OPT506 - L08 - RGP Over Refraction 2024-25 PDF

RGP Over refraction, the tear lens and fit amendment Dr Asma Zahidi Recap – RGP fit assessment / Edge position What else? / Edge position What you have learned so far… 1. Obtain the baseline measurements for selection of RGP lens  Keratometry  HVID  Pupil size...

RGP Over refraction, the tear lens and fit amendment Dr Asma Zahidi Recap – RGP fit assessment / Edge position What else? / Edge position What you have learned so far… 1. Obtain the baseline measurements for selection of RGP lens  Keratometry  HVID  Pupil size 2. Determine the initial trial lens for RGP fitting  BOZR  TD  BVP 3. Insertion and removal of RGP lens 4. Fit assessment of RGP lens  Dynamic Fit  Static Fit  Comfort  Vision  Fit conclusion What’s next??? Amending the fit Alignment- no change of BOZR  unless TD / BOZD needs changing Flat  Reduce BOZR Steep  Increase BOZR Decentred / lid attached lens  Increase TD (consider HVID & corneal astigmatism first, also increased lid attachment)  Reduce TD (can affect lens centration & reduce lid attachment Consider the amount it needs changing by Toric corneas Fig 15.3 Fluorescein patterns of rigid lenses of varying BOZR on with-the-rule corneas of varying astigmatism (From Contact Lens Practice, Nathan Efron (3rd Ed)) Toric corneas  Comfort reduced when area of alignment is reduced  Excessive edge clearance in steeper meridian will lead to unwanted lid interaction with the lens and discomfort  Poor centration  WTR – lens rocks along steeper meridian or decentre inferiorly  ATR – lens decentres horizontally  Lens flexure  Corneal moulding Toric corneas Consider toric lens when  Greater than 2.00D corneal astigmatism  More than 0.75D residual astigmatism (ocular astigmatism – corneal astigmatism)  Spherical lens unstable, excessive decentring  Patients cornea become significantly more toric towards the periphery  Large amounts of lens flexure Options for toric corneas  Altering BOZR depending on amount of corneal astigmatism  Minimises edge clearance in steeper meridian  Reduce TD  Minimises exaggeration between 2 different meridians to reduce edge clearance in steeper meridian  Spherical centre, toric periphery RGP  Considered when peripheral cornea is more astigmatic and reducing TD proved ineffective Options for toric corneas Aspheric RGP Narrower edge lift to reduce edge clearance in steeper meridian Toric RGP Variety of different options – back surface toric, front surface toric, bitoric  RGP and the tear lens The Tear Lens Air (n=1 ) Tear lens (n=1.336) Front radius CL back surface Backradius Cornealshape RGP Thickness depends (n=1.445) on the fit Conea (n=1.376) RGP and the tear lens -ve tear lens Plano tear lens +ve tear lens Fit amendment – flat lens ↓ BOZR Sag To amend a flat fit D Effectively Steeper Fit amendment – steep lens Sag ↑ BOZR To amend a Radius steep fit flatten the lens by D >R decreasing sag Flatter Original fit ↓ sag Increase BOZR R ↓ TD or D R Reduce TD than expected Rx: -3.00DS OR  Determine how much the trial lens is Keratometry: 7.80@170 / 7.82@80 steeper than the cornea Trial lens: 7.85/9.3/-3.00DS  Every 0.05mm = 0.25D Fit conclusion: Steep fit Expected OR: plano (1.00D / 0.25D) x 0.05mm = 0.20mm OR: -1.00DS Flatten lens by 0.20mm New BOZR = (7.85mm+0.20mm)=8.05mm  What to do next??? Use the OR to determine how much you need to change the BOZR Example 2:  OR found is +0.50DS > than expected Rx: -3.00DS OR Keratometry: 7.80@170 / 7.82@80  Determine how much the trial lens is steeper than the cornea Trial lens: 7.85/9.3/-3.00DS  Every 0.05mm = 0.25D Fit conclusion: Flat fit Expected OR: plano (0.50D / 0.25D) x 0.05mm = 0.10mm OR: +0.50 Steepen lens by 0.10mm New BOZR = (7.85mm – 0.10mm) = 7.75mm  Try this out! Use