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Effect of Anterior Capsulotomy Shape and Intraocular Lens Design on Lens Decentration
Effect of Anterior Capsulotomy Shape and Intraocular Lens Design on Lens Decentration GIUSEPPE RAVALICO, FABIO BACCARA and DANIELE IUSTULIN Eye Clinic, University of Trieste, Italy
OBJECTIVE: To determine whether the anterior capsulotomy shape (capsulorhexis or envelope) or the IOL design (polyHEMA, one-piece PMMA open and closed loops, three-piece polypropylene and PMMA loops) influence the lens decentration after intercapsular cataract surgery. STUDY DESIGN: Retrospective study of postoperative results 18 months after surgery. SETTING: University Eye Clinic of Trieste, Italy. PATIENTS: 150 patients divided into two groups: in 75 cases anterior circular continuous capsulorhexis (CCC) was performed and in 75 cases the envelope technique was used. Only patients with uncomplicated eyes and absence of posterior capsular opacification were included. MAIN OUTCOME MEASURES: Degree of IOL decentration was measured by projecting a slide of the anterior segment of the eye, taken after maximum dilatation, and calculating the distance between the circles formed by the corneosclerallimbus and the edge of the IOL. RESULTS: Including all IOLs, mean decentration was 0.28 ± 0.21 mm in the capsulorhexis group and 0.55 ± 0.32 mm in the envelope group; the difference is statistically significant (P < 0.01). One-piece IOLs decentred less than three-piece lenses when capsulorhexis was performed. There were no significant differences found with different types of IOLs implanted by the envelope technique. CONCLUSIONS: The loop material did not appear to influence the IOL centration. The loop design appears to influence the centration only when capsulorhexis is performed. In the envelope technique, the decentration relates more to the irregular opening of the capsular bag than to the loop design. Keywords: Capsulorhexis; Decentration; Envelope; IOL
INTRODUCTION
Extracapsular cataract extraction with implantation of an intraocular lens (IOL) is a surgical procedure with a high percentage of functional success. A patient with excellent visual acuity is not always asymptomatic; the most common complaints involve unwanted optical images caused by IOL decentration. These images most commonly result when a positioning hole, laser ridge or optic edge comes within the pupillary aperture [1, 2]. Hansen et al. [3] demonstrated that IOL implantation in the capsular bag improves lens centration. Many techniques for in-the-bag IOL implantation have been described over the years; the most common are Galand's envelope technique [4] and Correspondence to: Professor Giuseppe Ravalico, Clinica Oculistica dell'Universita, c/o Ospedale Maggiore, Piazza Ospedale 1, 34129 Trieste, Italy.
Neuhann and Gimbel's circular continuous capsulorhexis (CCC) [5, 6]. This study attempts to determine which technique is associated with better and more predictable centration, and to analyse the long-term decentration produced by different IOL styles. METHODS
We reviewed a consecutive series of patients who had uncomplicated extracapsular cataract extraction with in-the-bag IOL implantation between October 1990 and November 1991. The group of 150 eyes consisted of two subgroups of 75 eyes each. In one, CCC was made with capsulotomy forceps; if a continuous tear capsulotomy was begun but could not be completed for any reason the patient was excluded. In the other subgroup Galand's envelope technique was performed. A
0955-3681/94/010036+04 $08.0010 © 1994 W.B. Saunders Company Ltd
Eur J Implant Ref Surg, Vol 6, February 1994
37
Lens Decentration
Table 1 Characteristics of the IOLs tested Manufacturer Haptic diameter Model Pharmacia 720C 13.5 Pharmacia 801 A 9.0 Alcon 11.3 Iogel1103 P256E 13.5 Iolab 3M 13.8 815 LE
