Article

WHAT MAKES CUSTOM LENSES CUSTOM

And how to use them successfully in practice

Contact lenses at their core have the potential to be life-altering devices, whether they are molded and manufactured tens of thousands at a time or made one at a time to custom parameters. Many patients seek out contact lenses for cosmetic reasons to align with their hobbies or activities or to achieve their best corrected vision. In each case, practitioners must balance the wants and needs of their patients to their own.

Typically, the main reason a patient visits an eyecare professional is vision related. As eyecare professionals, we attempt to manage these visual needs and potential frustrations of our patients by any means necessary. There are times, however, when the exact parameter specifications needed to optimize a patient’s visual and wearing experience may escape us.

This article explores custom contact lenses and their justification, as to how these lenses may further enhance the lives of current contact lens wearers or those who are deemed “difficult to fit.” We demystify these challenges and explain how to overcome them with modern contact lens options, material science, ocular surface measurements, and manufacturing capabilities.

MAKING A CASE FOR CUSTOM

Soft contact lenses are the most often prescribed contact lenses in the industry. Most “off the rack” lenses are available in a wide range of powers but only one or two base curves, often only one diameter, and a center thickness predicated on the power of the lens. These parameters serve most patients well but fall short for patients who have larger or smaller than average corneas, large or very small refractive errors, or irregular astigmatism. The parameter range and reproducibility of custom soft lenses have dramatically improved in the last decade, as has the emergent option of lathing more oxygen permeable soft lens materials.

Many times, we encounter a patient who has a long history of unsuccessful contact lens wear. When evaluating these patients, we need to rule out any contraindication to contact lens wear, but once this is done, we should take a step back and take note of the patient’s corneal diameter. Is it average (11.8 mm) or larger or smaller than average? Corneas that measure outside the average often go unnoticed as a reason for contact lens intolerance. This seems particularly true for larger-than-average corneas where a soft contact lens may decenter or move inappropriately on the eye. A properly fitted soft contact lens has good centration, drapes approximately 1 mm onto the sclera, and demonstrates approximately 0.25 mm of movement in primary gaze (Figure 1). Larger corneas will have a greater sagittal depth and require a lens that is large and deep (larger overall diameter and/or steeper base curve).

Figure 1. Soft contact lens with appropriate scleral drape and centration.

Anatomical features that contribute to sagittal depth are corneal diameter, shape factor, and radius of curvature.1 Most traditional soft contact lenses have an overall diameter of 14.0 mm to 14.2 mm, which may not provide adequate coverage or centration for a cornea that is larger than 12.0 mm. These corneas tend to need a custom soft lens that is larger in diameter but also has a steeper base curve to accommodate the increased sagittal height. For regular refractive error, the power of the contact lens can be determined empirically by vertexing the spectacle refraction to the plane of the cornea. The initial base curve can be calculated by determining “effective K,” which uses the central corneal radius of curvature AND the corneal diameter. The general rule is that for every 0.2 mm smaller than 11.8 mm, subtract 1.00D from the mean keratometry value and for every 0.2 mm larger than 11.8 mm, add 1.00D to the mean keratometry value. The base curve of the contact lens can then be determined by factoring in the lens diameter needed to achieve a 1.0 mm drape (Table 1). Online calculators are available to help determine the starting power, base curve, and diameter.

TABLE 1 CALCULATING THE BASE CURVE FOR CUSTOM SOFT CONTACT LENSES
LENS DIAMETER (mm) FIT FLATTER BY (mm)
12.0 0.00
12.5 0.10
13.0 0.30
13.5 0.50
14.0 0.70
14.5 0.90
15.0 1.10
15.5 1.30
16.0 1.50
16.5 1.70
As the diameter of a contact lens increases, the sagittal depth increases. The fit factor helps to maintain an appropriate lens-to-cornea fitting relationship.

