Contact lenses are in immediate contact with the cornea. Although they are foreign bodies, most patients adapt to properly fitted contact lenses. Contact lenses differ from eyeglasses in that they correct the refractive error closer to the location of its origin. For this reason, the quality of the optical imageviewed through contact lenses is higher than that viewed through eyeglasses.Contact lenses have significantly less influence on the size of the retinal image than does correction with eyeglasses. Lenses do not cloud up in rainy weather or steam, and peripheral distortion is minimized. The cosmetic dis-advantage of thick eyeglasses in severe ametropia is also eliminated. Severeanisometropia requires correction with contact lenses for optical reasons, i.e.,to minimize aniseikonia.
Contact lenses are defined by the following characteristics:
❖ Diameter of the contact lens.
❖ Radius of curvature of the posterior surface.
❖ Geometry of the posterior surface, i.e., spherical, aspherical, complex cur-vature, or toric.
❖ Refractive power.
❖ Oxygen permeability of the material (Dk value).
The cornea requires oxygen from the precorneal tear film. To ensure this supply, contact lens materials must be oxygen-permeable. This becomes all the more important the less the contact lens moves and permits circulation of tear fluid. Contact lenses may be manufactured from rigid or flexible materi-als.
These contact lenses have a stable, nearly unchanging shape. Patients takesome time to become used to them and should therefore wear them often. Thegoal is to achieve the best possible intimacy of fit between the posterior sur-face of the lens and the anterior surface of the cornea (Fig. 16.19). This allows the contact lens to float on the precorneal tear film. Every time the patient blinks, the lens is displaced superiorly and then returns to its central position. This permits circulation of the tear film.
Previously, polymethyl methacrylate (PMMA) was used as a material.However, this is practically impermeable to oxygen. The lenses were fitted in small diameters with a very shallow curvature; the central area maintained contact with the cornea while the periphery projected. This allowed excellent tear film circulation, and patients were able to wear the lenses for surpris-ingly long periods. Today, highly oxygen-permeable materials such as silicone copolymers are available.
This eliminates the time limit for daily wearing. These lenses may also remain in the eye overnight in special cases, such as aphakic patients with poor coordination (prolonged wearing).
Rigid contact lenses can be manufactured as spherical lenses and toriclenses.Sphericalcontact lenses can almost completely compensate forcor-neal astigmatism of less than 2.5 diopters. This is possible because the spacebetween the posterior surface of the spherical contact lens and the anterior surface of the astigmatic cornea is filled with tear fluid that forms a “tear lens.” Tear fluid has nearly the same refractive index as the cornea. More severe cor-neal astigmatism or internal astigmatism requires correction with toric con-tact lenses. Rigid contact lenses can even correct severe keratoconus.
The material of the contact lens, such as hydrogel, is soft and pliable. Patients find these lenses significantly more comfortable. The oxygen permeability of the material depends on its water content, which may range from 36% to 85%. The higher the water content, the better the oxygen permeability. However, it is typically lower than that of rigid lenses. The material is more permeable to foreign substances, which can accumulate in it. At 12.5 – 16 mm, flexible lenses are larger in diameter than rigid lenses. Flexible lenses are often sup-ported by the limbus. The lens is often displaced only a few tenths of a milli-meter when the patient blinks. This greatly reduces the circulation of tear filmunder the lenses. This limits the maximum daily period that patients are able towear them and requires that they be removed at night to allow regeneration of the cornea. Deviation from this principle is only possible in exceptional cases under the strict supervision of a physician.
As the lenses are almost completely in contact with the surface of the cor-nea, corneal astigmatism cannot be corrected with spherical soft lenses. This requires toric soft lenses.
The following types of special lenses are available for specific situations:
In the presence ofcorneal erosion,soft ultra-thin (0.05 mm) contact lenses act as a bandage and thereby accelerate re-epithelialization of the cornea. They also reduce pain. Soft contact lenses may also be used in patients receiving topical medication as they store medication and only release it very slowly.
These are collagen devices that resemble contact lenses.These shields are gradually broken down by the collagenase in the tear film. They are used as bandages and substrates for topical medication in the treat-ment of anterior disorders, such as erosion or ulcer.
These colored contact lenses with a clear central pupil areused in patients with aniridia and albinism.
They produce good cosmetic results, reduce glare, and can correct a refractive error where indicated.
These lenses were developed to allow the use of con-tact lenses in presbyopic patients. As in eyeglasses, a near-field correction is ground into the lens. This near-field portion is always located at the bottom of the lens because the lens is heavier there. When the patient gazes downward to read, the immobile lower eyelid pushes this near-field portion superiorly where it aligns with the pupil and becomes optically effective. Another possi-bility is diffraction (bending of light rays as opposed to refraction) through concentric rings on the posterior surface of the contact lens. This produces two images, a distant refractive image and a near-field diffractive image. The patient chooses the image that is important at the moment. It is also possible to correct one eye for distance vision and the fellow eye for near vision (monocular vision).
Contact lenses exert mechanical and metabolic influences on the cornea.
Therefore, they require the constant supervision of an ophthalmologist.
Mechanical influences on the cornea can lead totransient changes in refrac-tion. “Spectacle blur” can result when eyeglasses suddenly no longer providethe proper correction after removing the lens. Contact lenses require careful daily cleaning and disinfection. This is more difficult, time-consuming, andmore expensive than eyeglass care and is particular important with soft lenses.
The macromolecular mesh of materialabsorbs proteins, protein breakdown products, low-molecular-weight sub-stances such as medications and disinfectants, and bacteria and fungi. Serious complications can occur where daily care of the contact lenses is inadequate. With their threshold oxygen permeability, soft contact lenses interfere with corneal metabolism. Contact lenses are less suitable for patients with symp-toms of keratoconjunctivitis sicca.
Complications have been observed primarily in patients wearing soft contact lenses. These include:
Infectious keratitis (corneal infiltrations and ulcers) caused by bacteria,fungi, and protozoans.
Acanthamoeba keratitis is a serious complication affecting wearers ofsoft contact lenses and often requires penetrating keratoplasty.
This is an allergic reaction of the palpebralconjunctiva of the upper eyelid to denatured proteins. It results in prolifera-tive “cobblestone” conjunctival lesions.
Corneal vascularization may be interpreted as the result of insufficientsupply of oxygen to the cornea.
This usually makes it impossible to continuewearing contact lenses.