Gas permeable (GP) lenses tend to be our “go-to” option for specialty contact lens fits. However, while their use is intuitive for cases of keratoconus and other irregular astigmatic corneas, many other patients would benefit from the crisp vision GP lenses provide or the additional health benefits associated with their use. The wide availability of soft lens modalities and parameters, coupled with the added time and complexity of fitting GP lenses rather than soft lenses, means that GPs comprise less than 10% of total fits today.
In reality, GP lens fitting is straightforward, especially when you have the appropriate tools of the trade—diagnostic GP fitting sets—at your fingertips. Forgoing fitting the lens empirically and observing it yourself on the eye will allow you to make a more informed decision on what parameters will work best for your patient. Let’s look at both empirical and diagnostic fitting methods.
This approach involves fitting a GP lens based on manufacturer guidelines, K readings and refractive measurements. As the simplest method, it saves chair time.1 Recent manufacturing improvements such as the advent of aspheric lathes and more consistent edge profile designs and minimal center thicknesses have further increased its success rate.
|Table 1. Fixed Diameter GP Spherical Diagnostic Fitting Set|
|• Diameter 9.4/8.0 (or 9.2/7.8)|
• Power -3.00D
• Peripheral curves are tetracurve design, similar to the Bennett system8
• Center thickness 0.14mm standardized
The downside of empirical fitting is that success with the first lens fitted is only about 40% when employing K readings and refractive measurements.2 Use of topographic software as well as incorporation of the Sim K information and the patient’s corneal e-values or asphericity can help make the lens design more accurate, however. Many corneal topographers are capable of generating a pseudo-fluorescein pattern based on the lens design (i.e., spherical, aspheric and even toric) desired, allowing practitioners to order lenses without the initial placement of a lens on the patient’s eye.
Empirical lens fitting can also be done using online resources (such as those found at GPLI.org) or smartphone apps like EyeDock (www.eyedock.com). However, empirical lens fitting without the advantages of topography and computer-assessed pseudo-fluorescein patterns is likely to result in more lens orders and exchanges until the patient is appropriately fit. This ultimately results in more office time and possibly less patient confidence in the practitioner than would have occurred had the fitting been done with the benefit of a diagnostic GP lens evaluation.
Diagnostic GP Lens Fitting
This technique provides the advantage of observing one or more test lenses, which are similar to the empirical lens values described above, on the patient’s eyes prior to ordering. In this case, the lenses that provide the best lens-to-cornea relationship, as well as good centration and movement, are the ones that should be chosen. Diagnostic lens fitting provides important information as to what visual acuity can be achieved. Residual astigmatism, if significant, is often observed along with reduced acuity; this improves once additional astigmatic correction is provided via over-refraction. Its presence may indicate that the lens is too thin, and flexing on the eye. Conversely, in some cases residual astigmatism may be higher with more rigid (thicker) lenses; thus, thinner lenses would in fact be more beneficial.3
Patients who undergo diagnostic lens fitting should be forewarned of the differences between GPs and soft contact lenses. Many GP lens patients experience excessive lacrimation and foreign body sensation as a result of initial lens placement, so use proparacaine during initial diagnostic lens fitting to numb the eye. Additionally, while the initial test lens applied to the patient’s eyes may not enable them to see clearly immediately, performing an over-refraction can assure them that vision will be correctable to excellent levels similar to what they may enjoy with their glasses.
Diagnostic lens fitting is highly likely to result in fewer lens changes or reorders, as the fitting gives the practitioner a great deal of information, increasing confidence in the lens fitting process. Patient satisfaction is also quite high, despite the added time commitment diagnostic fitting may entail compared with soft lens fittings.