linear incision into the anterior capsule was made with a needle between the 2 and 10 o'clock position and the capsulotomy was completed with McPherson forceps after IOL implantation; patients with unplanned tears were not included. Patients in the two subgroups were of similar sex and age: measurements were taken 18 months postoperatively. We included only uncomplicated eyes and we excluded patients who had posterior capsular opacification. We studied the following type of IOL (Table 1): one-piece polymethylmethacrylate (PMMA) modified C-loop or 360° encircling closed loops (compressible disc), one piece polyHEMA with flanges, threepiece PMMA with polypropylene or PMMA loops. For evaluation all eyes were dilated with one drop of 1% tropicamide and one drop of 10% phenylephrine. After maximum dilatation of the pupil a photograph of the anterior segment was taken with a slit-lamp camera. To determine decentration a 35 mm slide of each implanted IOL was projected onto a screen with concentric circles of 24, 26 and 28 em diameters; the IOL optic was aligned with the corresponding circle according to the diameter of the IOL (6, 6.5 and 7mm respectively) so that each mm of the IOL corresponded to 4 em on the screen. Decentration was defined as the distance between the centre of the IOL and the centre of the corneosclerallimbus divided by 40. The groups were compared using the Student's unpaired t-test to analyse the degree of statistical significance of the decentration of the IOLs. RESULTS
With all posterior chamber IOLs in the capsulorhexis group included the mean decentration was 0.28 ± 0.21 mm); 9.3% of the implanted IOLs were decentred between 0.5 and 1 mm, and 1.3% more than 1 mm. In the envelope group the mean decentration was 0.55 ± 0.32 mm; in 41.3% of cases IOLs decentration was found between 0.5 and 1 mm, and in 8% of eyes it was more than 1 mm. We compared these groups using the Student's unpaired t-test and the difference was found to be statistically significant (P < 0.01). . The results of the decentration measurements were analysed and are shown in Table 2 in which Eur J Implant Ref Surg, Vo/6, February 1994
Optic diameter 6.5 6.0 6.0 7.0 6.0
IOL design One-piece One-piece One-piece Three-piece Three-piece
Loop material PMMA PMMA PolyHEMA Polypropylene PMMA
Table 2 Mean decentration values for each of the IOL models used and results of Student's t-test comparing capsulorhexis vs envelope IOL models Capsulorhexis Envelope P value 0.003 0.49 ± 0.31 0.18 ± 0.21 One-piece/PMMA 0.013 0.54 ± 0.28 Compressible disc 0.21 ± 0.20 0.005 0.50 ± 0.28 0.24 ± 0.17 PolyHEMA Three-piece/ 0.018 0.63 ± 0.34 0.36 ± 0.20 polypropylene 0.029 0.62 ± 0.22 Three-piece/PMMA 0.38 ± 0.24
the mean decentration scores of the lens styles tested are compared with different anterior capsulotomy shapes. The average decentration value with each of the five types ofiOLs studied was less in the capsulorhexis group than in the envelope group and the difference was found to be statistically sigilificant. In both groups one-piece lenses provided the most consistent centration. Study of the influence of IOL style on the mean decentration values in eyes with the same anterior capsulotomy was considered. The mean decentration in the CCC group, shown in Table 3, was 0.18 mm for one-piece PMMA lenses, 0.21 mm for compressible disc lenses, 0.24 mm for polyHEMA lenses, 0.36 mm for three-piece polypropylene lenses and 0.38 mm for three-piece PMMA lenses. Centration of one-piece modified C-loop lenses was significantly better than that of the three-piece lenses. Table 3 Results of Student's t-test comparing mean decentration value of different styles vs one-piece modified C-loop in the capsulorhexis group IOL models Capsulorhexis P value 0.18 ± 0.21 One-piece/PMMA 0.692 0.21 ± 0.20 Compressible disc 0.397 0.24 ± 0.17 PolyHEMA 0.023 0.36 ± 0.20 Three-piece/polypropylene 0.022 0.38 ± 0.24 Three-piece/PMMA
The mean decentration in the envelope group, shown in Table 4, was 0.49 mm for one-piece PMMA lenses, 0.54 mm for compressible disc lenses, 0.50 mm for polyHEMA lenses, 0.63 mm for threepiece polypropylene lenses and 0.62 mm for threepiece PMMA lenses. Although the one-piece lenses were better centred than the three-piece ones, the difference was not statistically significant.
