Contact Lens Materials and Care Systems: Dk/t, Water Content, FDA Groups, Solutions and Wearing Schedules
Last updated: November 2026
RPC Rounds for Optometrists · B.Optom study guide
The material and the care system decide how much oxygen reaches the cornea, how deposits build up, and how likely infection is. Optometrists choose both for every patient, and most contact lens complications come back to one of them, or to poor compliance.
Oxygen: Dk and Dk/t
- Dk is the oxygen permeability of the material (D = diffusion coefficient, k = solubility).
- Dk/t (transmissibility) divides by lens thickness (t, in cm): the value that matters for the cornea. Minus lenses are thinnest centrally; high plus lenses are thick and transmit less.
- Holden and Mertz criteria: about 24 units for daily wear without corneal oedema, about 87 units for overnight wear. Harvitt and Bonanno suggested higher values (about 35 for daily, 125 for overnight) to avoid hypoxia across the whole cornea.
- In hydrogels, Dk rises with water content; in silicone hydrogels, oxygen passes through the silicone, so Dk is high even with lower water content.
Material families
| Material | Features | Typical use |
|---|---|---|
| Conventional hydrogel (HEMA-based) | Dk about 8–40; soft, comfortable, wettable | Daily wear, daily disposables |
| Silicone hydrogel | Dk about 60–160+; less hypoxia, more lipid deposits, stiffer | Daily wear, extended wear, monthly and daily lenses |
| RGP (fluorosilicone acrylates) | Dk about 30–150; rigid; corrects corneal astigmatism; long life | Astigmatism, keratoconus, ortho-k, high Rx |
| PMMA | Dk about 0; historical | Not used now (corneal hypoxia) |
| Hybrid | RGP centre, soft skirt | Irregular corneas |
| Scleral (high Dk RGP) | Large rigid lenses filled with saline | Irregular corneas, ocular surface disease |
FDA hydrogel groups
| Group | Water content | Ionic charge | Deposit tendency |
|---|---|---|---|
| I | Low (under 50%) | Non-ionic | Lowest protein deposits |
| II | High (50% or more) | Non-ionic | Moderate |
| III | Low | Ionic | Moderate |
| IV | High | Ionic | Highest protein deposits (lysozyme) |
| V | Silicone hydrogels (added later) | Various | More lipid deposits |
Wearing and replacement schedules
- Wearing schedule: daily wear (removed every night), extended wear (overnight, up to 6 nights), continuous wear (up to 30 nights, silicone hydrogels only). Overnight wear multiplies the risk of microbial keratitis several-fold.
- Replacement: daily disposable (safest, no care solution), two-weekly, monthly, and yearly (conventional or RGP).
Care systems
| System | Notes |
|---|---|
| Multipurpose solution (MPS) | Cleans, rinses, disinfects and stores; "rub and rinse" improves disinfection even with "no-rub" products |
| Hydrogen peroxide (3%) systems | Excellent disinfection including against Acanthamoeba; must be neutralised (catalytic disc or tablet) before insertion or it stings and burns |
| RGP conditioning and cleaning solutions | Separate cleaner and conditioner; enzymatic protein removal periodically |
| Saline | Rinsing and scleral lens filling only (preservative-free); not a disinfectant |
Never use tap water, saliva or homemade saline; never top up old solution; replace the case every month; no swimming or showering in lenses. These rules matter most because Acanthamoeba and Pseudomonas keratitis are linked to water exposure and poor hygiene.
Choosing for the patient
- First-time and occasional wearers, allergy sufferers and patients with poor compliance: daily disposables.
- Long hours, high minus: silicone hydrogel.
- Dry eye symptoms: try a different material, daily disposables, and treat lid disease.
- Astigmatism over about 0.75–1.00 D: soft toric or RGP; keratoconus: RGP, hybrid or scleral.
Practical pearls
- Ask patients to show you how they clean and store their lenses: watching reveals tap-water rinsing and topping up that they will not mention.
- Switching a patient with recurrent papillary conjunctivitis to daily disposables often solves it.
- Peroxide systems are the best choice for patients at high risk of Acanthamoeba (for example those who swim or live with poor water quality), but teach neutralisation carefully.
Where students slip
- Confusing Dk (material) with Dk/t (lens).
- Recommending saline for disinfection.
- Allowing overnight wear in hydrogel lenses.
Test yourself: flip cards
Quick quiz
Related: Contact Lens Calculator: Soft, Toric and RGP Power, Base Curve and Tear Lens, Contact Lens Quick Reference and Fitting Guide: Vertex, LARS, RGP Patterns, Irregular Cornea and Scleral Lenses, Contact Lens Types: Soft, Toric, RGP, Scleral and Bandage Lenses Explained, Contact Lens Care Tips, Acanthamoeba Keratitis: Early Diagnosis and Treatment. Back to the optometry curriculum.
Key sources
- Efron N. Contact Lens Practice. 4th ed. Elsevier; 2023
- Holden BA, Mertz GW. Critical oxygen levels to avoid corneal edema for daily and extended wear contact lenses. Invest Ophthalmol Vis Sci. 1984;25:1161-1167
- Stapleton F, et al. The incidence of contact lens-related microbial keratitis in Australia. Ophthalmology. 2008;115:1655-1662
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