With rising expectations of patients concerning predictability of refractive outcome, especially with multifocal and toric intraocular lenses (IOLs), there is an increasing interest in ocular biometry and IOL power calculation. In the days of ultrasound biometry, the measurement of axial length was the main source of error in biometry accounting for nearly two thirds of the refractive error. With the introduction of optical biometry by Fercher and co-workers from Vienna in the early 1990's, which became commercially available about a decade ago, results improved and axial length measurement errors are now only a minor contributing factor for postoperative refraction. However, even today, we are still confronted with a proportion of about 5 to 10% of eyes falling outside ±1 D of the desired refraction, as well as rare refractive surprises with high postoperative ametropia.

Today, there appear to be three main sources of biometry and power calculation inaccuracies: prediction of effective lens position, measurement of corneal power and prediction of the pupil size dependent optics of the pseudophakic eye.

IOL optic position

Predicting the postoperative axial position of the IOL optic, also known as effective lens position, is the main challenge of today's IOL power calculation. The currently available power calculation formulae differ mainly concerning this prediction. Even though some formulae use additional biometry measurements such as preoperative anterior chamber depth, lens thickness, or corneal diameter, the results are only a little better if at all than using only axial length and corneal curvature.

Another recently introduced approach is intraoperative measurements of refraction incorporated into the operating microscope, but this is only of limited help because the IOL is already implanted and would need to be exchanged, which still increases the risk of capsule tears, corneal endothelial cell loss and enlarging incision size and extends surgical time. It is not entirely clear what contributes more to the variability of IOL position: capsule bag position and size, or the capsule shrinkage process. Also, IOL haptic material and design may play an important role for reducing variability. Finding algorithms to better predict effective lens position will be one of the main challenges of biometry in the coming years.

Corneal power

Currently, we measure the anterior curvature of the para-central cornea with keratometry and try to describe the refractive power of the entire cornea from these measurements. Higher order aberrations as well as the effect of the posterior corneal surface contribute to errors in power calculation. More accurate and detailed assessment of corneal power using topographic or even better tomographic techniques may help to identify odd corneas that may lead to postoperative refractive surprises. Also, this would allow choice of the most adequate asphericity of the IOL, instead of the current 'one size fits all' approach by using the same aspheric IOL for all eyes.

Pupil size

Lastly, with the computational equipment to hand today, we should make the transition away from the simplified theoretical formulae to complete ray-tracing across the entire pupil size for power calculation and simulation of the retinal image before surgery. This appears to be a promising and powerful tool for a better choice of IOL type and power before surgery. However, it will be a challenge to simplify currently available ray-tracing software to be practical for routine use in high-volume cataract surgical centres.

The use of optical biometry and modern IOL power calculation formulae has markedly improved the accuracy of IOL power calculation, but we still have to face a considerably large amount of clinically relevant variability of refractive outcomes. In our effort to perfect cataract surgery and the use of modern IOL designs, such as aspheric, multifocal, or toric IOLs, further improvement in prediction of postoperative IOL position is required to optimize refractive outcome.

Dr Oliver Findl, MBA is group leader at the Vienna Institute for Research in Ocular Surgery (VIROS), Hanusch Hospital, Vienna, Austria and is a consultant ophthalmic surgeon at Moorfields Eye Hospital, London, UK. He can be reached by E-mail: oliver@findl.at

Dr Findl has no personal financial interests in any products or techniques mentioned. He is a scientific advisor to Carl Zeiss Meditec AG.