G. Ravalico eta/.
38
Table 4 Results of Student's t-test comparing mean decentration value of different styles vs one-piece modified C-loop in the envelope group IOL models Envelope P value 0.49 ± 0.31 One-piece/PMMA 0.722 0.54 ± 0.44 Compressible disc 0.927 0.50 ± 0.28 PolyHEMA 0.249 0.63 ± 0.34 Three-piece/polypropylene 0.196 0.62 ± 0.22 Three-piece/PMMA
DISCUSSION
Decentration of posterior chamber IOLs is a postoperative complication that can produce unwanted optical images, especially when a positioning hole, laser ridge or optical edge comes within the pupillary aperture. In an effort to resolve this problem increase of the optic size and the elimination of positioning holes and laser ridge have been advocated [7]. Friedberg et al. [8] reported that 57% of patients with lenses of 6 mm optic diameter and positioning holes complained of seeing unwanted images, but only 3% of patients with a 7 mm optic diameter without positioning holes noticed optical images. Several authors have reviewed complications resulting from malpositioned IOLs, including pupillary capture, sunrise or sunset syndromes with possible subluxation into the vitreous and also astigmatism induced by a tilted IOL [9-16]. To minimize these complications it is important to centre the IOL perfectly, symmetrical positioning of the lens in the capsular bag helps to achieve this goal. Brems et al. [17] studying autopsy eyes observed that in 71% of cases an optic edge or a positioning hole was situated either within the pupillary aperture or within 0.5 mm of the pupillary margin. This finding was present in 92% of the cases with asymmetric loop fixation and in 50% of cases with symmetrically placed loops. In a anatomicopathologic study of 222 eyes, Hansen et al. [3] reported that lenses implanted in the capsular bag had significantly less decentration than those lying in the sulcus and much less than those with asymmetrical positioning; the superiority of centration of in-the-bag lenses over asymmetrical lenses has been observed also in vivo. In this study we investigated the effect of two variables on IOL centration: (1) anterior capsulotomy and (2) IOL style. Decentration of capsular fixated IOLs in a bag with circular continuous capsulorhexis was minimal. Decentration increases significantly when IOLs are implanted in a bag with the envelope technique. The envelope technique predisposes to radial tears at 2 and 10 o'clock during nucleus expression; in this case the two radial tears of
the anterior capsule create a flap between the two breaks that often folds or collapses, creating a large defect in the continuity of the capsular circumference. An asymmetrical configuration of capsular circumference appears and, even if both loops are in the bag, uneven forces act on the IOL and a loop can escape from the capsular bag and result in bag/sulcus fixation with significant decentration. With a capsulorhexis an optimal IOL centration in the capsular bag is achieved and a resistance to displacement is widely maintained [18]. The resistance of the IOL loop to compression must be balanced by symmetrical forces on each loop at the capsular bag equator. Asymmetrically fixated IOLs will tend to decenter because the loop in the bag is compressed more than the loop in the ciliary sulcus and proliferative processes such as capsular bag fibrosis and posterior capsular opacification may displace the lenses easily [3, 19]. As the compressible disc lens is 9 mm in diameter, the outer ring of the IOL makes 360° contact with the equator of the capsular bag, which measures approximately 10.3-10.8 mm, thus creating a gentle symmetric stretch of the posterior capsule that inhibits the incidence of posterior capsular opacification. Tetz et al. found a positive correlation between posterior capsular opacification and IOL decentration [20]; our clinical results confirm that the compressible disc IOL appears to provide a more precise optic centration. Posterior capsular opacification, fixation sites and capsular tears are the main significant factors influencing IOL centration. Since every lens in the capsulorhexis group had both loops in the capsular bag, with no radial tears, the only factor that influenced decentration was the lens construction, as we excluded patients with posterior capsular opacification. The five IOL styles tested provide stable fixation, but compressible disc and one-piece IOLs have better centration than three-piece IOLs, and the differences are statistically significant. These results confirm those obtained by Hansen et al. [21] which observed less decentration with one-piece than with three-piece lenses in an experimental study in rabbits. With the envelope technique the integrity of the capsular bag is often damaged because radial tears occur; the loops exert a pressure on the capsular fornix and can come out of the bag if they are left near a tear or under an anterior capsular flap [22]. In the envelope group we found no significant difference in decentration with the different types of IOL studied. We believe that in these cases the anterior capsulotomy shape is the major factor influencing IOL decentration and neither the loop material nor the lens style appear to have a clinical effect on the Eur J Implant Ref Surg, Vol 6, February 1994
39
Lens Decentration
IOL centration. Our data suggest that the best centration is achieved after CCC and that IOL style has a significant effect; one-piece lenses appear to decrease IOL decentration. When the anterior capsule has one or more radial tears IOL decentration is related greater to the anterior capsulotomy shape than to the IOL style. In conclusion, the goal of good posterior chamber IOLs centration seems to be important since multifocal IOLs and various types of the small incision IOLs are extensively implanted. Our results suggest that the best IOL centration is achieved with CCC and a one-piece IOL implanted in the capsular bag.
