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Figure 10. Radionuclide arthrography. (Left): Loose right cemented femoral hip prosthesis depicting increased focal uptake of radiophosphate at the stem tip and in the intertrochanteric area. The intense deposit superior to the prosthesis neck is due to heterotopic bone formation (arrow). (Right): Combined 99mTc-colloid and radiocontrast injection into the joint cavity for athrography. 99mTc scan shows radioactivity seeping into the space surrounding the femoral component indicative of loosening (arrows).

are related to the hip, and there is general acknowledgement that radionuclide arthrography is more sensitive for the femoral component but unreliable for the acetabular component (Fig. 10).

Infection

Three-phase bone imaging is very sensitive for the disclosure of infected arthroplasties but it is not specific. In a group of 98 patients presenting with painful arthroplasties and suspected of having an infection, a sensitivity of 100% and specificity of only 18% was confirmed by surgical documentation. A number of test agents can be used to increase the specificity. Gallium was the first to be introduced. If the distribution of gallium is incongruent with that of the radiophosphate, or if it is intense and congruent, the likelihood of infection is high. Low grade congruent uptakes are equivocal and since this is reported to occur in about half the cases, the use of gallium is limited.

The results of labelled leukocytes are better, but vary with the intensity of the inflammatory process. Normally the labelled leukocytes, unlike gallium, do not accumulate at sites of increased bone turnover in the absence of infection. Sensitivity was found to be poorest in chronic osteomyelitis, and this was attributed to the low leukocyte attraction to the site. Specificity was compromised in chronic osteomyelitis by the occasional low grade uptake from the aseptic inflammatory reaction associated with healing fractures or by uptake in active marrow adjacent to the hip and knee prostheses. With regard to the latter, it has been shown that in asymptomatic patients with hip prostheses, mIn-leukocyte activity was present in 48% of the femoral tips at 24 months, and 37% had significant uptakes in the region of the acetabular component. The addition of a 99mTc-colloid scan will aid in identifying these areas of disturbed marrow function adjacent to hip and knee implants so that their concentration of labelled leukocytes will not be misinterpreted as infection (Fig. 6). Results from using combined leukocyte/colloid marrow imaging are reported to vary in accuracy from 89% to 98%, with the improvement being largely due to increased specificity.

Radionuclide Synovectomy

Treatment of chronic synovitis by surgical synovectomy is not always successful as recurrences occur with regeneration of the synovium and there may be an associated prolonged rehabilitation due to joint stiffness and limitation of motion. In surgical management of hemophilic synovitis and hemarthrosis the problem of maintaining hemostasis can be daunting and expensive. Intraarticular injection of chemicals such as nitrogen mustard, thiotepa and osmic acid, although less invasive, were not consistently successful. Repeated intraarticular injections of corticosteroids pose a risk for systemic toxicity.

Although modern arthroscopic synovectomy has made radionuclide synovectomy less popular, it remains a valuable therapeutic option. When a beta emitting particulate radiopharmaceutical is injected into the inflamed joint, the synovium is exposed to a high radiation dose and atrophies. Desirable attributes for the agent include little or no leakage from the joint cavity, high affinity binding of the beta emitter, uniform distribution throughout the joint cavity, synovial uptake without initiating an inflammatory reaction and a biological half-life within the joint that should not be less than the physical half-life of the radionuclide. Leakage from the joint cavity via the lymphatics can lead to deposition in the regional inguinal nodes. Leakage has also been attributed to synovial inflammation and joint movement; therefore premedication with intraarticular glucocorticoids (to reduce the synovial hyperemia) and bed rest have been advocated to decrease the frequency and amount of radiocolloid deposition in the regional nodes. Many suitable radiopharmaceuticals have been developed (Table 2). The most frequently used are 32P-chromic phosphate and 90Y-citrate which have pure beta emission with energy for good tisue penetration, large particle sizes and are commercially available.

Ideally, the penetration of beta particles should be limited to the thickness of the synovium to avoid radionecrosis of the cartilage and bone, but this may only be a theoretical concern as cartilage is relatively resistant to radiation and no cases of necrosis have been reported. Chromosomal abnormalities have been seen, but in all the years of experience with intraarticular radiation synovectomy no instance of induced malignancy have been reported.

Treatment Procedure

For the knee, an injection of a local anesthetic is obtained under aseptic conditions, and through a lateral approach an 18 to 20 gauge needle is inserted into the joint cavity. Smaller joints such as the wrist and elbow, may require x-ray fluoroscopic control, perhaps with contrast injection to ensure absence of leakage and loculation. At the knee, the ease in aspiration usually ensures proper needle placement. The

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