The Evolution of a Soft Translating Bifocal Lens
Author: Dr Donald Ezekiel. AM & Dr Daddi Fadel
Different contact lenses are available to correct presbyopia. The designs may be divided mainly into two groups: depending on the vision provided: Simultaneous vision and bifocal alternate vision. While multifocal simultaneous strategies provide satisfactory distance, intermediate and near visual acuity, adverse subjective phenomena may appear. Alternating bifocal lenses have two areas, one for distance and the other for near. The upper section of the lens is attributed to distance prescription and the lower section for a near prescription. Unlike simultaneous vision, translating lenses are independent of pupil size. The aim of this paper is to discuss the benefits of translating bifocal CLs strategies and to present the evolution of these lenses, presenting a novel design for better stabilization of the lens on the eye.
Keywords: bifocal contact lenses, multifocal contact lenses, presbyopia, simultaneous vision, translating bifocal contact lenses, vision comfort,
Presbyopia is an inevitable condition and affects over a billion people worldwide (1). This number is expected to grow further as the aging of the global population increases, when, by the year 2050, the average age could reach 40 years (2). Various definitions have been proposed to describe this phenomenon. The definitions provided by Gilmartin (3) and Millodot (4) are the most relevant and functional. Referring to Gilmartin: “Presbyopia is a condition of age rather than ageing and as such is devolved from the lamentable situation where the normal age-related reduction in amplitude of accommodation reaches a point when the clarity of vision at near cannot be sustained for long enough to satisfy an individual’s requirements.” Millodot proposed the following definition: “A refractive condition in which the accommodative ability of the eye is insufficient for near-vision work, due to ageing.” Wolffsohn and Davies stated that “The efficacy of a condition management option cannot be assessed if the condition is not defined.”(5). Thus, analyzing the origin of the term presbyopia, which derives from the ancient Greek πρέσβυς (translated into Latin présbus, “old man”) and ὤψ (ṓps, “Eye” or “see like”), Wolffsohn and Davies proposed a revised definition of presbyopia that is more appropriate and inherent to its etymology: “presbyopia occurs when the physiologically normal age-related reduction in the eyes focusing range reaches a point, when optimally corrected for distance vision, that the clarity of vision at near is insufficient to satisfy an individual’s requirements.”
The amplitude of accommodation gradually decreases with age, starting in the second decade of life or even earlier (6). Generally, presbyopia occurs between the ages of 38 and 45, and the prevalence is virtually 100% between 50 and 52 years (7). A recent survey showed that the average age of people wearing contact lenses is around 33 years, although it varies considerably from one country to another (8). This survey contradicts the previously widespread notion that CLs are most commonly prescribed to people 20 years of age. Females predominate among CL wearers, covering two-thirds of those fitted (8).
The results of another survey carried out on 500 presbyopic subjects are surprising, showing that only 8% of CL wearers reported having been informed about the availability of multifocal contact lenses when they complained of poor near vision. A third of the informed patients were willing to try multifocal CLs, and a third would have changed the clinician who had not informed them of this option (9). The main reason why professionals do not prescribe CLs for the correction of presbyopia could be a combination of three factors: lack of skills in the application of these lenses (10), awareness of the existence of a very high visual compromise (11), and lack of availability of a “perfect” multifocal CL that offers both comfort and good simultaneous vision at all distances (10). On the other hand, it is crucial to consider that 91% of patients aged between 35 and 55 want to continue to wear CLs (12). Today’s presbyopes have a strong desire to maintain their aesthetic and physical shape and can be embittered by the thought of wearing reading glasses or progressive glasses.
The correction of presbyopia with a soft CL has been a subject of discussion since the introduction of soft lenses. Indeed, the correction of presbyopia probably remains the most significant challenge in the CL field. Currently, there are various options for the correction of presbyopia with CLs. The designs can be divided into two groups, depending on the vision provided: Simultaneous vision and bifocal alternate vision. Although current multifocal simultaneous strategies provide satisfactory distance, intermediate and near visual acuity, adverse subjective phenomena may appear, such as haloes, ghosting, or glare. Success with multifocal CLs is mostly about managing expectations (13). Alternating bifocal lenses relates to lens design in which a vertical movement results in only one area of vision in front of the pupil. Unlike simultaneous vision, translating lenses are independent of pupil size. Thus there is no compromised vision in dim lighting or low contrast conditions. These designs are generally prism ballasted to stabilize the lens and keep the different powers in the proper position. Some of these lenses are also truncated. They depend on the lids to push the lens upward with the inferior gaze. This strategy is included in corneal lenses, as translation is much easier to obtain with rigid materials. Current soft translating CLs present different limitations caused by lens instability on the eye. This paper has the aim to discuss the benefits of translating bifocal CLs strategies and to present the evolution of soft translating bifocal lens, presenting a new design for better stabilization of the lens on the eye.
2. Simultaneous vs. Translating Contact Lenses
2.1. Simultaneous Contact Lenses
Simultaneous vision CLs comprise multiple powers places within the pupil. Thus, light rays from both distance and near converge on the retina (14). For a successful vision, the patient needs to selectively suppress the most blurred image that is not required for a specific task. Therefore, this concept is based on the blur interpretation and blur tolerance of different images on the retina formed by the various powers of the lens. Pupil diameter and lens stability play a significant role in the success of these lenses.The geometries that currently allow simultaneous viewing can be divided into two groups: concentric and aspheric.
