lenses are made from a mold taken of the anterior surface of the eye; the mold is made of an alginate mix, which hardens in the shape of the ocular surface. This alginate mold is then used to make a plaster mold, which, in turn, is used to make the actual scleral lens.
Patient Examination and Contact Lens Selection
As in all patient care, a complete history and eye examination are needed to rule out serious ocular problems such as glaucoma and macular degeneration.
Patient Examination
A clinician needs specific information to select a contact lens for a particular patient. This information includes the patient’s daily activities (desk work, driving, and so on) and reason for using contact lenses (eg, full-time vision, sports only, social events only, changing eye color, avoiding use of reading glasses). If a patient is already a contact lens user, the fitter must also find out the following: the number of years the patient has worn contact lenses, the current type of lens worn, the wear schedule, and the care system used. In addition, the fitter must determine whether the patient currently has or previously had any problems with lens use.
Factors that may suggest an increased risk of complications with contact lens use include diabetes mellitus, especially if uncontrolled; immunosuppression; long-term use of topical medications such as corticosteroids; and environmental exposure to dust, vapors, or chemicals. Other relative contraindications to contact lens use include an inability to handle and/or care for contact lenses; monocularity; abnormal eyelid function, such as with Bell palsy; severe dry eye; and corneal neovascularization. The primary indications for contact lenses in a patient with preexisting corneal disease are therapeutic or bandage lenses and a rigid contact lens for the correction of irregular astigmatism.
Key areas to observe during slit-lamp examination include the eyelids (to rule out blepharitis or mechanical lid abnormalities such as trichiasis, ectropion, and entropion), the tear film, and the ocular surface (to rule out dry eye). Eyelid movement and blink should also be observed. The cornea and conjunctiva should be evaluated carefully for signs of ocular surface disease, allergy, scarring, symblepharon, or other signs of conjunctival scarring diseases, such as ocular cicatricial pemphigoid (mucous membrane pemphigoid). Through refraction and keratometry, the ophthalmologist can determine whether there is significant corneal, lenticular, or irregular astigmatism. The identification of irregular astigmatism may suggest other pathologies, such as keratoconus, that would require further evaluation.
Contact Lens Selection
The selection of a contact lens for an individual patient and eye is a complex process. Optical, biological, mechanical, and social considerations are among the factors that enter into this process (see Table 4-1).
Soft contact lenses are currently the most frequently prescribed and worn lenses in the United States. They can be classified according to various characteristics. Given this variety, selection of the appropriate lens for each patient may be difficult. Typically, an experienced fitter knows the
characteristics of several lenses that cover the needs of most patients.
The main advantages of soft contact lenses are their shorter period of adaptation and their high level of comfort (Table 4-3). They are available with many parameters so that all regular refractive errors are covered. Furthermore, the ease of fitting soft lenses makes them the first choice of many practitioners.
Table 4-3
The decision about a replacement schedule may be made on a cost basis. Conventional lenses (changed every 6–12 months) are often the least expensive, but disposable lenses and conventional lenses that are replaced more frequently are typically associated with less irritation, such as red eyes, and more consistent quality of vision. Daily disposable lenses require the least amount of care, so less expense is involved for lens-care solutions. Disposable lenses are generally more expensive than reusable contact lenses, but they offer advantages to patients who are either unable or unwilling to properly care for and disinfect contact lenses. They are also helpful in patients who have unacceptable reactions to lens-care solutions or protein deposits on contact lenses.
Daily wear (DW) is the most favored wear pattern in the United States. Extended wear (EW)— that is, leaving the lens in during sleep—is less popular, primarily because of reports from the 1980s of the increased incidence of keratitis with EW lenses. However, improved materials that have far greater oxygen permeability (Dk = 60 to 140) have been approved for EW; use of these materials may decrease the risk of infection compared with the risk associated with earlier materials (although it is difficult to document the incidence because serious infections are rare with all lenses currently in use). Patients who want EW lenses should understand the risks and benefits of this modality. Specifically, patients are at increased risk of bacterial keratitis and other ocular infections (see BCSC Section 8, External Disease and Cornea.). Risk factors for EW complications include a previous history of eye infections, lens use while swimming, and any exposure to smoke. To avoid complications associated with EW lenses, the clinician should make sure that the lenses fit properly, that they feel comfortable to the patient, that the patient’s vision is good, and most importantly, that the patient is informed of and will adhere to care instructions. Patients should understand the need for careful contact lens care and replacement, as well as the signs and symptoms of eye problems that require the attention of a physician.
Rigid contact lenses continue to be used today, but only by a small percentage (<20%) of lens wearers in the United States. The original hard contact lenses, made of PMMA, are rarely used now because of their oxygen impermeability. Currently, commonly used RGP materials include fluorinated silicone acrylate with oxygen permeability ranging from the 20s to more than 250 and are manufactured with many parameters. Modern RGP lenses are approved for DW—some even for extended, overnight wear. Because of the manufacturing costs and the many parameters available, RGP lenses are not usually offered in disposable packs, but yearly replacement is recommended. The main advantages of RGP lenses are the quality of vision they offer and the ease with which they correct astigmatism (see Table 4-3). The main disadvantages are initial discomfort, a longer period of