MRI is arguably the most important investigative tool in knee pain. An emerging important finding with diagnostic, prognostic and therapeutic implications in early OA is Bone Marrow Edema (BME).
BMEs are found in other conditions, like trauma and osteonecrosis. In OA, it portends adjacent cartilage deterioration, it is a harbinger of disease progression, it correlates with pain, and it signifies repair failure if it persists after autologous chondrocyte implantation (ACI).
While MRI is particularly useful in the detection of early (or even "pre-") OA of the knee, ultrasound is a far cheaper and more convenient tool available at the point of care. For moderate OA, it is more sensitive than XRay in detecting osteophytes; and much of the femoral condylar cartilage can be visualised.
In addition, ultrasound can detect minimal effusions, synovial proliferation and inflammation, and cartilage calcifications and gout deposits. It can also accurately guide joint aspiration and injection.
Osteoarthritis is inflammatory; sometimes overtly so, with recurrent or persistent effusions. While the synovial fluid inflammatory cell count may not be stratospheric like what is seen in RA or Gout, a slightly higher white cell count correlates with synovial volume on MRI, and predicts for response to intra-articular steroid.