SH Lecture 3

For clear vision, where must light focus?

Light must be bent by the cornea and lens so it lands sharply on the retina.

What happens after light reaches the retina?

The retina sends signals to the brain for vision formation.

Which structure provides most of the eye’s focusing power?

The cornea provides about 2/3 of the eye’s focusing power.

Is the cornea fixed focus or variable focus?

The cornea is fixed focus.

Which structure provides the remaining focusing power?

The crystalline lens provides about 1/3 of the eye’s focusing power.

How does the eye change focal distance?

By changing the shape/curvature of the crystalline lens using the ciliary muscles.

What is the main physical function of the lens?

To change shape and adjust focus for objects at different distances.

What is the lens shape described as?

A transparent biconvex structure.

What is accommodation?

The process of changing lens curvature so the eye can focus on objects at different distances.

What controls lens curvature?

The ciliary muscles, through their effect on the zonular fibres/ligaments.

What is the amplitude of accommodation?

The maximum potential increase in optical power that an eye can achieve when adjusting focus.

What range does amplitude of accommodation refer to?

The range of object distances for which the retinal image can be sharply focused.

What is optical power?

Optical power is P = 1/f, where f is focal length in metres.

What are the units of optical power?

Dioptres, dpt, equivalent to m^-1.

How is accommodation amplitude written in the notes?

A = Px - Py, the difference between optical powers for two focusing states.

For distant vision, is the lens generally flatter or more curved?

Flatter/less curved.

For near vision, is the lens generally flatter or more curved?

More curved/thicker, giving greater optical power.

Why does increasing lens curvature help near vision?

It increases optical power, shortening focal length so nearby objects can focus on the retina.

What is myopia?

Nearsightedness: inability to view distant objects in focus.

In myopia, where is light focused relative to the retina?

In front of the retina.

What eye shape features can cause myopia?

A cornea that is too steeply curved or an eye that is too long front-to-back.

How is myopia corrected optically?

Using a concave/diverging lens.

What is hyperopia?

Farsightedness: inability to view close-up objects in focus.

In hyperopia, where is light focused relative to the retina?

Behind the retina.

What eye shape features can cause hyperopia?

An eye that is too short front-to-back or a cornea that is too flat.

How is hyperopia corrected optically?

Using a convex/converging lens.

What is presbyopia?

Age-related loss of accommodative ability, making near-field focusing difficult or impossible.

What material change causes presbyopia?

The eye’s lens loses natural flexibility.

At roughly what age does presbyopia affect almost all people?

By about age 50.

Why is presbyopia called a material problem in the notes?

Because the lens material stiffens/loses flexibility, so it becomes harder to deform for focusing.

How does accommodation power change with age?

It decreases with age.

What mechanical test is used for the lens in the notes?

Indentation testing across the lens surface.

How many points are tested across the lens surface?

Eight points.

How are test points arranged when possible?

In three rows with 1 mm spacing.

What material property is calculated from indentation testing?

Shear modulus, G, in Pa.

What formula gives lens shear modulus from indentation?

G = P(1 - ν)/(4Rd), where P is load, R is indenter radius, d is penetration depth, and ν is Poisson’s ratio.

What does P mean in the indentation formula?

Total load, in N.

What does R mean in the indentation formula?

Indenter radius, in m.

What does d mean in the indentation formula?

Maximum depth of penetration, in m.

What does ν mean in the indentation formula?

Poisson’s ratio.

How do the nucleus and cortex change with age?

They become much stiffer, with changes over orders of magnitude.

How does lens size change with age?

The lens becomes larger.

Why does lens stiffening reduce focusing ability?

A stiffer lens is harder for ciliary muscles to deform, so accommodation decreases.

In the lecture model, how is the lens treated mechanically?

As a passive material with linear elastic properties.

What is the engineering problem in myopia?

The eye shape makes light focus in front of the retina, so vision must be redirected/refocused.

Name three engineering repair strategies for myopia.

Glasses, contact lenses, and LASIK surgery.

Why do different myopia repair strategies need different criteria?

Each has different tissue/implant interaction requirements and different management for outcomes.

What does LASIK stand for?

Laser-Assisted In Situ Keratomileusis.

Which eye tissue does LASIK reshape?

The cornea.

Why can LASIK correct focusing errors effectively?

