Contact lens material, nomenclature, diameter, advantages and disadvantages

 CONTACT LENSES 

Contact lens is an artificial device whose front surface substitutes the anterior surface of the cornea.

Therefore, in addition to correction of refractive error, the irregularities of the front surface of cornea can also be corrected by the contact lenses.

Parts, curves, and nomenclature for contact lens

To understand the contact lens specifications following standard nomenclature has been recommended.

1. Diameters of the lens

i. Overall diameter (OD) of the lens is the linear measurement of the greatest distance across the physical boundaries of lens. 

It is expressed in millimetres. (It should not be confused as being twice the radius of curvature).

ii. Optic zone diameter (OZ) is the dimension of the central optic zone of lens which is meant to focus rays on retina.

2. Curves of the lens

i. Base curve (BC) or central posterior curve (CPC) is a curve on the back surface of the lens to fit the front surface of cornea.

ii. Peripheral curves. These are concentric to base curve and include intermediate posterior curve (IPC) and peripheral posterior curve (PPC). These are meant to serve as reservoir of tears and to form a ski for lens movements.

iii. Central anterior curve (CAC) or front curve (FC) is the curve on the anterior surface of the optical zone of the lens. Its curvature determines the power of contact lens.

iv. Peripheral anterior curve (PAC) is a slope on the periphery of anterior surface which goes up to the edge.

v. Intermediate anterior curve (IAC) is fabricated only in the high power minus and plus lenses. It lies between the CAC and PAC.

3. Edge of the lens. 

It is the polished and blended union of the peripheral posterior and anterior curves of the lens.

4. Power of the lens.

 It is measured in terms of posterior vertex power in dioptres.

5. Thickness of the lens. 

It is usually measured in the centre of the lens and varies depending upon the posterior vertex power of the lens.

6. Tint. 

It is the colour of the lens.

Types of contact lenses

Depending upon the nature of the material used in their manufacturing, the contact lenses can be divided into following three types:

1. Hard lenses,

2. Rigid gas permeable lenses, and

3. Soft lenses.

1. Hard lenses 

Hard contact lenses are manufactured from PMMA (polymethylmethacrylate). 

The PMMA has a high optical quality, stability and is light in weight, non toxic, durable and cheap. 

The hard corneal lenses have a diameter of 8.5-10 mm. 

Presently these are not used commonly.

Disadvantages 

Disadvantages of  PMMA hard contact lenses. 

(i) PMMA is practically impermeable to O2 thus restricting the tolerance. 

(ii) Being hard, it can cause corneal abrasions. 

(iii) Being hydrophobic in nature, resists wetting but a stable tear film can be formed over it.

Note : PMMA contact lenses are sparingly used in clinical practice because of poor patient acceptance.

2. Rigid gas permeable (RGP) lenses

 RGP lenses made up of materials which are permeable to oxygen. Basically these are also hard, but somehow due to their O2 permeability they have become popular by the name of semisoft lenses. 

Gas permeable lenses are commonly manufactured from copolymer of PMMA and silicone containing vinyl monomer. 

Cellulose acetate butyrate (CAB), a class of thermoplastic material derived from special grade wood cellulose has also been used, but is not popular.

3. Soft lenses 

Soft contact lenses are made up of HEMA (hydroxy methymethacrylate). These are made about 1-2 mm larger than the corneal diameter. 

Advantages: 

Being soft and oxygen permeable, they are most comfortable and so well tolerated. 

Disadvantages include 

problem of wettability, proteinaceous deposits, getting cracked, limited life, inferior optical quality, more chances of corneal infections and cannot correct astigmatism of more than 2 dioptres.

Note: In clinical practice soft lenses are most frequently prescribed.

Indications of contact lens use

1. Optical indications include anisometropia, unilateral aphakia, high myopia, keratoconus and irregular astigmatism.

 Optically they can be used by every patient having refractive error for cosmetic purposes.

Advantages of contact lenses over spectacles:

 (i) Irregular corneal astigmatism which is not possible to correct with glasses can be corrected with contact lenses. 

(ii) Contact lenses provide normal field of vision.

 (iii) Aberrations associated with spectacles (such as peripheral aberrations and prismatic distortions) are eliminated.

 (iv) Binocular vision can be retained in high anisometropia (e.g., unilateral aphakia) owing to less magnification of the retinal image. 

(v) Rain and fog do not condense upon contact lenses as they do on spectacles. 

(vi) Cosmetically more acceptable especially by females and all patients with thick glasses in high refractive errors.

2. Therapeutic indications are as follows :

i. Corneal diseases e.g., non-healing corneal ulcers, bullous keratopathy, filamentary keratitis and recurrent corneal erosion syndrome.

ii. Diseases of iris such as aniridia, coloboma and albinism to avoid glare.

iii. In glaucoma as vehicle for drug delivery.

iv. In amblyopia, opaque contact lenses are used for occlusion.

v. Bandage soft contact lenses are used following keratoplasty and in microcorneal perforation.

3. Preventive indications include 

(i) prevention of symblepharon and restoration of fornices in chemical burns

 (ii) exposure keratitis

 (iii) trichiasis.

4. Diagnostic indications include use during

 (i) gonioscopy

 (ii) electroretinography

 (iii) examination of fundus in the presence of irregular corneal astigmatism

 (iv) fundus photography

 (v) Goldmann's 3 mirror examination.

5. Operative indications. Contact lenses are used during 

(i) goniotomy operation for congenital glaucoma

 (ii) vitrectomy

(iii) endocular photocoagulation.

6. Cosmetic indications of contact lens include

 (i) unsightly corneal scars (colour contact lenses)

 (ii) ptosis (haptic contact lens)

 (iii) cosmetic scleral lenses in phthisis bulbi.

7. Occupational indications of contact lens include use by various profession

(1) sportsmen

(2) pilots

(3) actors

Contraindications for contact lens use

(i) Mental incompetence, and poor motivation

(ii) chronic dacryocystitis

(iii) chronic blepharitis and recurrent stye

(iv) chronic conjunctivitis; (v) dry eye syndromes

(vi) corneal dystrophies and degenerations

(vii) recurrent diseases like episcleritis, scleritis and iridocyclitis.

Principles of fitting and care of lenses

It is beyond the scope of this chapter. Interested readers are advised to consult some textbook on contact lenses.

OPTOMETRY-SHARP VISION

Optometrist