Comparison of shape recovery ratios in various intraocular lens hap tics Wataru Kimura, M.D., Tohru Kimura, M.D., Tatsu Sawada, M.D., Toshiharu Kikuchi, M.D., Hirotaka Toda, M.D., Yoshiharu Yamada, Ph.D., Hidenobu Nagai, Ph.D. ABSTRACT Since understanding the mechanical properties of intraocular lens (IOL) haptic materials can minimize decentration after surgery, we have examined shape recovery ratios of various intraocular lens haptics (polypropylene [PP], polyvinylidene fluoride [PVDF], extruded poly(methyl methacrylate) [PMMA]) currently on the market under conditions that approximate clinical use. The results using various Ascon lens-holding forceps and compression tests, during which the lenses were held in a cylindrical holder for seven days, one month, and three months, indicated that PVDF haptics had better shape recovery capability than PP and extruded three-piece PMMA haptics. Key Words: Ascon lens-holding forceps, compression, haptic material, polyvinylidene fluoride, shape recovery ratio

To maintain good centration of intraocular lenses (IOLs) for an extended period, the shape of the haptics must recover quickly from distortion at the time of insertion; that is, an IOL should be centered immediately after implantation. A few studies report the shape recovery of IOL haptics. 1 - 3 We have seen a case in which the haptics distorted in the lenticular capsule at the time of implantation using an Ascon lens-holding forceps and did not recover their original shape but remained decentered (Figure 1). Based on this experience, we described the effects of lens-holding forceps on haptics with the compression method. 3 In the current study, we looked at additional haptic materials and designed experiments to examine the shape recovery capability of IOL haptics under conditions simulating clinical situations.

Fig. 1.

(Kimura) Intraocular lens with PP haptics removed from an eye after asymmetric deformation of the haptics was observed during implantation (VTR of the surgery).

From Kimura Eye & Internal Medicine Hospital, Hiroshima, Japan (W. Kimura, T. Kimura, Sawada, Kikuchi, Toda) and Menicon Co., Ltd., Nagoya-city, Aichi, Japan (Yamada, Nagai) Presented in part at the 94th meeting of the Japanese Ophthalmological Society, Okayama, May 1990, and the Symposium on Cataract, IOL and Refractive Surgery, Boston, April 1991. Pharmacia, Surgidev, AMO, IOLAB, and ORC provided many intraocular lenses. Reprint requests to Wataru Kimura, M.D., Kimura Eye and Internal Medicine Hospital, 2-3-28, Nakadori, Kure City, Hiroshima, Japan 737.

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MATERIALS AND METHODS All IOL haptics used in these experiments were modified C shape. The haptic materials included polypropylene (PP) from four manufacturers, polyvinylidene fluoride (PVDF) from one manufacturer, and poly(methyl methacrylate) (PMMA) from one manufacturer. Table 1 shows IOL manufacturers' names, model numbers, haptic shape, material, and diameter, and the total length of the IOLs used in these experiments. Experiments using Ascon lens-holding forceps and compression tests were conducted on the IOLs. All experiments were performed one time on each sample by one trained individual.

Fig. 2.

Experiments Using Ascon Lens-Holding Forceps at Room Temperature 1. A Nikon projector was used to produce 20fold enlargements for IOL haptic length measurements. 2. Optics and haptics of IOLs were held simultaneously with Ascon lens-holding forceps (lensholding forceps AE-4208 without a peg from ASICO) (Figure 2). 3. Intraocular lenses were released into 2% methylcellulose (MC) after being held for a given time. 4. Haptic lengths were measured from projections at various times after release. 5. Shape recovery ratios (%) were computed using the following equation (Figure 3).

Compression Tests Using Cylindrical Holders at 37°C 1. After measuring their total length, the IOLs were compressed and placed in a cylindrical holder with a diameter of 10 mm (Figure 4). The holder was kept in balanced salt solution (BSS) at 37°C. 2. After a given time, the IOLs were removed from the cylindrical holder and released in BSS at 37°C. 3. Intraocular lens lengths were measured from projections at various times after release. 4. Shape recovery ratios (%) were computed using the following equation (Figure 3).

