Optical ACD Measurement with the Zeiss IOLMaster

1. What should I bear in mind for taking the measurements?

- Focus onto focus point (smallest dot)

- Make sure the dot is within the green square.

- Avoid too strong extraneous reflections and room light.

- The image of the lens should be well visible on the screen.

- Front surface of the lens should be visible inside the pupil

2. Why should the ACD be measured?

Most formulas use axial length and corneal power for IOL-calculation only and calculate a predicted ACD.

This may cause problems in case of short or long eyes.

According to J. Holladay 10% of short eyes have long ACD and 10% of long eyes short ACDs.

Therefore, especially in these cases, the use of a biometric formula with measured ACD should be better.

3. What is the principle of anterior chamber depth measurement?

The measuring principle of anterior chamber depth measurement is based on the "optical section" through the anterior chamber by means of a slit lamp with subsequent image analysis. The right eye is illuminated from the right; the left eye is illuminated from the left at an angle of approx. 30° relative to the optical axis.

The measurement is performed by image analysis.
The instrument measures the anterior-chamber depth (ACD) as it is usually measured in biometry. Anatomically, this is the anterior chamber depth plus corneal thickness.

4. What should I think of, if in successive anterior chamber depth measurements large deviations in the range of 0.1 mm occur?

Naturally, patient accommodation affects anterior chamber depth measurement and lens thickness. The patient should be told to look relaxed at a distant point. To eliminate accommodation and make measurements with small anterior chamber and small pupils easier, it is possible to measure under a mydriatic.

5. Is it possible to measure pseudophakic eyes with the anterior-chamber depth meter?

The measurement requires scattered light from the lens. The IOL of freshly operated patients is hardly scattering light. That's why measurements are not possible in this case.

6. How often may I measure an eye per day using the anterior chamber depth-measuring device?

The number of measurements is not limited. The light load on the patient remains uncritical even with a great number of measurements. A few patients however find the white light/flickering light too glaring.

7. What is the reproducibility on the eye?

Reproducibility is better than ±50 µm, this corresponds to a mean postoperative error of less than 0.05 D.

In clinical use, mean standard deviations of 31,9 µm (15 measurements, Dr.Haigis, University Eye Hospital Wuerzburg/Germany) and 33,4 µm (20 measurements, Dr. Vogel, University Eye Clinic, Mainz/Germany) were obtained. The variabilities between different examiners were 37.7 µm (4 examiners, Dr.Haigis, University Eye Hospital Wuerzburg/Germany) and 29.8 µm (5 examiners, Dr.Vogel, University Eye Clinic, Mainz/Germany).

Safety criterion:

If the results of the individual measurements differ by more than 100 µm, no mean value will be displayed, and the measurement must be repeated.

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

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Last revision : July 24, 2000