Concentric lenses are characterized by the simultaneous presence of annular zones within the pupil, one annulus for distance and the other for near. Concentric lenses can be center-distance or center-near. A geometry with center-distance has the correction for distance in the center and is surrounded by the addition zone.
This geometry is generally indicated for young presbyopes or for patients who need more vision for distance such as pilots, drivers, athletes. Nevertheless, in low light conditions, with mydriasis, the visual quality for the distance is reduced and disturbed by the presence of the peripheral rings of the lens containing the addition. A center-near lens has the addition zone in the center and is surrounded by the area for distance correction. This geometry allows a clear vision up close because the accommodation induces miosis by not allowing the pupillary diameter to exceed the addition zone during the reading. The near center geometry is the most widespread among soft concentric lenses and is ideal for subjects who prefer proximal activities.
Aspheric lenses allow a gradual change of power. Therefore, they can be adequately defined as progressive CLs. The variation in power can be from the center where the maximum positive power is positioned, which decreases moving towards the periphery (center-near) or with the maximum positive power placed in the periphery and decreases towards the center (center-distance). The asphericity can reach 1.6-1.8. When the geometry is center-distance, the addition is directly proportional to the asphericity and the pupil diameter. The asphericity increases in the lens periphery and with miosis. This facilitates the use of the lens in the peripheral section, where the addition is positioned.
- Lens comfort
- Relatively easy to fit.
- Multiple optics are presented simultaneously to the eye.
- Compromised vision because of the success of these dependent on pupil size, especially for high reading add powers.
- The stability of the lens is crucial for fitting success.
- With simultaneous vision lenses for a 54-year-old myope, the distance intermediate and reading optics are all within the variable center 3.5mm of the contact lens.
- The visual center of the eye is offset from the center of the contact lens. The optic centers of the lens need to be offset so that the two centers coincide.
2.2. Translating Contact Lenses
At the International Society of Contact Lens Specialists meeting in England, contact lens educator Irving Borish, OD, stated, “the only soft bifocal contact lens design that could provide optimal vision is a lens of translating design.”The concept of alternating image CLs is similar to that of bifocal ophthalmic lenses: they have two areas, one for distance and the other for near. The upper section of the lens is attributed to distance prescription and the lower section for a near prescription. These lenses allow vision through the two distinct areas by taking advantage of the vertical movement of the lens that moves upwards with the inferior gaze. For this reason, they are also called “translating.” Theoretically, looking at a distant object, the gaze is turned to the primary position, and the pupil is covered by the upper part of the lens that corrects the ametropia.
On the other hand, looking at an object closely, the gaze is turned downwards; the lens supported by the lower eyelid translates moving upwards, and at this point in front of the pupil is the lower part of the lens which contains the addition prescription.
A trial translating bifocal fitting lens provides the precise position of the reading segment. The absence of “no jump” optics allows an immediate clear vision when translating from distance to near, and vice-versa.
The rotational stability of the lens is essential to allow adequate vision at all distances and avoid a worsening of glare and the vision of halos. Generally, these lenses are stabilized with a ballast prism (14), which can be accompanied by a parting of the lens in the lower part for more significant support of the lower eyelid and, therefore, better stability. Maximum rotation of 10° temporally and 30° nasally is tolerable, but it is possible to reduce it by increasing the value of the ballast prism (15).
The effect of the truncation makes the lens fitting a slightly looser fit. No alteration in the normal lens fitting base curve is needed. The peripheral curvature of the lens has a flatter design to allow the lens to translate easily.
3. Background of the Evolution of the Translating Design
Having a busy contact lens practice and a manufacturing laboratory allowed a present author (DFE) to develop lenses for his patients. He had many patients wearing CLs that became presbyopic. The laboratory allowed him to make virtually any lens, so he tried many versions of simultaneous vision lenses. Patients were not satisfied with the lenses. None gave equivalent vision to their previous single vision CL or spectacle vision clarity, except for those who were fitted with a rigid translating bifocal lens. Many patients were wearing soft lenses, so why not copy the truncated rigid bifocal lenses?
While truncating a rigid lens had allowed enough lens bulk for lower lid interaction to hold the lens allowing to translate when the patient looked down to near vision, truncating a soft lens did not have sufficient bulk for the lower lid to engage the lens. Adding prism resulted in a bulky thick lens that was not acceptable. The problem was resolved with the inclusion of truncation and a ledge. The increase in the bulk of the ledge in the lower part allowed the lens to engage the lower lid permitting the lens translation.
The optics of soft translating bifocal lens provide optimal bifocal visual results. However, the early manufacturing CNC lathes and mills allowed to perform the design of the truncation shape but could not refine and round the junction of the truncation and the front optic edge to achieve patient comfort. This process was done manually. Some lenses provided patient comfort. Many did not. Today’s CNC lathe/mill combination allows the complete manufacture of the lens without any manual intervention.
