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Silicone and poly(methyl methacrylate) lens decentration associated with asymmetric capsule shrinkage in rabbits1
Silicone and poly(methyl methacrylate) lens decentration associated with asymmetric capsule shrinkage in rabbits Shunsaku Ohmi, MD, Saiichi Tanaka, MD, Shizuyz Saika, MD, Yoshitaka Ohnishi, MD ABSTRACT Purpose: To determine whether silicone intraocular lenses (IOLs) are readily affected by capsule shrinkage. Setting: Department of Ophthalmology, Wakayama Medical College, Wakayama, Japan. Methods: A D-shaped incision was made in the anterior capsule of 38 eyes of 20 white rabbits. One of 2 IOL types was implanted in the capsular bag: 3-piece silicone or single-piece all-poly(methyl methacrylate) (PMMA). Twenty-eight eyes were evaluated for IOL rotation and optic decentration. Results: The extent of postoperative decentration and rotation observed with the silicone IOLs did not significantly exceed that of the PMMA IOLs. Conclusions: Our results, coupled with the advantages of small incision surgery, indicate that a silicone IOL is an effective choice. J Cataract Refract Surg 1999; 25:1147–1150 © 1999 ASCRS and ESCRS
W
ith the increased performance of cataract surgery using a small incision, soft foldable intraocular lenses (IOLs) are being used more frequently. However, capsule shrinkage is suspected of having an unfavorable effect on IOLs made of soft materials. Good IOL centration is obtained in many cases by in-the-bag implantation.1–3 Decentration often occurs when 1 of the fixed
Accepted for publication April 27, 1999. From Ohmi Eye Clinic, Sakai (Ohmi), and the Department of Ophthalmology, Wakayama Medical College, Wakayama (Ohmi, Tanaka, Saika, Ohnishi), Japan. Presented at the 11th annual meeting of the Japanese Society of Cataract and Refractive Surgery, Nagoya, Japan, April 1996. Canon Starr Co. and Phamacia Co., Tokyo, Japan, provided the intraocular lenses. Reprint requests to Shunsaku Ohmi, MD, Ohmi Eye Clinic, 250 Naka 3-cho Hamadera Suwanomori, Sakai, Osaka 592-8348, Japan. © 1999 ASCRS and ESCRS Published by Elsevier Science Inc.
portions of the IOL escapes from the capsular bag while the other remains within.1,2 However, the IOL may become displaced postoperatively even when both fixed portions remain in the capsular bag. One possible cause is the asymmetric shrinkage of the capsule.1,3– 6 We assessed experimentally the effect of asymmetric shrinkage of the capsule on various IOLs6,7 and report a comparative study of silicone and poly(methyl methacrylate) (PMMA) lenses.
Materials and Methods A D-shaped anterior capsulotomy, half the size of a conventional continuous curvilinear capsulorhexis (CCC), was made in 38 eyes of 20 white rabbits weighing 2.5 to 3.0 kg using a previously reported surgical method.6,7 One of 2 IOL types was implanted and fixated in the capsular bag at a certain orientation (Fig0886-3350/99/$–see front matter PII S0886-3350(99)00140-6
SILICONE AND PMMA IOL DECENTRATION IN RABBIT EYES
Figure 1. (Ohmi) With a D-shaped anterior capsulotomy, the force of capsule contraction from the left exceeds that from the right because of the capsule’s asymmetry. The dotted line shows the position of IOL fixation immediately after surgery.
Figure 2. (Ohmi) In a rabbit eye with a 3-piece silicone IOL, IOL decentration was 0.47 mm and rotation was 0 degree 8 weeks postoperatively.
ure 1). The IOL was (1) a 3-piece lens with a 5.5 mm diameter silicone biconvex optic, modified C-type polyimide haptics with a 10 degree angle, and an overall length of 12.5 mm; or (2) a single-piece all-PMMA lens with a 6.0 mm diameter biconvex optic, modified Ctype haptics with a 6 degree angle, and an overall length of 12.5 mm. As in past studies,6,7 the goal was to implant an IOL in both eyes of the rabbit. When a good Dshaped anterior capsulotomy could not be performed (e.g., because the anterior capsule tore), an IOL was not implanted. In such cases, surgery was performed
in only 1 eye on the same side to complete the sample number. After 8 weeks, the rabbits were killed by an intravenous injection of pentobarbital sodium 5%. The eyes were enucleated and cut into halves along the equator. The anterior half of each eye was photographed on slide film from the posterior aspect. The slides were projected on a screen, and IOL decentration and rotation were measured. Decentration was measured as previously reported.6 – 8 To determine IOL rotation, the angle of rotation from the original fixation orientation (the 4 and 10 o’clock positions) was measured. The t test or Wilcoxon test was used for statistical analysis. A level of P ⬍ .05 was accepted as statistically significant.
