Introduction THE ART OF BIFOCAL CONTACT LENS FITTING Eef van der Worp BOptom PhD FAAO Bifocal contact lenses have high potential in contact lens practice. The contact lens wearer who turns presbyopic is looking for alternatives to glasses. Wearing lenses are a step back from what they were used to and would make contact lenses more or less redundant. It is a misconception that bifocal lenses do not work and it is also a misconception that they are difficult to fit. However, it is challenging and sometimes confusing to manage all the presbyopic correction options available today and the secret to success is to find the right design for the right patient. Many articles and presentations on this subject provide the practitioner with details about all presbyopic lens options that are available, with the risk of being unable to see the trees for the forest. This presentation aims on presenting an overview of basic principles in bifocal lens and providing a simple and useful flow-chart to help the contact lens practitioner in making this crucial decision and to make fitting of bifocals easier. STEP 1: BIFOCALS OR NOT? About half of the vision care population is presbyopic so there is hardly any discussion on the need for a presbyopic solution. Many bifocal wearers in fact report how satisfied they are to be the only ones in community that are able to read without glasses. The constant need for reading glasses, frequent application of the device and the risk of losing them is a serious consideration for many. Monovision is an option that is often regarded as a first option. And in fact: for beginning presbyopes this can be a good first step. Simply adding +0.50D to one (usually, but not necessarily, the dominant) eye will aid the presbyope in doing near work. However, practitioners have to make sure they are aware of this system, the progressive nature of presbyopia and the need for other solutions in the (near) future. At some point in time, monovision usually will not provide satisfactory visual results. There seems little doubt amongst practitioners and in the literature about the superiority of bifocal lens designs in both hydrogel and GP lens wear over monovision. STEP 2: GP vs HYDROGEL Even more so than in single vision contact lenses, there are many variables that need to be considered when deciding on GP lenses over hydrogels. However, much more than in the single vision fitting, the contact lens practitioner is in charge of the final decision.
Variables to consider are: HYDROGEL GP VISION High demand - + Contrast - + Complex cornea - + PHYSIOLOGY Tear film +/- +/- Hypoxia - + Extended wear - + COMFORT Dusty + - (Near) Emmetrope + - COST - + Taking all items into consideration, it can be concluded that hydrogel lenses do not provide the same optimal visual quality that GP lenses will give, and also are less ideal physiologically. In general: if superior vision is required, GP lenses are advised as a first lens of choice.
STEP 1 BIFOCAL STEP 2 HYDROGEL GAS PERMEABLE STEP 3 SIMULTANEOUS TRANSLATING STEP 4 ASPHERIC CONCENTRIC SEGMENT TRIANGLE TRIFOCAL CONCENTRIC STEP 5 Manufacturer Manucaturer Manufacturer Manufacturer Manufacturer Manufactuerer STEP 3: SIMULTANEOUS OR TRANSLATING? Following step 2, the focus will first be on GP lenses, covering step 3, 4 and 5 of the flow chart, after which hydrogel lenses will be discussed in the same matter and following the same steps. Typically simultaneous systems are truly bifocal, while translating systems are considered bifocal. However, a number of translating designs offer bifocal vision as well. When referring to the term bifocal, in fact bi- or bifocal is meant, unless specifically stated. RIGID GAS PERMEABLE LENSES For GP lenses, the most critical question that can and should be answered is whether a simultaneous or alternating design is desired by the patient. This can be decided upon based on several variables, but visual requirement is by far the number one. The basic decision is fairly simple: if good and stable vision is required for far and or for near, a translating system is the first method of choice. If flexible and good intermediate vision is required, simultaneous is the number one choice.
simultaneous translating Computer 35% > < 35% Work Mid Far-near Sport Mid Far-near Free time Mid Far-near Age Low addition High addition Contrast sensitivity OK Good Pupils 5mm< 5mm> Ac depth Small-normal Deep Comfort Normal Reduced No previous GP wear Best Less ideal Movement Limited movement Must move Topography Centration essential Centration desired Astigmatism Limited Possible Eccenicity cornea low e-value only (BS) Low and high e-value High myopia Possible Limited Hypoxia Good Decreased Tear film quality Less important More important Movement Not too much Must move Lid position (rotation) No influence Large influence Eyelid shape No influence Large influence Eyelid location No influence Around limbus Aperture size No influence Large influence Head position Any Only on down gaze Cost Moderate High Fitting Less challenging More challenging Bifocal lens fitting often relies on flexible and creative lens fitting, including combining different lenses or methods. Modified monovision is for instance an excellent way of expanding your options. However, one should be reserved to combine simultaneous and translating systems: essential differences in optics could result in optical confusion for the patient. STEP 4: DIFFERENT DESIGNS When the most important decision has been made in step three, it is time to finetune this choice. Within the system that has been chosen there are a number of possibilities.
