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Manual limbal markings versus iris-registration software for correction of myopic astigmatism by laser in situ keratomileusis

Shen, Elizabeth P. MD; Chen, Wei-Li MD, PhD; Hu, Fung-Rong MD

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Journal of Cataract & Refractive Surgery: March 2010 - Volume 36 - Issue 3 - p 431-436
doi: 10.1016/j.jcrs.2009.10.030
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Abstract

Cyclotorsion can occur when patients change from an upright seated position to a supine position.1–4 Although some studies found that most cyclotorsional errors are fewer than 2 degrees, others report significant cyclotorsion errors (>5 degrees).3–6 In a study by Swami et al.,3 8% of 240 eyes had more than 10 degrees of cyclotorsion. Although a small angle of cyclotorsion (<2 degrees) will not significantly affect postoperative results, larger rotational errors can lead to significantly worse outcomes after treatment for astigmatism.5–8 Cyclotorsional errors greater than 10 degrees will cause more than a 6% loss of treatment effect.3,9 Thus, accurate axis alignment and cyclotorsional checks during refractive laser procedures are crucial to ensure good postoperative outcomes.

To prevent cyclotorsional errors, some surgeons place marks on the limbus to assist with alignment during positional changes.10,11 Modern technologies, such as iris registration, have been used to ensure correct axis alignment for optimum visual outcomes.12,13 However, few studies have compared the efficacy of laser in situ keratomileusis (LASIK) astigmatic correction with manual limbal marking and LASIK with an iris-registration system. This study analyzed and compared the efficacy and safety of astigmatic correction by LASIK assisted by manual limbal markings or by iris-registration software.

PATIENTS AND METHODS

This retrospective study comprised eyes that had LASIK for myopic astigmatism at National Taiwan University Hospital from July 2004 to June 2005 using manual limbal markings (limbal-marking group) or iris-registration software (iris-registration group). Inclusion criteria were at least 18 years of age, stable myopia for at least 1 year, corrected distance visual acuity (CDVA) of 20/40 or better in both eyes, stable corneal topography after discontinuing soft contact lenses for at least 2 weeks and hard contact lenses for at least 3 weeks, postoperative follow-up of at least 6 months, and patient-provided informed consent. Exclusion criteria included previous ocular surgery, underlying ocular and systemic disease that might affect corneal wound healing, and irregular corneal topography, including a history of keratoconus or contact lens–related corneal warpage.

The Technolas 217z100 excimer laser (Bausch & Lomb) was used in all cases. This 193 nm laser has a repetition rate of 50 to 100 Hz and a fluence of 120 mJ/cm2. Eyes in the limbal-marking group had conventional LASIK with reference marks at the limbus for cyclotorsional alignment. In this group, the ablation was performed with the PlanoScan program, which uses a 2.0 mm laser beam diameter, or with the Zyoptix tissue-saving algorithm, which uses a combination of 1.0 mm and 2.0 mm spots (both Bausch & Lomb). The iris-registration group had had Zyoptix wavefront-guided ablation, which uses a combination of 1.0 mm and 2.0 mm spots, with automated cyclotorsional checks performed by iris-registration software. The software corrects for cyclotorsional error and pupil center shifts and is available for wavefront-guided ablations only. By customizing the corneal keratometry values for each patient, the tissue-saving algorithm reduces unnecessary tissue loss by yielding more precise ablations. A postoperative residual corneal bed thickness greater than 250 μm was planned in all cases.

All patients had a complete preoperative examination including keratometry; corneal topography; ultrasound pachymetry; manifest, subjective, and cycloplegic refractions; uncorrected distance visual acuity (UDVA); and CDVA. Subjective refractive data at the spectacle plane were converted to those at the corneal plane before vector analysis. Follow-up examinations were performed at 1, 3, and 6 months and included UDVA, CDVA, keratometry, corneal topography, and manifest and subjective refractions. The safety index (mean postoperative CDVA/mean preoperative CDVA) and the efficacy index (mean postoperative UDVA/mean preoperative CDVA) were determined using postoperative 6-month logMAR visual acuity data. The predictability of the procedures was assessed by determining the number of eyes within ±1.00 D of the targeted SE refraction and the number of eyes within ±0.50 D of the targeted refractive astigmatism.

