Centra PGA Keratoconus

Centra PGA Practitioner Fitting Guide

Applications

Design

Centra PGA Fitting Procedure

Optimal Fit

Step 1: Diagnostic Lens Selection

Objective: Select a base curve that achieves slight apical clearance over the apex of the cornea.

• Start with a base curve close to mid K. Consider starting closer to steep K, the larger the difference between flat and steep K.
• If there is apical touch, try a steeper BOZR until clearance is achieved.
• If there is excessive apical pooling and/or bubbles forming under the BOZD, flatten the BOZR until light clearance is achieved.

Note: You will need to assess the apical clearance whilst the lens is centered over the apex of the cornea.

Step 2: Paracentral Fit

Objective: To achieve adequate applanation on the paracentral cornea 360 degrees.

• If there is excessive stand-off in the paracentral area, order a decreased lift.
• If there is excessive pressure in the paracentral area, order an increased lift.
• If the applanation on the paracentral cornea is asymmetrical, consider a bi-sym, toric periphery, quad-sym or bi-toric.
• If a clear toric fitting pattern is observed, the lens consistently drops low, or there is excessive vertical movement, a toric periphery (TSP) may be required. A minimum amount of TSP to be considered is 0.6.
• See schematics at the end of the fitting guide that demonstrate TSP’s and Bi-syms.
• Please get in touch with one of our technical consultants for assistance or more information.

Step 3: Edge lift and Movement

Objective: To achieve an even edge width and lift and healthy lens movement for tear exchange.

A narrow edge width is typically associated with a paracentral area (cone angle) that is too steep, whilst an excessively wide edge lift is associated with a paracentral area that is too flat.

• A standard increased lift will increase the cone angle by 2 degrees.
• A standard decreased lift will decrease the cone angle by 1.5 degrees.
• The paracentral area can also be specified as an equivalent base curve.

E.g. A Centra PGA with a base curve of 7.0mm shows an optimal fit over the corneal apex but a tight paracentral and peripheral fit. A 7.0mm base curve can be ordered in combination with a mid-periphery (paracentral) of a 7.3mm base curve Centra PGA. This is equivalent to increasing the cone angle.

Edge lift (height) often goes hand-in-hand with edge width. However, if the edge width looks optimal but the height appears too low or high within the last 0.5mm of the overall lens diameter, a flatter or steeper RA (reverse aspheric geometry) can be requested.

• The standard RA effect of a Centra PGA is RA3.
• RA4 steepens the RA whilst RA2 flattens the RA.

N.B.
1) Edge lift is often best adjusted by changing the paracentral lift or cone angle rather than modifying the RA value.

2) The Centra PGA has a wider peripheral zone width incorporated into its reverse aspheric design than is used in many other contact lens designs.

Evaluating Vision

• Perform a sphere-cyl over-refraction over the chosen diagnostic lens.
• The final BVP is a combination of the trial lens BVP + over-refraction found.
• Consider the best sphere power if a low magnitude of cylinder is found.

Centra PGA Bi-sym and Toric Periphery (TSP)

WARRANTY

Corneal Lens Corporation NZ LTD guarantees all CLCNZ made lenses against all defects in materials and manufacturing for 30 days from the original invoice.