angulation in weight bearing bones is another cause of prolonged enhanced radiophosphate uptake. Less than 50% of the latter group will return to complete quiescence within 3 years.

The bone scan can be helpful in dating vertebral fractures seen on the radiograph. Old compression fractures are inactive by radiophosphate imaging, except when complicated by degenerative disease, infection and tumor. Enhanced uptake in a collapsed centrum suggests an injury of at most 2 years duration, but if the uptake is intense the fracture probably occurred within a year.

Radiophosphate planar and SPECT imaging of the knee for the assessment of meniscal and cruciate tears has received considerable attention, but where MRI facilities exist there is little need for it today. Planar imaging can present a confusing picture in the acute phase of internal knee injuries, because the major site of injury is often hidden by diffuse uptake due to synovitis and generalized hyperemic reaction to the hemarthrosis, trauma and stress reactions at musculo-ligamentous insertions. SPECT imaging is able to unravel and define the various contributions to total uptake, and thereby enhance the diagnostic yield. For injury to the meniscus, SPECT sensitivity and specificity are reported to be 88% and 87%, respectively (Fig. 4). Focal accumulation of radiophosphate at the medial femoral condyle peripherally signals an avulsion of the medial collateral ligament, and pure anterior and posterior cruciate ligament tears depict foci on the anterior and posterior tibial joint surface, respectively. Cruciate tears may also be accompanied by focal intercondylar uptake.

Stress fractures

Repetitive episodes of inordinate minor stress in individuals engaged in unaccustomed exercise induce an accelerated remodelling phase in the bones affected by this strenuous activity. Bone remodelling consists of osteoclastic resorption which reaches a peak bone loss at about 3 weeks. This is followed by a slower osteoblastic filling of the osteoclastic cavities, reaching completion in about 90 days. As a consequence there is an interval of imbalance when the bone is weakened and

Figure 4. Left knee meniscal tear. (A): Anterior planar images of the knees exhibit a marked increase of radiophosphate uptake in the periarticular bone of the left knee. Arrow points to a subtle higher uptake in the medial tibial plateau. (B): Coronal SPECT clearly identifies this focal lesion (arrow). (C): There is increased uptake along the entire surface of the medial tibial plateau on the transverse SPECT image, consistent with a meniscal tear.

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