33 Intraocular lens implant biocompatibility

Background

There are three major aspects to intraocular lens implant (IOL) biocompatibility within the human eye. These are the effect of the IOL on the blood aqueous barrier, the cellular reaction on the anterior surface of the IOL, and the effect on the lens capsule. Blood aqueous barrier changes can be assessed by the measurement of anterior chamber flare and cell levels using the laser flare and cell meter. Cells on the anterior surface of the implant can be examined in vivo postoperatively using specular microscopy and have been used extensively as a means of assessing the foreign body response to IOL. Postoperatively the cytology on the anterior surface of the lens implant is made up of three distinct cell populations: small cells, epithelioid/giant cells and lens epithelial cells. The effect of the IOL on the capsule consists of lens epithelial cell proliferation and metaplasia leading to anterior and posterior capsular opacification, and IOL decentration, all of which can affect visual outcome.

In this chapter the evidence for the effectiveness of different IOL designs and materials on biocompatibility is presented.

Question

Does the IOL type have an influence on the amount of blood aqueous barrier breakdown?

The evidence

Seven randomised controlled trials (RCTs)1–7 have commented on the amount of blood aqueous barrier breakdown with different IOL types (Table 33.1). Two studies showed that the heparin-surface-modified (HSM) polymethylmethacrylate (PMMA) lenses had a lower postoperative flare reaction than unmodified PMMA,1,2 and one showed no differences in the laser flare measurements between HSM and PMMA lenses at any stage.3

Two studies looked at laser meter readings of flare and cells after phacoemulsification with the implantation of either a silicone or a PMMA IOL.4,5 One showed a

difference in postoperative flare and cell measurements and the other did not. A smaller incision was used for the silicone IOL. Two studies compared HSM-PMMA and hydrophobic acrylic (AcrySof) IOLs in patients with diabetes and found no significant difference in the postoperative flare readings.6,7 With modern foldable lenses and good surgery postoperative blood aqueous barrier changes are largely influenced by the surgical technique and any effect of the IOL type on flare and cells is probably negligible.

Question

Does heparin-surface-modification have an influence on the amount of cells on the anterior surface of the IOL? What about the amount of IOL surface cells with foldable IOLs?

The evidence

The use of heparin-surface-modification of PMMA lens implants has been shown to reduce the postoperative cellular reaction on the surface of the lens implant after cataract surgery.3,8–14 These studies are summarised in Table 33.2. None of the studies showed any significant difference between the two groups with respect to the visual acuity of the patients.

There are four studies comparing foldable and PMMA IOLs with respect to the cellular reaction on the surface of the IOL (Table 33.3).15–18 In the first study by Hollick et al. the first-generation silicone IOL appeared to be the least biocompatible, and the hydrophobic acrylic the most.15 In the second study by Hollick et al. the hydrophilic acrylic IOL (Hydroview) appeared to be very biocompatible compared to PMMA and a second-generation silicone lens, but was associated with an unusual lens epithelial cell (LEC) response.16 Many LECs were found on the anterior surface of the Hydroview IOL and these did not regress as they do on other IOL types.

In Ravalico et al.’s study the conventional PMMA material exhibited the most cellular adhesion of the groups.17 The authors suggest that second-generation silicone and hydrophilic acrylic IOLs appear to be more biocompatible

Notes

Results

Outcomes

Laser flare and cells at 1 day, Lower flare and cells Smaller incision used

One-piece Starr n = 112

Martin and Sanders, RCT

than the three PMMA lens types. In Sveinsson and Seland’s study the amount of IOL precipitates, which were most likely to be epithelioid cells and giant cells, were the same on the hydrophilic acrylic and PMMA IOLs.18

Three studies compare different foldable IOLs (Table 33.4).19–21 These found IOL-related differences in the cellular reaction after cataract surgery. The incidence of lens epithelial cells was highest in the Hydroview group, and more giant cells were seen with hydrophobic acrylic IOLs.

