Rigid Gas Perm CLs
Contact lenses are technically considered thick lenses because they have very curved surfaces (very steep radius of curvature)
CLs are still often treated as thin lenses:
FV = F1 + F2
For CLs treated as thin lenses, just add front and back surface power to find back vertex power
Technically though, CLs are thick lenses, so if thickness is provided use the thick lens formula to find CL back vertex power:
FV = F2 + F1/1-(t/n)(F1)
If you are only provided front or back surface radius:
F = n2-n1/r
1-(t/n)(F1) is the same denominator as the thickness factor formula used to find SM of thick lenses
Vertex Distance
Effective power of a CL is different than effective power of specs because of the difference in vertex distance
Spec vertex distance: 12-14 mm
Plus lenses become weaker when they move towards the face/cornea
Hyperopes need more plus in their CLs than in their glasses
(+) prescriptions are more (+) in CLs than in specs
Minus lenses become stronger when they move towards the face/cornea
Myopes need less minus in their CLs than in their glasses
(-) prescriptions are less (-) in CLs than in specs
To find CL power of a spec prescription over 4.00D use effective vergence equation:
FCL = FS/1-dFS
d: vertex distance in m
RGP Optics
Lacrimal Lens (Tear Lens)
Layer of tears between cornea and RGP
Power of LL depends on front surface of cornea (gives back surface power of LL with opposite sign) and back surface of RGP (BC of RGP is same as front power of LL)
Exploded system: to find total refractive power of RGP on the cornea, add together each optical component as though it is isolated in air (corneal over-refraction, lacrimal lens, RGP)
FLL + FCL + FOR = RC
FOR: over-refraction on cornea
RC: refractive power at corneal plane
To find the power of each optical surface:
F = n2-n1/r
Lacrimal lens power
Determined by the front surface of the cornea and back surface of the CL
Back surface of the LL has a radius of curvature equal and opposite to the radius of curvature of the cornea
Front surface of the LL has a radius of curvature equal to the BC of the CL
To find the power of each surface of the tear film (front and back surface of LL), use F = n2-n1/r
ntearfilm = 1.336
ncornea = 1.3375
These 2 numbers are similar and keratometers use 1.3375 to convert radius to DK, so we can use k-readings to find power of LL
K-readings (corneal power) are equal to back surface power of the LL in air (measuring DK of cornea), but with opposite sign
BC of RGP given by K-reading is equal to front surface of LL
Not actual BC of RGP because machine uses n of 1.3375
To convert between power and radius of curvature:
F = 337.5/r
Answer in mm
Use n of cornea (1.3375)
1.3375-1/r = 0.3375/r
Convert into mm
Total power of LL depends on difference between front surface of LL (convex, plus) and back surface of LL (concave, minus)
If BC of CL equals curvature of cornea (K), front and back LL surface powers are equal and opposite, so total LL power is 0
If BC of CL is steeper than curvature of cornea, front surface power (+) is greater than back surface power (-) of LL, so total LL power is positive
If BC of CL is flatter than curvature of cornea, back surface power (-) is greater than front surface power (+) of LL, so total LL power is negative
Solving LL problems:
To solve LL problems:
Find the total LL power by finding front and back surface power (via BC of RGP (same power) and corneal curvature (equal and opposite power))
Flip the sign of the total LL power (due to SAMFAP) and add it to the subjective refraction for the total power of the RGP
If there is an over-refraction, add it to the LL power and refraction for total RGP power
If you need to find an on-K RGP fit (example):
Use K-readings to find power of LL
If K is 45.00 DK, back surface of LL is equal and opposite to cornea: back surface of LL is -45.00
Since the fit is on-K, BC of GP is equal to curvature of cornea: front surface of LL is +45.00
Total power of LL is -45.00 + 45.00 = 0
Add power of LL to power needed for corneal correction
If patient is a +1.00 D and LL is 0, a +1.00 D RGP should be chosen for an on-K fit
For on-K fits, LL is 0
To find the RGP power based on corneal curvature and CL BC (example):
Use corneal curvature to find back surface of LL
If corneal curvature is 7.5 mm, 337.5/7.5 = 45: back surface of LL is -45.00
Use CL BC to find front surface of LL
If RGP BC is 8.0 mm, 337.5/8 = 42.19: front surface of LL is +42.25 (need to use 0.25 steps)
Find total power of LL
-45.00 + 42.25 = -2.75 D
Add power of LL to subjective refraction needed for corneal correction
If subjective refraction is +1.00: +2.75 + +1.00 = +3.75 D
