With growing patient demand, multifocal contact lenses (MCLs) are an increasingly significant part of contact lens practice. However, fitting them has not so far enjoyed the popularity of standard single vision contact lenses, and are still considered by many to be a specialised area of practice, particularly the rigid gas permeable (RGP) material lenses.
Concerns over success rates and increased chair time (particularly for the more complex designs) are among the reasons cited for the reluctance of many optometrists to engage in fitting more MCLs. Much of the challenge stems from the fact that, in order to achieve acceptable vision with MCLs, both the optics of the lens and centration relative to the pupil need to be accurate.
Excellent vision may be achieved with well-fitting RGP translating design MCLs when the appropriate optical correction is positioned in front of the pupil in positions of gaze habitually used for tasks at far, intermediate and near working distances.
Most soft MCLs deliver optical correction via a concentric optical power profile system, conjugate with the ocular pupil. The images generated by this system are optically degraded, because they are perceived by the visual cortex simultaneously from an extended range of distances. This image degradation can be calculated and simulated when a known contact lens power profile is perfectly superimposed with a specific ocular pupil size.
The relationship between the pupil diameter and the power profile of concentric multifocal soft contact lenses directly affects the optical performance of these lenses.1,2 Optimal physical fit of MCLs will therefore not only improve lens comfort, but will generate the desired alignment of the lens power profile with the ocular pupil to prevent further degradation of image quality.
Unlike the custom-designed corneal RGP MCL, soft lenses are usually fitted from a limited range of mass-produced lens designs and power profiles. Sub-optimal lens fit is expected in 27 per cent of corneas which fall outside of normal profile range. Sub-optimal optical performance as a result of a poor fit and the decentration (usually temporal due to the steeper nasal scleral angle3) that results,4 further degrade the optical image of concentric soft MCL.
Correct conjugation of contact lens power profile and the ocular pupil is therefore key to the success of all types of MCL and can only be achieved by the optimisation of the physical contact lens fit and power profiles.
The physical fit of corneal RGP contact lenses is affected mainly by the corneal topography. An additional requirement for a successful RGP multifocal lens is the correct alignment of the relevant correction at the appropriate gaze position, and this is also influenced by the eyelid position with relation to the ocular pupil.
Soft contact lens fit is less affected by variations in corneal asphericity and radius. The most important measurement influencing soft contact lens fit is the corneal sagittal depth (SAG),3,5 which is influenced mainly by the size of the cornea or the horizontal visible iris diameter (HVID), and this is the most appropriate measurement to take in order to design an optimally fitting soft contact lens.6
Using the Volk Eye Check
The measurement in a clinical setting using traditional (ruler-based) techniques of pupil size, pupil horizontal and vertical eccentricity, HVID, corneal SAG and lid position is time-consuming and often inaccurate. The Volk Eye Check (VEC) (from Iriss Medical Technologies; irissmedical.com/volk-eye-check) is a camera device which produces instantaneous, accurate, repeatable and objective measurement of all of these key parameters.7
The in-built best fit analysis utilises algorithms, not only to establish the suitability of various standard contact lenses, but also to calculate custom lens parameters according to criteria stipulated by specialist contact lens laboratories. This allows the practitioner to order bespoke spherical and toric lenses, either single vision or multifocal.
Case studies
The following are examples of clinical cases in which standard soft MCLs performed poorly and were successfully refitted by utilising the VEC measurements.
Case study 1
A 53-year-old female professional, dissatisfied with the visual performance of her standard monthly disposable soft multifocal contact lenses, was keen to try RGP translating MCLs in order to improve vision at all distances. The following measurements were taken:
- Refraction: R -1.75/-0.75 x 60 (6/6) L -2.50/-0.50 x 115 (6/6) ADD +2.00 R&L (N5)
- Ks: R 7.75/7.60 L 7.68/7.52
- VEC report (Figure 1)
R&L: 7.70/9.50 -2.25DS ADD +2.00DS
The intermediate segment height was positioned 0.1mm under the geometrical lens centre and the reading segment 1.0mm under the geometrical lens centre. According to the VEC report, the distances from the lower pupil edge to the lower lid were 4.00mm and 3.50mm in the right and left eyes respectively, which positions the reading segment at 3.75mm from the lower lid, exactly at the lower margin of the pupil, with the intermediate segment encroaching by 1mm into the lower pupil area (simultaneous distance and intermediate vision). On downward gaze the reading segment translated upwards covering the whole of pupil (Figure 2).
