Biocompatibility of Intraocular Lenses

To make specific statements about intraocular lenses (IOLs) and facilitate the selection of the most suitable IOL for a specific indication, I believe it would be appropriate to introduce the terms uveal and capsular biocompatibility of IOLs. To date, it seems that in general, IOLs provide good uveal or capsular biocompatibility. Biocompatibility is one of the most important prerequisites of an intraocular implant. Therefore, this term is frequently used to describe specific characteristics of IOLs. However, the word biocompatibility is used rather arbitrarily and erroneously, sometimes causing misconceptions. A low incidence of capsular opacity is mentioned by some authors as a parameter of high biocompatibility. In other cases, a less pronounced foreign-body cell reaction is cited as a criterion. The amount of blood–aqueous barrier (BAB) change is also used as a measure of IOL biocompatibility. But the extent of BAB change is influenced by the IOL as well as other factors such as preexisting ocular pathology, surgical trauma, and perioperative therapy. Is an IOL that has a significantly lower incidence of macrophages on its surface than another IOL but a significantly higher incidence of lens epithelial cell (LEC) outgrowth biocompatible or not? Because of such discrepancies, authors have gone so far as to assert that an IOL could be too biocompatible since, on the one hand, significantly fewer macrophages were found on the lens surface of a hydrophilic IOL but, on the other hand, LEC outgrowth was observed on that lens type, especially in cases with low flare values after surgery. However, this statement is refuted by the definition of the term biocompatibility itself: “Biocompatibility is the capability of a prosthesis implanted in the body to exist in harmony with tissue without causing deleterious changes.”1 Biocompatibility is, by definition, a positive term. When an implant causes a specific tissue reaction in the eye that a different implant does not cause, it would be appropriate to conclude that the former implant is not as biocompatible as the latter. However, in no case can an IOL be too biocompatible because it proves to be superior with regard to a different parameter. As an implant reacts with different tissues of the eye, it is possible that a specific IOL will be better in relation to a specific tissue reaction of the body but have a less effective performance with regard to another reactive pattern. Therefore, it has become mandatory to systematize the terminology of biocompatibility in a tissue-dependent manner. Intraocular implants are primarily in the immediate vicinity of or in contact with uveal tissue and lens tissue. The implant itself causes a specific pathophysiological reaction of uvea and LECs.2–5 Therefore, a distinction should be made between uveal and capsular reactions of the eye. As both these reactive processes are influenced by the implant, it would be appropriate to introduce the terms uveal and capsular biocompatibility. The reactive pattern of both tissues influences BAB changes with its consequences. A further tissue-specific classification into corneal, vitreoretinal, and other forms of biocompatibility is conceivable but is of secondary importance for clinical outcome in comparison with the significance of the 2 mentioned tissues. Uveal Biocompatibility The iris, ciliary body, and choroid are composed of vascularized tissue and are in close vicinity to the IOL. In cases of iridolenticular synechias or sulcus position of the IOL, portions of the lens come into direct contact with uveal tissue. Because of the surgical trauma and the implanted foreign body, BAB changes develop, causing leakage of proteins and macrophages from the blood. Soon after IOL implantation, protein adsorption on the lens surface takes place. This membrane is composed of various proteins, and its composition is influenced by the lens material. The membrane, in turn, influences cell adhesion and activation on the IOL. Initially, small round cells and fibroblast-like cells adhere to this membrane. Later, as a result of fusion of the macrophages, they develop into epithelioid cells and foreign-body giant cells. Macrophages themselves produce various cytokines, which again influence the subsequent course of inflammation, resulting in BAB changes of varying intensity. It has been shown that the foreign-body reaction described above is influenced by lens material and lens design. Since the foreign-body cell reaction is the most important parameter of uveal biocompatibility, it seems appropriate to describe IOLs causing only a very mild foreign-body reaction as uveal biocompatible. Especially in compromised eyes, IOL with high uveal biocompatibility should be selected. Capsular Biocompatibility In most cases, the capsular bag and the LECs are the only ocular tissues in direct contact with the IOL. Contact between the LECs and the lens material causes proliferation, myofibroblastic metaplasia, and cytokine production. The secretion of cytokines may compromise the BAB and in turn influence LEC metaplasia. As a result of proliferation, LECs may spread onto the IOL material. However, the main complications of capsular reactions to the IOL are the development of opacities of the anterior capsule (ACO) and the posterior capsule (PCO) and capsule contraction. Anterior capsule opacification and PCO appear to be influenced by lens material as well as lens design. Thus, as the main parameters of capsular biocompatibility are LEC outgrowth, ACO, PCO, and capsular contraction, IOLs with good performance in relation to these aspects represent high capsular biocompatibility. The IOLs of the future should provide optimal uveal as well as capsular biocompatibility. Michael Amon MD aVienna, Austria