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Simple steep-axis marking technique using a corneal analyzer

Ng, Alex L.K. FRCS(Edin); Chan, Tommy C.Y. FRCS(Edin); Jhanji, Vishal MD, FRCOphth; Cheng, George P.M. FRCS(Edin)*

Author Information
Journal of Cataract & Refractive Surgery: February 2017 - Volume 43 - Issue 2 - p 153-155
doi: 10.1016/j.jcrs.2016.09.023
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Abstract

Correcting coexisting corneal astigmatism with arcuate keratotomy or toric intraocular lens (IOL) implantation during modern cataract surgery has recently become more popular.1 The accuracy of these methods depends on marking the steep axis in arcuate keratotomy or the desired toric IOL alignment axis accurately. Most methods in the literature are for toric IOL placement and involve marking a reference line (usually on the horizontal axis) under the slitlamp or at the surgical table shortly before surgery.2–4 The more popular methods use the horizontal slit-beam on the slitlamp or the 3-point marking method. The desired axis for toric IOL alignment can then be determined from the angular difference from the reference mark (usually the horizontal axis). Potential error can arise from differences in head positions during keratometry measurement and marking the axis under the slitlamp. There is a paucity of literature on marking the steep axis during arcuate keratotomy. We describe a simple marking method using the OPD III scan (Nidek Technologies Srl), which could eliminate the potential error with different head positions during marking and preoperative keratometry measurement.

Technique

The marking is performed in the surgery suite with the corneal analyzer placed next to the femtosecond laser platform. After topical anesthesia is instilled in the eye, a single reference mark is made at the temporal limbus using a marking pen visible under infrared, which is not necessarily on the zero-degree line. The patient can be in any position (supine or upright) during the marking, so the marking can be done by the surgical assistant. Next, a corneal analyzer scan is performed with the patient’s head in the proper upright position. An image of the cornea with a reference line along the steep axis is shown on the screen, together with the corneal marking (Figure 1). A second line is then made using the corneal analyzer scan screen tool connecting the center of the cornea to the reference mark. The angle between the steep axis and the reference point measured at the corneal center is recorded to determine the location of the axis for the arcuate keratotomy incision (Figure 1).

Figure 1
Figure 1:
An image captured from the corneal analyzer scan. The patient’s steep keratometry (K) is 45.18 @ 96 and the flat K, 44.0 @ 6. The limbal reference mark is at the temporal side, and the green reference line is placed using the corneal analyzer scan screen tool, connecting the pupil center and the reference mark. The red line and blue line along the steep axis (96 degrees) and flat axis (6 degrees) are also shown on the screen. The angle between the steep axis and the reference point measured at the cornea center (indicated by the red arrow, 99 degrees in this example) is then recorded to determine the location of the axis for the arcuate keratotomy incision.

The patient is transferred to the femtosecond laser platform. Under the microscope, the zero-degree mark of the Wallace Mendez degree gauge (Bausch & Lomb, Inc.) is placed at the reference mark. The steep axis can be marked directly at the predetermined angle (derived from the corneal analyzer scan) on the Wallace Mendez degree gauge (Figure 2)

Figure 2
Figure 2:
Diagram showing how to mark the steep axis under the microscope at the beginning of surgery. The limbal reference mark is indicated by the purple arrow. The zero-degree mark of the Wallace Mendez degree gauge is placed at this purple mark. Since the corneal analyzer scan image shows that the predetermined angle of the steep axis from our reference is 99 degrees, the steep axis (red line) could be easily located and marked at the 99-degree site of the Wallace Mendez degree gauge.

Results

The marking technique has been used with a good astigmatic outcome in 54 eyes that had combined phacoemulsification with arcuate keratotomy using the femtosecond laser.5,6 The steep axis was marked in all cases, and at 2 months postoperatively, the correction index (ratio of surgically induced astigmatism vector to target-induced astigmatism vector) was 0.86 diopters (D) ± 0.52 (SD) and the angle of error was 0.10 ± 26.10, indicating minimal systematic error of treatment alignment.5 All vector analysis parameters were well maintained over 2 years, including the correction index, which was 0.85 ± 0.48.6

Discussion

The traditional marking methods used in arcuate keratotomy involved preoperative measuring the steep axis, followed by marking the axis shortly before surgery and the arcuate keratotomy treatment. Errors could arise from any of these steps. In the technique we present, capturing the steep axis and marking the angle (between the steep axis and the reference line) were combined into 1 step using the corneal analyzer. There are fewer potential sources of error, and fewer patient transfers are required. Another major advantage of the technique is the elimination of errors that could arise from different head positions at the time of preoperative keratometry measurement and during the axis marking shortly before surgery. During the traditional marking method under the slitlamp or at the surgical bed, head tilt commonly occurs and leads to error in marking the horizontal reference mark. Locating the steep axis based on this inaccurate horizontal axis would result in an inaccurate steep axis; for example, if the horizontal axis was off by 10 degrees, the steep axis marked during surgery would also be 10 degrees off the preoperative keratometry measurement. In our technique, it is not necessary to locate the horizontal axis precisely. The precision derives from the angular difference between the limbal reference mark and the steep axis, and this angle remains the same regardless of the position of the limbal reference mark. Therefore, the limbal reference mark could be placed in any convenient position, which could also speed up the marking process.

