As parameters for mass-produced contact lenses expand, it would be easy to assume that the demand for custom soft contact lenses would be decreasing. However, as soft contact lens parameters have expanded, so has the prescriber awareness and available instrumentation to identify those patients who fall “outside of the bell curve” for a comfortable and visually stable lens fit with mass-produced lenses. Now, custom lenses are being prescribed for high refractive errors, precise astigmatic corrections, multifocal options, and for those patients who have atypical corneal sizes and curvatures. Our ability to truly design, assess, and prescribe a soft contact lens has never been better.
RECOGNIZING GREAT CANDIDATES
Custom soft lenses have a variety of indications. Patients who are likely to need custom soft contact lenses are those individuals who have a refractive error outside of a mass-produced soft contact lens fitting set, those who struggle to achieve a comfortable and stable lens fit, and those patients who desire better vision quality over their current soft multifocal contact lenses.
One of the most common users of custom soft contact lenses is to correct refractive errors that fall outside of mass-produced soft contact lens ranges. With regard to astigmatism, it is estimated that the prescription of about 90% of patients who have astigmatism would fall within a typical diagnostic set.1 However, that doesn’t mean that all 90% would find a diagnostic lens comfortable and visually stable, which introduces us to our next group of patients who could benefit from custom soft contact lenses; those who cannot achieve consistent vision or comfortable fit with mass-produced contact lenses are excellent candidates. Many times, these patients have astigmatism, and their vision inconsistency deriving from a poor-fitting contact lens is what drives their complaints.
It also is well-documented that many patients who wear multifocal contact lenses desire better visual acuity.2 For some, the zone sizes of the distance and near powers in their lens cause halos or blur. For others, lens decentration misaligns the contact lens optics with their line of sight.
Fortunately, we can address all of these patient hurdles with customizable optical zone sizes as well as with better-fitting lenses through custom base curves (BCs) and diameters. If this isn’t successful, we can even decenter the optical zone of multifocal contact lenses to line up with a patient’s line of sight.
BUILDING A CUSTOM LENS
During the initial evaluation, it can be valuable to examine a patient’s habitual contact lens as well as to measure the eye itself to design an empirical lens. With reliable measurements and some patient history, you can anticipate good success with an empirically designed lens from your preferred laboratory. The more patient-specific parameters and background information that we can record and provide to the laboratory, the better the likelihood of a successful first fit.
Lids Start by checking the tonicity and positioning of the eyelids against the globe. This is particularly important in astigmatic patients. Is the lower lid placed too high or too low to correctly utilize the prism ballasting? Are the lids very tight to the eye, spinning the lens with every blink? Both of these scenarios could be remedied with a custom soft contact lens. There are diameter and BC adjustments as well as increased prism ballasting options; these changes allow the lens to better align to the ocular surface and to remain self-aligned with less lid dependency, respectively. These are great ways to increase rotational stability for our astigmatic patients.
Ocular Surface We should evaluate not only for dry eye signs but the surface geography and how it plays a role in contact lens wear as well. This is critical for both our astigmatic and our multifocal lens wearers, as an optimal surface for lens positioning and stability is critical to the success of these designs.
Does the palpebral conjunctiva have significant papillae from a previous poor-fitting contact lens? Sometimes we need to treat the eye itself so that we can obtain reliable measurements for a new custom lens. It is tempting to want to measure and order new parameters on the first patient visit, but patients who have struggled with successful contact lens wear will be grateful to be provided with the reasons for their previous failures and will gladly return for repeat or fresh measurements if deemed necessary.
Other ocular surface findings could include pingueculas affecting lens centering or incomplete blinks causing poor tear spreading. When the lens is not wetting properly, it can get “sticky” and move excessively when the lid crosses the surface; it can also make the contact lens more prone to surface deposits. This would decrease vision quality in both astigmatic and presbyopic patients. Laboratories can make specific lens materials, coatings, treatments, and specific care system recommendations, which can help keep the contact lens surface in its best condition.
Cornea The cornea is where the real magic in custom lenses occurs! When considering corneal shape, we are not just talking about keratometric values and matching them with the contact lens BC. Matching sagittal depth of the cornea and the contact lens is the best predictor of a stable contact lens fit and comfort.3 Corneal sagittal depth is determined by corneal diameter, central corneal radius readings, asphericity of the cornea, and the radius and asphericity of the paralimbal sclera.2
Thankfully, we do not need to pull out our calculators to request a specific sagittal depth when consulting with our laboratories. As a general rule of thumb, it is helpful to note central corneal curvatures, eccentricity, and horizontal visible iris diameter (HVID). The corneal diameter is most impactful on sagittal depth, with larger diameters being deeper compared to smaller ones.
Eccentricity, or the rate of corneal flattening, is the second most impactful finding. Corneas with higher eccentricities flatten more quickly than do those with lower eccentricities. Most topographers will provide an eccentricity reading. If you are going to fit more custom lenses or are managing the patients who need them, a topographer is essential in your practice.
