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Testing for knee and hip problems

Sometimes a physical exam and your symptoms provide sufficient information for your doctor to make a diagnosis. But when that's not enough, a variety of imaging techniques and laboratory tests can clarify the situation.

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Imaging techniques

Medical technology has expanded doctors' ability to create images of joint damage far beyond the simple x-ray. Each imaging technique offers something different, and the choice will depend on the type of joint damage your doctor suspects.

X-ray. A standard x-ray, the most commonly used imaging technique, gives a two-dimensional picture of the bones in your joints (see Figure 11). X-rays can show bone fractures, spurs, loose fragments, dislocation, reduced space between bones, and bone damage from arthritis. But standard x-rays don't show cartilage, ligaments, or tendons; it requires more sophisticated imaging techniques to see these. For a standard x-ray of the knee, you may be asked to lie down or stand while a technician takes several views with your knee in different positions. For a hip x-ray, you will probably be asked to lie down. Even if only one knee or hip causes you problems, your doctor may order x-rays of both to compare the appearance of the joint and joint space.

CT scan. Doctors sometimes order a CT (computed tomography) scan to look for hidden fractures, bone on bone, and other structural abnormalities. A CT scan uses a rotating x-ray tube housed in a doughnut-shaped machine to take many cross-section x-rays of your anatomy. A computer assembles these "slices" into a three-dimensional picture. During the scan, which takes less than an hour, you lie on your back on a movable table that is raised, lowered and moved in and out of the scanner. The equipment doesn't touch you, and the test isn't uncomfortable. CT is expensive, but it provides an enhanced view of bone, allowing your doctor to better evaluate bone shape and diagnose some defects hidden on standard x-rays. CT does not show soft tissue.

MRI. This test uses a strong magnet and radio waves to evaluate cartilage, soft tissues, and bone marrow. MRI is more expensive than CT or standard x-rays, but it's the most accurate way to detect ligament damage, cartilage damage from arthritis, or tears in the cartilage or meniscus. It is also used to evaluate a possible pelvic fracture, tumor, or osteoporosis of the hip. During the exam, which lasts about 45 minutes, you lie on a scanning table that slowly moves you through the machine. (Many centers have smaller MRI machines that can focus on specific body parts, so if you're having your knee done you may need only insert your knee.) For a hip exam, your feet may be taped together to keep your hips in the desired position. For a knee exam, a wedge may keep your knees at the desired angle. In some cases, the radiologist may inject a dye to obtain better images. You cannot have an MRI if you have a pacemaker, aneurysm clips, or certain other metal implants.

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Laboratory tests

While imaging tests can give your doctor a good view of the damage in your knees or hips, laboratory tests are sometimes needed to determine what is causing the damage and how it might be halted.

Arthrocentesis. If you have sudden or unexplained swelling in a knee, hip, or other joint, your doctor may perform an arthrocentesis, removing a little synovial fluid for examination. Excess synovial fluid may indicate infection, crystal deposits, trauma, or inflammation. Before arthrocentesis, the skin is cleaned and an anesthetic spray or injection is used to numb the area. The doctor inserts a needle with a syringe attached into the joint space (you may hear a pop) and withdraws a fluid sample, which is sent to a laboratory for analysis. If you have pain afterward, your doctor may suggest ice and pain relief medication. The procedure may immediately lessen pain and pressure caused by excess fluid. Knee arthrocentesis can be done in the doctor's office. Hip arthrocentesis is performed by a radiologist, guided by fluoroscopy, a type of x-ray that shows internal structures in motion.

Rheumatoid factor. This blood test detects an antibody present in about 85% of people with rheumatoid arthritis, a systemic autoimmune disease. The same antibody is also present in other medical conditions and in about 3% of healthy people.

Erythrocyte sedimentation and C-reactive protein. These blood tests are general measurements of inflammation of any kind; the higher the result, the more severe the inflammation. Most people with osteoarthritis have normal values, but those who have inflammatory conditions, such as rheumatoid arthritis, usually have elevated levels. High rates may also be an early sign of infection after knee or hip surgery. High levels of C-reactive protein over the long term also indicate an increased risk for heart disease. The serum uric acid test measures uric acid in the blood, which can help diagnose gout, a condition caused by the accumulation of uric acid crystals in a joint.

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Review Date: 2006-09-01

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