Optometry Skills (OPT506) - RGP Lens Fitting

Optometry Skills (OPT506) - RGP Lens Fitting

• RGP Lenses: Rigid gas permeable contact lenses.
• Over Refraction: Refraction performed over the contact lens, used to refine BVS for the wearer and adjusting power when required.
• Flat Fit: Lens fitting characteristics including central touch, mid-peripheral pooling, excess edge clearance (>0.5mm), boundary between central touch and pooling, quick NaFl dissipation, unstable and inferiorly decentrating. Excessive movement on blinking with lens possibly crossing limbus. Typically poor comfort, adaptation severely compromised, and inconsistent vision.
• Steep Fit: Lens fitting characteristics including central pooling, trapped bubbles, excessive mid-peripheral touch, very narrow band edge clearance (<0.4mm), poorly defined boundary between central pooling and mid-peripheral touch, NaFl trapped under lens considerably. Limited or no movement on blink, and typically good initial comfort initially.
• Alignment: Apical clearance, slight mid-peripheral touch/alignment, narrow band edge clearance (0.5mm), boundary between centre and mid-periphery undefined.
• Centration: Good initial centration in primary and blinking, with gradual inferior movement with lids apart; 1-1.5mm of smooth vertical movement. Lens edge shouldn't move over limbus.

RGP Fit Assessment

• Fitting Characteristics (Recap): Flat, Steep fitting characteristics relating to fluorescein fit, center, mid-periphery, periphery and border regions, NaFl dissipation, centration, movement on blinking, coverage, and position.
• Over-refraction: Refraction over the contact lens, used to refine the best sphere and cylinder values and optimise vision.

Amending the Fit

• Alignment: No change to BOZR, increase or decrease TD depending on needs.
• Flat Fit Amendment: Reduce BOZR or Increase TD
• Steep Fit Amendment: Increase BOZR or reduce TD
• Decentred/Lid Attached Lenses: Increase TD (considering HVID & corneal astigmatism), increase lid attachment; Reduce TD (reduce lid attachment and lens centration).

Toric Corneas

• Comfort: Reduced when alignment area is reduced; excessive edge clearance in steeper meridians lead to unwanted lid interaction and discomfort; poor centration.
• RGP Behaviour: WTR - lens rocks along steeper meridian or decentrates inferiorly; ATR- lens decentres horizontally; lens flexures; corneal molding.
• When to Consider Toric Lenses: When corneal astigmatism is above 2.00D, or residual astigmatism above 0.75D (ocular minus corneal). Spherical lenses might become unstable with excessive decentrating, and the cornea might become significantly more toric toward the periphery. Large amounts of lens flexure should also be considered.
• Options for Toric Corneas: Alter BOZR based on astigmatism, minimize edge clearance in the steeper meridian; reduce TD; spherical center, toric periphery; aspheric RGP (narrower edge lift to reduce edge clearance); and toric RGP (variety of options including back, front surface toric, bitoric).

RGP and the Tear Lens

• Key factors: Air (n=1), Tear Lens (n=1.336), RGP (n=1.445), and Conea (n=1.376)
• Types of tear lenses: -ve tear lens, Plano tear lens, and +ve tear lens.

Fit Amendment – Flat/Steep Lens

• Flat: To amend a flat fit, steepen the lens through increasing sag. Reduce BOZR or Increase TD.
• Steep: To amend a steep fit, flatten the lens through decreasing sag. Increase BOZR or Reduce TD.

Adjusting the BOZR - Rules of Thumb

• BOZR increase: If increasing the BOZR (flatten), then increase the BVP by +0.25DS for every 0.05mm increase in BOZR
• BOZR decrease: If decreasing the BOZR (steepen), then increase the BVP by -0.25DS for every 0.05mm decrease in BOZR.

Adjusting the TD

• Increase TD to stabilise fit/reduce staining/increase lid attachment. Decrease TD to improve fit of spherical lenses on toric cornea / reduce lid attachment (encourage inter-palpebral fit), excluding amending a fit to steepen or flatten the fit..

Over-refraction

• Over-refraction Definition A refraction performed over the contact lens to refine the lens parameters.
• Procedure Details: VA measurement, Obtain BVS, Duochrome and ±1.00DS test, and finalize VA.
• Recording: Record the power of the final lens used in the trial frame during a refraction.

Over-refraction with RGP

• Alignment of RGP with tear lenses: When RGP lenses are aligned with the cornea, the tear lens does not create unintended power.
• Non-Aligned RGP lenses with Tear lens: If the RGP and cornea are misaligned the tear lens will create an over-power, with plus or minus lenses.

What to do next??

• Use the over-refraction (OR) results to determine corrections in BOZR needed.
• When OR is found to be less/more than the expected to be plano/plano result, the trial lens is determined to be steeper or flatter than the cornea, resulting in 0.05mm being added/subtracted for every 0.25D adjustment in power.

Additional Notes

• These notes cover various aspects of RGP lenses, from fitting characteristics to amending fits, and over-refraction.
• There are also techniques for determining lens parameters when the lens isn't perfectly fitted.
• Procedures are detailed for recording and interpreting results including adjusting the BOZR and TD as fitting lens parameters.

This quiz focuses on the fitting characteristics and techniques associated with rigid gas permeable (RGP) lenses. It covers over-refraction, flat and steep fit characteristics, and alignment considerations for optimal lens performance. Test your knowledge on the specifics of lens fitting and improve your optometry skills.