2. LENS DESIGN

3. Base Curve Radius

1. Selected to optimize the lens-to-cornea fitting relationship
2. Often fitted close to the flatter keratometry reading (i.e., "On K")
3. BASE CURVE SELECTION (Minus lenses)

Corneal Cylinder Fit

Pl – 0.50D 0.50D – 0.75D Flat

0.75D – 1.00D 0.25D – 0.50D Flat

1.25D – 1.50D On "K" – 0.25D Flat

1.75D – 2.00D 0.25D Steep

2.25D – 2.50D 0.50D Steep

2.75D – 3.00D 0.50D – 0.75D Steep

As a result of the more anterior center of gravity of plus lenses, hyperopes should be fitted 0.25D – 0.50D steeper than the BCR’s recommended for myopic patients.

4. Lens movement upon blinking is important to both:

3. Peripheral Curve Radii/Width

1. Functions:

•Ensure adequate tear pump

•Debris removal

•Adequate Fit

•Eliminate Bearing

2. Edge Lift/Clearance

1. Edge Lift
2. Peripheral Corneal Shape

Edge Lift – geometrical calculated values from lens edge to cornea; it is most often used as it is more easily determined. It approximates edge clearance although typically greater.

Axial Edge Lift – vertical distance from lens edge to an extension of the BCR

Radial Edge Lift (rarely used) – distance from lens edge perpendicular to an extension of the BCR

 

 

 

 

 

 

 

 

 

 

 

 

 

•Excessive edge lift/clearance acts in a funnel-like manner – drying out the surrounding tear pool

•"Lid Gap" may further exaggerate process

3. Edge lift is decreased via:

1. Peripheral Curve Design

•Recommended peripheral curve close to 11mm (not 12 – 12.25mm as with PMMA)

•Recommended peripheral curve width = .2-.3mm (not .4-.6 as with PMMA)

•Recommended several peripheral curves (not one)

 

Tetracurve Design (8.8mm + OAD)

SCR = BCR + 0.8mm/0.3mm

ICR = SCR + 1.0mm/0.2mm

PCR = ICR + 1.4mm/0.2mm

Example:

BCR = 8.01mm

SCR/W = 8.80mm/0.3mm

ICR/W = 9.80mm/0.2mm

PCR/W = 11.20mm/0.2mm

Tricurve Design (8.80mm OAD)

SCR = BCR + 1.0mm/0.3mm

PCR = SCR + 2.0mm/0.3mm

Example:

BCR = 8.01mm

SCR/W = 9.00mm/0.3mm

PCR/W = 11.0mm/0.3mm

2. Blend

3. Center Thickness

1. Important Considerations

1. thin (flexure) vs. thick (inferior decentration)
2. increases with increased Dk
3. increases with increasing corneal astigmatism
4. verify!!!!!

1. Specific Center Thickness Values:

1. Two Rules of Thumb

3. Edge Design

1. Thin, tapered edge preferred
2. Great variation between (and within) laboratories (Morris,Lowther study)
3. Evaluate frontally and in profile – look for bluntness, sharp or microchipped edges
4. Inspection very important: 2 methods

1. Comparator (Paragon Optical Ur-O-Vue) = preferred
2. Contact Lens Edge Profile Analyzer – attachment to radiuscope – provides good magnification but only profile

1. Lenticular Designs:

1. Plus lenticular to thin a high minus edge for –6.00D and greater powers
2. Minus lenticular to increase edge thickness for all plus and low (-1.50D and less) powers

3. Minus Versus Plus Lens Design Parameters

Parameter Minus Power Plus Power

BCR Flatter than "K" Steeper than "K"

OAD Smaller (8.8-9.6mm) Larger (9.2-9.8mm)

CT Lesser (<0.20mm) Greater (>0.20mm)

Edge Design Thicker = +lenticular Thinner = -lenticular

For all (>-6D) necessary for all powers

powers; - lenticular

with low (-1.50D)

powers

4. Other Design Considerations

1. Aspheric Designs

1. Definition – an aspheric posterior design consists of a gradual flattening of the lens to better approximate the corneal topography. For single vision lenses, an elliptical posterior surface is present.
2. Eccentricity – a measure of the rate of corneal flattening; ranges from .5 (similar to cornea to 0.8 – 1.1 (presbyopic designs). As the elliptical base curve flattens toward lens periphery, it has the optical effect of adding plus power to the lens
3. Claimed aspheric advantages:

1. Improved lens-to-cornea fitting relationship
2. Better pressure distribution
3. Comfort
4. Reduced flare
5. Valuable for atypical corneas

1. Claimed aspheric disadvantages:

1. Reduced vision
2. Variable vision with decentration
3. Difficulty in fabrication
4. Difficulty in verification
5. Different fitting techniques

