Most often the diagnosis of osteoarthritis of the hip can be made on a plain x-ray. Joint space narrowing, deformity, and spur formation are much more obvious on a plain film than on an MRI. MRI is used to diagnose certain problems in the hip which may cause pain similar to osteoarthritis. MRI stands for Magnetic Resonance Imaging.
When the technology was first developed, it was known as Nuclear Magnetic Resonance. The name was changed, however, because it was felt that the word nuclear implied the use of radioactive material.
In fact, there is no radioactivity with an MRI scanner at all. MRI has been utilized in orthopaedic surgery for more than 20 years. It is done mostly to evaluate soft tissue problems and ligament injuries but can be helpful in diagnosing problems of bones and joints as well. In the hip, several problems are often diagnosed first by MRI.
1. Avascular necrosis In its early stages, avascular necrosis is often not seen on a plain x-ray. The earliest changes of AVN in the femoral head are subtle changes in the bone which are best seen on an MRI. If AVN is diagnosed early enough and the femoral head has not collapsed, a core decompression procedure may be done and the hip joint preserved.
2. Fractures A fracture of the hip may be visible as fluid or edema within the bone before it can be seen as a fracture line on plain x-ray. In some situations, spontaneous stress fractures of the neck of the femur can cause pain in the groin and thigh similar to osteoarthritis. Stress fractures may be seen in young, active patients who are runners or in older patients with thin or osteoporotic bone. The pain of a spontaneous fracture may be similar to the pain of an arthritic hip and an MRI may be very useful in making the diagnosis.
3. Transient osteoporosis Transient osteoporosis of the hip is an inflammation of the head and neck of the femur that can cause acute pain similar to that of osteoarthritis. Transient osteoporosis develops spontaneously and resolves in several months. MRI will show fluid or edema in a wide area of the bone. A plain x-ray, how-ever, will be normal. This condition almost always resolves spontaneously with no need for surgery.
4. Tumors Bone tumors may be detected on MRI before they are seen on plain x-ray. A bone tumor may show subtle changes in the marrow of the bone which are best visualized on MRI. A tumor may be diagnosed and treatment begun in an early stage.
5. Septic arthritis If there is a large amount of fluid or pus in the joint, it is best seen on an MRI. This may help make the diagnosis of infection in the joint or septic arthritis. MRI may thus be helpful in diagnosing conditions not well seen on a plain x-ray. Most often, the diagnosis of osteoarthritis is clear, but sometimes an MRI can be helpful.
Some patients cannot have an MRI because of medical implants or metallic fragments within their bodies. Patients who have a pacemaker or defibrillator, a cochlear implant in the ear, or surgical clips following surgery for a brain aneurysm cannot have an MRI.
MRI can also not be done if there are small metal fragments within your body near vital organs such as the eyes. Most orthopaedic implants such as joint replacements, plates, and screws are not affected by an MRI and do not prevent a patient from having the study.