A Rational Approach to Fitting Multifocal Lenses
A Rational Approach to Fitting Multifocal Lenses
Meeting presbyopes' visual needs is possible with proper multifocal lens design and material selection.
By Robert L. Davis, OD, FAAO, & S. Barry Eiden, OD, FAAO
Dr. Davis is co-founder of Eye-Vis Eye and Vision Research Institute. He practices in a suburb outside Chicago. He is an advisor or consultant to CooperVision and SynergEyes, has received research funds from CooperVision and B+L, and has a proprietary interest in SpecialEyes and Alternative Vision Solutions.
Dr. Eiden, co-founder of Eye-Vis Eye and Vision Research Institute, is president of a private group practice in Illinois. He has a financial interest in Alternative Vision Solutions, LLC, is a consultant or advisor to Ciba Vision, CooperVision, SynergEyes, Alcon, and SpecialEyes, and has received research funds from Vistakon, CooperVision, and B+L.
Successful patient management with multifocal lenses can help establish one of the most rewarding segments of a contemporary contact lens practice. More patients presenting with presbyopia creates a great opportunity to meet their vision needs with contact lenses. Many of these patients have been wearing contact lenses for most of their pre-presbyopic years and desire to continue wearing them without the need for supplementary eyeglasses. Others are looking for a non-spectacle solution to their first true vision challenge as they have entered into their 40s.
Although eyecare practitioners have been continually advised by the contact lens industry that they must “establish realistic patient expectations” when prescribing presbyopic contact lenses, our patients often are intolerant to significant compromises in vision. With expanding and improving technologies available in multifocal contact lenses, we are getting closer to the goal of noncompromised vision correction. If we are able to meet or exceed our patients' vision needs with multifocal contact lenses, we have the opportunity to change the patient-practitioner relationship in a positive way. Conversely, an unsuccessful outcome can be equally damaging to this relationship.
If we can develop a methodology and science by which we can successfully match patients with the appropriate multifocal contact lens design, we can optimize their contact lens experience. This article will review the diagnostic data and treatment strategies that allow us to arrive at a scientific and rational approach to multifocal contact lens fitting.
An Overview of Multifocals
The literature has shown that our patients are interested in multifocal contact lenses. A 2007 survey conducted by the Contact Lens Council reported that 40 percent of patients who responded were unaware that their presbyopia could be corrected by multifocal contact lenses. Furthermore, 75 percent of contact lens wearers and 60 percent of the spectacle lens wearers responded that they were interested in trying multifocal contact lenses. The reason more patients don't wear them seems to lie in practitioners' lack of confidence in the ability of contact lenses to provide adequate vision correction for presbyopia.
Eiden and Davis (2009) enrolled only spectacle lens wearers into a bifocal research study and fitted them with the SynergEyes Multifocal (SynergEyes, Inc.) lens. The patients were selected from the general clinic population and were asked if they would like to wear multifocal contact lenses. Seventy percent of subjects who completed the study continued to wear the lens design after the study was completed. The results suggest that all we have to do is ask.
Alternating, simultaneous, annular, concentric, aspheric, and segmented are the hallmark terms associated with the multifocal contact lens vocabulary. All GP multifocal lenses utilize alternating vision effects to some degree because they move with the blink and will translate to varying degrees with inferior gaze. With isolated exceptions, soft and hybrid multifocal lens designs are simultaneous designs because they do not move significantly with the blink or translate with inferior gaze. The relative position of the line of sight to the power sections of a multifocal contact lens determines the success or failure in terms of distance and near vision clarity. This is especially critical with segmented bifocal contact lenses. This is true to a lesser degree with aspheric and concentric designs as well, because some translation with inferior gaze is still optimal. This results in a transfer of the line of sight into alternative power sections of the contact lens.
Although a comprehensive understanding of the mechanics and optics of multifocal lens designs is important, the key to fitting success is to appropriately match an individual patient's vision demands and visual/perceptual processing style with the proper design. All available multifocal lens designs can find a home within our practices. However, it is how we integrate the “tools of the trade” with patients that determines success or failure. In the following sections we hope to remove some of the mystery in fitting multifocal contact lenses. We will attempt to incorporate a rational and methodical approach to fitting presbyopic patients in available multifocal contact lens designs.
The first step in multifocal contact lens fitting has nothing to do with physical eye measurements, lens design or any other technical data. The essential initial step is to assess patients' vision requirements and expectations and to establish a clear understanding of how and when they want to wear multifocal lenses. Determine whether patients desire to wear contact lenses full time, daily yet limited hours, or occasionally. Discover what specific activities are most important. Consider the vision demands of those activities and how various designs might perform under those circumstances.
