It is believed that after cataract surgery, an intraocular lens (IOL) with rigid haptics has better fixation in the lens capsule than an IOL with flexible haptics.1,2 Experimental studies show 1-piece poly(methyl methacrylate) (PMMA) IOLs have better centration and shape recovery capability after long-term compression than 3-piece IOLs with flexible haptics.3–5 A clinical study shows that decentration of a 1-piece PMMA IOL is less than that of a 3-piece PMMA IOL.6
Recently, 1-piece acrylic IOLs with soft acrylic loops (SA30AL and SA60AT, Alcon Surgical) have become commercially available. These lenses have extremely flexible open loops composed of soft acrylic polymers. The haptic compression force of 1-piece acrylic IOLs has been reported to be approximately one seventh that of 3-piece acrylic IOLs.7,8 Indeed, 1-piece acrylic IOLs easily sway in the capsular bag when a balanced salt solution is injected during surgery. Based on these findings, a major concern is postoperative stability of 1-piece acrylic IOLs in the capsular bag.
The objectives of this prospective study were to compare the degrees of IOL decentration, tilt, and longitudinal movement as well as anterior capsule contraction after cataract surgery between eyes that received a 1-piece acrylic IOL with soft acrylic loops and those that received a 3-piece acrylic IOL with rigid PMMA loops. To ensure the clinical relevance of the results, changes in refractive state were also examined.
Patients and Methods
All patients who were admitted consecutively to the Hayashi Eye Hospital for bilateral cataract surgery between July 25 and December 24, 2002, were screened for inclusion in this study. Exclusion criteria were ocular pathology other than cataract that might cause loss of visual acuity, history of ocular surgery or inflammation, eyes scheduled for extracapsular cataract extraction (ECCE), a pupil diameter smaller than 6.0 mm after mydriasis, pseudoexfoliation syndrome, diabetes, difficulties in the analysis, and not available for follow-up. Screening was continued until 60 patients who were to have phacoemulsification and IOL implantation were recruited.
The institutional review board approved the study protocol, and all patients provided informed consent. The patients were randomly assigned the day before surgery to 1 of 2 groups: 30 patients who were to receive a 1-piece acrylic IOL (SA30AL) in the left eye and a 3-piece acrylic IOL (MA60BM, Alcon Surgical) in the right eye and 30 patients who were to receive a 1-piece IOL in the right eye and a 3-piece IOL in the left eye. The SA30AL is a 5.5 mm 1-piece acrylic IOL with soft acrylic loops and the MA60BM, a 6.0 mm 3-piece acrylic IOL with PMMA modified C-loops.
A clinical research coordinator generated randomization codes with equal numbers using random number tables and kept the assignment schedule concealed until all data were collected to ensure allocation concealment. The patients, examiners, and operating room staff who allocated the IOL to the patients were unaware of the type of IOL implanted. The data analyst, who was also the surgeon (K.H.) in all cases, did not know the type of IOL before surgery.
All surgeries were performed using a previously described technique.9 First, a 3.0 or 3.5 mm straight scleral incision was made for IOL implantation. Then, a continuous curvilinear capsulorhexis approximately 5.5 mm in diameter was created using a bent needle. Thorough hydrodissection, phacoemulsification of the nucleus, and aspiration of the residual cortex were performed. The wound was enlarged to 3.2 mm with a steel keratome for 1-piece IOL implantation and to 4.1 mm for 3-piece IOL implantation. The lens capsule was inflated with sodium hyaluronate 1% (Healon), after which the IOL was placed in the capsular bag with a forceps or an injector. After IOL insertion, the viscoelastic material was thoroughly evacuated. No sutures were placed in any eye. All surgeries were uneventful, and the IOLs were implanted accurately in the capsular bag.
The degree of IOL decentration and tilt, the anterior chamber depth (ACD), and the area of the anterior capsule opening were measured using Scheimpflug videophotography (EAS-1000, Nidek) 3 days and 1, 3, and 6 months after surgery (Figure 1); measurement methods of these parameters have been described.10,11 At each visit, the visual acuity, spherical equivalent (SE), and best corrected decimal visual acuity were recorded. The decimal acuity was converted to logMAR values for statistical analysis. The spherical and cylindrical powers were measured in diopters using an autorefractometer (KR-7100, Topcon). The SE value was determined as the spherical power minus half the cylindrical power in diopters. All measurements were done by experienced ophthalmic technicians who were unaware of the aim of the study.
