COMPARISON BETWEEN SILICONE AND ACRYLIC FOLDABLE
INTRAOCULAR LENSES, A PROSPECTIVE RANDOMISED STUDY.
Tarek Tawfik Soliman,MD. Shereef Abd El Wahab,MD. Hythem Mohamed Fayek,MD
AIM: To evaluate both the silicone and acrylic intra ocular lenses as regards the visual acuity, contrast sensitivity, IOL
decentration, aqueous flare, lens deposits and development of PCO.
METHODS: 55 eyes of 53 cataractous patients were included in this study, they were prospectively randomized to
receive silicone or acrylic intra ocular lenses (IOLs), 35 eyes received acrylic IOLs, and 20 eyes received silicone (IOLs
Soflex). Two types of acrylic IOLs were used in this study, the AcrySof acrylic lens (20 IOLs) and the Soflec acrylic lens
(15 IOLs), The patients were selected from outpatient clinic of the ophthalmic department of Benha Faculty of Medicine
Hospital. All cases underwent full ophthalmological examination preoperatively. Evaluation and analysis of operative and
postoperative findings were recorded and patients were followed up for one year.
RESULTS: The procedure of folding and implantation went straight forward in most of the cases. Acrylic IOLs, however
were found to unfold in a slower and better controlled manner, this made implantation of the Acrylic IOLs easier. The
postoperative inflammatory reaction was mild with both types of the IOLs but silicone group had a significantly higher
postoperative aqueous inflammatory cell count than the acrylic group. There was no statistically significant difference
either in the best corrected visual outcome, or in contrast sensitivity in either group of IOLs used. Posterior capsule
opacification developed in 5 cases and there was a significant increase in the incidence of PCO in association with silicone
IOLs as compared to Acrylic IOLs, there was however a non significant difference in the incidence of PCO in association
with either type of Acrylic IOLs.
SETTING: Benha Faculty of Medicine hospital in the period between February 2000 to January 2002.
CONCLUSION: Both silicone and acrylic intraocular lenses are well tolerated in the eye and result in a excellent final
outcome, Acrylic lenses however were found to be easier in their manipulation and were associated with a less incidence of
posterior capsule opacification.
INTRODUCTION:
Phacoemulsification has become the preferred technique for modern cataract surgery as it assists in a greater
control of the globe during the procedure, a rapid visual rehabilitation with less induced astigmatism and less postoperative
inflammation. Recent phacoemulsification hand-pieces enable cataract removal through a 3.2-mm or smaller incision.
(1).The use of flexible IOLs in small incision cataract surgery is increasing. These lenses retain the advantages of modern
cataract surgery such as less trauma, earlier recovery and less induced postoperative astigmatism (2). The evaluation of
new foldable IOls which are made of various materials including silicone, hydrogel and acrylic have been the subject of
continuing research (3). Silicone and hydrogel polymers were introduced in the early 1980 and acrylic polymer has been
used more recently. These materials have different biocompatibilities in the human eye and many of their characteristics
are not fully elucidated (4). These lenses, besides having excellent folding properties, should demonstrate biocompatibility
in the eye, as no material is completely inert within the body. The host response to lens implantation has three major
factors, postoperative break down of blood aqueous barrier (BAB), cellular response on the anterior surface of the IOL, and
cellular infiltration of the posterior capsule. A major complication after, cataract surgery and IOL implantation is
decentration of the IOL. Although improvement in surgical technique and proper placement of the IOl in the capsular bag
have minimized decentration,(5), many unrelated factors may contribute to its occurrence. The mechanical properties of
IOL haptics differ according to their material producing difference in intraocular haptic stability (6). Posterior capsule
opacification (PCO) is the most common complication of modern cataract surgery with an incidence of up to 50% by 2
years. It has been shown that the IOL biomaterial can influence the posterior capsule opacification ( 4 ) . Visually
significant pigmented cellular membrane may form on IOLs after implantation. Reduced vision secondary to such
pigmented cellular membranes is more with silicone IOLs. Dhaliwall (7) reported the occurrence of glistening in acrylic
intraocular lenses after their implantation with a difference in contrast sensitivity between patients having silicone and
acrylic foldable IOLs . In 1996 Oshika (8), reported that post operative flare is less with acrylic IOLs. Although foldable
IOLs have advantages over rigid IOLs, careful handling is required as alteration in IOL surface may occur during
unfolding of the IOL in the form of scratches and marks (9).