Simultaneous The term simultaneous vision is a bit misleading. Two (or more) images are projected on the retina simultaneously (not necessary all clear), but only one is clear to the patient. Simultaneous lenses are typically aspheric, concentric or diffractive. The first one is by far the most popular, but all systems will be covered in this presentation briefly. Simultaneous lenses are very user friendly, easy to fit, relatively inexpensive and popular: success rates up to 75% are not uncommon. In general, a simultaneous lens is the ideal lens to start with: both for the beginning presbyope and for the beginning practitioner. It should be fitted when critical vision is not required. For higher near additions simultaneous designs may have their limitations and translating designs should be considered. ASPHERIC Aspheric lenses can be subdivided in two main groups: front and back surface geometry. These have in common that the addition power increases towards the periphery and this automatically means that all aspheric GP bifocals are centre distance (CD) designs. They are also true bifocals, not bifocals. Be aware that the maximum near addition values provided by the manufacturer can not always be reached. Back surface aspheric The back surface of this lens has a prolate shape, meaning it flattens towards the periphery and thus adding plus power. This peripheral flattening should be compensated by fitting the lens significantly steep, otherwise the lens would be far too flat. But high aspheric lenses on relatively low eccentricity corneas (fitted 2D, 3D or even 4D steeper than k following the manufacturers guidelines) could lead to severe corneal distortion. Fortunately, newer lens designs only need to be fitted 1D steeper than k. Regular topography measurements are required with back surface aspheric bifocal lenses and the practitioner should be aware of any signs of spectacle blur. In the authors opinion, back surface aspheric bifocals should be avoided when alternatives are available that do not have the unwanted and uncontrolled ortho-k effect on the cornea. Front aspheric Front aspheric lenses do not to rely on the asphericity of the cornea. The shape of the cornea is respected as in any other GP lens. The front surface progressively becomes steeper towards the periphery (oblate shape), creating an increasingly progressive reading addition. CONCENTRIC
Also belonging to the simultaneous group but less common in everyday practice is the concentric lens design, often called annular. It uses a far zone and a distance zone and can be created both centre near (CN) and CD. To confuse things a bit: there are also concentric translating designs. In fact the concentric simultaneous designs might benefit from translating as well. But the aim with these simultaneous lenses is to fit them well centered on the cornea, not relying on the lower eyelid. Translating bifocals on the other hand aim to move excessively, rest on the lower eyelid and should translate. Down gaze is essential with translating lenses, which is less of a necessity in simultaneous concentric lenses. DIFFRACTIVE Diffractive lenses bend light that normally travels in a straight line and breaks up rays of light into dark and light bands or into the colors of the spectrum. The closer the spacing between the concentric circles, the higher the addition power. This construction has been used on both soft and GP lenses in the past and on IOL s, all with limited success. Disadvantages are the significant loss of contrast and the limited optical performance (especially with higher additions). Another disadvantage is the built up of debris in the prism rings, which are located at the back surface of the lens. The major positive effect of diffractive bifocals is that they are pupil independent unlike all other designs. Translating SEGMENT DESIGN Different segmented bifocal designs are available all with slightly different characteristics. They have in common that their visual performance is excellent. The long line/ executive segment bifocal is probably the most widely used lens, followed by crescent segments and D-shaped segments. The major advantage that GP lenses have over most hydrogel contact lenses, intra ocular lenses and other refractive surgery procedures is that they are capable of alternating (often used as a synonym for translating) and use different optics for different tasks, very similar to spectacle glasses. The goal is to create a situation where upon down gaze most of the reading section (not necessarily all of it) is in front of the pupil while in primary gaze the pupil is minimally bothered by the near segment. They are particular suitable for presbyopes with reading additions over +1.50D and/or those who require excellent near vision performance in general. Reading additions up to +3.50 are promised and with this lens design these are indeed feasible. They are fairly pupil independent, as long as the overall diameter is increased accordingly. All lenses except some of the concentric designs require stabilization, usually with prism ballast. Bear in mind that prism ballast works according to the watermelon
seed principle, not by gravity. This means the upper eyelids plays a crucial role in stabilizing these lenses. They are not as difficult to fit as often is thought, however: it is fairly difficult to predict how a lens will function on an eye (a little bit easier in existing GP lens wearers). Evaluation of the on-eye rotation is essential. Crescent shaped segments designs can reduce rotation problems. TRIANGLE The Triangle shaped translating bifocal is a monocentric lens design with the optical zones cut on the front of the lens that was developed in the Netherlands. Its design incorporates a spherical distance zone, spherical near zone and a triangle shaped aspheric intermediate zone. In contrast with straight top translating lenses, less translating is necessary, so with less movement the desired effect will be reached (28% less movement according to the manufacturer). This suggests that the amount of prism can be reduced and relatively large diameters can be used. It is less pupil diameter dependent than straight top bifocals. TRIFOCAL For patients who desire intermediate vision (such as computer workers) together high near and far demands, this design or the triangle shaped lens are the options to consider. True bifocals are just what they are: bi-focals. Trifocal designs offer the option to deal with the intermediate distance. It should be noted that this works only with small pupils, since the intermediate zone is usually only 1 mm large, and good centration is more critical than with normal bifocals. Some of the true executive/ long line bifocal designs also have an intermediate zone at the transition from distance to near. However, this should be regarded as a transition zone indeed, not providing a tri- or bifocal effect. CONCENTRIC As mentioned, concentric or annular design lenses are a bit confusing, since they can work as either simultaneous or translating. The central zone of a translating concentric lens is larger than in a simultaneous concentric lens and they should also move easily over the cornea and meet the under eyelid to translate. In translating designs the centre part is always for distance, the periphery for reading (remember, concentric simultaneous lenses can be both CN and CD). Some companies allow you to order the size of the central zone according to your desires; others have this zone diameter related to the height of the addition.
