Pelvic Insufficiency Fracture Imaging

Pelvic Insufficiency Fracture Imaging

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Insufficiency fracture is a subgroup of stress fracture. Unlike the other subtype (ie, fatigue fracture), insufficiency fracture is caused by normal or physiologic stress upon weakened bone. Loss of bone trabeculae decreases the bone’s elastic resistance. [1, 2, 3, 4, 5, 6, 7] (See the images below.)

Awareness is increasing concerning the occurrence of these fractures among older persons. Sites frequently affected by insufficiency fractures are the thoracic vertebra, the tibia, the fibula, and the calcaneus. [8, 9, 10, 1, 11]

Clinical assessment does not provide a definitive diagnosis of insufficiency fracture. Imaging has an important role in the detection and diagnosis of insufficiency fractures of the pelvis. [12, 13]  Bone scintigraphy and magnetic resonance imaging (MRI) are the imaging modalities of choice. [14, 15, 16, 17, 18, 19, 20, 21] MRI is sensitive for the detection of fractures, soft tissue edema, and marrow changes and may be better at demonstrating alternative diagnoses than bone scintigraphy or computed tomography (CT) scanning is. [22, 23, 24]

Cabarrus et al compared the sensitivities of CT scanning and MRI in detecting pelvic insufficiency fractures and found MRI to be substantially better in detecting such fractures. In patients undergoing both imaging modalities, 128 of 129 (99%) of fractures were detected by MRI, whereas only 89 of 129 fractures (69%) were detected by CT. [20]

CT provides further definition of the fracture, especially if MRI is unavailable or bone scintigraphy is inconclusive.

Bone scintigraphy is highly sensitive, but it relies on accurate interpretation of the uptake pattern, and atypical uptake patterns may be difficult to interpret. Abnormal uptake may persist for several months.

Failure to recognize insufficiency fractures of the pelvis may result in an incorrect diagnosis of malignancy. This may occur when areas of increased uptake are seen on bone scan or when marrow signal changes of the pelvis are observed on MRI, especially in patients who have undergone pelvic irradiation for gynecologic malignancies. [2] Care should be taken to recognize the bone scan uptake pattern, particularly atypical patterns; on MRI, altered bone marrow signal in the sacrum is cause for suspicion.

CT scanning may not accurately detect fractures oriented transversely.

Radiographic findings depend on the site of the fracture. Parasymphyseal and pubic ramus fractures may have an aggressive appearance, depending on the stage of fracture maturity.

Findings include sclerosis, lytic fracture line, bone expansion, exuberant callus, and osteolysis, although a lytic fracture line or cortical break rarely is observed. The most common finding is a sclerotic band or line. (See the image below.)

The degree of confidence is low. Sacral fractures are difficult to detect because of osteoporosis, overlying bowel gas, and calcified vessels. Parasymphyseal and pubic ramus fractures often are mistaken for malignant lesions. Sacral, iliac, and supra-acetabular fractures often are difficult to detect.

On CT scans, sacral fractures typically are oriented vertically and are located parallel to the sacroiliac joints. (See the image below.)

A linear fracture line with surrounding sclerosis is observed, and pubic fractures are seen as a lytic fracture line often surrounded by callus. (See the images below.)

Typically, a soft tissue mass is absent, bone destruction is lacking, and adjacent fascial planes are preserved. CT also is useful for detecting large bony sacral defects, such as Tarlov cysts (see the image below) and for the diagnosis of coexisting malignant lesions.

CT findings may be definitive for the diagnosis of insufficiency fractures of the pelvis. CT is specific and is useful as an alternative to MRI or bone scintigraphy when radiographs are inconclusive.

MRI shows decreased signal on T1-weighted images and increased signal on T2-weighted images. In the sacrum, signal changes are seen as linear bands within the sacral ala and body; such bands are parallel to the sacroiliac joints. (See the image below.)

On T2-weighted images, the fracture line may be seen if it is surrounded by adjacent marrow edema. (See the images below.)

MRI is highly sensitive and highly specific. However, it cannot be used in patients with pacemakers, which is a significant limitation in the elderly population.

In nuclear studies, the typical H-shaped or butterfly pattern of uptake in the sacrum is diagnostic of insufficiency fracture. The vertical limbs of the H lie within the sacral ala, parallel to the sacroiliac joints; the transverse limb of the H extends across the sacral body. (See the image below.) [22]

Other sacral variant uptake patterns occur frequently and include the unilateral ala, incomplete H, and horizontal linear dot patterns. (See the images below.)

Iliac fractures are seen as linear areas of uptake, while pubic and supra-acetabular fractures produce areas of linear or focal uptake.

Concomitant findings of 2 or more areas of uptake in the sacrum and at another pelvic site are considered diagnostic of insufficiency fractures of the pelvis. (See the image below.)

The degree of confidence may be high. Nuclear studies are highly sensitive and highly specific when a typical pattern of sacral uptake or concomitant sacral and pubic uptake is observed. If a typical pattern of abnormality is not present, the bone scan is much less specific. For variant or incomplete patterns of uptake, the findings may be mistaken as signifying malignancy or other diseases. CT or MRI is useful in such cases.

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Wilfred CG Peh, MD, MHSc, MBBS, FRCP(Glasg), FRCP(Edin), FRCR Clinical Professor, Yong Loo Lin School of Medicine, National University of Singapore; Senior Consultant and Head, Department of Diagnostic Radiology, Khoo Teck Puat Hospital, Alexandra Health, Singapore

Wilfred CG Peh, MD, MHSc, MBBS, FRCP(Glasg), FRCP(Edin), FRCR is a member of the following medical societies: American Roentgen Ray Society, British Institute of Radiology, International Skeletal Society, Radiological Society of North America, Royal College of Physicians, Royal College of Radiologists

Disclosure: Nothing to disclose.

Bernard D Coombs, MB, ChB, PhD Consulting Staff, Department of Specialist Rehabilitation Services, Hutt Valley District Health Board, New Zealand

Disclosure: Nothing to disclose.

William R Reinus, MD, MBA, FACR Professor of Radiology, Temple University School of Medicine; Chief of Musculoskeletal and Trauma Radiology, Vice Chair, Department of Radiology, Temple University Hospital

William R Reinus, MD, MBA, FACR is a member of the following medical societies: Alpha Omega Alpha, Sigma Xi, American College of Radiology, American Roentgen Ray Society, Radiological Society of North America

Disclosure: Nothing to disclose.

Felix S Chew, MD, MBA, MEd Professor, Department of Radiology, Vice Chairman for Academic Innovation, Section Head of Musculoskeletal Radiology, University of Washington School of Medicine

Felix S Chew, MD, MBA, MEd is a member of the following medical societies: American Roentgen Ray Society, Association of University Radiologists, Radiological Society of North America

Disclosure: Nothing to disclose.

Leon Lenchik, MD Program Director and Associate Professor of Radiologic Sciences-Radiology, Wake Forest University Baptist Medical Center

Leon Lenchik, MD is a member of the following medical societies: American College of Radiology, American Roentgen Ray Society, Radiological Society of North America

Disclosure: Nothing to disclose.

Pelvic Insufficiency Fracture Imaging

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