Contact Lenses

Anatomy and Physiology

Tear Film and Contact Lens Relationship

The functional integrity of the cornea is maintained primarily by the precorneal tear film. It helps in nourishment, lubrication, and protection of the cornea.[6] The refractive element of the eye is dependent on the cornea. The primary function of the cornea is to maintain transparency.[7] Contact lenses interfere with the interaction between tear film and the cornea. When this harmonious relationship is disturbed, it reduces the cornea's functioning and interferes with the efficiency and acceptability of the contact lens.[8]

Contact Lens Fitting Over Cornea

It is not practically possible to stick or clamp the lens over the corneal surface. The precorneal tear film has the property of surface tension and viscosity, which act as a glue to hold the lens together on the corneal surface.[9] When the lens is fitted over the cornea, the mucus of the conjunctival surface rubs the lens surface. The tear fluid will spread over the lens and maintain a uniform sheet, including the periphery, which helps hold the lens together.[10]

The cohesive force between the water molecule act as an adhesive force for the lens, and this tear film will act as a plastic sheet to hold the lens close to the corneal surface. A negative force develops between the lens and the cornea; the tear film and air interface form a surface membrane that acts as a reservoir for the lens.[11]

Visual Considerations

The lens and the tear film interact to form the tear lens. The tear film facilitates the functioning of the cornea. The refractive index of tear is 1.33, and that of the cornea is 1.37. The back of the tear film fills the corneal surface irregularities and presents a uniform surface for refractive elements. The difference of 0.04 refractive index indicated that contact lenses could correct up to 5D of astigmatism with a residual of 0.5D.[12]

Effects of Contact Lenses Over the Cornea

• Prevent tear evaporation and as a barrier to hypertonicity of tears
• Act as a barrier to the delivery of oxygen
• Helps in waste disposal by trapping the waste
• Trauma to corneal epithelial cells[13]

Hard contact lenses cover up to 50 to 80% of the cornea, and as the lens moves over the corneal surface, approximately 70 to 80% lens surface is covered all the time. The covered area of the cornea is dependent on the nutrition from the tear film.[14] The lens-tear film should constantly renew to meet the oxygen needs of the cornea. The blinking mechanism restores the lens tear film. The blink provides the pump mechanism, and the lid movement renews the lens-tear film interface, which is then formed again.[15]

Tear pump-action depends upon:

• Volume of tears
• Blink frequency
• Percentage of tear exchange with each blink[14]

Indications

Optical

• High myopia
• Unilateral aphakia
• Corneal scar
• Anisometropia
• Keratoconus
• Irregular astigmatism
• Refractive error (cosmesis)[16] 

Therapeutic

• Corneal Pathologies
• Pseudophakic bullous keratopathy
• Non-resolving corneal ulcer
• Recurrent corneal erosion syndrome
• Dry eyes
• Epithelial defect
• Post keratoplasty
• Perforation
• Post superficial keratectomy
• Post corneal tear repair- micro leak[16]

Iris Pathologies

• Aniridia
• Coloboma
• Albinism[16]

Glaucoma

• Drug delivery system

Pediatric

• Amblyopia-Opaque contact lens for occlusion 

Retina

• Post vitreoretinal surgery in some cases- epithelial defect

Diagnostic

• Gonioscopy
• Electroretinography
• Fundus examination in patients with astigmatism
• Photography of fundus
• Goldman 3 mirror examination[16] 

Preventive

• Trichiasis
• Exposure keratitis
• Prevention of symblepharon[16]

Occupational

• Sportsmen
• Police
• Pilots 

Cosmetic

• Corneal scar
• Phthisis bulbi 

Operative

• Pars plana vitrectomy
• Photocoagulation
• Goniotomy[16]

Contraindications

Absolute Contraindications

• Cerebral palsy or mentally challenged patients
• Chronic dacryocystitis
• Dry eyes
• Hordeolum Internum
• Hordeolum externum
• Blepharitis
• Trichiasis
• Conjunctivitis
• Corneal dystrophies
• Corneal degenerations
• Uveitis
• Episcleritis
• Scleritis
• Fifth nerve palsy
• Seventh nerve palsy
• Allergic conjunctivitis
• Superficial punctate keratitis
• Subepithelial keratitis
• Corneal ulcer except in perforation

Relative Contraindications

• Pregnancy
• Giant papillary conjunctivitis
• Strabismus[17]

Equipment

Classification

Based on Anatomical Position

• Scleral- Cover cornea, sclera, and conjunctiva 

• Corneal- Covers the cornea

• Semi-Scleral- Cover the cornea, bridge the limbus and partially cover the conjunctiva[5]

Based on Water Content

• Low - 0 to 40%
• Medium - 40 to 55%
• High - >55%[18]

Based on Wear Time

• Daily wear
• Extended wear
• Disposable[19]

Based on the Material

• Soft/hydrogel contact lenses - hydroxyethyl methacrylate
• Rigid gas permeable contact lenses - silicon and cellulose acetate butyrate
• Rigid non-gas permeable contact lenses - polymethylmethacrylate[4]

Designs of Contact Lenses

Single cut - These lenses have two surfaces, the front surface has a single continuous curve, and the back surface has a base curve as well as a peripheral curve.

