FormalPara Key Summary Points

In this laboratory experiment, we used nano-indentation for the mechanical assessment of acrylic and silicone intraocular lenses.

Hydrophobic acrylates with lower water content showed a higher relative stiffness.

As defects and penetration can occur with all materials, depending on the force applied, the principle of never touching the lens in the center of the optic should be in first place.

Introduction

Cataract surgery is a very successful procedure, and the implantation of an intraocular lens (IOL) is gold standard after the removal of an opacified lens by phacoemulsification.

Intraocular lenses are intended to remain in the eye for life following a successful insertion in the capsular bag. The material has to be biocompatible, and the biological response to a foreign material depends on both the chemistry of the material and the design or morphology. Therefore, it is particularly important that the material is durable and that IOLs are not damaged during preparation, or during the implantation process. An important step in increasing the safety of IOL handling was the development of preloaded injector systems, which has reduced the need to handle or touch the IOL prior to surgery [1].

Preloaded IOL injector systems showed several advantages: the elimination of manual, iatrogenic operator errors; the prevention of potential IOL loading errors and damage; the reduction of the risk of contaminating instruments with microorganisms or foreign bodies; lower cost and complexity and reduced surgical time with good OR workflow [1,2,3,4,5,6].

However, there are situations and opportunities during the implantation process where surgeons have to touch the lens for correct positioning in the capsular bag, or when intending to free the haptic when handshake phenomenon occurs. This means that the haptic is attached to the optic (sticking) and has to be released with an instrument. It is an important principle to never touch the central part of the optic of the lens as defects on the optics, such as scratches, can lead to deterioration of the image quality postoperatively. Several factors play a role in this, such as the position and size/extent of the defect and the design of the optics. Premium lenses (multifocals, toric, enhanced depth of focus IOLs) are particularly sensitive, due to their optics. A variety of IOL materials are currently available, including: collamer, hydrophobic acrylic, hydrophilic acrylic, polymethylmethacrylate (PMMA), PHEMA copolymer, and silicone. In the last market scope analysis, hydrophilic and hydrophobic acrylates accounted for > 85% of the global IOL market share for IOL materials, while PMMA accounted for 10% and silicone for 5% of the market share. IOL materials can be rigid (PMMA), or foldable made of hydrophobic or hydrophilic acrylics.

In the past, it has been shown that touching the IOL with instruments such as forceps or spatulas can damage the surface of the IOL [7]. It has also been shown with clinical cases that, depending on the location of the defects, this can lead to impairment of the optical imaging quality as well as to stray light and increased glare (Fig. 1).

Fig. 1
figure 1

Scanning electron microscopy of defects/scratches in the IOL material and surface. A–E Showing defects at the edge of the lenses, F–H showing severe scratches of forceps within the optic of IOLs. Slit lamp images (I–K) showing an implanted IOL with a defect right in the center of the optic causing clinical symptoms such as stray light and glare

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It seems that the extent of the defect depends on the force and the shape of the instrument, but also crucially on the material and its resistance. Acrylic lenses are produced either injection molded or with lathe-cut technique. A comparative analysis of YAG-laser induced damages on injection-molded and lathe-cut IOLs showed that the defects in lathe-cut IOLs lead to positive polarization and were correlated with increased potential for glare. Another laboratory experiment comparing hydrophobic and hydrophilic acrylic IOLs with YAG pits and different water content found differences regarding stray light and glare effect [8, 9].

The manufacturers of the IOLs must use materials that are soft and flexible enough to be folded in preloaded injector systems and inserted through small clear corneal incisions into the capsule. On the other hand, intraocular lenses should have enough stability in the capsule and stay in position permanently.

Nowadays, acrylic, one-piece IOLs are the most commonly used. Depending on the water content, a distinction is made between hydrophilic and hydrophobic materials.

Hydrophobicity is a measure of a particular material’s tendency to repel or separate itself from water. IOL materials are defined hydrophobic or hydrophilic according to the angle a drop of water makes with respect to the material surface. A water droplet is placed on the surface for testing, and the angle between the surface and the droplet is measured (Fig. 2). Hydrophilic surfaces have shallow angles, while hydrophobic surfaces have larger angles [9].

