Cataract surgery has evolved into a refractive procedure as surgeons strive to provide postoperative emmetropia. As 16% of eyes have corneal astigmatism of 1.5 diopters (D) or more and 8% have 2.0 D or more,1 cataract surgeons must correct the astigmatism to achieve postoperative spectacle freedom for these patients. Precise astigmatism reduction is also required for multifocal intraocular lenses (IOLs) as postoperative astigmatism of 1.0 D or more can significantly compromise both near and distance vision.2 With the advent of toric IOLs for astigmatism reduction, ophthalmic surgeons must place the IOL in the optimal position to completely correct the refractive error.3 A misaligned toric IOL axis can result from misplacement at the time of surgery or rotation after the initial placement. Misalignment of a toric IOL limits the effectiveness of astigmatism reduction and also induces a hyperopic spherical change and a shift of the astigmatism axis.4 Initial postoperative misalignment of more than 10 degrees typically requires surgical intervention to reposition the IOL before scarring that can prohibit IOL rotation occurs.
Optimal postoperative toric IOL axis position remains a challenge for surgeons. Accurate and clinically efficient methods of measuring the preoperative intended position and postoperative IOL axis are essential. The toric IOL axis can be determined with the angle of the slitlamp beam. However, many commonly used slitlamps do not provide a precise scale for measuring the slitlamp beam angle. The axis measurement strip provides a method for obtaining more precise measurements for the slitlamp beam angle and thus improves the accuracy of determining the toric IOL axis.
Optimizing the toric IOL position typically involves preoperative axis selection via an online calculator,A marking the eye at the intended axis of placement prior to IOL implantation and confirming the IOL axis position postoperatively. The postoperative IOL axis measurement can be performed at the slitlamp by aligning the angle of the slitlamp beam with the toric IOL axis. Some slitlamp manufacturers provide numbered axis markings that correspond to the angle of the slitlamp beam, including some newer Haag-Streit designs (eg, the Haag-Streit BC900 has a numbered scale in 5-degree increments). However, the popular Haag-Streit BM900 slitlamp series has a scale that is represented only by dots separated by 10 degrees, which are not labeled. As this scale is difficult to use clinically, we developed an axis measurement strip (Figure 1) that can be placed on the Haag-Streit BM900 slitlamp to measure the slitlamp beam angle in 1-degree increments, with larger numbered lines every 10 degrees (Figure 2). The 1-degree scale provides a more precise clinical reading for measuring toric IOL position.
Previous work by Carey et al.5 showed that observations at the slitlamp correlated well with measurements from a corneal analyzer. Thus, to show that using the axis measurement strip is an accurate technique, measurements made with the strip were compared with measurements derived from images obtained with a Galilei G2 corneal analyzer.
The axis measurement strip was designed on Adobe Photoshop CS4 by dividing the length of the curve of the BM 900 series slitlamp scale for axis position into 180-degree increments (Figure 1). The measurement strip was printed and applied to the slitlamp (Figure 2; Video 1, available at: http://jcrsjournal.org). The applied strip correlated with the simple dot scale provided on the BM 900 in that the larger 10-degree marks aligned with the dots. A printed protractor confirmed that the axis measurement strip was accurate at several positions (Video 2, available at: http://jcrsjournal.org).
To obtain the postoperative axis of a toric IOL with the axis measurement strip, the patient’s head is properly positioned behind the slitlamp without any tilting. The narrow slitlamp beam is then focused on the IOL surface and rotated until it is parallel with the markings on the toric IOL. The angle is indicated by the position of the silver pin over the axis measurement strip.
The axis measurement strip was applied to several BM 900 slitlamps and used to determine the position of postoperative toric IOLs in 47 eyes. After the pupil was dilated, the surgeon aligned the slitlamp beam to the axis of the toric IOL marks and recorded the angle from the axis strip. The Galilei images were subsequently obtained and the observed angle of the toric IOL was determined using the ruler tool in Adobe Photoshop CS4, as it provided more precise measurements than the scale provided with the Galilei images.
The toric IOL angles measured by the Galilei method and by the axis measurement strip method showed a significantly direct relationship, as shown in Figure 3, the fitted line plot. Regression analysis showed a slope of 0.9741, and the R2 value of 0.9944 indicates that the results are significantly related. The mean difference between angles measured by each method was less than 1 degree at 0.87 degrees. The standard deviation between the 2 groups was 3.98.
The axis measurement strip on the Haag-Streit BM 900 slitlamp provides a precise and clinically efficient technique for measuring toric IOL position. The strip provides 1-degree increments for measuring the slitlamp beam angle when it is aligned with toric IOL markings.
Measurements from the axis measurement strip were, on average, within a 1-degree difference from those derived from images obtained on the Galilei G2 corneal analyzer. A linear regression analysis (Figure 3) showed a direct 1-to-1 correlation between angle measurements obtained by the 2 methods. The differences in measurements between the methods may relate to patient head position, observer error, patient movement, or other factors.
Although the axis measurement strip is scaled to fit a Haag-Streit BM900 series slitlamp, the scale can easily be altered to fit other slitlamps by determining the length of the scale that is required and adjusting the size of the image before printing. Confirming the scale has proper calibration before use (Video 2, available at: http://jcrsjournal.org) is recommended. Although the strip was used for postoperative measurements in the study, we believe that it could be used for preoperative corneal marking of the intended IOL axis with comparable accuracy. The axis measurement strip provides a technique for determining toric IOL axis position that is clinically efficient and precise.
What Was Known
- Cataract surgeons using toric IOLs need an efficient and accurate technique for measuring toric IOL position.
- Many commonly used slitlamps do not provide a slitlamp beam angle measurement scale that is clinically useful.
What This Paper Adds
- The axis measurement strip provided precise measurement of the postoperative toric IOL position for the popular Haag-Streit MB 900 series slitlamp.
- The axis measurement strip pdf can be downloaded, printed, and attached to a Haag-Streit BM900 series slitlamp. Video instructions for application are also provided.
1. Hoffmann PC, Hütz WW. Analysis of biometry and prevalence data for corneal astigmatism in 23,239 eyes. J Cataract Refract Surg
2. Hayashi K, Manabe S-I, Yoshida M, Hayashi H. Effect of astigmatism on visual acuity in eyes with a diffractive multifocal intraocular lens. J Cataract Refract Surg
3. Visser N, Bauer NJC, Nuijts RMMA. Toric intraocular lenses: historical overview, patient selection, IOL calculation, surgical techniques, clinical outcomes, and complications. J Cataract Refract Surg
4. Jin H, Limberger I-J, Ehmer A, Guo H, Auffarth GU. Impact of axis misalignment of toric intraocular lenses on the refractive outcomes after cataract surgery. J Cataract Refract Surg
5. Carey PJ, Leccisotti A, McGilligan VE, Goodall EA, Moore CBT. Assessment of toric intraocular lens alignment by a refractive power/corneal analyzer system and slitlamp observation. J Cataract Refract Surg
Video 1 Instructions for placement of the axis measurement strip.
Video 2 Calibration and confirmation of the axis measurement strip.
Axis Measurement Strip
Other Cited Material
A. Alcon, Inc. AcrySof®
Toric IOL Web Based Calculators. Available at: http://www.acrysoftoriccalculator.com
. Accessed June 20, 2014