Lenticular cut - These lenses have a front surface with a central optical and peripheral carrier portion. The peripheral portion is thinner, and the radius of curvature is flatter. The back surface has a peripheral curve as well as a base curve.[20]

Terminologies Related to Contact Lens Material

Wettability

It is the property by which liquid binds to the surface of a solid despite cohesive force between the water molecules. Wettability is inversely proportional to the wetting angle. The lower the angle, the better is the wettability and vice versa.[21]

• No wetting - Angle 150 degree
• Partial wetting - Angle 70 degree
• Complete wetting - Angle 0 degree

Water content

The percentage of water constituting the contact lens is called the water content. The contact lens has cross-linked monomers, which lead to the formation of pores. The pores absorb the water increasing the water content of the lens. Water content increases the lens's oxygen transmission, permeability, and thickness and increases the lens's mechanical strength.[22]

Transmission of Light

This is an optical property of contact lenses.[23]

Refractive Index

This is dependent on the density of contact lenses.[24]

Heat Resistance

This is the property of contact lens material to resist high temperatures.[25]

Dimensional Stability

Depending on various external factors of the contact lenses.[26]

Flexural Stability

Oxygen Permeability

By this property, contact lenses transmit gases like oxygen. This is denoted by the coefficient of variable DK, where D is the diffusion coefficient, and K is the solubility coefficient. This property doesn't mean the actual permeability of oxygen through the contact lenses. This is a unit by which oxygen passage for a specific lens is obtained.

Oxygen Transmission

This is the oxygen passage of a contact lens of a particular thickness. This is denoted by DK/L, where L indicates the thickness. The central thickness is taken as the average thickness for all practical considerations, and -3D is taken as the standard thickness of the lens by the manufacturers.[27]

Personnel

The optometrist, paramedical contact lens expert or mid-level ophthalmic personnel, the examining ophthalmologist or the cornea and contact lens expert to expected to know the basics of contact lens, anatomy, and physiology associated with contact lens and tear film interaction, the basics of contact lens prescription, fitting, hygiene, variety of contact lens materials, cost, indication, contraindications and complications associated with contact lens use.[28]

Nomenclature

• Diameter of the lens - Overall diameter, Optic zone diameter
• The curve of the lens - Base curve, peripheral curve, central anterior curve or front curve, peripheral anterior curve, and intermediate anterior curve.
• Blend - Light, medium, or heavy
• Edge of the lens
• Power of the lens
• The central thickness of the lens
• Tint or color of the lens[4]

Nomenclature Types of Contact Lenses

Preparation

Material for Contact Lenses

Earlier contact lenses were made of glass, but they remained unpopular due to the weight, brittleness of the material, and manufacturing problems. Obrig and Muller launched the first true methyl methacrylate transparent lenses in 1938, and later in 1948, plastic lenses were made by Kevin Tuchy. Wichterle, in 1961, launched the first soft hydrophilic contact lenses, thus changing the management of contact lens prescriptions and outcomes.[4]

Classification

• Filcons - These are hydrophilic non-rigid lens materials. These are actually silicon rubber elastomers and have been grouped as Group 1a, 1b, 2a, 2b, 3a, 3b, 4a, 4b, and 5
• Focons - These are hydrophobic rigid lens materials. These are PMMA and RGP lenses. These are grouped as 1a, 1b, 2, 3, 4, and 5.[29]

Ideal Contact Lens Material

An ideal contact lens material should have the following properties

Biocompatibility

This means that the contact lens material should not be harmful to the eye and should not contain any harmful material potentially dangerous to the eye.[30]

Stability

The curvature and thickness of the contact lens should be stable to provide an excellent visual function.[31]

Molding

The contact lens material should be amenable to reshaping and duplication to be manufactured easily.[32]

Sterility

The contact lens material should be sterilizable or should be resistant to contamination.[33]

Tolerance

The tolerance of contact lenses depends on the design and oxygen permeability.[34]

