Contact Lenses: Fit Assessment

RGP

• Let lens settle for a few minutes
• Can check centration, TD and movement under white light first - want good centration, staying within limbus, slow and smooth drop on blink, TD should allow ~1mm each side
• Then check flu pattern w/blue light - areas of dark blue = touch and brighter green = clearance with thicker tear film

Alignment Fit

• Even thickness of tears
• Good edge
• Hint of apical clearance
• Centres well
• Drops slowly & smoothly
• Stays within limbus in all dirs of gaze

Steep Fit

• Central pooling suggesting BOZR is less than k reading
• Inadequate edge
• Centres well
• Comfort often fine
• Drops slowly
• +ve liquid lens
• Could see bubble if very steep

Flat Fit

• Central touch and wide edge
• Discomfort
• Centres poorly and moves too much
• Drops quickly and in arc shape on push-up
• -ve liquid lens

Strategies

• Lid Attachment - common now that GP lenses are bigger. Has good comfort. The edge of the lens is in contact with the lid during blinking and in the primary position
• Lid attachment is the natural fit w/minus lenses and gives good tear exchange and comfort. It's more difficult with a plus lens (a -ve carrier is an option). Beware of corneal exposure and moulding
• Interpalpebral was more common when more people were using the smaller PMMA lenses. The centration is good and there's less flare. This strategy is useful if the patient has an irregular peripheral cornea as it only fits the regular central part.
• Alignment fit is used for modern lenses - multicurves, aspherics
• Apical clearance gives better centration if using a small lens.

Overrefraction

• A steep lens results in a positive tear lens so the minus power of the lens itself needs to increase
• A flat lens results in a negative tear lens so the plus power of the lens itself needs to increase

Edge Clearance

• This is the gap between the cornea and the lens edge
• Poor EC = stagnant tears, binding, staining, discomfort, hard to remove
• Good EC = easy removal of lens, good tear exchange, improves lens movement

Soft Lenses

• Can first check comfort and vision - is vision same as best spec Rx?
• If the lens blurs straight after blinking and then clears = flat fit
• If vision starts off clear and then blurs this suggests the lens is too steep
• Can do this subjectively or with keratometer
• Assess coverage (lens 1mm over each side ideally)
• Centration - if bit off to one side but all of cornea is still covered = acceptable. If cornea not covered can either fit a larger TD or smaller BOZR
• Good blink induced movement is 0.2-0.5mm upwards lens recovery straight after blink
• Excess blink induced movement is >0.5mm upwards recovery following blink - fit larger TD or smaller BOZR
• Inadequate = <0.2mm>
• Lag on upgaze - similar params to blink induced movement
• Push-up Test - lens should move w/slight resistance when pushed through lids
• If lens is totally free then larger TD or smaller BOZR
• If movement is sluggish then smaller TD or larger BOZR

Using K Mires

• STEEP - Mires immediately clear post-blink due to lid compression of lens, then mires blur as lens distorts
• FLAT - Mires are blurred immediately post-blink due to prismatic effect of lens movement, then mires clear and lens stabilises

Effect of TD/BOZR on Sag

• Greater BOZR = less sag, flatter fit, looser lens
• Greater TD = more sag, steeper fit, tighter lens

Misc

• Thinner lenses move less than thicker lenses
• Spun cast lenses move less than lathe cut/molded