The crosslinking of tarsal collagen induced by controlled exposure to UVA radiation (365 nm) was developed in our laboratories as a potential treatment for eyelid laxity. It is known that the pathophysiologic mechanism of palpebral laxity involves the enzymatic degradation and ensuing disorganization of the network consisting of elastin and collagen fibres. In this study, tarsal specimens excised from sheep eyelids were subjected in vitro to elastolysis or collagenolysis for variable durations. The degraded samples were then crosslinked by irradiation with UVA light at an irradiance of 75 mW/cm2 for 3 min, corresponding to a radiant exposure of 13.5 J/cm2, and their mechanical properties were measured before and after enzymolysis. The samples exposed to elastase for the shortest time (30 min) displayed enhanced stiffness when compared to non-degraded samples, both before and after further crosslinking, probably due to an initial major depletion of elastin associated with a transient predominance of collagen that was not yet degraded. However, at longer durations of digestion, the measured values for stiffness reached a plateau that remained constant. Following crosslinking, the degraded samples recovered mechanically, although a net increase was not supported statistically. Collagenolysis of tarsal specimens led to values that plateaued at the very beginning of digestion. The stiffness was restored by subsequent crosslinking, without a statistically definite increase. The study showed that the UVA-induced crosslinking can reverse the effect of enzymolytic degradation of the tarsus.
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