Over-Refraction: Determining Final Contact Lens Power

What Is Over-Refraction?

An over-refraction is a subjective refraction performed while the patient is wearing a diagnostic (trial) contact lens. The purpose is to determine the additional optical correction needed to achieve best-corrected visual acuity, which, when combined with the diagnostic lens power, gives the final contact lens prescription.

Over-refraction is a standard step in diagnostic contact lens fitting. It accounts for the tear lens effect, vertex distance changes, and any differences between the estimated and actual lens performance on the individual eye.

Spherical Over-Refraction

A spherical over-refraction determines only the additional sphere power needed. It is used when fitting spherical contact lenses (no astigmatic correction).

Procedure

1. Place the diagnostic contact lens on the patient's eye and allow it to settle
2. Confirm the lens is fitting acceptably (adequate movement, centration, coverage)
3. With the diagnostic lens in place, perform a standard subjective refraction using trial lenses or a phoropter
4. Determine the additional sphere power that provides best distance acuity

Calculating Final Power

Final CL power = Diagnostic lens power + Over-refraction result

Example:

• Diagnostic lens power: -3.00 D
• Over-refraction result: -0.50 D (patient needed an additional -0.50 D for best acuity)
• Final CL power: -3.00 + (-0.50) = -3.50 D

Another example:

• Diagnostic lens power: -4.00 D
• Over-refraction result: +0.75 D (patient was over-corrected by the diagnostic lens)
• Final CL power: -4.00 + (+0.75) = -3.25 D

Sphero-Cylinder Over-Refraction

A sphero-cylinder over-refraction determines both sphere and cylinder power, and is used when fitting toric contact lenses or when residual astigmatism needs to be quantified.

Procedure

1. Place the diagnostic toric contact lens and confirm rotational stability
2. Note the lens orientation marking position (to determine if the lens is rotating)
3. Perform a full subjective refraction (sphere, cylinder, and axis) with the lens in place
4. Record the complete over-refraction result

Calculating Final Toric Power

For toric lenses, the calculation is more complex because the cylinder powers from the diagnostic lens and over-refraction may be at different axes. In many cases, the over-refraction result indicates whether the toric lens cylinder and axis need adjustment.

If the over-refraction shows:

• Sphere only (no cylinder): The toric lens is properly correcting the astigmatism. Add the sphere to the diagnostic lens sphere
• Cylinder at the same axis as the lens: The toric lens has too little or too much cylinder power. Adjust accordingly
• Cylinder at a different axis: Suggests the lens is rotating or the lens axis needs compensation

GP Lens Over-Refraction

Over-refraction with GP lenses requires additional consideration because the tear lens between the GP and the cornea contributes to the overall correction:

• A spherical GP on an astigmatic cornea creates a toric tear lens that partially corrects corneal astigmatism
• Any cylinder found in the over-refraction represents residual astigmatism (astigmatism not corrected by the tear lens, typically lenticular astigmatism)
• If significant residual astigmatism is found, a front-surface toric GP may be needed

Common Over-Refraction Scenarios

Sources of Error

Several factors can affect over-refraction accuracy:

• Lens movement: A loose lens shifts during refraction, creating inconsistent readings
• Lens rotation: For toric lenses, rotation during the over-refraction changes the effective axis
• Tear film disruption: Excess tearing or dry spots can alter the optical system temporarily
• Accommodation: As with any refraction, controlling accommodation is important for accurate results, especially in younger patients

Key Takeaways

• Over-refraction determines the additional power needed over a diagnostic contact lens
• Final CL power = diagnostic lens power + over-refraction result (algebraic addition)
• Spherical over-refraction is used for spherical lens fitting; sphero-cylinder for toric fitting
• For GP lenses, the tear lens contributes to the correction; residual cylinder indicates lenticular astigmatism
• Always verify toric lens orientation before interpreting the over-refraction
• Allow the lens to settle 10-15 minutes before performing the over-refraction