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Capsular bag shrinkage after implantation of a capsular bending or capsular tension ring

Kurz, Sabine MD; Krummenauer, Frank PhD; Hacker, Philipp MS; Pfeiffer, N. MD; Dick, Burkhard H. MD

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Journal of Cataract & Refractive Surgery: October 2005 - Volume 31 - Issue 10 - p 1915-1920
doi: 10.1016/j.jcrs.2005.06.046
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Abstract

Shrinkage of the capsular bag and capsulorhexis opening occurs frequently after cataract surgery. These complications can lead to intraocular lens (IOL) decentration, deformation, and tilting, resulting in reduced or double vision.1 These problems can be severe, especially with multifocal and toric lenses. Capsular bag shrinkage usually desists after 3 to 6 months.2

Capsular tension rings (CTRs) have proved valuable for an increasing variety of applications during the past 13 years, including for use in cases of zonular weakness and dialysis, pseudoexfoliation, lens colobomas, phacodonesis, and severe nystagmus in complicated anterior segment surgery.3–7 Furthermore, CTRs can be used to prevent lens decentration, for example, under conditions of nonideal capsulorhexis and implantation of a multifocal or aspheric IOL.8,9 Other indications include the reduction of posterior capsule opacification (PCO)10,11 in cases with a potential for IOL exchange, relief of primary posterior capsulorhexis, and iris defects.11–14 The aim of this prospective randomized study was to evaluate the influence of the implantation of a capsular bending ring (CBR) or a CTR on capsular bag shrinkage.

PATIENTS AND METHODS

Included in the study were 92 eyes of 92 patients (median age 78 years; range 37 to 93 years). Inclusion criteria were otherwise healthy eyes with cataract and an axial length (AL) between 21 mm and 26.5 mm. Other inclusion criteria were intact zonular apparatus and intact capsular bag. Exclusion criteria were intraoperative complications (ie, zonulysis, capsule tears), prior intraocular surgery, lenticular pseudoexfoliation, proliferative diabetic retinopathy, uveitis, corneal diseases, trauma, vitrectomy, neoplasm of the eye, and pupils <8.0 mm after medical mydriasis. Each patient signed informed consent forms. Preoperatively, slitlamp examination of the anterior and posterior eye segments was performed in mydriasis with tropicamide. Biometric characteristics such as AL and the anterior chamber depth were determined by means of the Zeiss IOLMaster. Following uneventful phacoemulsification, a capsular measuring ring (CMR) was implanted into all eyes. On the day of cataract surgery, eyes were randomized into 3 groups: The first group received a CMR and a CBR, the second group received a CMR and a CTR, and the third group served as a control sample and received only a measuring CMR.

The CMR first introduced by H.R. Koch (MicroPlex KMR 12, HumanOptics) is composed of soft and thin poly(methyl methacrylate) (PMMA) with round edges. It allows determination of the capsular bag in vivo (Figure 1).2 In its relaxed state, the CMR is a ring with a diameter of 12.0 mm, possessing measuring extensions at both ends instead of eyelets. Determining the relative positions of these extensions via slitlamp biomicroscopy allows for calculation of the diameter of the capsular bag. The measuring extensions are 0.2 mm thick; 1 of the 2 extensions has a 1.0 mm long transverse extension and represents an intraocular reference distance (Figures 2 and 3). The distance between the extensions can thereby be measured accurately. By using a conversion table, the current capsular bag diameter can be determined.2 When the capsular bag shrinks, the ends of the CMR move above each other. The CBR (Ø 11.0 mm, Nishi/Menapace 1E, Morcher) is an advancement over the CTR with a lateral height of 0.7 mm and a sharp-edged design. It is made of PMMA and has a diameter of 11.0 mm. A CTR made of PMMA and 11.0 mm in diameter was implanted (SR11, Acritec).

Figure 1.
Figure 1.:
In vivo slitlamp photograph of the CMR (type KMR12, HumanOptics). The rectangular end passed the other end due to capsular bag shrinkage. Both ends lie on the other; the actual capsular bag diameter is 10.0 mm.
Figure 2.
Figure 2.:
Scanning electron microscopy of the 2 ends of the CMR. One end measures exactly 0.2 mm in width, and the second rectangular end is 1.0 mm in width, acting as an intraocular measuring reference.
Figure 3.
Figure 3.:
The CMR at different compression stages (distance between the ends 2.0 mm in relaxed state and 1.7 mm in compressed state) and the resulting capsular bag diameter (12.0 mm versus 10 mm).

