Final answer:
Crepitus in a patient with osteoarthritis is caused by the erosion of articular cartilage leading to the bones rubbing against each other, generating a grating sound during movement.
Step-by-step explanation:
The audible grating sound, known as crepitus, that a client with osteoarthritis experiences when moving his knees is caused by the rough surfaces of the knee joint rubbing together. The underlying cause of this sound can be attributed to the erosion and loss of the articular cartilage that covers the surfaces of the bones within a synovial joint.
Osteoarthritis, characterized by joint pain and stiffness, is believed to result from mechanical stress on the joints combined with inadequate self-repair of cartilage and low-grade inflammation of the joints. Over time, as articular cartilage is increasingly lost, the bones may rub directly against each other when the joint is moved, causing the crepitus that the nurse observes.