+3.75 D is the power of the RGP
Note SAMFAP: because CL was flatter than cornea, (7.5 mm cornea vs. 8 mm BC), the LL was minus
Plus was added in the CL to compensate for minus LL
Higher radius of curvature is flatter: imagine how a larger circle is flatter
SAMFAP (Steep Add Minus, Flat Add Plus)
Changing the BC of the CL changes the power of the LL
If you steepen the BC, the LL becomes more plus so RGP power becomes more minus to compensate
If you flatten the BC, the LL becomes more minus so RGP power becomes more plus to compensate
Steep and flat refers to the BC of the RGP (compared to corneal curvature)
For every 1 mm change in BC of RGP, the power of the RGP changes by 0.50 D
If you steepen the BC by 1 mm, subtract 0.50 from the total power
BC is inversely related to radius of curvature
Larger radius of curvature is flatter
Astigmatism and RGPs
When a spherical RGP is put onto an astigmatism, the LL fully corrects the corneal astigmatism
Patients with corneal astigmatism can be fully corrected with spherical RGPs
Residual astigmatism = total astigmatism - corneal astigmatism
If a patient has internal/lenticular astigmatism (not due to corneal shape), a spherical RGP will not correct it
Residual astigmatism is the amount of astigmatism a patient has left over when they are corrected with a spherical GP (corrected for corneal astigmatism)
If a patient has more than -1.00D WTR or -0.75 ATR/oblique astigmatism, they will need a toric RGP
To figure out residual astigmatism:
Find the difference between K readings (axis at smaller K): corneal astigmatism
Subtract total corneal astigmatism from overall refractive astigmatism for residual astigmatism
If the axes are opposite, flip the sign
Javal’s Rule
Predict the total astigmatism correction using K-readings (corneal astigmatism):
Arx = 1.25Ac + (-0.50 × 090)
Average amount of internal astigmatism: -0.50 × 090
Use K-readings to find astigmatism and axis
Multiply astigmatism power by 1.25
If the axis is the same (x090), subtract 0.50 from the astigmatism power
If the axis is opposite (x180), flip the sign and add 0.50 to the astigmatism power
Over-refraction helps find the amount of lenticular astigmatism
Doing an OR with the RGP on the eye finds the amount of residual astigmatism left over
Add the OR to the RGP power to find new CL power
When doing problems, pay attention to whether it is asking you to predict the OR or predict the power of the RGP
To predict the RGP power, add the OR to the power of the RGP with the corneal refraction and the LL power
To predict the OR, subtract the power of the RGP (with LL accounted for) from the power of the corneal refraction
RGP Fit & Design
Lens Parameters
Optic Zone Diameter (OZD): usable area of optics in the CL
Average: 7.6-8.2 mm
Increasing OZD increases sag depth (larger = deeper)
Causes steeper fit (deeper = steeper)
Larger diameter = lower radius of curvature (larger circle is not as curved)
To maintain alignment, if CL OZD is increased, BC needs to be flattened
This is because larger diameter = lower radius of curvature
For every 0.4 mm change in OZD, BC should be changed by 0.25D
Overall Diameter (OAD): diameter of CL edge to edge
Average: 9.4-9.6 mm
Adjusted in 0.4 mm steps (like OZD)
Selected to minimize flare, avoid bottom lid, help with lid attachment, and maximize comfort
Peripheral Curves: align CL edge and peripheral cornea
CL can have 1 or multiple peripheral curves that get flatter the further they go towards the periphery
Prevent edge bearing onto the cornea, promote tear exchange, and support a tear meniscus to promote centration
Edge Thickness: ideal edge thickness to promote lid attachment is -3.00D
(+) CLs are thinner on the edges and often drop inferiorly due to poor lid attachment
Can happen in anything more (+) than -1.50D
May need (-) carrier lenticular
CLs that are too (-) can have excessive lift causing the lens to ride too high
Can happen in anything more (-) than -5.00D
May need (+) lenticular or beveling to decrease edge thickness
Edge Lift: distance between cornea and edge of CL
Adjusted by changing peripheral curve by 0.1mm steps
Excessive edge lift: excessive Fl pooling, decreased centration, increased CL awareness
Either steepen peripheral curve radius or reduce peripheral curve width
Inadequate edge lift: minimal Fl pool, debris trapped under CL, poor CL movement, poor tear exchange, vascularized limbal keratitis
Either flatten the peripheral curve radius or increase peripheral curve width
Central Thickness (CT): influences oxygen transmissibility and flexure of CL
Changed in 0.03 mm steps
Thinner CT: greater oxygen transmission & better centration, but more flexure
