Phaco Training: Interesting Questions

By | September 3, 2012

I am putting here some questions which are encountered by ophthalmologists in their daily clinical practice.

Question:  Will there be any difference when one calculates IOL power in a dilated pupil & a

                      non dilated pupil?

Answer: As none of the power calculation formulae incorporate pupil size, the IOL power calculation will not

change with the size of the pupil. Measuring the pupil size is important in younger patients. Those with

pupils larger than 4 mm in diameter in mesopic illumination will benefit from aspheric optics.

Question: Should calibration of Goldmans applanation tonometer be done after a certain time period?

                    What is the protocol routinely followed?

Answer: Some say one should do it daily and some say once a month.

It also depends on the usage.

In general, we can only verify the calibration of tonometers and cannot actually calibrate it.

In case we find that it is not proper during the calibration check , the seller or the engineer of the

company has to be notified to take the corrective action.  I do not think that any calibration error

can be corrected by us!! The one thing in our OPD that we can actually

calibrate is the manual keratometer B&L type.

Having said this, I am posting a video made by Aravind Eye Hospital which shows their expert

engineer making the calibration correction.

The copyright belongs to Aravind eye hospital, I am only putting it here for educational purpose.

Dr. Ronnie George gave her opinion too:

The manufacturer suggests an arbitrary monthly check. Some author’s recommend an arbitrary annual check and others suggest an arbitrary monthly check. In their study, (Choudhari NS, George R, Baskaran M, Vijaya L, Dudeja N. Measurement of Goldmann applanation tonometer calibration error. Ophthalmology. 2009 Jan;116(1):3-8.) reported that the tonometers initially found faulty, were repaired and subsequently released for use. Four months later, 11 (20.75%) of the 53 initially faulty Goldmann tonometers were found to have drifted out of calibration. On this basis, they recommend at least 4 monthly formal calibration error testing with the use of the calibration error check weight bar is advisable. A simple way of assessing calibration is at 0 mmHg, calibration error testing can be performed easily without use of the calibration error check weight bar- the revolving knob of the measuring drum should be rotated forwards. The reading at which the feeler arm with the applanation prism in place moves forward freely should be recorded.) It should be at zero, any deviation from this indicates a calibration error at that value. Formal testing should be done at 20 mm and 60 mm Hg. Repair of these instruments is a concern since no facilities are available in India. Most of the calibration errors were rectified by lubrication and cleaning of the faulty instrument in this study.


With some of my fellow scientists at the Wilmer Eye Institute, Johns Hopkins, Baltimore. 

Debasish Sinha PhD is the Assistant Professor at the Wilmer Eye Institute. He is a cellular and

molecular biologist.  He is an expert on lens, atrocytes and nanoparticles. Debashish Sinha

can be contacted at . Sam Zigler is an assistant professor and is a

protein biochemist working on nanoparticle synthesis. He can be contacted at                                                                                                                                                                                                                               

The Mystery of the A-Constant

How is the A-Constant of the IOL decided?

The A constant refers to the effective power of the IOL inside the eye initially for the SRK formula

IOL Power = A 2.5L 0.9K;  L= axial lenth in mm, K is corneal power in diopters

The most common A constants used are

#. Anterior chamber lenses 115.0-115.3

#. Posterior chamber lenses in the sulcus 115.9-117.2

#. Posterior chamber lenses in the bag 117.5-118.8

Here are 2 links for further reading:



A power point presentation By Dr. Chi-Wah (Rudy) Yung