Shape recovery ratio (%) =

Shape recovery ratio (%) =

Length of haptics after being held (Y 1) Length of haptics before being held (Yo)

x 100

(Kimura) An IOL held with a pegless Ascon lensholding forceps.

Total length of IOL after being compressed (L 1 )

------~------------~--~----~~xl00

Total length of IOL before being compressed (Lo)

Table 1. Intraocular lenses used in the study.

Manufacturer Company A ORC Company B Pharmacia Company C IOLAB Company D AMO Company E Menicon Company F SURGIDEV * Manufactured by Rohm 548

Model Number UV41A4

Haptic Shape Modified C

Haptic Material PP

Haptic Diameter (mm) n = 12 0.143 ± 0.003

Total Length (mm) n=6 13.2 ± 0.1

052D

Modified C

PP

0.141 ± 0.004

14.2 ± 0.1

106G

Modified C

PP

0.141 ± 0.003

13.6 ± 0.1

PC85T

Short C

PP

0.142 ± 0.006

13.9 ± 0.4

P-25

Short C

PVDF

0.126 ± 0.003

12.9 ± 0.1

20-24

Modified C

PMMA*

0.142 ± 0.003

13.8 ± 0.1

and Haas and had low molecular weight (about 400,000).

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2. Compression tests

1. Experiments using Ascon lens holding forceps

Fig. 3.

(Kimura) Definition of shape recovery ratio for IOL haptics.

These experiments were conducted between December 1989 and May 1990. RESULTS Before the full-scale experiments were initiated, preliminary experiments were conducted on the shape recovery ratios of Men icon P-25 IOL haptics after being held for five minutes with two types of Ascon lens-holding forceps (Figure 5). The shape recovery ratio of Ascon lens-holding forceps with a peg (AE-4209) was higher than those without a peg (AE-4208) in 2% methylcellulose and also in the air (Figure 6). Pegless forceps can maintain a fixed force regardless of the position of the manipulating hole in the IOL. Since their shape recovery ratio is lower than forceps with a peg, as shown in Figure 6, it is possible to set up more rigorous

A

C

B

experimental conditions. Thus, Ascon lens-holding forceps without a peg were chosen for our experiments. Although there was no substantial difference in shape recovery ratios measured at room temperature either in 2% methylcellulose or in air, our experiments were conducted in 2% methylcellulose to provide conditions closer to clinical use.

Shape Recovery Ratios of IOL Haptics Held One Time for Five Minutes with Ascon Lens-Holding Forceps After being held for five minutes (one time), IOLs were placed in 2% methylcellulose. Haptic lengths were measured after one, five, ten, 30, 60 minutes, and 24 hours. Although the IOLs with PP haptics recovered their shape well and showed relatively high recovery ratios for all four manufacturers the ratios were not as high as those for PVDF (Figure 7). Intraocular lenses with PVDF haptics showed excellent shape recovery ratios (91.6% to 96.2%) at all measurement times. In contrast, shape recovery ratios of IOLs with three-piece PMMA haptics were between 56.5% and 87.3%, which were much lower than any others at each measurement time. The shape recovery ratio for extruded PMMA was 87.3% after 24 hours, while the shape recovery ratios for haptics made of other materials exceeded 92.0%. Shape RecoverlJ Ratios of IOL Haptics Held Five Times for One Minute with Ascon Lens-Holding Forceps Intraocular lenses with PVDF haptics recovered their shape very well (83.7% to 95.4%) at all measurement times (Figure 8). Lenses with three-piece PMMA haptics had the lowest shape recovery ratios (50.7% to 87.7%) during the first 60 minutes after release. The results for extruded PMMA hap-

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D

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2% methylcellulose to measure the haptic lengths. The results of this experiment and the pattern of shape recovery ratios (Figure 10) were similar to those of Experiments 1, 2, and 3.

