Findings:

1. Bone scan demonstrates increased activity in the region of the left sacroiliac joint on the delayed image and on the SPECT images.
2. CT scan demonstrates a vertically oriented linear area of sclerosis through the left sacral ala.

Diagnosis: Stress fracture.

Discussion: Two types of stress fractures are recognized: insufficiency and fatigue fractures. Insufficiency fracture esults from normal physiologic stress applied to abnormal bone. Fatigue fracture results from abnormal stress applied to normal bone.

Causes of insufficiency fracture include osteoporosis, rheumatoid arthritis, osteomalacia, rickets, Paget’s disease, hyperparathyroidism, and radiation therapy. Fatigue fractures are usually caused by strenuous repetitive activity. Examples of fatigue fracture includes fractures of metatarsal bones in military recruits ("march" fracture) and lower extremities in athletes and joggers.

Clinically, stress fractures are characterized by activity related pain that is relieved by rest. Soft tissue swelling along with focal tenderness can be seen. The lower extremities are usually more frequently involved than the upper extremities.

Radiographically, stress fractures may be difficult to identify especially in the early stages. A linear cortical radiolucent area with cortical thickening may be seen. On MRI, signal intensity pattern is consistent with edema in the soft tissues and bone marrow (decreased on T1 and increased on T2).

Scintigraphy can be very helpful in evaluation of stress fractures which may be difficult to identify on radiography. Typically, stress fractures will be seen on scintigraphy much sooner than on radiography. Abnormalities can be seen within 24 hours of the injury although this can be delayed in older patients, particularly those with osteoporosis. A focal fusiform area of increased activity will be seen in all three phases of a bone scan. It may take up to 5 top 7 months for the delayed images to return to normal.

References:

1. Resnick D and Niwayama G. Diagnosis of Bone and Joint Disorders.
   2nd edition. W. B. Saunders Company, 1988:2773-2791.

2. Mettler F and Guiberteau M. Essentials of Nuclear Medicine Imaging.
   3rd edition. W.B. Saunders Company, 1991:209-236.