Thicker CT: less oxygen transmission & less flexure, drops inferiorly on eye
Higher Dk lenses need thicker CT to minimize flexure
Center of Gravity: center of weight of the GP
Affected by CL power, diameter, CT, and BC
More posterior center of gravity = better CL centration
Thicker, smaller, more (+), flatter CLs have more anterior center of gravity and tend to drop inferiorly
Fluorescein Patterns
Alignment: ideal pattern
Even green pattern in the optic zone
Thicker, brighter green ring in the periphery
Flatter-Than-K: corneal touch/bearing
Center of the CL will touch the cornea causing absence of Fl in the OZ
Broad green ring in mid-periphery and periphery
Steeper-Than-K: apical clearance
Fl will pool centrally
Corneal bearing in the mid-periphery and periphery
Spherical Fit Over WTR: horizontal dumbbell shape
Fl pools along the vertical meridian (steepest meridian)
Corneal touch along horizontal meridian (flatter meridian)
Creates dark horizontal dumbbell shape
Spherical Fit Over ATR: vertical dumbbell shape
Fl pool along horizontal meridian (steepest meridian)
Corneal touch along vertical meridian (flatter meridian)
Creates dark vertical dumbbell shape
GP Lens Designs
While GPs can fully correct corneal astigmatism, patients with high corneal astigmatism might experience excessive rocking over steepest meridian
This and residual astigmatism may cause patients to need toric GPs
Indications for GP types:
Spherical GP
No astigmatism
Corneal astigmatism under 2.50D
Back surface toric
More than 2.50D of corneal astigmatism and spectacle astigmatism that = 1.5 x corneal cylinder
Front surface toric
Lenticular astigmatism with corneal astigmatism under 2.50D
Bitoric
Corneal astigmatism over 2.50D and spectacle astigmatism that does not = 1.5 x corneal cylinder
Bitoric GP Lenses
If you have over 2.50D of corneal astigmatism, you need some sort of back surface toric (back surface or bitoric) to combat rocking over steep meridian
Back surface toric GPs over-correct for astigmatism, so if the spectacle astigmatism does not = 1.5 x cylinder, you need to compensate for correction with front surface toric (bitoric)
Fitting bitoric GPs:
On-K fits are often too steep/tight, so it is recommended to fit 0.25 flatter than K with a 9.4 OAD
Saddle Fit: both principal meridians are equally aligned and fit 0.25D flatter than K
Used in ATR and oblique fits
May cause poor tear exchange
Less common than low toric simulation fit
Low Toric Simulation: principal meridians are not equally aligned, so the flat meridian is fit on-K or 0.25 flatter than K, and the steep meridian is fit 0.75 to 1.00 flatter than K
Creates 0.75-1.00D WTR astigmatism
Helps with ideal tear exchange and movement
Most common fitting philosophy
Back Surface Toric GPs
Only used when corneal cylinder is over 2.50D and astigmatism = 1.5 x corneal cylinder
In these patients, amount of cylinder correction of toric back surface aligns with spectacle astigmatism correction, and there is no need to compensate for back surface over-correction
Occurs in ATR astigmatism patients
Front Surface Toric GPs
Used when corneal astigmatism is under 2.50D and there is residual astigmatism
Most patients cannot tolerate over 0.50D ATR or oblique and over 0.75D WTR astigmatism
Spherical back surface corrects corneal astigmatism, while toric front surface corrects for residual astigmatism
Because of spherical back surface, these lenses are susceptible to rotation (ruins astigmatic front surface correction)
Prism ballasting: incorporate BD prism at the bottom of the CL to minimize rotation during blink
Greater prism necessary for low minus/plus, and small diameter CLs
Aspheric GPs
Progressively flatten towards the periphery (matches corneal flattening)
Improved alignment and better centration
Decrease spherical aberrations
Back surface aspheric CLs: ATR astigmatism, irregular cornea
Increases capillary attraction between tears and CLs for better centration
Front surface aspheric CLs: excessive residual astigmatism, high VA demands
Decreases spherical aberrations improving VA
Primary disadvantage of aspheric GPs is decrease in tear exchange due to decrease in CL movement on eye
Especially in patients with back surface aspherics
Multifocal GPs
Simultaneous Design
Distance & near powers located within the pupil
Centration of the CL is critical (must be over the pupil to work)
Dependent on pupil size which determines add power
Must choose center-distant or center-near CLs
Translating Design
CL is segmented and has different focal zones for different viewing distances
Lens is designed to move up when patient looks down so patient is looking through the reading portion of the CL
