Laser biometry more reliable with experts and
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
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
"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
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
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
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
The one ultrasound failure occurred when the patient could not co-operate
well enough for contact biometry. The patient successfully underwent
"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