Optics of Contact Lenses

Scott A. Tomasino, OD

Contact Lens Materials

Contact lenses are made from a variety of polymers that can be separated into two broad categories: soft, or hydrogel, lenses and rigid lenses.

Rigid lenses
Rigid lenses, commonly known as rigid gas permeable (RGP) lenses, typically contain silicone (to allow oxygen transmission) and fluorine (to improve the deposit resistance of the lens surface). Fluorine and silicone are cross-linked with the original hard lens plastic, PMMA, to form copolymers known as fluorosilicone acrylate lenses.

Hydrogel Lenses
As the name implies, hydrogel lenses contain water, which imparts oxygen transmission and flexibility to the material. All soft lenses are hydrophilic and contain water in varying percentages. The higher the percentage of water in the lens, the greater the amount of oxygen that is transmitted through the lens. Thinner lenses also maximize oxygen transmission. A soft lens is inherently more comfortable than a rigid lens due to a soft lens’ flexibility and larger diameter. The soft lens’ natural flexibility allows it to conform to the corneal surface without resorting to complex curve designs. The larger lens size allows less lid interaction during blinking.

Hybrid Lenses and New Polymers
Attempts have been made over the years to combine the properties of soft and rigid lenses to form hybrid lenses. One such hybrid that has been available for several years is the Softperm lens (CIBA Vision Ophthalmics, Atlanta, Ga.). The lens has a silicone acrylate center button and a hydrophilic skirt. In theory, it should provide the visual benefits of a gas permeable design with the comfort of a soft lens. However, the lens has limited clinical applications due to lens tightening and a tendency for the materials to separate at the junction zone.

A new hybrid lens combines silicone with hydrogel materials to form highly permeable soft lenses. The Purevision (Bausch & Lomb, Claremont, Calif.) lens for monthly replacement is currently available. These hybrid lenses make extended wearing schedules safer and more tolerable due to higher oxygen transmission and are comparable to soft lenses in fitting technique and edge comfort.

Fitting Methods and Philosophies

Optics of Contact Lenses
It is important to understand the optical properties of contact lenses before discussing fitting philosophies. In many cases, the choice of contact lens depends on the optical goal for each patient.

When light enters the eye, it is refracted first by the corneal surface. The refractive power of the cornea is determined by its curvature and refractive index. At 1.375, the refractive index of the cornea is greater than water and significantly less than contact lens materials, which have a refractive index ranging from 1.42 to 1.52. The radius of curvature of the cornea averages approximately 7.8 mm, or 43.25 D. This measurement is obtained by taking keratometric readings.

The refractive power of a contact lens is based on the index of refraction of the material and the radius of curvature of the front and back surfaces. Plus power lenses have a front surface that is steeper than the back surface, and minus lenses have a back surface that is steeper than the front surface.

In clinical practice, the dioptric power of a contact lens is determined by measuring it with a lensometer. This is not always accurate, because the lens measured in air will be different than when the back surface is immersed in tears. If the back curvature of the lens matches the curvature of the cornea exactly, the effective power of the contact lens on the eye will match the lensometer power. This is the case with soft lenses because they are flexible and will conform to the curvature of the eye in most cases. With rigid lenses, it is important to understand the relationship between the lens and the cornea when calculating the power of the lens needed.

The Tear Lens
The tear layer between the back surface of a rigid contact lens and the cornea forms a tear lens that contributes to the total system power. This tear lens has no power if the back curve of the contact is identical to the corneal curvature. If the back curve is flatter than the cornea, a minus tear meniscus lens is formed. If the back curve is steeper than the cornea, a plus tear lens is formed (Slide 1). The power of the tear lens can be easily determined by calculating in diopters the difference between the radial curvature of the cornea and the radial curvature of the lens. This value is then combined with the desired system power to arrive at the appropriate lens power.

Slide 1

This lens will fit 0.5 D flatter than the corneal curvature (K), which will induce a tear lens of –0.50 D. To obtain the desired system power, the contact lens power to order would be –1.00. If the desired contact lens curvature was 43.50, a plus tear lens would be created and the contact lens power would need to change to –2.00 to obtain the same system power. If the desired contact lens curvature matches the corneal K (called "on K"), the tear layer has no power and the contact lens power desired would be –1.50.

The example above illustrates how the system power will vary based on the fitting relationship between the rigid contact lens and the cornea. This fitting relationship determines the power contributed by the tear lens between the cornea and the lens and must be considered to arrive at the proper system power.

When fitting soft lenses, the tear lens is not an important consideration because the flexibility of the material allows the lens to conform to the cornea.