Surface Chemistry

The contact lens material should be wettable so that tear film can cover it and stay over it.[35]

Gas Permeability

The contact lens material should have good gas permeability so that it doesn't interfere with corneal oxygen transmission. In this way, the contact lens can be worn for a more extended period, and its contact time and tolerance can be increased.[36]

Optical Property

The contact lens material should be transparent, and its refractive index should be near the tear film.[37]

Lens Material

Silicone Hydrogel Lenses

These are lenses with high DK and low water content. The silicon content of the lens determines its oxygen transmission capacity.[37]

Hydrogel Lenses

• Hydroxyethyl methacrylate (HEMA)

          These are the soft contact lenses that are highly prevalent and widely used. These are resistant to any chemical or enzymatic reaction biodegradation and can tolerate chemical and thermal sterilization.[38]

• Hydroxyethyl methacrylate with vinyl pyrrolidone (HEMA-VP)

         These lenses were designed to attain higher hydration as normal HEMA lenses have a hydration capacity of only 40%. Copolymerisation of HEMA with VP and cross-linkage with ethylene glycol dimethacrylate (EGDM) yields PHP. Examples include hydrocure, hydro flex, and flexicon, with a hydration of 45%. These lenses change color with age.[39] 

• MMA-PVD

         These lenses are made up of PVP (polymer), VP (monomer), and MMA (hydrophilic). Examples include Sauflen 70, Medigel 70 and Vizilen 70[4]

• Glycidyl Methacrylate

         These are combinations of HEMA with VP or MMA with VP. These are usually hydrophobic monomers.[40]

Hydrophilic Soft Contact Lenses

Soft contact lenses are made up of hydrogels. These contain a hydrophilic moiety such as the hydroxyl group. These have a large number of cross-linked polymers that can retain a large quantity of water. These can change shape and swell to form labile soft lenses. The polymerization of hydrophilic monomers creates these by cross-linking with EGDM.[41]

High Water Content Contact Lenses

These lenses are helpful and are preferred because of increased exposure of the cornea to atmospheric gases. Increased water content and reduction in lens thickness result in a much larger delivery of oxygen to the lenses. However, too thin lenses may be fragile and may be damaged by the blinking movement of the eye.[42]

Rigid Gas Permeable Lenses

They are also called semisoft lenses. They are more permeable to oxygen than PMMA lenses. Examples of RGP lenses used earlier include cellulose acetate butyrate (CAB), Silicon, and Styrene.[43]

Cellulose Acetate Butyrate (CAB)

They are made up of thermoplastic material, a derivative of wood cellulose. These were the first RGP lenses to be used. The advantage of these lenses includes good wetting ability, but the disadvantage includes the tendency to wrap. Because of this disadvantage, these lenses became unpopular.[44]

Silicon

These have high oxygen permeability, but these also became unpopular because of a lack of wetting ability.[45]

Styrene

The material used for lens manufacture is T-Butyl styrene, but these lenses were brittle and had less surface durability; hence became unpopular and are not used now.[46]

Currently used RGP Lenses

Silicon Acrylate

These are copolymers of PMMA lenses and silicon vinyl monomers. The combination is known as siloxanyl methacrylate lenses. The MMA helps in wetting ability, and silicon helps in oxygen transfer. By altering the silicon content, a variety of lenses can be manufactured with varying DK values and oxygen permeability. If the silicon content is high along with oxygen permeability, there will be increased difficulty with the surface properties of lenses.[47]

Fluoropolymers

These are RGP lenses with a copolymer containing fluorine. Due to high oxygen permeability and suitable surface properties, they are used for extended wear. There are no surface deposits in these lenses as they occur with silicon lenses. The three types of fluorinated lenses are 

• Pure fluoropolymer - cast molded
• Fluorosiloxanyl methacrylate - lathe-cut contact lenses
• Perfluoroalkyl itaconate siloxane - Alberta N lenses from Canada[48]

Rigid Non-Gas Permeable Lenses

These are hard lenses made of plastic and thermoplastic. PMMA lenses were the first commercially available plastic contact lenses. They are a derivate of acrylic acids hence also labeled as acrylic. PMMA lenses are stable, light in weight, good optical quality, and easily mouldable and non-toxic. PMMA lenses have reduced tolerance, can cause corneal abrasions, and resists wetting. PMMA hard contact lenses are not available these days.[49]

Technique or Treatment

Manufacturing Techniques

Lathe Cutting

The lathe is employed to curve the anterior and posterior surface in the technique. There are various lathes used for each surface.