Lastly, central corneal curvatures have some effect on corneal depth, with steeper readings indicating increased sagittal depth. As noted in Figure 1, a central curvature reading tells very little about sagittal depth and alone is not sufficient for selecting back optical zone radius (BOZR),4 also known as BC. On a personal note, the awareness of appropriate lens diameter (and therefore sagittal depth) has been the most impactful adjustment for my patients who have struggled with lens comfort, centration, and rotational stability.
One of the most frustrating moments as a contact lens prescriber is when a lens looks perfect on the eye, there is no over-refraction, and the acuity charts can be read quite well, but patients tell you that they can’t see well. Typically, this scenario arises from our multifocal lens-wearing groups, which has lead many patients and prescribers to believe that multifocals just don’t work.
Pupil Size While many of my custom soft multifocal-wearing patients are those who also need stable astigmatism correction, there is a presbyopic patient group that can benefit from a custom lens solely due to their pupil size. It is well known that pupil size decreases with age.5 If patients have a particularly small pupil, they are unable to utilize the lens zones because the different powers are not distributed across their line of sight. They will complain about poor near vision if the lens is center-distance and about poor distance vision if the lens is center-near. A custom soft lens can create greater eccentricity across the lens center to generate more add power within a smaller aperture so that all of the needed optics can be utilized.
The same considerations can be made for patients who have larger pupils, which are typically our younger presbyopes. Higher-order aberrations (HOAs) from large pupils can decrease vision quality,6 making fitting an effective multifocal even more difficult. Creating a multifocal lens with add zones that are more proportionate in size and rate of power change can balance vision quality. A final suggestion pertaining to pupil size is to measure your patients’ pupil diameters at a given working distance—as working distance also affects pupil size—to ensure that the lens is providing best-quality optics for their most important tasks.5
Refractive Error The discussion of pupil size ties nicely into the discussion of refractive error considerations, as refractive error plays a role in pupil size. You can expect to see larger pupil sizes in myopes and smaller pupils in emmetropes and hyperopic patients of similar age.7 With that knowledge, it would make sense that emmetropes and hyperopes would need lenses with greater generation of add power across a smaller lens surface area at an earlier age when compared to myopes.
It’s also worth mentioning the benefits of correcting astigmatism with custom soft contact lenses, as our astigmatic patients have much to gain from custom soft lens technologies. First and foremost, the parameter availability in custom soft lenses in both the sphere and cylinder powers at 1º axis steps is very advantageous over mass-produced designs. Common reasons for which I will switch a patient from a mass-produced lens design include that he or she is an astigmatic presbyope, when the refractive error falls outside of a fitting set, and finally, rotational instability of a toric contact lens. My favorite customization for patients who have astigmatism is a custom diameter to better align the lens to the ocular surface. Larger diameters allow the lens to extend further onto the sclera for more stability, and smaller diameters reduce edge fluting (Figure 2) or tight-fitting contact lenses.
Another customizable parameter of custom soft lenses for our patients who have astigmatism is the amount of prism ballasting. When I see a lens that rotates excessively with the blink but vision is good when the lens settles, adding just an extra one-quarter prism diopter of ballasting can really lock vision into place. This is obviously particularly beneficial with highly astigmatic patients. Patients who have low astigmatism, especially those who want a multifocal contact lens, can appreciate a low-astigmatism correction to really fine-tune their distance and near vision.
Imagine the impact that a custom lens can have on patients who previously thought that they couldn’t wear contact lenses because they’ve struggled to see clearly, when all they’ve previously been given the opportunity to wear is a one-size-fits-all design. Table 1 suggests some easily obtained patient parameters to maximize the patient experience with the first lens ordered.
|Blink patterns, tonicity
|Diameter or base curve (BC) to negate lid interactions
|Dry eye findings, allergies
|Lens material, coatings, care system
|Horizontal visible iris diameter, Ks, topography
|Diameter, BC, sagittal depth
|Large or small sizes
|Optical zone size
|High sphere or astigmatism
|Center thickness, prism amount
|Previous contact lens experience
|Previous lens rotation, instability, allergy, awareness
|Based on finding, a lab consultant can suggest proactive changes
|Low but visually significant cylinder or fine-tuned axis
|Sphere and cylinder amounts in 0.25D steps, 1º axis changes
All of this mention of customization and working with any lens parameter can be a little overwhelming, especially on the first few fittings. Evaluating custom soft lenses is the same process as is evaluating a mass-produced soft lens, but knowing what you can change to improve the lens fitting relationship and when you should do it is the difference in approach.
At a dispense or follow-up visit on a toric custom soft lens, I will wait just a few minutes before evaluating vision, settling, and position of rotation. If the lens appears in the desired rotational position, I will perform my over-refraction and recheck lens rotation after the power assessment to confirm lens stability. When I troubleshoot vision quality in a toric lens, I look for one of three scenarios:
- If the lens takes a significant amount of time to settle into place but eventually provides good-quality vision, some extra prism ballasting can be added to encourage clear vision shortly after lens application.
