Postoperative rotation of a 3-piece loop-haptic acrylic intraocular lens

ARTICLE

Postoperative rotation of a 3-piece loop-haptic acrylic intraocular lens Mario J. Saldanha, DO, MRCOphth, FRCS, Larry Benjamin, FRCOphth, FRCS, Chetan K. Patel, FRCOphth

PURPOSE: To assess the rotational stability of the AcrySof MA60BM acrylic intraocular lens (IOL) with and without aspiration of residual lens epithelial cells (LECs). SETTING: Stoke Mandeville Hospital, Aylesbury, United Kingdom. METHODS: Routine phacoemulsification was performed with (Group A) or without (Group B) aspiration of residual LECs (random allocation). The baseline position of the IOL was determined from a video frame acquired at the conclusion of surgery. Postoperative IOL position was documented using digital retroillumination images at 2 weeks and 3 months. Early IOL rotation (surgery to 2 weeks postoperatively) was graded as mild (30 degrees) by semiobjective comparison of the images. Late IOL rotation (2 weeks to 3 months) was measured more precisely using purpose-designed software. RESULTS: Group A had incomplete aspiration of residual LECs. The only between-group demographic difference was significantly more men in Group A than in Group B (P.05). Between 2 weeks and 3 months, counterclockwise rotation occurred in 50% of all cases. CONCLUSIONS: Postoperative rotation of the loop-haptic IOL was stable; LEC clearance did not adversely affect performance. The tendency toward counterclockwise rotation would not preclude good performance of a toric model of the IOL. J Cataract Refract Surg 2009; 35:1751–1755 Q 2009 ASCRS and ESCRS

Approximately 20% of patients having routine cataract surgery have visually significant corneal astigmatism.1 To achieve emmetropia, these patients may have combined corneal refractive surgery and

Submitted: March 5, 2009. Final revision submitted: May 1, 2009. Accepted: May 5, 2009. From Oxford Eye Hospital (Saldanha, Patel), Oxford, and Stoke Mandeville Hospital (Benjamin), Aylesbury, United Kingdom. No author has a financial or proprietary interest in any material or method mentioned. Corresponding author: Chetan K. Patel, FRCOphth, Oxford Eye Hospital, West Wing, John Radcliffe Hospital, Headley Way, Headington, Oxford OX3 9DU, United Kingdom. E-mail: ckpatel@ btinternet.com. Q 2009 ASCRS and ESCRS Published by Elsevier Inc.

spherical intraocular lens (IOL) implantation or toric IOL implantation alone. Although plate-haptic and loop-haptic toric IOLs are in use, results in a study by Chang2 suggest that loop-haptic acrylic toric IOLs are more stable than plate-haptic silicone toric IOLs in terms of intraocular rotation, especially in the early postoperative period. Our present study discusses the theoretical basis for postoperative IOL rotation and concludes that the interaction between the biomaterial of the IOL optic and the anterior capsule could affect rotational stability through variation in friction. The friction could be affected by the presence of residual lens epithelial cells (LECs) and the associated postoperative metaplastic response. The purpose of this study was to document the postoperative rotational stability of a nontoric IOL with an acrylic optic and loop haptics and to determine the effect of aspirating residual LECs on postoperative IOL rotation. 0886-3350/09/$dsee front matter doi:10.1016/j.jcrs.2009.05.024

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POSTOPERATIVE ROTATION OF ACRYLIC IOL WITH LOOP HAPTICS

PATIENTS AND METHODS Patients attending for cataract surgery at Stoke Mandeville Hospital, a district general hospital in the United Kingdom, were prospectively assessed for the study, which had full institutional review board approval (reference NC849). Exclusion criteria included previous intraocular surgery, evidence of previous uveitis, and the presence of pseudoexfoliation, iridodonesis, diabetes, or marked corneal opacity. Patients were included in the study if they provided informed consent. The comparative demographics and preoperative clinical variables in this cohort have been published.3 Variables included visual acuity, postoperative inflammation, and capsulorhexis size. Preoperatively, the pupil was dilated with cyclopentolate 1% and phenylephrine 10%. Tetracaine 0.5% was used to achieve ocular surface anesthesia. In the operating room with the patient seated upright, the head was stabilized in a standardized position before the 6 o’clock meridian on the limbal conjunctiva was dried and marked. Peribulbar anesthesia was then administered. Details of the surgical technique have been published.3 Briefly, the same surgeon (C.K.P.) performed surgery using a standard bimanual clear corneal phacoemulsification technique, including bimanual aspiration of soft lens material using separate disposable irrigating and aspirating handpieces (Steriseal, Maersk Medical) inserted through 2 paracenteses 180 degrees apart. Cases in which the capsulorhexis was incomplete or in which posterior capsule rupture occurred were excluded before patients were randomized to bimanual aspiration of residual LECs (Group A) or to no aspiration of residual LECs (Group B). An AcrySof MA60BM IOL (Alcon, Inc.) was implanted in the bag. A video clip of the eye was acquired before a subconjunctival injection of betamethasone 2 mg and gentamicin 20 mg was given. Postoperatively, dexamethasone 0.1% and chloramphenicol 0.5% were used 4 times a day for 2 weeks and then tapered over 2 weeks or as clinically indicated. Patients were assessed 2 weeks and 3 months after surgery to determine early IOL rotation and late IOL rotation, respectively, which were the primary outcome measures. Rotation was assessed using digital imaging analysis as previously described.4 Statistical analysis was performed on a Macintosh computer (Apple Inc.) using JMP statistical software (SAS Institute Inc.). Categorical data were compared using the chi-square test. The Mann-Whitney U test was used to compare mean late rotation between groups. A P value less than 0.05 was considered significant.

