Imaging for Osteoarthritis: An Overview

Osteoarthritis: A Common Joint Disorder

Osteoarthritis (OA) is one of the most common disorders of joints. The joints most frequently affected are the hip, knee, shoulder, the big toe and the base of the thumb. Degeneration of articular cartilage, leading to osteoarthritis, can occur naturally over time with a age or as a secondary condition to an associated injury, such as a trauma or repetitive-stress or occupational injury. Articular cartilage degeneration is a gradual process.

The findings from radiographic imaging depend on the stage of a person's OA. The doctor who orders the imaging will use the description and gradation of osteoarthritis ("localized or diffuse" and "mild, moderate or severe," etc.) to determine treatment options. Treatments include conservative, nonsurgical therapy or surgery.

• Nonsurgical treatments can include physical therapy, modifications to physical activity, oral medications and/or intra-articular steroid injections
• Surgical options include osteotomies or joint replacement.

What are the different types of imaging examinations for osteoarthritis?

Conventional radiographs – Routine X-ray examinations

Regardless of the joint that is affected, osteoarthritis is revealed on conventional radiographs (X-rays) by characteristics that are distinct from other joint disorders, such as rheumatoid arthritis. Specifically, an X-ray of a joint with osteoarthritis will show a narrowing of the space between the bones of the joint where the cartilage has worn away, as shown in the image below.

Anteroposterior (front to back) X-ray image of the knee showing osteoarthritis. Note the narrower spacing on the right side of the image, where cartilage has degenerated.

When cartilage is lost, bone rubs against bone. This can cause to cysts or fluid-filled cavities can form in the bone, which will also be visible in an X-ray. The body also responds with sclerosis (increased bone density), in which more bone grows in where the cartilage used to be. The joint surfaces become misaligned and osteophytes (bone spurs) may form. There are basic routine X-ray views for imaging each joint:

• An anteroposterior (AP) view (front-to-back view)
• A lateral (outer side) view
• One or two oblique (45-degree) views

In order to detect early cartilage wear, HSS uses special X-ray views in place of or in addition to these standard views. These specialized views are designed to increase the sensitivity of the conventional radiographic study.

What kinds of specialized imaging can help diagnose osteoarthritis?

Symptoms of osteoarthritis may arise before the damage can be seen in standard X-rays. For this reason, radiologists at Hospital for Special Surgery often use the more sensitive MRI, CT and ultrasound forms of imaging, which are superior for detecting early osteoarthritis.

• MRI (magnetic resonance imaging) is very sensitive imaging that can reveal subtle changes in bony and soft tissues. An MRI can show a reactive bone edema (fluid build-up in the bone marrow, which causes swelling), inflammation of soft tissues, as well as degenerated cartilage or bone fragments lodged in the joint. HSS uses a protocol of specific MRI pulse sequences to identify early evidence of cartilage degeneration. When evidence of cartilage wear is detected early, treatment can begin to prevent or delay progression. This in turn can postpone or eliminate the need for surgery.
  
  MRI of the hips showing osteoarthritis and edema of the femoral head and acetabulum.
• CT (computed tomography) examinations, also called CT scans, are excellent for showing osteophytes (bone spurs) and the ways they affect adjacent soft tissues. CT examinations are also useful in providing guidance for therapeutic and diagnostic procedures.
  
  CT scan of an arthritic showing hip bony femoral head debris
• Ultrasound is extremely sensitive for identifying synovial cysts that sometimes form in people with osteoarthritis. Ultrasound is excellent for evaluating the ligaments and tendons around the joint, which can be stretched or torn because of osteoarthritis.
  