After ring implantation, a hydrophobic acrylic OptiEdge IOL (AR40e, AMO) was implanted in all eyes. The same surgeon (H.B.D.) performed all surgeries in this study. The following measurements were performed in all eyes: The capsular bag size and the horizontal and vertical capsulorhexis diameters were measured intraoperatively. The capsular bag diameter was assessed at the end of surgery when the CMR and the CBR or CTR and the IOL were in the bag, after complete removal of viscoelastic substance and after the chamber was filled with balanced salt solution (BSS). Intraocular pressure (IOP) after the anterior chamber was filled with BSS did not influence capsular bag diameter within a range of 5 to 35 mm Hg. The capsular bag diameter was measured after an IOP of 15 to 20 mm Hg was achieved. At visit 1 (1 to 3 days postoperatively), capsular bag size and capsulorhexis diameter were measured again after mydriasis by means of slitlamp biomicroscopy. Additionally, in all eyes, retroillumination photography was performed to confirm the slitlamp results by measuring with a ruler the distance between the measuring ends of the CMR and the capsulorhexis diameter. At visit 2 (1 month postoperatively) and visit 3 (3 months postoperatively), these parameters were measured again.

Primary Clinical End Point

The primary clinical end point of this investigation was intraindividiual difference (in millimeters) in capsular bag diameter on the day of surgery versus visit 3 (3 months after implantation). A clinically relevant shrinkage of capsular bag diameter was defined as a reduction of 0.3 mm or more in the statistical analysis plan of this investigation. This benchmark was introduced into sample size calculation, which affords predetermination of clinically relevant changes due to the intervention under consideration. Regarding the results of previous pivotal trials, a median shrinkage of 0.3 mm was expected and also corresponded to clinically relevant changes in capsular bag shrinkage (the latter indicating possible loss in clinical benefit after surgery). Clinical relevance was therefore defined before the start of the investigation.

Study Design

Ninety-two eyes of 92 patients were included to achieve a group size of more than 30 patients per subsample (CMR, CTR+CMR, CBR+CMR). Patients were randomized without stratification for possible confounders; random numbers were generated using SPSS software (release 10.0 for Windows). The sample size (n = 33) was chosen to achieve a statistical power of 80% or more in pairwise group comparisons along this investigation's primary clinical end point. A multiple significance level of 5% (ie, a pairwise comparison level of 1.7%) was determined in the statistical analysis plan to reveal a group difference ≥0.3 mm in the clinical end point as statistically significant in pairwise comparisons. The Independent Ethics Committee (Landesärztekammer Rheinland-Pfalz) approved this study design in March 2003.

Statistical Analysis

Data description was based on medians and quartiles for continuous end points and on absolute and relative frequencies for categorical end points. Comparisons of repeated measurements in continuous end points were evaluated by means of intraindividual differences and ratios. Graphic representation of continuous data was based on nonparametric box whisker plots, accordingly.

For significance comparisons in continuous repeated measurements, the sign test was applied, and the 2-sample Wilcoxon test was applied for group comparisons along continuous end points. Results of these tests were summarized with P values for which the evaluation of the primary clinical end point was due to a multiple significance level of 5%. Bonferroni correction was applied to the P values, which arose from pairwise comparison along the primary clinical end point. P values derived by analysis of secondary end points were not adjusted for multiplicity and therefore indicate local statistical significance.

RESULTS

Preoperatively, no statistically significant differences were found between groups in the AL (pooled median 23.3 mm, range 21.4 to 26.8 mm) and in the ACD (median 3.1 mm, range 2.8 to 3.4 mm). Postoperatively, no statistically significant differences were found in the capsulorhexis diameter within groups (intraindividual comparison by means of sign tests: group 1, P = .06; group 2, P = .56; group 3, P = .19) and between groups (interindividual comparison by means of 2-sample Wilcoxon tests, pooled median 5.5 to 5.6 mm, range 4.4 to 6.8 mm). The eyes implanted with the CBR (group 1) showed small but statistically significant shrinkage of the median capsular bag from 10.55 mm (range 9.85 to 11.4 mm) to 10.4 mm (range 9.7 to 11.6 mm) after 3 months (sign test, P = .023). The eyes with a CTR (group 2) showed a statistically significant shrinkage of the median capsular bag from 10.45 mm (range 9.95 to 11.25 mm) to 10.25 mm (range 9.15 to 10.85 mm) (P<.001). The largest shrinkage from a median of 10.45 mm (range 9.8 to 11.55 mm) to 10.03 mm (range 8.0 to 11.1 mm) was found in the control group (CMR alone, group 3, P<.001) (Table 1 and Figure 4).