In addition to the traditional marking methods using the horizontal slit-beam or the 3-point marking system, devices such as the pendulum marker or bubble marker have been described.2 However, errors from different head positions during marking and preoperative keratometry measurement persist. A method similar to ours, the iris pattern marking method, which uses an anterior segment optical coherence tomography system to identify an area of distinctive iris for placing the reference mark, has been described.7 The limitations of the method are that it is machine-dependent and in the study, not all patients had identifiable distinctive iris patterns.7 A smartphone application (toriCAM) that helps to reorientate the limbal reference mark with the horizontal axis is also available. The main advantage of the method we describe is that marking the reference line (using the reference mark), measuring the steep axis, and marking the angular difference (the critical number) are performed at the same time on the corneal analyzer screen and can be easily visualized (Figure 1) with no additional calculation. Thus, our method is quick and can minimize potential calculation errors.

Machine dependency is the main limitation of our technique. However, the technique is simple, quick, and straightforward as it combines capturing and marking the image in 1 step. The technique also eliminates the need to accurately mark the horizontal axis under the slitlamp or at the surgical table and is independent of head position and cyclotorsion, which are common sources of error. A comparative study of the traditional marking method versus OPD III marking would establish the advantages we describe. Although we used the technique for arcuate keratotomy during femtosecond laser–assisted cataract surgery, the technique could also be used to identify the IOL alignment axis in toric IOL cases.

What Was Known

  • Accurate axis marking is essential for astigmatic correction during cataract surgery with arcuate keratotomy or a toric IOL.
  • Currently popular marking methods involve marking a reference line, usually the horizontal axis, under the slitlamp or at the surgical table. If the horizontal axis is inaccurate, the subsequent intraoperative markings would be inaccurate and different from the axis determined during the preoperative keratometry measurement, resulting in a major source of error.

What This Paper Adds

  • The marking technique we describe uses a corneal analyzer. It is simple and eliminates the need for marking before surgery because it is not necessary to determine the zero-degree line.
  • The method has fewer patient transfer steps and is independent of the patient’s head position or cyclotorsion.

References

1. Kessel L, Andresen J, Tendal B, Erngaard D, Flesner P, Hjortdal J. Toric intraocular lenses in the correction of astigmatism during cataract surgery; a systematic review and meta-analysis. Ophthalmology. 123, 2016, p. 275-286, Available at: http://www.aaojournal.org/article/S0161-6420(15)01148-3/pdf. Accessed December 17, 2016.
2. Farooqui JH, Koul A, Dutta R, Shroff NM. Comparison of two different methods of preoperative marking for toric intraocular lens implantation: bubble marker versus pendulum marker. Int J Ophthalmol. 9, 2016, p. 703-706, Available at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4886884/pdf/ijo-09-05-703.pdf. Accessed December 17, 2016.
3. Woo YJ, Lee H, Kim HS, Kim EK, Seo KY, Kim TI. Comparison of 3 marking techniques in preoperative assessment of toric intraocular lenses using a wavefront aberrometer. J Cataract Refract Surg. 2015;41:1232-1240.
4. Onishi H, Torii H, Watanabe K, Tsubota K, Negishi K. Comparison of clinical outcomes among 3 marking methods for toric intraocular lens implantation. Jpn J Ophthalmol. 2016;60:142-149.
5. Chan TCY, Cheng GPM, Wang Z, Tham CCY, Woo VCP, Jhanji V. Vector analysis of corneal astigmatism after combined femtosecond-assisted phacoemulsification and arcuate keratotomy. Am J Ophthalmol. 2015;160:250-255.
6. Chan TCY, Ng ALK, Cheng GPM, Wang Z, Woo VCP, Jhanji V. Corneal astigmatism and aberrations after combined femtosecond-assisted phacoemulsification and arcuate keratotomy: two-year results. Am J Ophthalmol. 2016;170:83-90.
7. Watanabe K, Negishi K, Torii H, Saiki M, Dogru M, Tsubota K. Simple and accurate alignment of toric intraocular lenses and evaluation of their rotation errors using anterior segment optical coherence tomography. Jpn J Ophthalmol. 2012;56:31-37.

Disclosure

None of the authors has a financial or proprietary interest in any material or method mentioned.

© 2017 by Lippincott Williams & Wilkins, Inc.