In addition to helping diagnose osteoarthritis, radiographic technologies can also be used to guide your doctor while applying treatments. For example, anesthetics and/or steroids injected into an arthritic joint can help reduce the pain of osteoarthritis. It is helpful to perform such injection procedures using image guidance, because the doctor can see that the needle tip appropriately inserted into the joint space. This kind of direct visualization can be performed using CT, ultrasound or fluoroscopy (a continuous, real-time X-ray that works like an X-ray movie). A joint injection performed under fluoroscopy or CT guidance, is called an arthrogram. The radiologist will inject a small amount of contrast agent into the joint to better visualize it and confirm accurate needle placement. With ultrasound, the radiologist directly visualizes the needle inside the joint, as well as neighboring muscles, arteries and veins.

The faculty members of the HSS Department of Radiology and Imaging at HSS are board certified radiologists with fellowship specialty training in musculoskeletal imaging and cross-section imaging (MRI, CT and ultrasound). Although other types subspecialty physicians perform some of these procedures, radiologists are doctors who are trained in the use of all forms of imaging. They also have specific training in the physics behind the imaging, the safe use of ionizing radiation (fluoroscopy/CT), as well as in MRI and ultrasound safety.

What is the imaging process for hip and knee osteoarthritis?

Hip joint imaging

Standard X-rays of the hip

• The standard, conventional radiographic views obtained when a patient has hip pain from osteoarthritis are the anteroposterior (front-to-back) and lateral (outer side) view. At Hospital for Special Surgery, a view of the pelvis is usually ordered as part of the initial examination to visualize other potential sources of hip pain, such as the low back or sacroiliac joints.
• Positioning for these views is very specific, as reproducibility is invaluable to follow progression of the condition, response to treatment and/or for preoperative planning.

Additional, optional X-rays of the hip

Optional radiographs are obtained fairly routinely by HSS joint replacement surgeons. They include the Lowenstein lateral, cross-table lateral, false-profile and elongated femoral neck views. All of these help to identify the precise site where early joint space narrowing/or bony changes occur and are either responsible for, or secondary to, osteoarthritis.

Findings of hip osteoarthritis from X-rays

• Typically, joint space narrowing is focal in the superolateral direction, apical (up and into the pelvis) or medial (into the center of the midline).
• As the cartilage loss becomes severe, bone cysts and sclerotic remodeling (increased density of bone to fill the space formerly occupied by cartilage) is evident on the radiographs.
• Osteophytes (bone spurs) form opposite to the direction of cartilage loss and joint space narrowing. The osteophytes form in an attempt to stabilize the joints and compensate for the cartilage loss and narrowed joint space.

Knee joint imaging

Standard X-rays of the knee

The standard X-ray examinations obtained to evaluate the knees in patients with osteoarthritis are:

• Anteroposterior (A) view (front-to-back view). At HSS, this view is usually obtained with the patient standing and bearing weight on the joint.
• Lateral (outer side) view.
• Merchant view of both patellofemoral joints (top-angled view, looking down at the knee caps).

Additional, optional X-rays of the knee

Optional radiographs are commonly ordered at Hospital for Special Surgery. Most often, these will be a posteroanterior (back-to-front) X-ray, taken while the knee is weightbearing (standing position) and in slight flexion (knees slightly bent). This position helps the radiologist evaluate whether there is cartilage loss in the posterior (rear) portion of the of the medial (inside) or lateral (outer side) knee joint compartments. Positioning for these view is very specific.

Findings of knee osteoarthritis from X-rays

Typically, the cartilage in one of the joint compartments (that is, the medial, lateral, or anterior patellofemoral joint compartment) is most severely affected. The standing X-rays may show narrowing of the involved joint space of the knee. Altered alignment of the knee joint is very common as either the cause or as a result of osteoarthritis. The patellofemoral joint (at the front of the knee) can be the focus of osteoarthritis of the knee or part of a more generalized condition. Lateral subluxation or patellar malalignment is very common.

As the disease progresses, osteophytes (bone spurs) form along the joint margins and bone cysts and sclerosis are typically present. When osteochondral or cartilage fragments break off they can remain in the joint or can get larger and are referred to as intra-articular loose bodies. Fluid can form in the joint secondary to the arthritis and is referred to as an effusion.

Updated: 5/24/2013

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Reviewed by Helen Pavlov, MD, FACR