Table 1
Table 1:
Medians and quartiles for capsular bag diameter distribution and intraoperative and 3-month postoperative measurements.
Figure 4.
Figure 4.:
Nonparametric box plots for the capsular bag shrinkage distribution 1 to 3 days and 1 and 3 months postoperatively (pre-post; mm) in groups 1 (CBR+CMR), 2 (CTR+CMR), and 3 (CMR). Horizontals indicate medians and quartiles (minimum and maximum values), which do not exceed a deviation from the quartiles of more than 1.5 times the interquartile ranges. ○ = statistical outliers with deviation from the quartiles of more than 1.5 times the interquartile range. = statistical extreme values with deviation from the quartile of more than 3 times the interquartile range.

For interindividual comparison between groups 1 and 3, a significantly larger shrinkage of the capsular bag was found in group 3 within the first 3 postoperative days (P = .01), after 1 month (P<.001), and after 3 months (P<.001). Comparing groups 2 and 3, no locally statistically significant differences were found within the first 3 postoperative days (P = .663) but was seen after 1 month (P = .008) and 3 months (P = .020). The latter group difference did not remain statistically significant after Bonferroni correction for multiplicity and was therefore due to local statistical significance. The median changes in the capsular bag diameter between groups are displayed in Figure 5.

Figure 5.
Figure 5.:
Nonparametric box plots for the relative shrinkage (post/pre) of the capsular bag diameter in groups 1 (CBR+CMR), 2 (CTR+CMR), and 3 (CMR). Horizontals indicate medians and quartiles (minimum and maximum values), which do not exceed a deviation from the quartiles of more than 1.5 times the interquartile ranges. ○ = statistical outliers with deviation from the quartiles of more than 1.5 times the interquartile range. = statistical extreme values with deviation from the quartile of more than 3 times the interquartile range.

DISCUSSION

Capsular tension rings are frequently used for the management of intraoperative complications such as zonulolysis. A CTR is able to reduce folds of the posterior lens capsule after IOL implantation and therefore helps to restore quality of vision. Moreover, it can be useful to support optimal centration of the IOL. The CBR is more rigid with a higher spring constant15 and is implanted to reduce PCO because of its sharp, OptiEdge design.16,17 Capsule opacification and capsular bag shrinkage may lead to IOL decentration and tilt. These complications can cause a decrease in visual acuity. Resistance to lens decentration and tilt and the occurrence of capsule opacification greatly depend on the design and the material of the IOL.18–20 The latter also influence the clinical presentation of capsular shrinkage.19 Particularly for toric IOLs, rotational stability and centering are necessary to achieve the desired refractive correction. Additionally, optimal centration is needed in aspheric as well as multifocal IOLs.

The goal of this prospective randomized masked study was to find out whether a CTR or a more rigid CBR is able to inhibit the forces of capsular bag shrinkage under normal circumstances. To our knowledge, this is the first report that analyzed the influence of a CTR compared with a CBR on capsular bag shrinkage. The influence of a CBR or CTR on capsular bag shrinkage over a 3-month follow-up was quantified. Shrinkage prevention of the capsular bag by the CBR, and to a lesser extent by the CTR, over 3 months was evaluated. The most shrinkage occurred between the first postoperative day and the first postoperative month. During a multivariate reanalysis of the data by means of multiple logistic regression modeling, no significant association was observed between the preoperative biometric characteristics (ie, AL, anterior chamber depth), capsulorhexis diameter, and capsular bag shrinkage. The differences in capsular bag shrinkage among groups (CBR+CMR, CTR+CMR, CMR only) were statistically significant but not clinically relevant in terms of the prespecified order of 0.3 mm. However, the median differences supported the expected tendency toward a preventive nature of CBRs and, to a somewhat lesser extent, of CTRs.

The CBR group showed few cases of capsular expansion within the first 3 postoperative months. This may be related to baseline being the intraoperative measurement, whereas postoperative examinations were performed at the slitlamp.

One limitation of this conclusion is that it can only be drawn for the specific types of CBRs, CTRs, and hydrophobic acrylic IOLs used in the study. Another point of view must be considered concerning the control sample: The CMR was used as a reference to determine the capsular bag size. Although it is made of a soft acrylic material, we do not know exactly how much capsular bag shrinkage is prevented by the CMR. In this case, one would expect even larger differences among treatment groups. At present, the CMR is used solely for clinical research purposes.

