3 Steps of Over refraction Calculation are:
- Step-1: Identifying Tear lens power using the formula:
Tear Lens = Base Curve of Trial Contact Lens – Base Curve of Cornea
- Step-2: Identifying Effective power of Trial CL using the formula:
Effective Power = Trial Contact Lens Power + Tear Lens Power
- Step-3: Identifying Over Refraction power using the formula:
Over Refraction = Prescription Power + Effective Power of Trial Contact Lens.
- Before starting those steps, we need to know some basics about Tear Lens, Effective Power of contact lens and Over refraction.
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What is Tear Lens in over calculation of Contact lens fitting?
- The lens made of tear is called tear lens.
- When a contact lens fit is not optimum, it creates a space in between the cornea and contact lens.
- In that space tear accumulate and form a tear lens.
- Tear lens can be of two types:
Convex Tear Lens- That is formed in Steep Fit.
Concave Tear Lens- That is Formed in Flat Fit.
Let’s know details mechanism of how Steep Fit forms a Convex Tear lens and how a flat forms a Concave Tear lens:
Remember, tear lens is mostly formed in Rigid lenses not in soft lenses. Because soft lenses are flexible that takes the shape of cornea, whereas Rigid lens create a space in between cornea and contact lens as they are less flexible.
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Convex Tear Lens formation in Steep of Rigid Permeable (RGP) Lens:
- In Steep fit, the periphery of the Contact Lens is attached with peripheral cornea and there is gap between central cornea and central contact lens.
- In this gap, tear accumulate, and a lens is formed which is called tear lens.
- As the contact lens is attached at periphery and create a gap in the center, it forms a convex tear lens (Precisely, Concavo-Convex Tear lens, where inner surface of the lens is concave- that is in corneal side and outer surface is convex- that is in contact lens side).
- A convex tear lens will converge the rays that will decrease the power of minus trial Contact lens, and it will increase the power of plus trial contact lens.
- So, in steep fit minus power of contact lens is decreased and plus power of contact lens is increased.
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Concave Tear Lens formation in Steep of Rigid Permeable (RGP) Lens:
- In Flat fit, the center of the contact lens is attached with the center of the cornea and there is gap between peripheral anterior surface of cornea and peripheral posterior surface of contact lens.
- In this gap, tear accumulate, and a lens is formed which is called tear lens.
- As the lens attached at center and create a gap in the periphery, it forms a concave tear lens (Precisely, Convexo-Concave Tear lens where inner surface of the lens is concave- that is in corneal side and outer surface is convex- that is in contact lens side).
- A concave tear lens will diverge the rays that will increase the power of minus trial Contact lens, and it will decrease the power of plus trial contact lens.
- So, in flat fit minus power of contact lens is increased and plus power of contact lens is decreased.
Remember: When a lens is not fit properly, it will create a tear lens and tear lens change the Effective power of the trial lens that we get in Over refraction.
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Step-1: Identifying Tear Lens Power in Contact Lens Fitting:
- The difference between base curve of Trial lens and base curve of cornea is the Tear Lens Power.
- The formula used to identify Tear Lens power:
Tear Lens = Base Curve of Trial Contact Lens – Base Curve of Cornea.
Example-1: Suppose Corneal Base Curve is 44.50D but the trial Contact lens base curve is 44.0D. What will be the Tear Lens Power?
- As the trial contact lens base curve is Flatter, it will create a Concave Tear lens, and the power of the concave tear lens will be:
- Tear Lens = Base Curve of Trial Contact Lens – Base Curve of Cornea.
- Tear Lens Power = 44.0D – 44.50D
- Tear Lens Power = -0.50D.
Example-2: Suppose Corneal Base Curve is 44.50D but the trial Contact lens base curve is 45.0D. What will be the Tear Lens Power
- As the trial contact lens base curve is Steeper, it will create a Convex Tear lens, and the power of the convex tear lens will be:
- Tear Lens = Base Curve of Trial Contact Lens – Base Curve of Cornea.
- Tear Lens = 45.00D – 44.50D
- Tear Lens = + 0.50D.
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Step-2: Identifying Effective Power of Trial Contact Lens:
- The summation of Trial lens Power and Tear Lens Power is the effective power of Trial contact lens.
- The formula used to identify Effective power:
Effective Power = Trial CL Power + Tear Lens Power.
Example-1: Suppose Corneal Base Curve is 44.50D and trial lens power is -2.0D but the trial Contact lens base curve is 44.0D instead of 44.50D. What will be the Effect Power of the Contact Lens?
- As the trial contact lens base curve is Flatter, it will create a Concave Tear lens, and the power of the concave tear lens will be:
- Tear Lens = Base Curve of Trial Contact Lens – Base Curve of Cornea.
- Tear Lens = 44.0D – 44.50D
- Tear Lens = -0.50D.
- So, the Effective Power will be:
- Effective Power = Trial CL Power + Tear Lens Power.
- Effective Power = -2.0 + (-0.50)
- Effective Power = -2.50D.