Fig. 2
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Schematic diagram of a hydrophobicity measurement. A water droplet is placed on the surface for testing, and the angle between the surface and the droplet is measured. Hydrophilic surfaces have shallow angles, while hydrophobic surfaces have larger angles

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Hydrophilic materials are always packaged wet and typically contain 22–26% water content. A material is generally considered to be hydrophobic if it has a water content equal to or less than 5%. Traditionally hydrophobic materials are packaged dry, but to help mitigate the effect of glistening they increasingly have a small amount of hydrophilic character incorporated into their composition. Some of the “glistening-free” hydrophobic materials with water contents around 5% are supplied in saline.

Hydrophobic material (even with low water content such as 4%) is well known to be hard to fold, with tackiness, and prone to glistening/vacuoles . This material is providing the best results in terms of low posterior capsule opacification (PCO).

Hydrophilic material has the advantage of preventing glistening due to its higher water content, but has the reputation of having a higher rate of PCO. This material is the easiest to inject into the bag especially with small incision and micro-incision (Table 1).

Table 1 Advantages and disadvantages of different materials (PMMA, hydrophilic acrylates, hydrophobic acrylates, silicone)
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In this laboratory experiment, we used nano-indentation for the mechanical assessment of acrylic and silicone intraocular lenses. We wanted to determine whether there are differences in the materials and whether some react more sensitively to touching/handling than others. The information is intended to assist surgeons and scrub nurses in assessing the importance of not touching lenses in sensitive areas such as central optic parts.

Methods

Intraocular Lenses (Samples)

We have included the following intraocular lenses in our experiment: A: Primus HD (OphthalmoPro, Germany), B: CT Lucia 621P (Zeiss Meditec, Germany), C: Acunex AN6 (Teleon Surgical, the Netherlands), D: Aspira aXA (HumanOptics, Germany), E: AT709M (Zeiss Meditec, Germany), F: C-Flex ADV970 (Rayner Surgical, UK), G: LI61AO Sofport (Bausch Lomb, USA). The IOL design of the seven IOLs is shown in Fig. 3, and the specifications of the seven lenses are summarized in Table 2. IOLs A, B, and C are made of hydrophobic acrylates, whereas IOLs D, E, and F are made of hydrophilic acrylates and IOL G is made of silicone. All acrylic IOLs are one-piece IOLs; only IOL G is a three-piece IOL. In all cases IOLs with the same power (22.0 D) were used. The exact manufacturing process and detailed specifications of the material are usually kept secret by companies. The most obvious difference in the six acrylic lenses is the water content (0.3–26%) and differences in the manufacturing process.

Fig. 3
figure 3

The hydrophobic and hydrophilic acrylic IOLs (A–F) are all foldable, one-piece IOLs packaged in preloaded injector systems. The silicone IOL (G) is a three-piece IOL with PMMA haptics. Note: The pictures were made during the laboratory experiment/preparation in balanced salt solution; therefore, the optics do not look completely clear or stray light can be seen

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Table 2 Seven IOLs have been tested
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Ultra Nanoindentation Tester

The Bioindenter (UNHT Bio, Anton Paar, Austria), an ultrahigh-resolution nanoindenter with real force and depth sensor, was used to investigate the mechanical properties of the material in nanoscale measurement ranges (Fig. 3).

The instrumented indentation technique involves pressing an indenter of known geometry into the surface while both penetration depth and normal load are monitored. The indentation elastic modulus (EIT) and other mechanical properties such as the creep were obtained from the force displacement curve. The analysis of this curve is done automatically according to the ISO 14577 standard [10,11,12,13].

Measurement Protocol

For measuring penetration depth and testing of possible damage to the intraocular lenses, the samples were measured at room temperature. A 200-µm-diameter ruby spherical tipped indenter was used for all the tests. Indentations were made to three different maximum loads, namely 5 millinewtons (mN), 15 mN, and 30 mN to increase the penetration depth up to a limit of 100 µm. The loading rate and unloading rate was 25 mN/min with a hold period of 30 s. The IOL samples were fixed with duct tape between a plastic disc with a 6-mm-diameter hole and a fused silica disc. The disc was placed in a Petri dish and fully submersed in balanced salt solution (Fig. 3). Measurements were taken on a 3 × 3 matrix indentation grid (three indentation points per maximum load) in the center of the optics of the IOLs. All measurements and exactly the same procedure were repeated three times.