Most part-time wearers will be better suited to wearing soft multifocal lenses because of the ease of physical adaptation and the reduced initial lens awareness. If ultimate convenience is desired, consider a single-use disposable multifocal lens design. Although the currently available single-use multifocal options are very limited and often result in less than pristine vision, many patients might find this an acceptable option for occasional social wear.
Patients who want to wear multifocal contact lenses full-time have many options: disposable soft multifocals, customized soft multifocals, GP multifocals, and hybrid multifocals are available to meet the growing needs of the presbyopic market. Success is based on the realization that no one lens design is superior to another, and that the lens selection that satisfies a patient's individual vision requirements will provide the best outcome.
Discussions regarding initial comfort, cost, and convenience must occur during this pre-fitting phase in order to properly select appropriate multifocal lens candidates. If you are able to discover that a given patient is not able to afford multifocal lenses, or that he would not accept the vision outcomes available, or would not be able to tolerate physical lens adaptation, then you could avoid wasting his time and yours. However, it is equally critical to present the information in a clear and non-judgmental way. For example, presenting the expected vision with a soft multifocal lens to a patient as “somewhat hazy” or the comfort of a GP multifocal as “initially painful” is setting up for failure. Present multifocal options to patients in an easy-to-understand and informative way. Make sure they understand the need to replace lenses at pre-determined intervals, the necessity of ongoing professional eye care, and the maintenance issues involved in healthy and successful contact lens wear.
Before you begin the lens selection, note certain anatomical factors that will help you choose an appropriate lens design. Physical examination of the ocular surface is critical to determine candidacy and, more specifically, the lens material and care system that will result in the highest probability for success. Examine the tear film quality and quantity. It must be adequate in volume as well as clear from debris and solids such as mucus strands. Check the lid margins for evidence of anterior and/or posterior blepharitis. The presence of anterior blepharitis or meibomian gland dysfunction will limit patients' likelihood for comfortable long-term lens wear, so manage these conditions before you start fitting.
Examine your patients for signs of Rosacea or active dermatological involvement. Any allergic conditions, if present, should be under control, and it is also important to check for active ocular involvement. Also be cognizant of all prescribed medications and their ramifications on the ocular surface.
Pupil size is important, especially in relative extremes. Measurement of pupil size in normal room illumination and in the dark is helpful. Using an infrared pupillometer is the most accurate method to document pupil size; however simple pupil gauge measures are sufficient in most cases. A larger pupil will help in translation effects with GP lenses whereas in simultaneous design soft lenses and hybrid lenses large pupils can create ghosting and glare. A small pupil in GP, hybrid, and soft lenses can potentially cause visual problems due to the inability to see within the peripheral optical zone.
A major problem relating to pupil size is the dilation effect of scotopic vision at night. Patients can be successful during daytime viewing only to fail at night. One potential management strategy involves the use of Alphagan ? (brimonidine 1% ophthalmic solution, Allergan), a selective alpha-2 adrenergic agonist, in an off-label manner to inhibit pupil dilation. Prescribing Alphagan at dusk will eliminate some of the problems resulting from pupil dilation at night.
Another potential cause of nighttime vision problems with multifocal lenses may be associated with hydrophilic lenses. Patients often complain of nighttime visual problems caused by the diffraction of light through water molecules. They can be refit to a GP or hybrid lens modality to address this problem.
Lid position is another important anatomical consideration. Lid influence on the lens positioning can be a source of fitting problems. Note the lower lid position relative to the limbus as well as the space between the globe and the lid. As individuals age, the amount of orbital fat increases, muscle tone decreases, and the elasticity of the skin changes. These factors affect contact lens movement and tear distribution. The lower lid loosens its position against the globe and the punctal apposition changes, affecting tear drainage. Loose lids may interfere with the spreading of tears across the cornea as well as with lens movement and translation in GP lenses. Tight lids might cause excessive lens movement in soft and hybrid lens designs or position a rigid segmented multifocal lens superiorly.
Corneal diameter is another anatomical factor to assess. Obtain corneal diameter measures by determining horizontal visible iris diameter (HVID), or obtain direct corneal diameter measures on anterior segment OCT, Pentacam (Oculus), or other similar anterior segment imaging systems. Caroline et al (2001) have shown that corneal diameter is the major contributing factor to matching the sagittal height of the contact lens to the anterior portion of the eye.
