It is the first time in literature that the PPC signal was proved in vivo in this study to be originated from friction between the articular surfaces of the patellofemoral joint.Seventeen patients(17 knee joints) of knee injuries whose joints were inspected by arthroscopy and 24 patients(36 knee joints) of degenerative osteoarthritis(OA) to whom total knee replacement was peformed were included in this study.physiological patellar crepitus(PPC) signals which were obtained preoperatively were compared with the operative findings of the knee joint.Five young and five old normal volunteers were also included for age controlled comparison.Normal PPC signals occurred throughout the whole cycle of knee flexion and extension in the individual with intact patellofemoral articular cartilage.The mean root mean square(RMS) of the PPC signal which represents the amplitude and energy content of the vibration waves of the normal young individuals was 67.3 mv and 48.9 mv for the normal old individual s.The mean RMS for those knee injury patients whose patellofemoral joints were intact was 46.0 mv and for the OA patients was 18.4 mv.The mean RMS value for the latter was significantly smaller than the others(P<0.001).For OA patients,three types of PPC signals can be identified.Each represents a unique pattern of degeneravive involvement of the knee joint.PPC measurement is a simple non-invasive diagnostic technique for evaluating the integrity of articular cartilage of the patellofemoral joint.Not only the severity of pathology but also the area of involvement in the knee joint can be assessed.There is a geat future for PPC to be used clinically.