Fig. 5.

(Kimura) Ascon lens-holding forceps with a peg (left) and without a peg (right).

tics were similar to those in Experiment 1 (Figure 7). However, after 24 hours the ratio was the same (91.3%) as that of IOLs with PP haptics.

Five-Minute Shape Recovery Ratios of IOL Haptics Held for One, Three, and Five Minutes with Ascon Lens-Holding Forceps Longer holding times were associated with lower shape recovery ratios (Figure 9). The shape recovery ratios for IOLs with PP haptics after being held for five minutes were 83.2% to 90.8%. Lenses with PVDF haptics had a recovery ratio of93.9%, while extruded PMMA haptics showed an extremely low ratio of 67.2%. Five-Minute Shape Recovery Ratios of IOL Haptics Held for One, Three, or Five Times with Ascon LensHolding Forceps Intraocular lenses were held with forceps for one minute for one, three, or five times and released in

Shape Recovery Ratios of IOL Haptics Compressed for Seven Days Intraocular lenses were compressed for seven days in BSS at 37°C and taken out of the holder. Shape recovery ratios were obtained at one, five, ten, 30, 60 minutes, and 48 hours after release. A 10.0 mm cylindrical holder was used as a compression holder. Measurements at 48 hours after release showed that PVDF had the best recovery ratio (85.8%) followed by PP (82.3% to 85.2%) and extruded PMMA (77.9%) (Figure ll). These results agreed well with those of the experiments using various Ascon lens-holding forceps. Shape Recovery Ratios of IOL Haptics Following Long-Term Compression (Seven Days, One Month, and Three Months) Intraocular lenses were released following compression periods of seven days, one month, and three months. Shape recovery ratios were measured 30 minutes after release. After three months of compression, which is considered to be similar to clinical conditions in the eye, the shape recovery ratios were 76.2% to 80.4% for PP, 81.9% for PVDF, and 73.3% forextrudedPMMA(Figure 12). These results showed the pattern observed in Experiments 1, 2, 3, 4, and 5. A statistical workup was not done in this study. DISCUSSION When the IOL is inserted in the lenticular capsule, the haptics receive some form of compressive force. Insertion with the dialing method is thought

100

~~

~

~

o

'iii a: 90 Fig. 6.

(Kimura) Summary of experimental conditions using the Ascon lens-holding forceps at room temperature.

__--------------;x--------

~x-

~

~

a:

CD

o

Co

~ (j)

with peg (in 2% MC)

• with peg (in the air)

80

X without peg (in the air) P-25 used as

10

5 Recovery Time in Minutes

550

J CATARACT REFRACT SURG-VOL 18, NOVEMBER 1992

an IOL and held for five minutes

90

~

:

~

0

~

•

0

~ 80

.Q

0;

a:

Fig. 7.

(Kimura) Shape recovery ratios for IOL haptics after being held one time for five minutes at room temperature with the Ascon lens-holding forceps.

Fig. 8.

(Kimura) Shape recovery ratios of IOL haptics after being held five times for one minute at room temperature with the Ascon lens-holding forceps.

>-

~ 70

0



(])

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a:

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o Company 0 (PP) o Company E (pVDF) •

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30

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Company F (PMMA)

24 hours

Recovery Time

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~

a:

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~

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70

(])

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24 hours

Recovery Time

100

~ 90 0

~

Fig. 9.

a:

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~

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(Kimura) Five-minute shape recovery ratios for IOL haptics after being held for one, three, and five minutes at room temperature with the Ascon lens-holding forceps.

3

Company F (PMMA)

5

Holding Time (minutes)

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90

Fig. 10.

(Kimura) Five-minute shape recovery ratios for IOL haptics after being held for one, three, and five times at room temperature with the Ascon lensholding forceps.

• Company A (PP)

t.