Elastic Modulus

The force–depth indentation curves during unloading were used for evaluation of stiffness parameters of the materials (Fig. 4). The E-moduli EIT were determined by fitting the unloading profile according to Oliver and Pharr [11]. The EIT is a measure for the elasticity of the material as determined by indentation. The creep analysis was used to show the progression of the deformation during the 30 s holding time under constant force. The creep was calculated as CIT = (hm − hi)/hi, where hi and hm denote the indentation depth at the beginning and the end of the holding period, respectively.

Fig. 4
figure 4

The Bioindenter (UNHT) by Anton Paar was used in our laboratory experiment. Schematic illustration showing the instrumented indentation technique

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Note: For the discussion of the elastic moduli and the creep during the holding period, only the results of the measurements at the lowest maximal load (5 mN) were taken into account.

The laboratory study was exempt from ethical committee approval as it is a laboratory experiment without humans involved.

Results

Data Analysis and Statistics

The results of the penetration depth, hm, in nanometers (nm) with forces of 5, 15, and 30 millinewtons (mN) are shown in Fig. 5. The lowest penetration depth (12 μm) with 5 mN force was observed with IOL B. However, IOL A, D, and F showed similar low penetration depths (20, 18, 23 μm). Lenses C and E showed slightly higher penetration depths of 36 and 39 μm, respectively. The comparison lens of silicone, G, showed the greatest penetration depth of 54.6 μm at a maximum load of 5 mN. With higher maximal loads (15 and 30 mN) the penetration depth increases significantly. In case of lens G, the maximal penetration depth of 100 μm was already exceeded at the maximal load to 15 mN. Lens C, however, showed the same penetration depth both at 15 and 30 mN with no increase of penetration depth. This could fit well with the construction process of the lens (lathe-cut technique) or a specific material design of the lens.

Fig. 5
figure 5

The force–depth indentation curves during unloading were used for evaluating the stiffness parameters of the materials

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During the holding time of 30 s at constant force, all six acrylic lenses showed a significant increase of the penetration depth (CIT 21–43%). The silicone lens (G) showed the smallest creep with 14%.

Elastic Moduli

The elastic moduli of the IOLs showed, as expected, a rather low stiffness of the materials. Clearly, IOL B (hydrophobic acrylic with lowest water content) was the stiffest of all seven tested lenses followed by A, C, D, and F. IOLs E and G showed the highest elasticities.

The indentation tests showed a similar trend and were in accordance (Tables 3, 4). The EIT values ranged from 1 to 37 MPa. IOL B had the largest EIT of 37 MPa, which could be caused by the low water content. In comparison, the EIT of lenses A, C, and D was found at 10.6, 10.2, and 11.8 MPa, respectively. Lens F had a lower EIT of 5 MPa, and the lowest EIT was measured for lens E (1.6 MPa) and lens G (1.0 MPa). At a maximal load of 15 mN, the EIT for lens G could not be determined anymore due to the exceeded maximal penetration depth. Most other lenses showed a similar EIT at 15 mN, with the exception of lens C, where the EIT increased. Again, this is in accordance with the results of penetration depth and might be explained with a layered lens structure or different water contents with a coat and core structure or the manufacturing process (Figs. 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16).

Table 3 Elastic moduli and initial and maximal indentation depth after 30 s hold at a maximum load of 5 mN
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Table 4 Elastic moduli and initial and maximal indentation depth after 30 s hold at a maximum load of 15 mN
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Fig. 6
figure 6

The results of the penetration depth, hm, in nanometers (nm) with forces of 5, 15, and 30 millnewtons (mN). A–C are hydrophobic acrylic IOLs, D–F are hydrophilic acrylic IOLs, and G is the silicone IOL

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Fig. 7
figure 7

Penetration depth (µm) versus force (mN). Comparison of all tested IOLs

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Fig. 8
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Material stiffness measured with Oliver–Pharr method

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Fig. 9
figure 9

Results of IOL A sample with maximum load of 5.00, 15.00, and 30 mN

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Fig. 10
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Results of IOL B sample with maximum load of 5.00, 15.00, and 30 mN

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Fig. 11
figure 11

Results of IOL C sample with maximum load of 5.00, 15.00, and 30 mN

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Fig. 12
figure 12

Results of IOL D sample with maximum load of 5.00, 15.00, and 30 mN

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Fig. 13
figure 13

Results of IOL E sample with maximum load of 5.00, 15.00, and 30 mN

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Fig. 14
figure 14

Results of IOL F sample with maximum load of 5.00, 15.00, and 30 mN

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Fig. 15
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Results of IOL G sample with maximum load of 5.00, 15.00, and 30 mN