Similarly, custom soft lenses are available for patients with irregular astigmatism. Generally, these lenses are designed with an increased center thickness to help manage the irregular corneal astigmatism (Figure 2). Previously, these lenses were manufactured in hydrogel materials only, but with the increased center thickness the oxygen permeability of the lens decreased. Silicone hydrogel is a sticky material and difficult to lathe. Being able to reliably lathe a material is essential, because spin-casting or molding custom parameters for individual patients is not cost effective. Currently, there are a few FDA-approved lathable silicone hydrogel materials available as custom soft contact lenses.

Figure 2. Irregular astigmatism secondary to keratoconus. Note the irregularity of the mires in (A) with no contact lens and a smoothing of the mires in (B) with a custom soft contact lens.

DIVERSE CORNEAL SHAPE PROFILES

Different corneal shape profiles require some design modifications to achieve an appropriately fitted soft contact lens. For example, custom soft lenses specifically designed for keratoconus have a central base curve to allow for a steeper-than-average central corneal curvature with a flatter “skirt” curve to accommodate the relatively unaffected sclera (Figure 3). Custom soft lenses that are reverse geometry in shape are best for aligning with an oblate, post refractive surgery, or post penetrating keratoplasty cornea. Both of these lenses will have a center thickness of approximately 0.3 mm, much thicker than the average 0.1 mm (Figure 4).

Figure 3. The central base curve of the custom soft contact lens is steeper than the “fitting curve,” which lands on the more regular sclera.

Figure 4. An optic section will show the thickness of the soft contact lens relative to the corneal thickness. The average corneal thickness in both photos is 0.550 mm. Figure 4A is a silicone hydrogel lens with a thickness of 0.07 mm. Figure 4B is a custom soft lens with a thickness of 0.40 mm.

To determine the power of these lens designs, a diagnostic lens must be placed on the eye and an over-refraction performed. If vision is still not ideal, consider using a GP lens design to optimize the patient’s vision. Often, when we come to this crossroad, we consider the patient’s history of contact lens wear, vocation, and hobbies.

The largest population of patients who will benefit from custom contact lenses are those with a history of irregular astigmatism. Regardless of the cause of the astigmatism — injury, surgery or disease — the irregular corneal topography can be a challenge to fit with traditional contact lens designs. The visual effects left behind by these conditions may be devastating and often do not allow the patient to function in their daily lives with traditional spectacles or contact lenses. Therefore, it is imperative that we as the prescribing eyecare professionals are knowledgeable of the various custom contact lens options and modalities to provide the patient with creative contact lens designs that best match their unique corneas or refractive errors.

One of the more common conditions treated with specialty contact lenses is keratoconus. Owing to the asymmetrical nature of the disease, considerations must be made for the different visual and anatomic challenges of each eye. In an initial effort to determine if a patient is ideally served with traditional corneal GP lenses or if scleral lenses would be necessary, it is best to use the elevation display topographical map.

Traditionally, we are accustomed to analyzing the corneal surface with the axial display map. While the axial display map is useful for determining the corneal curvature, it reveals little about the elevation or height differences within the cornea. The relative height differences in the cornea are important to assess overall shape and are good indicators of whether a cornea can support a corneal GP lens or would be better served with the vault of a scleral lens.

In studies by Zheng and colleagues, patients who were successfully fitted with corneal GP contact lenses were evaluated. Elevation differences were measured along the greatest meridian of change.2 It was determined that if the greatest meridian of change (highest to lowest) was less than 350 microns, patients had an 88.2% chance of being well fitted with — and successfully wearing — a corneal GP lens. If the greatest meridian of change was greater than 350 microns, however, we predicted the corneal profile would not ideally support the physical fit of a corneal GP lens and would likely need the significantly larger overall diameter of a scleral contact lens.

The sclera is relatively unaffected by most conditions that affect the corneal shape. A scleral lens fits and lands on the sclera and allows the central portion of the lens to vault the central and peripheral cornea and avoid interaction with the highly asymmetric corneal elevation differences (>350 microns), thereby allowing the optical correction of the irregular astigmatism and providing justification for a scleral lens.

