With the advent and increasing popularity of small-incision cataract surgery, surgeons now have numerous choices of soft foldable implants and insertion methods. These implants and insertion tools can be described as lens systems, and they all have advantages and disadvantages. The question is, of the choices available today, which is the ideal lens system? This article explores some of these issues, and provides my reasoning as to which system I believe is best.
THE BASICS
Foldable lens implants are composed of acrylic, silicone, collamer, or hydrogel material, and the designs of these lenses can be classified as either three-piece, plate haptic, or one-piece lenses. Lens haptics are typically made of PMMA, prolene, polyimide, or acrylic. Lenses can be delivered either with insertion forceps or cartridge-based insertion devices. Some of these devices include the Monarch II (Alcon Laboratories, Fort Worth, TX), the Unfolder series (Allergan Surgical, Irvine, CA) the STAAR Microstar (STAAR Surgical, Monrovia, CA), the Mport & Passport (Bausch & Lomb Surgical, San Dimas, CA). The Monarch series is used for Alcon's new one-piece acrylic lens, the SA30AT and the SA60AT.
INJECTION SYSTEMS VERSUS FORCEPS
The advantage of inserting lenses with forceps is that the surgeon has more control, especially if he or she manually folds the implant. However, this technique usually requires enlarging the incision. Nick Mamalis, MD, conducted a study in which he compared incision widths for lenses that were implanted with forceps against lenses implanted with injectors.1 He measured the incision width after phacoemulsification and compared that with the width of the same incision after a three-piece silicone lens had been inserted using forceps, and found that the incision width had increased from 4.4% to 6.2%. When Dr. Mamalis examined three-piece acrylic lenses inserted in a similar fashion, there was a 5% to 6% increase in incision width after forceps insertion. This range is due to the fact that higher lens powers require greater incision widths. However, when Dr. Mamalis studied three-piece silicone lenses inserted using an injector, he found that there was only a 3.2% increase in the incision size, and the lens power was inconsequential when passed through an injector. Because an increased incision width may induce astigmatism and lessen wound stability, it would appear that lens injectors are superior to forceps. The best injection systems, of course, would be those which insert the IOL without requiring the surgeon to enlarge the wound, and implant the lens in the bag without extra manipulation of the trailing haptic. This is otherwise known as single-incision, one-step implantation, and is the most efficient means of insertion.
ADVANTAGES OF ACRYLIC MATERIAL
Which material produces the best lens? I believe that a number of characteristics are important. First, biocompatibility is very important, because a lens that integrates seamlessly in the capsular bag with minimal inflammation, proliferation, and migration of the lens epithelial cells will produce a low rate of posterior capsular opacification (PCO). Another characteristic of an excellent lens is the ability to be implanted safely and easily via an insertion device using single-incision, one-step technology.
THE AcrySof LENS
According to peer-reviewed, published data, the lens associated with the lowest YAG capsulotomy rate appears to be Alcon's AcrySof. David Apple, MD, conducted a postmortem study of 5,416 eyes2 in which the YAG rate with the AcrySof lens was 0.9%. The next lowest rate corresponded to a one-piece silicone plate lens at 12.1%, and the other lenses ranged from 12.1% for a one-piece silicone lens up to 33% for a three-piece PMMA lens. Takayuki Akahoshi, MD, from Tokyo, Japan, has conducted a study of approximately 20,000 eyes implanted with the AcrySof lens, and has concluded that the YAG rate of the AcrySof lens is approximately 1% (information presented at ASCRS, San Diego, 2001). It is thought that this lens, with its square-edge design, provides a barrier to migrating lens epithelial cells. In addition, this lens has been shown to create a bioadhesive bond with the collagenous capsule, which assists in reducing the PCO rate.3 Figures 1A and B show the right and left eye of the same patient approximately 18 months after uncomplicated phacoemulsification with an IOL. The right eye was implanted with a three-piece silicone lens, and the left was implanted with a one-piece AcrySof lens. The eye with the acrylic lens had a substantially clearer posterior capsule, and the patient noted better vision with that eye.
Lower rates of anterior capsule opacification in turn decrease the decentration rate. The ability to eradicate PCO is extremely important because of the tremendous economic benefits in addition to reducing patients' exposure to the risks associated with laser capsulotomy. Thomas Newland et al performed a study comparing YAG laser damage to silicone, acrylic, and PMMA lenses.4 He found that the silicone lens had the lowest threshold of energy before damage occurred, which means that it was the easiest lens to harm by the laser. Acrylic had the highest threshold for damage and the least amount of collateral harm. In other words, the acrylic was the hardest lens to damage, and it developed less damage at a distance away from the point of laser impact.
CRITERIA FOR DELIVERY SYSTEMS
The ideal lens system is one that offers easy delivery of the most biocompatible lens available through a single, small incision. The Monarch II accomplishes these criteria and is easy for the staff to learn to use. Although other injector systems perform well, they sometimes require extra manipulation within the eye, such as rotation of the entire cartridge. Also, it is sometimes difficult to implant the trailing haptic in one step, which reduces efficiency and may increase the risk of the haptic introducing contaminants into the eye. With the Monarch II, the surgeon can implant the entire lens into the bag virtually every time.
BIOMECHANICS
The SA60AT is a one-piece, 6-mm, completely acrylic IOL that can be implanted through an unenlarged incision. One aspect of the SA60 that I appreciate is that even with small posterior capsular tears, the surgeon can implant this lens safely without enlarging the tear, thereby avoiding placing the lens in the sulcus. The SA60 lens opens gently within the capsular bag, and integrates well with the ocular environment. It has low lens epithelial cell migration and proliferation due to its combination of material and square edge.
PLATFORM FOR THE FUTURE
The SA60AT/Monarch II system is ideal for positioning in the capsular bag, as its delivery places minimal tension on the bag. The optic and haptics adhere strongly to the posterior capsule, which I believe will provide long-term stability and satisfactory centering performance. This rotational stability and centration, combined with first-rate biocompatibility, seems to create an excellent platform for other refractive designs, such as toric and multifocal implants.
1. Mamalis N: Incision width after phacoemulsification with foldable intraocular lens implantation. J Cataract Refract Surg 26:237-241, 2000
2. Apple D, Peng Q, Visessook N, et al: Eradication of posterior capsule opacification: Documentation of a marked decrease in Nd:YAG laser posterior capsulotomy rates noted in an analysis of 5416 pseudophakic human eyes obtained postmortem. Ophthalmology 108:505-518, 2001
3. Linnola R, Sund M, Ylonen R, et al: Adhesion of soluble fibronectin, laminin, and collagen type IV to intraocular lens materials. J Cataract Refract Surg 25:1486-1491, 1999
4. Newland T, McDermott M, Eliott D, et al: Experimental neodymium:YAG laser damage to acrylic, poly(methylmethacrylate),and silicone intraocular lens materials. J Cataract Refract Surg 25:72-76, 1999
