articles Loop memory of posterior chamber intraocular lenses of various sizes, designs, and loop materials Ehud I. Assia, M.D., Ulrich F.e. Legler, M.D., Victoria E. Castaneda, M.D., David

J.

Apple, M.D.

ABSTRACT Looped intraocular lenses (IOLs) fixate by exerting centripetal pressure on the ocular tissues. The ability of the flexible loops to maintain pressure depends on their rigidity (i.e., resistance to flexion) and their "memory" (i.e., ability to restore original configuration after a long period of compression). We studied the memory of 30 different posterior chamber IOLs, with loops made of polypropylene (PP) and poly(methyl methacrylate) (PMMA), of various diameters, styles, and designs. The lenses were inserted into plastic wells, 9.5 mm in diameter, and immersed in water (37°C) for one month. They were then placed on an open plate and allowed to re-expand for one month. Loop memory was calculated as the difference in diameter between the initial (pretest) measurement and measurements taken during the compression and release periods. The results showed that short (12.0 to 12.5 mm) IOLs had relatively better memory than longer (13.5 to 14.0 mm) IOLs. Those with PP loops expanded more and for longer periods than those of comparable size and design with PMMA loops. One-piece, all-PMMA lenses exhibited the best loop memory. These lenses have the high rigidity of the PMMA material and the good memory of the design. Thus, the total IOL diameter can be reduced to 12.0 mm while providing long-term constant pressure on the capsular bag to maintain stable fixation. Key Words: flexible looped intraocular lens, memory, poly(methyl methacrylate), polypropylene, posterior chamber lens

Flexible looped intraocular lenses (IOLs) fixate by exerting pressure on the surrounding ocular tissues, i.e. , the capsular bag or ciliary sulcus. The ability of the loops to maintain pressure for a long period is largely dependent on the loop material and the IOL design. For many years it has been assumed that loops made of polypropylene (PP)

have less "memory" (i.e., the ability to restore and maintain their original configuration) than loops made of poly(methyl methacrylate) (PMMA). This impression was based on the clinical observation that PP loops are easier to manipulate during implantation than PMMA loops and that PP loops have a tendency to bend toward the optic when the

From the Departments a/Ophthalmology and Pathology, Storm Eye Institute, Medical University a/South Carolina, Charleston. Supported in part by an unrestricted grant from the Research to Prevent Blindness, Inc. , New York. Reprint requests to David J. Apple, M.D., Department a/Ophthalmology, Medical University a/South Carolina, 171 Ashley Avenue, Charleston, South Carolina 29425.

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capsule contracts from late fibrosis. The present study was conducted to investigate the effects of loop material, IOL design, and total diameter on loop memory.

measurements between lenses of the same model. Thus, 64 samples were examined. The IOLs and their characteristics are listed in Table 1. The IOLs were placed in plastic wells with an inner diameter of 9.5 mm (a device used for preparing specimens for scanning electron microscopy) (Figure 1). After the IOL was inserted, each well was sealed with a metal cylinder to prevent flotation of the lenses and exit of the flexible loops. The wells were placed in a water tab which was placed in an incubator (Fisher Scientific, Isotemp Incubator, Model 630D). The temperature was

MATERIALS AND METHODS Thirty different IOLs made by three manufacturers were investigated (Iolab-ll lenses, AIcon-l0 lenses, Pharmacia-9 lenses). Each lens model was examined in duplicate. Four samples of two IOLs (Iolab GI06E and Pharmacia U37BC) were analyzed to study the intravariability of the Table 1. Characteristics of the IOLs. Manufacturer Iolab Iolab Iolab Iolab Iolab Iolab Iolab Iolab Iolab Iolab Iolab Alcon Alcon Alcon Alcon Alcon Alcon Alcon Alcon Alcon Alcon Pharmacia Pharmacia Pharmacia Pharmacia Pharmacia Pharmacia Pharmacia Pharmacia Pharmacia * P = piece t Mod = modified 542