Empirical lens design is not advised in cases where specialized lenses are needed, such as with keratoconus or post-keratoplasty patients, and with ortho-keratology fittings. In these cases, specialized diagnostic fitting sets are required to properly assess lens performance, vision and proper lens-to-cornea fitting relationships. This is especially true for patients with keratoconus due to the different cone shapes and locations; patients with pellucid marginal degeneration who require much larger GP lenses to assist in lens centration; and patients who wear bifocal lenses due to variations in pupil size and GP lens designs (i.e., center distance, center near, concentric, spherical and aspheric.)
|Table 2. Variable Diameter GP Diagnostic Fitting Set |
|• Diameter varies with base curve (BC)|
• Back Vertex Power (BVP) varies with BC
• PCs standardized to provide relatively constant edge lift (0.09-0.11mm)
|BCR||Back Vertex Power||Diameter/OZD||SCR/W||ICR/W||PCR/W|
One exception is the ReClaim HD lens (Blanchard), a multifocal GP lens designed to be fit empirically. Blanchard does not make diagnostic fitting sets available; instead, an online calculator employing K readings (or Sim Ks) and refractive data will inform the practitioner about the lens parameters that will be provided, and give him/her the ability to adjust add power and other parameters if desired. Note, if residual astigmatism is calculated to be excessive or if inadequate levels of plus reading power would be produced by the lens rear surface aspheric geometry, other options need to be explored through consultation.
Toric GP lenses are often calculated empirically, and frequently with the assistance of the Mandell-Moore Bitoric Fitting Guide (available at www.gpli.info/mandell-moore). This step-by-step empirical method does all the calculations necessary to produce a first bitoric lens for the moderate to highly astigmatic contact lens patient. It provides a good starting point, and in many cases works well in designing a lens that does not need a lot of additional parameter manipulation.4 Use of diagnostic spherical power effect (SPE) fitting sets, in which each lens in the set is pre-designed with spherical lens optics, makes diagnostic lens fitting extremely easy; the fluorescein analysis that an SPE bitoric fitting set provides enables the practitioner to rapidly select the right amount of rear surface toricity, and then determine the correct power rapidly through over-refraction.5-7
Diagnostic Fitting of Conventional GP Lenses
When fitting conventional corneas (i.e., to correct astigmatism, provide sharper visual acuity or slow pediatric myopic refractive error), it is recommended to have more than one diagnostic GP lens set—for example, an average diameter GP fitting set and a larger diameter set for “under-the-lid” fitting, which is typically more comfortable and more easily centered. Conventional GP fitting sets on average measure 9.2 to 9.4mm in size, with an optic zone diameter 1.4mm smaller than the total diameter.8 Powers for these lenses are typically made in -3.00D, as this is close enough for most patients’ refractive errors to be within a range such that over-refractions do not need to be adjusted further.
While there can be variations with high or low Dk GP lens materials, diagnostic sets are best manufactured in a mid-range Dk material with a low wetting angle to ease tear spreading on the lens surface. While many practitioners may have fitting sets in both low Dk and high Dk materials, a Dk of somewhere between 30 and 60 is recommended. As there are many peripheral curve formulas available, it is best to standardize these in the set’s diagnostic lenses using a formula that ensures the secondary curve (SCR), intermediate curve (ICR or TCR) and peripheral curve (PCR) have the same flattening relationship predicated on the selected base curve.
An example of a 20-lens diagnostic fitting set is seen in Table 1. These can be manufactured by any CLMA contact lens laboratory, and are typically provided to the practitioner at very lost cost (assuming of course that the practitioner orders their lenses through the manufacturing CLMA laboratory).
|Storing and Maintenance of Diagnostic Sets|
While it is possible to store diagnostic GP lenses in disinfecting solution, which also serves to improve on-eye wettability, it is not practical. Lenses that are not used regularly are more likely to adhere to the case and subsequently warp, as wet storage is likely to dry out via evaporation. Secondly, as disinfecting solutions are not approved for use for more than a 30 days, lenses would have to be recleaned and placed in fresh solution regularly. In a busy practice, this is unlikely to happen; thus, storing diagnostic lenses dry is more practical. Lenses can be kept in flat-packs, which take up very little space.
After a diagnostic lens has been used, it should be rubbed and rinsed with a lens cleaner and then disinfected before being blotted and and stored dry. The CDC (Centers for Disease Control and Prevention) recommends hydrogen peroxide to disinfect GP lenses. While many practitioners suggest a five- to 10-minute soak with an approved peroxide disinfection system such as ClearCare (Alcon) or PeroxiClear (Bausch + Lomb), proper disinfection generally requires a full four hours.
When a lens is to be reused, it should be first cleaned using an approved cleaner (alcohol-based cleaners are very effective for this purpose), rinsed and applied with a wetting or conditioning solution. Use of a conditioning solution is also recommended for six to 24 hours prior to delivery when dispensing a new set of lenses to the patient to ensure the base curve of the lens is properly hydrated.