10 11
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REFERENCES 1 M Rosner, M Sharir, M Blumenthal. Optical aberrations from a well-centred intraocular lens implant. Am. J. Ophthalmol., 1986; 101: 117-118. 2 DJ Apple, SB Lichtenstein, K Heerlein, SL Letchinger et al. Visual aberration caused by optic components of posterior chamber intraocular lenses. J. Cataract Refract. Surg., 1987; 13: 431-435. 3 SO Hansen, MR Tetz, KD Solomon et al. Decentration of flexible loop posterior chamber intraocular lenses in a series of222 postmortem eyes. Ophthalmology, 1988; 95: 344-349. 4 A Galand. Implantations dans le sac capsulaire. J. Fr. Ophtalmol., 1983; 6: 533-535. 5 T Neuhann. Theorie und Operationstechnik der Kapsulorhexis. Klin. Monatsbl. Augenheilkd., 1987; 190: 542-545. 6 HV Gimbel, T Neuhann. Development, advantages and methods of the continuous circular capsulorhexis technique. J. Cataract Refract. Surg., 1990; 16:31-37. 7 RA Landry. Unwanted optical effects caused by intraocular lens positioning holes. J. Cataract Refract. Surg., 1987; 13: 421-423. 8 HL Friedberg, OR Kline, AH Friedberg. Comparison of the unwanted optical images produced by 6 mm and 7 mm intraocular lenses. J. Cataract Refract. Surg., 1989; 15: 541-544. 9 KD Solomon, DJ Apple, N Mamalis et al. Complications of intraocular lenses with special reference to an analysis of
Eur J Implant Ref Surg, Vol 6, February 1994
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18 19
20
21
22
2500 explanted intraocular lenses (IOLs). Eur. J. Implant Refract. Surg., 1991; 3: 195-200. UFC Legler, DJ Apple, P Hund, WS Kirkconnell. Chronic ciliary pain secondary to posterior chamber intraocular lens loop incarceration. Am. J. Ophthalmol., 1991; 111: 513-515. WS Chambless. Incidence of anterior and posterior segment complications over 3000 cases of extracapsular cataract extractions: intact and open capsules. J. Am. Intraocul. Implant Soc., 1985; 11: 146-148. DF Goodman, WJ Stark, JD Gottsch. Complications of cataract extraction with intraocular lerts implantation. Ophthalmic Surg., 1989; 20: 132-140. A Balmer, R Andenmatten, CA Hiroz. Complications in cataract surgery. A retrospective study of 1304 cases. Klin. Monatsbl. Augenheilkd., 1991; 198: 344-346. AS Jolson, FJ Seidl. Postoperative astigmatism induced by intraocular lens tilt. Am. Intraocul. Implant Soc. J., 1984; 10: 213-214. P Erickson. Effects of intraocular lens position errors on postoperative refractive error. J. Cataract Refract. Surg., 1990; 16: 305-310. DA Atchison. Refractive errors induced by displacement of intraocular lenses within the pseudophakic eye. Optom. Vis. Sci., 1989; 66: 146-152. RN Brems, DJ Apple, BR Pfeffer et al. Posterior chamber intraocular lenses in a series of 75 autopsy eye. Part III: Correlation of positioning holes and optic edges with the pupillary aperture and visual axis. J. Cataract Refract. Surg., 1986; 12: 367-371. DM Colvard, SA Dunn. Intraocular lens centration with continuous tear capsulotomy. J. Cataract Refract. Surg., 1990; 16: 312-314. A Caballero, M Losada, JM Lopez et al. Decentration of intraocular lenses implanted after intercapsular cataract extraction (envelope