Lens Materials & Design

- Dk = oxygen permeability, a property of the material (D = diffusion coefficient, k = solubility). Units are ×10⁻¹¹.

- Dk/t = oxygen transmissibility = Dk divided by lens thickness (t, in cm); it describes the finished lens and is what actually reaches the cornea. Units are ×10⁻⁹.

A high-Dk material lathed into a thick lens can still deliver low Dk/t, so a high-plus lens (thick center) or a high-minus lens (thick edge) may starve the cornea locally.

Holden-Mertz criteria (the classic exam numbers) give the minimum Dk/t needed:

- ~24 ×10⁻⁹ for daily wear (avoids daytime edema)

- ~87 ×10⁻⁹ for extended (overnight) wear (holds overnight edema near the ~4% level of a non-wearing eye)

The stricter Harvitt-Bonanno values (~35 daily, ~125 extended wear) aim to avoid anoxia in every corneal layer.

Silicone hydrogels transmit oxygen through the silicone rather than the water, breaking the old "more water = more oxygen" rule of traditional hydrogels. They dramatically raised Dk/t, cutting hypoxia and enabling safer extended wear — at the cost of a stiffer, more lipophilic, sometimes less-wettable surface that manufacturers treat with plasma coatings or internal wetting agents.

The FDA sorts hydrogels by water content (the 50% line) and surface charge (ionic vs nonionic), because those properties drive deposits and solution compatibility:

Ionic, high-water Group IV lenses attract the most protein because their negative surface charge binds positively charged tear proteins. In 2018 the FDA added a fifth category (Group V) for silicone hydrogels, which do not fit the water-based scheme.

Replacement modalities:

- Daily disposable — worn once and discarded; no case or solution, lowest infection and deposit risk, best for allergy and dry-eye patients.

- Reusable (2-week or monthly) — require cleaning, disinfection, and case care.

- Extended/continuous wear — FDA-cleared overnight wear (up to 6 nights, or 30 days for approved lenses), demanding high Dk/t silicone hydrogel.

Core parameters to specify: base curve, overall diameter (~13.5-15 mm for soft), and power; torics add cylinder and axis, multifocals add an add.

Multifocal / presbyopic contact lenses:

- Simultaneous vision — distance and near powers reach the pupil at once and the brain selects; delivered as concentric rings or aspheric gradients. Sub-types are center-near (common in soft) and center-distance; pupil size affects performance.

- Translating (alternating) — the lens shifts on downgaze so the wearer looks through a distance zone up top and a segment/crescent near zone below; stabilized by prism ballast and truncation, this is mainly a GP design.

- Monovision — dominant eye corrected for distance, non-dominant for near; simple but reduces stereoacuity and depth perception. Modified monovision blends a multifocal on one eye.

Success depends on identifying the dominant eye, setting realistic expectations, and fine-tuning add and sphere.

RGP materials: modern rigid lenses are fluorosilicone acrylate — silicone raises oxygen permeability while fluorine improves wettability and resists deposits. Higher-Dk GP materials are softer and warp or scratch more easily, so handling matters. PMMA, the original rigid material, has essentially zero oxygen permeability and is obsolete for corneal wear. Hybrid lenses pair a GP center with a soft skirt to combine GP optics with soft-lens comfort.

Knowing how lenses are made explains their cost, reproducibility, and available parameters.

Lathe cutting: a hard button (or dehydrated hydrogel blank) is mounted on a computer-controlled lathe that cuts the back and front curves; it is then polished and, for hydrogels, hydrated to final size. Lathing allows almost any custom base curve, power, and diameter, so it is used for GP lenses, custom torics, and specialty soft lenses. Because hydration expands the lens, blanks are cut oversized to a calculated dry dimension.

Cast molding: liquid monomer is placed between a matched pair of precision plastic molds, then cured (heat or UV) and hydrated. It is fast, cheap, and highly reproducible, making it the standard for mass-produced disposable soft lenses — but each design needs its own tooling.

Spin casting: monomer is spun in a single concave mold; centrifugal force and surface tension shape the front surface while the mold sets the back. It yields smooth, comfortable lenses but offers less parameter flexibility.

Many disposables combine methods (a cast-molded front with a lathed or molded back). The finished lens is inspected, sterilized (autoclaved in buffered saline), and blister-packed.