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WHILE many patients seem to be satisfied with their accommodative intraocular lens implants, re-searchers are struggling to find objective methods to demonstrate the physiological mechanisms responsible for this success.

Gerd Auffarth MD, Heidelberg University Eye Clinic, Germany proposed that researchers redefine ‘accommodation’ for pseudophakic eyes during a presentation at the 17th Annual Meeting of the German-Speaking Association for Intraocular Lens Implantation and Refractive Surgery (DGII).

"The outcome of over two years of experience with accommodative IOLs has shown that the definition of ‘phakic’ accommodation and the terminology that derives from the physiological events in ‘phakic’ eyes do not apply to ‘pseudophakic’ eyes.

"Accommodation measurements must be made more reliable and reproducible as the data that researchers have collected does not corroborate with the subjective results, hinting that other factors may play a role in pseudophakic accommodation," he said.
Dr Auffarth stressed that a strictly objective assessment of accommodative IOLs is required to explain their efficacy and allow the replication of results.
There are a number of different accommodative IOLs now on the market. Most of them are based on the focus-shift principle, which involves movement of the lens within the capsular sac.

Dr Auffarth’s team and that of T Küchle at Erlangen University, Germany used the Miyake-Apple technique to verify the potential efficacy of the 1CU (HumanOptics) accommodative IOL. He reported that the extent and strength of accommodation was still difficult to predict, as it varied inter-individually and depended on several factors, including age.

Although Dr Auffarth’s studies revealed good functional results with 1CU lenses, the measurements made with the IOLMaster and Orbscan II system gave false positives, including overstated anterior chamber depth changes. Ultrasound biomicroscopy revealed a far less accommodative movement.

Dr Auffarth explained that anterior chamber depth measurements had to be viewed critically, since clinical trials have revealed that automated methods of measurement on the slit lamp were imprecise.

He said that axial length could be reliably measured and calibrated using optical coherence biometry with a highly precise ultrasound-immersion method. The Zeiss IOLMaster combines this method with a technique which measures the corneal radii and anterior chamber depth. According to its manufacturer, the IOLMaster is not designed to measure anterior chamber depth.

The Orbscan is a 3D slit-lamp topography system which can be implemented to analyse the corneal surface and anterior chamber structures (iris, lens). Although the Orbscan and IOLMaster show good agreement on anterior chamber depth measurements in phakic eyes, measurements on aphakic eyes do not concur. Ultrasound measurements by contrast were always reliable.

The Orbscan often recognised the pupillary border instead of the lens surface, with drug-induced miosis. A recent study by Dr Auffarth clearly showed that all three methods corresponded before but not after pilocarpine drops.

He found that ultrasound was useful for showing the position of the lens and iris as well. Nonetheless, Dr Auffarth warned that the results of the high-resolution 20MHz ultrasound biomicroscopy head should be critically evaluated as the anterior chamber depth did consistently correlate with near visual acuity values.

"It is hard to measure accommodation in pseudophakes effectively. Many studies should reconsider their results as they are not reproducible and too often rely on subjective and apparent data," Dr Auffarth said.

Burkhard Dick and Peter Kaiser MD introduced a means to objectively evaluate accommodative IOLs in their study on dynamic aberrometry. They dynamically recorded aberration changes within the optical system with wavefront analysis and compared examinations done on healthy phakic eyes with pseudophakic patients, using both accommodative and not-accommodative type IOLs.

Again, the good results seen with the 1CU did not always correlate with the anterior chamber measurements made with pilocarpine or with dynamic aberration measurements.
Dr Auffarth suggested that, aside from slight myopic rest refractions or astigmatism, curvature changes of the IOL optic or slight asphericity of the lens anterior surface might account for the discrepancies.

Another approach to restoring accommodation is ‘lens refilling’. Many experimental studies were done on the topic, using silicone in primate eyes (A Häfliger MD and co-workers) or with a silicone balloon and plug to fill the mini capsulorhexis (O Nishi MD and co-workers).

They achieved up to 4.6 ± 2.5 D accommodation amplitudes in monkeys. Other trials involved substances which changed shape when exposed to different kinds of light or to aqueous humour.

"The basic problem with lens refilling is calculating the right lens size, curvature radii and refraction for individual patients. In spite of a very encouraging start, there has been limited resonance in this direction," he noted.

Dr Auffarth commented that although the first clinical trials with accommodative IOLs are now reaching two and three years follow-up with good or very good functional results, long-term complications such as capsular sac fibrosis and PCO are still under investigation.

Physiological accommodation ranges from 1.0 D to 3.0 D. In spite of the fact that the currently marketed accommodative lenses only offer accommodation amplitudes of 0.75 D to 1.5 D, they have shown remarkably good clinical results so far. New methods of measurement and long-term clinical studies are necessary to validate them objectively, he stressed.

The development of accommodative IOL implants reflects an important step in the progression in modern cataract surgery. The implications of such developments are beginning to touch fields such as refractive surgery and presbyopia treatment, he said.
"Despite the fact that the functional principle of accommodative IOLs relies on changing the refraction of the entire optical system through the axial sliding of the implant, this mechanism has only been physiologically proven to exist in fish and amphibians. Research on axial sliding in humans revealed it does occur in children, but only in tenths of a millimetre," commented Rudolf Guthoff MD in a related presentation.

Refractive changes of the entire ocular system of 1.0 D correspond to approximately 1.0mm of anterior chamber depth difference. Dr Guthoff stressed that there is a pressing need for objective measurements of the extent of accommodation. He agreed that large randomised studies were needed to differentiate between the wishful thinking of patients and physicians alike and the reality of the so-called ‘accommodative lenses’.
"All publications to date on commercially available accommodative IOLs tend to ignore the objective evidence regarding the refractive outcome and highlight the subjective results. What we need is a system that can describe accommodation precisely. We also need to be able to measure accommodative success objectively."

According to Dr Guthoff, accommodation was accurately shown with the use of a ‘finite element model’, a modern imaging method that simulates image material of higher dimensions. He said that the Oxford research group successfully used the same model to illustrate the zonular fibres and other optical tissues involved in accommodation.
He explained that the key measures of human accommodation were anterior chamber depth, lens thickness and the coincidence of striped patterns with the coincidence refractometer. Changes in anterior chamber depth of around 0.8mm indicate good accommodative power. This was not the case in the patients studied in Rostock with the Human Optics 1CU lens. He said that axial sliding only theoretically helps in presbyopia. Research in shark models showed that accommodation also works by axial sliding in that system. This, however, is not relevant in humans, Dr Guthoff stressed.

The HumanOptics 1CU accommodative IOL has mobility of the haptics, evidenced by a change in haptic angulation. This could not be demonstrated using high frequency ultrasound imaging.

Dr Guthoff pointed out that further developments in both materials and techniques are required to discover more about physiological accommodative mechanisms.

Gerd Auffarth MD
University Clinic, Heidelberg, Germany
Email: gerd_auffarth@med.uni-heidelberg.de

Rudolf Guthoff MD
University Clinic, Rostock, Germany
Email: rudolf.guthoff@medizin.uni-rostock.de

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