Hard contact lens - These lenses are polished with convex (posterior surface) and concave (anterior surface) wax lenses.

Soft contact lenses are polished with aluminum oxide and lubricated with distilled paraffin.

This is followed by edges, the peripheral, and the intermediate curve modification. Contact lenses are then modified and verified based on these parameters. The significant advantage of lathe cutting is that in soft lenses, the lenses can be modified and engraved for identification, good quality surface finishing can be obtained nearly equal to that of PMMA lenses, the dimensional accuracy can also be maintained by lens evaluation in the dry state.[50]

Molding

This means that the lens can be heated and curved to a mobile phase, and then it can be placed on a mold to obtain a desired shaped contact lens.[51]

Spin Casting

In this technique, centrifugal forces modify the hydrogel lenses and polymerization. The monomer component, a cross-linking agent, and the initiators are placed in the concave mold that rotates constantly. The polymerization and change in shape result from the spinning of the mold. The lens's outer surface is spherical, the inner surface is aspherical, and the curvature is determined by the shape and speed of rotation of the mold. The lens is swollen, and the unpolymerized component can be removed by lens placement in water for 24 hours.[52]

Stages in Contact Lens Manufacturing

• Stage of contact lens blank-  It includes a section of clear PMMA, ethyl acrylate, or copolymer of methyl and ethyl acrylate rod or sheet, which has a diameter of 12.7 and 15.9 mm with a thickness of 3 and 6.5 mm
• Stage of semifinished blank- Contact lens blank having a posterior curve
• Stage of semifinished blank- This includes an anterior and posterior curve with a polished surface having optical quality
• Stage of finished corneal contact lens- This is a finished lens with modified peripheral and intermediate areas on the posterior surface and the diameter and edge of the lens.[51]

Complications

Conjunctival

• Allergic conjunctivitis
• Giant papillary conjunctivitis
• Superior limbic keratoconjunctivitis[5] 

Corneal

• Epithelial edema
• Epithelial defect
• Epithelial microcysts
• Abrasions
• Superficial punctate keratitis
• 3 and 9 o clock staining
• Sterile corneal infiltrates
• Marginal keratitis
• Toxic keratitis
• Corneal neovascularization
• Microbial keratitis (contact lens-induced keratitis- CLIK) - bacterial, acanthamoeba
• Warpage
• Endothelial changes[5] 

 Mechanical- Contact Lens Related

• Physical damage
• Lens discoloration
• Lost lens
• Lens deposits
• Tight lens syndrome
• Acute hypoxia
• Chronic hypoxia[5]

Clinical Significance

Contact lenses have revolutionized the optical industry. Many patients now prefer contact lenses over spectacles for cosmetic and aesthetic purposes. Young patients favor contact lenses due to workplace demand, cosmesis, and independence from spectacles.

In elderly patients, the significant indications are treatment secondary to trauma, corneal perforation, pain relief in pseudophakic bullous keratopathy, and unilateral corneal scars. The major limitations are cost, patient compliance, and complication associated with contact lenses. Contact lenses are also avoided in the pediatric age group at some centers due to the risk of contact lens-induced keratitis and compliance to treatment.[53]

Enhancing Healthcare Team Outcomes

Patients presenting to the clinic for routine ocular examination and diagnosed with a refractive error are the primary candidates for contact lenses. The other main indications in the younger age group are keratoconus and recurrent corneal erosion syndrome. The optometrist, contact lens paramedical experts, cornea and contact lens specialists, and the counselor have a crucial role in patient management.

In a patient with contact lens-related complications, the examining cornea specialist plays a vital role in the diagnosis. The nursing staff helps explain the medication required and the need for regular follow-up. The pharmacists help in arranging the required medicines. The patient's visual outcome results from interprofessional communication and coordination among the staff.[54]

Nursing, Allied Health, and Interprofessional Team Interventions

The nursing and allied health staff play an essential role in managing patients in the contact lens clinic. The paramedical staff trained in contact lens assessment, prescription, and application also assess the patients and help apply the varied type of contact lenses based on the indication. The nursing staff also assesses the patients on slit lamp after contact lens fit to determine the fitting. The nursing staff and contact lens paramedical experts can also inform the patient regarding the varied contact lens solutions, hygiene, the timing of wear, and how to store the lenses.[55]

Nursing, Allied Health, and Interprofessional Team Monitoring

The nursing team and contact lens expert help regularly monitor patients prescribed the contact lenses. The patients are evaluated and monitored on every visit for contact lens fit, comfort, any problems associated with contact lens application, hygiene, and the visual outcome related to contact lens use.[55]