RGP Lens Design Impact on Fit (OPT506) 2024-25 PDF

RGP Impact of lens design on fit Dr Asma Zahidi Principles of RGP design Fitting Sets vs Empirical Fitting Empirical Fitting Fitting Sets  Order what you like  On the spot...

RGP Impact of lens design on fit Dr Asma Zahidi Principles of RGP design Fitting Sets vs Empirical Fitting Empirical Fitting Fitting Sets  Order what you like  On the spot  More than one session  Fitting in one session  Various BOZRs (0.05)  Limited BOZRs (0.10)  Various TDs  Usually a fixed TD  Power to suit  Usually a fixed power  ‘Made to Measure’ option Principles of RGP design  Single curve  Bicurve – C2  Tricurve – C3  Tetracurve/Multicurve – C4  Aspheric  Constant Axial Edge Lift (CAEL) Single curve First lens design 1947, PMMA single curve  Fitted 0.3mm flatter than flattest K Limited success due to physiological problems  Lens did not fit properly Cornea isn’t spherical, increased flattening towards the limbus Improvements made by adding peripheral curves to lenses 3mm area of cornea curvature measured RGP with K reading 12mm Bicurve (C2) lens Consists of central radius and one flatter peripheral curve r0 r1 ØT BVP E.g. 7.80/8.60/-3.00D Sharp transition between curves Tricurve (C3) lens Consists of central radius and flatter peripheral curves E.g. 7.70 / 8.50 / 9.65 /-3.00D Basic design of most modern lenses Final peripheral curve is much flatter than the first peripheral radius Multicurve Forces controlling lens design  Center of gravity  Frictional forces  Capillary attraction  Specific gravity  Thickness and lenticulation  Refractive index of materials  Edge shape Centre of gravity Minus lens Plus lens Steep lens Flat lens Frictional forces  Viscosity of tear film  Thinning of tear film/increase in aqueous component reduces centration ability Capillary attraction  Good alignment = good capillary attraction = stability  In reality, we are looking to achieve a balance  Flat lenses have less capillary attraction  Steep lenses create negative pressure (suction effect)  Tear meniscus at edge aid in centration (the greater the meniscus, the greater the centration Steep Alignment Flat Specific gravity  Lenses of the same specifications but with different specific gravity may behave differently on the eye  Gravitational forces greater than fluid forces may cause the lens to drop  Prism ballast can contribute to centration Thickness and lenticulation Thickness Lenticulation >-6.00D or +4.00 BVP Reduce excess thickness and mass Design Make the front optic zone diameter (FOZD) Material smaller BOZD should be 0.50mm smaller than FOZD Carrier can be plano, + or - Refractive index of materials The higher the refractive index, the thinner the lens can be made PMMA (1.49) CAB (1.47) High refractive index plastics are used Silicon acrylate (1.47-1.48) for bifocal segments Fluorosilicon acrylate (1.42-1.53) Silicone (1.43) A material with high refractive index can provide lenses with reduced weight to improve fitting characteristics RGP lens design EC = edge clearance REL = radial edge lift AEL = axial edge lift EC is the gap between the cornea and the back surface of the peripheral curves (observed during fluorescein fit) EL is a design characteristic of the lens – definable in either axial or radial form Edge clearance and edge lift Edge clearance - gap between the cornea and the back surface of the peripheral curve (observed during fluorescein fit) Edge lift - design characteristic of the lens degree of flattening of the lens Specified as the distance of the lens edge between the extension of the BOZR and the peripheral curve  Radial edge lift – measured along radius  Axial edge lift – measured along axis of lens Edge shape  Related to comfort  Must be smooth and well finished  Blend into final peripheral curve  Can help lens removal What we are aiming for… Alignment fit (once lens has settled) Adequate centration no limbal overlap in excursions, BOZD centred over pupil 1-1.5mm smooth, vertical movement with blink stable VA, comfortable, effective tear exchange Acceptable comfort which improves after adaptation The RGP fit Recap…. Alignment fit  Central alignment or slight apical clearance over central region  Mid peripheral alignment/ slight touch  Narrow, even band of edge clearance approx. 0.5mm width / Edge position Dynamic fit assessment Centration Position of centre of lens compared to centre of cornea Recorded on fitting cross May depend on lid geometry Lid interaction Interpalpebral Lid attached Low riding Edge position Crossing or touching limbus in excursions of gaze Dynamic fit assessment Lid interaction  Inter-palpebral  Lid attached  Low riding Dynamic fit assessment Movement on blink  Lens movement  Direction: Vertical / oblique/ diagonal  Speed: Fast / average/ slow  Type: Smooth / apical rotation/ arcuate/ jerky/ rocky  Quantify movement in mm  Very flat lenses may move diagonally or arcuate  Spherical lens on toric cornea may move arcuate  Often depends on lid geometry and corneal toricity Dynamic fit - MOB Observation Dynamic fit - MOB Observation Dynamic fit - MOB Observation Static fit- fluorescein pattern  Fluorescein & cobalt blue / Burton lamp  Be careful of using too much/little fluorescein  Wratten filter  Consider  Central, mid periphery & periphery  Boundaries between them  Avoid using term “staining” for pooling  Use green pen/ pencil / highlighter  Label diagram & quantify thickness Static fit (Fluorescein pattern descriptions) The centre  Pooling - fluorescein under the centre of the lens  Touch - no fluorescein under the centre of the lens  Apical clearance – ideal amount of fluorescein under lens The mid periphery  Touch / pooling / alignment  Width  Boundaries The periphery  Edge clearance Mid Centre Periphery periphery  Width Static fit- fluorescein pattern Observe how pattern changes with tear exchange Steep fit looks the same for number of minutes, alignment fit may only look the same for a few seconds Why? Fluorescein may not be able to pass under a very steep lenses with poor edge clearance Avoid misinterpretation by re-centering the lens Flat, decentred lens can look like a steep fluorescein fit Estimating the amount of movement & width of NaFl pattern Vertical movement Width of NaFl pattern 7.5 mm 5.5 mm 4.5mm 3 mm 0.75 mm 1.5 mm 1.5 mm 7.5 mm 0.75 mm 3 mm 5.5 mm 4.5mm “Typical” fluorescein patterns Fit Centre Mid-periphery Periphery Very narrow edge Steep Central pooling Central touch clearance 0.5mm Fluorescein pattern recording Draw and label your observation for each of the fluorescein pattern above Static fit- fluorescein pattern Dimple veiling Simplified fluorescein fit of steep RGP Cornea- following lens removal Bubbles within area of central pooling Uniform fluorescence Negative staining- of tear film epithelium temporarily indented Fit assessment - dynamic fit Movie clip- Fit 1 Centration Lid interaction Movement Edge position Fit assessment- static fit 1 Centre Mid periphery Periphery Fit conclusion Lens specification 7.80/9.3/-3.00 Combine your observation of the dynamic fit from the video of Fit 1 and the static fit 1  What is your fit conclusion?  How would you amend the fit?  What would be the new lens specification? Fit assessment- dynamic fit Movie clip- Fit 2 Centration Lid interaction Movement Edge position Fit assessment- static fit Centre Mid periphery Periphery Fit conclusion Avoid making a decision before carefully assessing the fluorescein fit Decide if lens is flat / alignment / steep Unsure if alignment?  try steepening fit to confirm  easy to recognise steep fluorescein pattern 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 Questions?