The surgeries were performed by 1 of 2 surgeons in the hospital's cornea service (F-R. Hu, W-L. Chen.) Topical anesthesia of proparacaine 0.5% was instilled preoperatively. In the limbal-marking group, reference marks were made on the limbus with a fine pen at the 3 o'clock and 9 o'clock positions of the horizontal meridians with the patient seated upright at the slitlamp. With the patient supine on the operative bed of the excimer laser machine, the patient's head position was adjusted to check for possible cyclotorsion. In the iris-registration group, the cyclotorsional error was corrected automatically by the iris-registration software and the patient's head position was not manually adjusted.

Laser in situ keratomileusis was performed with a superior hinged corneal flap using an M2 microkeratome (Moria). The subjective manifest refraction giving the CDVA was used as the targeted attempted correction. In both groups, 0.2 diopter (D) of sphere was deducted for 1.00 D of myopic astigmatism correction to compensate for the coupling effect.

Vector analysis was performed using postoperative 6-month refractive and topographic data. In both groups, the surgically induced astigmatism (SIA) vector, target-induced astigmatism (TIA) vector, astigmatic correction index, index of success, angle of error, torque, flattening effect, and flattening index were analyzed using methods described by Alpins.9,14–16 The SIA is the vector of the actual change induced by surgery. The TIA is the vector of the intended change after surgery. The astigmatic correction index is the ratio of SIA to TIA. The difference vector is the magnitude and axis of astigmatic correction from the achieved result required to obtain the targeted goal. The index of success is calculated by dividing the difference vector by the TIA. An astigmatic correction index of 1.00 and an index of success of 0 mean that the desired results have been obtained. The magnitude of error is the difference between the magnitude of SIA and TIA (ie, SIA − TIA). The angle of error is the difference between the angles of the SIA and TIA. The flattening effect is the amount of astigmatism reduction achieved by the effective proportion of the SIA at the intended meridian (flattening effect = SIA cos2 × angle of error). The flattening index, which preferably equals 1, is obtained by dividing the flattening effect by the TIA. These vector analysis results were calculated and compared between the groups.

Patient data were entered into an Excel spreadsheet (Microsoft, Inc.) and a VECTrAK astigmatic vector calculator (Assort Pty. Ltd.), and SPSS statistical software for Windows (11.0, SPSS, Inc) was used for analysis. The Student t test was used to compare results between the 2 groups. Simple logistic regression was used to compare the number of eyes within ±1.00 D of the targeted spherical equivalent (SE) refraction and the number of eyes within ±0.50 D of the targeted refractive astigmatism in the 2 groups. Simple linear regression was used to determine the relationship between the astigmatic correction index or index of success and the SE of the attempted correction.

RESULTS

Of the 118 eyes (77 patients) completing the 6-month follow-up, 67 eyes (43 patients; 11 men, 32 women) were in the limbal-marking group and 51 eyes (34 patients; 2 men, 32 women) were in the iris-registration group. The mean age of the patients was 34 years (range 23 to 46 years) and 32 years (range 21 to 55 years), respectively. There were no statistically significant differences between the groups in age.

Table 1 shows he preoperative and postoperative refractive data by group. The preoperative SE correction at the spectacle plane and the mean cylinder magnitude at all time points were similar in both groups. At 6 months, there was a statistically significant reduction in SE and astigmatism in both groups (P<.001).

Table 1
Table 1:
Refractive data in limbal-marking group (n = 67 eyes) and iris-recognition group (n = 51 eyes).

Regarding predictability, 59 eyes (88%) in the limbal-marking group and 40 eyes (78%) in the iris-registration group were within ±1.00 D of the targeted SE refraction at 6 months (Figure 1); the difference between groups was statistically significant (P = .04). At 6 months, 58 eyes (87%) in the limbal-marking group and 45 eyes (88%) in the iris-registration group were within ±0.50 D of the targeted refractive astigmatism; the difference between groups was not statistically significant (P = .93).

