Excessive mechanical load to the knee during a blunt impact can initiate a degenerative disease process. Laboratory experiments have shown that the stresses and strains generated in the knee joint during impact can cause gross mechanical tissue damage. There are two basic types of damage reported in the knee joint under impact loading:​ fissures on the surface of the cartilage and/or microcracks in the subchondral bone or calcified cartilage. A mechanical damage criterion is developed in this dissertation using new experimental data, old observations and some new analytical results using the finite element method. A Coulomb-Mohr damage criterion is applied to study mechanics of biological tissue trauma. The failure envelop reveals that the damage of cartilage from the tibial plateau is controlled by two variables:​ shear stress and hydraulic pressure. In fact, the effect of static hydraulic pressure is significant and can not be ignored. The criterion is robust and can be used to help explain other types of biological tissue damage during blunt impact loading.