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Axis measurement strip applied to slit lamp enables precise measurement of toric IOL position
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Some newer slit lamp designs include a scale to measure the angle of the slit lamp beam, but many classic designs do not provide a precise scale.
In particular, the Haag-Streit BM 900 series slit lamp has a scale of dots located 10° apart. An axis measurement strip applied to the BM 900 slit lamp provided precise measurement of postoperative toric IOL placement, according to a study.
Measuring the toric IOL position in this manner corresponds well with the true IOL position, David S. George, MD, the corresponding author, told Ocular Surgery News.
“Measuring postoperative toric IOL position can be accurately performed by aligning the angle of the slit lamp light beam to the toric IOL markings and reading the axis from the axis measurement strip,” he said.
The axis measurement strip can also be altered to fit other slit lamps.
Tools and technique
The axis measurement strip was designed on Adobe Photoshop CS4 by dividing the length of the curve of the BM 900 series slit lamp scale for axis position into 180° increments. The measurement strip was printed and applied to the slit lamp.
The axis measurement strip can measure the slit lamp beam angle in 1° increments, with larger numbered lines every 10°. The 1° scale enables a more precise reading for measuring toric IOL position, the authors said.
To obtain the postoperative axis of a toric IOL, the patient’s head is positioned behind the slit lamp with no tilting. The narrow slit lamp beam is focused on the IOL surface and rotated until it is parallel with the markings on the IOL. The angle is indicated by the position of a silver pin over the axis measurement strip.
Study methods and results
Axis measurement strips were applied to several BM 900 slit lamps to assess postoperative toric IOL position in 47 eyes.
Measurements made with the axis measurement strip were compared with measurements derived from images captured with the Galilei G2 corneal analyzer (Ziemer).
After dilating the pupil, the surgeon aligned the slit lamp beam to the axis of the toric IOL marks and recorded the angle from the axis strip. Images were obtained with the corneal analyzer; the observed angle of the toric IOL was determined using the ruler tool in Adobe Photoshop CS4, which provided more precise measurements than the scale provided with the Galilei images.
On average, measurements from the axis measurement strip were within 1° of measurements taken with images from the corneal analyzer. Linear regression modeling showed a direct one-to-one correlation between angle measurements taken with both methods.
The mean difference between angles measured by each method was less than 1°. The standard deviation between the measurement strip and corneal analyzer groups was 3.98. Small differences in measurements between the two methods may be attributed to the patient’s head position, observer error, patient movement or other factors, the authors said.
The study showed that postoperative toric IOL position can be accurately measured at the slit lamp by aligning the beam angle with the IOL markings and reading the axis from the measurement strip.
George said that he and his colleague do not plan to market the axis measurement strip. – by Matt Hasson
Reference:
George VE, et al. J Cataract Refract Surg. 2014;doi:10.1016/j.jcrs.2014.08.024.For more information:
David S. George, MD, can be reached at The Eye MDs, 418 Grand Park Drive, Suite 315, Parkersburg, WV 26105; email: george200@aol.com.Disclosure: George has no relevant financial disclosures.