WITH THE INTRODUCTION of innovative designs from custom soft lens manufacturers, eyecare practitioners have significantly expanded opportunities to better meet the challenges of patients needing or desiring improved contact lens performance. To take advantage of the versatility and problem-solving benefits these lenses offer, clinicians must be familiar with the clinical indications and fitting goals of specialty soft lenses and be very competent in the medical management of the underlying ocular condition. This article will detail the principles of fitting various corneal and refractive conditions with custom-designed soft contact lenses, utilizing the most current designs and methods.
The prescribing of custom soft lenses allows clinicians to provide or maintain patients in soft lenses when conventional lenses are not available in parameters or designs that meet their specific refractive, anatomical, or topographical needs. Pertinent examples include high myopic astigmatism, scleral irregularities such as pterygia, or mild to moderate ectasia where acceptable acuity can be achieved. Additionally, custom soft lenses are an excellent option for patients whose physical sensitivity, visual demands, and/or environmental challenges are not adequately resolved with conventional lens brands.
REASONS TO “FIT” A SOFT CONTACT LENS
Given the ease of selecting from mass-produced molded contact lenses available in one or two “base curves” per brand, it’s easy to forget that soft contact lenses should ideally meet definitive fitting standards. Ill-fitted soft contact lenses may cause problems ranging from mild discomfort to long-term or permanent undesirable medical complications.1Poorly fitting soft lenses are more likely to alter ocular physiology than well-fitting lenses and can contribute to the discontinuation of contact lens wear.2,3
Both tight- and loose-fitting lenses are associated with greater fluorescein staining.3 Loose-fitting lenses can cause bulbar and limbal hyperemia.3
A good tear layer on the contact lens surface is critical for maintaining clear vision, comfort, and ocular health.4 The presence of an abnormally thin tear film on both sides of the lens or less tear exchange underneath the lens may result in a dry eye sensation that also represents a common cause of contact lens wear discontinuation.5
A well-fitted soft contact lens will exhibit excellent centration with 0.2mm to 0.4mm movement on blink and full corneal coverage in all positions of gaze.6 The clinician should confirm regular edge alignment with the conjunctiva and easy movement on push-up, as a tight-fitting lens can be more difficult for the patient to remove and may induce rebound hyperemia as well as injection.
Communication with the patient regarding his or her comfort, upon application and at day’s end, is helpful in determining necessary fit modifications. For example, chronic lens edge awareness might necessitate alteration of lens diameter, base curve, or edge design.
Achieving crisp, stable vision with an accurate over-refraction is not only important for patient satisfaction but is also a key indicator of lens fit. A flat-fitting lens may move excessively, increase patient lens awareness, demonstrate fluting edges, and induce astigmatism. A flat toric lens will show rotation upon application with stabilizing after a settling period. An over-refraction exhibiting more plus power may also indicate a flat-fitting lens.
Vision will be inconsistent with a steep fitting lens, with clearing briefly post blink. Toric lenses will show progressive movement of the cylinder axis away from its prime position and over-refraction will show more minus power. Customization of all contact lens physical and optical parameters ensures that the lens is fit to the patient, rather than simply being “matched.”
MATERIALS AND MODALITY
Custom soft lenses are lathe cut and are available in both traditional hydroxyethylmethacrylate (HEMA)-based polymers and silicone hydrogel materials. Hioxifilcon, a glycerol methacrylate polymer, offers the advantage of being nonionic and dehydration resistant.7 It exhibits reduced flexure and better optical properties.8 Methafilcon, an older conventional material with good durability, may also be selected. When increased oxygen permeability is indicated or desired, many laboratories will manufacture lenses using efrofilcon A, a high-water, low-modulus, latheable silicone hydrogel material. Manufacturers offer the option of multi-lens purchasing with most custom lens designs prescribed on a monthly or quarterly replacement basis.
CUSTOM “ARCHITECTURE”
In common with their molded counterparts, custom lathed lenses are defined by base curve, diameter, and power. What sets them apart, is the availability of lenses manufactured to exacting specifications rather than the limited choice in conventional designs of two base curves in one diameter. Additional customizable parameters include a virtually limitless prescription range, optical zone diameter, material (and water content), thickness, asphericity, and prism.
Optical zone customization is useful in the management of presbyopia and myopia. Custom multifocal lens designs are available with both near-center and distance-center options. Additionally, manipulation of the central optic zone with respect to both diameter and “placement” can be critical to achieving a successful multifocal fit. Current technology permits customization to pupil size as well as to “angle kappa.” In other words, by “offsetting” the optics, laboratories can now manufacture lenses to better align the optical center of the lens with the patient’s fixation.
Customization of lens curvatures, including central and peripheral radii, are key to the managing contact lenses for regular and irregular corneas alike. Normal, prolate corneas falling outside the range of conventional lenses may benefit from slightly steeper or flatter central curves to optimize visual performance.
Management of irregular corneas is best approached through topographical analysis. It’s important to recognize that the clinical diagnosis, while important to ongoing corneal monitoring, is not the central component in contact lens fitting.
