[Long-term stability of heparin-coated PMMA intraocular lenses. Results of an in vitro study].

UNLABELLED

The surface modification of PMMA-intraocular lenses (IOLs) demonstrated a blood-aqueous-barrier protective effect and reduced the incidence of IOL depositions and postoperative fibrin exudation, especially in risk patients (e.g. pediatric cataract, diabetes mellitus, recurrent uveitis). The long-term stability of the surface modification via phenylimines, which permit a covalent surface linkage of heparin to synthetic polymeric materials by reductive amination, is still unknown.

MATERIAL 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 daily rotation. After 4 years the concentration of surface-bound heparin on two HSM-IOLs of this mixture and two brand-new HSM-IOLs were determined using an orcin-assay after initial heparinase treatment. Four years 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-molecular heparin layer even under critical conditions because of its homogeneous staining. Light and scanning electron microscopic examination of the IOL surfaces were subsequently performed.

RESULTS

The concentration of heparin (microgram/cm2) on the IOL surface after 4 years of incubation 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 coarsegrained 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-IOLs showed a homogeneous heparin structure and coating after 4 years of in vitro storage. No signs of desorption or reduced reactivity of the heparin were observed in comparison with new HSM-IOLs. The unmodified PMMA-IOLs did not stain, as expected.

CONCLUSION

The heparin-modified 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 postoperative period, especially for risk patients, is therefore suggested.