Figure 1
Figure 1:
Attempted versus achieved SE correction at 6 months. Thee error bars indicate ±1.00 D.

All eyes in the iris-registration group, except 1 with myopic regression, had a UDVA of 20/40 or better 6 months postoperatively. The UDVA was 20/25 or better in 62 eyes (93%) in the limbal-marking group and 43 eyes (85%) in the iris-registration group and 20/20 or better in 45 eyes (67%) and 35 eyes (69%), respectively. The efficacy index was 0.97 in the limbal-marking group and 0.98 in the iris-registration group. Figure 2 shows the lines of CDVA gained or lost 6 months postoperatively. No patient in either group lost more than 1 line of Snellen visual acuity. The safety index was 1.04 in the limbal-marking group and 1.07 in the iris-registration group.

Figure 2
Figure 2:
Changes in lines of CDVA at 6 months.

Table 2 shows the vector analysis results using 6-month refractive data and Table 3, using 6-month keratometry data. Based on refractive data, there was no statistically significant difference between the groups in the astigmatic correction index (P = .99), index of success (P = .47), or flattening index (P = 0.44), although there was a trend toward a slightly more favorable outcome in the limbal-marking group. Slight undercorrection of astigmatism occurred in the iris-registration group, with the TIA exceeding the SIA at 6 months. There was no statistically significant relationship in either group between the amount of attempted SE correction and the astigmatic correction index value (limbal marking, P = .45; iris registration P = .70) or the index of success value (P = .56 and P = .39, respectively). The angle of error was within ±10 degrees in 49 eyes (73%) in the limbal-marking group and 38 eyes (75%) in the iris-registration group (P = .87).

Table 2
Table 2:
Vector analysis using postoperative 6-month refractive data.
Table 3
Table 3:
Vector analysis using postoperative 6-month keratometry data.

DISCUSSION

The most common causes of cyclotorsion include incorrect axis alignment that occurs with the patient's head tilted, unintentional rotation of the operating microscope, a cyclophoria that is unmasked, pupil shift, and distortion of the globe by the eyelid speculum.3–5,17 Methods commonly used to detect cyclotorsion include manual reference limbal markings and laser-assisted software programs such as iris registration, which is also known as iris recognition. Manual limbal markings are usually made with the patient seated upright at the slitlamp.11 Farah et al.11 marked only the 6 o'clock position at the limbus and found it improved the refractive outcomes of photoastigmatic refractive keratectomy for astigmatism greater than 1.25 D. In our study, 2 limbal markings were made at the horizontal 3 o'clock and 9 o'clock positions in the limbal-marking group to ensure proper eye position. This method was safe and accurate in adjusting cyclotorsion, and postoperative visual and vector analysis showed no statistically significant difference between manual limbal marking and iris registration, especially in the astigmatic correction index, index of success, and flattening index. The preoperative SE and astigmatism were similar between the groups. We did not find a significant correlation between the attempted correction and the astigmatic correction index or index of success. Thus, for myopia with mild to moderate astigmatism, manual limbal markings were as effective as iris-registration software in preventing significant cyclotorsional errors.

Marking the limbus preoperatively is an easy and fast method to detect cyclotorsional errors. However, the disadvantages of limbal markings are that the marks can fade with time and manual methods are prone to human error. Thus, most laser machines are now equipped with programs that can detect and correct rotational errors automatically using iris-registration or limbal-registration techniques. However, these programs may fail preoperatively in patients without significant iris details, with large rotational angles, or with pupils smaller than 3.0 mm or when pupil movements are greater than 0.5 mm or the iris registration fails to engage during surgery. Chang18 recently reported an iris-registration failure rate of 43% in 245 eyes having LASIK. In addition, programs that automatically correct cyclotorsional error are usually available only on newer machines and for wavefront-guided ablations. Thus, the manual limbal-marking technique provides valuable backup protection and can also be used to supplement automatic rotation-detection programs.