Astigmatism
Most corneas are not spherical and have some degree of toricity or astigmatism. Refractive astigmatism is most often caused by this corneal toricity and, in other cases, may be caused by lenticular toricity or a combination of both (mixed astigmatism). When a spherical rigid lens is placed on a nonspherical cornea, the tear layer that is created has a cylindrical power. When this cylindrical power favorably matches the refractive cylinder, the rigid lens neutralizes the astigmatism. When the corneal cylinder does not match well with the refractive cylinder, a toric lens is required to correct the residual, or left over, astigmatism. A lens is called toric when it uses two curves and two powers to correct astigmatism. Toric lenses are similar in design to spectacle lenses. The cylinder power is added to the lens at a specific axis. The lens must remain in a stable position on the eye for the power to line up properly. If the lens rotates significantly, the resultant power is undesirable. Rigid toric lenses are available as front toric (cylinder on the front of the lens), back toric (cylinder on the back of the lens), or bitoric (cylinder on the front and back of the lens). Front toric lenses use prism to ballast or stabilize the lens. Back toric and bitoric lenses match up to the corneal curves in a saddle fashion that prevents the lenses from rotating significantly.

Soft lenses that correct astigmatism also have a toric design. If spherical soft lenses are used on an eye with significant refractive astigmatism, the acuity is inadequate due to the residual astigmatism. Generally, if the refractive cylinder is higher than 1 D, a toric lens is required. The two most common means of stabilizing soft toric lenses are base down prism and thin zone lenses (superior and inferior), which position underneath each lid. The higher the astigmatism, the more important lens stability becomes. Thin zone lenses work well in lower cylinder powers and are comfortable. Most toric lenses and all higher cylinder powers in custom designs require prism stabilization to limit rotation. Although initially there can be some lower lid sensation, adaptation is fast with full-time wear.

Soft toric contact lenses are marked with ridges to allow a clinician to determine the lenses’ position on the eye. Some lenses have marks at the 3 o’clock and 9 o’clock positions, and others at the 6 o’clock position. If a lens consistently rotates such that the marks are out of position, the cylinder axis can be adjusted to compensate optically for this rotation. The rule used is called the LARS rule: left, add, right, subtract. Left and right are designations of the deviation of a 6 o’clock mark from the true 6 o’clock position, as observed at the slit lamp. If the lens on the patient’s eye rotates 5° clockwise, 5° must be added to the refractive axis to compensate when ordering the next lens. The new lens should also appear to rotate 5° clockwise like the original lens, but now the power will line up properly due to the axis compensation. This simple method of adjusting axis can make a big difference in fitting soft toric lenses successfully.

Presbyopic Correction
Correction of presbyopia with contact lenses is a challenging and often frustrating proposition, although it can be the most rewarding area of contact lens practice. The key to success is preparing the patient for compromise and instilling realistic expectations.

Options for correcting presbyopia include distance contacts with near overcorrection in spectacles, monovision correction, simultaneous vision bifocal designs, and alternating vision bifocal designs.

All forms of presbyopic contact lens correction require some compromise. For patients who are unable or unwilling to compromise, distance contact lenses with reading glasses may be the best solution. This is particularly true for patients with high refractive errors who will gain substantial vision by wearing contacts for distance. They may have no problem with wearing a pair of reading glasses as needed.

Monovision involves full correction of the dominant eye for distance and correction of the nondominant eye for near. The brain learns to suppress the blurred image in favor of the clear one at any working distance. When fitted correctly, monovision is successful. It takes approximately 2 weeks for full adjustment for most people, but some adjust quickly and others take longer. There are some compromises in depth perception and binocularity, but each eye receives a clear image, albeit one for distance and one for near. An advantage to fitting monovision is that all contact lenses in an examiner’s arsenal are available for use.

Simultaneous vision bifocals are available in both rigid and soft designs. They are numerous in design and concept. Some bifocal lenses have concentric zones of power with near power in the center and distance in the surround, whereas other bifocal lenses have the distance zone in the center. Bifocal contact lenses can also have alternating zones of power (e.g., the Acuvue Bifocal, Johnson & Johnson, Jacksonville, Fla.), or aspheric curves to create different regions of power (e.g., Progressive, CIBA Vision). The principle is the same with all bifocal designs. The lens brings images from two or more distances to focus on the retina at the same time. The visual system then selects the clear information for any viewing distance from the rest. Many patients have difficulty with these designs due to the annoyance of an unfocused image that is always present. The key to fitting these lenses successfully is learning the strengths and weaknesses of the various designs and using them creatively. It is not unusual to use different designs on each eye to gain a desired result.

Alternating vision bifocals work like bifocal glasses. There are two discrete areas of power and the lens must translate to allow vision through the reading area in downgaze and the distance area when looking straight ahead. These lenses exist primarily in the rigid lens arena due to the inability of a soft lens to translate effectively on the eye. These rigid bifocals are either segmented similar to a flat top bifocal in spectacles or single cut like an executive bifocal. The lenses are stabilized either with prism or truncated on the bottom allowing them to rest on the lower eyelid. On downgaze, the lens translates upward allowing vision through the near portion of the lens. On straight gaze, the lens drops and the distance portion is in place. When fitting these lenses, proper segment height must be set and adequate translation obtained. This can be accomplished only with a fitting set, which is available from most manufacturers of these lenses.

