At first glance, the soft contact lens market appears to be comprised of stock lenses from various manufacturers, each offering designs in a specific material, with typically one diameter, one or occasionally several base curves, and a range of power options. While these parameters may address the needs of many of your patients, they will not meet the visual or anatomic requirements of every patient. When a standard soft contact lens falls short in satisfying a patient’s needs, a custom soft lens can fill the gap.

INDICATIONS AND APPLICATIONS

The clinical scenarios for which custom soft lenses are indicated are numerous and varied. Typically, their utility begins where the parameters available for standard soft lenses end, most often in two main categories: lens power and fitting characteristics. A patient who needs a sphere, cylinder, or axis combination that’s not offered in standard soft lenses is an excellent candidate for a custom soft lens.

Custom soft lenses can be ordered in extreme sphere and cylinder powers as well as for visually discriminative patients. In addition, when vertex-correcting without rounding, custom soft lenses can be ordered in sphere and cylinder powers of 0.10D steps versus the traditional 0.25D steps for sphere powers and 0.50D steps for cylinder powers. In terms of the axis, lenses can be ordered in as fine as 1º increments. This can be helpful when the cylinder correction in the lens increases or when minor misalignments induce significant amounts of cross-cylinder effect. In the latter case, being able to order lenses more precisely than the 10º intervals that are standard for soft toric lens axes may be what’s necessary to achieve visual success for prescriptions that require higher amounts of astigmatic correction. One of the most common reasons why eyecare practitioners prescribe custom soft lenses is because the cylinder correction was not available in a standard soft toric lens design, which typically happens above –2.25D cylinder.

Custom soft lenses are also indicated when the size and shape of the eye yield a sagittal height profile that cannot be matched with standard soft lens base curve and diameter options. Custom soft lenses are manufactured in a wide range of base curves and diameters, often in as little as 0.1mm steps. This enables practitioners to fit a broad range of corneal curvature and diameter combinations.

Custom Soft Multifocals A significant advantage of custom soft multifocal contact lenses is that, depending on the manufacturer, the optical profile within the lens can be optimized for an individual patient’s ocular characteristics and visual needs. For example, the sizes of the near and distance zones can be adjusted to respect a patient’s unique pupil size and vision demands (Figure 1). Also, some manufacturers can further customize the multifocal correction by decentering or offsetting the optics in the lens so that they align with a patient’s line of sight, which may not be lined up with the geometric center of the lens.

In addition, as is the case with other lens parameters, custom soft lenses offer a wider range of add powers beyond the usual offerings of standard soft multifocal designs. And, when patients have presbyopia coupled with a significant cylinder component in their refractive error at the plane of the cornea, custom soft toric multifocal contact lenses are an option.

Irregular Astigmatism Soft contact lenses and irregular astigmatism may not seem to go hand-in-hand, as managing irregular astigmatism caused by injury, surgery, or disease is typically reserved for the non-flexing GP lenses (i.e., corneal GP, scleral, and hybrid lens designs). There are, however, indications and design considerations with regard to custom soft lenses for patients who have irregular astigmatism.

Typically, manufacturers address irregular astigmatism by increasing the center thickness of a custom soft lens beyond what the power of the lens would normally dictate. The rationale for doing this is to try to replicate the non-draping nature of a GP lens to mask some of the corneal irregularity. The center thickness of a custom soft lens designed to manage irregular astigmatism is generally around 0.4mm. In comparison, the center thickness of a standard soft lens with a power of –3.00D is typically in the range of 0.08mm (Figure 2).

To determine how much irregular astigmatism a custom soft lens can correct, perform manual keratometry or corneal topography first without the lens and then over the lens on the eye. Analyze the reflected mires and their regularity/symmetry with the lens versus the corneal irregularity of those reflected mires without the lens on the eye (Figure 3). When the mires begin to clear and to increase in regularity over top of the soft lens as the center thickness is increased, that is the point at which irregular astigmatism begins to be masked. Once you observe appropriate regularity, perform a spherocylindrical over-refraction and add the result to the contact lens power for optimal vision correction.