Results
Figure 3. (Ohmi) In a rabbit eye with a single-piece PMMA IOL, IOL decentration was 0.74 mm and rotation was 85 degrees postoperatively (arrows ⫽ haptic peaks).
1148
Three rabbits died by 8 weeks postoperatively. Any animal that had marked endophthalmitis or asymmetric IOL in–out fixation was excluded from analysis. The mean postoperative decentration in 16 eyes with silicone IOLs was 0.60 mm ⫾0.25 (SD) while that in 12 eyes with PMMA IOLs was 0.55 ⫾ 0.25 mm (Table 1). The difference was not significant. Mean rotation of the silicone IOLs was 42.2 ⫾ 25.2 degrees and of the PMMA IOLs, 38.8 ⫾ 27.3 degrees (Table 2). The difference was not significant. Figures 2 and 3 are representative slides used in evaluation.
J CATARACT REFRACT SURG—VOL 25, AUGUST 1999
SILICONE AND PMMA IOL DECENTRATION IN RABBIT EYES
Table 1. Postoperative IOL decentration of IOLs 8 weeks after a D-shaped anterior capsulotomy.
Table 2. Postoperative IOL rotation 8 weeks after a D-shaped anterior capsulotomy.
Decentration (mm)
Rotation (Degrees)
Eye
Silicone IOL
PMMA IOL
Eye
Silicone IOL
PMMA IOL
1
0.47
0.32
1
0
85
2
0.71
0.74
2
20
85
3
0.94
1.14
3
60
35
4
0.23
0.41
4
20
15
5
0.74
0.41
5
25
50
6
0.34
0.51
6
75
20
7
1.13
0.92
7
65
5
8
0.45
0.36
8
65
50
9
0.67
0.51
9
25
55
10
0.56
0.59
10
75
50
11
0.87
0.30
11
60
10
12
0.96
0.39
12
25
5
13
0.27
In–out fixation
13
5
In–out fixation
14
0.64
In–out fixation
14
50
In–out fixation
15
0.43
Severe inflammation
15
30
Severe inflammation
16
0.70
No IOL
16
75
No IOL
In–out fixation
17
17
In–out fixation
Discussion Capsule shrinkage occurs because of collagen fiber contraction that results from the proliferation and metaplasia of lens epithelial cells.7,9,10 When the anterior capsulotomy is not circular, particularly when it is Dshaped, contraction will be greater on the capsule’s relatively intact side, probably decentering the IOL toward the opposite side. When anterior capsulotomy was performed by conventional CCC in an earlier study,8 no asymmetric in–out fixation occurred; it occurred in 4 eyes in this study. The effect of asymmetric capsule shrinkage caused by a D-shaped anterior capsulotomy is thus presumed to be very strong, but the extent of postoperative decentration in eyes with silicone IOLs did not significantly exceed that in those with PMMA IOLs. Differences may become apparent over a longer follow-up because of the progressive nature of capsule shrinkage. The material used for the silicone IOL haptics in this study was polyimide, whose ability to recover its shape is comparable to that of PMMA.11 Escape of the haptic from the capsule in an in–out fixation occurred in only 1 eye with a silicone IOL,
In–out fixation
In–out fixation
whereas it occurred in 3 eyes with a PMMA IOL. Earlier single-piece silicone IOL models, which were not used in this experiment, were implanted in capsules using a can-opener anterior capsulotomy. In such cases, postoperative capsule shrinkage often resulted in dislocation of the IOL into the anterior chamber or out of the capsule12 because no cicatricial synechias developed between the silicone IOL and intraocular tissue. Thus, this type of IOL is not commonly used now in Japan. With open-loop IOLs, capsule shrinkage can be avoided by the haptics’ flexibility, which prevents bending of the optic. The problem of capsule shrinkage may also be prevented by using 3-piece silicone IOLs. There was no statistically significant difference between the 2 IOL types in mean degree of rotation. Because the silicone IOL’s haptic is polyimide, not silicone, these IOLs seem to adhere well to the capsule, to the same extent as PMMA lenses. These observations, coupled with the advantages of small incision surgery for astigmatism and for protection against external forces, indicate that the silicone IOL is an effective choice.
