Spherical Soft Contact Lens Fitting & Assessment

Complete process of Soft Contact lens fitting can be divided into 3 segments:

• A. Baseline Measurement.
• B. Trial Lens parameter choice.
• C. Fitting Assessment.

A. Baseline Measurement in Soft Contact Lens Fitting:

• In baseline measurement, we evaluate the eligibility of patient’s eye whether patient’s eyes are healthy enough to wear contact lens. Following tests will be done in baseline measurement:
  • 1. Slit-Lamp Examination.
  • 2. Horizontal Iris Visible Diameter.
  • 3. Palpebral Fissure Height (PFH).
  • 4. Pupil Size Measurement.
  • 4. Keratometry.
  • 5. Additional Tests.

1. Slit-Lamp Examination:

• To identify any pathology in eye and its surrounding structures.
• Observe Accordingly: Eyelash –> Lid Margin –> Conjunctiva –> Limbus –> Corneal Tear Film –> Cornea –> Anterior Chamber –> Pupil Lens.

2. Horizontal Iris Visible Diameter (HVID):

• HVID is important for identifying the total diameter of soft contact lenses.
• It can be measured horizontally through the center of cornea by several instruments even with normal mm scale in primary gaze.
• Available Methods are:
  • Normal mm scale.
  • Slit-lamp.
  • Corneal Topographer.
  • Vernier Scale.

• Normal Value of HVID:
  • Normal HVID varies 10-14 mm.
  • Average HVID 11.7 mm.

3. Palpebral Fissure Height (PFH):

• It is the distance between upper and lower lid margin vertically through the center of cornea when patient looking at the primary gaze.
• Higher PFH patients are suggested for large diameter contact lens and vice versa.
• Available methods are:
  • Normal mm scale.
  • Slit-lamp.
  • Corneal Topographer.
  • Vernier Scale.

• Normal Value of VVID:
  • Normal PFH varies 9-11 mm.
  • Average PFH 10 mm.

4. Pupil Size:

• Pupil size is important to identify optical zone of the soft contact lens.
• Normal Pupil Size:
  • In Bright Light: 2-4 mm.
  • In Dim Light: 4-8mm.
• In contact lens practice pupil size is measured in dim light condition.

Optical Zone (OZ) = Pupil Size (dim light) + 2 mm

• Available Methods are:
  • Normal mm scale.
  • Corneal Topographer.
  • Vernier Scale.

5. Keratometry:

• To identify Base Curve (B.C) of the soft contact lens.
• Formula:

B.C = 337.5/D

• Here, D = Dioptric value of Avg K

6. Additional Tests:

a. Iris Color.

b. Tear Film Evaluation:

• Schirmer Test, 
• Tear Break-Up Time Test, 
• Tear Meniscus Height, 
• Non-invasive Tear Break-Up Time Test.

B. Trial Lens Parameter Choice:

• In this step we will calculate 3 parameters to identity a trial lens that will be fitted in patient’s eye.
• Later in step-3, we will assess the fitting outcome whether the fitting is optimal, steep or flat fit.
• Following parameters need to be calculated to identify a trial lens:
  • 1. Base Curve (B.C).
  • 2. Total Diameter (TD).
  • 3. Estimation of Contact Lens Power.

1. Base Curve (B.C):

• Base curve is the radius of curvature of back surface of the contact lens.
• Base Curve should be 0.6 to 1.0 mm flatter than average K reading.
• For beginner it’s better to start with 1.0 mm flatter than the average K reading.
• B.C. is identified by using the formula:

B.C = 337.5/D (D= Dioptric value of Avg K)

• Suppose Keratometer readings are: K1 44D @ 180 & K2 46D @ 90, will be the Base Curve?
• Avg K is = K1 + K2/2 or 44 + 46/2 or 90/2 or 45.
• So Average K is 45D.
• According to our formula Base Curve will be:

B.C = 337.5/45= 7.50mm.

• Now as we have to choose 0.6 to 1.0 mm flatter Base curve, so our final Base Curve will be,

7.50+1 mm = 8.50 mm

• So, the final Base Curve of this Soft Contact Lens is 8.50 mm.

2. Total Diameter (TD):

• Total Diameter is the width of the contact lens from edge to edge through the center of CL & it is expressed in mm.
• The Total Diameter (TD) should be 2 mm greater than the HVID.
• Formula:

TD = HVID + 2 mm.

• If HVID is 12 mm than our Total Diameter will be:

12+2 = 14 mm

• So, Total Diameter of this soft contact Lens is 13 mm.