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Fig. 16
figure 16

Results of elastic moduli (Hertz) and comparison of the seven tested IOLs

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Discussion

The aim of the study was to compare the mechanical response of the seven IOLs and test for possible damage using increasing maximal penetration forces (loads). Intraocular lens manufacturers must use a material that is not only biocompatible and meets the highest medical requirements, but is also soft and flexible enough to be implanted through a small clear corneal incision with an injector in the folded state. On the other hand, the lens should be as stable as possible in the eye, achieve a good centered position in the capsular bag, and remain unchanged over a long period of time. For these reasons, a compromise must always be found to combine as many advantages as possible. Since the introduction of foldable lenses and the development of preloaded injectors, the workflow during surgery has been significantly increased and safety has been further enhanced. Handling of the lens for preparation purposes has been minimized for most IOL types and injectors. Nevertheless, there are situations where the surgeon or scrub nurse is required to touch the lens. Even if no defects are visible macroscopically, it has been shown several times in the past that changes (defects or scratches) can be detected at the microscopic level or by means of electron microscopy. These defects can lead to different severe effects depending on the position in the IOL, size, and depth. Various studies have shown that the manufacturing process (lathe-cut versus molded) also plays an important role and has advantages and disadvantages. The most commonly used materials today are hydrophobic and hydrophilic acrylates. Whether hydrophobic or hydrophilic, opacification might be a problem with all acrylate material lenses. In hydrophobic IOLs, this takes the form of “glistenings,” microvacuoles that result from fluid seeping into the empty spaces within the material’s polymer structure [14]. In hydrophilic IOLs, opacification is an outcome of calcification, which can result from defects in the material, and changes in the aqueous milieus may occur during surgery and in patients with conditions such as uveitis and diabetes [15]. Optical bench tests and clinical cases showed that glistenings have little effect on modulation transfer function (MTF) but significantly increase stray light [16,17,18,19,20,21]. Modern hydrophilic and hydrophobic IOLs on the market have been able to show good optical quality and clarity, therefore leading to very good refractive results. Some manufacturers try to take advantage of material properties by building up the lens layer by layer, or wrap/coat the lens on the surface to incorporate additional features. Although there are countless clinical case series and control studies on the individual lenses, there are very few laboratory studies that are limited purely to material differences and compare these material properties and evaluate special features of lenses. It would be beneficial if there were more lab trials in this area in the future.

This experimental laboratory study aimed to compare the materials (hydrophobic acrylic and hydrophilic acrylic) with each other and also with the silicone lens in terms of behavior against pressure. Some differences could be found. However, it is important to emphasize that with any material, depending on the pressure, a defect can also occur at some point.

Limitations of the Study

Surgeons and scrub nurses should be careful with any implant/IOL material and try to avoid any contact with the center of the optic. This laboratory experiment has shown that acrylic materials with different water content and manufacturing process behave slightly different under indentation and are therefore more or less sensitive to external force. No direct correlation to clinical data can be drawn with these laboratory results. It is also not possible to give a ranking of the tested lenses from good to bad based on the results. All seven tested lenses have confirmed their quality through various studies and in clinical practice since their market launch. It is a pity that various details of the lens (details of the material formulation, manufacturing details) are kept extremely secret and one is partly dependent on information provided by the manufacturers even for scientific reasons.

Manufacturers try to balance the advantages and disadvantages of flexibility and stability. This explains that there are differences in the water content of the lens models, which are reflected in marginal differences in the behavior of our indentation tests. The key message is that knowledge about the vulnerability of modern, foldable acrylic, and silicone lenses seems to be important to achieve best postoperative results in clinical routine.

Conclusion

Since the three hydrophilic and three hydrophobic acrylic lenses are constructed of very similar material, it is not surprising that the results are very similar and there are no significant differences. Nevertheless, minor differences could be shown in this laboratory experiment. It was found that these correlate very well with the initial water content of the material . Even though hydrophobic materials with lower water content showed higher relative stiffness, penetration and defects can also occur with these. The manufacturing process of the lens (lathe-cut versus molded) is also crucial and seems to play an influence.

The surgeon and scrub nurse should always be aware that macroscopic changes are difficult to detect but could theoretically lead to clinical effects. The principle of not touching the center of any IOL optic at any time, either when preparing the lens or when positioning the IOL in the capsular bag, should be taken seriously.