Model 4291B P2S0D 3291B 7260S 6669B UI02D 6693B GI06E GI03E G276E GlS6H SK60CM SK22UO MC60CP MC60CM MC60BP MC60BD JF3BUO MCSOBM J403 C403 UV34L UBM2F UBM2B UB89 UB7l UB120 UVS2A UVS2F U37BC

Optic Diameter (mm) 6.0 7.0 6.0 6.0 6.0 6.0 6.0 6.0 6.0 6.0 7.0 6.0 6.0 6.0 6.0 6.0 6.0 6.0 6.S 6.S 6.S 6.0 6.S 6.S 7.0 6.0 6.0 6.0 6.0 6.0

Loop Diameter (mm) 12.S 14.0 12.S 14.0 14.0 14.0 12.S 13.S 13.0 13.S 13.S 14.0 14.0 13.S 13.S 14.0 13.S 13.S 14.0 14.0 14.0 14.0 13.S 13.S 14.0 13.S 12.0 14.0 13.S 13.S

Loop Material PMMA PMMA PMMA PMMA PMMA PP PMMA PP PP PMMA PP PMMA PP PP PMMA PP PMMA PMMA PMMA PMMA PMMA PMMA PMMA PMMA PMMA PMMA PMMA PP PP PP

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Style* 3P 3P 3P 1P IP 3P 1P 3P 3P 3P 3P 3P 3P 3P 3P 3P 1P 1P 3P 1P IP 3P 3P 3P 1P 1P IP 3P 3P 3P

Designt ModC ModJ ModJ ModJ C ModJ C ModC Mod J ModC ModC ModJ ModJ ModC ModC ModC ModC JAFFE(C) ModC M6dJ ModC ModC ModC ModC Modc ModC ModC ModC ModC ModJ

Fig. 1.

(Assia) A three-piece, modified-C, PP-Iooped posterior chamber IOL with a total diameter of 14.0 mm (Alcon/Cilco MC60BP) was placed in a 9.5 mm plastic well. The hole at the bottom provided access of water into the covered well.

maintained at 37°C (range 35°C to 38°C). A small opening at the bottom of each well allowed the IOLs to be constantly immersed in water. The total diameter of the loops was measured with a metric ruler and a vernier dial-type caliper with a metric scale (Manostat). Readings were taken by two observers (KI.A., U.F.C.L.) and averaged for each lens. Measurements were taken before the compression test and at two and four weeks of IOL compression. These measurements were taken within five minutes of the IOLs' removal from the wells. After the third set of measurements, the IOLs were placed on an open plate at room temperature (approximately 24°C). Additional measurements were taken at two days and one month without compression (one and two months from the initiation of the study, respectively). The ability of the loops to re-expand (loop "memory") was calculated as the difference between the initial measurement (pretest) and the reading at each testing period. We used the pretest reading rather than the IOL diameter given by the manufacturer as the reference value. RESULTS The pretest readings were usually close to the values given by the manufacturer. The differences between the measured and given values were within ±0.1 mm in 92% of the IOLs from lolab, 85% of the IOLs from Alcon, and 65% of the IOLs from Pharmacia. The largest difference was 0.3 mm