KEY MEASUREMENTS

In our academic setting, we have the benefit of using instrumentation to measure the sagittal depth of the cornea and the sclera at various chord lengths. In most practices, a topographer will be the primary resource for determining sagittal depth. Many topographers can extrapolate the depth of the cornea at a chord length of 10 mm. The measurement taken from 10 mm to 15 mm falls outside of the measuring ability of the corneal topographer, as the human cornea is approximately 12 mm in diameter and the sclera itself is not reflective and cannot be measured by current topographers.

A NEW PARAMETER TO CUSTOMIZE

For a contact lens to be “custom,” not all parameters need to be customized, and it could be argued that a lens that has been modified in any way is custom. For this reason, we wanted to mention a surface treatment that recently received FDA approval. It is a 90% water polyethylene glycol (PEG)-based polymer coating that is bound to the front and back surface of the lens, creating a mucin-like layer. This aids in surface wetting of the lens with the potential to aid in comfort and visual quality. This surface treatment is applied on a newly manufactured lens prior to shipping the lens to your office.

In a study by Achong-Cohen and colleagues, an anterior segment optical coherence tomographer (Visante OCT, Carl Zeiss Meditec) was used to measure the elevation from 10 mm to 15 mm.3 This evaluation showed that normal eyes had an elevation of 1,992 microns, and eyes with moderate-to-advanced keratoconus had an elevation of 1,970 microns. When evaluating the eye to determine the overall sagittal height, the initial diagnostic lens may be estimated by using the corneal topographer and measuring 10 mm to the apex, then adding the average sagittal height from 10 mm to 15 mm of 2,000 μm, and then the desired initial clearance (range 300 μm to 400 μm).

Our ability to vault the cornea with a contact lens has been game-changing for many patients and practitioners, and this has intensified research to better understand the shape of the sclera. Clinically, we see that the sclera has asymmetry to it, but is there a way to predict the scleral shape by measuring the corneal shape? Unfortunately, this may not be possible. In a small study evaluating varying amounts of corneal toricity, there was no correlation to the presence or absence of scleral symmetry as measured by the sMap3D (Precision Ocular Metrology).4

This evaluation of the sclera has challenged our understanding and description of the sclera, because traditionally, we have described corneal astigmatism in terms of the dioptric power from the keratometer or axial display topography map. When fitting a scleral lens, we see that the scleral shape is highly asymmetric and confirms the presence of variations in elevation and curvature, but can we describe it in the same terms as corneal height differences? As contact lens designs are developed and intended to fit out onto a broader portion of the sclera, it may be necessary to image both the cornea and the sclera in an attempt to optimize the asymmetric scleral shape.

Some scleral lens designs have a predetermined asymmetry on the haptic portion of the lens to better align the sclera. In one design, an ocular impression is used to determine the exact contour of the cornea and the sclera.

It is important that we all gain from these insights when designing and prescribing custom contact lenses for our patients to optimize vision, fit, and comfort, particularly those who need them the most, so that they can thrive in their activities of daily living and we may all share in the reward of satisfying their visual needs.

CONCLUSION

Good contact lens practice involves considering all options for your patients, including lifestyle, vision needs, and anatomical features. Recognizing that some patients will fall outside the norm will help to expand your contact lens practice and potentially minimize contact lens dropout.

Although there is still a lot that we do not know, we have an industry and profession that is constantly trying to improve and innovate, so keep your eyes open for what is on the horizon. CLS

REFERENCES

  1. Young G. Ocular sagittal height and soft contact lens fit. J Br Contact Lens Assoc. 1992;15:45-49.
  2. Zheng F, Caroline P, Kojima R, Lampa M, Kinoshita B, Andre M. Corneal Elevation Differences in the Initial Selection of Therapeutic Scleral Contact Lenses. Poster presented at: Global Specialty Lens Symposium; 2015; Las Vegas, NV.
  3. Achong-Coan R, Caroline P, Kinoshita B, et al. How Do Normal and Keratoconic Eyes Differ in Shape? Poster presented at: Global Specialty Lens Symposium; 2012; Las Vegas, NV.
  4. Kinoshita B, Caroline P, Morrison S, Lampa M, Kojima R, Andre M. Corneal Toricity and Scleral Asymmetry — Are They Related? Poster presented at: Global Specialty Lens Symposium; 2016; Las Vegas, NV.