Once the design obtained a patent, various laboratories were approached. Several were interested but asked if the lens could be cast-molded. A lens that was truncated and had a front surface, including a bump on the front surface, had never been cast-molded before. Ezekiel had to prove that it could be cast-molded.
Returning to Perth (Australia), Ezekiel set about establishing a cast-molding laboratory. There are no cast-molding laboratories in Australia; it was an expensive and interesting exercise. He started making standard lenses and then the translating bifocal lenses with cast-molding inserts made in metal. While still having to manually round the junction of the truncation and the front optic, the lenses provided patient comfort as doing this process on the metal inserts allowed this manual process to be done accurately.
4. The Evolution of the Translating Design
The characteristics and the fitting procedure of the modified soft translating bifocal lens are as follow (16):
- Features The translating bifocal lens
- Back surface design that allows the lens to translate easily
- Unique lower area design that facilitates lens/ lid interaction
- Toric back surface if needed
- Bi-prism design to allow for lens stability and location
4.2. Lens fitting procedure:
Like a bifocal spectacle lens, the position of the reading segment can be varied to the correct position for optimal bifocal vision. This is achieved with the use of a translating bifocal diagnostic fitting set. The trial lens has overall diameters of 14.50-13.40 mm, with the reading segment positioned 0.50 mm below the optical center. The top of the bifocal junction is marked with dots placed at the lens periphery to allow the estimation of the correct position of the lens segment, which is in line with the lower edge of the pupil. From the position of the location dots, an accurate estimation of the correct segment position is established. When the patient looks down during the lower gaze when reading, the lower lid engages the truncated portion of the lens, positioning the segment for near vision within the pupil area (figures 4 & 5).
The lens is appropriately fit when the lower lid is located at the limbus, and the contact lens location dots areas the lower edge of the pupil.
When the lid is positioned above or below the limbus, the position of the reading segment can be modified by raising or lowering the segment position in the same way a spectacle bifocal lens segment can be positioned.
If the location dots are 0.50 mm above the lower edge of the pupil, the segment height needs to be ordered at 1.00 mm below center (0.50 mm plus the trial lens segment below 0.50 mm center (1.00 mm). When the dots are 0.50 mm below the lower edge of the pupil, the reading setting is then 1.00 mm below center. The full astigmatic correction at the precise axis for both distance and near powers is included in the lens back optic zone.
- The design is similar to that of a rigid translating bifocal lens with a clearly defined superior distance portion and a clearly defined lower area for near vision. Rigid bifocal CLs have shown a high rate of success, 60% of eyes were fitted successfully with the first lens ordered, and an additional 22.4% met success criteria after a second lens order (17).
- Optimum visual acuity at far and at near. There are no modifications to the patients’ full prescription. There are no modifications of the patient’s full prescription to compromise optimal vision.
- The segment height can be varied to relate to patients’ visual needs.
- The wide width of the optics of the lens results in comfortable clear vision at all distances.
- Easy to fit. The diagnostic fitting set defines the segment height.
- The inclusion of the patients’ full prescription for all distances. No modification of the prescription is needed.
- Indicated for the treatment of accommodative esotropia with high AC/A ratio (18,19).
- Subjects wearing the translating bifocal lenses recover their vision significantly faster than the subjects wearing no lens or the simultaneous vision bifocal lens. (20).
- The rotational stability of the lens is crucial.
- Patients who work on computers may have a posture issue since they need to look through the lower portion of the lens for near vision. For a comfortable working posture for computer viewing, the lens segment can be raised. This is normally done in the non-dominant eye.
- The abrupt change in power, from far to near or vice versa, can generate significant nuisances. The translating lens incorporates “no Jump” optics. A small head movement will locate the optics in the correct position for immediate clear vision at the required distance.
- Lower lid position and tonicity are crucial. Patients whose lower lids are more than 1.50mm below the lower limbus, and patients with loose lids are not a good candidate for this design.
Presbyopia is an inevitable condition when reaching about 38-40 years old. Generally, patients drop out from contact lens wear because of dry eye and poor vision. Most patients complaining of poor near vision are not informed of the possibility of correcting presbyopia with contact lenses. Practitioners do not prescribe these lenses because of a combination of multiple factors, including lack of skills in the application and assessment of patients’ expectations since multifocal lenses cannot provide contemporary “perfect” vision and comfort.
There are several strategies to correct presbyopia with soft CLs; the most two conventional methods used are simultaneous and translating the bifocal vision lenses. Current available soft CLs available in the market are conceived with simultaneous vision, which still represents a significant challenge in providing optimal visual acuity for either distance and near vision. Patients are more likely to experience glare and vision halos since simultaneous vision relates closely to pupil size and stability of the lens on the eye. With translating bifocal CLs, patients have shown to recover their vision significantly faster than the subjects wearing no lens or the simultaneous vision bifocal lens. Ezekiel et al. have performed a new design that overcomes discomfort offering a lens that provides stability when translating from the upper section for distance vision to the lower vision for near vision. Furthermore, the benefits of soft translating bifocal CLs include easy fit, comfort, and optimal visual acuity for both distance and near vision.
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