If one has been used to the ultrasound biometry and manual keratometry, transitioning to IOLMaster is at times a leap of faith. For so long, one spends much time looking at the reliability of the A-scans and the art of perfecting the manual keratometry, that simply passing over that control to a "box" may seem as difficult as giving away one's first child. It is natural for many doctors, for long periods of time, to continue using both techniques, until they have been proven the clinical validity of the IOLMaster. On the other hand, for those that are now IOLMaster users, it is virtually impossible to consider going back to the old A's and K's style. So here are a number of items to consider:
The ultrasound systems used for measuring the axial length of the eye, generate an A-Scan image. The concern here is always the quality of the A-Scan. Additional concerns arise from making sure that the A-Scan is not catching the optic nerve, or a staphyloma or other retinal pathologies. The other issue is that the echo from the ultrasound is based on the "surface" which could be an elevated epi-retinal membrane and not the layer of the photoreceptors!
The IOLMaster allows for partial coherence interferometry, which allows for about a 10 fold increased resolution compared to ultrasound. Additionally, the patient is able for fixate on the target and therefore a true cornea to macula distance is measured. The other main issue is that the measurement is done to the level of the retinal pigmented epithelium (RPE). This avoids the entire issue of retinal pathologies such as ERM's and Staphylomas.
Unfortunately in 5-10 percent of the patients, the density of the cataract does preclude the use of the IOLMaster axial length measurement. In these case an US A-Scan can be performed and the data then is inputed into the IOLMaster for proper calculation.
Keratometry depends heavily on the instrument used. Many keratometers use a ring at 3 mm on the central cornea to do the measurements. The measurements are heavily dependant on the skill of the technician and patient cooperation.
The Cornea is bell shapes and the it flattens in the periphery. The IOLMaster uses the central 2.5 mm with a sphere of approximately 47D. This is one main reason why manual and IOLMaster K's may differ.
The IOLMaster uses a 2.5 mm circle with six points of measurement. This allow for a more central measurement and therefore a better clinically more relevant corneal power for IOL calculation. Additionally, the IOLMaster acquires multiple sets in very little time, allowing for a much better patient cooperation and reduced variability due to patient tear film instability. These factors allow for a much better quality of data in much shorter acquisition time and better patient experience. For more information visit the Keratometry page.
Additionally, the IOLMaster measures the Anterior Chamber Depth and the Horizontal White-to-White. These measurements are progressively becoming more important in IOL calculation and the newer generation of formulas.
Other items that are also evaluated are angle Kappa, and pupil size. The fact that the instrument is also non-contact, there are no issues with compression errors and artifacts. IOLMaster can measure through silicone oil, where are A-Scan is inadequate.
Users of IOLMaster will consistently state that their experience is very positive. The IOLMaster is easy to use and very good data can be obtained with minimal training. As such, the dependency on technician skills is significantly reduced. The acquisition time is significantly reduced. All the examination in non-contact. The acquired data show much greater reliability of the measurements. Modern formulas are all used and in an instant, all the data and calculations are all printed out.
In the process of tranisition, one should proceed with the leap of faith as it has been done by so many thousands of other ophthalmologist around the world. That does not mean, one should blindly accept a simple print-out. There are about 5-10% of cases where one may need to revert to some, or all, standard old A's and K's methods.