Normality of data distribution was assessed using the Kolmogorov-Smirnov test. Differences in the degrees of IOL decentration and tilt and of other continuous variables that showed normal distribution between the 1-piece and 3-piece acrylic IOL groups were compared using the unpaired t test. Continuous variables without a normal distribution were compared using the Mann-Whitney U test. A repeated-measures analysis of variance was used to compare differences at the various examinations. When a statistically significant difference was found, the differences between time points were further compared using the Mann-Whitney U test with Bonferroni adjustment. Discrete variables were compared using the Fisher exact probability test. Differences with a P-value less than 0.05 were considered statistically significant. The results are given as the mean ± SD.
Of the 372 patients screened for inclusion, 139 declined to be enrolled and 173 were not enrolled according to the exclusion criteria. The major reasons for exclusion were other ocular pathology (68 patients, 39.3%), history of ocular surgery or inflammation (9, 5.2%), eyes scheduled for ECCE (7, 4.0%), poor mydriasis (4, 2.3%), pseudoexfoliation syndrome (11, 6.4%), diabetes (18, 10.4%), and not available for follow-up (56, 32.4%). Accordingly, 60 patients were enrolled in the study. There was no significant difference in age (P=.3904) or sex (P>.9999) between the patients who were enrolled and those who were excluded. By 3 months after surgery, 3 patients did not appear for follow-up, 2 because of a scheduling conflict and 1 because of being hospitalized; furthermore the EAS-1000 analysis presented problems in 1 patient. Thus, 56 patients (93.3%) completed the 6-month follow-up.
The mean age of the patients was 70.7 ± 9.2 years (range 36 to 88 years). There were 18 men (32%) and 38 women (68%). There was no statistically significant difference between the 1-piece IOL group and 3-piece acrylic IOL group in the ACD (P=.5576) or axial length (P=.3765) before surgery.
Figures 2 and 3 show the mean degrees of decentration and tilt, respectively, in the 1-piece and 3-piece IOL groups. There was no significant change during follow-up in decentration or tilt in the 1-piece group (P=.4918 and P=.2416, respectively) or the 3-piece group (P=.4591 and P=.2965, respectively). There was no significant difference between the groups in IOL decentration or tilt throughout the follow-up.
Figure 4 shows the change in the mean ACD. The ACD did not change significantly in the 1-piece IOL group (P=.0950), while the anterior chamber showed significant shallowing in the 3-piece group (P<.0001). In the 3-piece group, the ACD 3 days after surgery was significantly greater than the depth at the other time points. The anterior chamber was more shallow in the 1-piece group than in the 3-piece group; however, the difference was statistically significant only at 3 days (P<.0001).
Figure 5 shows the changes in the mean SE. The SE did not change in the 1-piece group (P=.5367), while the SE showed a significant myopic shift of approximately 0.4 diopter (D) in the 3-piece group (P<.0001). In the 3-piece group, the SE at 3 days was significantly less myopic than at the other time points. The SE was more myopic in the 1-piece group than in the 3-piece group; however, the difference was significant only at 3 days (P=.0036).
There was no significant difference between the 2 groups in the mean area of the anterior capsule opening or in the mean percentage reduction in the anterior capsule opening area (Figure 6) throughout the follow-up.
Figure 7 shows retroillumination photographs of bilateral eyes of a representative patient 1 month after surgery. In the eye with a 1-piece acrylic IOL (Figure 7, left), the posterior capsule was completely clear. In the fellow eye with a 3-piece acrylic IOL (Figure 7, right), posterior capsule folds parallel to the axis of the apices of the loops were apparent.
Our study demonstrated that IOL decentration and tilt in eyes with a 1-piece acrylic IOL did not change after surgery and was similar to the decentration and tilt in eyes with a 3-piece IOL. In addition, no eye in either group had marked decentration (>1.0 mm) or tilt (>5.0 degrees). These results indicate that the 1-piece acrylic IOLs were well centered in the capsular bag.