Excellent monocular (6/4.8, N4) and binocular (6/3.8 N4) distance and near acuity along with satisfactory (N8) intermediate vision was now achieved.
[CaptionComponent="2061"]Case Study 2A 62-year-old office worker, who had worn soft, spherical MCLs for many years, had noticed a deterioration in her vision due to gradually increasing lenticular astigmatism, early bilateral nuclear sclerosis and the development of an epiretinal membrane in the left eye. The following measurements were taken:
- Refraction: R -8.00/-1.75 x 82 (6/7.5-1) L -8.75/-2.00 x 90 (6/9) ADD +2.25 (N4 R+L)
- Ks: R 7.15/6.99 L 7.14/7.09
- VEC report
The VEC report (Figure 3) was sent to SpecialEyes contact lens laboratory (where the SAG-based contact lens fitting system, known as the Arc Length Algorithm, has been developed and incorporated into the VEC).
[CaptionComponent="2062"]The power profile of the resultant multifocal contact lens is designed to optimally fit the pupil size of the patient based on the information provided by VEC and may be further modified to optimise individual patients’ needs utilising a unique image simulation software called ‘multifocal simulator’ (http://specialeyesqc.com/multifocal-simulator-landing.php).
Custom MFC ( hioxifilcon D, 54 per cent water content) contact lenses were supplied with a centre distance (CD) design for the dominant right eye and centre near (CN) design for the left eye along with the following parameters:
R 7.50/13.90 -7.50/-1.50 x 82
ADD +2.25 CD 2.0mm IZ* 3.0mm
L 7.50/13.90 -8.00/-1.50 x 90
ADD +2.25 CN 1.80mm IZ 3.0mm
(*Intermediate zone)
Note the unusual parameters of the optic zone radius (OZR) and total diameter (TD) calculated and the difference in the optical profile (designed to best fit the relatively small pupils) between the centre near and centre distance designs.
The lenses fitted well, centred well, were comfortable and the vision achieved was significantly better than with previous lenses at all distances: binocular acuity 6/7.5, N6, with comfortable intermediate vision, essential for this patient’s significant computer demands.
Case study 3
A 51-year-old professional was no longer satisfied with her modified monovision and, due to the poor centration of existing standard MCLs, was getting unstable and inadequate vision. The following measurements were taken:
- Refraction: R -10.25/-0.25 x 10 6/7.5 L -11.25/-0.75 x 165 6/6 ADD +1.75 R&L
- Ks: R 7.71/7.61 @ 82 L 7.70/7.50 @ 100
- L eye dominant
- VEC report (Figure 4)
Note the large 12.30mm HVID and deep corneal SAG revealed in the VEC report, which were the reasons for poor fit of standard lenses.
Lenses with the following parameters were supplied by SpecialEyes as detailed:
R 8.0/14.90 -9.25DS ADD +1.75 CN 1.8mm IZ 3.0
L 8.0/14.90 -10.0/-0.50 x 165
ADD +1.75 CD 2.0mm IZ 3.0
The lenses fitted well and gave clear vision at all distances after a minor -0.50DS adjustment was incorporated into the left lens.
Case study 4
A 14-year-old female with rapidly progressing myopia, who at the time was wearing single vision soft contact lenses, was interested in trying any methods that offered the potential of slowing down myopia progression. She had been unable to tolerate orthokeratology lenses when having tried these previously, and it was therefore decided to fit her with centre distance soft multifocal lenses in an attempt to simulate the optical effect of orthokeratology. The following measurements were taken:
- Refraction: R -5.25DS L -3.50/-1.75 x 65,
- Ks: R 7.70/7.62 L 7.80 @ 169 / 7.51 @ 79
- VEC report (Figure 5)
Note from the VEC report the large sagittal depth due to the patient’s large HVID (12.32mm).