In the rare case that a newly-dispensed GP lens is non-wetting, it is usually due to a hydrophobic zone on the lens caused by a waxy or pitch-like residue left from the lens blocking process. This can be readily remedied by cleaning the lens in-office; the author recommends FluoroSolve (Paragon), which dissolves waxy residue to enable tears to spread more effectively on the lens surface. The FluoroSolve should then be rinsed off and the lens cleaned and hydrated using an alcohol-based lens cleaner and conditioning solution, respectively, before re-insertion. If FluoroSolve is not available, an alcohol-based cleaner like as Miraflow or a generic equivalent can be used alone to restore the lens surface.
In addition to the standard fitting set mentioned above, it is prudent to have another in high minus powers (with plus lenticulated edges) and one in high plus powers (with minus carrier lenticulated edges). These supplementary sets provide more accurate information regarding how lenses center, as the average thickness of the lenses in these various powers would differ and thus affect lens positioning. In cases where this is not economically feasible, a standard set of -3.00D power will usually suffice, as it reflects the most typical patient refractive powers seen in practice.
Another option for a practitioner-designed supplementary diagnostic fitting set is one with a variable diameter (i.e., larger lenses for flatter corneas and smaller lenses for steeper corneas). This set could also have variable lens powers, with lower powers for flatter lenses and higher powers for steeper base curves. Table 2 represents a 20-lens diagnostic set designed by the author and manufactured by Valley Contax for use in an optometric training program.
When cost is not a factor, such as in an institutional practice or large group practice, having a large inventory of GP lenses made in a variety of base curves for the most commonly prescribed myopic powers (i.e., from -1.25D through -5.00D in 0.25D steps). For example, having a 200-lens inventory set would make it easy to dispense lenses directly out of inventory even on the first visit, as typically is done with hydrogel replacement lenses. This would help reduce dispensing delay and make parameter changes easy, as well as enable instant lens replacement for patients who lose or damage their lenses. Maintaining the inventory is critical to this method’s success; while initially expensive, it does provide good economy of time and in patient satisfaction.9
While there is a place for empirical fitting of GP lenses, practitioners should remember the use of diagnostic lens fitting sets, whether for conventional lens design, or for more complex lenses, can only lead to more accurate and satisfying use of these wonderful devices.
Dr. Silbert is a professor of optometric medicine at Pennsylvania College of Optometry, Salus University. He is also the director of cornea and specialty contact lens services at the Eye Institute of Salus University. He has no financial interests in any of the lens materials or designs discussed in this article, nor any financial interests in any of the companies’ products mentioned herein.1. Benoit DP, Ames KS. Diagnostic versus empirical fitting. Contact Lens Spectrum. 2010;25(4):12-13.
2. van der Worp E, de Brabander J, Lubberman B, et al. Optimising RGP fitting in normal eyes using 3D topography data. Cont. Lens Anterior Eye. 2002;11:1-5.
3. Harris MG. Contact lens flexure and residual astigmatism on toric corneas. J Am Optom Assoc. 1970;41(3):247-248.
4. Mandell RB, Moore CF. A bitoric lens guide that really is simple. Contact Lens Spectrum. 1988;3(11):83-85.
5. Sarver MD, Kame RT, Williams CT. A bitoric rigid gas permeable contact lens with spherical power effect. J Am Optom Assoc. 1985;56(3):184-189.
6. Silbert JA. Rigid Contact Lens Correction of Astigmatism. In: Bennett ES, Weissman BA (eds.): Clinical Contact Lens Practice, 2nd ed. Butterworth-Heinemann, 2004.
7. Silbert JA. Benefits of diagnostic fitting for bitoric GP lenses. Optometric Management. 2007;42(2):54-59.
8. Bennett ES, Sorbara L, Kojima R. Gas-Permeable Lens Design, Fitting, and Evaluation. In: Clinical Manual of Contact lenses, 4th ed. Wolters Kluwer, Lippincott Williams & Williams, 2014:112-156.
9. Keech P. The top 10 reasons to inventory RGPs. Contact Lens Spectrum. 1996;11(10):32-36.