LONG-TERM STABILITY OF THE HEPARIN SURFAC E ON PMMA-INTRAOCULAR LENSES - RESULTS OF AN IN-VITRO STUDY

The surface modification of PMMA-intraocular lenses (IOLs) demonstrate d a blood-aqueous-barrier protective effect and reduced the incidence of IOL depositions and postoperative fibrin exudation, especially in r isk patients (e.g. pediatric cataract, diabetes mellitus, recurrent uv eitis). The long-term stability of the surface modification via phenyl imines, which permit a covalent surface linkage of heparin to syntheti c polymeric materials by reductive amination, is still unknown. Materi al and methods: Four heparin surface-modified (HSM) monofocal and two unmodified monofocal sterile PMMA-IOLs were stored in an aqueous-serum mixture at 37 degrees C over a period of 4 years and 1 months with da ily rotation. After 4 years the concentration of surface-bound heparin on two HSM-IOLs of this mixture and two brand-new HSM-IOLs were deter mined using an orcin-assay after initial heparinase treat ment. Four y ears after incubation, the modified toluidine blue staining method was used to examine the surface-bound heparin on synthetic polymers. This staining technique with toluidine blue, a non-protein basic substance , enables examination and analysis of the homogeneity of the mono-mole cular heparin layer even under critical conditions because of its homo geneous staining. Light and scanning electron microscopic examination of the IOL surfaces were subsequently performed. Results: The concentr ation of heparin (mu g/cm(2)) on the IOL surface after 4 years of incu bation and treatment with heparinase was valued at 0.51 +/- 0.05 and 0 .53 +/- 0.04 in the two brand-new HSM-IOLs. A slightly coarse-grained complex agglutination on the IOL surface was detected by the toluidine blue staining method. light and spectral microscopy of the surface of the stained IOLs as well as scanning electron microscopy of all HSM-I OLs showed a homogeneous heparin structure and coating after 4 years o f in vitro storage. No signs of desorption or reduced reactivity of th e heparin were observed in comparison with new HSM-IOLs. The unmodifie d PMMA-IOLs did not stain, as expected. Conclusion: The heparin-modifi ed surface of the examined PMMA-IOLs was intact even after 4 years of storage in an aqueous serum solution. A long-term benefit, in addition to the advantages of the hydrophilisation in the immediate postoperat ive period, especially for risk patients, is therefore suggested.