Company B (PP)

• Company C (PP) o Company D (PP) o Company E (pVDF) 50~________________________________________________ _

• Company F (PMMA)

5

3 Times Held

90 o

Fig. 11.

(Kimura) Shape recovery ratios for IOL haptics after being compressed for seven days at 37°C.

•

__--------~D~--------------~G--•

l

~

~

~--~----------------~---------

!80~.;

a:

&

o

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~

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a.

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en

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70

• Company F (PMMA)

1 5 10

30

60 minutes

48

hours

Times Since Release

100

• Company A (PP)

t.

Company B (PP)

• Company C (PP) o Company D (PP)

~ 90

o Company E (pVDF) •

0

Company F (PMMA)

~

Fig. 12.

Thirty-minute (Kimura) shape recovery ratios of IOL haptics by compression period at 37°C.

a: ~

~

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a:

Q)

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co ~

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3

days

Compression Period

5.52

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to reduce the compressive force somewhat. However, all compression methods subject haptics to substantial forces. Improper insertion methods can subject them to strong forces. Large compression forces affect haptic shape, which in turn influences IOL centration during and after IOL surgery. Therefore, maintaining the haptic shape is an important factor in retaining better and longer centration. Haptics that exhibit good shape-retention capability should retain good centration not only when the form of the capsule is maintained in relatively good conditions after continuous circular anterior capsulotomy but also when a radial tear is generated in the capsule. Based on many studies and pathologic reports, investigators have concluded that the C type and modified C type are the most suitable haptic designs. 4 However, researchers have not agreed about the materials and mechanical properties of haptics. In this study, various experiments on PP, PVDF, and three-piece extruded PMMA were conducted to study shape recovery capabilities under the conditions simulating clinical situations. Although a statistical work up was not done, almost the same results as in the previous report were obtained, showing that PVDF had the best recovery capabilities among the materials examined. Additional IOLs with PP haptics manufactured by four different companies were used in the experiments. In a general evaluation of physical properties, the elasticity of IOL loops can be rated from highest to lowest as extruded PMMA, PVDF, and PP. It is understood that PMMA has a higher elastic modulus with regard to material properties. In this study, PMMA showed lower shape recovery capability than PP. This may be because the extruded three-piece PMMA (manufactured by Rohm and Haas) used in this study had low molecular weight

and was subjected to force that exceeded its elastic modulus. Various factors other than choice of material are involved in the shape recovery of haptics. These include shape (design) and total haptic length, the angle between haptic and optic (5° to 10° in this study), the connection of haptic to optic, and the shape and diameter of the cross section of haptics. The mechanical properties of IOL haptics are closely related to IOL centration for both short and long periods after surgery. For example, soft haptics with low shape recovery capability can be inserted easily but are sometimes distorted because oflenticular capsule shrinkage, which can result in decentration. Since harder materials with higher shape recovery capability place more force on Zinn's zonule and do not alleviate lenticular capsule shrinkage, decentration can occur. With recently developed multifocal lenses, decentration can cause serious complications. Therefore, it is necessary to study the mechanical properties of haptics including investigations of the most appropriate shape recovery ratios. Further studies are required to examine the mechanical properties of haptics made of softer materials (including silicone) and one-piece PMMA, which were not reviewed in this report. REFERENCES 1. Hayano S. Intraocular lenses-on the basis of high polymers. Acta Soc Ophthalmol Jpn 1986; 90:25-42 2. Drews RC, Kreiner C. Comparative study of the elasticity and memory of intraocular lens loops. J Cataract Refract Surg 1987; 13:525-530 3. Kimura W, Kimura T, Sawada T, et al. Effects of lens holding forceps on the shape of IOL haptics using the compressed implantation method. Ringan 1990; 44: 504-505 4. Apple DJ, Tetz MR, Hansen SO, et al. Intercapsular (endocapsular) intraocular lens implantation: results of animal studies. Jpn IOL Soc J 1988; 2:45-61

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