Postoperative capsular bag shrinkage may lead to change in the axial intraocular lens (IOL) position,1–4 with an unsatisfactory refractive outcome after uneventful biometry and cataract surgery, or may lead to IOL decentration and tilt.5 Intraocular lens design, especially haptic and optic edge designs, as well as IOL material may be responsible for changes in the postoperative IOL position. Knowledge of the factors influencing a postoperative IOL position change may allow better prediction of the final IOL position in patients and could, therefore, improve IOL power calculation. The results could help determine the earliest time the anterior chamber depth (ACD) stabilizes after cataract surgery and thus the time for the first spectacle prescription. The outcome could also help refine IOL design for optimal, predictable performance in the capsular bag.
Increases6,7 as well as decreases8 in the ACD after posterior chamber IOL implantation have been reported. In a study of the AR40e IOL (AMO),1 significant forward movement was found from the first day to the first week after surgery. However, it is not clear when this movement took place, directly after surgery or continuously within the first week.
To describe the dynamics of capsular bag shrinkage after surgery, Strenn and coauthors9 used an open poly(methyl methacrylate) (PMMA) capsular tension ring (CTR) with eyelets that can be visualized by gonioscopy to allow measurement of the capsular bag diameter (CBD). The time course of the distance between the eyelets was assessed to quantify the postoperative shrinkage of the capsular bag.
This study examined the time course of change in the axial IOL position, especially within the first postoperative week. The influence of the capsular bag size on the change in axial IOL position was also assessed.
Patients and Methods
Twenty-nine eyes of 29 patients were included in this prospective study performed at the Department of Ophthalmology at the Vienna General Hospital. Inclusion criteria were age-related cataract and good overall physical condition. Exclusion criteria were history of ocular trauma or intraocular surgery, laser treatment, diabetes requiring medical control, pseudoexfoliation syndrome, glaucoma, uveitis, and retinal pathology that would make a postoperative visual acuity of 20/40 (0.5) or better unlikely. All the research and measurements followed the tenets of the Helsinki Declaration, and informed consent was obtained from all patients.
One surgeon (O.F.) performed all cataract surgery in a standardized fashion. Each patient received a Sensar OptiEdge AR40e IOL in the examined eye. The IOLs were implanted with an injector system through a temporal 3.5 mm self-sealing incision after phacoemulsification. After the IOL was implanted in the capsular bag, an ophthalmic viscosurgical device was used to irrigate the capsular bag, including behind the IOL, and was meticulously aspirated. The AR40e is a foldable, hydrophobic acrylic, open-loop 3-piece IOL with an optic diameter of 6.0 mm, an overall length of 13.0 mm, and a haptic angulation of 6 degrees. The capsulorhexis rim covered the edge of the IOL along the entire circumference in all cases.
In study 1, 15 eyes of 15 patients had cataract surgery with implantation of an AR40e IOL, as described above. Follow-up examinations were performed 2 hours; 1, 3, and 5 days; 1 week; and 1 month after surgery. On each occasion, the ACD (defined as the distance from the posterior corneal surface to the anterior IOL surface) was measured.
In study 2, the same surgical technique was performed in 14 eyes of 14 patients. Additionally, a PMMA CTR (type 14, Morcher Inc.) was inserted in the bag before IOL implantation. Since axial eye length correlates with capsular bag size,10 primarily myopic and hyperopic patients were included in study 2 (dioptric range of IOLs, 13.0 to 29.0). Follow-up examinations included ACD measurements 2 hours, 1 day, 1 week, and 1 month postoperatively and gonioscopy 1 day, 1 week, and 1 month postoperatively.
Anterior chamber depth measurements were performed with the dual-beam version of partial coherence interferometry (PCI), which has been described.11–14 Partial coherence interferometry yields optical distances that have to be converted to geometrical ones by dividing them by the group refractive index of the respective medium. A group refractive index of 1.3454 was used for the aqueous humor.15
The CTR eyelets at the ends of the ring were visualized through the gonioscopic mirror of a 3-mirror contact glass, and the distance between the 2 eyelets was measured by adjusting the slit height.9 The capsular bag circumference was obtained by adding the measured distance between the eyelets (E) multiplied by the magnification factor (M=1.12) to the length of the CTR (30.3 mm).10 From this, the capsular bag diameter (CBD) was calculated (CBD=circumference/π).