For example, a cornea having undergone penetrating keratoplasty may have an oblate or prolate profile, depending on the age of the graft. Reverse geometry designs should be considered for oblate corneas after surgeries such as penetrating keratoplasty (PKP), radial keratotomy (RK), phototherapeutic keratotomy (PRK), and laser-assisted in situ keratomileusis (LASIK).
While conventional nominal contact lens thickness of 0.06mm to 0.10mm is desirable when fitting “regular” corneas and correcting “simple” refractive errors, custom soft lenses with increased thickness are often indicated and beneficial in the presence of irregular astigmatism. Oxygen delivery in these thicker designs is facilitated through adequate tear exchange resulting from proper lens movement of 1.0mm to 2.0mm with the blink.6
FITTING APPROACH
Soft lenses, like their scleral counterparts, are ideally fit based upon sagittal depth. Sagittal depth is influenced chiefly by horizontal visible iris diameter (HVID) and to lesser degrees by corneal eccentricity, corneal radius, and scleral shape.
For normal, prolate corneas, reference nomograms are available for selecting the base curve based upon the overall corneal diameter as well as lens material. One suggested guideline is to use an “adjusted” base curve in millimeters + 0.3mm for the initial base curve selection.6 The lens diameter is calculated by adding 0.3mm to the HVID.6
Another approach is based on a method proposed by Caroline and Andre, using further adjustments for corneas which fall outside the average range of 11.6mm to 12.0mm.9 With this approach, for every 0.2mm larger than 12.0mm, 1.00D is added to the flat K value, and for every 0.2mm smaller than 11.6mm, 1.00D is subtracted from the flat K value.
When fitting custom soft lenses to irregular corneas, either highly prolate or oblate, the base curve is selected to align with the central cornea with the mid-peripheral curves and edge curves fit to the “normal” paracentral and limbal corneal curves. The ideal base curve (within the central optic zone) will provide the necessary refractive power, while the mid-peripheral fitting curve ensures proper lens draping and stability.
Troubleshooting “tight” or “loose” fitting lenses can be accomplished by adjusting the peripheral curve system. Poor centration may require a larger lens diameter. Localized edge “fluting” may be diminished by “sector” control offered by some laboratories.
GETTING STARTED
Custom soft lens fitting is easily integrated into everyday contact lens practice. Traditionally, diagnostic fitting has been the best approach, but computer-assisted software is now being utilized by several manufacturers to enable empirical design.
In either case, having a basic understanding of lens design and fitting goals will ensure greater proficiency and better outcomes. Finally, laboratory consultants are an invaluable resource.
CASE 1: PRESBYOPIA
A 60-year-old professional photographer presented with a history of unsuccessful contact lens wear. He previously attempted monovision and experienced unstable vision and chronic lens awareness. His goal was to be able to see his subject as well as the camera settings at various distances. The initial clinical evaluation data is charted in Table 1.
Utilizing a custom laboratory’s proprietary online arc length calculator and multifocal simulator, the following lenses were empirically ordered and dispensed:
OD: 8.2 (14.7) +6.75 –1.50 x 100 +2.50 near-center (NC) 1.8 54% hioxifilcon
OS: 8.2 (14.7) +8.00 –1.25 x 065 +2.50 NC 1.8 54% hioxifilcon
The patient achieved monocular visual acuities of 20/25+2 with binocular distance vision of 20/20 and near vision 20/25. He replaces his lenses on a semiannual basis and continues to report excellent comfort and wearing time after several years.
CASE 2: MICROCORNEA
A 12-year-old male patient was referred by an outside provider (pediatric ophthalmologist) for a contact lens evaluation. There was no history of ocular surgery. He was previously unsuccessful with corneal GP lenses as they frequently dislocated off the cornea. He was active in sports and desired contact lenses for full-time wear and to avoid the poor cosmesis of his thick spectacle lenses.
The examination findings are shown in Table 2.
The external examination revealed no strabismus or oculomotor deficits. Intraocular pressures were 12 mmHg OD and OS by applanation. Trace corneal neovascularization was noted upon slit lamp examination. There was an indistinct border between the sclera and cornea. The posterior segment examination was normal bilaterally.
The corneal topography and axial length measurements are shown in Figure 1. The patient was diagnosed with cornea plana. Findings included significant flattening of the cornea, hazy limbus, normal axial length, and high hyperopia. Cornea plana exhibits a genetic inheritance pattern that can be autosomal dominant (mild forms) or autosomal recessive (severe forms).
Various contact lens options including soft, scleral, and corneal GPs were discussed with the patient and his parents. Because he had been previously unsuccessful with corneal GP lenses, he was reluctant to try large-diameter scleral lenses. Additionally, given the required high-plus power, scleral lenses would be thick and could present challenges to his corneal health secondary to oxygen deprivation and/or lens decentration.
Custom soft lenses were ordered using the manufacturer’s lens calculator. Considerations included his high hyperopia, extremely flat Ks, and very small HVID. Lenses were ordered with the following parameters:
Lens parameters: base curve 9.0mm OD and OS; diameter 13.5mm OD and OS; and +12.50 sph OD and +11.50 sph OS.