for each of the three companies. Maximal intravariability (difference between IOLs of the same model) was 0.1 mm for lolab, 0.2 mm for Alcon, and 0.2 mm for Pharmacia. No differences between IOLs were evident in 83%, 50%, and 50%, respectively. The results of the re-expansion of the loops (differences between pretest and test measurements) for each manufacturer are shown in Figure 2. The lower the difference in values, the better the loop memory (loop diameter closer to the original diameter). Thus a lower line represents the group of IOLs with a better memory. For all three manufacturers the three-piece IOLs with both PMMA and PP loops had similar rates of re-expansion during the compression phase (two and four weeks). Following removal from the wells, the three-piece PMMA-Iooped lenses expanded less than the three-piece PP-Iooped lenses (one month). The differences were even more significant at the last measurement (two months) after the IOLs were allowed to re-expand for a month. The one-piece design always exhibited a better memory than any similar-sized three-piece IOL. Since the results among the three manufacturers were generally the same, we grouped all the IOLs and compared lenses according to their total diameter, loop material, and lens design. We did not have IOLs with 12.0 to 12.5 mm total diameter and three-piece, PP-Ioop design; therefore, there were a total offive groups (Figure 3): (1) 12/1P/PMMA, (2) 12/3P/PMMA, (3) 14/1P/PMMA, (4) 14/3P/ PMMA, and (5) 14/3P/PP (12 and 14 indicate lenses with a total diameter of 12.0 to 12.5 mm and 13.5 to 14.0 mm, respectively; 1P and 3P indicate one-piece and three-piece design). Statistical analysis revealed the following: Total IOL diameter: 12.0-12.5 mm vs 13.514.0 mm. Statistically significant differences (P < .01) were recorded between 12/1P/PMMA and 14/1P/PMMA and between 12/3P/PMMA and 14/3P/PMMA at all measurement periods. In general, small IOLs had better memory than large IOLs. Loop design: one-piece vs three-piece. Differences between 12/1P/PMMA and 12/3P/PMMA and between 14/1P/PMMA and 14/3P/PMMA were statistically significant at all test periods (P < .0001). Thus, the one-piece PMMA had a significantly better memory than the three-piece design. Loop material: PMMA vs PP. When measured immediately after removal from the wells (two and four weeks) there were no differences between the 14/3P/PMMA and 14/3P/PP lenses in re-expansion. However, significant differences in re-expan-

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IOlAS t::.

ALCON .c:.

DIAMETER (mm)

4.-·------~----------------------------__,

DIAMETER (mm)

4~--~~----

3~-----------------

2~~~--------~~~------~~~------~

oL------------L~--------~----------~

2 WEEKS

4 WEEKS

1 MONTH (2 DAV$ OUT)

--f3--

-+-

1 P PMMA

3 P PMMA

-+-

2 MONTH

o

--Q--

3 P PP

Fig. 2.

DIAMETER (mm)

3~----------------~~~~----------~

2~--------------~~------~--~~----~

o

~

_

_____.L._lJ'_____'_____---' 4 WEEKS

1 MONTH

12 DAVS OUT)

- - 1 P PMMA

-+-

3 P PMMA

-+-

2 MONTH 11 MONTH OUT)

3 P PP

sion were noted after the IOLs were allowed to expand with no external pressure. The maximum difference was seen after the IOLs were out of the wells for one month (P< .0001). Loops made ofPP generally had better memory than those ofPMMA. Poly(methyl methacrylate) loops had almost reached their maximum diameter after two days whereas PP loops continued to re-expand. DISCUSSION Fixation oflooped IOLs is achieved by the spring effect of the flexible loops. During insertion the loops are bent centrally and re-expand as the forces exerted on the loops are released. Stable fixation is maintained by the centripetal pressure of the loops against the ocular tissues. Positioning the IOL within the capsular bag is associated with a lower rate of complications than fixating the loops in the ciliary region. 1,2 Capsular fixation can be 544

1 MONTH (2 DAYS

4,------------,-------------------

2 WEEKS

4 WEEKS

2 WEEKS

(t MONTH OUT)

PHARMACIA .c:.

2~----------------~~------------~~

1 P PMMA

-+- 3 P PMMA

ouT)

2 MONTH (1 MON 11'4 OUT'

-+- 3 P PP

(Assia) Re-expansion of the loops after compression in the well. Loop memory was calculated as the difference in total diameter between a measured value and the pretest value. Thus, a lower value indicates better memory (more re~expansion). The IOLs were placed in the wells for one month and measured at two weeks and four weeks. The lenses were then placed on a plate (arrow) and additional measurements were taken at two days and one month without external pressure (one and two months from initiation of the study). Lenses from the three manufacturers, Iolab, Alcon, and Pharmacia, exhibited similar re-expansion patterns. Three~piece IOLs of both PP and PMMA demonstrated comparable expansion during the compression phase (two and four weeks), but PP loops expanded significantly more after release from external pressure (one and two months). One-piece, all-PMMA lenses showed the best loop memory.