Since the change in refraction induced by a change in ACD in a pseudophakic eye depends on several factors (eg, IOL power, axial length, and absolute ACD values), the refractive shift was calculated individually for each eye in the study. This method is more exact than using a rule of thumb for all eyes. To do this, recently introduced ray-tracing software (Okulix, Der Leu Technische Produkte) was applied to the biometric data.16 Input variables were axial eye length and keratometry readings from preoperative biometry (IOLMaster, Carl Zeiss Meditec AG), lens type and power (characteristics of anterior and posterior IOL optic curvature, optic thickness for all IOL powers, and refractive index of the optic material are included in the software), and ACD was measured with PCI 2 hours, 1 day, 1 week, and 1 month after surgery. The refractive shift induced by the IOL movement was calculated for each eye.
Data are presented as mean values and standard deviations. The CBD was correlated with the change in ACD using the Pearson correlation coefficient. A P value of 0.05 or less was considered statistically significant. The precision of PCI is defined as the standard deviation of multiple recorded consecutive measurements of the ocular distance under investigation.
The mean age of the patients was 71 years (range 41 to 83 years).
The change in ACD in the first postoperative month in study 1 is presented in Figure 1. Two hours after surgery, the mean ACD was 4.185 mm ± 0.288 (range 3.772 to 4.849 mm). Significant forward movement was seen after that (Table 1), especially in the first week (P<.005), leading to a mean ACD of 3.942 ± 0.309 mm (range 3.592 to 4.491 mm) after 1 month.
As there was a high interindividual variability of IOL movement in the early postoperative period, ACD changes were divided into 3 groups: IOLs showing a forward movement of more than 0.1 mm over the first postoperative week (Figure 2, top), IOLs showing a slight forward movement of less than 0.1 mm (Figure 2, middle), and IOLs showing an initial backward movement (Figure 2, bottom).
In study 2, with the CTR, the mean ACD was 4.314 ± 0.540 mm (range 3.540 to 5.732 mm) 2 hours postoperatively. There was a significant decrease in ACD of −81.00 ± 77.00 μm (P<.005) in the first postoperative week, with only slight changes after that (Table 2). One month after surgery, the mean ACD was 4.100 ± 0.559 mm (range 3.165 to 5.521 mm). Figure 3 shows the change in ACD compared to the change in CBD in the first month. The mean CBD on the first postoperative day was 10.24 ± 0.24 mm (range 9.64 to 10.59 mm). In the first week, the CBD shrank by −0.17 ± 0.11 mm (range −0.16 to 0.00 mm) (P<.005). The subsequent decrease of −0.10 ± 0.19 mm (range −0.54 to 0.29 mm (P=.07) from 1 week to 1 month led to an overall CBD shrinkage of −0.29 ± 0.15 mm (range −0.55 to −0.07 mm) (P<.005).
The correlation between the change in ACD from the first postoperative day to 1 month and the (baseline) CBD at 1 day was 0.67 (P<.05). Myopic eyes with a large capsular bag showed less ACD change postoperatively (Figure 4).
In the first postoperative week, the myopic shift was −0.31 ± 0.19 D (range −0.70 to −0.05 D) (P<.001) in study 1 and −0.20 ± 0.16 D (range −0.62 to −0.01 D) (P<.001) in study 2. After that, up to the first month, the refractive shift was −0.13 ± 0.13 D (range −0.41 to 0.04 D) (P<.05) and −0.07 ± 0.11 D (range −0.21 to 0.10 D), respectively (P=.07).
This study showed a linear forward movement of the IOL in the first postoperative week, followed by a relatively stable IOL position. The high interindividual variability of IOL movement in the early postoperative period was correlated with the CBD. Myopic eyes with large capsular bags induced less IOL shift in the early postoperative period than hyperopic eyes with small capsular bags.
