if reusable).

5. Contact Lens Care and Maintenance

• Daily Disposable Lenses: Discard after single use.

• Reusable Lenses: Clean with multipurpose solution.

• Never Use Tap Water for rinsing.

• Replace Lens Case Every 1-3 Months.

Conclusion

The process of contact lens prescription, selection, and fitting is critical for ensuring optimal vision correction and eye health. Proper calculations, trial fitting, and patient education help in achieving successful contact lens wear. Regular follow-ups with an optometrist are necessary to monitor eye health and make adjustments if needed.