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A NEW diagnostic imaging technique, anterior chamber optical coherence tomography, is providing a wealth of useful clinical and physiological information about everything from phakic implants to accommodation, according to a French ophthalmologist. Georges Baikoff MD, told delegates at the XXI Congress of the ESCRS that he is very impressed with the performance of the anterior chamber optical coherence tomography device (AC OCT) currently under development by Carl Zeiss Meditec.
"This device provides a range of useful data for the anterior chamber and is relatively straightforward to use, giving surgeons a non-contact means of viewing high resolution images of the anterior segment. My belief is that in the near future this equipment will be as necessary for preoperative assessment of phakic refractive IOL patients as topography currently is for corneal refractive surgery," said Dr Baikoff. Dr Baikoff reported on trials carried out earlier this year at the Clinic Monticelli in Marseilles, France, using the AC OCT device to analyse the anatomical relationship between the structure of the anterior segment and four different types of phakic implants: angle supported lenses, NuVita™ (Bausch & Lomb) and GBR/Vivarte™ (IOLTech/CIBA Vision Surgical), the Artisan ®/Verisyse™ iris-fixated IOL (Ophtec/AMO), and the PRL™ posterior chamber implants (Medennium/CIBA Vision Surgical). Dr Baikoff said that a cross-sectional scan was performed on different meridians, helping to underline the anatomical relationship between the implants and the iridocorneal angle, the cornea, the iris and the crystalline lens. Measurements were taken in an accommodated and unaccommodated state. "With this device we can measure angles, curvature radius and the distances in the anterior segment. What we really wanted was to answer one key question raised by the use of refractive phakic IOLs – do they touch the crystalline lens and do they provoke cataracts?"Early phakic IOL models were linked to problems of endothelial cell loss, epithelial damage, pupil ovalisation, and pupillary distortion. In the posterior chamber, IOLs ran the risk of inducing cataracts as well as chafing the iris pigment, potentially leading to glaucoma. Design modifications, which increased the "vaulting" or clearance from the crystalline lens have since reduced these problems, but the long-term implications of the implants intermittently coming into contact with ocular tissue remains at issue. Dr Baikoff highlighted the case of one patient who had been implanted with the Artisan phakic IOL. Under standard
observation on a dilated pupil, there appeared to be no apparent contact between the IOL and the crystalline lens. A scan with the AC OCT device, however, demonstrated that there was a definite contact between the implant and the natural lens. "This was due to the tilting of the lens when the pupil is dilated, as the IOL tilts and the down-edge comes into contact with the crystalline lens. I asked this patient to return in a few days and we observed that during accommodation there was a high tendency of the crystalline lens to move forward and increase the risk of contact with the IOL," he noted. Dr Baikoff said that his team also established that some posterior chamber phakic IOLs were also coming into contact with the crystalline lens during accommodation. Similar problems with angle-supported lenses had also been observed in certain cases. Illustrating the case of one patient implanted with a ZB5M lens over a decade ago, Dr Baikoff said it was clear that there was contact between the phakic IOL and the crystalline lens. "It was evident that after 10 years, the thickening of the crystalline lens had brought it into contact with the phakic IOL. This patient had been checked every year but this was the first time I could actually observe that there was a clear contact," he said. Dr Baikoff said that the AC OCT device has also been used on healthy eyes to allow researchers to learn more about how accommodation and ageing affected the anatomical composition of the anterior segment. "We noted that during accommodation there is a forward movement of the crystalline lens, in the region of about 300 microns for every 10 dioptres of accommodation," said Dr Baikoff. He said that this forward displacement decreases over time as the elasticity of the crystalline lens diminishes and there is less anterior movement. However, this was offset by the fact that the anterior chamber depth also decreases over time due to thickening of the crystalline lens. Dr Baikoff noted that the end result of this dual process was that there is "less and less room in the anterior segment for foreign bodies as the patient gets older". He concluded that it was vital to adhere to the safety criteria that were established for anterior chamber implants. "The edges of the optical lens must remain at a minimum distance of 1.5 mm from the corneal endothelium." Dr Baikoff said that there were still many unresolved questions concerning the risk of phakic IOL-induced complications in the anterior chamber – questions that the new AC OCT device would go some way towards answering. "We now know the importance of correct sizing and anatomical positioning of the phakic implants in the anterior chamber. We will also need new inclusion criteria for refractive phakic IOLs that take into account the dynamic evolution and the ageing of the anterior segment," he said. The device is expected to be on the market in the next 18 months or so after completion of European safety trials. Georges Baikoff MD
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