J CATARACT REFRACT SURG—VOL 25, AUGUST 1999
1149
SILICONE AND PMMA IOL DECENTRATION IN RABBIT EYES
References 1. Apple DJ, Kincaid MC, Mamalis N, Olson RJ. Intraocular Lenses; Evolution, Designs, Complications, and Pathology. Baltimore, MD, Williams & Wilkins, 1989; 133–168 2. Hansen SO, Tetz MR, Solomon KD, et al. Decentration of flexible loop posterior chamber intraocular lenses in a series of 222 postmortem eyes. Ophthalmology 1988; 95:344 –349 3. Rochels R, Nover A. Untersuchung der Ha¨ufigkeit und Entstehung der Dezentrierung Kapselsackfixierter Hinterkammerlinsen. Klin Monatsbl Augenheilkd 1988; 193:585–588 4. Kimura W, Kimura T, Sawada T, et al. Intraocular lenses decentration after two years of envelope technique ECCE. Jpn IOL Soc J 1989; 3:185–192 5. Miyake K. Some considerations on methods of anterior lens capsulotomy and mechanisms of lens capsule opacification. Jpn IOL Soc J 1989; 3:236 –239 6. Ohmi S. Decentration associated with asymmetric capsular shrinkage and intraocular lens size. J Cataract Refract Surg 1993; 19:640 – 643
1150
7. Ohmi S, Uenoyama K. Decentration associated with asymmetric capsular shrinkage and intraocular lens design in a rabbit model. J Cataract Refract Surg 1995; 21:293–296 8. Ohmi S, Uenoyama K. Experimental evaluation of posterior capsule opacification and intraocular lens decentration: comparison of intraocular lenses of 12.5 mm and 14.0 mm diameter. J Cataract Refract Surg 1993; 19: 348 –351 9. Nishi O, Nishi K. Fibrous opacification of the anterior capsule after cataract surgery. Jpn J Clin Ophthalmol 1991; 45:1811–1815 10. Namiki M, Yamamoto N, Tagami Y, et al. Risk factors for anterior capsular shrinkage in intraocular lens implantation. Jpn J Clin Ophthalmol 1991; 45:1828 – 1831 11. Taguchi H, Kimura W, Kimura T, et al. Comparison of shape recovery ratios of polyimide IOL haptics. Jpn IOL Soc J 1992; 6:323–329 12. Sakka Y, Nishi O, Yamamoto M. Intraocular lens: soft lens. In: Masuda K, Oguti Y, Kozaki M, eds, Compact Ophthalmology: Clinic of Cataract. Tokyo, Kanahara & Co Ltd, 1992; 93–94
J CATARACT REFRACT SURG—VOL 25, AUGUST 1999
W
ith the increased performance of cataract surgery using a small incision, soft foldable intraocular lenses (IOLs) are being used more frequently. However, capsule shrinkage is suspected of having an unfavorable effect on IOLs made of soft materials. Good IOL centration is obtained in many cases by in-the-bag implantation.1–3 Decentration often occurs when 1 of the fixed
Accepted for publication April 27, 1999. From Ohmi Eye Clinic, Sakai (Ohmi), and the Department of Ophthalmology, Wakayama Medical College, Wakayama (Ohmi, Tanaka, Saika, Ohnishi), Japan. Presented at the 11th annual meeting of the Japanese Society of Cataract and Refractive Surgery, Nagoya, Japan, April 1996. Canon Starr Co. and Phamacia Co., Tokyo, Japan, provided the intraocular lenses. Reprint requests to Shunsaku Ohmi, MD, Ohmi Eye Clinic, 250 Naka 3-cho Hamadera Suwanomori, Sakai, Osaka 592-8348, Japan. © 1999 ASCRS and ESCRS Published by Elsevier Science Inc.
portions of the IOL escapes from the capsular bag while the other remains within.1,2 However, the IOL may become displaced postoperatively even when both fixed portions remain in the capsular bag. One possible cause is the asymmetric shrinkage of the capsule.1,3– 6 We assessed experimentally the effect of asymmetric shrinkage of the capsule on various IOLs6,7 and report a comparative study of silicone and poly(methyl methacrylate) (PMMA) lenses.