technique). J. Cataract Refract. Surg., 1986; 12: 124-129. MR Tetz, DJC O'Morchoe, TD Gwin et al. Posterior capsular opacification and intraocular lens decentration. Part II: Experimental findings on a prototype circular intraocular lens design. J. Cataract Refract. Surg., 1988; 14: 614-623. SO Hansen, KD Solomon, GT McKnight et al. Posterior capsular opacification and intraocular lens decentration. Part 1: Comparison of various posterior chamber lens designs implanted in the rabbit model. J. Cataract Refract. Surg., 1988; 14: 605-612. JA Davison. Analysis of capsular bag defects and intraocular lens positions for consistent centration. J. Cataract Refract. Surg., 1986; 12: 124-129.
OBJECTIVE: To determine whether the anterior capsulotomy shape (capsulorhexis or envelope) or the IOL design (polyHEMA, one-piece PMMA open and closed loops, three-piece polypropylene and PMMA loops) influence the lens decentration after intercapsular cataract surgery. STUDY DESIGN: Retrospective study of postoperative results 18 months after surgery. SETTING: University Eye Clinic of Trieste, Italy. PATIENTS: 150 patients divided into two groups: in 75 cases anterior circular continuous capsulorhexis (CCC) was performed and in 75 cases the envelope technique was used. Only patients with uncomplicated eyes and absence of posterior capsular opacification were included. MAIN OUTCOME MEASURES: Degree of IOL decentration was measured by projecting a slide of the anterior segment of the eye, taken after maximum dilatation, and calculating the distance between the circles formed by the corneosclerallimbus and the edge of the IOL. RESULTS: Including all IOLs, mean decentration was 0.28 ± 0.21 mm in the capsulorhexis group and 0.55 ± 0.32 mm in the envelope group; the difference is statistically significant (P < 0.01). One-piece IOLs decentred less than three-piece lenses when capsulorhexis was performed. There were no significant differences found with different types of IOLs implanted by the envelope technique. CONCLUSIONS: The loop material did not appear to influence the IOL centration. The loop design appears to influence the centration only when capsulorhexis is performed. In the envelope technique, the decentration relates more to the irregular opening of the capsular bag than to the loop design. Keywords: Capsulorhexis; Decentration; Envelope; IOL
INTRODUCTION
Extracapsular cataract extraction with implantation of an intraocular lens (IOL) is a surgical procedure with a high percentage of functional success. A patient with excellent visual acuity is not always asymptomatic; the most common complaints involve unwanted optical images caused by IOL decentration. These images most commonly result when a positioning hole, laser ridge or optic edge comes within the pupillary aperture [1, 2]. Hansen et al. [3] demonstrated that IOL implantation in the capsular bag improves lens centration. Many techniques for in-the-bag IOL implantation have been described over the years; the most common are Galand's envelope technique [4] and Correspondence to: Professor Giuseppe Ravalico, Clinica Oculistica dell'Universita, c/o Ospedale Maggiore, Piazza Ospedale 1, 34129 Trieste, Italy.
