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Figure 6. Left total knee arthroplasty. (Left): Radiophosphate image exhibits intense periarticular uptake. (Middle): There are three areas of 99mTc-white blood cell deposition (arrows). (Right): The 99mTc-colloid scan demonstrates uptake in the same areas as the 99mTc-white blood cells, indicating active marrow and weighing against infection.

Figure 6. Left total knee arthroplasty. (Left): Radiophosphate image exhibits intense periarticular uptake. (Middle): There are three areas of 99mTc-white blood cell deposition (arrows). (Right): The 99mTc-colloid scan demonstrates uptake in the same areas as the 99mTc-white blood cells, indicating active marrow and weighing against infection.

but it may mask a co-existent osteomyelitis. Poor spatial resolution of mIn contributes to the false negative and false positive scan interpretations.

In comparisons between combined three-phase bone and mIn-leukocyte scanning versus MRI, the latter was more sensitive in disclosing osteomyelitis in the forefoot, but was less sensitive in the presence of a neuropathic (Charcot) joint. In a study of 24 diabetic patients with foot ulcers, 13 had proven osteomyelitis. Sensitivity for clinical evaluation was 54%, 38% for plain radiography, 77% for combined radiophosphate and 99mTc-leukocytes and 100% for MRI. Combined radionuclide and MRI studies registered an 82% specificity.

Vascular Disorders

There are multiple causes of avascular necrosis (AVN), both traumatic and nontraumatic. Fractures and dislocations can lead to AVN, the most common site being the femoral epiphysis following femoral neck fractures. Nontraumatic causes of epiphyseal AVN and intramedullary infarcts are thromboemboli to bone in disorders such as hemoglobinopathies, sickle cell disease (Fig. 7), sickle cell trait and sickle cell thalassemia. Decompression states (Caisson disease) that result from deep sea diving and high-altitude flying or ballooning can lead to AVN due to nitrogen emboli. Pancreatitis and alcoholism can be associated with fat necrosis and fat emboli that may lead to AVN. Systemic lupus erythematosis, polyarteritis nodosa and giant cell arteritis are connective tissue disorders involving small vessels which are consequently susceptible to occlusion and AVN of dependent tissue. Gaucher's disease, Cushing's disease and steroid therapy are associated with swollen fatty marrow cells which may compromise vascularity and cause AVN.

Spontaneous AVN commonly occurs in the medial condyle of the femur, but it may develop in the lateral condyle, tibial plateau and elsewhere. Its etiology is unknown and usually middle aged men and women are affected, in contrast to osteochondritis dissecans which is typically found in adolescent years. Other bones that can be afflicted with AVN are the humeral epiphysis, metacarpal and metatarsal

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