RESULTS One hundred six patients entered the study. Four patients had an incomplete capsulorhexis and 2 had a posterior capsule rupture, leaving 100 eyes for randomization. Demographics

significant differences between the 2 groups in distribution of eye color, axial length (AL), IOL power, and emulsification-parameter distribution.

Early and Late Rotation The proportion of patients not included in the analysis of early or late IOL rotation (for failure to attend follow-up or IOL was not imaged adequately) was not different between the 2 IOL groups (early rotation, P Z .41; late rotation, P Z .81) (Table 1). Early IOL rotation was mild in 75 (91.5%) of all eyes and moderate in 7 eyes (8.5%). There were no cases of severe early IOL rotation. The mean late rotation in all cases was 0.03 degrees G 3.06 (SD) (range 7.8 to 8.7 degrees). Table 2 shows a comparison of IOL rotation between Group A and Group B. Lens epithelial cell clearance did not affect IOL rotation.

DISCUSSION Astigmatism greater than 1.50 diopters (D) occurs in approximately 22% of cataract patients and astigmatism greater than 4.00 D, in approximately 2%.1,5 Targeting astigmatism during cataract surgery facilitates emmetropia as the refractive outcome. If a spherical IOL is used, correction is achieved by creating limbal or corneal incisions or by performing laser refractive surgery postoperatively. Incisional approaches are associated with variable refractive outcomes. Toric IOLs are reported to provide less variable results if they do not rotate postoperatively. The magnitude of cylinder corrected is inversely proportional to the degree of axis misalignment between 0 degree and 30 degrees.6 It has been estimated that approximately 1 degree of off-axis IOL rotation results in a loss of up to 3.3% of IOL cylinder power. Complete loss of cylinder power occurs when the IOL is misaligned by more than 30 degrees.7 Although cylinder is not fully corrected when rotation is less than 30 degrees, rotation up to this level was well tolerated in clinical studies of 2.00 D and 3.00 D toric IOLs.8,9

Table 1. Comparison of follow-up data. Number (%) Variable

Although there were no statistically significant differences between the 2 IOL groups in mean age, there was a trend toward more right-eye operations and there were significantly more men in Group A than in Group B (P!.05). There were no statistically

Lost to follow-up at 2 weeks Lost to follow-up at 3 months Inadequate imaging for early rotation Inadequate imaging for late rotation

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Group A (n Z 46)

Group B (n Z 50)

1 (2.2) 2 (4.4) 7 (15.2) 1 (2.2)

0 3 (6) 5 (10) 2 (2)

POSTOPERATIVE ROTATION OF ACRYLIC IOL WITH LOOP HAPTICS

Table 2. Comparison of IOL rotation. Variable

Early rotation (% eyes)* Mild Moderate Mean late rotation (degrees) G SD† Cases with late counterclockwise rotation, n (%)†

Group A

Group B

P Value

.29 92.9 85.7 7.1 14.3 0.14 G 2.77 C0.20 G 3.33 23 (53.5)

21 (46.7)

.29 .44

*Group A, n Z 42; Group B, n Z 42 † Group A, n Z 43; Group B, n Z 45

The theoretical basis for IOL rotation is worth discussing. Translational stability of the IOL improved significantly after the development of capsulorhexis. Several clinical and ex vivo studies show that IOL decentration is less common when there is symmetric in-the-bag IOL fixation.10,11 If rotation occurs when the IOL is entirely sequestered in the bag, it results in torque, which acts on the IOL. When the center of the torque and the center of the IOL coincide, it generates rotation alone. If this were not the case, translation would be expected in addition to rotation. The forces generating torque are gravity and those resulting from ocular movement and events associated with epithelial proliferation. The tension developed in the haptics and friction, which can be equatorial or central, resist this torque. Central friction is thought to be caused by the interaction between the capsule and the anterior and posterior surfaces of the IOL optic. Rotational instability and the possible need for early repositioning of plate-haptic toric IOLs12 led to the evaluation of the AcrySof SN60TT toric IOL in a U.S. Food and Drug Administration (FDA) clinical trial (Available at: http://www.fda.gov/cdrh/pdf/ p930014s015.html. Accessed June 24, 2009), which assessed IOL rotation, residual astigmatism, uncorrected distance visual acuity (UDVA), and spectacle freedom for distance vision. Twelve months after surgery, the toric IOL had excellent rotational stability in the capsular bag, with mean rotation of fewer than 4 degrees from the axis of initial IOL placement. Chang’s2 comparative study of 2 FDA-approved toric IOLs found that the AcrySof SN60TT IOL was stable; 90% of the IOLs were aligned within G5 degrees of the astigmatic axis. Only 1% had rotated more than 10 degrees off axis. The AcrySof SN60TT IOL was modeled on the 1-piece nontoric AcrySof SA60AT. Weinand et al.13 assessed the rotational stability of the nontoric version

