Radiographs show a sclerotic region, which is linear and extends from the cortex. Despite this, sacral stress fracture is often referred for biopsy or more imaging, based on concern over the patient’s age and the presence of a sclerotic lesion. Stress fracture has a characteristic appearance on most modalities and should not be confused with tumor or another pathology. The latter may be prophylactically treated with pin fixation. If the fracture occurs laterally, it is more unstable (this is the tensile aspect weight-bearing tends to pull the fracture apart). Since bony apposition is essential for healing, stress fractures in this location have a good prognosis for healing. They tend to occur most frequently by far at the base of the medial femoral neck (the compressive aspect, where each step pushes the fracture sides together). Stress fractures at the femoral neck tend to be solitary and unilateral. In older patients, the sacral alae and supra-acetabular region are more common. In young patients, common sites for stress fracture around the pelvis include the superior and inferior pubic ramus and the femoral neck. If MRI is ordered for a stress fracture, which is suspected either based on the clinical history or hip pain in a susceptible population, it is recommended to use a large field of view because these injuries often are multiple and bilateral in expected locations around the pelvic ring. MRI with in-phase and out-of-phase imaging can be used to document residual marrow fat if the diagnosis remains unclear on routine sequences. However, on fluid-sensitive images the low-signal fracture line is seen, set apart by surrounding edema. On T1-weighted images, the fracture and surrounding edema can resemble a malignancy. These patients may be treated differently.įigure 11-13 Acetabular insufficiency fracture. Osteoarthritis is the last stage of AVN, but distinction should be made between patients with AVN, collapse and subsequent osteoarthritis and those with incidental AVN, no collapse, and osteoarthritis from some other cause. TOPH is in the differential for diffuse femoral head edema, but underlying established AVN (or AVN on the other side) helps exclude this possibility. Like AVN, the collapse is usually anterosuperior. If established AVN is seen in addition to this finding, certainly acute-on-chronic AVN is a possibility, but subtle articular surface collapse should be sought. The advantage of MRI is visualization of the diffuse marrow edema that results from collapse, edema that extends to the intertrochanteric region simulating early AVN.

MRI can also be used, although CT has higher resolution. If no radiographic signs of collapse are seen, CT with sagittal and coronal reformats is very useful, especially when using multidetector CT with thin cuts. Flattening of the normally spherical femoral head indicates collapse. On radiographs, a subchondral lucent line may be seen, representing a fracture. When a patient with established AVN presents with acute pain, the suspicion and imaging should be directed toward finding articular collapse, which is often very subtle. Patients may go on for many years at the prior stage. At this point, patients may be asymptomatic. Typically, the surrounding marrow edema has resolved. The margins demonstrate the classic “double line sign” of Mitchell, representing the interface between living and dead bone with a rim of granulation tissue. The central signal is variable, often high on T1 (trapped/mummified fat) but occasionally bright on T2 or dark on all sequences (fibrotic). In stage 2 in which increased radiographic density is seen, MRI demonstrates a focal subchondral geographic signal abnormality, usually centered anterosuperiorly. In later stages, it is easier to document the finding as AVN. Table 11-1 Modified Ficat Staging System for Avascular Necrosis of the Hip

Normal hip xray teen