Diagnostic Versus Empirical Fitting

point TOPIC: Diagnostic Versus Empirical Fitting

Diagnostic Fitting is a Precise and Confident Way to Fit Lenses


It seems that the debate about the merits of diagnostic contact lens fitting versus empirical fitting lives on. There are varied opinions about which approach is better, with advocates for each method adamant that their way is the best way. I would have to say that I am still in the diagnostic fitting camp, although I have become a bit more flexible.

Benefits of Diagnostic Fitting

Using a diagnostic lens has many benefits. First, it allows patients a chance to experience the feel of the contact lens on their eye. This can dispel any apprehension about the comfort of the lens, which is particularly important with GP lenses.

Second, a diagnostic lens makes it possible to assess the fit of the lens. A soft contact lens needs to center well, with full coverage of the cornea. It also needs to demonstrate adequate movement in all positions of gaze and not impinge upon the conjunctiva. For a standard GP lens, look for proper positioning on the cornea. I generally prefer a slight lid-attachment approach to ensure good movement and to provide a better tear pump. With a more specialized GP lens design, such as semi-scleral or full scleral lenses, there needs to be good centration with the lens resting gently on the conjunctiva (again, you do not want to compress the conjunctiva). During the diagnostic fitting of a GP lens, you can assess the lens-to-cornea fitting relationship with the use of fluorescein, allowing for further refinement of the final lens parameters.

Diagnostic fitting also provides both you and the patient with an idea of the visual acuity that the final lens will impart. Over-refraction using the diagnostic lens refines the final lens power in a precise fashion.

Empirical Fitting is Diagnostic

I know that there are instances in which using a diagnostic lens is not practical. In my October 2009 article “Empirical Fitting of GP Multifocal Lenses,” I mentioned that the very large lens parameter availability of some contact lenses, such as soft toric multifocals, makes it impossible to inventory every diagnostic lens you might need. With some designs, such as the newer GP multifocals, a diagnostic inventory is not necessary. Labs can make lenses based on their fitting nomogram from patients' spectacle prescription, keratometric readings, corneal diameter, and lid aperture height.

But remember that an empirically ordered contact lens is really just a custom diagnostic lens for that individual patient. Once the lens arrives from the lab, it is placed on the patient's eye so that vision, fit, and comfort can be assessed. Refinements are then made using over-refraction and slit lamp evaluation of the lens on the eye. Sounds like a diagnostic fitting approach to me.

Fitting With Confidence

From this I believe it can safely be said that diagnostic fitting is a better way to approach contact lens prescribing. Using a diagnostic lens provides more precise information during the evaluation visit. You learn how the particular lens will perform on the eye and how good the vision will be. The patient gets to experience the on-eye feel of the lens and has the opportunity to glimpse the spectacle-free world with clarity. The benefits of all of these facets of diagnostic contact lens fitting cannot be overstated.

Having a confident approach to the process instills patients with the understanding that their practitioner is knowledgeable about contact lenses and is paying special attention to their needs. Using diagnostic lenses reinforces this perception because you are refining their vision with what amounts to a custom lens made just for them. Once the process is over and patients are released from immediate care, their enthusiasm will spread to family and friends who may then seek you out for their care. CLS

Dr. Benoit practices in a multi-subspecialty ophthalmology group in Concord, NH. He is a Diplomate of the Section on Cornea, Contact Lenses and Refractive Technologies of the American Academy of Optometry. He is currently vice-chair of the Section.


Empirical Fitting can Save Time for Both You and Your Patients


Gas permeable (GP) lenses offer significant advantages for many patients who have high refractive errors, significant astigmatism, presbyopia, extended wearing schedules, and irregular corneas. In spite of these advantages, GP lenses are significantly underutilized in North America. I believe a principal reason for this is the cumbersome and unpleasant way that GP lenses are frequently delivered through extensive in-office diagnostic fitting. Empirically designing lenses and delivering them without in-office trial fitting is an efficient, patient-friendly, and technically superior method of fitting for most GP candidates who do not have irregular corneas.

Empirical Fitting is Not New

Empirically designing a contact lens and dispensing it for a trial period of wear is a familiar technique to all of us. We do it every day with our soft lens patients! The ease of soft lens fitting has played a critical role in their success. Why not offer our GP patients the same advantages?

Remember that when you empirically design a GP lens and dispense it for a trial period of wear, you are trial fitting. You just happen to be dispensing your diagnostic lens. By thoughtfully designing your initial lens, you will ensure high success rates with minimal reorders.

Rules for Success

Empirical fitting is not guess work. High success rates can be achieved by following a few basic rules and respecting patients' ocular anatomy. Simply observing the upper lid positioning will direct your empirical fitting approach. If a patient's upper lid covers the superior limbus and will support lid-attachment, fit a larger-diameter contact lens (9.4mm to 9.8mm) slightly flatter than flattest K. If the upper lid is positioned high and not capable of supporting lid-attachment, fit a smaller-diameter contact lens (9.0mm to 9.2mm) on or slightly steeper than flattest K. Exact base curve selection will depend on corneal toricity and can be determined with readily available guideline charts.

This basic fitting paradigm will work for specialty toric fitting as well. Once the diameter and fitting approach are selected, base curves can be determined to optimize either a lid-attached or centered fit. During my years in industry research and professional services, I encountered thousands of practitioners who fit toric GP lenses empirically with high success rates. Yet even many of these practitioners still insisted on non-dispensing diagnostic fitting for their routine fits. If you can fit “difficult” corneal shapes empirically, why not use this same efficient method to fit “easy” eyes?

Multifocal GP lenses, especially non-ballasted aspheric designs, are ideal for empirical fitting. Some designs are intended to be fit lid-attached and others more centered. A patient's anatomy will help direct you to the design that is most appropriate for that patient. Select the base curve based on back surface geometry and the guideline charts provided by the manufacturers. Reorder rates will be higher than with single vision fitting, but allowing patients to partially adapt will permit more accurate design and power adjustments when needed.

Even ballasted GP multifocals can be fit empirically if all of the stars align: lens positioning will be predictable and optimal for patients who have a wide aperture with a lower lid at the limbus to provide support and a low minus or plus refractive error.

Best for You and Your Patients

Empirical fitting offers many technical and management advantages for you, but remember that your patients' time is also valuable. Offering GP fitting that is more pleasant and less time-consuming will be a win-win for both you and your patients. CLS

Dr. Ames is currently in private group Optometrie practice in Chillicothe, Ohio. He graduated from The Ohio State University in 1977. Previously he served in Army optometry, he was a clinical faculty member of the University of Waterloo College of Optometry, a research optometrist and manager of the research clinic at Bausch & Lomb, and director of Technical Affairs at Polymer Technology Corporation. He has published and lectured extensively on contact lenses.