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and found rotation between 0.58 degrees and 1.08 degrees (range 0.10 to 1.88 degrees) in 47% of patients. Similarly, in our study of the 3-piece version of the nontoric IOL, approximately 91% of patients had mild early rotation. The mean late rotation was 0.03 G 3.06 degrees. This shows that the AcrySof MA60BM IOL is as stable as the AcrySof SA60AT IOL. Weinand et al.13 used a precise photographic method to measure the amount of postoperative rotation between 2 serial examinations (1 day postoperatively and 6 months later) in 23 eyes with a nontoric AcrySof SA60AT IOL. However, their images were affected by inclination of the patient’s head, which in turn affected the angle of measurement of the axis. They were also limited by the learning curve of the method and poor quality of the images obtained through the operating microscope. Viestenz et al.14,15 and Quentin and Gene´e.16 used simultaneous slide projection to evaluate the rotation of the toric IOL. This method is more accurate in calculating rotational stability because it accounts for autorotation of the eye up to 11.5 degrees (mean 2.3 degrees).15,17 In the present study, early rotation was determined using 2 imaging systems with categorical grading as a reasonable compromise between accuracy and simplicity.4 Late rotation was measured using a continuous variable because our imaging system ensures accurate image registration between visits. Large-diameter capsular bags might be associated with a reduction in equatorial friction for a given IOL and therefore a decrease in IOL stability. Using AL as an indicator of capsular bag size,18 we found no association between AL and rotation with the loop-haptic IOL. This is not surprising given that the haptic diameter was 13.5 mm. Chang’s study2 also found this to be true. The other variable that affects IOL rotation in uneventful cataract surgery is capsular bag shrinkage.19 This mainly occurs in the first 3 months after IOL implantation.20 Anterior capsule contraction in our patient cohort was previously reported.3 Using capsule contraction as an indication of LEC response, we found no relationship between IOL rotation and LEC proliferation in our assessment of the loop-haptic IOL. The initial IOL position could affect how gravity influences early rotation. Although we found no relationship between the 2 variables, the small size of our patient cohort could have significantly diminished the power of the study. The IOL we used has flexible open-loop haptics of a soft acrylic polymer.21–23 The haptic compression force of 1-piece acrylic IOLs is reported to be approximately one seventh that of 3-piece acrylic IOLs.

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POSTOPERATIVE ROTATION OF ACRYLIC IOL WITH LOOP HAPTICS

Although low compression forces are desirable, they may compromise IOL stability in the capsular bag. The more bulky haptics of the 1-piece AcrySof SA60AT IOL extend directly from the posterior optic surface, leaving a section of the circumference of the optic without a sharp edge. This could cause a loss of barrier function to migrating LECs in this region, which may result in less inhibition of posterior capsule opacification (PCO).24 This was confirmed by Leydolt et al.25 The AcrySof SA60AT IOL has a more curved anterior surface and a less curved posterior surface than the AcrySof MA60BM IOL. Second, the latter IOL is vaulted posteriorly and the former is not vaulted. This may result in a higher PCO rate in the eyes with the 1-piece model with a longer follow-up. This could also be true with the toric model because it is based on the 1-piece nontoric model. Bylsma26 found that inserting a silicone plate-haptic toric IOL (Staar Surgical Co.) so that its cylindrical anterior surface is in contact with the posterior capsule limits rotation. Although this technique improved UDVA, it may result in a theoretical loss of toric power of 8%. In our study, rotation of the acrylic loop-haptic 3piece IOL was minimal and there was increased IOL stability in the bag. This supports the hypothesis that the AcrySof MA60BM IOL could be developed into a toric model that would not only be stable but would also have decreased rates of PCO and anterior capsule contraction. The ideal toric IOL should not rotate at all, although it is argued that the effects of early instability could be easily corrected by modifying components of the IOL.27 There is evidence that 3-piece silicone toric IOLs have increased rotational stability.28 Similar considerations apply to acrylic loop IOLs. How the IOL could be modified while maintaining the ease with which it is implanted must be evaluated. Whatever change in haptic design is contemplated, the present study suggests that the AcrySof MA60BM platform could be used to develop a toric IOL. REFERENCES

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First author: Mario J. Saldanha, DO, MRCOphth, FRCS Oxford Eye Hospital, Oxford, United Kingdom