achieved in practically all cases when the continuous curvilinear capsulorhexis technique is used. 3 - 6 Two factors contribute to the ability of IOL loops to maintain their original symmetrical configuration: 1. Loop rigidity/flexibility-the resistance of the haptic to external forces that act to bend the loops centrally. 2. Loop memory-the ability of the loops to re-expand laterally to their original size and configuration. Many surgeons feel that the rigidity of loops made of PP is significantly lower than that of loops made of PMMA. This is based on the clinical impression of the ease with which PP loops are bent at the time of IOL implantation and the effect of secondary capsular fibrosis on these loops. In our study we had a similar impression while inserting the IOLs into the wells. Lenses with PP loops were

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~

DIAMETER (mm)

4,-- -- - - - - -- - - -- -- - - -- - - -- - - ------

3r---------------~~-=~--~==~"~ / 3P~IP=MM:1A

t2/3" ...... A.

Table 2. Characteristics of loop material and design. Characteristic

3-Piece PP

3-Piece PMMA

I-Piece PMMA

Rigidity

Low

High

High

Memory

High

Low

High

2~--------------~~------~~----~ .4/3 ..'''P

oL---------~~--

2 WEEKS

4 WEEKS

______

~

______

1 MONTH 12 DAYS DUn

Fig. 3.

2 MONTH It WONTH OUT)

(Assia) Expansion curves ofIOLs from all three manufacturers grouped according to the total diameter, lens design , and loop material. This graph demonstrates that IOLs with a total diameter ofl2.0 to 12.5 mm (marked as 12) had better memory than 13.5 to 14.0 mm lenses (marked as 14). One-piece (IP) design provided better memory than three-piece (3P) design. Polypropylene (PP) loops expanded more and for a longer period of time than loops made of PMMA of IOLs with similar design and total diameter.

inserted most easily; lenses with PMMA loops required greater force. This was most marked with the long, one-piece, all-PMMA IOLs which tended to pop out of the wells; therefore, the wells had to be sealed with a cover. The lower rigidity of the PP loops was often manifested as an asymmetrical bending of the loops and as slight decentration (which was manually corrected). The higher memory of the PP loops, as evidenced when tested without external pressure, did not generate enough force to recenter the optics. In contrast, PMMA lenses always centered well within the wells. Guthoff and associates 7 studied the resistance of various loop materials to compression forces . They did not find any significant differences between PMMA and PP haptics, whereas soft materials were generally more resistant. However, the lenses in their study were made by 12 different manufacturers and included various lens designs. They did not differentiate between the one-piece and the three-piece loop designs. The results of their compression study showed a large variability in the physical behavior of both PP and PMMA lenses. It is possible that other factors, such as loop design, played a significant role, thus masking the net effect of the material. The ability of the loops made of PMMA and PP to re-expand after long-term compression was studied by Drews and Kreiner. 8 In their elegant study, all factors were identical except for the loop

material. They have shown that PP has a significantly lower memory than PMMA and that memory in the PP group was maintained for a longer period (three years for PP and three months for PMMA loops). In addition, the re-expansion pattern of the two materials differs in such a way that most of the PMMA re-expansion was achieved within the first day, whereas PP continued to expand even after one week. These results agree with our results, indicating that PP, as a material, has a better memory than PMMA in total re-expansion and in length of time in which additional expansion is recorded. Since all IOLs in our study were compressed to the same well diameter (9.5 mm), the longer IOLs sustained relatively more compression than the shorter IOLs and had a greater distance to reexpand. Therefore, it is not surprising that the shorter lenses had a relatively better memory (lower curve) . With both short and long IOLs, the three-piece PMMA design exhibited a flat expansion curve, indicating that most of the expansion was achieved within the first two days after release of external pressure. In contrast, the PP continued to expand for a longer period. However, the most significant factor in loop memory was the onepiece IOL design. The low rigidity of the PP loops allows them to "give" more easily to external forces such as capsular fibrosis. For this reason it is more common to see central bending or asymmetrical distortion of the configuration ofPP loops than ofPMMA loops. In recent years there has been a trend toward downsizing the total diameter of IOLs to 12.0 to 12.5 mm. This is supported by studies showing that the diameter of the evacuated bag, after removal of the lens substance, is 10.5 mm.9-11 The average diameter of the ciliary sulcus is 11.1 mm. 12, 13 In the presence of intact zonules the capsular bag cannot stretch to more than 12.0 mm even when an IOL with a total diameter of 14.0 to 14.5 mm is placed in the bag. 13 Therefore, since capsular fixation can be assured by using the capsulorhexis technique, an IOL diameter greater than 12.0 mm is not necessary. Small lenses are easier to implant because less bending of the loops is required. They also cause less stretching and distortion of the capsular bag than larger IOLs, maintaining the circular

