Position-Induced Cyclorotation Between Wavefront Measurement and Refractive surgery by Iris Registration Method Yu-Chih Hou, MD, Yao-lin Liu, MD, I-Jong Wang, MD, Ph. D, Fung-Rong Hu, MD, and Wei-Li, Chen, MD, Ph. D Department of Ophthalmology, National Taiwan University Hospital, Taipei, Taiwan Authors have no financial interest in any product mentioned herein
Purpose • Cyclorotation of eyes may be affected by motion and orientation of the head and body. 1 The difference of cyclotorsion between seated and supine positions may range from 2°- 7° and is considered to be one of the possible causes of residual astigmatism or aberrations after laser refractive surgery. Many investigators have attempted to measure the amount of cyclorotation by various kinds of methods. 3, 4, 5 In the present study, we used an automated alignment method by iris registration technique to assess the amount and orientation of position-induced cyclorotation between preoperative wavefront measurement and laser refractive surgery. We also estimated the amount of rotational misalignment between dilated and undilated pupils in the seated position.
Materials and Methods • A retrospective, consecutive study was performed to measure the degree and direction of cyclorotation in 186 eyes in 93 patients in laser refractive surgery. Cyclorotation (angle X) between pupillary changes in the seated position was derived from a comparison of iris images between undilated and dilated pupils using the Zyoptix system (Bausch & Lomb). • Overall cyclorotation (angle Y) was detected by a comparison between iris images of undilated pupils in the supine position and dilated pupils in the seated position before laser treatment. The degree and direction of position-induced cyclorotation (angle Z= X+Y) and cyclorotation (angle X) between pupillary changes were assessed.
Results • The distribution of position-induced cyclorotation is shown in Figure 1. The mean value of cyclorotation was – 0. 42° ± 3. 84° (– 10. 87° to 10. 01°) in right eyes and 1. 74° ± 3. 97° (– 6. 40° to 13. 51° ) in left eyes. The correlation of cyclorotation between both eyes was statistically significant (correlation coefficient = 0. 418, P < 0. 0001; Figure 2). The distribution of the absolute values of cyclorotation for all 186 eyes is depicted in Figure 3. The average absolute value of cyclorotation was 3. 22° ± 2. 53° (right eye, 3. 04° ± 2. 37°; left eye, 3. 40°± 2. 68°). 112 eyes (60. 2%) had significant cyclorotation > 2°, and 39 eyes (21. 0 %) had cyclorotation > 5°. • The distribution of absolute values of cyclorotation between undilated pupils and dilated pupils in the seated position was shown in figure 4 with the mean of 1. 15° ± 0. 91° (– 4. 57° to 4. 22°). When we compared the difference of cyclorotation between positional change and pupillary change, the difference was statistically significant (P = 0. 03).
Figure 1 Right eyes Excyclotorsion Incyclotorsion Left eyes Excyclotorsion Incyclotorsion Distribution of position-induced cyclorotation
Figure 2 Correlation coefficient = 0. 418, P<0. 0001 The correlation of position-induced cyclorotation between both eyes
Figure 3 Distribution of absolute value of position-induced cyclorotation
Figure 4 Distribution of absolute value of cyclorotation between dilated and undilated pupils
Discussion • Possible causes of ocular misalignment include rotation of the head and body, ocular cyclotorsion, unmasking of a cyclophoria, and distortion of the globe by application of a lid speculum in the refractive surgery. 6, 8 Iris registration is a method of comparing the detail images of the iris, then subtle changes in rotational orientation can be measured very accurately. This method has been used by Chernyak 7 and Kim et al. 8 Both of their studies showed a mean of 2 degrees of cyclotorsion and a maximum up to 9. 5 degrees. Swami et al. 6 used a different non-automatic method by measuring the rotational deviation of two limbal marks at the horizontal position between positional changes and found a mean of approximately 4 degrees cyclorotation and a maximum up to 16 degrees. Our study showed a mean of 3. 22° ± 2. 53° of cyclorotation with a maximum of 13. 5°. Cyclorotation was > 2° in 60. 2% eyes and > 5° in 21. 0% eyes. (Table 1).
Table 1 Case number Mean absolute value of cyclorotation Results Maximum of cyclorotation Percentage of excyclotorsion Measurement method Chernyak 51 eyes/26 patients Approximate 2° 56% eye >2° 21. 6% eye >5° 9. 5° 79. 2% Iris registration Kim et al. 140 eyes/70 patients 2. 59° ± 1. 91° >50% eye >2° 13. 0% eye >5° 9. 5° 54. 3% Iris registration Swami et al. 240 eyes/169 patients 4. 1° ± 3. 7° 65% eye >2° 28% eye >6° 16° N/A* Marking at limbus Hou et al. 86 eyes/93 patients 3. 22° ± 2. 53° 60. 2% eye >2 ° 21. 0% eye >5° 13. 5° 60. 8% Iris registration *N/A: non-available
Conclusions • Approximate 1 degree of rotational change between dilated and undilated pupil may be due to the torsional or shifting change of iris features after dilatation when using an iris registration technique or head position change between each measurement. • Significant postsurgical aberrations could be induced in > 2 degrees rotational misalignment. 2 Moreover, alignment of wavefront-guided treatments should be better performed with a torsional precision of approximately 1°. 2 According to our results, the amount of positioninduced cyclorotation is significantly greater than the rotational misalignment between pupil changes and more than 60% patients can have a significant negative effect on the outcome of laser refractive surgery if the rotation is not corrected, especially when dealing with high astigmatism and high-order aberrations. Therefore, iris registration technique can achieve a more accurate correction than rough centration by the surgeon’s judgment.
References 1. 2. 3. 4. 5. 6. 7. 8. Chernyak DA. Cyclotorsional eye motion occurring between wavefront measurement and refractive surgery. J Cataract Refract Surg 2004; 30: 633– 638 Bueeler M, Mrochen M, Seiler T. Maximum permissible torsional misalignment in aberrationsensing and wavefront-guided corneal ablation. J Cataract Refract Surg 2004; 30: 17– 25 Smith EM Jr, Talamo JH. Cyclotorsion in the seated and supine patient. J Cataract Refract Surg 1995; 21: 402– 403 Becker R, Krzizok TH, Wassill H. Use of preoperative assessment of positionally induced cyclotortion: a video-oculographic study. Br J Ophthalmol 2004; 88; 417 -421 Suzuki A, Maeda N, Watanabe H, Kiritoshi A, Shimomura Y, Tano Y. Using a reference point and videokeratography for intraoperative identification of astigmatism axis. J Cataract Refract Surg 1997; 23: 1491 -1495 Swami AU, Steinert RF, Osborne WE, White AA. Rotational malposition during laser in situ keratomileusis. Am J Ophthalmol 2002; 133: 561– 562 Chernyak DA. Iris-based cyclotorsional image alignment method for wavefront registration. IEEE Trans Biomed Eng 2005; 52: 2032 -2340. Kim H, Joo CK. Ocular cyclotorsion according to body position and flap creation before laser in situ keratomileusis. J Cataract Refract Surg 2008; 34: 557– 561
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