Materials and Methods A D-shaped anterior capsulotomy, half the size of a conventional continuous curvilinear capsulorhexis (CCC), was made in 38 eyes of 20 white rabbits weighing 2.5 to 3.0 kg using a previously reported surgical method.6,7 One of 2 IOL types was implanted and fixated in the capsular bag at a certain orientation (Fig0886-3350/99/$–see front matter PII S0886-3350(99)00140-6
SILICONE AND PMMA IOL DECENTRATION IN RABBIT EYES
Figure 1. (Ohmi) With a D-shaped anterior capsulotomy, the force of capsule contraction from the left exceeds that from the right because of the capsule’s asymmetry. The dotted line shows the position of IOL fixation immediately after surgery.
Figure 2. (Ohmi) In a rabbit eye with a 3-piece silicone IOL, IOL decentration was 0.47 mm and rotation was 0 degree 8 weeks postoperatively.
ure 1). The IOL was (1) a 3-piece lens with a 5.5 mm diameter silicone biconvex optic, modified C-type polyimide haptics with a 10 degree angle, and an overall length of 12.5 mm; or (2) a single-piece all-PMMA lens with a 6.0 mm diameter biconvex optic, modified Ctype haptics with a 6 degree angle, and an overall length of 12.5 mm. As in past studies,6,7 the goal was to implant an IOL in both eyes of the rabbit. When a good Dshaped anterior capsulotomy could not be performed (e.g., because the anterior capsule tore), an IOL was not implanted. In such cases, surgery was performed
in only 1 eye on the same side to complete the sample number. After 8 weeks, the rabbits were killed by an intravenous injection of pentobarbital sodium 5%. The eyes were enucleated and cut into halves along the equator. The anterior half of each eye was photographed on slide film from the posterior aspect. The slides were projected on a screen, and IOL decentration and rotation were measured. Decentration was measured as previously reported.6 – 8 To determine IOL rotation, the angle of rotation from the original fixation orientation (the 4 and 10 o’clock positions) was measured. The t test or Wilcoxon test was used for statistical analysis. A level of P ⬍ .05 was accepted as statistically significant.
Results
Figure 3. (Ohmi) In a rabbit eye with a single-piece PMMA IOL, IOL decentration was 0.74 mm and rotation was 85 degrees postoperatively (arrows ⫽ haptic peaks).
1148
Three rabbits died by 8 weeks postoperatively. Any animal that had marked endophthalmitis or asymmetric IOL in–out fixation was excluded from analysis. The mean postoperative decentration in 16 eyes with silicone IOLs was 0.60 mm ⫾0.25 (SD) while that in 12 eyes with PMMA IOLs was 0.55 ⫾ 0.25 mm (Table 1). The difference was not significant. Mean rotation of the silicone IOLs was 42.2 ⫾ 25.2 degrees and of the PMMA IOLs, 38.8 ⫾ 27.3 degrees (Table 2). The difference was not significant. Figures 2 and 3 are representative slides used in evaluation.
J CATARACT REFRACT SURG—VOL 25, AUGUST 1999
SILICONE AND PMMA IOL DECENTRATION IN RABBIT EYES
Table 1. Postoperative IOL decentration of IOLs 8 weeks after a D-shaped anterior capsulotomy.
Table 2. Postoperative IOL rotation 8 weeks after a D-shaped anterior capsulotomy.
Decentration (mm)
Rotation (Degrees)
Eye
Silicone IOL
PMMA IOL
Eye
Silicone IOL
PMMA IOL
1
0.47
0.32
1
0
85
2
0.71
0.74
2
20
85
3
0.94
1.14
3
60
35
4
0.23
0.41
4
20
15
5
0.74
0.41
5
25
50
6
0.34
0.51
6
75
20
7
1.13
0.92
7
65
5
8
0.45
0.36
8
65
50
9
0.67
0.51
9
25
55
10
0.56
0.59
10
75
50
11
0.87
0.30
11
60
10
12
0.96
0.39
12
25
5
13
0.27
In–out fixation
13
5
In–out fixation
14
0.64
In–out fixation
14
50
In–out fixation
15
0.43
Severe inflammation
15
30
Severe inflammation
16
0.70
No IOL
16
75
No IOL
In–out fixation
17
17
In–out fixation
Discussion Capsule shrinkage occurs because of collagen fiber contraction that results from the proliferation and metaplasia of lens epithelial cells.7,9,10 When the anterior capsulotomy is not circular, particularly when it is Dshaped, contraction will be greater on the capsule’s relatively intact side, probably decentering the IOL toward the opposite side. When anterior capsulotomy was performed by conventional CCC in an earlier study,8 no asymmetric in–out fixation occurred; it occurred in 4 eyes in this study. The effect of asymmetric capsule shrinkage caused by a D-shaped anterior capsulotomy is thus presumed to be very strong, but the extent of postoperative decentration in eyes with silicone IOLs did not significantly exceed that in those with PMMA IOLs. Differences may become apparent over a longer follow-up because of the progressive nature of capsule shrinkage. The material used for the silicone IOL haptics in this study was polyimide, whose ability to recover its shape is comparable to that of PMMA.11 Escape of the haptic from the capsule in an in–out fixation occurred in only 1 eye with a silicone IOL,
In–out fixation
In–out fixation
whereas it occurred in 3 eyes with a PMMA IOL. Earlier single-piece silicone IOL models, which were not used in this experiment, were implanted in capsules using a can-opener anterior capsulotomy. In such cases, postoperative capsule shrinkage often resulted in dislocation of the IOL into the anterior chamber or out of the capsule12 because no cicatricial synechias developed between the silicone IOL and intraocular tissue. Thus, this type of IOL is not commonly used now in Japan. With open-loop IOLs, capsule shrinkage can be avoided by the haptics’ flexibility, which prevents bending of the optic. The problem of capsule shrinkage may also be prevented by using 3-piece silicone IOLs. There was no statistically significant difference between the 2 IOL types in mean degree of rotation. Because the silicone IOL’s haptic is polyimide, not silicone, these IOLs seem to adhere well to the capsule, to the same extent as PMMA lenses. These observations, coupled with the advantages of small incision surgery for astigmatism and for protection against external forces, indicate that the silicone IOL is an effective choice.
J CATARACT REFRACT SURG—VOL 25, AUGUST 1999
1149
SILICONE AND PMMA IOL DECENTRATION IN RABBIT EYES
References 1. Apple DJ, Kincaid MC, Mamalis N, Olson RJ. Intraocular Lenses; Evolution, Designs, Complications, and Pathology. Baltimore, MD, Williams & Wilkins, 1989; 133–168 2. Hansen SO, Tetz MR, Solomon KD, et al. Decentration of flexible loop posterior chamber intraocular lenses in a series of 222 postmortem eyes. Ophthalmology 1988; 95:344 –349 3. Rochels R, Nover A. Untersuchung der Ha¨ufigkeit und Entstehung der Dezentrierung Kapselsackfixierter Hinterkammerlinsen. Klin Monatsbl Augenheilkd 1988; 193:585–588 4. Kimura W, Kimura T, Sawada T, et al. Intraocular lenses decentration after two years of envelope technique ECCE. Jpn IOL Soc J 1989; 3:185–192 5. Miyake K. Some considerations on methods of anterior lens capsulotomy and mechanisms of lens capsule opacification. Jpn IOL Soc J 1989; 3:236 –239 6. Ohmi S. Decentration associated with asymmetric capsular shrinkage and intraocular lens size. J Cataract Refract Surg 1993; 19:640 – 643
1150
7. Ohmi S, Uenoyama K. Decentration associated with asymmetric capsular shrinkage and intraocular lens design in a rabbit model. J Cataract Refract Surg 1995; 21:293–296 8. Ohmi S, Uenoyama K. Experimental evaluation of posterior capsule opacification and intraocular lens decentration: comparison of intraocular lenses of 12.5 mm and 14.0 mm diameter. J Cataract Refract Surg 1993; 19: 348 –351 9. Nishi O, Nishi K. Fibrous opacification of the anterior capsule after cataract surgery. Jpn J Clin Ophthalmol 1991; 45:1811–1815 10. Namiki M, Yamamoto N, Tagami Y, et al. Risk factors for anterior capsular shrinkage in intraocular lens implantation. Jpn J Clin Ophthalmol 1991; 45:1828 – 1831 11. Taguchi H, Kimura W, Kimura T, et al. Comparison of shape recovery ratios of polyimide IOL haptics. Jpn IOL Soc J 1992; 6:323–329 12. Sakka Y, Nishi O, Yamamoto M. Intraocular lens: soft lens. In: Masuda K, Oguti Y, Kozaki M, eds, Compact Ophthalmology: Clinic of Cataract. Tokyo, Kanahara & Co Ltd, 1992; 93–94
J CATARACT REFRACT SURG—VOL 25, AUGUST 1999