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configuration of the bag. However, reducing the total diameter of the haptic also reduces the spring effect of the flexible loops. To assure stable and long-term fixation, more resistance to compression forces, by high rigidity and high memory, is required. The one-piece all PMMA loop design achieves both conditions (Table 2). The one-piece loop design (high memory design) compensates for the quality of the PMMA (low memory material). The desired loops should have adequate rigidity to resist contraction forces, yet they should have the flexibility to allow easy insertion and smooth accommodation to the circular shape of the capsular bag. Even though the optimal parameters of factors such as loop rigidity and memory and haptic material are not determined, it seems that a onepiece design, all-PMMA construction, broad Cloop style and downsized total loop diameter provide a good combination for flexible looped IOLs.

5.

6.

7.

8. 9. 10.

REFERENCES 1. Apple DJ, Kincaid MC, Mamalis N, Olson RJ. Intraocular Lenses; Evolution, Designs, Complications and Pathology. Baltimore, Williams & Wilkins, 1989 2. Apple DJ, Mamalis N, Loftfield K, et at. Complications of intraocular lenses. A historical and histopathological review. Surv Ophthalmol1984; 29:1-54 3. Assia EI, Apple DJ, Barden A, et at. An experimental study comparing various anterior capsulectomy techniques. Arch Ophthalmol1991; 109:642-647 4. Assia EI, Apple DJ, Tsai JC, Lim ES. The elastic prop-

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II . 12.

13.

erties of the lens capsule in capsulorhexis. Am J Ophthalmol 1991; 111:628-632 Apple DJ, Assia EI, Wasserman D, et at. Evidence in support of the continuous tear anterior capsulectomy (capsulorhexis technique). In: Cangelosi CC, ed, Advances in Cataract Surgery; New Orleans Academy of Ophthalmology. Thorofare, NJ, Slack Inc, 1991; 2147 AppleDJ, Tsai JC, Castaneda YE, et at. Posterior chamber intraocular lens (PC IOL); a clinical goal with bifocal and multifocal IOLs. In: Maxwell WA, Nordan LT, eds, Current Concepts of Multifocal Intraocular Lenses. Thorofare, NJ, Slack Inc, 1991; 219-231 Guthoff R, Abramo F, Draeger J, Chumbley L. Measurement of elastic resisting forces of intraocular haptic loops of varying geometrical designs and material composition. J Cataract Refract Surg 1990; 16:551558 Drews RC, Kreiner C. Comparative study of the elasticity and memory of intraocular lens loops. J Cataract Refract Surg 1987; 13:525-530 Assia EI, Apple DJ. Side-view analysis of the lens. Part I: The crystalline lens and the evacuated capsular bag. Arch Ophthalmol1992; 110:89-93 Galand A, Bonhomme L, Collee M. Direct measurement of the capsular bag. Am Intra-Ocular Implant Soc J 1984; 10:475-476 Richburg FA, Sun HS. Size of the crushed cataractous capsule bag. Am Intra-Ocular Implant Soc J 1983; 9: 333-335 Assia EI, Castaneda VE, Legler UFC, et at. Studies on cataract surgery and intraocular lenses at the Center for Intraocular Lens Research. Ophthalmol Clin North Am 1991; 4:251-266 AssiaEI, Legler UFC, Castaneda VE, et at. Anatomical changes of the capsular bag after lens removal and intraocular lens implantation. ARVO Abstracts. Inv Ophthalmol Vis Sci 1991; 32(4):797

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