Prism-ballasted to minimize rotation
Flexure
GP can lose some of its shape when on the eye
More likely when GP is thinner, has over 1.50D astigmatism, increased OZD/sag depth, high Dk material, and steep BC
Plus lenses or lenses with higher central thickness tend to maintain their shape
Flexure increases residual astigmatism because less astigmatic correction is provided when lens shapes to the cornea
Warpage is flexure on or off the eye, while flexure only occurs when lens is on the eye
Warpage is characterized by permanently-induced toricity, most likely from excessive digital cleaning
Warpage can be measured with radiuscope: if measurements show toricity, it is warpage and not flexure, since lens is not on the eye
K-reading can be taken with GP on the eye to check for flexure
If the reading is spherical there is no flexure
If the reading is toric, some flexure is occurring
Flexure can minimize astigmatism in WTR patients because a small amount of WTR flexure can counteract average astigmatism of -0.50×090
GP Fitting Techniques
Most patients can be fit well with average OAD (9.4-9.6) and OZD (8 mm)
2 main fitting philosophies:
Lid-attached: patients whose upper lid is at or below the limbus
Average OAD (9.4-9.6) is appropriate
Interpalpebral: patients whose upper lid is above the limbus
Smaller OAD is recommended (also recommended for patients with small interpalpebral fissures)
Design:
Sphere: no astigmatism or corneal astigmatism under 2.50D
Back surface toric: corneal astigmatism over 2.50D and spec cyl = 1.5 x cyl
Front surface toric: corneal astigmatism under 2.50D and residual astigmatism
Bitoric: corneal astigmatism over 2.50D and spec cyl not = 1.5 x cyl
Aspheric: ATR astigmatism or high visual demands
Steeper BCs are needed for higher amounts of corneal astigmatism
Increase center thickness by 0.03 mm to minimize flexure of steeper lens
Higher Dk lenses have greater permeability but also more flexure
Increase center thickness by 0.03 mm to minimize flexure of higher Dk lenses
High Dk indicated for extended wear
Base Curve
For a lid attached fit with an OAD of 9.4 mm
If corneal astigmatism is over 0.75D, fit 0.50D flatter than K
For every -0.50D, steepen BC
Gas Permeability
Gasses pass through CLs by dissolving into the material and then diffusing through the CL, combining to determine permeability of CL
P = Dk
P: permeability of CL
D: diffusivity of gas in CL material
k: solubility of gas in CL material
High Dk (51-99) or hyper Dk (100+) are recommended for EW CLs
Total amount of gas that passes through a CL (transmissivity) can be measured by:
T = Dk/t
T: transmissivity of gas through CL
t: CL thickness in cm
Amount of oxygen reaching cornea increases when permeability (Dk) of CL material increases, or thickness of CL decreases
GP Lens Materials
GPs have little water, so their permeability depends on silicone or fluorine concentration
PMMA: original hard CL material with Dk of 0
Great optics, but lots of corneal damage
Silicone Acrylate (SA): improved oxygen permeability, but can have flexure, warpage, deposits, and poor wettability
Dk = 12-56
Fluoro-silicone acrylate (FSA): better wettability, less deposits, and less flexure
Dk = 18-163
Patients previously fit in PMMAs should be refit into low Dk CLs
Myopes need low Dk for daily wear and high Dk for EW
Hyperopes need high Dk for daily wear and hyper Dk for EW
Corneal Edema
Can occur if cornea is not getting enough oxygen
To fix, reduce wear time, fit with looser CLs, or choose higher Dk
Solve RGP Problems
If you need to find an apically aligned BC using K-readings:
Find average of the 2 K readings and subtract 0.75D
Based on idea that cornea is aspheric by 0.50D, and the fit needs to be slightly flatter than that
To choose type of RGP:
Sphere Power Effect (SPE) Bitoric: If residual astigmatism is under 0.75D but corneal astigmatism is over 1.50D
Toric BC/Back Surface Toric: if you multiply the BC astigmatism by 1.5 and it equals the refractive astigmatism
To decrease debris/mucin: steepen BC
Center of thickness increases with increase in diameter and increase in BC/steepening
For every 0.5 mm increase in OAD or OZD, BC has to be flattened by 0.25D
Increasing diameter steepens lens, so BC has to be flattened to compensate
If toric lenses have prism ballast, it is most likely an F1 toric
Needs back surface stabilization
To loosen a tight GP: flatten BC, decrease OAD or OZD, widen or flatten peripheral curves
For hyperopes, convergence and accommodative demand decreases in CLs
Opposite for myopes
3-9 staining/corneal desiccation comes from edge lift/flat CLs
Steepen peripheral curves or BC