In another study,15 we compared the elastic boundaries of 14 commercially available CTRs. We found great differences in the spring constants among the various types and designs, ranging from 0.82 to 4.55 mN/mm. However, CTRs of the same model had comparable mechanical properties.

To date, exact measurement of the capsular bag diameter in vivo remains a challenge. Vass et al.21 measured the distance between the ends of a CTR through a gonioscopy lens by adjusting the slit height. This distance was added to the known length of the CTR to calculate the capsular bag circumference and from this, the capsular bag diameter by means of a regression formula. However, magnification of the cornea by 3-mirror contact glass and material depending magnification constrain the measuring results.

The method used in this study was first introduced by Tehrani et al.,2 who found, using a CMR, capsular bag shrinkage of almost 14% in 55 patients, from a mean of 10.53 mm at the time of surgery to 9.07 mm after 3 months. Capsular bag shrinkage in our control group (in which only a measuring ring was implanted) was only about 5%. Tehrani et al.2 found an increase in the capsular bag shrinkage with eyes of greater length. Therefore, only eyes with an AL of 21.5 to 26.5 mm were included in our study.

Nishi et al.16 investigated the inhibitory effect of a CTR on migrating lens epithelial cells (LECs) in rabbit eyes. These authors used a CTR with a 14.0 mm diameter (1.0 mm wide and 0.2 mm thick). After cataract surgery, a PMMA IOL was implanted in the capsular bag in 5 rabbit eyes. The authors found an accumulation of LECs at the equatorial corner outside the haptic or ring, showing the inhibition of LEC migration. The control group showed markedly less inhibition of LEC migration on macroscopic and microscopic observations.16

Nishi and coauthors17 examined the effect of an equatorial PMMA CBR in another study. They performed a prospective randomized trial of 100 patients requiring bilateral cataract surgery. Each patient received a Hydroview hydrogel IOL (Storz Ophthalmics) with a CBR in 1 eye and a Hydroview IOL without the CBR in the fellow eye. After 6 months, there was significantly less PCO in the eyes with the CBR, with no constriction of the anterior capsule opening and fewer capsule folds.17

Lee and coauthors8 examined the effect of a CTR on the tilting and decentration of IOLs after cataract surgery in 40 eyes of 20 patients. They performed an intraindividual comparison; 1 eye received a hydrophobic IOL and a CTR, and the fellow eye received only the IOL. The postoperative IOL position was measured with a Scheimpflug camera anterior eye segment analysis system. The eyes with both an IOL and a CTR had significantly less IOL decentration and tilting than eyes with an IOL only. Lee and coauthors concluded that a CTR seemed effective in preserving capsule integrity and IOL position.

Strenn and coauthors9 described 19 eyes (13 patients) that had clear corneal cataract surgery with implantation of a foldable, open-loop, silicone posterior chamber IOL. At the same time, an open PMMA CTR (Morcher type 14 or 14A) was inserted to allow measurement of capsular bag circumference and diameter. They measured the distance between the ends of the 2 eyelets of the CTR gonioscopically at the slitlamp by adjusting the slit height to the distance in between as described by Vass et al.21 The authors found a significant decrease in the capsular bag circumference of 2% ± 1.5% within a 3-month follow-up. These results are similar to our results of capsular bag shrinkage with the CTR. We found a shrinkage of 2.9% in this group. Strenn and coauthors further reported that although the decrease in the capsule bag circumference was similar with both ring types from 1 week to 1 month, capsule contraction continued for 2 months with the smaller ring (type 14 = 10.0 mm in diameter) but ceased with the larger ring (12.0 mm in diameter). These authors concluded that the diameter of the capsular bag could be calculated in the living eye and that the influence of the CTR itself on capsular bag shrinkage remained to be established.

In conclusion, capsular bag shrinkage can be inhibited by a CBR and, to a lesser extent, by a CTR. The effect of a CTR on capsular bag shrinkage in a normal eye is dependent on the design and the diameter of the CTR. It is probably further dependent on the type, diameter, and material of the IOL. Larger eyes of >27.0 mm respond with greater capsular bag shrinkage.2 Therefore, in these eyes, a larger CTR diameter is preferred. Further study is necessary to evaluate the influence of other CTR designs on capsular bag shrinkage. The reduction in the capsular bag shrinkage after CTR implantation may reduce the incidence of IOL dislocation and tilt, helping to maintain postoperative visual acuity.

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