1
PATIENTS AND METHODS:
The patients in this study were prospectively randomized to receive silicone or Acrylic IOLs, 35 eyes received
Acrylic IOLs, 20 AcrySof, 15 Softec, (another type of acrylic IOL) and 20 eyes received silicone IOLs (Soflex). Patients
included in this study were those having senile cataract in an otherwise normal eye and patients with a history of ocular
disease or intraocular surgery, gluaucoma, previous uveitis, and significant posterior segment pathology were excluded
from the study. Preoperatively all patients underwent a full ophthalmological examination including best-corrected visual
acuity measurement, applanation tonometry, and slit lamp examination to evaluate the cornea for opacities, the anterior
chamber for inflammatory reaction, the iris for color, pattern, patches of atrophy, vascularization or posterior synechia,
pupil for shape, regularity and reaction to light and lens for type of cataract and position. Indirect ophthalmoscopy to
examine the fundus of the eye intended for surgery was done (if the lens opacity is not so dense). Keratometry was
performed to measure the refractive power of the cornea, and biometry was performed for IOL power calculation using the
SRK-II formula. Three different types of IOLs were used in this study the acrylic Acrysof , and Softec and the silicone
Soflex . Table 1 shows the characteristics of the acrylic Acrysof lens, table 2 shows the characteristics of the acrylic Softec
lens and table 3 shows the characteristics of the silicone Soflex lens.
Table (1) : AcrySof IOL characteristics :
AcrySof IOL characteristics
Model
Optic diameter
: MA60BM
: 6.0mm.
Overall length
Haptic angle
: 13.0 mm.
: 10 degrees.
UV cut off at 10% transmission
: 398 nm-400 nm
Index of refraction
: 1.55
Configuration
: Biconvex
Haptics : Configuration
: Modified-C
Material
: PMMA
Colour
: Blue
A-constant
: 118.9
Central optic thickness for a 20D lens = 0.82 mm
Table (2) : Softec IOL characteristics :
Model
Softec 1
Opitc diameter
5.75mm
Overall length
12 mm
Haptic angle
Zero degrees
Index of refraction
1.46
Haptics
1 piece EMA
A constant
118.0
Table (3): Soflex IOL characteristics :
2
Model
: Soflex 2
Optic diameter
: 6.0mm.
Overall length
: 12.5 mm.
Haptic angle
: Zero degrees
Configuration
: Biconvex
Index of refraction
: 1.429
Haptics : Configuration
: Modified-C
Material
: PMMA
Colour
: Blue
A constant
: 118.1
Postoperatively patients were examined on the 1st postoperative day and after one week, one month, 2 months, 3 months, 6
months, to 12 months. During each visit comment was made on best corrected visual acuity, inflammatory reaction,
presence of deposits on IOL by the slit lamp, IOL centration in relation to the centre of dilated pupil. IOL surface for
presence of fissures, scratches or permanent marks, posterior capsule for posterior capsule opacification and contrast
sensitivity. Contrast sensitivity was measured by Cambridge low contrast gratings. This test comprises 12 pairs of plates.
The first pair serves as a demonstration. The next ten pairs of plates are numbered 1-10 and form 10 test stimuli. The last
pair is unnumbered and is not usually used. Table 4 showes the contrasts of the plates under optimum lighting conditions
Table (4) : Contrasts of the plates of Cambridge low contrast grating
Plate No.
1
2
3
4
5
6
7
8
9
10
5.0
2.7
1.6
1.0
0.72
0.52
0.37
0.27
0.19
0.14
Demo
Contrast %
13
Contrast thresholds of 0.3% are typical, contrast sensitivity is the reciprocal of contrast threshold
Figures 1and 2 show the steps of implanting the Acrysof IOL in the eye
RESULTS: This study comprised 55 eyes of 53 patients with a mean age of
Figure1
The lens (Acrysof IOL is advanced
through the incision
62.9±6.9 years, (range; 43-73 years). It included
30 2(56.6%) male patients
Figure
with a mean age of 61.7±7.4 years,
(range; IOL)
43-73is years)
andplaced
23 (43.4%)
lens (Acrysof
perfectly
in the bag
3
female patients with a mean age of 64.5±5.9 years, (range; 51-73 years). The preoperative visual acuity ranged between
6/18 and 2/60. The 55 eyes included in the study were randomly allocated in two groups
I. Silicon IOL group :
This group comprised 20 eyes of 19 patients. They were 11 males and 8 females. Their mean age was 62.5 ±7.1 years,
(range; 49-73 years), and their preoperative visual acuity ranged between 6/24 and 2/60.