Keratometric readings or topography measurements excluding the consideration of corneal diameter can be misleading when selecting a base curve radius for a contact lens. Fortunately most patients' corneal diameters fall between 11.60mm and 12.00mm. Lens manufacturers have based their fitting nomograms on this constant. SpecialEyes Contact Lens Company has developed a business around this one single fitting philosophy. Outside the norm of 11.60mm to 12.00mm, an adjustment needs to be made on the base curve radius selection to accommodate the influence of corneal size on overall sagittal height. For patients who have a smaller HVID than 11.60mm, the lens needs to be flatter. For patients who have a larger HVID than 12.00mm, a steeper base curve radius is necessary to fit the sagittal height of the cornea.
Figure 1 illustrates the measures helpful in initial lens design selection.
Figure 1. Measures helpful in initial lens design selection.
Design Options: Aspheric Multifocals
Aspheric multifocal lenses are designed with the aspheric shape on either the anterior or posterior surface of the lens. Occasionally “bi-aspheric” designs are used that have aspheric surfaces on both the front and back of the lens. The degree of asphericity is directly correlated to the effective add power and is mathematically defined by the eccentricity value or e-value. Higher-eccentricity lenses will generate greater effective add powers; however, lens centradon becomes critically important for higher e-value lens designs. Pupil size also is important in aspheric lenses. Once again, if an aspheric lens with a high e-value is required to meet the add demand of a particular patient, pupil size can influence how much of the full power range becomes available to that individual patient.
Posterior-surface aspheric multifocal lenses will have their maximum distance power in the center of the lens optics, and the near power will increase as you move from the center to the periphery of the lens optics. Most posterior-surface aspheric multifocal lenses are fabricated with GP materials; although there are a limited number of soft versions (mostly lathe-cut custom soft multifocals). These lenses generally provide good distance and intermediate vision.
Near add powers are often limited with these designs due to the need for high e-value designs, which often compromise distance vision because of aberrations that increase toward the periphery and because of potential lens centration issues. Night vision flare and glare are not as great a problem when compared to concentric designs. The asphericity creates an optical smoothing effect when compared to the distinct and abrupt optical transition that occurs with concentric design lenses.
For this design to be successful, the progressive asphericity needs to match the patient's near requirements. The lens should be fit in a relatively centered position. Posterior-surface aspheric GP multifocal lenses are fit steeper than the flat keratometry reading. How much steeper is relative to the flat-K and is directly correlated to the required add power and, as such, to the e-value of the posterior lens surface.
Keep in mind that the portion of the lens being fit steep is limited to the center of the posterior lens surface—posterior apical radius versus a base curve radius that is consistent within the entire optical zone. As the lens flattens away from the center, the resultant lens-to-cornea relationship is far less steep as compared to a spherical posterior-surface lens fit steep to the same degree. Posterior aspheric multifocal GP lenses are available in proprietary fixed eccentricity designs as well as in customizable designs for which you can specify all shape factors.
Anterior-surface multifocal lenses have their maximum near power in the center with progressive distance power toward the periphery of the lens optics. Most contemporary soft aspheric multifocals utilize these forms of this design. Some GP multifocals are pure front-surface aspheric designs; however, they provide limited effective add powers.
As with posterior-surface multifocal designs, pupil size and centration become more important with higher e-value lenses. In all aspheric designs the pupil area incorporates a ratio of near power versus distance power. Whereas distance vision is compromised with larger pupils in posterior-surface aspheric designs, the near vision will be compromised with anterior-surface aspheric designs and larger pupils. However, for very small pupils with anterior-surface aspheric designs, the near power will be emphasized and distance vision will be compromised. Fit front-surface aspheric GP lenses with an alignment philosophy similar to single vision GPs. Once again, obtaining a well-centered lens is important, especially with higher e-value designs. Due to the relatively limited effective add powers found with all anterior surface aspheric multifocal lenses, the use of relative “mono-plus” and “mono-minus” is common. In cases of higher required add effects, you can use a lens that accentuates distance vision slightly more in the dominant eye or a lens that accentuates near vision slightly more in the nondominant eye.
The difference between the two lenses' effective add powers is commonly limited to one diopter or less. For example, the nondominant eye may have less than or equal to no more than one diopter of additional relative plus power compared to the dominant eye. In lens designs with two or three add powers, you might need to use a low or a median add on the dominant eye and a high add on the nondominant eye.
Material selection in aspheric GP multifocals is important to create the greatest effective add power. Materials that have higher refractive indices will create a greater effective near add compared to lower-refractive-index materials (all other parameters being equal). Selecting high-index GP lens materials such as SGP material (1.48) (The Lifestyle Company, Inc.), Contamac HR (1.505) or Paragon HDS HI (1.544) (Paragon Vision Sciences) will deliver the greatest effective add power in a given lens design.