Comment

To summarise, the foldable IOLs, with the exception of first-generation silicone lenses, appear to be at least as biocompatible in terms of surface cytology as PMMA, if not more biocompatible. The second-generation silicone IOLs with a sharp edge appear to have a particularly low incidence of all cell types.

Question

Does IOL material have an influence on the anterior capsule reaction?

The evidence

The changes in the anterior capsule occurring after cataract surgery consist of opacification and fibrosis, which can lead to capsulorhexis contraction (phimosis), IOL decentration and tilt. These are caused by LECs undergoing fibroblastic transformation when the anterior capsule comes into contact with the IOL, and can be considered to be an index of biocompatibility. These changes have been shown to be influenced by the type of IOL used. Five studies have looked at anterior capsule opacification (ACO) (Table 33.5).2,19,21,22 Two studies have shown less ACO with HSM-PMMA lenses than unmodified PMMA at three months.2,22 In Tognetto and colleagues’ study comparing three hydrophilic acrylic IOLs, a significantly higher rate of ACO was found with the ACR6D lens.21 Miyake et al. showed less ACO with hydrophobic acrylic AcrySof IOLs than second-generation silicone, and less with the Memory hydrophilic acrylic lens compared to AcrySof.22 They suggest that the more hydrophobic lenses induce more ACO. In the Abela-Formanek study the hydrophobic acrylic AR40 lens showed the most ACO, followed by silicone, then Hydroview, AcrySof, with the least ACO with MemoryLens.19 With the exception of AcrySof, ACO was more predominant in the hydrophobic IOL groups.

The second change in the anterior capsule reflecting IOL biocompatibility is the contraction of the capsulorhexis opening. Three RCTs have studied this and these are

summarised in Table 33.5.23–25 It appears that the contraction of the anterior capsule opening is greatest with silicone IOLs, with relative stability of the anterior capsule on the surface of AcrySof IOLs.

Decentration of IOLs has been investigated in four RCTs, which are summarised in Table 33.6.26–29 All surgeries consisted of phacoemulsification after an intact continuous curvilinear capsulorhexis (CCC), with the IOL located in the capsular bag. The study by Dick et al. revealed very little difference in decentration between plate-haptic silicone, silicone disc lens and PMMA.26 Hayashi et al.’s study suggests that when the IOL is placed within the capsular bag there is a small degree of tilt and decentration just after surgery, but this does not increase significantly with time in any of the three IOLs.27 The results were similar in Jung et al.’s and Wang et al.’s studies, showing no significant increase in decentration and tilt up to two months and six months after surgery respectively, and no difference between the hydrophobic acrylic and silicone IOLs used in Jung et al.’s study and the silicone and PMMA IOLs in Wang et al.’s.28,29

Comment

The hydrophilicity of the IOL appeared to have a loose association with the amount of ACO. Contraction of the capsulorhexis opening was greatest with silicone IOLs. All the IOLs studied showed good stability within the bag with an intact capsulorhexis.

Question

What influence does design and surface modification of PMMA lenses have on posterior capsule opacification?

The evidence

Eighteen RCTs have looked at the effect of IOL type on the amount of PCO.6,10,18,19,30–44 Many of these suffer from the disadvantage of not having an objective measure of PCO, and relying on Nd:YAG laser posterior capsulotomy rates as an indirect measure of PCO, or using subjective grading techniques.6,10,18,19,30–34,38,39,44 More recently some objective measures for quantifying PCO have been

developed.35–37,40–43

The first group of papers looked at various PMMA designs and modifications to investigate their effect on PCO.10,30–34

Their results are presented in Table 33.7. Three of the studies looked at the effect of heparin-surface-modification on PCO, and their conclusions differ. In Lai and Fan’s paper there was a trend to lower PCO with HSM but this was not significant.10 In Winter-Nielson et al.’s study there was no difference in the frequency of PCO between patients with

Results

Specular microscopy: giant % not discussed in text. Significantly fewer giant

n = 367

vPMMA -HSM

Trocme and Li, RCT

at

2 cells/mm25·4

cases:all

smallHSM incells

Specular smallmicroscopy:

n = 53

vPMMA -HSM

RCT

.,al etYgge

cellsgiant

4at Noweeks.