The postoperative ACD contributes to the refractive outcome after cataract surgery. Capsular bag shrinkage in combination with IOL design, especially haptic and optic edge designs, and IOL material may be responsible for changes in the postoperative IOL position.4,6–8 A recent study assessed the change in ACD with a 3-piece silicone IOL with nonangulated modified C-loop haptics and compared it with the change with an IOL with a sharp optic-edge design.2 Both IOLs showed a slight forward movement in the first postoperative week (sharp edge, −72 ± 114 μm; round edge, −50 ± 187 μm) with no significant difference between the edge designs, suggesting that the haptic angulation is more important. Another study reported a significant forward movement of a 3-piece acrylic IOL with 10-degree angulated modified J-loop haptics in the first postoperative week, but only minimal changes in the axial position of a 1-piece, open-loop, acrylic IOL with no haptic angulation.3 Data from a study of 104 eyes show a significant forward movement of −139 ± 74 μm (range −284 to 10 μm) in the first week with the AR40e IOL, the 3-piece lens used in this study.1 Although this study assessed a smaller population of 29 eyes, the results are similar to those obtained earlier.
One important factor in the initial forward movement of the optic in the first postoperative week is the loss of haptic memory in the case of an angulated IOL. When the IOL is positioned in the capsular bag, the haptic loops are compressed by the bag equator since the CBD is smaller than the overall diameter of the IOL. Over time, the compression force needed to keep the loops in this position decreases. The loss of haptic memory influences not only the IOL's overall diameter but also the angulation of the haptics. Several laboratory studies have performed lens compression tests to assess the loop memory of haptic material and design.17,18 In vitro studies of the AR40 IOL, in which the IOLs were compressed to a 10.0 mm diameter, showed a haptic compression force decay of more than 50% after the first day of compression (S. Lane, MD, “A Comparison of the Biomechanical Behavior of Foldable IOLs,” presented at the ASCRS Symposium on Cataract, IOL and Refractive Surgery, San Diego, California, USA, April 2001).
The high interindividual variability of IOL movement in the early postoperative period is related to the CBD, as confirmed by the good correlation between the change in IOL movement and the CBD. Large capsular bags showed only small changes in the IOL position in the first week. However, a pronounced forward shift of the IOL was detected with small capsular bags. This may be explained by the stronger compression of the haptics in a small bag, which leads to a more pronounced initial posterior placement of the IOL because of the 6-degree haptic angulation. After the loss of haptic memory in the first postoperative days, as described above, the IOL optic moves forward, as seen in this study.
Another factor responsible for axial IOL movement is the interaction between capsular bag fusion and the fibrotic reaction following IOL implantation that leads to capsular bag shrinkage. A CTR has been reported as a measuring gauge for quantification of capsule dimensions and postoperative capsule shrinkage.9 Our results of mean CBD are comparable to those in a previous study of 70 eyes.10 We found significant shrinkage of the CBD in the first postoperative week, which correlated with the forward IOL shift in this period and continued until the first month. Obviously, the influence of the CTR on the fibrotic capsule shrinkage and the concomitant change in IOL position cannot be excluded. However, a previous study with various types of CTRs shows no change in circumference with the PMMA ring used in this study.9 Comparing the change in ACD in both groups, there was no significant difference in IOL movement from 2 hours to 1 week and from 1 week to 1 month after surgery. Nevertheless, from 1 day to 1 week, a small but statistically significant difference was detected; the eyes without a CTR showed a more pronounced IOL forward movement (−149 ± 62 μm) than the eyes with a CTR (−80 ± 76 μm) (P=.01). The difference might also be explained by the higher variability in the CBD in the selected myopic and hyperopic eyes in study 2.
A myopic shift of about 0.25 D in the first postoperative week, followed by a relatively stable refraction was calculated with the ray-tracing software program.16 The following examples of some patients should underline the clinical relevance of the high variability of IOL movement found in this study: One hyperopic eye (axial eye length 21.0 mm) showed a remarkable decrease in ACD of 375 μm over the first month, causing a myopic shift of 0.76 D. Most of the shift (−0.62 D) took place within the first week. On the other hand, the ACD in a myopic eye (axial eye length 25.8 mm) decreased by only 89 μm. The resulting myopic shift of 0.08 in that eye is negligible.
In conclusion, the open-loop, 3-piece acrylic IOL in this study showed a linear forward movement in the first 5 days after surgery, presumably due to the loss of haptic memory of an angulated IOL following bag implantation. After 1 week, only slight changes in IOL position were detected, indicating a stable refraction. This may be clinically relevant for the early prescription of glasses with this IOL. We also demonstrated the influence of capsular bag size on the extent of IOL position change; myopic eyes with a large capsular bag showed less IOL movement postoperatively.
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