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Example-2: Suppose Corneal Base Curve is 44.50D and trial lens power is -2.0D but the trial Contact lens base curve is 45.0D instead of 44.50D. What will be the Effect Power of the Contact Lens?
- As the trial contact lens base curve is Flatter, it will create a Concave Tear lens, and the power of the concave tear lens will be:
- Tear Lens = Base Curve of Trial Contact Lens – Base Curve of Cornea.
- Tear Lens = 45.0D – 44.50D.
- Tear Lens = 0.50D.
- So, the Effective Power will be:
- Effective Power = Trial CL Power + Tear Lens Power.
- Effective Power = -2.0 + (+0.50).
- Effective Power = -1.50.
So, Concave tear lens will increase the effective power minus trial lens and convex tear lens will decrease the effective of minus trial contact lens.
And Convex tear lens decreases the effective power minus trial lens and concave tear lens will increase the effective of plus trial contact lens.
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Step-3: Identifying Over Refraction Power:
- The difference between the Prescription Contact lens power and the effective Power of Trial contact lens is called Over Refraction Power.
- The formula used to identify Over Refraction power:
Over Refraction = Prescription Power – Effective Power of Trial CL.
Example-1: A patient’s prescription power is – 2.50 DS and his K-reading is 44.25 DS. If a trial rigid lens of BC 44.50 and power – 2.00 is placed on the eye, what is the expected over-refraction?
Here,
- Trial Contact Lens Base Curve = 44.5D.
- Corneal Base Curve = 44.25D.
- Prescription Power = 2.50D
- Trial Lens Power = -2.0D
Step-1: Identifying Tear Lens power:
- Tear Lens = Trial Contact Lens Base Curve – Corneal Base Curve
- Tear Lens = 44.50 – 44.25
- Tear Lens = +0.25
Step-2: Identifying Effective Power of Trial CL.
- Effective Power = Trial Contact Lens Power + Tear Lens Power.
- Effective Power = -2.0D + 0.25
- Effective Power = -1.75D
Step-3: Identifying Over Refraction Power:
- Over Refraction = Prescription Power – Effective Power.
- Over Refraction = -2.50 – (-1.75)
- Over Refraction = -2.50 + 1.75
- Over Refraction = -0.75D
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Example-2: A patient’s K-reading is 43.50D. A RGP lens with a base curve of 43.50D & power of – 3.0D is placed on the eye. If the patient’s prescription power is – 2.0D sphere, what over-refraction would you expect with this lens in place?
Step-1: Identifying Tear Lens power:
- Tear Lens = Trial Contact Lens Base Curve – Corneal Base Curve
- Tear Lens = 43.50D – 43.50D
- Tear Lens = 0.00D
Step-2: Identifying Effective Power of Trial CL.
- Effective Power = Trial Contact Lens Power + Tear Lens Power.
- Effective Power = -3.0D + 0.00D
- Effective Power = -3.0D
Step-3: Identifying Over Refraction Power:
- Over Refraction = Prescription Power – Effective Power.
- Over Refraction = -2.0D – (-3.0D)
- Over Refraction = -2.0 + 3.0D
- Over Refraction = 1.0D
If Base curve of trial, contact lens and cornea is same then no tear lens will form and the effective power of the trial contact will not change. So, you can directly shift to step-3 to identify over refraction power.
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Example-3: A patient’s prescription power is – 2.50 DS and his K-reading is 44.25 DS. If a trial rigid lens of BC 44.50 and power – 2.50 is placed on the eye, what is the expected over-refraction?
Step-1: Identifying Tear Lens power:
- Tear Lens = Trial Contact Lens Base Curve – Corneal Base Curve
- Tear Lens = 44.50 – 44.25
- Tear Lens = +0.25
Step-2: Identifying Effective Power of Trial CL.
- Effective Power = Trial Contact Lens Power + Tear Lens Power.
- Effective Power = -2.5D + 0.25
- Effective Power = -2.25D
Step-3: Identifying Over Refraction Power:
- Over Refraction = Prescription Power – Effective Power.
- Over Refraction = -2.50 – (-2.25)
- Over Refraction = -2.50 + 2.25
- Over Refraction = -0.25D
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Example-4: A patient’s K-reading is 43.50D. A RGP lens with a base curve of 43.0D & power of – 3.0D is placed on the eye. If the patient’s prescription power is – 2.0D sphere, what over-refraction would you expect with this lens in place?
Step-1: Identifying Tear Lens power:
- Tear Lens = Trial Contact Lens Base Curve – Corneal Base Curve
- Tear Lens = 43.0D – 43.50D
- Tear Lens = -0.50D
Step-2: Identifying Effective Power of Trial CL.
- Effective Power = Trial Contact Lens Power + Tear Lens Power.
- Effective Power = -3.0D + (-0.50)
- Effective Power = -3.0D – 0.50D
- Effective Power = -3.50D
Step-3: Identifying Over Refraction Power:
- Over Refraction = Prescription Power – Effective Power.
- Over Refraction = -2.0D – (-3.50D)
- Over Refraction = -2.0 + 3.50D
- Over Refraction = 1.50D
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