Neuhann and Gimbel's circular continuous capsulorhexis (CCC) [5, 6]. This study attempts to determine which technique is associated with better and more predictable centration, and to analyse the long-term decentration produced by different IOL styles. METHODS
We reviewed a consecutive series of patients who had uncomplicated extracapsular cataract extraction with in-the-bag IOL implantation between October 1990 and November 1991. The group of 150 eyes consisted of two subgroups of 75 eyes each. In one, CCC was made with capsulotomy forceps; if a continuous tear capsulotomy was begun but could not be completed for any reason the patient was excluded. In the other subgroup Galand's envelope technique was performed. A
0955-3681/94/010036+04 $08.0010 © 1994 W.B. Saunders Company Ltd
Eur J Implant Ref Surg, Vol 6, February 1994
37
Lens Decentration
Table 1 Characteristics of the IOLs tested Manufacturer Haptic diameter Model Pharmacia 720C 13.5 Pharmacia 801 A 9.0 Alcon 11.3 Iogel1103 P256E 13.5 Iolab 3M 13.8 815 LE
linear incision into the anterior capsule was made with a needle between the 2 and 10 o'clock position and the capsulotomy was completed with McPherson forceps after IOL implantation; patients with unplanned tears were not included. Patients in the two subgroups were of similar sex and age: measurements were taken 18 months postoperatively. We included only uncomplicated eyes and we excluded patients who had posterior capsular opacification. We studied the following type of IOL (Table 1): one-piece polymethylmethacrylate (PMMA) modified C-loop or 360° encircling closed loops (compressible disc), one piece polyHEMA with flanges, threepiece PMMA with polypropylene or PMMA loops. For evaluation all eyes were dilated with one drop of 1% tropicamide and one drop of 10% phenylephrine. After maximum dilatation of the pupil a photograph of the anterior segment was taken with a slit-lamp camera. To determine decentration a 35 mm slide of each implanted IOL was projected onto a screen with concentric circles of 24, 26 and 28 em diameters; the IOL optic was aligned with the corresponding circle according to the diameter of the IOL (6, 6.5 and 7mm respectively) so that each mm of the IOL corresponded to 4 em on the screen. Decentration was defined as the distance between the centre of the IOL and the centre of the corneosclerallimbus divided by 40. The groups were compared using the Student's unpaired t-test to analyse the degree of statistical significance of the decentration of the IOLs. RESULTS
With all posterior chamber IOLs in the capsulorhexis group included the mean decentration was 0.28 ± 0.21 mm); 9.3% of the implanted IOLs were decentred between 0.5 and 1 mm, and 1.3% more than 1 mm. In the envelope group the mean decentration was 0.55 ± 0.32 mm; in 41.3% of cases IOLs decentration was found between 0.5 and 1 mm, and in 8% of eyes it was more than 1 mm. We compared these groups using the Student's unpaired t-test and the difference was found to be statistically significant (P < 0.01). . The results of the decentration measurements were analysed and are shown in Table 2 in which Eur J Implant Ref Surg, Vo/6, February 1994
Optic diameter 6.5 6.0 6.0 7.0 6.0
IOL design One-piece One-piece One-piece Three-piece Three-piece
Loop material PMMA PMMA PolyHEMA Polypropylene PMMA
Table 2 Mean decentration values for each of the IOL models used and results of Student's t-test comparing capsulorhexis vs envelope IOL models Capsulorhexis Envelope P value 0.003 0.49 ± 0.31 0.18 ± 0.21 One-piece/PMMA 0.013 0.54 ± 0.28 Compressible disc 0.21 ± 0.20 0.005 0.50 ± 0.28 0.24 ± 0.17 PolyHEMA Three-piece/ 0.018 0.63 ± 0.34 0.36 ± 0.20 polypropylene 0.029 0.62 ± 0.22 Three-piece/PMMA 0.38 ± 0.24