Because about 2/3 of the eye’s focusing power comes from the cornea.

What is the first surgical step in LASIK described in the notes?

The cornea is opened as a flap to expose the stroma.

What does the laser do in LASIK?

It vaporizes selected regions of the corneal stroma.

What happens after the laser reshapes the stroma?

The flap is closed and naturally re-adheres.

Are sutures normally required in LASIK according to the notes?

No, sutures are not required.

What has dominated the history of contact lenses?

Materials and fabrication developments.

What did Leonardo da Vinci describe in 1508?

Altering corneal power by immersing the eye in a bowl of water.

Who first published contact lens use and when?

Fick, in 1888.

What early materials were used for contact lenses?

Glass, then PMMA from 1936.

When were corneal lenses first demonstrated?

1948

What was the problem with early silicone rubber soft lenses?

Poor results due to a hydrophobic surface.

What did Wichterle produce in 1961?

A prototype soft lens made from hydrophilic gel.

Which company acquired Wichterle’s soft lens patent?

Bausch & Lomb.

What limited rigid PMMA contact lenses?

Lack of gas permeability.

What material development improved rigid gas-permeable lenses?

Silicone acrylates.

What did Johnson & Johnson improve for disposable contact lenses?

They overhauled the moulding process to make low-cost disposable lenses.

What practical issues remain important for contact lenses?

Infection and compliance.

What is the elastic modulus of glass contact lenses?

About 70 GPa.

What is the elastic modulus of PMMA contact lenses?

About 4 GPa.

What is the elastic modulus range of silicone rubber lenses?

About 1-10 MPa.

What is the elastic modulus range of hydrogel lenses?

From kPa to MPa.

What is the approximate elastic modulus of the cornea?

About 1-15 MPa, depending on location and orientation.

Why does material stiffness matter for contact lenses?

The lens must interact comfortably and mechanically with the cornea.

What was Wichterle’s original manufacturing process?

Spin casting.

Is Wichterle’s spin-casting process still used?

Yes, it is still in use.

What is the dominant contact lens manufacturing process?

Cast moulding.

Why are manufacture, packaging and sterilization important?

They affect safety, quality, infection risk and practical use.

What is the common symptom linking cataract, glaucoma and corneal opacity?

Loss of sight.

What is a cataract?

A cloudy/opaque lens.

Why are cataracts clinically important?

They are extremely common in the elderly and are the leading cause of blindness, almost half of cases.

What is corneal opacity?

A cloudy/opaque cornea.

What usually causes corneal opacity?

Injury, inflammation, or infection.

How does corneal opacity onset compare with cataracts?

Corneal opacity is often sudden, while cataracts usually cause gradual lens deterioration.

What is glaucoma associated with?

Excess intraocular pressure causing damage to the optic nerve.

How common is glaucoma as a cause of blindness?

It is the second most common form of blindness.

What is the repair strategy for cataracts?

Remove the cloudy lens and implant an artificial intraocular lens, IOL.

How many IOLs are implanted per year according to the notes?

About 10,000,000 per year.

How large is the unmet cataract surgery need in the developing world?

The notes estimate 35 million more operations could occur per year compared with the 10 million currently.

When was the first IOL surgery done?

1950, at St Thomas’ Hospital.

When did IOL surgery gain significant popularity?

In the 1970s, with improved IOL materials.

What material were early intraocular lenses made from?

PMMA.

What is the approximate modulus of PMMA IOLs?

About 3 GPa.

Why are modern acrylate IOLs useful surgically?

They are flexible and can be folded for surgical insertion.

Name two categories of modern acrylate IOL polymers.

Hydrophobic acrylates and hydrogel/hydrophilic polymers.

What other flexible material has been used for IOLs?

Silicone rubber.

What is the approximate modulus of silicone rubber IOLs?

About 4 MPa.

How has IOL mass changed from early PMMA to modern lenses?

Modern lenses average about 20 mg, compared with about 110 mg for early PMMA lenses.

How did reduced IOL stiffness change surgery?

It enabled a minimally invasive approach requiring no stitches.

What are benefits of modern flexible IOL surgery?

Faster recovery and significantly improved outcomes.

Why are IOL materials often doped with UV absorbers?

To protect the retina from radiation damage.

What limitation do traditional monofocal IOLs have?

Accommodation is lost.