Therapeutic contact lens use has been present for over a century with early descriptions in 1886 of gelatine shields used as bandage lenses after cataract surgery for postoperative anesthesia and antisepsis.1 Wichterle and Lim2 raised awareness of a variety of potential clinical applications available for therapeutic lens wear in their landmark article in 1960. Reports ensued describing the use of therapeutic soft contact lenses for treatment of corneal disorders such as bullous keratopathy, cicatricial pemphigoid, exposure keratitis, chemical burns, and neurotrophic keratitis.2, 3 Subsequently, the use of therapeutic contact lenses was approved for use in the United States in 1979 followed shortly by approval for extended soft contact lens wear for cosmetic purposes in 1981. Since the inception of approved use, therapeutic lens technology has further progressed with advancements in production, fitting techniques, and lens material. The latest hydrogel lens innovation is the development of silicone hydrogel contact lenses. Silicone hydrogel lenses afford the availability of increased oxygen transmission with increased rigidity while providing improved safety profiles for overnight therapeutic use compared to traditional hydrogel lens material. With the newer generation of silicone hydrogel contact lenses, therapeutic hydrogel lens use continues to be a highly effective treatment modality for ocular surface disease. These advances make therapeutic contact lenses firmly entrenched as a highly effective device for management and treatment of a variety of corneal and external diseases.
HYDROGEL LENS CLASSIFICATION
Hydrogel lenses are formed by the cross-linking of chains of hydrophilic monomeric units into a matrix-like polymer. Each polymer has unique attributes defined by the interaction of chemical groups and the degree of crosslinking. The main component of conventional hydrogel lenses is poly 2-hydroxyethyl methacrylate (polyHEMA), and additional hydrophilic monomers are added to improve wettability and enhance oxygen transport of the lens. The addition of substances such as methacrylic acid or higher proportions of hydrophilic neutral groups such as polyvinyl alcohol can add increased water content to the contact lens.
Silicone hydrogel lenses were introduced into the market in 1998. These lenses include the addition of siloxane monomers to the hydrophilic monomers of hydrogel lenses. The addition of siloxane into the polymer provides extremely high oxygen permeability, but also lowers the proportion of hydrophilic monomers that can be incorporated, leading to a reduction in water content. The hydrogel component in silicone hydrogel lenses counteracts the reduction in water content from siloxane polymers by facilitating increased flexibility, wettability, and fluid transport, a function that enhances lens movement.
The U.S. Food and Drug Administration (FDA) divided hydrogel contact lenses into four groups in 1985 based on ionicity and water content. Group 1 consists of nonionicpolymers with low water content. Group 2 consists of nonionic polymers with high water content. Group 3 consists of lenses with ionic polymers and low water content, and group 4 includes lenses with ionic polymers and high water content. Silicone hydrogels are currently included into groups 1 and 3, but a new classification may occur in the future to account for the unique attributes of these lenses. A variety of hydrogel lenses are currently FDA approved for use in extended wear (Table 11.1), some of which are also approved as therapeutic devices (Table 11.2).4 While not all of the approved extended wear hydrogel lenses are approved for therapeutic use, the improved lens parameters of oxygen permeability and increased water content afford off-label use of some of the newer hydrogel contact lenses despite the lack of FDA labeling for therapeutic use.
HYDROGEL LENS PHYSIOLOGY
Hydrogel lenses can significantly impact the normal architecture, metabolism, and physiology of the cornea and ocular surface. Most of the physiologic changes induced by therapeutic contact lenses are related to the decreased oxygen delivery to the cornea that occurs during lens wear. While a contact lens is in place, oxygen delivery to the cornea is dependent on oxygen dissolved in the tear film after diffusion of oxygen through the lens matrix or from limbal vasculature. The oxygen dissolved in the tear film is exchanged around the lens as the lens moves with each blink. A hypoxic environment can be created if the oxygen tension decreases to a level that leads to significant decreased oxygen availability to the cornea. When the eyes are open without a contact lens, oxygen available to the cornea through the tear film is derived from atmospheric oxygen with a partial pressure of 155 mm Hg. In the setting of closed eyelids, the oxygen available to the cornea is derived from the limbal blood vessels and can range from only 40 to 61 mm Hg.5, 6 Traditional therapeutic hydrogel lenses can potentially create a situation in which the partial pressure of oxygen available to the cornea may be as low as 10 to 15 mm Hg.7 Thus wearing a contact lens can further decrease the amount of oxygen available to the cornea when the eyelid is both open and closed. This hypoxic state can create a variety of undesirable physiological changes within the ocular surface, including increased carbon dioxide, glycogen depletion, lactate accumulation, and a potential for intracellular epithelial edema and stromal corneal edema.6, 8