the mean decentration scores of the lens styles tested are compared with different anterior capsulotomy shapes. The average decentration value with each of the five types ofiOLs studied was less in the capsulorhexis group than in the envelope group and the difference was found to be statistically sigilificant. In both groups one-piece lenses provided the most consistent centration. Study of the influence of IOL style on the mean decentration values in eyes with the same anterior capsulotomy was considered. The mean decentration in the CCC group, shown in Table 3, was 0.18 mm for one-piece PMMA lenses, 0.21 mm for compressible disc lenses, 0.24 mm for polyHEMA lenses, 0.36 mm for three-piece polypropylene lenses and 0.38 mm for three-piece PMMA lenses. Centration of one-piece modified C-loop lenses was significantly better than that of the three-piece lenses. Table 3 Results of Student's t-test comparing mean decentration value of different styles vs one-piece modified C-loop in the capsulorhexis group IOL models Capsulorhexis P value 0.18 ± 0.21 One-piece/PMMA 0.692 0.21 ± 0.20 Compressible disc 0.397 0.24 ± 0.17 PolyHEMA 0.023 0.36 ± 0.20 Three-piece/polypropylene 0.022 0.38 ± 0.24 Three-piece/PMMA
The mean decentration in the envelope group, shown in Table 4, was 0.49 mm for one-piece PMMA lenses, 0.54 mm for compressible disc lenses, 0.50 mm for polyHEMA lenses, 0.63 mm for threepiece polypropylene lenses and 0.62 mm for threepiece PMMA lenses. Although the one-piece lenses were better centred than the three-piece ones, the difference was not statistically significant.
G. Ravalico eta/.
38
Table 4 Results of Student's t-test comparing mean decentration value of different styles vs one-piece modified C-loop in the envelope group IOL models Envelope P value 0.49 ± 0.31 One-piece/PMMA 0.722 0.54 ± 0.44 Compressible disc 0.927 0.50 ± 0.28 PolyHEMA 0.249 0.63 ± 0.34 Three-piece/polypropylene 0.196 0.62 ± 0.22 Three-piece/PMMA
DISCUSSION
Decentration of posterior chamber IOLs is a postoperative complication that can produce unwanted optical images, especially when a positioning hole, laser ridge or optical edge comes within the pupillary aperture. In an effort to resolve this problem increase of the optic size and the elimination of positioning holes and laser ridge have been advocated [7]. Friedberg et al. [8] reported that 57% of patients with lenses of 6 mm optic diameter and positioning holes complained of seeing unwanted images, but only 3% of patients with a 7 mm optic diameter without positioning holes noticed optical images. Several authors have reviewed complications resulting from malpositioned IOLs, including pupillary capture, sunrise or sunset syndromes with possible subluxation into the vitreous and also astigmatism induced by a tilted IOL [9-16]. To minimize these complications it is important to centre the IOL perfectly, symmetrical positioning of the lens in the capsular bag helps to achieve this goal. Brems et al. [17] studying autopsy eyes observed that in 71% of cases an optic edge or a positioning hole was situated either within the pupillary aperture or within 0.5 mm of the pupillary margin. This finding was present in 92% of the cases with asymmetric loop fixation and in 50% of cases with symmetrically placed loops. In a anatomicopathologic study of 222 eyes, Hansen et al. [3] reported that lenses implanted in the capsular bag had significantly less decentration than those lying in the sulcus and much less than those with asymmetrical positioning; the superiority of centration of in-the-bag lenses over asymmetrical lenses has been observed also in vivo. In this study we investigated the effect of two variables on IOL centration: (1) anterior capsulotomy and (2) IOL style. Decentration of capsular fixated IOLs in a bag with circular continuous capsulorhexis was minimal. Decentration increases significantly when IOLs are implanted in a bag with the envelope technique. The envelope technique predisposes to radial tears at 2 and 10 o'clock during nucleus expression; in this case the two radial tears of
the anterior capsule create a flap between the two breaks that often folds or collapses, creating a large defect in the continuity of the capsular circumference. An asymmetrical configuration of capsular circumference appears and, even if both loops are in the bag, uneven forces act on the IOL and a loop can escape from the capsular bag and result in bag/sulcus fixation with significant decentration. With a capsulorhexis an optimal IOL centration in the capsular bag is achieved and a resistance to displacement is widely maintained [18]. The resistance of the IOL loop to compression must be balanced by symmetrical forces on each loop at the