A significant limitation of GP posterior-surface aspheric multifocals is the tendency to cause temporary corneal molding and irregularity following removal. This results in lens-induced spectacle blur that can last for many hours. The degree of corneal distortion is related to both lens eccentricity and to individual corneal deformation properties. Higher-eccentricity lenses will distort the cornea to a more dramatic degree; however, there is significant individual variation in the cornea's potential to become distorted. This complication has limited the use of these lenses and motivated lens designers to develop alternative multifocal mechanisms for GPs.
Aspheric multifocal lenses in general provide a smooth progressive vision effect that simulates pre-presbyopic vision function. Limited by the degree of presbyopia, pupil size, and the need for lens centration, they can be considered the design of choice for early-to-moderate presbyopia. When utilized for more advanced presbyopia, high-eccentricity designs or modified “mono-minus” and “mono-plus” designs often required. Table 1 gives some examples of different aspheric lens designs.
Design Options: Concentric Lens Designs
Concentric lens designs incorporate distinct annular zones of power to create their multifocal effect. The power value in an annular zone abruptly changes either from distance to near in a distance-center lens or from near to distance in a near-center lens. This abrupt change in power has a tendency to create ghosting or double vision. You can minimize ghosting by placing a transition aspheric zone in between the two spherical annular zones.
Concentric designs can incorporate the add power on the front or on the back of the lens. These lenses typically allow for more effective add compared to aspheric lenses. With concentric lenses, finding the ratio of distance and near zone sizes will provide a successful visual outcome. The center-distance front-surface annulars are fabricated as lenticulars with a flatter central curve as the distance zone surrounded by a steeper carrier that corresponds to the near power.
The distance portion of the lens covers approximately 60 percent of the pupil area, which corresponds to 3.00mm to 5.00mm. In front concentric designs, an extensive pool of tears in front of the pupil minimizes aberrations. The back-surface de Carle concentric design has a very steep central back curve. The surrounding annulus for near is used as a bearing surface. Back concentric bifocals must have a smooth transition between the zones or staining will result. Placing the near power in the center causes interference with distance vision if the zone selection size is not optimized. The near portion of the lens typically represents about 30 percent of the pupil area, which corresponds to 2.00mm to 3.50mm.
Table 2 provides some examples of concentric multifocal designs.
Design Options: Alternating Designs
Alternating or translating designs have the advantage of distinct and separate areas dedicated to vision for distance and for near. The positioning of the line of sight in alternating multifocals is entirely dependent on lens positioning in primary gaze and translation upon inferior gaze. The distance and near portions of the lens are never used simultaneously (Figure 2).
Figure 2. BiExpert GP lens (Art Optical/Blanchard).
In primary gaze the lens typically will position inferior to the center of the cornea, thus allowing the line of sight in primary gaze to pass through the superior distance portion of the lens. It is critical that the near segment is not positioned within any portion of the pupil zone. A blink in primary gaze will typically raise the lens; however, it must drop back down to the inferior position relatively quickly to avoid transient blur with the blink.
With inferior gaze the lens must translate enough to allow the line of sight to pass through the inferior near portion of the lens. The lens has to be stable and is controlled by incorporating a base-down prism and/or truncation. Pupil size, lid position and fissure width will be influential in selecting the optimal parameters to position the lens correctly.
You can adjust the segment line by the size of the lens as well as the base curve radius selection. A flatter base curve radius can cause the lens to ride higher, whereas a steeper base curve radius will cause the lens to drop. Lens weight will also affect lens positioning. You can use the specific gravity value of a lens material to influence positioning. Truncation, prism ballast and lenticularization will also help you position the lens correctly. With a proper physical fit, these designs provide the clearest and best vision quality of all multifocal lenses.
One potential problem with this bifocal arrangement is the missing intermediate zone. If the amplitude of accommodation is absent, attempt a trifocal design or place an aspheric curve in between the distance and near portion or on the posterior surface of the lens. Even these designs are “gaze dependent” and rely on looking down for near vision. This can be a problem for some due to occupational or avocational near gaze requirements.
