University of Missouri-St. Louis

School of Optometry

I. Definition-A hydrogel contact lens is a type of flexible lens which is made from plastic and water. The lens will conform to the shape of the cornea, can be folded so that the edges meet and when released, will return to its normal shape (i.e., excellent memory). Also called a soft contact lens

II. Advantages & Disadvantages

A. Advantages
1. "Initial" Comfort
2. Visual acuity or comfort affected little by lens movement
3. Simple to fit
4. May inventory
5. Little adaptation
6. Occasional wear possible
7. Little chance of dislodging the lens
8. Chance of trapped foreign body or abrasion small
9. Rarely causes photophobia or excessive tearing
10. Cosmesis
11. Minimal spectacle blur

B. Disadvantages
1. Bacterial contamination and infection
2. Increased solutions/complexity and solution cost
3. Increased care time
4. Durability
5. Greater risks with noncompliance
6. Prone to deposits/GPC
7. Reduced oxygen permeability (corneal edema)
8. Quality of vision may be reduced from a couple of letters to a line on Snellen chart & reduced CSF
9. Difficult to verify
10. Limitations of correction

III. Properties

A. Material
1. Consists of chemical monomers, linked together to form chains of polymers which are linked by cross-linking units. With few cross-linking molecules, the polymer may absorb a solvent; in this case water.
2. Original hydrogel contact lenses made of polyHEMA (2-hydroxy-ethyl methacrylate) crosslinked with ethylene glycol dimethacrylate (EDMA).
3. Attempts to improve the properties have resulted in using polyHEMA; but, in addition, using other monomers such as NVP (n vinyl pyrollidone), to improve properties such as water content, strength or resistance to deposits.
4. Lenses may also be made of different polymers which are related to HEMA.

B. Water Content
1. Water content = weight of water in lens X 100 /total weight of lens
2. Range of water content 38-79%
3. HEMA by itself produces a water content of 38%.
4. Monomers used in the copolymers are hydrophilic or hydrophobic.

a. Hydrophilic monomers (Listed in greatest to lowest ability to increase water content): MAA (methacrylic acid), NVP, HEMA

1) Increase water content
2) Decrease lens modulus (strength)
3) Decrease lens elasticity

b. Hydrophobic monomers: MMA (methylmethacrylate), isobutyl methacrylate, pentyl methacrylate

1) Decrease water content
2) Increase lens modulus
3) Increase lens elasticity

5. Water content plays a role in determining:

a. Pore size
b. Ability to regain shape after blink
c. Strength
d. Elasticity
e. Oxygen transmissibility
g. Adsorption of tear solutes

6. FDA recommended hydrogel class system (Useful for identifying lens physical properties & response to various solutions)

a. Group 1 - nonionic polymers, low water (polyHEMA & hydrophobic monomers, no MAA)
b. Group 2 - nonionic polymers, high water (poly NVP & hydrophilic monomers, no MAA)
c. Group 3 - ionic polymers, low water (MAA)
d. Group 4 - ionic polymers, high water (poly HEMA & hydrophilic monomers & increased levels of MAA, no NVP)
e. Bottom line: Groups 2 & 4 avoid heat disinfection, sorbic acid, potassium sorbate, extended time in papain enzyme and patients with deposit problems; Group 1 has greatest resistance to protein deposits and Group 4 the lowest.

FDA Groups:

Group 1 Group 2 Group 3 Group 4
tefilcon lidofilcon B etafilcon bufilcon A
tetrafilcon A surfilcon bufilcon A perfilcon
crofilcon lidofilcon A deltafilcon A etafilcon A
dimefilcon ofilcon A droxifilcon A ocufilcon B
hefilcon A & B xylofilcon A phemfilcon A phemfilcon A
phemfilcon A scafilcon A ocufilcon tetrafilcon B
isofilcon methafilcon mafilcon vifilcon A

Reference: Stone RP. Why Contact Lens Groups? Contact Lens Spectrum 1988; 3(12): 38-41.

C. Oxygen Permeability = Dk Transmission = Dk/thickness
1. Oxygen permeability increases logarithmically with an increase in water content.
2. Oxygen Transmission increased by decreasing center thickness.
3. Dk values for daily wear hydrogel lenses range from 8.3 to 18.8.
4. Dk values for extended wear hydrogel lenses range from 8.4 to 38.9. This does not take into account oxygen transmission.
5. A thicker high water content lens can have an equal oxygen transmission as a thin low water content lens.
6. Tear exchange behind a hydrogel contact lens is only 1-2% of the tear volume per blink compared to 10-20% for RGP’s so oxygen must be obtained by transmission.