capsular bag equator. Asymmetrically fixated IOLs will tend to decenter because the loop in the bag is compressed more than the loop in the ciliary sulcus and proliferative processes such as capsular bag fibrosis and posterior capsular opacification may displace the lenses easily [3, 19]. As the compressible disc lens is 9 mm in diameter, the outer ring of the IOL makes 360° contact with the equator of the capsular bag, which measures approximately 10.3-10.8 mm, thus creating a gentle symmetric stretch of the posterior capsule that inhibits the incidence of posterior capsular opacification. Tetz et al. found a positive correlation between posterior capsular opacification and IOL decentration [20]; our clinical results confirm that the compressible disc IOL appears to provide a more precise optic centration. Posterior capsular opacification, fixation sites and capsular tears are the main significant factors influencing IOL centration. Since every lens in the capsulorhexis group had both loops in the capsular bag, with no radial tears, the only factor that influenced decentration was the lens construction, as we excluded patients with posterior capsular opacification. The five IOL styles tested provide stable fixation, but compressible disc and one-piece IOLs have better centration than three-piece IOLs, and the differences are statistically significant. These results confirm those obtained by Hansen et al. [21] which observed less decentration with one-piece than with three-piece lenses in an experimental study in rabbits. With the envelope technique the integrity of the capsular bag is often damaged because radial tears occur; the loops exert a pressure on the capsular fornix and can come out of the bag if they are left near a tear or under an anterior capsular flap [22]. In the envelope group we found no significant difference in decentration with the different types of IOL studied. We believe that in these cases the anterior capsulotomy shape is the major factor influencing IOL decentration and neither the loop material nor the lens style appear to have a clinical effect on the Eur J Implant Ref Surg, Vol 6, February 1994
39
Lens Decentration
IOL centration. Our data suggest that the best centration is achieved after CCC and that IOL style has a significant effect; one-piece lenses appear to decrease IOL decentration. When the anterior capsule has one or more radial tears IOL decentration is related greater to the anterior capsulotomy shape than to the IOL style. In conclusion, the goal of good posterior chamber IOLs centration seems to be important since multifocal IOLs and various types of the small incision IOLs are extensively implanted. Our results suggest that the best IOL centration is achieved with CCC and a one-piece IOL implanted in the capsular bag.
10 11
12 13 14 15
REFERENCES 1 M Rosner, M Sharir, M Blumenthal. Optical aberrations from a well-centred intraocular lens implant. Am. J. Ophthalmol., 1986; 101: 117-118. 2 DJ Apple, SB Lichtenstein, K Heerlein, SL Letchinger et al. Visual aberration caused by optic components of posterior chamber intraocular lenses. J. Cataract Refract. Surg., 1987; 13: 431-435. 3 SO Hansen, MR Tetz, KD Solomon et al. Decentration of flexible loop posterior chamber intraocular lenses in a series of222 postmortem eyes. Ophthalmology, 1988; 95: 344-349. 4 A Galand. Implantations dans le sac capsulaire. J. Fr. Ophtalmol., 1983; 6: 533-535. 5 T Neuhann. Theorie und Operationstechnik der Kapsulorhexis. Klin. Monatsbl. Augenheilkd., 1987; 190: 542-545. 6 HV Gimbel, T Neuhann. Development, advantages and methods of the continuous circular capsulorhexis technique. J. Cataract Refract. Surg., 1990; 16:31-37. 7 RA Landry. Unwanted optical effects caused by intraocular lens positioning holes. J. Cataract Refract. Surg., 1987; 13: 421-423. 8 HL Friedberg, OR Kline, AH Friedberg. Comparison of the unwanted optical images produced by 6 mm and 7 mm intraocular lenses. J. Cataract Refract. Surg., 1989; 15: 541-544. 9 KD Solomon, DJ Apple, N Mamalis et al. Complications of intraocular lenses with special reference to an analysis of
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18 19
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