2 cells/mm13·3

week,1

Age-related cataract only and giant cells at 1 and

unmodified

4 FUweeks

13 1990

at 1 or 4 weeks. PMMA small cells in all cases:

4 weeks

Swedish

PMMA

ECCE

at

2 cells/mm37·8

week,1

at

2 cells/mm43·2

n = 40

vPMMA -HSM

Zetterstrom RCT

<0·001)

P( 1/12:At

<0·001)

P( 1/12:At

Specular

1 Hydrophilic n = 50

RCT

.,al etRavalico

cells at 1/12:

H – 0

H 40–

related-Age smallmicroscopy:

(Hydroview) = H

6 FUmonths

.,al etEidenbock

21 2002

(Continued)

Results

Participants Outcomes

Slit lamp grading of ACO % at 1/12:

n = 73

Hydrophilic acrylics

RCT .,al etTognetto

21 2002

Less change in CCC area with A than P

Area inside CCC

1 Hydrophobic acrylic (Alcon n = 90

RCT

.,al etUrsell

CCC size measured with More contraction of the CCC opening

1 Hydrophobic acrylic (Alcon n = 240

RCT

.,al etHayashi

24 1997

6 months FU 30) = S Phaco 2 PMMA = P

29 1998

modified and unmodified IOLs overall, but comparing the patients with laser-ridge (LR) IOLs, there was more PCO with modified LR IOLs compared to unmodified LR lenses.33 In Zetterstrom’s study on patients with pseudoexfoliation there was significantly less PCO and fewer YAGs with HSM lenses.34

Laser-ridge IOLs were investigated in three of the studies. Martin et al. showed that LR IOLs were associated with significantly more YAGs than biconvex, but not planoconvex IOLs, but they did not show any difference in PCO grades.30 Westling and Calissendorff et al. showed that LR lenses had more PCO, but the same amount of YAG capsulotomies as PMMA lenses without a laser-ridge.32 In Winter-Nielson et al.’s study there was no significant difference between biconvex and laser-ridge groups with respect to the need for YAG capsulotomy, but the heparin- surface-modified laser-ridge IOLs were associated with more YAGs compared to unmodified LR, HSM biconvex and unmodified biconvex IOLs.33

The convexity of the IOL optic was compared in three studies. In Martin et al.’s study biconvex IOLs were found to have lower YAG rates than plano-convex and laser-ridge IOLs.30 In Sellman and Lindstroem’s study IOLs with the convex surface posterior (PC) had less pearls than IOLs with the convex surface anterior (CP).31 In Winter-Nielson et al.’s study there was no significant difference between biconvex and laser-ridge groups.33 It has been proposed that the increased IOL surface contact with biconvex or IOLs with a posterior convex surface produce less PCO due to a barrier to lens epithelial cell migration.

Question

What influence do the lens design and material of foldable IOLs have on posterior capsule opacification?

The evidence

Eight of the twelve RCTs looking at this question (Table 33.8) compare one or two foldable IOLs to PMMA,18,35–37,39–43 and the other four studies compare foldable IOLs.19,38,42,44 All of the foldable IOLs were found to be associated with less PCO than PMMA except for one hydrophilic acrylic lens (Storz Hydroview). As we have seen when comparing the cytology on the surface of IOLs, this Hydroview IOL gets very few small cells and epithelioid cells on its surface but becomes covered with lens epithelial cells. It appears that the LECs grow onto the anterior surface and posterior capsule producing significant PCO.

