Laser biometry more reliable with experts and novices

By Roibeard O'hÉineacháin

NICE - Laser biometry with the IOLMasterTM (Carl Zeiss Meditec, Germany) produces more reliable axial length measurements than ultrasound biometry, whether in the hands of experts or novices - but it also has a higher failure rate, a UK Clinical Nurse Specialist told the XX ESCRS Congress.

In a study comparing axial length measurements carried out by biometrists of varying expertise, the correlation between the measurements of expert and non-experts was 10 times higher with laser biometry than it was with ultrasound biometry, Mike Butcher BSc said.

The axial length measurements made by experts and intermediate users in the study differed by a mean of only 0.024 mm within a range of between 0.0 mm and 0.14 mm when using laser biometry, compared to 0.22 mm within a range of between 0.03 mm and 1.29 mm when using ultrasound biometry.

Furthermore, the difference between the measurements made by experts and novices with laser biometry was only 0.02 mm within a range of between 0.0 mm and 0.4 mm, Mr Butcher said.
"Our study indicates that even the novice user can provide comparable and dependable consistency in axial length measurement using laser biometry.

"In contrast, the axial length variation on the ultrasonic system between expert and intermediate users had a broad range, with a potential for up to 1.29 mm difference, which can translate to over 3.0 D of postoperative refractive error," he explained.
Moreover, even experts had a mean difference of 0.12 mm in axial length measurements when measuring the same eye with the two systems.

However, this was significantly better than that of intermediate users who registered a mean difference of 0.18 mm, Mr Butcher pointed out.
"Experts had significantly better laser/ultrasound consistency when compared with the intermediate level user, demonstrating the need for greater expertise when using ultrasound systems.

"Therefore, we recommend that only expert audited biometrists should be depended upon when using ultrasound systems," he said.
The randomised prospective study involved 68 eyes of 39 patients. Forty-five eyes underwent axial length measurement by experts and intermediate users with both laser and ultrasound biometry.

The remaining 13 eyes underwent axial length measurement by experts and novices with laser biometry alone.
The expert biometrists in the study all had audited consistent and accurate results with both ultrasound and laser systems in over 1,000 patients.

The intermediate level user had non-audited experience with both systems and had biometric experience of about 100 cases. The novice users were completely inexperienced with both systems but trained in the use of the equipment.
Laser biometry is inherently more accurate than ultrasound because it measures the true axial length along the line of sight, since the patient actually fixates on the laser beam.

Ultrasound depends more on the biometrist's skill since it requires correct alignment to measure the eye's true axial length, Mr Butcher noted.
Laser biometry uses the principle of partial coherence interferometry. A laser diode emits infrared light which is split into two partial beams reflected at the cornea as well as the retina.

Interference occurs if the path between the partial beams is smaller than the coherence length. The interference signal measured is the optical path length between the anterior surface of the pre-corneal tear film and the retinal pigment epithelium.
Ultrasound biometry uses the 200-year-old principle of the Italian scholar Lazzaro Spallanzani who discovered that bats utilise ultrasound for locating purposes.

The same principal is used to assess the time delay in the echo received from the corneal surface, the anterior lens surface and the retinal surface, from which the anterior chamber depth and the axial length are then calculated. The probe sends and receives the ultrasound signal to measure the axial length.

The main drawback with laser biometry is its higher failure rate. Mr Butcher and his associates were unable to obtain axial length measurements with laser biometry in eight eyes and with ultrasound in one eye.

Of the IOLMaster's failures, five (63%) were due to dense cataracts where the laser light could not penetrate the eye, and three (37%) occurred when the patients were not able to maintain their visual axis.
The one ultrasound failure occurred when the patient could not co-operate well enough for contact biometry. The patient successfully underwent laser biometry.

"As the IOLMaster becomes more commonplace it is essential that biometrists do not completely rely on laser biometry but maintain their ultrasound skills as well," Mr Butcher stressed.

Siddartha Goel FRCSEd, FRCSGlas is the principal author in this study.

Top