Clinically Important Features of Contact Lens Optics
Contact lenses and conventional lenses have 4 parameters in common: posterior surface curvature (base curve), anterior surface curvature (power curve), diameter, and power (see Fig 4-1). However, unlike for spectacle lenses, the shape of contact lenses’ posterior surface is designed primarily to have certain fitting relationships with the anterior surface of the eye.
The refractive performance of contact lenses differs from that of spectacle lenses for 2 primary reasons: (1) contact lenses have a shorter vertex distance and (2) tears, rather than air, form the interface between the contact lens and the cornea. Unique optical considerations that are related to contact lenses include field of vision, image size, accommodation, convergence demand, the tear lens, correction of astigmatism, and correction of presbyopia. Each type of contact lens has unique optical considerations (Table 4-1).
Table 4-1
Field of Vision
Spectacle frames reduce the field of vision by approximately 20°. Owing to their proximity to the entrance pupils and lack of frames, contact lenses provide a larger field of corrected vision and avoid much of the peripheral distortion, such as spherical aberration, created by high-power spectacle lenses.
Image Size
Retinal image size is influenced by the vertex distance and power of corrective lenses. Contact lenses have shorter vertex distances than do spectacles, so image size changes less with contact lenses than
with spectacles.
Anisometropia and image size
Axial ametropia is predominant in eyes with higher (non–surgically induced) refractive errors. Theoretically, the anisometropic aniseikonia of such eyes is minimized when the corrective lens is placed in the eyes’ anterior focal plane (see discussion of Knapp’s law in Chapter 1), which is, on average, approximately 15.7 mm anterior to the corneal vertex. In axial myopia, moving the corrective lens posterior to the eye’s focal plane (closer to the cornea) increases the size of the retinal image compared with that of an emmetropic eye. The reverse is true in axial hyperopia. In practice, however, using contact lenses to correct the refractive error of the eyes is usually best for managing anisometropia because anisophoria generated by induced prism in off-axis viewing of spectacle lenses is eliminated. In addition, the greater separation between the elements in the stretched retinas of larger myopic eyes may explain the less-than-perceived magnification observed with contact lenses. Surgically induced anisometropia (resulting from, for example, cataract or refractive surgery) without an axial component is usually managed best through use of contact lenses or additional surgery; in either method, the images will be closer in size than if spectacles are used.
Monocular aphakia and aniseikonia
Minimizing aniseikonia in monocular aphakia improves the functional level of binocular vision. An optical model of surgical aphakia can be represented by inserting a neutralizing (minus-power) lens in the location of the crystalline lens and correcting the resulting ametropia with a forward-placed plus-power lens. Doing so effectively creates a Galilean telescope within the optical system of the eye. Accordingly, magnification is reduced as the effective plus-power corrective lens (corrected for vertex distance) is moved closer to the neutralizing minus-power lens (the former site of the crystalline lens). This model illustrates why contact lens correction of aphakia creates significantly less magnification than does a spectacle lens correction; a posterior chamber intraocular lens creates the least magnification of all.
Although the ametropia of an aphakic eye is predominantly refractive, it can also have a significant preexisting axial component. For example, the coexistence of axial myopia would further increase the magnification of a contact lens–corrected aphakic eye (compared with the image size of the spectacle-corrected fellow phakic myopic eye). Even if the image size of the fellow myopic eye were to be increased by fitting this eye with a contact lens, the residual aniseikonia might still exceed the limits of fusion and cause diplopia (Clinical Example 4-1). Divergent strabismus can develop in aphakic adult eyes (and esotropia may develop in children) if fusion is interrupted for a significant period. If diplopia does not resolve within several weeks, excessive aniseikonia should be suspected and confirmed by demonstration of the patient’s inability to fuse images superimposed with the aid of prisms. Such patients are usually aware that the retinal image in the aphakic eye is larger than that in the fellow phakic eye.
Clinical Example 4-1
Fitting a unilateral aphakic eye causes diplopia that persists in the presence of prisms that superimpose the 2 images. The refractive error of the fellow eye is –5.00 D, and the image of the aphakic eye is described as being larger than that of the fellow myopic eye.
How can the diplopia be resolved?
The goal is to reduce the aniseikonia of the 2 eyes by magnifying the image size of the phakic eye and/or reducing the image size of the contact lens–corrected aphakic eye. To achieve the former, correct the myopic phakic eye with a contact lens to increase its image size. If doing so is inadequate, overcorrect the contact lens by 5.00 D and prescribe a spectacle lens of – 5.00 D for that eye, thereby introducing a reverse Galilean telescope into the optical system of the eye. (If, however, the phakic eye were hyperopic, its image size would be increased by correcting its refractive error with a spectacle lens rather than a contact lens.)
