gms | German Medical Science

With a novel biometry device (Zeiss ACMaster) it has now become possible to perform laser interference measurements of the anterior ocular segment. Since the primary measurement parameters in laser biometry are optical path lengths, the group refractive indices of the respective ocular media are needed for the translation into geometrical distances. By comparative measurements using well-established precision ultrasound methods, we determined these indices for cornea, anterior chamber and crystalline.

For the measurement of corneal thickness a Tomey AL 2000 pachometer (ultrasound frequency: 20 MHz) was applied, while a Grieshaber Biometric System GBS (ultrasound frequency: 10 MHz) was used for the comparison of anterior chamber depth and lens thickness. Patients and volunteers were examined prospectively by ultrasound and laser interference (ACMaster prototype).

The following group refractive indices were determined for the applied wavelength of λ=850 nm: cornea: 1.3616 ± 0.0015, n=104, R=98.5%; lens: 1.3669 ± 0.0051, n=50, R=97.4%; anterior chamber: 1.3873 ± 0.0049; n=56, R=99.1%. In this context, anterior chamber depth is defined from the anterior corneal surface to the anterior lens surface.

The Zeiss ACMaster allows highly precise measurements of the anterior ocular segment. By means of the refractive indices deduced it is easily possible to relate laser interference results to those of clinically well-established ultrasound methods.