Lenses were designed by Ultravision CLPL with the following parameters:
R 8.60/15.0 -4.75DS
ADD +5.00 CD segment 4.0mm
L 8.60/15.0 -3.50/-1.50 x 165
ADD +5.00 CD segment 4.00mm
Their comfort level was similar to that of her habitually worn soft contact lenses and an excellent 6/4.8 distance acuity was maintained in both eyes. The orthokeratology-like optical effect can be seen in the topography plots in Figure 6 (topography performed with lenses in).
[CaptionComponent="2065"]Case study 5
I had attempted to fit a 54-year-old male nurse with soft MF contact lenses prior to having the VEC contact lens module. The following measurements were taken:
- Refraction: R +0.75/-0.25 x 120 (6/4.8), L +0.75/-0.25x135 (6/4.8)
- Ks: 8.01/7.84 R&L
- VEC plot (Figure 7)
None of the standard soft multifocal contact lenses tried provided satisfactory vision. After performing the VEC it became apparent why this was the case; a combination of small pupils (2.66mm) with a deep corneal sag (HVID 12.52mm), as well as significant nasal pupil decentration (0.37mm) in the right eye were conspiring to compromise vision through the lenses.
[CaptionComponent="2066"]Having these measurements prior to embarking on contact lens fitting would have saved chair time as it would have been obvious that standard lenses were unlikely to fit well and centre optimally on a cornea with a sagittal depth of 3.00mm. Such a sagittal depth, in combination with small and nasally decentred pupils (opposite to the expected temporal decentration of contact lenses), makes optimal alignment with available multifocal contact lens power profiles a remote possibility. The patient was not interested in RGP contact lenses, nor in custom-made soft contact lenses, so was refitted with single vision daily disposable contact lenses in a monovision prescription and this proved adequate for his purposes.
Conclusion
The VEC has already been incorporated into fitting algorithms by some speciality contact lens manufacturers and is being internally tested by others. It has become an invaluable tool in my contact lens practice and impresses patients who appreciate the technological advances in measurement of ocular parameters and custom contact lens manufacture. They also are able to become much more involved in the decision-making and thought process in the fitting of their contact lenses.
References
1 Madrid-Costa D, Ruiz-Alcoce, J, Garcia-Larcia-Lazaro S, Ferrer-Blasco T and Montes-Mico R. Optical power distribution of refractive and aspheric multifocal contact lenses: Effect of pupil size. Contact Lens & Anterior Eye , 2015; 38(5), pp317-321.
2 Madrid-Costa D, Dominguez-Vicent A, Belda-Elda-Salmeron L and Ferrer-Blasco T. In vitro power profiles of multifocal simultaneous vision contact lenses. Contact Lens & Anterior Eye, 2014; 37(3):162-7.
3 Van der Worp E. Decoding soft lens fitting. Contact Lens & Anterior Eye 2014; Jun 37(3):162-7, pp391-393.
4 Hall LA, Young G, Wolffsohn JS and Riley C. The influence of corneoscleral topography on soft contact lens fit. Investigative Ophthalmology & Visual Science, 2011; 52(9), pp6,801-6,806.
5 Van der Worp E. Reviving the art of soft lens fitting. Contact Lens Spectrum, 28 (Sept 2013), pp44-55.
6 Douthwaite WA. Initial selection of soft contact lenses based on corneal characteristics. CLAO, 2002; 28(4),
pp202-205.
7 Rosen C, Ramdass S, Norman C, Buckingham R. Visible Iris Diameter with the Volk Eye Check Device, 2015. Michigan College of Optometry, Vision Research Institute.
Alex Levit practises at Barnard Levit Optometrists in London