E. Deposit Resistance
1. Crofilcon A- (PBH/WJ - CSI Clarity DW, CSI Clarity FW & CSI Clarity toric) - This material has been found to be more deposit resistant than other materials. It is nonHEMA, {GMA/MMA (glycerol methacrylate)}.
2. Planned Replacement - Replacement of lenses at regular intervals such as weekly, bi-weekly, monthly, bi-monthly or quarterly.
        a. Frequent Replacement: a. Optima FW Medalist, Gold Medalist Toric, Occasions                Multifocals - Bausch & Lomb
        b. Focus, Focus Toric, Focus Softcolors - Ciba Vision
        c. Preference DW/FW or Preference Std., Preference Toric, Frequency 55 & 38, Hydrasoft,               Frequency 55 Toric, Hydrasoft Toric - Coopervision
        d. Hydron Proactive 55 & Edge III Proactive DW - Ocular Sciences
        e. Surevue, Vistavue - Vistakon
        f. Gentle Touch DW/FW - WJ
        g. Multiples, Multiples Toric, Additions - Sunsoft
        h. Proclear, Proclear Compatibles - Biocompatibles
        i. Specialty T-FRP, Specialty Progressive - Specialty Ultravision
        j. Flexlens, Flexlens Toric - Flexlens (Paragon)
        k. Misc. companies:Accugel (Sph & Toric), Metro Optics (Satureyes), California Optics          (COSoft Toric), Blanchard (Quattro), Lifestyle (XTRA), Unilens (E.M.A.), World Optics          (ABerCon)

3. Disposable lenses
        a. Acuvue, 1 day Acuvue, Acuvue 2, Acuvue Bifocal - Vistakon
        b. Focus 1-2 wk., Focus 1-2 wk. Softcolors, Focus Dailies - Ciba Vision
        c. Seequence, Optima FW, Soflens 66, Soflens 66 Toric, Soflens One Day - Bausch & Lomb           d. Hydron Biomedics 38 & 55 - Ocular Sciences
        e. Fresh Look, Fresh Look Colors, Fresh Look Color Enhancers, Fresh Look Toric, Fresh           Look Color Blends. Precision UV - WJ
        f. Specialty Choice A.B. - Specialty Ultravision

4. New Hydrogel Hybrids
        a. Pure Vision-Bausch & Lomb
        b. Focus Night and Day -Ciba

F. Wettability
1. Aids in lid closure and comfort
2. Contaminants adhere to surface
3. Increased sensitivity to preservatives

IV. Lens Types

A. Daily Wear
1. Probably the most popular lens available
2. Worn daily, generally 12-14 hours; however, can be worn occasionally or for shorter periods of time.
3. Disinfection systems include heat, chemical & oxidation.
4. Largest variety of torics and bifocals fall in this category

B. Flexible/Extended Wear
1. Lenses are worn continuously for 3-7 days or can be worn occasionally extended wear (i.e., flexible wear); for example, weekends.
2. Generally require chemical or oxidation disinfection.
3. Decreased selection of torics and bifocals

C. Planned Replacement (FRP & Disposables)
1. Disinfection
2. Wear time

D. Tints
1. Handling tints
        a. Light blue tint added to lens to aid in handling of lens.
2. Color Enhancers/Transparent Tints
        a. Used to enhance an eye color already present.
3. Opaque Tints
        a. Can change eye color
        b. Lenses available:
            Durasoft 3 Complements (FW), Durasoft 3 Colors (FW), Durasoft 2 Colors (DW), Fresh               Look Colors (FW) -WJ Illusions (DW) - Ciba Vision Natural Touch (DW) CooperVision

V. Lens Design

A. Lathe-cut
B. Cast-molded
C. Spun-cast
D. Lathe-cut back surface and Spun-cast front surface

VI. Factors to Consider in Material Selection

A. Patients with Ocular Compromise
B. Refractive Error
C. Deposit-Prone Patients
D. Dry Eye
E. PMMA/RGP Refits
F. Age
G. Part-time Wearers

VII. Lens Selection

A. Base Curve Radius
1. Typically available in 2-4 BCR
2. Some manufacturers assign sag or vault # (B&L - Sag I Flatter, Sag II Steeper)
3. 2 ways to fit
        a. 4-5D flatter than flat or low K
        b. Steepest BCR - K greater than 45D
        c. Middle BCR - 41-45 D
        d. Flat BCR - K less than 41D

B. Lens Diameter
1. HVID + 2 = Lens Diameter
2. Generally available in 13.8-15mm diameters

C. Power
1. Final power is based on predicted power and OR + diagnostic lens power
2. Predicted power
        a. Vertex back to cornea Rx’s greater than + 4D
        b. Spherical equivalent for patient’s with low amounts of spectacle cylinder.
        c. Residual astigmatism = Refractive cylinder Residual corneal cylinder is not apparent as              the hydrogel lens drapes cornea resulting in little to no tear lens.

D. Water Content
1. Low water content - less than 40% water
2. Medium water content - 50-60% water
3. High water content - greater than 60% water
4. Increasing water content increases oxygen permeability & decreases durability

E. Center Thickness
1. Center thickness of -3.00D lens will be approximately 0.08mm
2. Affects handling & oxygen transmission

VII. Cases

A patient wears extended wear high water content lenses. He is continually returning to the clinic to have his lenses replaced due to the fact that he tears them. What might be a solution for him?

Mr. GPC is prone to deposits and giant papillary conjunctivitis. You decide to refit him from his current HEMA lenses. What are his options?

Mrs. Presbyope is being fit for contact lenses for the first time. She loves monovision; however, being a presbyope she has difficulty seeing the lenses to insert or clean. What would you do for her?

Dr. E. Bennett is a first year medical resident. She spends long hours awake and may wear her daily wear contact lenses 18-20 hours. Upon examination, you determine that she needs more oxygen to the cornea. What are the options you have for refitting her?

A patient has K readings of 43.00/43.50 @ 90. Based on this information, what base curve would you initially select?

A patient has a spectacle Rx of OD -2.00 -1.00 X 180 and OS -5.00 DS. What would be the predicted power for hydrogel lenses?

V.A.H. Revised 1999