The instructions to the patient included quarterly replacement of his lenses and no extended wear. A multipurpose soft contact lens cleaning solution was prescribed for nightly disinfection and storage.
The patient was successfully fit in custom soft lenses with resulting best-corrected visual acuity (BCVA) of 20/40+ OU and full-day wearing time.
CASE 3: MEGALOCORNEA
A 17-year-old patient was referred by his outside eyecare provider for contact lenses. He had tried soft contact lenses a few years prior to his initial visit but had been unable to successfully apply and remove them. He was planning on going to college in the fall and was interested in trying contact lenses again. There was no pertinent ocular or medical history.
His BCVA in spectacles was 20/20 bilaterally. The external examination was unremarkable. Intraocular pressure was 13 mmHg in both eyes. The lids and lashes were of normal appearance. The cornea exhibited symmetric 360º thinning of the limbus. The posterior segment was remarkable for bilateral optic disc drusen (Figure 2).
The refractive information is:
The corneal topography maps are shown in Figure 3. The patient was diagnosed with megalocornea, a rare, typically bilateral condition with corneal diameter greater than 13mm. There is no corneal edema, scarring, or buphthalmos in patients who have megalocornea. It is typically inherited as an X-linked genetic pattern and can be associated with Alport Syndrome, Marfan syndrome, Ehlers-Danlos syndrome, and Down syndrome.10 Scleral, corneal GP, and soft contact lens options were discussed with the patient.
We attempted a diagnostic corneal GP evaluation in the office. However, multiple diagnostic lenses were unstable on the eye. Scleral GP lenses were considered, but due to the large corneal diameter, custom freeform lenses most likely would be required. The patient and his father requested soft contact lenses first.
Soft contact lenses were ordered using the manufacturer’s fitting calculator in the following parameters:
Hioxofilcon 59% material;
OD: 9.0mm base curve, 16.5mm diameter, –5.75D sph, VA 20/20
OS: 8.8mm base curve, 16.5mm diameter, –5.75D sph, VA 20/20.
Diagnostic lenses had appropriate fitting characteristics. There was appropriate movement and centration with 20/20 visual acuity in both eyes. The lenses were dispensed with a multipurpose cleaning and disinfection solution. A one-week follow-up visit showed good corneal physiology and excellent visual acuities in both eyes. The patient continues to successfully wear the custom soft lenses on a full-day basis.
SUMMARY
Custom soft contact lenses can play an important role in our clinical management of patients with special visual needs, intolerance to other modalities, or challenging anterior segment anatomy. Offering specialty soft lens services is professionally rewarding and promotes patient loyalty and practice growth.
References
1. Gurnani B, Kaur K. Contact Lens–Related Complications. [Updated 2023 Jun 11]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2023 Jan. Available at ncbi.nlm.nih.gov/books/NBK587443.
2. Alipour F, Khaheshi S, Soleimanzadeh M, Heidarzadeh S, Heydarzadeh S. Contact Lens-related Complications: A Review. J Ophthalmic Vis Res. 2017 Apr-Jun;12:193-204.
3. Young G, Coleman S. Poorly fitting soft lenses affect ocular integrity. CLAO. 2001 Apr;27:68-74.
4. Shen M, Cui L, Riley C, Wang MR, Wang J. Characterization of soft contact lens edge fitting using ultra-high resolution and ultra-long scan depth optical coherence tomography. Invest Ophthalmol Vis Sci. 2011 Jun 9;52:4091-4097.
5. Muntz A, Subbaraman LN, Sorbara L, Jones L. Tear exchange and contact lenses: a review. J Optom. 2015 Jan-Mar;8:2-11.
6. Davis R, Becherer D. Techniques for Improved Soft Lens Fitting. Contact Lens Spectrum. 2005 Aug. Accessed 2024 Mar 21. Available at clspectrum.com/issues/2005/august-2005/techniques-for-improved-soft-lens-fitting.
7. Jones L, Brennan NA, González-Méijome J, et al. The TFOS International Workshop on Contact Lens Discomfort: report of the contact lens materials, design, and care subcommittee. Invest Ophthalmol Vis Sci. 2013 Oct 18;54:TFOS37-TFOS70.
8. SpecialEyes. The Benefits of Hioxifilcon Contact Lens Material. 2014 Sept. Accessed 2024 April 6. Available at blog.specialeyesqc.com/choosing-a-contact-lens-material-the-benefits-of-hioxifilcon.
9. Frogozo J, Harthan J, Sonsino J. Custom Soft vs. Hybrid vs. Rigid Lenses. Contact Lens Spectrum. 2022 Oct;37:32-34,36,37. Accessed 2024 Mar 21. Available at clspectrum.com/issues/2022/october/custom-soft-vs-hybrid-vs-rigid-lenses.
10. Ong APC, Zhang J, Vincent AL, McGhee CNJ. Megalocornea, anterior megalophthalmos, keratoglobus and associated anterior segment disorders: A review. Clin Exp Ophthalmol. 2021 Jul;49:477-497.
