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Until recently it was not possible to measure or correct these imperfections for many of these patients, symptoms manifest when driving at night or while in reduced light conditions. Typical experiences are 'comet tails', halos, glare around lights at night or in reduced light conditions and these effects result in compromised near and distance vision.
Approximately 50 per cent of people have HOA that compromise subjective vision, while a further 17 per cent have seriously compromised vision as a result of HOA. HOA can make up as much as 20 per cent of refractive error. Until recently it was not possible for practitioners to easily test for and correct this component of refractive error.
Last year Veni Vidi launched a new type of contact lens asphere technology on the UK market that utilises individualised wavefront corrected surfaces to correct both lower and higher-order aberrations (see Optician, February 3 2006). These lenses are available in a daily, monthly and now multifocal format, and require no extra technology for the practitioner to prescribe. Recent technological advances have also resulted in the company launching the Z View aberrometer for use in practice in the UK to measure HOA.
Comparison of Definition HD Vision lenses with others
This study utilised a Z View aberrometer to test and compare the performance of seven current contact lens designs in their ability to reduce HOA. The Z View aberrometer uses 3D wavefront technology to measure lower- and higher-order aberrations by firing an aberration free beam of light onto the patient's retina, measuring the reflection from each eye in turn. The reflected distortions are measured and quantified for each eye. This results in a complete refractive map of the patient's error, creating an optical finger print with lower-order and HOA measurements (down to sixth order). These are quantified using Zernicke polynomials and converted into dioptre equivalents for each measurement with an accuracy of ±0.01D. The information is shown in a refractive map that is colour coded (green = average, yellow = above average, red = high) and the shape of the error is then shown.
It is important to recognise that this is not topography. Figure 1 shows an example of a patient's refractive map, first uncorrected and then corrected with a Definition contact lens.
Method
Using a Z View aberrometer the subject eye was measured without any correction for HOA. This determined the power of the soft contact lenses required.
Each of the study contact lenses was placed on the patient's eye and re-measured with the Z View aberrometer. This determined the amount of corrected or induced HOA.
This was a masked study for both patient and investigator such that the investigator measured each eye without knowing which brand was worn at that time. A refractive map was printed (Figure 1) showing the amount of HOA that existed on each patient's eye wearing each lens. The resulting HOA maps were then compared and evaluated.
If total HOA had increased over the amount present in the unaided eye, then that lens had induced HOA. If the amount of HOA had decreased from the amount present in the unaided eye then that lens design reduced HOA. Figure 3 shows a summary of how the lens compares with other major lenses on the market.
A similar evaluation was done just for spherical aberration (Figure 4). Pupil size was not controlled during the study and all testing took place in a darkened room.
Pupil size (as measured by the Z View aberrometer at time of testing) ranged from a minimum of 4mm to a maximum of 7.2mm. Two pupils were both measured less than 4.5mm and one was 4.8mm. The pupils of the remaining 13 eyes measured greater than 5mm at time of testing.
The full evaluation HOA maps for the study are available from Veni Vidi in CD format. Of the 16 unaided eyes evaluated, one had below average HOA, eight had average amounts of HOA, and seven eyes demonstrated above average amounts of HOA. The range of aberration for the 16 eyes was between 0.17D of total HOA to 0.45 of total HOA.
All of the lenses evaluated did an acceptable job of correcting the spherical requirement on each eye study. The average amount of HOA per study eye was 0.281D.
The following is a breakdown of the spherical requirements of each of the 16 eyes:
-1.00six eyes
-2.00three eyes
-3.00three eyes
-5.00two eyes
-6.00two eyes
Results: HOA decrease/increase
DefinitionAC lowered HOA in 14/16 subject eyes, more than all other designs tested (Figure 2). Indeed, in this study, DefinitionAC provided the greatest decrease in HOA, four times greater than the nearest competitor. Table 1 summarises the sample base.
Results of spherical aberration Reduction
DefinitionAC lowered SA in 11/16 subject eyes - again more than all other contact lens designs tested (Figure 3).
Conclusion
The Definition product with its individualised asphere wavefront optics offers a new and better alternative in aberration control than current first generation aspheres for patients and practitioners alike.
Uncorrected or induced HOA may result in patient visual discomfort, particularly in mesopic or scotopic conditions. Although many currently available lens designs incorporate advanced aspheric optics in an attempt to reduce HOA on the eye, Definition reduced HOA by four times that of its nearest competitor.
Despite being designed to reduce spherical aberration many of the lenses in this survey showed a surprising difference in their ability to achieve that task. Some of the lens brands evaluated actually induced HOA while others induced spherical aberration.
In attempting to offer a visual upgrade, current asphere technology does not deliver the vision result achieved with individualised aspheres at each power curve utilising wavefront optics that are incorporated into Definition lenses. Though many factors need to be considered when prescribing soft contact lenses for any individual, to achieve crisper, clearer, vision, lens designs that demonstrate the greatest amount of HOA correction should be considered. ?
? Gordon Hughes is UK & Eire professional relations & sales & marketing manager, Contact Lens Division, Veni Vidi