Pentacam an excellent device with limitations

The Pentacam (Oculus) is an excellent device, but it has limitations, the audience heard during a debate of the system's pros and cons in corneal diagnosis at the JCRS Symposium, Controversies in Cataract & Refractive Surgery.

"The Pentacam takes advantage of the Scheimpflug principle, taking up to 50 scans in 2.0 seconds as it rotates around the eye. Light scatter is used to provide an image of the curvature of the anterior and posterior cornea," said Dr Jack Holladay, speaking pro.

He said the data is important because the posterior surface has a negative dioptre effect on the power of the eye. "The anterior surface may be +50 D, but if the posterior is -5 D, you need to take that into account. The posterior is normally 10% of the anterior, but it varies, and that variable is not accounted for in topography. That's why you need the tomography Pentacam."

"The imaging of both surfaces means the Pentacam documents and quantifies corneal scars and lenticular opacities. It measures corneal pachymetry, maps front and back cornea height, determines accurate corneal power for normal and post refractive eyes, and offers a sensitive detection of thinning disorders," Dr Holladay noted, summing up.

Speaking against the device, Dr Cynthia Roberts noted that there are powerful uses for a schleimpflug imaging system, "Because you have a beautiful image of the anterior segment in a particular meridian." "Anterior segment biometry and the cataract densities that Jack discussed are beautifully illustrated and quantified. It's just in the reconstruction of the surface curvature that these limitations exist," she said.

She said in her talk that some readings, for example, true net power and keratometric power deviation, have little physical meaning outside of the central 2 to 3 mm. "In ophthalmology, power and curvature are sometimes used interchangeably, this is not appropriate outside the central 2 to 3 mm of the cornea," she said. Further, she added, the angle of incidence upset these calculations.

Other limitations could be potential eye movement during the two-second scan. She noted curvature calculation from elevation has severe resolution requirements and was not sure the Pentacam met those requirements. She compared it with Placido disk, which does have the sensitivity and is well characterised in the literature.

In his rebuttal, Dr Holladay said that there was no question that an instant picture with a Placido disk is much better in terms of one sample than the 50 samples in two seconds from the Pentacam. However, because of the improvements made to the second-generation Pentacam, the error introduced by eye movement is roughly five to seven microns. "Not a case where you're going to have to worry about ectasia or going too thin," he said. "And the Placido disk cannot measure the back surface. That's 5.6 D of power, plus or minus a quarter," he concluded.