Clear-Cut Classification

The evolution of intraocular lenses with a variety of mechanisms of action, each attempting to improve lens performance, has resulted in complex and poorly understood terminology. Much of this has stemmed from manufacturers trying to differentiate themselves from the competition in an attempt to capture market share. In my opinion, they should perhaps be more concerned about collaborating with their rivals to grow the overall business – but that is a topic for another discussion.

Universally accepted nomenclature is needed to communicate effectively. Often mechanisms of action are confused with optical and visual performance, the classic example being the term EDoF or “extended depth of focus,” whereby many ophthalmologists associate EDoF with fewer dysphotopsias. The ANSI standard for EDoF lenses requires implants to have intermediate vision (67 cm) of 20/30 in 50 percent or more of recipients, but there is no mention of dysphotopsias. However, returning to fundamentals, the term EDoF, borrowed from photography, is in reality “extended depth of field” and should really only be applied to small aperture lenses.

Observing manufacturers all wishing to jump on the bandwagon of having an EDoF offering was a source of amusement to many ophthalmologists. However, the mechanisms used to achieve the phenomenon of improved range of focus did not really follow the principle of elongating the conoid of Sturm and used diffractive, zonal refractive or a combination along with others like optimizing chromatic aberration. This resulted in similar dysphotopsias caused by bifocal and trifocal lenses, though professed to be less in magnitude. More concerning was the misinformation provided to patients who were informed that having so-called EDoF lenses would result in fewer halos, starburst and glare. This misinformation set false expectations for patients, who still faced dysphotopsias and compromises in their reading ability.

Several members at the European division of the American European Congress of Ophthalmic Surgery (AECOS) informally discussed the myth of EDoF equating to less dysphotopsias and the arising confusion with the variety of lenses being manufactured. This led to the creation of a committee by me and four other members of AECOS Europe with the objective of looking further into the issue and identifying ways to simplify terminology and classify lenses. The purpose of this initiative was to provide a systematic approach that would deliver cognitive ease for all stakeholders: patients, ophthalmologists, and manufacturers, avoiding misunderstanding.

Considering the defining principles of good nomenclature and classification, we determined on parameters for our system: simple, stable, uniform set of rules, room for further additions with language that could be easily understood, where relevant, by all parties and in particular patients. An example of good classification is the 4C system created by Shipley and employed by the Gemological Institute of America for classifying diamonds. The 4Cs of Color, Clarity, Cut and Carat simplified what was previously a complex process.

To decide the components needed, the issues concerning each stakeholder group were considered. Component issues of lens behavior were identified and differentiating terminology was crafted by the members of the committee.

The common denominator “Lens Performance” was broken down into three main components: range of focus, mechanisms of actions, and dysphotopsias. The committee acknowledged that lens performance was not just about the optical device alone and included a number of other important variables, but, to avoid complexity, decided that the variables could not be incorporated. These included corneal optical performance (presence of aberrations), postoperative pupil size, axial length and conoid of Sturm, neuroadaptation, and patient expectation management.

Classified outcomes
 

Range of focus was classified using the following terms: Monofocal, Monofocal Plus, Increased Range of Focus (IRoF) and Full Range of Focus (FRoF).

Mechanism of action of the lens included Accommodation, Small Aperture (true EDoF), Diffractive, Zonal Refractive, Combined, and Other.

Dysphtopsias were classified with the following terms: Glare, Halos, Starbursts, and Other (to include less frequent issues like waxy vision syndrome).

Attempting to classify available high-performance lenses, specifically regarding range of focus, we invited manufacturers to complete a questionnaire. Responses were unfortunately not as good as hoped and could not be reliably considered. The inclusion criteria for each category are something that will need further clarification and consensus from the board of AECOS.

Accepting that no classification is perfect, especially where elements are subjective and variables, such as neuroadaptation and expectation management, have considerable influence, the proposed classification system does fulfil its objective of simplifying intraocular lenses, based on the range of focus provided, mechanism of action, and magnitude of dysphotopsias. We are very pleased to see good acceptance of this terminology, which is now becoming commonplace amongst ophthalmologists.