II. Acrylic IOL group :
This group comprised 35 eyes of 34 patients who were further subdivided into two groups according to the type of acrylic
IOL implanted:
1. Acrylic subgroup A1 :
This group comprised 20 eyes of 19 cataractous patients. They were 12 males and 7 females. The patients included in this
Acrylic subgroup had a mean age of 62.1±7.3 years, (range; 43-73 years). Their preoperative visual acuity of these ranged
between 6/18 and 3/60.
2. Acrylic subgroup A2 :
This group comprised 15 eyes of 15 cataractous patients. They were 8 males and 7 females. The patients included in this
Acrylic subgroup had a mean age of 64.6±5.9 years, (range; 51-71 years). Their preoperative visual acuity ranged between
6/18 and 3/60.
There was a non-significant difference between the three groups as regards age of patients, male to female ratio and
percentage of eyes with variable preoperative visual acuity. Postoperatively, these 55 eyes were followed-up for a period of
12 montths. There was a significant (P<0.001) improvement in post-operative visual acuity as compared to preoperative
visual acuity of eyes included in both the silicone and acrylic groups. However, there was a non-significant (P>0.05)
difference between post-operative visual acuity in Acrylic subgroup A2 as compared to post-operative visual acuity of eyes
included both in the silicone group and in Acrylic subgroup A1. Table 5 shows the best corrected post operative visual
acuity by the end of the follow up period.
Table (5) Postoperative visual acuity of the studied groups at end of the 6th month.
Groups
Visual acuity
Silicone
Acrylic subgroup
A1
Acrylic subgroup A2
6/6
12 (60%)
15 (75%)
9 (60%)
6/9
6 (30%)
4 (20%)
5 (33.3%)
6/12
1 (5%)
1 (5%)
1(6.7%)
6/18
1 (5%)
0
0
Postoperative aqueous inflammatory cells :
At the first postoperative day there was a significant (P<0.05) increase in the number of eyes showing grade I in
acrylic subgroup A1, with a significant (P<0.05) reduction in the number of cases showing grades II and III, as
compared to silicone group. One week after surgery, there was a significant (P<0.05) increase in the number of
cases showing grade 0, with a significant (P<0.05) reduction in the number of cases showing grades I, II and III in
acrylic subgroups as compared to silicone group. Up to 3 months after surgery, there was a significant (P<0.05)
increase in the number of eyes showing grade 0 and a significant (P<0.05) reduction in the number showing
grades I, II and III in acrylic subgroups as compared to silicone group However, 6 months after surgery, there was
a non-significant (P>0.05) difference in the number of eyes showing grade 0 between the three groups (table 6).
Table (6): The post-operative aqueous inflammatory cells in the studied groups
4
Grade
Group
D1
W1
M1
M3
M6
0
0
5%
30%
95%
Acrylic subgroup A1
0
5%
60%
90%
100%
Acrylic subgroup A2
0
6.70%
66.70%
86.70%
93.30%
Silicon group
5%
15%
20%
55%
5%
Acrylic subgroup A1
35%
45%
40%
10%
0
Acrylic subgroup A2
46.70%
66.70%
33.30%
13.30%
6.70%
Silicon group
70%
65%
70%
15%
0
Acrylic subgroup A1
55%
40%
0%
0
0
Acrylic subgroup A2
33.30%
13.30%
6.70%
0
0
Silicon group
25%
20%
5%
0
0
Acrylic subgroup A1
10%
10%
0
0
0
Acrylic subgroup A2
20%
13.30%
0
0
0
Grade 0
Grade 1
Grade 2
Grade 3
Timing of follow-up examination
Silicon group
D1=day one, W1= week one, M1= month one, M3= month 3, M6= month 6
Postoperative posterior capsule opacification (PCO)
Posterior capsule opacification was diagnosed in 3 eyes out of 20 eyes (15%) eyes in which silicone IOLs were implanted
and in 2 eyes out of the 35 eyes (5.6%) where acrylic IOL was implanted.There was a significant (P<0.05) increase in the
frequency of PCO in association with silicon IOL implantation compared to acrylic IOL implantation, but There was a
non-significant (P>0.05) difference in the frequency of PCO in association with either type of acrylic IOL.