The foldable IOLs associated with lowest rates of PCO in the studies are the second-generation silicone IOLs and the

AcrySof hydrophobic acrylic lens. In six studies the AcrySof IOL was associated with very low amounts of PCO and YAG capsulotomy rates.19,35,36,38,40,43,44 Hayashi et al.’s study and the one by Pohjalainen et al. compared the AcrySof to a secondgeneration silicone IOL and found no significant difference in PCO.35,43 Kucuksumer compared the hydrophobic AcrySof to a hydrophilic acrylic lens, the MemoryLens, and found that PCO was worse with the MemoryLens and YAG rates were higher.38 In Abela-Formanek et al.’s study PCO was significantly greater with the hydrophilic, round edged IOLs (Hydroview, MemoryLens) than with hydrophobic sharp-edged IOLs (CeeOn 911A, AcrySof).19 The difference in PCO between CeeOn 911A and AcrySof was not significant. In the hydrophobic acrylic group the AcrySof performed better than the round-edged AR40. In the second-generation silicone group, the CeeOn 911A performed better than the round-edged CeeOn 920.

The AcrySof IOL was the first foldable lens implant to be manufactured with a square optic edge. The square edge has come to be regarded as important in the prevention of PCO, by forming a barrier to LEC migration. The importance of the square-edge is supported by the study by Buehl et al. comparing two hydrophobic acrylic IOLs that were identical except for optic edge design.42 The IOL with sharp posterior optic edge was associated with significantly less PCO than one with the round optic edges. Similarly in Abela-Formanek et al.’s study the CeeOn silicone IOL with a square-edge was associated with less PCO than the roundedged lens.19

The AcrySof material is more adhesive than other lens materials, which may also be important in the limitation of PCO and the stability of the anterior capsule on the surface of the IOL.23 Hollick et al. showed that LECs that migrate onto the posterior capsule under the optic were more likely to regress in patients with AcrySof IOLs, whereas with firstgeneration silicone and PMMA IOLs these cells were more likely to progress.45 They propose that this was most likely to be due to differences in the IOL material.

Comment

The hydrophobic IOLs tended to have a lower incidence of PCO than the hydrophilic ones, and IOLs with sharpedged optics led to a lower incidence of PCO than roundedged IOLs. IOL material and optic edge design have a significant influence on PCO.

Question

What influence does IOL type have on visual acuity, contrast sensitivity, and glare?

Clinical exam (not graded) At 1 year fibrosis present in:

n = 99

1 HSM IOL = H

Lai and Fan, RCT

Clinical PCO grade (0–3) Moderate or severe PCO at 1 year in:

n = 600

piece-3

piecev -1

1

RCT

.,al etMartin

American

sulcusv Bag 3

ECCE

32 1991

Clinical PCO grade (1–4) Graphs in paper

n = 250

1 HSM biconvex = HB

Winter-Nielson RCT

Mean PCO score at 2 years (SD): A = 11·7

Scheimpflug

1 Acrylic (AcrySof) = A n = 300

Hayashi, RCT

(Continued)

Participants used to measure PCO) PCO

Outcome (technique

= NS)

P( =76%

=S A61%,

(0–4)grade PCO:No

PCOClinical

n = 80

1 Silicone (SI 30) = S

Pohjalainen RCT

A = 11·8%, S = 33·5%, P = 43·7%;

imagesretroillumination

No ocularother

Silicone gen.)-(first = S

2

2 FUyears

.,al etUrsell

(intensity) at 1 year P. Central 3 mm of posterior capsule

pathology

Phaco

Woung,

Chinese

41 2000

For abbreviations see Table 33.1.

