2.2.1 Acrylic IOLs: Polymethyl Methacrylate

Nonfoldable acrylic lenses are made of polymethyl methacrylate (PMMA), which was the original material used by Harold Ridley for the first IOL implanted in a human being in the late 1940s (see Chapter 1). ² PMMA lenses are hard and rigid and require a 5.5 to 6 mm or larger corneoscleral incision for insertion into the eye. ³ PMMA lenses are inherently hydrophobic, so they are more likely to adhere to corneal endothelial cells during insertion, resulting in potential loss of those cells. Overall, PMMA lenses are known for their superior optical quality. ⁴ They are available in one-piece versions (optic and haptics made from one mold) and three-piece versions (optic and haptics made from separate materials and attached together). PMMA IOLs were well suited for extracapsular cataract extraction techniques where larger incisions were necessary for nucleus removal. However, with the advent of phacoemulsification and eventual foldable IOLs that allowed for smaller incisions, PMMA lenses are now the least commonly implanted lenses for routine cataract surgery. ⁵

2.2.2 Acrylic: Foldable Hydrophobic

By 1998, foldable acrylic lenses had become the most common IOL inserted by American surgeons. ⁵ Foldable acrylic IOLs come in both hydrophobic and hydrophilic varieties (Table 2-1). Hydrophobic foldable acrylic lenses contain very little water (< 1%) (Fig. 2.1, Fig. 2.2). ¹ These hydrophobic acrylics are known for lower posterior capsule opacification (PCO) rates than other foldable lenses. ⁶ Hydrophobic acrylic lenses are offered as one-piece or three-piece lenses, with three-piece lens haptics most often made of flexible PMMA. ⁵

2.2.3 Acrylic: Foldable Hydrophilic

Foldable acrylic lenses can also be hydrophilic in nature (Table 2-1). These hydrophilic acrylic lenses can be called hydrogels if they contain polyhydroxyethylmethacrylate (PolyHEMA). ⁷ Hydrogels contain a significantly higher amount of water (18–38%) compared to their hydrophobic acrylic counterparts. ¹ As a result, these hydrophobic acrylic lenses must be packed in either distilled water or balanced salt solution. ⁵ Because of the higher water content of these lenses, they have a low refractive index. This results in theoretically decreased glare and other light distortions by the lens. Early hydrophilic lenses were associated with calcifications that could opacify the optic, but the rate of calcification has dropped significantly since the production techniques have been revised and the packaging improved. ⁶ These lenses are highly foldable and can be injected through incisions approximately 1.8 mm in length or smaller, ⁵ allowing for microincision cataract surgery (MICS) (Fig. 2.3).

Another subset of hydrophilic foldable acrylics is the Collamer lens (Fig. 2.4). This Collamer material is a patented copolymer of hydrophilic acrylic and porcine collagen. In theory, the porcine collagen improves the biocompatibility of the lens when implanted in human eyes. ⁵ These lenses contain about 34% water. ¹

2.2.4 Silicone IOLs

The other broad category of lenses is silicone, which was the first foldable IOL. ⁵ These lenses have a common polyorganosiloxane backbone and lower refractive index than acrylic lenses, so they are traditionally thicker than acrylic lenses for the same refractive power.s. Literatur ^{,​ 4} These lenses are usually made with either haptics with modified C-loops or plate haptics. In the case of modified C-loop haptics, the arms can be composed of PMMA, polypropylene, polyamide, or polyvinylidene fluoride (PVDF). The plate haptics, however, have a higher tendency to decenter in eyes with anterior capsule defects. Silicone lenses are easily folded. ⁵

2.3.1 One-Piece (Single-Piece) IOLS

One-piece IOLs vary dramatically in their overall composition, with both large inflexible styles made of PMMA and foldable IOLs capable of fitting through the smallest available incisions. Single-piece flexible lenses that can be folded include hydrophobic (Fig. 2.1, Fig. 2.2) and hydrophilic acrylics (Fig. 2.3), along with silicone lenses with haptics composed of the same optical body material seamlessly radiating from the optic.

Although one-piece PMMA IOLs have fallen out of favor as primary IOL implants due to the large incision size, due to their thin haptic design these IOLs are compatible with both in-the-bag (capsular) and sulcus placement. The haptic design can result in excessive in-the-bag rotation though, leaving this IOL style a poor choice for toric IOL design or any other optical design that requires minimal postoperative rotation.

One-piece foldable IOLs are known for their good in-the-bag fixation due to their thicker haptics that are continuous with the optic material. The haptics of hydrophobic acrylic one-piece lenses also have greater stiffness and memory than three-piece lenses with PMMA haptics, providing more tension on the capsular bag, along with better centration and less tilt of the optic. ⁸ These qualities make one-piece foldable lenses ideal for optics with toric and multifocal qualities that require strict placement and limited rotation once placed in the capsular bag. ⁹ As will be discussed later in the Sulcus Placement section of this chapter, single-piece foldable IOLs are poor candidates for implantation in the ciliary sulcus because of their tendency to chafe against the posterior iris, causing iritis and even uveitis. ¹⁰

Plate haptics, a single-piece design (Fig. 2.4), in which the haptics form a uniform edge around the optic, were designed to achieve better fixation of the lens to the capsular bag.s. Literatur The first toric IOL designs were plate haptic styles due to this capsular stability as seen in STAAR Surgical’s original Elastic Silicone Toric IOL AA4203TF. ¹¹ These lenses are typically extremely stable, except in eyes with capsule defects. ⁵