Investigating the best way to directly measure phakic IOLs, why sizing is so critical, potential problems and useful tips to overcome them
Sizing of phakic IOLs: Is there a better way of doing it than direct measurements by high frequency/ultrasound scanning? This was the question posed by Professor Jorge Alió of Spain during the clinical research symposium at this year's ESCRS congress in Paris. Today, phakic intraocular lenses (PIOLs) are a widely accepted alternative for the correction of moderate and high refractive errors with excellent levels of efficacy, predicatability and safety, said Professor Alió. He emphasized that sizing of a PIOL has critical stages both preoperatively and postoperatively.
Dr Alió was speaking at the 2010 Congress of the European Society of Cataract and Refractive Surgeons. The session emphasized the importance of adequate sizing to ensure successful phakic IOL treatment alongside that of careful patient selection and choice of the right IOL.
Dr Alió's team at Vissum Alicante at the Instituto Oftalmologico de Alicante, in Spain has published and undertaken studies investigating how anterior segment biometry using ocular coherence tomography (OCT) and VHF/ultrasound scanning have compared against other optical methods (Journal of Cataract & Refractive Surgery). When comparing the Artemis 2 system, which is a very high frequency digital ultrasound scanner with the Visante OCT their results found measurements of the anterior chamber depth (ACD), the central corneal thickness (CCT) and the angle-to-angle distance (ATA) were equivalent and repeatable and could be easily interchanged in that role. However, when it came to measuring iridocorneal angular size (IAS) at the nasal and temporal positions the two systems were not interchangeable as the range of agreement between devices for nasal and temporal values were clinically relevant.
"However," he confirmed, "if an ultrasound system is not available, sulcus-to-sulcus can be predicted from iris pigment end to iris pigment end obtained with the Visante OCT by means of a linear equation with an error between 0.5 and 1.0 mm in 50% of cases, an error slightly larger than IOL-size steps.
"Our conclusions were that VHF/US is the only way available today to know the precise interior dimensions of the eye and related structures such as phakic IOLs. Obtaining measurements with today's OCT instruments provide an approximation of the real measurements and are, therefore, perhaps only valid at the anterior chamber level. Posterior chamber measurements and distances calculations, such as sulcustosulcus, are only possible with VHF/US. Extrapolating measurements from OCT are neither a precise nor a reliable method for posterior chamber PIOL sizing."