A bone scan is an imaging test that can show damage to the bones and joints caused by conditions like arthritis. It may be used to help diagnose arthritis or evaluate its progression. However, a bone scan alone cannot definitively diagnose most types of arthritis. It provides additional information that complements other tests and the clinical exam.
Quick facts on bone scans and arthritis
- A bone scan takes images of the bones using a radioactive tracer injected into the body. Areas of increased uptake of the tracer indicate bone remodeling and inflammation.
- In arthritis, bone scans show increased joint activity that may indicate cartilage damage and inflammation.
- Bone scans are more sensitive for detecting abnormalities than plain x-rays but less specific. They may detect joint changes very early in arthritis.
- Bone scans are often used for evaluating difficult to diagnose joint pain,arthritis progression, and determining the best locations for joint injections.
- Bone scans do not show soft tissue well and do not give fine anatomical detail. MRI and ultrasound provide better imaging of joint structures.
How a bone scan works
A bone scan uses a very small amount of radioactive material called a tracer. The tracer is injected into a vein and travels through the bloodstream. Areas of bone that are undergoing rapid turnover and repair absorb the tracer at a higher rate than normal bone.
About 2-3 hours after the injection, the patient lies still on a table while a special camera detects radiation emitted by the tracer in the body. It creates images of the skeleton highlighting areas of increased tracer uptake. These “hot spots” suggest increased blood flow and osteoblastic activity, which may indicate bone injury and healing.
What arthritis looks like on a bone scan
In arthritis, damaged cartilage and inflamed synovium (joint lining) trigger increased bone remodeling. On a bone scan, this appears as increased uptake of the tracer at joint margins and surfaces.
The pattern of uptake may suggest the type of arthritis:
- Rheumatoid arthritis – Symmetric involvement of small hand and foot joints
- Osteoarthritis – Asymmetric involvement of large weight-bearing joints like hips, knees and lower spine
- Psoriatic arthritis – Asymmetric involvement, especially in the fingertips (dactylitis)
- Gouty arthritis – Asymmetric involvement of joints, often in the feet
Increased bone scan activity correlates with clinical assessments of inflammation and damage. However, a normal bone scan does not rule out arthritis since it may only detect moderate to severe joint abnormalities.
Advantages of bone scans for arthritis evaluation
Bone scans have the following benefits for arthritis diagnosis and management:
- High sensitivity – Bone scans detect inflammation and joint damage earlier than x-rays. They may visualize abnormalities months to years before changes become apparent on plain films.
- Whole body imaging – Bone scans screen the entire skeleton, allowing doctors to assess multiple joints at the same time. This aids in determining the pattern and extent of arthritis.
- Early arthritis detection – Increased bone scan activity may reveal arthritis before erosions are visible on x-rays, allowing earlier treatment.
- Pain localization – Identifying the precise anatomical source of joint pain can guide injections and other treatments.
- Treatment response – Reduced bone scan activity over time indicates a positive response to therapy.
Limitations of bone scans in arthritis
Bone scans do have some disadvantages:
- Not arthritis-specific – Increased bone activity is non-specific. Trauma, fracture, infection, orthopedic hardware and some tumors can also cause hot spots.
- Poor anatomical detail – The tracer highlights areas of bone turnover but does not give fine anatomical information.
- irradiation – Radioisotopes used expose patients to a small amount of ionizing radiation.
- No soft tissue visualization – Bone scans do not show joint structures like cartilage, ligaments and tendons well.
Interpreting bone scan findings in arthritis
The presence and pattern of joint abnormalities on a bone scan provide clues about arthritis, but clinical correlation with signs, symptoms and other tests is needed. Some key points of interpretation include:
- Symmetrical involvement of multiple joints favors inflammatory types like rheumatoid arthritis.
- Asymmetrical involvement limited to a few joints raises suspicion for osteoarthritis or post-traumatic arthritis.
- Areas of intense, focal uptake suggest erosive damage.
- Mild to moderate diffuse uptake usually indicates earlier or less severe arthritis.
- Changes are often more extensive than expected from x-rays alone, indicating clinically silent disease progression.
However, bone scans should not be the sole basis for diagnosing arthritis. Correlation with a clinical exam, laboratory tests like rheumatoid factor or anti-CCP antibodies, and imaging such as plain films and MRI is essential.
How bone scans compare to other arthritis imaging tests
Other imaging modalities used in arthritis evaluation include:
- Plain x-rays – Inexpensive and widely available, good for visualizing bone. Less sensitive than bone scanning for detecting early arthritis changes.
- MRI – Visualizes bone marrow, soft tissues, and subtle erosions. More specific than bone scans. MRI has become the advanced imaging modality of choice for assessing arthritis.
- Ultrasound – Relatively inexpensive way to view soft tissue joint structures and inflammation. User-dependent. Not useful for bones.
- CT – Excellent visualization of bone. Not used routinely in arthritis but may supplement plain films.
Comparatively, bone scanning offers greater sensitivity for detecting arthritis earlier but less anatomical specificity than tests like MRI and ultrasound. It continues to play a supplementary role in arthritis diagnosis and management.
Conclusion
Bone scanning is a nuclear medicine imaging test that highlights areas of increased bone turnover caused by arthritis inflammation and damage. It serves as an adjunct to other diagnostic tests and the clinical exam rather than definitive proof of arthritis by itself.
Bone scans are more sensitive but less specific than x-rays. They may detect joint abnormalities earlier in the disease process before erosions are visible on plain films. However, they do not provide the fine anatomical detail given by soft tissue imaging like MRI.
Overall, bone scanning provides a unique global picture of skeletal involvement in arthritis. It offers complementary information to help rheumatologists diagnose, characterize and monitor arthritis in a more timely way.