STEP 5: MANUFACTURER Once a lens concept and design have been assigned to a patient, a manufacturer should be chosen. The amount of GP lens designs available worldwide is almost everlasting, and it is impossible to cover all available lenses. However, all lens designs available can be classified according to the presented schedule, making it easy for the practitioner to categorize all bifocal lenses available worldwide. The GPLI in the USA has a website (www.gpli.org) where all available lenses are listed and one can search on manufacturer, lens design or on brand to find what you need. STEP 1 BIFOCAL STEP 2 HYDROGEL GAS PERMEABLE STEP 3 SIMULTANEOUS TRANSLATING STEP 4 ASPHERIC CONCENTRIC SEGMENT STEP 5 Manufacturer Manufacturer Manufacturer HYDROGEL The same questions as with GP lenses can be applied to hydrogel lenses: is a simultaneous or a translating design desired for this patient? The options with hydrogels compared to GP lenses are however very limited. There are two translating hydrogel lenses available worldwide. Although they can be helpful for some patients that do not get enough reading addition with simultaneous hydrogel lenses, the visual outcome is not comparable to GP lens designs. For simultaneous lenses in general it is a rule that they should be stable on an eye during lens wear in order to work well. From that perspective, hydrogel lenses are very suitable for simultaneous lens designs. Different types of simultaneous hydrogel lens designs will be discussed.
Within the group simultaneous hydrogel group there are a number of options. Basically they can be subdivided into aspheric and concentric designs as with GP s. But even within these two groups essential differences exist. In this section, disposable lens designs on the market will be mentioned as examples. Tear film deposits and hypoxic conditions is even more important in presbyopic lens fitting than normally and frequent replacement systems are desired and should be considered as a first choice. Bennett, ES. Building a Successful Presbyopic GP Practice. Contact Lens Spectrum, Febr. 2004 Brunstetter TJ, Fink BA, Hill RM. What is the oxygen environment under an encapsulated segment bifocal GP contact lens? J Am Optom Assoc. 1999 Oct;70(10):641-6 Davis, R. Pinpoint success with GP bifocal lenses. Contact Lens Spectrum October 2003 Ghormley NR.New bifocal designs in hyper-oxygen materials. Eye Contact Lens. 2003 Jan;29(1 Suppl):S180-1; discussion S190-1, S192-4 Hansen DW. Rigid bifocal contact lenses. Optom Clin. 1994;4(1):103-19 Kirschen DG, Hung CC, Nakano TR.Comparison of suppression, stereoacuity, and interocular differences in visual acuity in monovision and acuvue bifocal contact lenses. Optom Vis Sci. 1999 Dec;76(12):832-7. Norman, C. Eight ways to maximize presbyopic contact lens fitting success. Contact Lens Spectrum April 2004 Situ P, Du Toit R, Fonn D, Simpson T. Successful monovision contact lens wearers refitted with bifocal contact lenses. Eye Contact Lens. 2003 Jul;29(3):181-4 Soni PS, Patel R, Carlson RS. Is binocular contrast sensitivity at distance compromised with bifocal soft contact lenses used to correct presbyopia? Optom Vis Sci. 2003 Jul;80(7):505-14. du Toit R, Situ P, Simpson T, Fonn D.The effects of six months of contact lens wear on the tear film, ocular surfaces, and symptoms of presbyopes. Optom Vis Sci. 2001 Jun;78(6):455-62. Woods RL, Saunders JE, Port MJ Optical performance of decentered bifocal contact lenses. Optom Vis Sci. 1993 Mar;70(3):171-84. Woods C, Ruston D, Hough T, Efron N. Clinical performance of an innovative back surface bifocal contact lens in correcting presbyopia. CLAO J. 1999 Jul;25(3):176-81.