Consider these factors in an alternating multifocal contact lens fit:
- Lens positioning in primary gaze (an inferiorly positioned lens is desired)
- Segment position relative to the inferior pupil (positioning at or slightly below the inferior pupil edge is desired)
- Segment rotation (less than 20 degrees of rotation is desired along with stable positioning)
- Lens movement with blink in primary gaze (adequate lens movement is necessary for appropriate physiological response; however, the lens must drop back to the original inferior position quickly enough not to induce perceived vision fluctuation with blink)
- Translation (adequate translation with inferior gaze is required to position the line of sight within the near segment of the lens)
Recessed Lens Technology
Recessed lens technology may prove to be the optimal solution to the challenges of multifocal contact lenses. Fusion Contact Lens Inc., along with Eye Vis Eye and Vision Technologies and Research Institute, has been developing a novel patented contact lens system. The Recessed Lens System utilizes a hydrophilic base lens with integrated anterior recessed geometry that allows for a customized GP lens to be placed within the recess (Figure 3). Once the rigid lens is positioned within the recessed soft lens, the system works as one. The system moves as a single unit. The movement of the system is determined by the movement characteristics of the soft recessed lens. It is the positioning and movement of the recessed soft lens that dictates what portion of the rigid lens the line of sight will pass through. It is the optics of the rigid lens, however, that drive the system.
Figure 3. Recessed lens.
In summary, movement and positioning are based on the recessed soft lens, while the optics are determined by the rigid lens. With the use of various soft lens materials (variable modulus characteristics), shape factors, and curvatures, we can either promote or inhibit movement. Similarly, we can also create variable degrees of translation. As such, virtually all modalities of GP multifocal lens designs can be utilized with the Recessed Lens System. Simultaneous designs such as anterior and posterior surface aspheric lenses, near- or distance-centered concentrics, combination designs, and even alternating design lenses have been used in the Recessed Lens System with success. Seidner et al (2009) have discovered that only small increments of vertical movement are required to reach the bifocal portion in downward gaze. Transfer of the line of sight from the distance portion of the GP lens into the near portion can be achieved with both simultaneous distance-center GP lenses and alternating segmented rigid lenses in the Recessed Lens System.
Beyond the ability to provide superior multifocal optical performance due to precise lens centration, the Recessed Lens System also can address ocular surface complications associated with posterior aspheric GP lens-induced corneal molding, 3 o'clock and 9 o'clock staining, vascularized limbal keratitis, corneal dellen, and GP comfort issues as well as solving complications from keratoconus and post surgery therapies. Besides the advantage of the soft carrier acting as a bandage lens on the surface of the cornea, the optics of the recess system offer the same optics of a GP lens and therefore can solve many distortion problems associated with soft lenses. Further development of this lens system may open up a new arena for contact lens uses.
The field of multifocal lens technologies is in a constant state of flux. We continue to develop new designs, materials, and fitting methods. Of equal, if not greater, importance is the fact that our patients' world is also changing. Our patients face a more complex visual environment. Driving, laptop computers, and cell phone/PDA use all require different working distances.
As our patients' visual demands are changing, so do the demands placed on our visual system. Examination room testing is not a good indicator of contact lens multifocal success. Do not base your clinical decisions on the diagnostic evaluations performed in the examination room, but on real life situations.
The choice of the multifocal lens material is certainly more definitive than the specific design options. The soft lens versus GP selection comes down to vision, initial and long-term comfort, cost, and convenience. A dialogue with your patient will develop a partnership to arrive at a joint decision.
The explosion of new multifocal lens designs is allowing us to address issues that have limited success with multifocal lenses in the past. New GP designs are attending to issues such as lens-induced molding by placing more effective multifocal optics on the front surface of the lens. Systems are combining various optical approaches such as combined concentric and aspheric or combined translating/alternating and aspheric designs.
Soft multifocals are addressing the optical needs of presbyopic lens wearers by incorporating novel approaches and addressing the physiological needs of the cornea by using silicone hydrogel materials.
Our role remains paramount in both appropriate patient selection and lens design. Working with various parameters available within a design and using more customized fitting approaches allow for greater success rates with presbyopic contact lenses.
Multifocal fitting is the one contact lens modality that has been minimally affected by the economy. It is a great referral source from successful patients. Whether soft, GP, hybrid, or recessed technologies, the rules for concentric, aspheric, and segmented multifocal lens fitting are the same. They need to provide adequate distance, intermediate, and near acuity to perform daily routines. Any of the presbyopic options either in spectacles or contact lenses have some compromise. We must partner with patients to find the solution that meets their vision demands. This is our rational approach to multifocal lens fitting. This is our methodology. We believe it can be yours as well. CLS
For references, please visit www.clspectrum.com/references.asp and click on document #171.
Contact Lens Spectrum, Issue: February 2010