TABLE 11-1 Hydrogel Soft Contact Lenses That Have Received FDA Approval for Extended Wear
Lens Brand
Material
Manufacturer
Dk Value
Dk/t (1×10-9)
Water Content (%)
Power (Diopter)
BC/Diameter (mm/mm)
Kontour 55
Methafilcon A
Kontour Kontact
18.8
16.0
55.0
+20.00 to −20.00
8.3, 8.6, 8.9/12-24
CO Soft 55
Methafilcon A
Advanced Ultra Vision
18.8
16.0
55.0
+30.00 to −30.00
6.5-11/12-25
Silsoft
Elastofilcon A
Bausch & Lomb
340.0
71.0
0.2
+11.50 to +20.00
7.5, 7.7, 7.9, 8.1, 8.3/11.3, 12.5
LL-55
Methafilcon A
Lombart
18.8
16.0
55.0
+5.00 to −10.00
8.4, 8.7, 9.0/14.5
Softcon
Vifilcon A
Lombart
16.0
20.0
55.0
+9.50 to −8.00
8.1, 8.4, 8.7/14.0, 14.5
Sof-Form 55
Methafilcon A
Unilens Corp
18.8
14.5
55.0
+9.75 to −10.00
8.3, 8.6, 8.9/14.0, 15.0
Soflens 38
Polymacon
Bausch & Lomb
8.4
24.3
38.0
+4.00 to −9.00
8.4, 8.7, 9.0/14
Preference
Tetrafilcon A
Cooper Vision
9.3
20.0
43.0
+6.00 to −10.00
8.4, 8.7/14.0, 14.4
Frequency 55
Methafilcon A
Cooper Vision
18.8
16.0
55.0
+8.00 to −10.00
8.4, 8.7, 9.0/14.2
Hydrasoft
Methafilcon B
Cooper Vision
18.8
24.0
55.0
+10.00 to −20.00
8.3, 8.6, 8.9, 9.2/14.2, 15.0
Fresh Look
Phemfilcon A
Alcon
16.1
20.0
55.0
+6.00 to −8.00
8.6/14.5
BioMedics 55
Ocufilcon D
Cooper Vision
19.7
28.0
55.0
+6.00 to −10.00
8.6, 8.8, 8.9/14.2
Acuvue-2
Etafilcon A
Vistakon
28.0
33.3
58.0
+8.00 to −12.00
8.3-9.1/14.0
Air Optix Night & Day Aqua*
Lotrafilcon A
Alcon
140.0
175.0
24.0
+6.00 to −10.00
8.4, 8.6/13.8
Air Optix Aqua*
Lotrafilcon B
Alcon
138.0
138.0
33.0
+6.00 to −10.00
8.6/14.2
PureVision*
Balafilcon A
Bausch & Lomb
91.0
110.0
36.0
+6.00 to −12.00
8.6/14.0
Acuvue Oasys*
Senofilcon A
Vistakon
103.0
147.0
38.0
+8.00 to −12.00
8.4, 8.8/14.0
Biofinity*
Comfilcon A
Cooper Vision
128.0
160.0
48.0
+8.00 to −12.00
8.6/14.0
BC, base curve; Dk, oxygen permeability; Dk/t, oxygen transmissibility; *, Silicone hydrogel lenses; ± FDA approval for extended wear.
TABLE 11-2 Hydrogel Soft Contact Lenses Receiving FDA Approval for Therapeutic Use
Lens Brand
Material
Manufacturer
Dk Value
Dk/t (1 ×10-9)
Water Content (%)
Power (Diopter)
BC/Diameter (mm/mm)
Air Optix Night & Day Aqua*
Lotrafilcon A
Alcon
140.0
175
24.0
+6.00 to −10.00
8.4, 8.6/13.8
Air Optix Aqua*
Lotrafilcon B
Alcon
138.0
138
33.0
+6.00 to −10.00
8.6/14.2
PureVision*
Balafilcon A
Bausch & Lomb
91.0
110
36.0
+6.00 to −12.00
8.6/14.0
Acuvue Oasys*
Senofilcon A
Vistakon
103.0
147
38.0
+8.00 to −12.00
8.4, 8.8/14.0
Sof-Form 55
Methafilcon A
Unilens Corp
18.8
14.5
55.0
+9.75 to −10.00 8.3, 8.6, 8.9/14.0, 15.0
BC, base curve; Dk, oxygen permeability; Dk/t, oxygen transmissibility; *, Silicone hydrogel lenses; ± FDA approval for extended wear.
The degree to which oxygen tension and oxygen availability to the cornea are diminished during contact lens wear is dependent on a number of factors including polymer type, thickness, water content, diameter, base curve, peripheral curvature, and edge design.9 Oxygen delivery to the cornea with a hydrogel lens in place is maximized by high water content, reduced lens thickness, and adequate movement of the lens during blinking. Additional factors in assessing the risk of hypoxia with a particular lens include the oxygen permeability of a lens and its oxygen transmissibility. The oxygen permeability of a contact lens is expressed as the Dk value. As oxygen flow into the cornea increases, the Dk value increases. The Dk value is partially dependent on water content, and as the water content increases by 20% in contact lenses with over 40% hydration, the Dk value doubles.9 Oxygen transmissibility is determined by dividing the Dk value by the lens thickness. This parameter is extremely important when evaluating lens safety for adequate oxygen delivery during overnight wear, the time in which hypoxic conditions are greatest because of eyelid closure.
Therapeutic lenses can alter the corneal and conjunctival epithelium by creating direct mechanical trauma to the ocular surface. The induced microtrauma accelerates the epithelial regenerative capacity, leading to depleted glycogen stores, accumulation of lactic acid, and increased tear film acidity. Important factors to limit ocular surface trauma include adequate lens fit, adequate tear levels, and less contact of the lens with the cornea.8 Therapeutic lenses also alter tear film composition by creating decreased tear circulation and tear turnover under the contact lenses. This disturbance decreases the advantage of mechanical removal of ocular surface microbes and tear film aggregates from the normal tear film migration, flushing, and cleansing action.
THERAPEUTIC LENS FITTING
The fitting process for a therapeutic hydrogel lens is not difficult, yet several differences exist between cosmetic and therapeutic hydrogel lens fitting. One key difference is the lack of reliability of central keratometry in therapeutic lens fitting as the corneal surface is typically irregular, leading to inaccurate measurements; thus trial fitting is most effective to achieve a proper fit. Important fitting parameters for therapeutic lenses include lens diameter, centration, and movement. Desired characteristics of a therapeutic hydrogel lens include high water content, complete corneal coverage along with at least 1 mm of limbus coverage, lack of lens edge fluting, lack of conjunctival molding, and approximately 1 mm of excursion with blinking.
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