Measurement with the IOLMaster - Keratometer

 

1.    When do I get faulty or no measurement results with the keratometer?

You will get faulty or no measurement results, if the

-         instrument is not properly focused on the eye,

-         measuring marks are partly obscured by eyelid or eyelashes,

-         the eye is being closed during the measurement,

-         strong reflections are additionally produced at the anterior surface of the IOL with pseudophakic eyes,

-         the anterior surface of the cornea is affected by scars, local or other irregularities,

-         the tear film is strongly impaired.

 

2.    What can I do to nevertheless obtain results after an error message was displayed?

-         Focus the instrument exactly onto the light dots (mostly, additionally a fine circle is visible around the dots)

-         Ask the patient to open the eye wide, then repeat the measurement.

-         Let the patient close and open the eye several times to improve the tear film, then quickly start the measurement.

-         If measuring marks are obstructed, carefully hold the eyelid.

-         Slightly decentre the instrument by moving the instrument towards the eye to measure, for instance, beside local scars.

You should observe however, that with strongly irregularly formed corneas or after refractive surgery of the cornea the measured values cannot be used for the calculation without thorough consideration. In this respect, the built-in keratometer does not differ from any other manual keratometers or autokeratometers commercially available.

 

3.        How do I recognize "appropriate" or "inappropriate" reflections (risk of erroneous

       measurements)?

If you can see six peripheral circular light dots on the screen, everything is OK. If one of the dots is irregularly shaped, this might be caused, for instance, by a local corneal scar. If several dots are deformed, this might be due to a poor tear film. In this case, tell the patient to shortly close and open her or his eyes again.

If, for instance, the top dot is missing, you should consider a hanging down eyelid or eyelashes.

 

4.    Is it possible to measure pseudophakic eyes with the keratometer?

This depends on the intensity of reflections additionally produced at the IOL. If the additionally visible six light dots are faint, you can measure the eye without any problems. If they are strong, measurements are impossible.

This depends on the curvature of the implanted IOL.

If the additional light dots are so strong, that the measurement is impossible, you can slightly defocus the IOLMaster (approx. 1...2 mm). In this way, the additional reflections become so faint that they do not disturb the measurement. On the other hand, defocusing the instrument by 1 to 2mm will affect the results only insignificantly.

 

5.    What is to be observed for patients after refractive surgery (PRK, LASIK, ...)?

In the calculation of the future IOL at least the measured axial length and the corneal curvatures (refractive power of cornea) are entered. The changed corneal geometry does not affect the measurement of the axial length of the eye but the keratometer values. The formulas presently used for IOL calculation base on the “Gullstrand Eye“ and thus presuppose defined conditions of the radii of curvature of anterior and posterior surfaces of the cornea. This “geometry“ however has been changed by refractive surgery. For this reason, the data obtained with the keratometer cannot be included unchanged in the IOL calculation.

 

6.    What should I bear in mind when I compare corneal power (K values) obtained with keratometers of different manufacturers?

Keratometers/ophthalmometers always measure the radius of the cornea. On one and the same eye (equal radius), different diopter values will be displayed due to the different eye models used by different manufacturers (refer to the table below for corneal refractive indices). This may result in deviations of 0.5 to 0.8 D that are to be attributed only to the eye model used, but not to the actual measuring accuracy.

Influence of the measurement with different keratometers using different corneal refractive indices:

 

 

Corneal refractive index

Resulting deviation of refraction, referred to 1st line

Topcon, Zeiss

       1.332

       0 D

Haag-Streit, American Optical

       1.336

       0.5 D

Rodenstock, Humphrey

       1.3375

       0.7 D

Hoya

       1.338

       0.8 D

 

 

 

 

If you want to compare the results of keratometers of different manufacturers with the IOLMaster, you will obtain comparable diopter results, if you enter the same corneal refractive indices in the menu "Options–>setup–>program settings->Keratometer".

 

 

 

Dr. Wilfried Bissmann                               

Carl Zeiss

Ophthalmic Instruments Division

 

Phone                               +49 3641 64 2058

Fax                                   +49 3641 64 2917

E-mail                               bissmann@zeiss.de

 

 

The telephone hotline          +49 3641 64 2030

 

---------------------------------------

Last revision: July 24, 2000