Subfracture trauma to articular joints is a routine occurrence in sports, falls, and car accidents. In some cases, the trauma can produce microscopic fractures of bone and cartilage. Approximately 25% of patients sustaining subfracture injuries will experience chronic joint tissue degeneration within several years following the initial injury. Because of the small size of the fractures, subfracture injuries are difficult to detect acutely and patients are typically not treated. Unfortunately, there is no protocol for assessing the injury risk associated with subfracture traumatic events. An established joint injury criterion does exist, however, it is based on gross fracture of bone. The criterion is used by the automotive industry and is based on impact experiments on human cadavers. The criterion states that loads exceeding 10 kN will likely result in gross fracture of bone. Unfortunately, this criterion does not address subfracture injuries. This has led to the current investigation in which theoretical and experimental models were used to develop a stress-based, subfracture injury criterion. The patellofemoral joint served as a representative joint to develop the criterion. Numerous experimental and theoretical investigations were conducted to determine the impact response of the knee to fracture and subfracture level traumas. Impact loads were directed at the patella of 62 pair of isolated human cadaver knees to simulate impact loading which can occur during realistic trauma scenarios such as falls or car accidents. During such traumatic events, body kinematics and the contacting impact surfaces can vary, thus, a range of impact interfaces were used to impact the knee flexed at 60° , 90° , or 120°. Fracture level injuries typically involved transverse and comminuted fractures of the patella. Subfracture impact experiments at approximately 70% of the fracture load produced microscopic cartilage fissures and occult subchondral bone microfractures. These injuries were documented via histology and were observed for all flexion angles. It was also observed that higher impact loads could be tolerated before fracture or subfracture level injuries were produced with increased contact area over the knee. Increased contact area typically resulted from experiments conducted with padded impact interfaces. A mathematical model was developed to estimate the stresses in the bone and cartilage in an effort to understand the injury mechanism of subfracture injuries. It was found that elevated shear stresses at the cartilage surface were associated with impact induced fissures. Occult injuries at the subchondral plate were associated with elevated shear and tensile stresses. The model showed that increased contact area over the knee significantly reduced tensile stresses in the subchondral plate. The degree to which the tensile stresses were reduced was dependent on the magnitude of the impact load and large contact areas. The reduced incidence of fracture and subfracture injuries was significantly associated with reductions in the tensile stresses in the subchondral bone. This suggested that the load and contact area data could be used as a gross estimation of the stresses in the bone. This concept was expanded to develop an injury criterion based on peak impact load and the contact area on the knee. This criterion is useful for predicting injury in the human cadaver knee, however, experiments conducted by automotive companies typically utilize anthropomorphic dummies for crash simulations. Because studies have suggested that the load-area response of the dummy knee significantly differs fi'om the human knee, a transformation protocol was developed to allow the load-area data from the cadaver experiments to be transposed to the dummy knee. Thus, experimentally derived load-area data points {Tom the dummy were compared to injurious load-area data points fiom the cadaver. The load-area criterion can be applied in experimental or theoretical studies to optimize the design of car instrument panels, sports padding, etc. Implementation of such improvements may reduce the overall incidence of joint injuries and the associated sequelae of chronic joint morbidity.