Postoperative total contrast sensitivity testing
Mean of the total score is 25.3±4.87 (range, 20-29) in silicone group, 26.5±3.27 (range, 20-30) in acrylic subgroup A1 and
26.3±4.1 (range, 20-28) in acrylic subgroup A2, Analysis of these results revealed the presence of non-significant (P>0.05)
difference between the three study groups .Table 7 shows the mean contrast sensitivity score of the patients included in the
studied groups
Table (7): The mean contrast sensitivity score of the patients included in the studied groups :
Group
Silicone
Acrylic subgroup A1
Acrylic subgroup A2
Mean (±SD)
25.4±4.87
26.6±3.27
26.3±4.1
Range
20-29
20-30
20-28
IOL scratches
Throughout this study, scratches occurred in one silicone IOL (5%), 3 Acrysof IOLs (15%) and 2 softec IOLs (13.3%).
Statistical analysis revealed a significant difference in the frequency of occurrence of IOL scratches between both
silicone and acrylic (both subgroups) IOLs, (X2= 7.88, P<0.05) with scratches occurring more with acrylic IOLs.
DISCUSSION: In this study 20 foldable silicone posterior chamber IOLs, and 35 foldable Acrylic IOLs (20- Acrysof &
15 softec) were randomly implanted after small incision phaco emulsification, the procedure of folding, implanting and
centering of the IOLs went straight forward, in all except 4 cases. One of these cases needed more manipulations during
IOL implantation and the incision size was increased about 1mm, this patient needed a + 26D IOL, and a silicone IOL was
used. In another case where also a silicone IOL was used, folding of the IOL was very difficult as the IOL was accidentally
wett with saline before folding which made it very slippery & difficult to hold, it was dried with a microsponge and all the
instruments were dried, and implantation was completed with no further complication, this was one of the first silicone
IOLs implanted in this study, after that all the needed precautions were made to avoid wetting any silicone IOLs. In the
other 2 cases in which one silicone and one acrylic IOL were used, implantation was made into the bag over a small tear in
5
the posterior capsule with no further complications intra-operatively or during the follow up period. The difficulty
encountered during implantation of the +26 D silicone IOL may be attributed to its high diopeteric power, which markedly
increased its thickness and made its folding and implantation more difficult. This problem did not occur with any of the
Acrylic lenses used in this study and this may be attributed to the high refractive index of Acrylic which makes the optic
thinner than that of silicone. This also goes with the results of (4) who mentioned that Acrysof lenses are thinner than
silicone lenses because of their higher refractive index.
Acrylic lenses unfold slower than silicone lenses and due to their somewhat tacky behavior, an iris spatula was
used to help release the IOL from the implantation forceps, on the other hand silicone IOLs unfolded rapidly and slipped
out of the folding forceps easily without need for any other instrument, this made implantation of the Acrylic IOL easier
over the torn posterior capsule tear where the IOLs’ unfolding was slow and controlled, while implantation of the silicone
IOL over the tear required meticulous handling of the folding forceps with very slow release to avoid the sudden unfolding
of the silicone IOL. Both IOLs were, however, implanted safely in the bag with no further complications.
During the follow up period in this study, postoperative aqueous inflammatory cells were found to be more with
silicone IOLs until the end of the first postoperative week were most of eyes with Acrylic IOLs showed almost absence of
inflammatory cells compared to eyes with silicone IOLs, this significant difference in presence of postoperative
inflammatory cells continued throughout the follow up period till the 4th months were the presence postoperative
inflammatory cells found in eyes with silicone IOLs starded to decline markedly and almost disappeared by the 6th month.
In this study both IOL types produced a mild degree of inflammatory response that resolved over the study period without
residual effects. Thus both IOLs are biocompatible to allow them to function well as clean refractive elements in normal
eyes after cataract extraction. The study however identified statistically significant differences among both groups. The
silicone group had a significantly higher postoperative aqueous inflammatory cell count than the Acrylic group. These
results suggest that Acrylic IOLs have a higher degree of biocompatibility as compared to silicone IOLs.