When a patient who has irregular astigmatism is already wearing corneal GP lenses but is not satisfied with the comfort, one troubleshooting technique is to piggyback the lenses. Typically, a piggyback system uses a standard soft contact lens. When patients have an ocular condition and a corneal sagittal height that preclude the use of a standard soft lens, a custom soft lens that fits the patients’ ocular surface can be of great benefit in achieving a comfortable fit. Custom soft lenses for piggyback purposes are most helpful when the sagittal height of an eye is excessive, as, for example, in cases of severe keratoconus or corneal transplantation.

DESIGNING, FITTING, AND PRESCRIBING CONSIDERATIONS

Once you’ve identified a candidate for custom soft contact lenses, the process of designing and prescribing the lens begins. Young evaluated the ocular anatomic features under a typical soft contact lens and described their contribution to the overall height of the eye.¹ An understanding of these features and how they contribute to the height of the eye helps us to better align the lens to the ocular surface and to avoid poorly fitting lenses. In analyzing corneal radius, corneal shape factor, and corneal diameter, Young found that corneal diameter has the greatest impact on sagittal height. In addition, when evaluating the on-eye performance of a soft contact lens, he determined that the lens should extend beyond the corneal limbus to aid in centration, to limit movement, and to optimize stabilization. When collecting corneal information for the design of custom soft lenses, be aware that the corneal diameter and center keratometry readings are key measurements that will enable the manufacturer to arrive at an ideal custom base curve and diameter.

Many custom soft lenses are ordered for patients who have significant refractive errors; therefore, it’s important to vertex-correct both primary meridians of the spectacle prescription to the plane of the cornea. Of note, for myopia, the amount of cylinder correction at the plane of the cornea decreases; for hyperopia, the amount of cylinder correction increases versus the spectacle plane.

Once the initial diagnostic lens has been applied to the eye and allowed to settle and the physical fit deemed appropriate, measure visual acuity. If patients’ visual acuity with the new lens is not the same as their previous best-corrected visual acuity, perform an over-refraction. For simplicity, I recommend a spherical over-refraction; however, if this doesn’t produce a visually significant improvement, I recommend a spherocylindrical over-refraction. If a spherocylindrical over-refraction improves patients’ visual acuity, a cross-cylinder calculator can be extremely helpful in determining the next power to order. Often, when using a cross-cylinder calculator for standard soft toric lenses, we must round the sphere, cylinder, and axis to the nearest lens option provided by the manufacturer. For custom soft toric lenses, we often can order the power in the exact parameters suggested by the calculator, as powers are typically available in 0.10D steps and in axes in 1º increments, giving us the potential to truly optimize visual acuity.

TROUBLESHOOTING GLARE, FLARE, AND HALOS

Occasionally, a lens fit is optimal and an over-refraction has been performed, but a patient still cannot resolve the letters on the chart with the desired visual clarity. Symptoms such as flare, glare, or halos may result from a too-small optic zone. To establish this, after over-refracting, partially occlude the eye that does not have visual symptoms, leave the eye that has visual symptoms unoccluded, and shine a light into the partially occluded eye. This will cause miosis in both eyes. If the patient reports that the image has improved in the unoccluded eye, then the optic zone of that lens should be increased. Provide this information to the manufacturer, and request an increase in the optic zone size to alleviate the symptoms of flare, glare, and halos (Figure 4). Depending on the lens design, the optic zone can generally be increased to 10mm to 11mm, and 1.0mm changes would be significant.

IN CLOSING

For optimal vision correction and lens-to-ocular-surface fit, custom soft contact lenses offer numerous creative solutions, enabling patients to remain successful in contact lenses for years to come. CLS

REFERENCES

1. Young G. Ocular sagittal height and soft contact lens fit. Cont Lens Anterior Eye. 1992;15(1):45-49.