Studies comparing hydrogel and GP lenses have found significantly better visual performance with GP lenses. This includes both subjective patient preference⁴,⁵ and contrast sensitivity function.⁶,⁷ The superior optical quality provided by a stable refractive surface with little to no water content is the primary reason for this visual difference between contact lens types. In comparing both soft and GP lenses, it was found that whereas both soft and GP lenses induce more aberrations for the eyes that have low wavefront aberrations, soft lens wear tends to induce more higher-order aberrations and GP lens wear tends to reduce higher-order aberrations.⁸,⁹,¹⁰ GP lenses also maintain surface wettability better than hydrogel lenses. This can lead to improved long-term comfort and less deposit formation, although the benefits of this is less with the popularity of disposable lenses, especially daily disposable lenses. GP lenses represent a very good option if the sphere-to-cylinder refractive error ratio is ≤2:1. When the corneal cylinder is 2.5 D or greater, a bitoric design often provides a stable and nonfluctuating vision correction.

The benefits of a small overall diameter lens that does not compress the limbus, lens movement typically resulting in good tear exchange and debris with the blink, potentially (depending on the material) unparalleled oxygen permeability, and good surface wettability have resulted in numerous clinical studies that have found GP lenses to be a safer alternative to soft lenses. GP lenses have resulted in less corneal staining⁴,¹¹ and are less likely to result in peripheral corneal infiltrates, not uncommon with tight-fitting soft lenses.¹²,¹³ The prevalence of microbial keratitis has been found to be less with GP lenses, with an incidence of 1 to 1.48 per 10,000 eyes as compared to 3.50 per 10,000 eyes with soft daily-wear lenses and 20 per 10,000 eyes with extended-wear soft lenses.¹⁴,¹⁵ In several studies conducted in the United States in which the relative risk of wearing extended-wear lenses was evaluated, GP lenses resulted in the lowest rate of infectious keratitis.¹⁶,¹⁷,¹⁸ There is also less binding of Pseudomonas aeruginosa¹⁹,²⁰ and
acanthamoeba²¹ to GP lenses than soft lenses. In addition, giant papillary conjunctivitis (GPC, also known as contact lens papillary conjunctivitis, CLPC) is less likely to occur with GP than with soft lenses.²²

Unlike soft lenses, GP lenses do not tear or easily change shape or coloration; therefore, frequent lens replacement is not necessary.

One of the challenges facing contact lens practitioners today is patient retention and revenue from replacement contact lenses. The Fairness to Contact Lens Consumers Act (FCLCA) mandates that practitioners provide the contact lens prescription to their patients. With the increasing number of Internet sites offering replacement lenses, many patients think that they can bypass the professional care provided by eye care practitioners. However, as a custom device, GP lenses are much more difficult to obtain through these unconventional channels; in fact, it has been reported that only 1% of mail-order lenses were GP.²³ That percentage may not be any higher via the Internet for the same reason. The variety of parameters specified in a GP lens prescription (including base curve radius, overall and optical zone diameters, peripheral curve widths and radii) helps to demonstrate the specialty nature of the device to patients. Likewise, although the FCLCA requires that every contact lens patient is entitled to their prescription, they can only be provided this information once it has been determined, which may be as long as 1 to 3 months after dispensing for a GP patient.

It has been found that contact lens patients are approximately 50% more profitable to the eye care practice than non-contact lens wearers.²⁴ Further, it has been found that GP wearers generate greater revenue to the practice than soft lens wearers.²⁵ This was attributed to several factors, including the fact that GP lens patients return more frequently for eye examinations and purchase eyeglasses more often than soft lens patients.

Several studies have found that GP lenses slow down the progression of myopia,²⁶,²⁷,²⁸ although this was not the conclusion of a recent study by Katz et al.²⁹ The most comprehensive and best controlled study was the Contact Lens and Myopia Progression (CLAMP) study.²⁸ In this study, children were adapted to GP lenses before being randomized to GP or soft lens groups. After 3 years, GP-wearing young patients increased by 1.56 D in myopia, whereas soft lens wearers increased by 2.19 D. However, most of the refractive error change occurred during the first year and no difference was found between soft and GP lens wearers in axial growth of the eyes. Therefore, it would be safe to conclude that GP lenses may slow down the progression in young people.

A more important benefit of GP lenses, particularly with—but not limited to—young people, pertains to overnight orthokeratology (OOK). Recent studies have found that OOK reduces myopia³⁰ and slows down axial growth.³¹,³² In fact, both the Longitudinal Orthokeratology Research in Children (LORIC)31 and the Corneal Reshaping and Yearly Observation of Nearsightedness (CRAYON) studies found that OOK subjects had 57% less axial growth than control subjects (i.e., consisting of spectacle wearers in LORIC and both soft lens and non-OOK GP wearers in CRAYON) over a 2-year period.