When the fellow phakic eye is significantly myopic, correcting it with a contact lens increases its image size and often reduces the aniseikonia sufficiently to resolve the diplopia. If excessive aniseikonia persists, the clinician should aim to further reduce the image size of the contact lens– corrected aphakic eye. Overcorrecting the aphakic contact lens and neutralizing the resulting induced myopia with a forward-placed spectacle lens of appropriate minus power can achieve the additional reduction in image size. In effect, this process introduces a reverse Galilean telescope into the optical system of that eye. Empirically, increasing the power of the distance aphakic contact lens by +3 D and prescribing a –3 D spectacle lens for that eye usually suffice. Alternatively, if it is impractical to fit the fellow myopic eye with a contact lens, the clinician may elect to add plus power to the aphakic contact lens by an amount equal to the spherical equivalent of the refractive error of the fellow eye, in effect equalizing the myopia of the 2 eyes. The resulting decrease in the residual aniseikonia usually improves fusional potential and facilitates the recovery of fusion even of significant aniseikonic exotropia over several weeks. However, the resolution of aphakic esotropia or cyclotropia is less certain.
In contrast with axial myopia, coexisting axial hyperopia reduces the magnification of a contact lens–corrected aphakic eye. Residual aniseikonia can be further mitigated by correction of the fellow hyperopic eye with a spectacle lens (rather than a contact lens) to maximize image size.
Infantile aphakia
Management of aphakia in infants and young children represents a challenge because of the possibility of amblyopia and permanent vision loss. Contact lens wear may be ineffective in children because of poor patient adherence; therefore, intraocular lens implants may be a better option. Aphakia may be corrected in infants with contact lenses or lens implants. The optimal method in this group of patients is not yet known. The rapid change in axial length and corneal power during infancy (see Chapter 2) may make the selection of implant power difficult. Aggressive management of both optical correction and amblyopia treatment is necessary to achieve an optimal outcome in such young patients.
Autrata R, Rehurek J, Vodicková K. Visual results after primary intraocular lens implantation or contact lens correction for aphakia in the first year of age. Ophthalmologica. 2005;219(2):72–79.
Infant Aphakia Treatment Group; Lambert SR, Buckley EG, et al. The infant aphakia treatment study: design and clinical measures at enrollment. Arch Ophthalmol. 2010;128(1):21–27.
Accommodation
Accommodation is defined as the difference in vergence at the first principal point of the eye (1.35 mm behind the cornea) between rays originating at infinity and those originating at a near point.
This disparity creates different accommodative demands for spectacle and contact lenses. Compared with spectacles, contact lenses increase the accommodative requirements of myopic eyes and decrease those of hyperopic eyes in proportion to the size of the refractive error. The difference between the accommodative efficiency of spectacle lenses and that of contact lenses results from the effect of these 2 modalities on the vergence of light rays as they pass through the respective lenses. Contact lens correction requires an accommodative effort equal to that of emmetropic eyes. In other words, contact lenses eliminate the accommodative advantage enjoyed by those with spectaclecorrected myopia and the disadvantage experienced by those with spectacle-corrected hyperopia. The accommodative advantage observed in patients with spectacle-corrected myopia is consistent with the clinical observation that patients with spectacle-corrected high myopia can read through their distance correction at older ages than can patients with emmetropia. The opposite is true of patients with spectacle-corrected hyperopia (Clinical Example 4-2).
Clinical Example 4-2
What is the accommodative demand of a –7 D myopic eye corrected with a spectacle lens compared with a contact lens? A 7 D hyperopic eye? Assume a vertex distance of 15 mm and a near-object distance of 33.3 cm.
The myopic refractive error of the first eye is –7 D at a vertex distance of 15 mm, and the object distance is 33.3 cm. The vergence of rays originating at infinity and exiting the spectacle lens is –7 D. Due to the vertex distance, the vergence of these rays at the front surface of the cornea (which is approximately the location of the first principal point) is – 6.3 D. Use the focal point of the –7 D spectacle lens, 1/7 = 0.143 m, plus the vertex distance of 0.015 m (0.158 m) to find the vergence at the corneal surface: 1/0.158 m = –6.3 D (Fig 4- 2A).