- If the lens has rotational instability but there are moments of clear vision, I will manually rotate the lens toric marker into place and then quickly have patients evaluate their vision. If vision quality is good, increased prism ballasting can be beneficial. If vision quality is poor, diameter and/or BC adjustments are more impactful. Over-refracting a toric lens that is not stable is very unreliable, and it is recommended to avoid changing the lens power too much at this stage. Poor vision quality after an over-refraction is often an indicator of a poor fit.
- If the lens rotation is stable and I can obtain a clear visual endpoint on over-refraction, then I will adjust the lens power based on the over-refraction and will use the steeper add minus/flatter add plus (SAM-FAP) lens rules. When all of the above seem to fail, repeating manifest refraction and topography are valuable steps.
For patients who are wearing custom multifocal lenses, my first goal is to achieve a perfect fit. That means utilizing the above scenarios if I am working with a toric multifocal lens. Once I am confident that we have the fit and distance power locked in, we will start to work on the multifocal optics.
The two most impactful pieces to my multifocal assessments are using a topographer to determine positioning of the lens’ optical center and using loose lenses to determine whether a patient can utilize the optical distribution of power. As the front surface of a lens is modified by eccentricity to create add power, these curvature changes are measurable by topography. As Figure 3 shows, the center-near optics of the multifocal lens are inferior-temporal to the center of a patient’s pupil. If a patient is struggling to read, I would take one of the following steps:
- If distance vision is excellent, I would most likely increase the size of the near zone to allow easier access to the near optics rather than adjusting the fit to attempt to improve lens centration, as some distance vision quality may be lost.
- If distance quality is poor, I would then consider attempting to modify the lens fit to improve centration.
- If attempts at better lens centering fail, some laboratories offer multifocal lenses that have decentered optics to bring that optical zone into prime view for the patient. Decentered optics can also be utilized on a lens that is centered but perhaps the patient has a larger-than-average angle kappa.
Another important piece of data that I frequently utilize is a loose lens over-refraction. If the distance over-refraction results in the need for a high amount of minus power, similar to the amount of add power, this tells me that the distance zone is not being accessed by the patient’s line of sight. Similarly at near, if patients need their full plus add power in a loose lens assessment, the near zone is misaligned or too small for functional use. During these situations, a zone design change is needed over power adjustments.
When troubleshooting contact lenses, some techniques could be similar to Sir William Osler’s approach. A physician in the 19th century, he once told his students to “Listen to your patient; he is telling you the diagnosis.”8
Patients present to our clinics telling us the issue with their contact lenses, and it is up to us to know what to look for so that we can make positive changes in their lens-wearing experience. Over my years of working with custom soft lenses, I’ve identified some common complaints with particular lens issues to guide my lens evaluation. As you can see in Table 2, most of the complaints are vision related and less to do with lens comfort. This confirms how well-made custom soft lenses can be when made specifically for your patient; the latest materials support the tear layer with sufficient oxygen transmission. Table 2 is a quick reference that lists the complaint/lens remedy pairings that I have used with the most success.
|POSSIBLE LENS ISSUE
|Over-refraction, repeat manifest refraction
|Lens movement or rotation
|Increase lens stability with better fit or prism
|Diameter mismatch creating tight or loose fit
|Custom diameter based on horizontal visible iris diameter and anterior chamber depth
|Halos with multifocal optics
|Zone size mismatch, decentered lens
|Custom zone sizes based on pupil size, consider decentered optics
|Poor near or distance vision with multifocal optics
|Patient unable to access all power zones
|Custom zone sizes based on pupil size, switch center-near or -distance
Working with custom soft lenses has improved my skills at evaluating all contact lenses. Much like our irregular cornea patients trying scleral lenses for the first time, custom-designed soft lenses can be just as impactful on our astigmatic and presbyopic patients. CLS
- Young G, Sulley A, Hunt C. Prevalence of astigmatism in relation to soft contact lens fitting. Eye Contact Lens. 2011;37:20-25.
- Pérez-Prados R, Piñero DP, Pérez-Cambrodí RJ, Madrid-Costa D. Soft multifocal simultaneous image contact lenses: a review. Clin Exp Optom. 2017 Mar;100:107-127.
- Caroline P, André M. The Effect of Corneal Diameter on Soft Lens Fitting, Part 1. Contact Lens Spectrum. 2002 April. Available at www.clspectrum.com/issues/2002/april-2002/contact-lens-case-reports . Accessed May 10, 2021.
- Young G. Ocular Sagittal Height and Soft Contact Lens Fit. J Brit Contact Lens Assoc. 1992;15:1:45-49.
- Cardona G, López S. Pupil diameter, working distance and illumination during habitual tasks. Implications for simultaneous vision contact lenses for presbyopia. J Optom. 2016 Apr-Jun;9:78-84.
- Suliman A, Rubin A. A review of higher order aberrations of the human eye. African Vis Eye Health. 2019 Oct;78:a501.
- Guillon M, Dumbleton K, Theodoratos P, Gobbe M, Wooley CB, Moody K. The Effects of Age, Refractive Status, and Luminance on Pupil Size. Optom Vis Sci. 2016 Sep;93:1093-1100.
- Pitkin RM. Listen to the patient. BMJ. 1998 Apr 18;316:1252. Available at https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1113013 . Accessed May 10, 2021.