Results

Participants Outcomes

VA and CS at 2 months No significant difference

1 Acrylic (AcrySof) = A n = 86

RCT

.,al etAfsar

= 0·03

PMMA,P opticoval withMore

German

The evidence

Four RCTs have vision as their primary outcome measure (Table 33.9), although their follow up was very short (six weeks to four months).46–49 Afsar et al. found no significant difference in visual acuity or contrast sensitivity with AcrySof to PMMA, as would be expected at two months before the potential influence of PCO development.46 Anderson et al. found that patients with oval-optic PMMA IOLs reported significantly more visual symptoms such as reflections, halos and ring around lights, than those with round-optic silicone IOLs.47 They suggest that the oval IOL’s truncated and thick edge may scatter light. In Johansen et al.’s study (comparing PMMA and silicone) and Kohnen et al.’s study (comparing PMMA, hydrophobic acrylic, and silicone), contrast sensitivity was lower for silicone IOLs than PMMA, with no significant difference between acrylic and silicone.48,49 Neither study showed a significant difference in visual acuity between the groups. These comparisons suggest worse visual function results for silicone.

In the studies on PCO with foldable IOLs longer follow-up is reported and vision is discussed in the majority. In Hayashi et al.’s study comparing AcrySof, silicone and PMMA, visual loss of one or more lines was seen in 27·4% of those with AcrySof IOLs, 27·1% with silicone, and 63·8% with PMMA (P <0·001).35 In Hollick et al.’s study comparing AcrySof, silicone and PMMA, the visual acuity and contrast sensitivity were lowest for patients with PMMA IOLs, but this was not significant, perhaps due to the smaller numbers in this study.36 In Hollick et al.’s study comparing Hydroview, silicone, and PMMA there was no significant difference in visual acuity at two years, but the patients with the silicone IOLs had significantly better contrast sensitivity.37 Kucuksumer et al.’s study on AcrySof and MemoryLens showed no significantly better acuity at three years for AcrySof (0·03 + − 0·06) than MemoryLens (0·12 + −0·34), P = 0·02.38 In the study comparing PMMA and Iogel by

Sveinsson and Seland, no significant difference was shown in the visual acuity between the two IOLs.18

Buehl et al.’s study comparing two hydrophobic acrylic IOLs, differing only in the shape of the posterior edge of the optic, showed no significant difference in visual acuity despite differences in PCO, which might be because the PCO had yet to spread across the central region.42 There have been reports from non-randomised studies of edge glare from sharp IOL optic edges. The “half-rounded” edge profile of the AR40e (where the posterior edge is sharp and the anterior optic edge is round) did not cause more visual side effects than the round edge of the AR40 lens.

The early results suggest a possible worse visual outcome with silicone IOLs compared to PMMA and AcrySof, with similar results for PMMA (except for those with oval optics) and AcrySof. The long-term results show a different picture, largely reflecting which IOL type was associated with more

PCO. In some studies no difference in acuity was shown despite differences in PCO, which may reflect the fact that acuity is a poor measure of the disability caused by PCO, and contrast sensitivity better; or may be due the small numbers of patients in some of these studies.

Implications for practice

It appears that there are two clinically important aspects to IOL biocompatibilty: the cytological reaction and the capsular reaction. An IOL may have excellent cytological biocompatibility, but poor capsular biocompatibility, such as the Hydroview IOL. The goal is to find an IOL which has both cytological and capsular biocompatibility. Material composition and design features, such as a sharp optic edge, appear to be important. The current hydrophobic IOLs offer good cytological and capsular biocompatibility. To date the second-generation silicone and the hydrophobic acrylic AcrySof IOLs appear to show the best overall performance. The least PCO was found with square-edged IOL designs such as the AcrySof and CeeOn 911A.

Implications for research

The debate continues over the relative influence of design and material on biocompatibility. A study comparing the hydrophobic IOLs with sharp optic edges made from different materials (such as AcrySof, Sensar OptiEdge AR40e and silicone CeeOn 911A), might help answer the question of whether the square edge is all important, or whether material also plays a role in determining the amount of PCO. Future research on IOLs should use an objective measure of PCO.

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