The results of this study are close to the results of Hollick (10)who studied the biocompatibility of silicone and
Acrysof IOLs and mentioned that the mean postoperative inflammatory cell reaction reached its peak at 1 week and fell to
“zero” by 6 month in both IOLs and that the silicone IOLs had the highest mean postoperative inflammatory cell reaction
throughout the follow up period. According to these results he pointed out that the Acylic IOLs are more biocompatible
than silicone IOLs. They attributed the presence of the postoperative inflammatory cells to a “mild degree of non specific
foreign-body response” which differed according to the material of the used IOL.
The results of this study also compare to the results of a study by Oshika (8) who conducted a 2 year study on
Acrylic IOLs, and used a laser-flare cell meter to measure intensity of Aqueous flare after Acrylic IOL implantation and
compared his results to those obtained after silicone IOL implantation. Their analysis revealed significant differences
among both groups at day 1, week 2 and month 1, and 3 but was not significant thereafter. They attributed the differences
between Acrylic and silicone IOLs to the high refractive index of soft Acrylic IOLs that makes the optic of this lens very
thin leaving less space between the anterior and posterior capsules (the site of migration/proliferation of lens epithelial
cells) and to the acrylic biomaterial that is thought to adhere to the lens capsule resulting in encapsulation of lens epithelial
cells from the immunologically active aqueous humor, resulting in reduction of postoperative inflammatory response and
posterior capsule opacification.
In this study there was no decentration or tilt in either groups of IOLs used, (Acrylic and silicone) The absence of
tilt or decentration in this study may be attributed to the meticulous IOL implantation and care to implant both haptics in
the bag, the results of this study also similar to the results of Jung (5) who reported that no significant difference between
silicone and acrylic IOLs when the IOLs were placed properly in the capsular bag.
In 5 of the acrylic lenses (3 Acrysof and 2 softec) and in only one silicone IOL, scratches were seen on the IOL
optic, the scratches were peripheral and no scratches were seen on the center of the lenses. The lenses that showed
scratches were of high diopteric powers this may explain why the scratches occurred as the high diopteric power increased
the IOL thickness and thus more pressure was used to fold the optic, this was however noticed after wards and was
avoided. The presence of these scratches did not affect the final visual outcome in any of the three cases who attained a
visual outcome of 6/6 and 6/9 after 1 week.
These findings compare to the results of Milazzo (9) who reported the presence of scratches on 2 Acrysof IOLs
after implantation and attributed their presence to the fragile behaviour of acrylates in general where simple pressure with
the forcep alteres the lens material this leaving a mark on the lens that usually diappears in a few minutes, However if the
inner side of the forceps is not smooth, or if minimal pressure is applied, permanent scratches may occur. The results also
resemble the results of Oshika (8) who pointed out that the presence of scratches on Acrylic IOLs did not alter the visual
outcome unless they resulted from extreme manipulations such as prolonged folding for 60-30 minutes, or extremely tight
folding for 5 minutes. They also mentioned that IOL of more than + 18 Diopter have thicker optics and may be more
susceptible to folding manipulations. They concluded that folding procedures within the range of clinical settings may
cause scratches which are unlikely to affect the visual function.
After 6 months of follow up, 90% of patient with silicone IOLs had a visual acuity of 6/9 or better, 95%of patients
with Acrysof had a visual acuity of 6/9 or better and 93.3% of patients with softec Acrylic IOL had a visual acuity of 6/9 or
better. These results show a statistically significant (P < 0.001) improvement in postoperative best corrected visual acuity
6
as compared to preoperative best corrected visual acuity as compared to preoperative best corrected visual acuity as
compared to preoperative best corrected visual acuity in all groups. However non statistically significant (P > 0.05)
difference between the groups as regards the best corrected visual outcome was observed, these results are similar to the
results of some authors (11) who compared the best corrected visual acuity results in acrylic, silicone & PMMA lenses and
found no statistically significant difference between the groups (ANOVA, p= 0.13).