the OR to determine how much you need to change the BOZR in the following case Rx: -4.50DS (BVD 12mm) Keratometry: 7.50@170 / 7.70@80 Trial lens: 7.85/9.3/-3.00DS Fit conclusion: Flat fit OR: -0.75DS  Solution Rx: -4.50DS (BVD 12mm) Step 1: Compensate Rx for BVD Keratometry: 7.50@170 / 7.70@80 Trial lens: 7.85/9.3/-3.00 Fit conclusion: Flat fit Step 2: Determine the expected OR if alignment fit OR: -0.75DS Step 3: Determine the difference between the expected OR and OR obtained Step 4: Determine the amount of BOZR to alter  Solution Rx: -4.50DS (BVD 12mm) Step 5: Determine the final BVP Keratometry: 7.50@170 / 7.70@80 Trial lens: 7.85/9.3/-3.00 Fit conclusion: Flat fit OR: -0.75DS Adjusting the TD of an alignment fit Adjusting the TD  Increase TD to stabilise fit / reduce 3 & 9 o’clock staining / increase lid attachment  Decrease TD to improve fit of spherical lens on toric cornea / reduce lid attachment (encourage inter-palpebral fit) Not in the case of amending a fit (ie. to steepen or flatten the fit) Amending TD In the video above, the CL is decentred due to In the video above, although there is some the lid attachment and the lack of movement movement on blink, the CL is decentred on blink. superiorly due to the lid attachment which can affect the vision when the px blinks as the optical zone further from the center of the pupil. Amending TD CL is decentred inferior-nasally. Vision affected? CL is decentred inferior-nasally. Vision affected? Cross limbus on up-gaze? Reduce TD and change Lack of lens movement could be due to steep fit? BOZR if fit is alignment Change BOZR then check centration. If still decentred then change TD Adjusting the TD- equivalencies Sag If we change the TD we alter the sag of the lens Therefore, we must change the BOZR to maintain equivalency (in order to keep alignment fit) Radius (BOZR) Rule of thumb For every 0.50mm change in TD, change BOZR by 0.05mm to maintain the equivalent fit Example  7.85 / 9.00 / -2.50DS – Alignment fit  Lens doesn’t centrate very well so we choose to increase TD to 10.00mm to help lens centration What BOZR is needed to maintain an equivalent alignment fit?  Increase TD by 1.00mm, increases sag effectively steepening lens so we must increase BOZR by 0.10mm to compensate (flatten) & maintain equivalent alignment fit ↑ TD Final lens specification: 7.95 / 10.00 / -2.50DS R >D Effectively Steeper Examples ↓ TD 8.00 / 10.00 / +3.00DS – Alignment fit R Lens is too large on cornea so reduce TD to < 9.00mm D Reduce TD by 1.00mm, reduce BOZR by 0.10mm Effectively Flatter Final lens specification 7.90 / 9.00 / +3.00DS 7.75 / 8.50 / -4.00 DS– Alignment fit increase TD to 10.00mm ↑ TD Increase TD by 1.50mm, increase BOZR by 0.15mm Final lens specification 7.90 / 10.00 / -4.00DS R >D Effectively Steeper Rules of thumb For every 0.50mm increase in TD, increase BOZR by 0.05mm to maintain the equivalent fit Recording your findings Over-refraction R VA OR Final VA L VA OR Final VA 6/9 +0.50DS 6/6 6/7.5 -0.25DS 6/6 Final Bin VA after OR: Fit assessment RE LE Dynamic fit: Record Lid interaction Edge position centration x here with MOB an ‘x’ Static fit: Draw the NaFl Center pattern here in Mid periphery GREEN Periphery Fit conclusion: Flat / Alignment / Steep Fit conclusion: Contact Lens Specification – Lenses issued / lenses to be ordered Manufacturer and lens name BC TD Sph Cyl Axis RE B&L Maxim Fit 7.80mm 9.00mm -2.50DS - - LE B&L Maxim Fit 7.80mm 9.00mm -3.25DS - - Action Prescribe or refit? Which eye? If re-fit – what are the parameters of the lens to be refitted? Why are you re-fitting with the parameters above? THE OSCE Try out some RGP fitting exercise at the following link: https://xerte.plymouth.ac.uk/xerte/pl ay.php?template_id=5994 Questions?