3. Estimation of Contact Lens Power:

• Contact Lens power is estimated using the formula:

Contact Lens Power= Spectacle Power/ 1- d X Spectacle Power

• Here; d = Vertex Distance in meter.
• Suppose Spectacle power is  OU -5.0D and Vertex Distance is 12 mm

Here,

• Spectacle Power = -5.0D
• Vertex Distance = 12 mm or 0.012 m (12 / 1000)
• According to formula,
  • Contact Lens Power = -5/1- {0.012 x (-5)}
  • Contact Lens Power= -5/1+ (0.012 x 5)
  • Contact Lens Power= -5/1+ 0.06 = -5/1.06 
  • Contact Lens Power= -4.72D or 4.75D
• So, estimated contact power for -5D is – 4.75D

• Finally, Our Trial Lens will be:
  • Base Curve: 8.5 mm
  • Total Diameter: 13 mm
  • CL Power: 4.75DS

C. Fitting Assessment:

• In this step, we examine the trial lens fitting on patient’s eye.
• Our target is to get a optimal fit but if the fitting is steep or flat than we have to make modifications in the trial lens parameters to get an optimal fit.
  • 1. Centration.
  • 2. Coverage.
  • 3. Movement (Vertical).
  • 4. Lag
  • 5. Push Up.
  • 6. Comfort.
  • 7. Edge.
  • 8. Modification.

• Before going for fitting assessment, lets know in short about what are the ways of fitting a contact lens:
• Contact Lens fitting can be divided into two types:

1. Empirical:

• Empirical fitting involves fitting of contact lens based on total diameter, keratometer readings, back vertex calculation and manufacturer guidelines without use of any trial contact lens.

2. Diagnostic Fitting:

• Diagnostic fitting involves fitting of contact lens based on total diameter, keratometer readings, back vertex calculation & manufacturer guidelines with use of trial contact lens on patient’s eye before the final prescription.
• Diagnostic Fitting is further divided into two:

a. Dynamic Fitting:

• Type of Diagnostic fitting where CL fitting is assessed without any dye (Fluorescein).

b. Static Fitting:

• Type of Diagnostic fitting where CL fitting is assessed with dye (Fluorescein).

Here, We will discuss about Diagnostic Fitting (Dynamic) assessment of Soft Contact Lens

• The outcome after fitting of a contact lens can be:

• A. Optimal Fit/Normal Fit.
• B. Steep Fit/Tight Fit.
• C. Flat Fit/ Loose Fit.

B. Steep Fit/Tight Fit:

• Lens edge is tightly attached to the cornea as compared to the central portion. 
• Tight peripheral Attachment

C. Flat Fit/ Loose Fit:

• Central portion of lens is attached with central cornea and edge is floating.
• Tight Central Attachment.

a. Centration:

• Centration means superimposition of the center of cornea and center of contact lens.
• The amount of limbal overlap is normal.
• Corneal coverage is maintained in all positions.
• In optimal and steep fit lenses will be centered.
• In flat fit lenses will be de-centered.

b. Coverage:

• About 1 mm symmetrical overlap is ideal.
• Ideal coverage is important for:
  • Shape regularity.
  • Prevent trauma to cornea, limbus & conjunctiva.
  • Prevent corneal exposure & desiccation.
  • Prevent tear film disturbance.
• In optimal fit, it should be complete cover.
• In steep fit de-centered and poor recovery.
• In flat fit, often rides low. 

c. Movement (Vertical):

• Movement is required due to:
  • Removes & disperses ocular debris.
  • Promotes tear exchange.
  • Epithelial wetting by mucin spreading.
• Lens movement depends on:
  • Lens type.
  • Lens design.
  • Lens material.
  • Fitting relationship.
  • Lid factors.
  • Anterior eye topography.
• In optimal fit, 0.3 mm.
• In steep fit, less than > 0.2mm.
• In flat fit, more than > 1.0 mm.

d. Lag:

• Lag refers to the resistance of the lens to move with the eye in any gaze from its primary gaze.
• Lens centration is noted with the eye looking up-gaze while patient is instructed to maintain their eye position and not to blink.
• In optimal fit, 1.5 mm.
• In steep fit, little or none
• In flat fit, more than > 2.0 mm.

e. Push Up:

• In this test the lens is pushed upward with  the help of lower lid and then lens recovery is observed.
• In optimal fit, Quick recovery (40-60%)
• In steep fit, Slow or Nil recovery (65-100%)
• In flat fit, Quick recovery (35-10%)

f. Comfort:

• In optimal fit, Good.
• In steep fit, Initially Good.
• In flat fit, Poor.

g. Edge:

• In optimal fit, No indentation.
• In steep fit, indentation.
• In flat fit, fluting.

h. Modification:

• In optimal fit, Not required.
• In steep fit, Flatter BC/decrease TD
• In flat fit, Steeper BC/increase TD

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