Although we did not measure the angle of cyclotorsion, several studies have. Smith et al.1 and Smith and Talamo2 found no significant difference in astigmatic angle change between the seated and supine positions using the cylinder-rocking, Jackson cross-cylinder, and Maddox double-rod techniques. The mean difference in angles due to positional change was 4.3 degrees with the cylinder-rocking method, 2.3 degrees with the Jackson cross-cylinder method, and 0.2 degrees with the Maddox double-rod method.1,2 However, the range of cyclotorsional angle change was 0 to 13 degrees, 0 to 7 degrees, and –2 to 4 degrees, respectively.1,2 Smith et al.1 also found that 25% of the 50 eyes evaluated had a change in axis of 7 to 13 degrees. This means that 1 in 4 eyes will have significant cyclotorsional errors that require correction before laser treatment. Chernyak6 measured cyclotorsional eye motion occurring between wavefront measurement and refractive surgery. He found a mean cyclotorsional angle of approximately 2 degrees in each eye and a maximum cyclotorsion error up to 9.5 degrees. In a study by Swami et al.,3 20 (8%) of 240 eyes had rotational malpositioning greater than 10 degrees during LASIK. Thus, although most studies report cyclotorsion of approximately 2 degrees, there is a significantly large cyclotorsion angle to cause poor postoperative results in a small proportion of patients.

Further study of our vector analysis results showed that the SIA was less than the TIA, especially in the iris-registration group, using both refractive data and keratometry data. This indicates an overall undercorrection in the iris-registration group. Analysis of the difference vector using the refractive arithmetic mean and the summated vector mean found a systemic trend in treatment errors in approximately 41% of eyes in the limbal-marking group and 46% of eyes in the iris-registration group. The astigmatic correction index, index of success, and flattening index vector analysis results using refractive data were more favorable than those calculated using keratometry data. A higher angle of error and torque from keratometry data indicated less optimum treatment of topographic astigmatism, especially in the limbal-marking group. Taken together, these vector analysis results suggest that better refinement of our nomogram for wavefront treatment using iris registration and careful adjustment of astigmatism axis treatments with iris registration and limbal markings are required to optimize postoperative results.

The UDVA was better postoperatively in both groups in our study, and the degree of safety and predictability was high. In a study comparing LASIK performed with and without iris-registration software, Ghosh et al.13 found a mean preoperative astigmatic error of −1.22 D in the iris-registration group and −1.34 D in the control group and that wavefront-guided LASIK using iris-registration software achieved significantly better visual and refractive outcomes 3 months postoperatively. However, manual limbal markings were not routinely used in the control cases. Our astigmatic and vector analysis results were comparable between the 2 groups, probably because reference limbal markings were routinely used in cases without iris registration. Limbal markings helped refine the treated cylinder axis and reduce the probability of measurement error. In a study by Shaikh and Manche10 of LASIK for myopia and compound myopic astigmatism using the laser and conventional ablation program we used in this study, the index of success was 0.32 ± 0.46 at 3 months in the myopic astigmatism group; horizontal limbal markings were used in this group. Our index of success values using refractive data at 3 months was 0.22 in the limbal-marking group, which is comparable to the result of Shaikh and Manche.10

A limitation of our study was that different laser ablation programs were used in the 2 groups. The different algorithms may have affected the accuracy of the astigmatic treatment and thus the vector analysis results. Nevertheless, because the iris-registration software and the laser in our study cannot be used for conventional LASIK treatments, a comparison of the effectiveness of limbal marking versus iris registration with the same algorithm was not possible.

In conclusion, myopic astigmatic treatments with LASIK assisted by manual limbal reference markings and wavefront-guided ablation with iris-registration software were both safe and effective. There were no statistically significant differences in vector analysis results between the 2 groups. Thus, with the excimer laser used in our study, limbal markings to prevent cyclotorsional errors gave astigmatic correction results comparable to those by the iris-registration software of the laser. We suggest that limbal markings be routinely used in all LASIK procedures for astigmatic correction to ensure the least errors and provide the best postoperative outcomes.

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