These results are similar to the results of a study by Oshika (8) who conducted a 2 year study on soft acrylic IOLs,
96.9% of patients had BCVA of 20/40 (6/12) or better and 50% had 20/20 or better at 6 months 98.4% of cases achieved
20/40 or better, 86.9% of cases achieved 20/20 or better, at 2 years all patients had an acuity of 20/40 or better and 86.3%
had 20/20.
Ursel (4) reported that BCVA were 20/40 or better in 81% of silicone patients and 94% in Acrylic patients, there
was no significant difference between the two groups.
Mengual (2) reported that BCVA in patients with acrysof 9 months postoperatively were 20/40 or better in 100%
of patients without pathology.
All the cases in this study were followed up on regular basis for a minimum of 12months, during this follow up
period 5 cases showed posterior capsule pacification, PCO was first seen on the 9th month of follow up in one of the cases
in which a silicone IOL was used, this patient had discontinued follow up after the 7th month with a best corrected visual
acuity of 6/12 and returned after 2 months due to diminution in his best correct visual acuity which reached 6/24, two other
patients with silicone IOLs showed P.C.O. causing marked decrease in best corrected visual acuity through out this study,
these patients were sent for YAG laser posterior capsulotomy on the 12th postoperative month. Their best corrected visual
acuity had decreased from 6/6 and 6/12 on the 6th postoperative month to 6/24 and 6/36 respectively on the 12th month.
P.C.O. was seen in two of the cases with acrylic lenses, the first was in the softec group and was first noticed at
the 11th postoperative month. The other occurred in the Acysof group and was seen on the 12th postoperative month. The
incidence of occur of P.C.O. was 15% in the silicone IOLs group, 5% in the Acrysof group & 6.7% in the Softec group.
These results show a significant (P < 0.05) increase in the incidence of PCO in association with silicone IOLs as compared
to Acrylic IOLs. There was however, a non-significant (P > 0.05)difference in the incidence of PCO in association with
either type of acrylic IOL.
The results of this study are close to the results of Kucuksumer (12), who reported an incidence of P.C.O. with
Acrysof IOLs of 4.7% after a 2 year follow up study. Oner (13), also found the incidence of PCO with Acylic IOL to be
8.7% at a mean postoperative follow up period of 17.8 months, the slight increased in incidence in their study may be
attributed to the longer follow up period (16-22 months for all of their cases). In 1999 a group of researchers (14)
mentioned that the average percentage of PCO was 40% with silicone IOLs and 10% with Acylic IOLs, after 3 years
follow up, there was a significant difference in percentage PCO among the lens types (p = 0.0001). Again the increase in
incidence of PCO may be attributed to the longer follow up period (3ys for 72% of their patients).
The low incidence of PCO with acrylic IOLs in this study may be attributed to more than one factor, mainly the
tight bioadhesion between the anterior and posterior capsules and the IOL optic. This was assurred by finding almost no
space between the IOL optic and the posterior capsule, this adhesion head to less PCO as less space for the cells to grow,
tight adhesion may sequester the IOL and LECS from the aqueous which may contain factors that promote PCO (4). Tight
adhesion between lens and capsular bag was found to be completed within approximately 8 days after cataract surgery with
silicone IOLs and 11 days with acrylic IOLs (15).
In this study contrast sensitivity was measured by Cambridge low contrast gratings and the mean of total score is
25.3 + 4.87 for silicone IOLs group, 56.5 + 3.27 in Acrysof IOLs group and 26.3 + 4.1 in softec IOLs group and there was
no significant (P > 0.05) difference between the three studied groups, theses results are similar to the results of Kohnen et
al. (1996) who studied contrast sensitivity in patients receiving silicone and acrylic IOLs by using 25% and 11% regan
charts, he reported that no significant difference between acrylic and silicone IOLs in either test (P = 0.26 for the 25% and
P = 0.17 for the 11% Regan charts).
Hollick (10) studied contrast sensitivity of patients with silicone and acrylic IOLs by using pelli robson contrast
charts, median pelli Robson contrast sensitivities were 1.35 for silicone and 1.46 for acrylic (P = 7.4), there was no
significant difference among the two groups.
CONCLUSION: Both silicone and acrylic intraocular lenses are well tolerated in the eye and result in a excellent final
outcome, Acrylic lenses however were found to be easier in their manipulation and were associated with a less incidence of
posterior capsule opacification.
7
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