Insufficiency fractures of the fibula are typically found in patients with underlying rheumatic diseases, mainly rheumatoid arthritis (Alonso-Bartolome et al. 1999; Yamamoto et al. 2002) (Figs. 8, 9). Insufficiency fractures are also frequently found at the metatarsal bones in particular in the setting of inflammatory arthropathies (Fig. 10). Also these may be diagnosed in patients with severe osteoporosis, high-dose corticosteroids, or methotrexate therapy. Patients with marked joint deformity are also at high risk for developing insufficiency fractures (Maenpaa et al. 2002). Less frequently, in up to 25 %, insufficiency fractures at the femoral shaft can occur, predominately in elderly patients with osteoporosis (Martin-Hunyadi et al. 2000) (Fig. 11). Insufficiency fractures at the tibia (Fig. 12a–c) may be found as an early manifestation of bone failure in patients after renal transplantation as reported by a previous study (Franco et al. 2003). The main causes are preexisting renal osteodystrophy, glucocorticoid therapy, and hyperparathyroidism, whether residual or secondary to imperfect graft function (Franco et al. 2003). Longitudinal stress fractures of the tibia can also occur in patients with healed chronic osteomyelitis (Feydy et al. 2000). Even postpartum osteoporosis was found to be a cause of insufficiency fractures around the knee (Clemetson et al. 2004).

One of the important differential diagnoses of subchondral insufficiency fractures of the femoral head may include osteonecrosis. Typical MRI findings in insufficiency fractures include a pattern of bone marrow edema with a low-signal-intensity line (resp. fracture) on the T1-weighted images parallel to the subchondral bone. In general, the circumscribed lesions on MRI, which are commonly observed in osteonecrosis, are not found (Yamamoto et al. 2007). Histopathologically fracture callus, reactive cartilage, and granulation tissue is seen without any evidence of antecedent osteonecrosis. The subchondral insufficiency fracture of the femoral head (SIF) is a recently recognized cause of acute onset arthritis mostly in elderly women, which previously had been commonly considered either as osteonecrosis or osteoarthritis.

Insufficiency fractures at the spine are a leading cause for acute low back pain without an acute traumatic event. Usually a concave or wedge-shaped deformity of the affected vertebra is found and a wide range of the vertebral height ratios and fracture distribution were reported (Kawaguchi et al. 2001). Once an initial vertebral fracture is sustained, the risk of subsequent vertebral fracture increases significantly. However, this effect cannot be explained by low bone mass alone, suggesting that factors independent of this parameter contribute to this occurrence (Briggs et al. 2007). The assessment of vertebral fractures using a semiquantitative approach has been described, grading osteoporotic fractures into type 1 (20–25 % deformity), type 2 (25–40 %), and type 3 (>40 %) (Genant et al. 1993). Accurate radiographic diagnosis of osteoporotic vertebral fractures is important. Several studies indicated a false-negative rate of up to 34 % in reports of lateral radiographs of the thoracolumbar spine (Delmas et al. 2005). Radiologists should be aware of the importance of vertebral fracture diagnosis in assessing future osteoporotic fracture risk. Vertebral fractures incidental to radiologic examinations done for other reasons should be identified and reported, in particular vertebral fractures should be assessed in lateral chest radiographs. Proper training of radiologists is necessary to improve detection of vertebral fractures (Lentle et al. 2007). In oncologic patients differentiation from benign and malignant vertebral fractures is important and can be achieved by MRI or PET-CT (Figs. 13, 14).

A common site of fragility fractures is the distal forearm. In addition, fractures involving the wrist are known to be strongly associated with osteoporosis. It is well-known that patients with distal radius fracture who are otherwise healthy have a preferential bone loss at the distal forearm. Distal radius fractures are also associated with generally low bone mass and elevated fracture risk at other skeletal sites. In these subjects pharmacotherapy for osteoporosis is warranted (Mallmin and Ljunghall 1994). Interestingly in osteoporosis the proximal humerus may also be a fracture site at risk (Guggenbuhl et al. 2005).

An important differential diagnosis of a stress/insufficiency fracture is a fracture due to malignant disease. MRI features of a malignant fracture are diffusely or focally abnormal bone marrow signal which may be either well-defined or ill-defined and does not follow fracture lines. In addition abnormal intracortical, periosteal, or muscle signal intensity are found as well as endosteal scalloping and soft-tissue masses. The features seen on CT are bone marrow abnormality which may be well-defined, ill-defined, permeative or moth-eaten, endosteal scalloping, periosteal reaction, and a soft-tissue mass. Accuracy for differentiating malignant fractures from stress fractures was reported to be highest with MRI (93–98 %) followed by CT (82–88 %) and radiographs (88–94 %) (Fayad et al. 2005).

Radiographs are the initial imaging test in patients with pain localized to the skeleton. If the radiographs are inconclusive and pain persists, either MRI or CT will be performed. Multidetector computed tomography (MDCT) is currently standard and allows multiplanar reconstruction, near isotropic 3D reconstructions of anatomical structures, reduction of artefacts as well as thin-section high-resolution imaging which is beneficial to visualizing also subtle fracture lines. It should be considered, however, that MRI is more sensitive and the imaging modality of choice if the patient history suggests malignant disease and metastasis may be responsible for fracture. Though MRI is very sensitive for detection of fractures, bone marrow changes, and related soft-tissue edema, it should be considered that in the absence of fractures lines or a typical history MRI may also be misleading and suggest other bone marrow pathology such as malignant infiltration. While MRI is the most sensitive technique in the visualization of insufficiency fractures, CT sometimes tends to depict the extent and stability of these fractures better. Bone scintigraphy is highly sensitive, but not specific. Atypical uptake patterns may be difficult to interpret and abnormal uptake may persist for several months.