1. Topical Anesthetic Use

1. Controversial
2. Concerns: Staining, effect of eye rubbing, misleading patient
3. Potential Benefits: Improved initial comfort, less reflex tearing, less initial chair time, greater patient satisfaction
4. Pilot Study (Schnider & White): Greater east of adaptation, patient satisfaction and no staining problems
5. Multicenter Study: 80 subjects (40 placebo at fit; 40 anesthetic). One month study with first-time wearers, 8/10 dropouts were placebo subjects. Other results similar to pilot study.
6. Recommended for all first-time wearers, especially apprehensive, children and keratoconics

1. Designing for Comfort

1. Diameter: several studies have confirmed that large diameter (i.e., 9.5-10.5mm) are more comfortable initially, especially if a good lens-to-cornea fitting relationship is present
2. Blend: not a very important factor; nevertheless, the enhanced tear flow and fitting relationship created by blended pc’s may enhance initial comfort.
3. Lid Attachment edge (i.e., Fluorocon 9.5mm diameter Korb design) may be beneficial when lenses are decentered inferiorly.
4. Lenticular Designs: +lenticular for high minus designs; -lenticular for plus and low minus power lens designs.
5. Thin lens design: verify center thickness and edge (according to survey by Bennett and Grohe, 44% of practitioners do not verify center thickness.
6. Edge Design: The most important factor must be carefully verified although edge quality is getting better.

2. FITTING

1. Evaluation

1. Fluorescein Patterns

1. Alignment – light, even pooling of fluorescein of fluorescein; dense peripheral pooling due to flat PCR
2. Apical Clearance – steep fitting relationship – BCR usually 0.50D steeper than "K". Mid-peripheral bearing is present.
3. Apical Touch – Flat fitting relationship – BCR is usually 0.50D or more flatter than "K". Mid-peripheral pooling is present.
4. Astigmatic – dumbbell pattern due to absence of alignment via high corneal toricity
5. False Fluorescein Patterns:

1. Small Corneal Cap = steep pattern
2. Steep BCR = Adherence
3. Steep PCR = peripheral sealoff
4. Quick dissipation of fluorescein
5. Psuedosteep pattern with high minus lenses – thickness of edge blocks fluorescence

1. Methods of Observation

1. Biomicroscope

1. Most effective
2. Flexibility with magnification, illumination, and beam width
3. Use low magnification, wide beam, high intensity
4. Used with cobalt filter, Wratten #12 filter

1. Burton Lamp

1. Ultraviolet Fluorescent Lamp
2. Utilizes a +5.00 lens
3. Inexpensive, simple and allows good overall view
4. Does not allow for variable magnification or illumination and is ineffective with materials incorporating UV filters

1. Use Diagnostic Lenses
2. Fluorescein Application
3. Alignment does not equal "on K"

•Remember: the cornea gradually flattens from center to periphery; the base curve does not

4. Attempt to achieve alignment; if not accept the "least bad" fitting relationship
5. Lens movement is always along the steeper meridian; likewise the lens will always move toward the steepest region of the cornea
6. Effect of base curve radius and optical zone diameter change:

To maintain the same lens-to-cornea fitting relationship:

•Flatten base curve 0.25D for every 0.5mm increase in optical zone

•Steepen base curve 0.25D for every 0.5mm decrease in optical zone

7. When making a lens design change to effect the fitting relationship, make it a significant change:

1. Other Important Fitting Considerations

1. Empirical Vs. Diagnostic

1. According to a survey by Andrasko and report by Harris, diagnostic fitting accounts for only about 505 of all new fittings.
2. A Clinical Comparison of Empirical Versus Diagnostic Fitting of Daily Wear Fluoro-silicone/acrylate Lenses (Bennett, Henry, Davis, Kirby)

1. N = 39 (18 Diagnostic, 21 Empirical)
2. Large differential in time of dispensing:

Dx Group = 77.8% within 5 weeks

Empirical = 38.1% within 5 weeks

3. Reorders:

Dx Group = 9 lenses

Empirical = 20 lenses (BCR in 12 cases – usually to a flatter BCR)

4. Conclusions:

1. Diagnostic Fitting: Material/Design

1. Same material as to be ordered due to factors such as flexure and specific gravity

 

 

 

2. Same design is important:

OAD 8.4-9.2 8.8-9.8

1. Inventory

1. need 200 lenses minimum
2. benefits include:

1. fit out of stock
2. lens replacements
3. parameter changes

1. Standard Versus Custom Designs

1. typically you only order BCR, OAD and power
2. advantages:

1. less time consuming
2. easier

1. disadvantages:

1. loose control
2. possibly compromise fit

1. store dry in flat packs
2. if hydrated, leakage and adherence
3. hydrate prior to dispensing
4. clean after use

1. Determining Contact Lens Power

Example One: