- Email: [email protected]
Effect of intraocular lens haptic compressibility on the posterior lens capsule after cataract surgery1
Effect of intraocular lens haptic compressibility on the posterior lens capsule after cataract surgery William R. Meacock, FRCOphth, David J. Spalton, FRCP, FRCS, FRCOphth ABSTRACT Purpose: To evaluate the effect of intraocular lens (IOL) haptic compressibility on the posterior capsule after cataract surgery. Setting: Teaching hospital, London, United Kingdom. Methods: In this randomized prospective study, 60 patients had standardized phacoemulsification with in-the-bag placement of a poly(methyl methacrylate) (PMMA) (Storz P497UV) or hydrogel (Storz Hydroview H60M) IOL. Both IOLs had PMMA haptics of identical configuration and length. The IOL haptic compressibility was measured in air and then during incubation in saline at 37°C over 1 month. Digital retroillumination imaging was performed 1, 7, 28, 90, 180, 360, and 720 days postoperatively. The presence and duration of postoperative capsule folds were recorded and correlated with the haptic compressibility measurements, lens epithelial cell (LEC) growth patterns on the posterior capsule at 6 months, and the extent of posterior capsule opacification. Results: On the first postoperative day, 21 patients (88%) in the Hydroview group had posterior capsule folds that persisted in 12 patients (50%) for 2 years. Nineteen patients (68%) in the PMMA group had folds at day 1 (P ⫽ .01), with 1 patient (3%) still having folds at 1 month (P ⫽ .0002) and no patient having folds at 3 months. At 6 months, 11 patients (46%) in the Hydroview group and no patient in the PMMA group had LEC growth in the direction of the folds. The PMMA IOLs showed a greater decrease in haptic compressibility during incubation. Conclusions: Haptic compressibility should be an important consideration in IOL design. The results suggest that to avoid posterior capsule folds, the compressibility should be less than 2.5 mN. J Cataract Refract Surg 2001; 27:1366 –1371 © 2001 ASCRS and ESCRS
P
osterior capsule opacification (PCO) is a common and important complication of cataract surgery caused by the wound-healing response induced by the surgery and characterized by proliferation and fibrous Accepted for publication January 31, 2001. Reprint requests to D.J. Spalton, FRCP, FRCS, FRCOphth, The Eye Department, St. Thomas’ Hospital, London SE1 7EH, United Kingdom. E-mail: [email protected]. © 2001 ASCRS and ESCRS Published by Elsevier Science Inc.
metaplasia of residual lens epithelial cells (LECs). Several studies have shown that surgical technique, intraocular lens (IOL) material, and IOL design influence the formation of PCO. For example, a capsulorhexis that is smaller in diameter than the IOL optic produces less capsule wrinkling and is more effective in preventing LEC proliferation than a capsulorhexis that lies off the IOL.1 Different PCO rates are seen with polyacrylic, poly(methyl methacrylate) (PMMA), silicone, and hy0886-3350/01/$–see front matter PII S0886-3350(01)01024-0
EFFECT OF IOL HAPTIC COMPRESSIBILITY ON POSTERIOR CAPSULE
drogel IOL materials,2,3 and factors such as apposition of the posterior optic surface to the capsule (no space, no cells) and the effect of the IOL edge profile on PCO prevention are known to be important design features.4,5 Clinical experience has shown that with some types of IOLs, folds can be seen postoperatively in the posterior capsule in the axis of contact between the haptics and bag. These may cause visual symptoms by light scattering or act as conduits for LEC growth. This study evaluated the relationship between the presence and persistence of folds postoperatively and IOL haptic compressibility as well as the effect of folds on later development of PCO. The 2 IOLs compared were of different optic material with haptics of the same material, length, and configuration but of different rigidity.
Patients and Methods The patients in this study formed part of a larger cohort for which some aspects have been reported.3 After approval from the hospital Ethics Committee, patients were recruited in a continuous cohort and prospectively randomized at surgery to the IOL type. All patients were seen preoperatively by the same clinician. Inclusion criteria were the presence of senile cataract in an otherwise normal eye in patients older than 55 years. Exclusion criteria were a history of ocular disease, intraocular surgery, laser treatment, diabetes requiring medical control, glaucoma, uveitis, and posterior segment pathology that would preclude a postoperative visual acuity of 20/40 or better. Patients using topical medications (apart from lubricants) were excluded. Patients who had cataract surgery in the contralateral eye in the previous 4 months and those unable to give informed consent were also excluded. All patients had phacoemulsification with continuous curvilinear capsulorhexis (CCC) performed by a single surgeon (D.J.S.) using peribulbar anesthesia. The surgical technique and medication were standardized. A superior scleral tunnel incision was made, and the anterior chamber was reformed with sodium hyaluronate 1% (Healon威). A CCC between 5.0 and 6.0 mm was created with a forceps. The nucleus was removed by a phaco-chop technique and soft lens material, by irrigation/aspiration (I/A) with balanced salt solution (BSS威) containing vancomycin and epinephrine. No attempt
was made to remove LECs by polishing the anterior capsule. The bag was reformed with Healon and the section enlarged. The patients were then randomized to receive a 3-piece PMMA IOL (Storz P497UV) or a 3-piece hydrogel IOL (Storz, Hydroview H60M), which was implanted in the bag. Both IOLs had an optic diameter of 6.0 mm and an overall diameter of 12.5 mm with C-loop PMMA haptics (Figure 1). The Healon was removed by I/A with BSS. Surgical complications such as capsulorhexis rim tear, zonular dehiscence, posterior capsule rupture, or vitreous loss led to patient exclusion and replacement. Postoperatively, all patients used neomycin, polymyxin B sulfates, and dexamethasone drops (Maxitrol威) 4 times a day for 1 month. No nonsteroidal antiinflammatory preparation was used before, during, or after surgery. Postoperatively, all IOLs were confirmed to have in-the-bag placement. Patients were examined 1, 7, 30, 90, 180, 360, and 720 days postoperatively. Digital retroillumination imaging of the posterior capsule was performed at each visit through fully dilated pupils using a purpose-built, highresolution, digitized retroillumination camera system.7 All images were analyzed by 2 experienced ophthalmologists. The images from the first postoperative day were analyzed to determine the presence and number of folds in the posterior capsule. The duration of the folds was then established by serial examination of the images. Images taken 6 months after surgery were analyzed to characterize the growth patterns of the LECs and classified by forced-choice into 4 major categories: peripheral ⫽ LECs growing in from the periphery in a
Figure 1. (Meacock) A Hydroview (right) and Storz P497UV PMMA (left) IOL (original magnification ⫻2).
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circumferential pattern; plaque ⫽ LECs in continuous sheets; scattered ⫽ isolated islands of cells; directional ⫽ cells showing signs of growth in the orientation of the capsule folds. Examples of these are shown in Figure 2. At 180, 360, and 720 days, the percentage area of PCO in each image was calculated objectively using dedicated software. The software produces a segmentation of the image using an operator based on the texture of the image.8 The force required to reduce the overall IOL hapticto-haptic diameter to 10.0 mm by compressing the hap-
tics was measured for the same models of IOLs using 3 PMMA and 3 Hydroview IOLs, each with a dioptric power of 22.00. Measurements were made first at room temperature. The IOLs were then incubated in saline at 37°C for 1 month. At weekly intervals during the incubation period, the lenses were removed from the saline bath for force measurements and then returned to incubation. The data for percentage of PCO were normally distributed and analyzed by the Student t test. Binary data were analyzed by the Fisher exact test.
Figure 2. (Meacock) Lens epithelial cell growth patterns on the posterior capsule. A: Peripheral. B: Plaque. C: Scattered. D: Directional. 1368
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Table 1. Lens epithelial cell growth patterns. Pattern, n (%) Group
Clear Peripheral Scattered Plaque Directional
Hydrogel
1 (4)
5 (20)
PMMA
2 (7)
17 (61)
0
7 (29)
5 (18)
4 (14)
11 (47) 0
PMMA ⫽ poly(methyl methacrylate)
Figure 3. (Meacock) Percentage of patients with postoperative capsule folds versus time.
Results Thirty patients received a PMMA lens and 30, a Hydroview IOL. The mean age of the patients was 73.6 years (range 60 to 90 years). There was no difference in age or sex distribution between the 2 groups. Six patients (20%) in the Hydroview IOL group were excluded from analysis, 3 (10%) because of no day 1 images, 2 (6%) because the day 1 image was of poor quality, and 1 (3%) because there was no 6 month image. In the PMMA IOL group, 2 (6%) patients were excluded because they had no day 1 image. On the first postoperative day, 21 patients (88%) in the Hydroview group and 19 (68%) in the PMMA group had posterior capsule folds (P ⫽ .01). Figure 3
Figure 4. (Meacock) Haptic compressibility forces before and after incubation in saline at 37°C for Hydroview and PMMA IOLs.
shows what happened to the capsule folds during the 2 years after surgery. The folds in the Hydroview group persisted for the full duration of follow-up. One patient (4%) in the PMMA group had folds at 1 month; no patient had folds after this (P ⫽ .0002). The haptic compression force of both the Hydroview and PMMA IOLs before and after incubation in saline at 37°C is shown in Figure 4. Both IOLs had similar rigidity before incubation in saline; however, the decay in haptic rigidity (or decreased compressibility) was less in the Hydroview group over the entire incubation period. No patient in the PMMA group had capsule folds by the time haptic rigidity had fallen below 2.5 mN at the 1 month incubation. Table 1 shows the LEC growth patterns at 6 months in the PMMA and Hydroview groups. The PMMA group tended to have peripheral LEC growth onto the capsule. In the Hydroview group, the growth was orientated to the axis of the haptic–capsule contact, particularly when there were multiple folds in the capsule (Figure 5). Table 2 shows the mean area of PCO measured by the image-analysis software. By 3 months, there was significantly more PCO in the Hydroview group than in
Figure 5. (Meacock) Percentage of Hydroview patients with directional LEC growth at 6 months in relation to the number of folds at day 1.
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Table 2. Mean percentage area of PCO postoperatively. Mean % PCO (SD) Group
3 Months
6 Months
1 Year
2 Years
Hydrogel
53.7 (28.3)
61.2 (28.9)
62.5 (31.0)
62.6 (23.7)
PMMA
26.9 (17.6)
26.3 (16.0)
29.7 (22.6)
45.6 (32.0)
PMMA ⫽ poly(methyl methacrylate)
the PMMA group (P ⫽ .01). However, there was no relationship in either IOL group between the extent of PCO at 6 months and the number of capsule folds at day 1 or the duration of the folds.
Discussion In this prospective study, we compared 2 IOL models and their effect on the posterior capsule. Although the 2 IOLs were of different optic materials, the PMMA haptics had the same length and configuration but had different memory compressibility characteristics. Folds in the posterior capsule are commonly seen after cataract surgery and may lead to increased PCO and cause optical aberrations. They lie parallel to the axis of the haptic loops and are caused by the contact of the haptic loops stretching the bag. The human lens capsular bag has a diameter of 10.5 mm after removal of the crystalline lens and can be stretched to a maximum diameter of 12.0 mm after IOL implantation.9 Most posterior chamber IOLs are manufactured with a haptic length of 12.5 to 13.5 mm. Their effect on the bag depends on the degree of haptic compressibility, the bag diameter, and how the haptic configuration distributes the force along the equatorial bag (eg, C-loop versus J-loop). The first day after surgery, patients in both the PMMA and Hydroview IOL groups had a high incidence of capsule folds, although there was a significantly larger proportion of patients with folds in the Hydroview group (P ⫽ .01). In both groups, the rate of disappearance of the folds was greatest during the first month, falling from 88% at day 1 to 57% at 1 month in the Hydroview group and from 68% to 4% in the PMMA group (P ⫽ .0002). There was no further loss of folds in the Hydroview patients, whereas in the PMMA group, all folds were gone by 3 months. We think the most likely reason for this is that there are differences between the 2 IOLs in haptic memory or compressibility. Before incubation, the haptics of the 1370
PMMA IOLs were more rigid and required a greater compressibility force. To simulate in vivo conditions, the IOLs were incubated in saline at 37°C for 1 month. With PMMA IOLs, there was a greater than 50% loss in haptic rigidity after 1 day of incubation in saline, and the compressibility force decreased throughout the incubation period. In contrast, although the Hydroview lens had a lower compressibility force before incubation, this force decreased to a lesser extent during incubation. Thus, these IOLs were more rigid than the PMMA IOLs throughout the study. Although the 2 IOLs had different optic materials, we think this is unlikely to relate to capsule folds. We believe that the most important difference between the 2 IOLs was the maintenance of haptic rigidity with the Hydroview lenses, leading to the higher proportion of patients with folds at day 1 and their persistence for up to 2 years postoperatively. Our results suggest that a compression force of less than 2.0 to 2.5 mN is required to prevent folding of the posterior capsule by the IOL. This has practical implications for IOL design and manufacture. No information is available on whether lens bag size is related to axial length; that is, whether smaller eyes have smaller lens bags. However, this did not appear to be a confounding factor in our study. The difference between groups in axial length was not significant. The mean axial length was 23.04 mm ⫾ 0.94 (SD) in the Hydroview group and 23.14 ⫾ 0.94 mm in the PMMA group. Three eyes (12%) in the Hydroview group and 2 (7%) in the PMMA group had an axial length of less than 22.0.0 mm. There was, therefore, no evidence that axial length had an effect on capsule folds. The retroillumination camera we used makes it possible to study the temporal changes in the posterior capsule and to analyze LEC growth patterns at any time by virtue of the high-resolution of the images (25 000 pixels per mm2). We determined the morphology and extent of LEC growth on the posterior capsule at 6 months to identify whether the folds in the 2 IOL groups influenced the pattern of LEC growth and the extent of PCO. Patterns of LEC growth were classified into 4 types of PCO morphology. The most important difference was the presence of LEC growth in the orientation of the axis of the folds in 47% of patients in the Hydroview group; this pattern was not present in any patient in the PMMA group. In some cases, the LECs were within the folds (Figure 6). In other cases, the directionally
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In conclusion, this study shows that the persistence of folds in the posterior capsule is likely attributable to the maintenance of haptic rigidity after cataract surgery. The results suggest that if the compressibility force threshold is below 2.5 mN, folds are unlikely to occur, a finding that has practical implications for IOL design.
References
Figure 6. (Meacock) A Hydroview IOL with LECs growing within a fold (small arrows) and LECs at a distance from the fold (large arrows).
orientated LECs were at a distance from the folds, suggesting that the entire posterior capsule was linearly tensioned. At 6 months, no eye in the PMMA group had an orientational growth pattern; the LECs were confined to the periphery of the IOL optic or were in a scattered, broken-up pattern. The results in the Hydroview group were used to determine whether the number of folds on day 1 significantly influenced the directional growth of LECs. All patients with more than 2 folds showed evidence of directional growth; however, in both groups, neither the number of capsule folds at day 1 nor the duration of the folds had an effect on the percentage area of PCO at 6 months. Hydroview IOLs have been shown to produce more PCO than PMMA IOLs,3 and we believe that the propensity of Hydroview IOLs to develop LEC growth probably masks the influence of capsule folds. Although persistent capsule folds made no difference in the extent of PCO formation in Hydroview lenses, they may be an important factor with IOL materials. The results from the PMMA group suggest that folds need to be present for longer than 1 month to be a significant factor in promoting PCO formation. To test this hypothesis, it would be necessary to repeat this study with 2 identical PMMA IOL models, 1 in which the haptic compressibility force decreases postoperatively and the other in which haptic rigidity is retained. For reasons of expense and regulatory approval, this is unlikely to be possible.
1. Hollick EJ, Spalton DJ, Meacock WR. The effect of capsulorhexis size on posterior capsular opacification: oneyear results of a randomized prospective trial. Am J Ophthalmol 1999; 128:271–279 2. Hollick EJ, Spalton DJ, Ursell PG, et al. The effect of polymethylmethacrylate, silicone, and polyacrylic intraocular lenses on posterior capsular opacification 3 years after cataract surgery. Ophthalmology 1999; 106:49 –54; discussion by RC Drews, 54 –55 3. Hollick EJ, Spalton DJ, Ursell PG, et al. Posterior capsular opacification with hydrogel, polymethylmethacrylate, and silicone intraocular lenses: two-year results of a randomized prospective trial. Am J Ophthalmol 2000; 129:577– 584 4. Nishi O, Nishi K, Sakanishi K. Inhibition of migrating lens epithelial cells at the capsular bend created by the rectangular optic edge of a posterior chamber intraocular lens. Ophthalmic Surg Lasers 1998; 29:587–594 5. Nishi O, Nishi K, Mano C, et al. The inhibition of lens epithelial cell migration by a discontinuous capsular bend created by a band-shaped circular loop or a capsule-bending ring. Ophthalmic Surg Lasers 1998; 29: 119 –125 6. Hollick EJ, Spalton DJ, Ursell PG. Surface cytologic features on intraocular lenses; can increased biocompatibility have disadvantages? Arch Ophthalmol 1999; 117: 872– 878 7. Pande MV, Ursell PG, Spalton DJ, et al. High-resolution digital retroillumination imaging of the posterior lens capsule after cataract surgery. J Cataract Refract Surg 1997; 23:1521–1527 8. Barman SA, Hollick EJ, Boyce JF, et al. Quantification of posterior capsular opacification in digital images after cataract surgery. Invest Ophthalmol Vis Sci 2000; 41:3882– 3892 9. Assia EI, Legler UFC, Apple DJ. The capsular bag after short- and long-term fixation of intraocular lenses. Ophthalmology 1995; 102:1151–1157
From the Department of Ophthalmology, St. Thomas’ Hospital, London, United Kingdom. Alcon helped measure intraocular lens haptic compressibility.
J CATARACT REFRACT SURG—VOL 27, SEPTEMBER 2001
1371
P
osterior capsule opacification (PCO) is a common and important complication of cataract surgery caused by the wound-healing response induced by the surgery and characterized by proliferation and fibrous Accepted for publication January 31, 2001. Reprint requests to D.J. Spalton, FRCP, FRCS, FRCOphth, The Eye Department, St. Thomas’ Hospital, London SE1 7EH, United Kingdom. E-mail: [email protected]. © 2001 ASCRS and ESCRS Published by Elsevier Science Inc.
metaplasia of residual lens epithelial cells (LECs). Several studies have shown that surgical technique, intraocular lens (IOL) material, and IOL design influence the formation of PCO. For example, a capsulorhexis that is smaller in diameter than the IOL optic produces less capsule wrinkling and is more effective in preventing LEC proliferation than a capsulorhexis that lies off the IOL.1 Different PCO rates are seen with polyacrylic, poly(methyl methacrylate) (PMMA), silicone, and hy0886-3350/01/$–see front matter PII S0886-3350(01)01024-0
EFFECT OF IOL HAPTIC COMPRESSIBILITY ON POSTERIOR CAPSULE
drogel IOL materials,2,3 and factors such as apposition of the posterior optic surface to the capsule (no space, no cells) and the effect of the IOL edge profile on PCO prevention are known to be important design features.4,5 Clinical experience has shown that with some types of IOLs, folds can be seen postoperatively in the posterior capsule in the axis of contact between the haptics and bag. These may cause visual symptoms by light scattering or act as conduits for LEC growth. This study evaluated the relationship between the presence and persistence of folds postoperatively and IOL haptic compressibility as well as the effect of folds on later development of PCO. The 2 IOLs compared were of different optic material with haptics of the same material, length, and configuration but of different rigidity.
Patients and Methods The patients in this study formed part of a larger cohort for which some aspects have been reported.3 After approval from the hospital Ethics Committee, patients were recruited in a continuous cohort and prospectively randomized at surgery to the IOL type. All patients were seen preoperatively by the same clinician. Inclusion criteria were the presence of senile cataract in an otherwise normal eye in patients older than 55 years. Exclusion criteria were a history of ocular disease, intraocular surgery, laser treatment, diabetes requiring medical control, glaucoma, uveitis, and posterior segment pathology that would preclude a postoperative visual acuity of 20/40 or better. Patients using topical medications (apart from lubricants) were excluded. Patients who had cataract surgery in the contralateral eye in the previous 4 months and those unable to give informed consent were also excluded. All patients had phacoemulsification with continuous curvilinear capsulorhexis (CCC) performed by a single surgeon (D.J.S.) using peribulbar anesthesia. The surgical technique and medication were standardized. A superior scleral tunnel incision was made, and the anterior chamber was reformed with sodium hyaluronate 1% (Healon威). A CCC between 5.0 and 6.0 mm was created with a forceps. The nucleus was removed by a phaco-chop technique and soft lens material, by irrigation/aspiration (I/A) with balanced salt solution (BSS威) containing vancomycin and epinephrine. No attempt
was made to remove LECs by polishing the anterior capsule. The bag was reformed with Healon and the section enlarged. The patients were then randomized to receive a 3-piece PMMA IOL (Storz P497UV) or a 3-piece hydrogel IOL (Storz, Hydroview H60M), which was implanted in the bag. Both IOLs had an optic diameter of 6.0 mm and an overall diameter of 12.5 mm with C-loop PMMA haptics (Figure 1). The Healon was removed by I/A with BSS. Surgical complications such as capsulorhexis rim tear, zonular dehiscence, posterior capsule rupture, or vitreous loss led to patient exclusion and replacement. Postoperatively, all patients used neomycin, polymyxin B sulfates, and dexamethasone drops (Maxitrol威) 4 times a day for 1 month. No nonsteroidal antiinflammatory preparation was used before, during, or after surgery. Postoperatively, all IOLs were confirmed to have in-the-bag placement. Patients were examined 1, 7, 30, 90, 180, 360, and 720 days postoperatively. Digital retroillumination imaging of the posterior capsule was performed at each visit through fully dilated pupils using a purpose-built, highresolution, digitized retroillumination camera system.7 All images were analyzed by 2 experienced ophthalmologists. The images from the first postoperative day were analyzed to determine the presence and number of folds in the posterior capsule. The duration of the folds was then established by serial examination of the images. Images taken 6 months after surgery were analyzed to characterize the growth patterns of the LECs and classified by forced-choice into 4 major categories: peripheral ⫽ LECs growing in from the periphery in a
Figure 1. (Meacock) A Hydroview (right) and Storz P497UV PMMA (left) IOL (original magnification ⫻2).
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circumferential pattern; plaque ⫽ LECs in continuous sheets; scattered ⫽ isolated islands of cells; directional ⫽ cells showing signs of growth in the orientation of the capsule folds. Examples of these are shown in Figure 2. At 180, 360, and 720 days, the percentage area of PCO in each image was calculated objectively using dedicated software. The software produces a segmentation of the image using an operator based on the texture of the image.8 The force required to reduce the overall IOL hapticto-haptic diameter to 10.0 mm by compressing the hap-
tics was measured for the same models of IOLs using 3 PMMA and 3 Hydroview IOLs, each with a dioptric power of 22.00. Measurements were made first at room temperature. The IOLs were then incubated in saline at 37°C for 1 month. At weekly intervals during the incubation period, the lenses were removed from the saline bath for force measurements and then returned to incubation. The data for percentage of PCO were normally distributed and analyzed by the Student t test. Binary data were analyzed by the Fisher exact test.
Figure 2. (Meacock) Lens epithelial cell growth patterns on the posterior capsule. A: Peripheral. B: Plaque. C: Scattered. D: Directional. 1368
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EFFECT OF IOL HAPTIC COMPRESSIBILITY ON POSTERIOR CAPSULE
Table 1. Lens epithelial cell growth patterns. Pattern, n (%) Group
Clear Peripheral Scattered Plaque Directional
Hydrogel
1 (4)
5 (20)
PMMA
2 (7)
17 (61)
0
7 (29)
5 (18)
4 (14)
11 (47) 0
PMMA ⫽ poly(methyl methacrylate)
Figure 3. (Meacock) Percentage of patients with postoperative capsule folds versus time.
Results Thirty patients received a PMMA lens and 30, a Hydroview IOL. The mean age of the patients was 73.6 years (range 60 to 90 years). There was no difference in age or sex distribution between the 2 groups. Six patients (20%) in the Hydroview IOL group were excluded from analysis, 3 (10%) because of no day 1 images, 2 (6%) because the day 1 image was of poor quality, and 1 (3%) because there was no 6 month image. In the PMMA IOL group, 2 (6%) patients were excluded because they had no day 1 image. On the first postoperative day, 21 patients (88%) in the Hydroview group and 19 (68%) in the PMMA group had posterior capsule folds (P ⫽ .01). Figure 3
Figure 4. (Meacock) Haptic compressibility forces before and after incubation in saline at 37°C for Hydroview and PMMA IOLs.
shows what happened to the capsule folds during the 2 years after surgery. The folds in the Hydroview group persisted for the full duration of follow-up. One patient (4%) in the PMMA group had folds at 1 month; no patient had folds after this (P ⫽ .0002). The haptic compression force of both the Hydroview and PMMA IOLs before and after incubation in saline at 37°C is shown in Figure 4. Both IOLs had similar rigidity before incubation in saline; however, the decay in haptic rigidity (or decreased compressibility) was less in the Hydroview group over the entire incubation period. No patient in the PMMA group had capsule folds by the time haptic rigidity had fallen below 2.5 mN at the 1 month incubation. Table 1 shows the LEC growth patterns at 6 months in the PMMA and Hydroview groups. The PMMA group tended to have peripheral LEC growth onto the capsule. In the Hydroview group, the growth was orientated to the axis of the haptic–capsule contact, particularly when there were multiple folds in the capsule (Figure 5). Table 2 shows the mean area of PCO measured by the image-analysis software. By 3 months, there was significantly more PCO in the Hydroview group than in
Figure 5. (Meacock) Percentage of Hydroview patients with directional LEC growth at 6 months in relation to the number of folds at day 1.
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Table 2. Mean percentage area of PCO postoperatively. Mean % PCO (SD) Group
3 Months
6 Months
1 Year
2 Years
Hydrogel
53.7 (28.3)
61.2 (28.9)
62.5 (31.0)
62.6 (23.7)
PMMA
26.9 (17.6)
26.3 (16.0)
29.7 (22.6)
45.6 (32.0)
PMMA ⫽ poly(methyl methacrylate)
the PMMA group (P ⫽ .01). However, there was no relationship in either IOL group between the extent of PCO at 6 months and the number of capsule folds at day 1 or the duration of the folds.
Discussion In this prospective study, we compared 2 IOL models and their effect on the posterior capsule. Although the 2 IOLs were of different optic materials, the PMMA haptics had the same length and configuration but had different memory compressibility characteristics. Folds in the posterior capsule are commonly seen after cataract surgery and may lead to increased PCO and cause optical aberrations. They lie parallel to the axis of the haptic loops and are caused by the contact of the haptic loops stretching the bag. The human lens capsular bag has a diameter of 10.5 mm after removal of the crystalline lens and can be stretched to a maximum diameter of 12.0 mm after IOL implantation.9 Most posterior chamber IOLs are manufactured with a haptic length of 12.5 to 13.5 mm. Their effect on the bag depends on the degree of haptic compressibility, the bag diameter, and how the haptic configuration distributes the force along the equatorial bag (eg, C-loop versus J-loop). The first day after surgery, patients in both the PMMA and Hydroview IOL groups had a high incidence of capsule folds, although there was a significantly larger proportion of patients with folds in the Hydroview group (P ⫽ .01). In both groups, the rate of disappearance of the folds was greatest during the first month, falling from 88% at day 1 to 57% at 1 month in the Hydroview group and from 68% to 4% in the PMMA group (P ⫽ .0002). There was no further loss of folds in the Hydroview patients, whereas in the PMMA group, all folds were gone by 3 months. We think the most likely reason for this is that there are differences between the 2 IOLs in haptic memory or compressibility. Before incubation, the haptics of the 1370
PMMA IOLs were more rigid and required a greater compressibility force. To simulate in vivo conditions, the IOLs were incubated in saline at 37°C for 1 month. With PMMA IOLs, there was a greater than 50% loss in haptic rigidity after 1 day of incubation in saline, and the compressibility force decreased throughout the incubation period. In contrast, although the Hydroview lens had a lower compressibility force before incubation, this force decreased to a lesser extent during incubation. Thus, these IOLs were more rigid than the PMMA IOLs throughout the study. Although the 2 IOLs had different optic materials, we think this is unlikely to relate to capsule folds. We believe that the most important difference between the 2 IOLs was the maintenance of haptic rigidity with the Hydroview lenses, leading to the higher proportion of patients with folds at day 1 and their persistence for up to 2 years postoperatively. Our results suggest that a compression force of less than 2.0 to 2.5 mN is required to prevent folding of the posterior capsule by the IOL. This has practical implications for IOL design and manufacture. No information is available on whether lens bag size is related to axial length; that is, whether smaller eyes have smaller lens bags. However, this did not appear to be a confounding factor in our study. The difference between groups in axial length was not significant. The mean axial length was 23.04 mm ⫾ 0.94 (SD) in the Hydroview group and 23.14 ⫾ 0.94 mm in the PMMA group. Three eyes (12%) in the Hydroview group and 2 (7%) in the PMMA group had an axial length of less than 22.0.0 mm. There was, therefore, no evidence that axial length had an effect on capsule folds. The retroillumination camera we used makes it possible to study the temporal changes in the posterior capsule and to analyze LEC growth patterns at any time by virtue of the high-resolution of the images (25 000 pixels per mm2). We determined the morphology and extent of LEC growth on the posterior capsule at 6 months to identify whether the folds in the 2 IOL groups influenced the pattern of LEC growth and the extent of PCO. Patterns of LEC growth were classified into 4 types of PCO morphology. The most important difference was the presence of LEC growth in the orientation of the axis of the folds in 47% of patients in the Hydroview group; this pattern was not present in any patient in the PMMA group. In some cases, the LECs were within the folds (Figure 6). In other cases, the directionally
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EFFECT OF IOL HAPTIC COMPRESSIBILITY ON POSTERIOR CAPSULE
In conclusion, this study shows that the persistence of folds in the posterior capsule is likely attributable to the maintenance of haptic rigidity after cataract surgery. The results suggest that if the compressibility force threshold is below 2.5 mN, folds are unlikely to occur, a finding that has practical implications for IOL design.
References
Figure 6. (Meacock) A Hydroview IOL with LECs growing within a fold (small arrows) and LECs at a distance from the fold (large arrows).
orientated LECs were at a distance from the folds, suggesting that the entire posterior capsule was linearly tensioned. At 6 months, no eye in the PMMA group had an orientational growth pattern; the LECs were confined to the periphery of the IOL optic or were in a scattered, broken-up pattern. The results in the Hydroview group were used to determine whether the number of folds on day 1 significantly influenced the directional growth of LECs. All patients with more than 2 folds showed evidence of directional growth; however, in both groups, neither the number of capsule folds at day 1 nor the duration of the folds had an effect on the percentage area of PCO at 6 months. Hydroview IOLs have been shown to produce more PCO than PMMA IOLs,3 and we believe that the propensity of Hydroview IOLs to develop LEC growth probably masks the influence of capsule folds. Although persistent capsule folds made no difference in the extent of PCO formation in Hydroview lenses, they may be an important factor with IOL materials. The results from the PMMA group suggest that folds need to be present for longer than 1 month to be a significant factor in promoting PCO formation. To test this hypothesis, it would be necessary to repeat this study with 2 identical PMMA IOL models, 1 in which the haptic compressibility force decreases postoperatively and the other in which haptic rigidity is retained. For reasons of expense and regulatory approval, this is unlikely to be possible.
1. Hollick EJ, Spalton DJ, Meacock WR. The effect of capsulorhexis size on posterior capsular opacification: oneyear results of a randomized prospective trial. Am J Ophthalmol 1999; 128:271–279 2. Hollick EJ, Spalton DJ, Ursell PG, et al. The effect of polymethylmethacrylate, silicone, and polyacrylic intraocular lenses on posterior capsular opacification 3 years after cataract surgery. Ophthalmology 1999; 106:49 –54; discussion by RC Drews, 54 –55 3. Hollick EJ, Spalton DJ, Ursell PG, et al. Posterior capsular opacification with hydrogel, polymethylmethacrylate, and silicone intraocular lenses: two-year results of a randomized prospective trial. Am J Ophthalmol 2000; 129:577– 584 4. Nishi O, Nishi K, Sakanishi K. Inhibition of migrating lens epithelial cells at the capsular bend created by the rectangular optic edge of a posterior chamber intraocular lens. Ophthalmic Surg Lasers 1998; 29:587–594 5. Nishi O, Nishi K, Mano C, et al. The inhibition of lens epithelial cell migration by a discontinuous capsular bend created by a band-shaped circular loop or a capsule-bending ring. Ophthalmic Surg Lasers 1998; 29: 119 –125 6. Hollick EJ, Spalton DJ, Ursell PG. Surface cytologic features on intraocular lenses; can increased biocompatibility have disadvantages? Arch Ophthalmol 1999; 117: 872– 878 7. Pande MV, Ursell PG, Spalton DJ, et al. High-resolution digital retroillumination imaging of the posterior lens capsule after cataract surgery. J Cataract Refract Surg 1997; 23:1521–1527 8. Barman SA, Hollick EJ, Boyce JF, et al. Quantification of posterior capsular opacification in digital images after cataract surgery. Invest Ophthalmol Vis Sci 2000; 41:3882– 3892 9. Assia EI, Legler UFC, Apple DJ. The capsular bag after short- and long-term fixation of intraocular lenses. Ophthalmology 1995; 102:1151–1157
From the Department of Ophthalmology, St. Thomas’ Hospital, London, United Kingdom. Alcon helped measure intraocular lens haptic compressibility.
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