Tissue fusion devices apply heat and pressure to ligate or ablate blood vessels during surgery. Although this process is widely used, a predictive finite element (FE) model incorporating both structural mechanics and heat transfer has not been developed, limiting improvements to empirical evidence. This work presents the development of a novel damage parameter, which incorporates stress, water content and temperature, and demonstrates its application in a FE model. A FE model, using the Holzapfel-Gasser-Ogden strain energy function to represent the structural mechanics and equations developed by Cezo to model water content and heat transfer, was created to simulate the fusion or ablation of a porcine splenic artery. Using state variables, the stresses, temperature and water content are recorded and combined to create a single parameter at each integration point. The parameter is then compared to a critical value (determined through experiments). If the critical value is reached, the element loses all strength. If the value is not reached, no change occurs. Little experimental data exists for validation, but the resulting stresses, temperatures and water content fall within ranges predicted by prior work. Due to the lack of published data, additional experimental studies are being conducted to rigorously validate and accurately determine the critical value. Ultimately, a novel method for demonstrating tissue damage and fusion in a FE model is presented, providing the first step towards in-depth FE models simulating fusion and ablation of arteries.