Photocrosslinking of Tarsal Collagen for Treating Eyelid Laxity: Applying the Exposure Reciprocity Law to Establish an Optimal Irradiant Profile

The aim of this study was to develop an irradiant exposure strategy able to control validity of the exposure reciprocity law upon exposing tarso-conjunctival tissue to ultraviolet A (UVA) radiation as a treatment for palpebral laxity, and thus to establish a research-based criterion for implementing optimal irradiation levels in clinical settings. Tarso-conjunctival specimens were harvested postmortem from ovine and caprine eyelids and exposed to UVA radiation (365 nm) in specified conditions. Thermographic infrared analysis was applied to monitor the photothermal response. By performing irradiation in the presence of a photosensitizer (riboflavin) a photochemomechanical response was also determined based on changes in mechanical properties as evaluated in a mechanical tester. A graphical method was employed to define the failure zones of reciprocity law when applied to radiation-induced photoresponses. Discontinuities in law’s validity were found and confirmed by descending values of Schwarzschild’s p -coefficients. The photothermal response showed that law’s failure zone centered on an irradiance of 30 mW/cm 2 in both sheep and goat tissues, for radiant exposures between 6 and 18 J/cm 2 . The photochemomechanical response indicated a failure onset around 60 mW/cm 2 . Anticipating the concept of safest application route, an irradiance of 30 mW/cm 2 appeared an appropriate upper limit for the validity zone of reciprocity law, as indicated by the photothermal response, at exposure durations assuring a radiant exposure between 5 and 20 J/cm 2 .