GP lenses are most often the material of choice when fitting postsurgical and irregular corneas. The optical quality and rigid nature of these lenses allow for a more regular refractive surface because of the ability of these lenses to exhibit some molding ability and sphericalization
of an irregular cornea. It has been found in the Collaborative Longitudinal Evaluation of Keratoconus (CLEK) study, with over 1,100 keratoconus subjects, that 73% were GP lens wearers.³³ Patients who have undergone refractive surgery and still require an optical correction benefit from the increased oxygen delivery of GP lenses versus soft lenses. Numerous types of reverse geometry lens designs—incorporating a steep secondary curve radius—have been developed to allow a GP lens to align better and exhibit satisfactory centration on a postrefractive surgery patient who has a significantly flatter central than midperipheral cornea. Likewise, with the use of corneal topography instrumentation, it is possible for laboratories to fabricate a lens to closely match the corneal irregularity resulting from keratoconus, trauma, postpenetrating keratoplasty, postrefractive surgery, and other causes of irregular cornea. The availability of hyperpermeable GP lens materials also allows for optimum oxygen delivery to the cornea.

Presbyopic patients benefit from any one of several GP presbyopic designs. Aspheric multifocal and segmented and annular translating designs have resulted in success rates of over 75%.³³,³⁴,³⁵,³⁶,³⁷ In addition, when compared to progressive addition lenses, monovision lenses, and soft bifocal lenses, aspheric GP multifocal lenses resulted in significantly better high- and low-contrast acuity and contrast sensitivity function as compared to the other contact lens options and exhibited visual parity to spectacle wear.³⁸ As a result of improvements in manufacturing technology, higher-add aspheric multifocal and segmented translating designs with an intermediate correction have been introduced.

GP lenses have the natural ability to correct anterior corneal astigmatism by allowing the tear lens to compensate for the corneal astigmatism.

It has been reported that patients who failed with soft lens wear because of factors such as poor vision or giant papillary conjunctivitis (GPC) have been successfully refitted into GP lenses.³⁹,⁴⁰ Likewise, individuals who have had a history of eye infections are also good candidates and often are motivated to be refitted into a potentially safer modality. Patients who are not satisfied with their vision from soft toric lenses are often successful with GPs. Practitioners who continue to fit a series of soft toric lenses on astigmatic patients—especially high astigmatic patients or individuals who have <2:1 sphere-to-cylinder ratio for their refraction—despite reduced vision because of lens rotation are doing a disservice to their patients. These patients often observe an immediate improvement in vision when refitted into GP lenses. As will be discussed in the next chapter, the use of a topical anesthetic before the initial lens application will be beneficial in minimizing initial awareness.

The most commonly reported cause of discontinuation of GP lens wear is discomfort.⁴,³⁹,⁴² The initial sensation experienced by new GP wearers varies from mild awareness to much discomfort and tearing. Conversely, soft lenses are more comfortable initially, primarily a result of their larger overall diameter resulting in less movement with the blink. Andrasko and Billings⁴³ evaluated numerous factors in new GP lens wearers after 20 to 30 minutes of wear. If patients reported that they experienced either poor comfort or itching (or both) after this time period, they were deemed poor candidates for GP lenses. Whether discomfort is going to be problematic can sometimes be determined during the prefitting evaluation. If the patient exhibits apprehension during primary care examination procedures such as lid eversion, fluorescein application, or tonometry, soft lenses should be considered or the patient should be provided with a slow build-up schedule for adaptation. The authors have a three-step program to minimize initial awareness that consists of how GP lenses are presented to the patient, the use of a topical anesthetic before the initial application, and, if possible, allowing the patient to see optimally with the first lenses applied (i.e., via fitting empirically or from an inventory).⁴⁴ This will be explained in much more detail in the next chapter.

GP wearers should always have a backup pair as lenses can be lost and, on occasion, warped or possibly broken. Soft lenses have the benefit of being available in, at minimum, six to a package in most cases.

The simplicity of soft lens design lends itself very well to fitting from an inventory and being able to make small changes easily and determine if the lenses have successfully improved vision and/or fitting relationship. Patients can be sent home in trial lenses for which they experience good vision and can be assessed sometime afterward to determine the final lenses to be ordered. The custom nature of GP lenses often results in practitioners ordering lenses for the patient and, if changes are indicated, new lenses have to be ordered.

Soft lenses have the benefit of allowing occasional or intermittent wear with relatively little effect on comfort. Soft lenses also can be used to change or enhance eye color, whereas the smaller overall diameter of GP lenses all but preclude iris color changes.

Another limitation of GP lenses is increased susceptibility for dust and debris to become trapped underneath the lens. Patients who work exclusively outdoors in a dusty, windy environment may be better candidates for soft lenses. Likewise, individuals who participate in sports often benefit from soft lenses.⁴⁵ If GP lenses are indicated in these athletes, the use of a large overall diameter, low edge clearance design would be indicated.

It has been found that GP lenses can result in a smaller palpebral aperture size versus soft lens wearers and non-contact lens wearers.⁴⁶ In addition, one study reported 15 cases of long-term rigid lens wearers who presented with blepharoptosis.⁴⁷ It was hypothesized that this problem may be the result of chronic lens removal in which the pulling of the lids laterally over time may lead to levator aponeurosis dehiscence.