The percentage of area reduction in the anterior capsule opening in eyes with the 1-piece acrylic IOL was approximately 10%, a change similar to that in eyes with the 3-piece acrylic IOL. Based on previous studies,11,12 this percentage is also similar to that in eyes with a PMMA IOL10 and less than in eyes with a silicone10 or hydrogel IOL. Thus, the 1-piece acrylic IOL appears to withstand substantial postoperative capsular shrinkage.
To further prove clinical equivalence between the 1-piece and 3-piece acrylic IOLs, we calculated the power of our study to detect a clinically meaningful difference. When we considered decentration of 0.1 mm and tilt of 1.0 degree to be differences of a clinically meaningful magnitude, the power was calculated as 93% for decentration and more than 99% for tilt. Furthermore, when we considered a percentage of anterior capsule contraction of 10% to be a clinically meaningful difference, the power was determined to be 93%. Thus, the powers were high enough to detect differences of a clinically meaningful magnitude.
The haptic compression force of 1-piece acrylic IOLs is reported to be significantly less than that of 3-piece acrylic IOLs.7,8 Despite the flexible haptics, decentration, tilt, and anterior capsule contraction with the 1-piece acrylic IOL were similar to the results with the 3-piece acrylic IOL and, based on previous studies,10,11 to results with 1-piece PMMA IOLs. This suggests that rigid haptics may not be necessary for capsule fixation of an IOL as long as the lens capsule is not pathologic.
A previous study6 found that decentration of a 1-piece PMMA IOL was less than that of a 3-piece IOL, although the difference was only 0.1 mm.6 The results of the present study appear to be inconsistent with these previous results. The possible inconsistency between the 2 studies may be reasonably explained by the following: Rigidity of the haptics, which is basically determined by haptic compression force, is not necessarily associated with better IOL centration. We now believe that shape-recovery capability is a more relevant factor in good IOL centration. Indeed, it has been shown that 1-piece acrylic and PMMA IOLs have a better shape-recovery ratio than 3-piece IOLs (unpublished data).
In the present study, the ACD did not change postoperatively in eyes with the 1-piece IOL. However, eyes with the 3-piece IOL had shallowing of the anterior chamber in the early postoperative period. These results suggest that axial movement of the 1-piece IOL in the capsular bag is less than movement of the 3-piece IOL. Consequently, the SE in eyes with the 1-piece IOL did not change after surgery, while the SE in eyes with the 3-piece IOL had a myopic shift of approximately 0.4 D. Thus, the postoperative refractive status was more stable with the 1-piece acrylic IOL than with the 3-piece IOL.
The diameter of the empty capsular bag is approximately 10.0 to 11.0 mm.13–15 However, the overall length of most currently used IOLs is 12.5 to 13.0 mm. Therefore, the haptics are contracted by the lens capsule because of the discrepancy in diameter.16 An experimental study8 found that axial displacement of the optic, which occurs when the open loops are compressed, is much less with 1-piece acrylic IOLs than with the other types of IOLs. The probable reason is that most of the compression force is by the soft loops, which leads to less axial movement of the optic in the clinical situation.
In conclusion, IOL decentration and tilt and anterior capsule contraction in eyes that received a 1-piece acrylic IOL were similar to the results in eyes that received a 3-piece acrylic IOL. Furthermore, longitudinal movement of the 1-piece IOL was significantly less than movement of the 3-piece IOL, which led to less myopic change. This shows that fixation of the 1-piece acrylic IOL is stable when the IOL is accurately placed in the capsular bag. In this series, the target postoperative refraction was slight myopia. The target was based on our long-term experience that refraction immediately after 3-piece acrylic IOL implantation tends to be more hyperopic than expected. However, the target with the 1-piece acrylic IOL should be emmetropia. Further study is necessary to examine the stability of the 1-piece acrylic IOL in eyes with pathology such as retinitis pigmentosa,17 pseudoexfoliation syndrome,18 or angle-closure glaucoma.19
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