1. In 78 per cent. of adult knees, a suprapatellar synovial plica exists, representing the fusion of the suprapatellar bursa with the knee joint.
2. In pneumo-arthrograms the density of the suprapatellar plica serves as an index of synovial reaction, and is of value in prognosis.
3. Fibrosis, hyalinization, or calcification of the suprapatellar plica mechanically interferes with the extensor apparatus, producing a syndrome of pseudo "locking", pain, and chronic effusion, which is to be differentiated from other intra-articular disabilities.