How beneficial is aspheric lens design?

April 1, 2007

A study reveals that the negative spherical aberration-inducing quality of the aspheric Tecnis Z9000 results in reduced ocular spherical aberration and improved mesopic contrast sensitivity.

Heralded as a major turning point in the history of refractive surgery, the aberrometer has not only proved to be an invaluable diagnostic tool, helping refractive surgeons create tailor-made ablation patterns for superior visual outcomes, but it has proven its worth in predicting optical performance by objectively measuring aberrations present in pathologic and surgically-altered eyes. This, improved knowledge of ocular aberrations through the advance of wavefront technology, has naturally led to the evolution of improved refractive correctors, in particular laser ablation devices and intraocular lenses (IOLs).

During this journey of discovery, contrast sensitivity measurements have emerged as important indicators of quality of vision, helping researchers determine visual function across a wide range of distances and conditions and allowing them to use this knowledge to develop methods to correct failing parts of the visual system.

The fact that the ageing population is expanding means that the number of cataract patients will continue to rise. Indeed, expectations for quality of vision in older patients are now higher than ever before. Thankfully, good vision post-cataract surgery is now attainable with surgical techniques becoming less invasive and associated with fewer complications, and the new generation of IOLs also taking ocular spherical aberrations into account, thus improving contrast sensitivity and hence quality of vision.

With this in mind, the Tecnis Z9000 IOL (AMO) was designed to have a modified prolate anterior surface that induces a negative spherical aberration to offset the positive aberrations of the cornea.

Pitching the aspheric Tecnis lens against the spherical CeeOn Edge

On a mission to establish the effect of contrast sensitivity and wavefront aberration improvements on everyday quality of vision, a team, led by Alexandre Denoyer, MD of Bretonneau University Hospital, Tours, France, compared pseudophakic eyes bilaterally implanted with either the aspheric Tecnis IOL or the conventional spherical IOL, CeeOn Edge 911 (AMO).

Both lenses share basic design features, including a three-piece, biconvex, 6.0 mm square-edged optic with a refractive index of 1.46 and angulated cap C polyvinylidene fluoride haptics. The fact that the CeeOn Edge 911 lens is identical to the Tecnis IOL in all aspects except it has a spherical surface, allowed the investigators to precisely elucidate the influence of the aspherical surface on ocular performance and quality of vision.

The team randomly assigned 20 cataract patients to undergo bilateral implantation of the prolate aspherical Tecnis lens or the spherical CeeOn Edge 911 IOL. The mean age of patients in the Tecnis group was 79.3±8.2 years and 77.9±5.2 years in the CeeOn Edge group. Mean IOL power was 21.40±1.80 D and 21.90±1.60 D in the Tecnis and CeeOn Edge groups, respectively. Study inclusion criteria included myopia or hyperopia <4.00 D spherical equivalent, no ocular surface pathology, normal 24-hour intraocular pressure (IOP), no neural or retinal pathology, and no systemic disease. The same surgeon performed each procedure.

A full preoperative examination was performed seven days in advance of the surgery, whilst an independent physician conducted postoperative evaluations at six months. Tests included refraction before and after pharmacologically-induced mydriasis; best-corrected visual acuity (BCVA) tests using ETDRS charts; pupillometry, contrast sensitivity under photopic, mesopic and glare conditions; and aberrometry without pupil dilation. The team also assessed subjective quality of vision using the 22-item Activities of Daily Vision Scale (ADVS) questionnaire, which evaluates day and night driving, best-corrected distance and near vision and glare disability.

Tecnis outperformed CeeOn Edge in mesopic conditions

When evaluating contrast sensitivity, the team found little difference in contrast sensitivity enhancements between the groups under photopic conditions and with glare at all tested spatial frequencies. However, Tecnis outperformed the CeeOn Edge under mesopic conditions at high spatial frequencies (mean 9.9±5 dB versus 4±2.69 dB).