Other Power Considerations
In powers above +/– 4 D, the power of a contact lens at the corneal plane must be different from the refractive power at the spectacle plane to obtain the same resultant power. The vertex distance is the difference between the spectacle and corneal planes. The following formula can be used to convert the spectacle refraction to the desired contact lens power to obtain the same refractive result at the corneal plane.

Fc = Fs/1 – (d) (Fs)

Where Fc is contact lens power, Fs is spectacle lens power, and d is vertex distance in meters.

Contact lens power will always be higher in plus power or less in minus power when converted for vertex distance.

The desired contact lens prescription would be –8.50/1 – (0.012 x –8.50) = –7.71

If the refraction was +8.50, the result would be +8.50/1 – (0.0012 x +8.50) = +9.47

More plus power (or less minus power) is needed at the corneal plane to obtain the effective power as measured at the spectacle plane.

Retinal image size is also affected by contact lens correction. In myopes, images will be magnified approximately 1% per diopter when compared to spectacle wear. In a patient with 10 D of myopia, there will be approximately a 10% increase in image size. On the acuity chart, this often translates into measured improvement. In reality, these patients will note that initially their feet appear larger and, when reaching for objects, their perception may be altered. When wearing contact lenses full-time, adjustment to these differences is rapid as the brain compensates for the size difference. Conversely, high hyperopes see objects less magnified and often report an expanded field of view and initially their feet appear smaller. Adaptation is rapid with full-time wear.

When myopes wear contact lenses, they must exert more accommodative and convergence effort when working at the near point, whereas hyperopes use less. This is partly due to the loss of base in prism effect at near from spectacles with myopes and the loss of base out prism effect with hyperopes. Contact lens wearers will present with reading difficulty at an earlier age if they are myopes, and a later age than expected if they are hyperopes.

Material Selection
Many factors are involved in selecting the appropriate contact lens material for your patient. Each new contact lens patient has his or her own unique history, physiology, expectations, and occupational and recreational requirements. For this reason, to successfully fit contact lenses, the examiner must listen to the patient and then formulate a solution to the problem with best acuity and maintenance of healthy physiology being the primary goals.

Most new patients who have spherical or low cylinder prescriptions are fit in soft materials. Advantages include initial comfort and clear and consistent image quality. Athletes and physically active patients are well suited for soft lens materials. Soft lenses are not as easily dislodged as rigid lenses due to the soft lenses’ larger diameters and stability on the eye. Therefore, soft lenses tend to remain stable during physical contact and exposure to the elements (wind and water).

When fitting patients with soft materials, examiners should prescribe the most disposable program available. Vision and comfort are optimal when the materials are new. One-day disposable lenses are gaining popularity for this reason, and toric and bifocal options are now available in 2-week disposable designs in most prescriptions.

Rigid materials are a better option for patients with irregular corneas and astigmatism, which is primarily caused by corneal toricity. When patients with astigmatism use spherical rigid lenses, rotation has no effect on image quality. This can be an advantage when correcting higher amounts of astigmatism where any significant rotation of a soft toric lens can cause image blur. If corneal astigmatism exceeds 3 D, a back or bitoric rigid design is the best choice. These lenses remain stable on the eye and generally provide superior acuity compared to spectacles or other contact lens options. Rigid lenses also provide the best bifocal image quality when fitted properly.

A trial lens fitting is always preferred when time and inventory allow. Most large contact lens practices maintain a large inventory of lenses and many patients can be fit with trial lenses to wear home that day. Soft toric lenses have become so predictable that empirical fitting by ordering trial lenses has become standard practice. Most rigid lenses should be custom designed, but a good fitting set is helpful in making decisions on diameter, base curve, and resultant power before placing the order.

Whether fitting rigid or soft, spherical or toric, or single vision or bifocal contact lenses, the examiner must understand the optics involved to achieve a desirable result.

Practice Calculations

Question 1
Calculate the soft contact lens power needed for the following patient:

Answer 1
To calculate the contact lens power, the formula Fc=Fs/1 – (d)(Fs) should be used. This results in powers of –6.28 and –8.40. These powers are then rounded to the nearest 0.25 D step. The appropriate contact lens powers are –6.25 and –8.50. The K readings are not involved in this calculation because there is no significant tear lens effect when fitting soft lenses.

Bibliography

Bennett E, Henry V. Clinical Manual of Contact Lenses, 2nd ed. Philadelphia, PA: Lippincott; 2000:63-67.

Koetting RA. Contact lenses, Chap 11. In: Podos SM, Yanoff M, eds. Textbook of Ophthalmology. Vol. 1: Optics and Refraction. 1987.

Mandell RB. Contact Lens Practice, 3rd ed. Springfield, Ill: Charles C. Thomas; 1981:496-497.

Tyler’s Quarterly soft contact lens parameter guide, March 2001. Tyler’s Quarterly Inc., Little Rock, Ark.