Pelvic fractures represent 3% of all skeletal fractures and are exceeded only by skull fractures in their associated complications and mortality.1 These fractures range from low-energy stable fractures to high-energy unstable injuries. The mortality rate for high-energy pelvic fractures ranges from 10% to 20%, but in hemodynamically unstable patients or after open fractures, it increases to 50%.2,3 Motor vehicle collisions account for approximately two-thirds of all pelvic fractures. Pedestrians struck by automobiles are responsible for 15% of cases. Crush injuries, motorcycle crashes, and falls each account for an additional 5%.4
Pubic rami fractures are the most common pelvic fractures with the superior ramus more frequently involved than the inferior ramus. Pubic rami fractures account for over 70% of all pelvic fractures.5 The incidence of fractures of the remaining pelvic bones in descending order is the ilium, ischium, and acetabulum. Sacroiliac (SI) fractures are associated with the most significant bleeding. Both the mechanism of injury and the fracture pattern identified on imaging studies are important in predicting associated injuries.
In humans, the pelvic ring serves two important functions: weight support (stability) and protection of the viscera. There are essentially three bones that combine to form the pelvic ring: two innominate bones (composed of the ischium, ilium, and pubis) and the sacrum (Fig. 17–1). The coccyx is a fourth bone, but it is not incorporated into the pelvic ring. The two innominate bones and the sacrum are united by the formation of three joints (the symphysis pubis and the two SI joints). The ligaments that form the pelvic ring are the strongest in the body.
The osseous structures of the pelvis.
Weight bearing is transmitted through the bony pelvis along two pathways (Fig. 17–2). When standing, weight is transmitted through the spine to the sacrum, SI joints, and along the arcuate line to the superior dome of the acetabulum and down the femur. In the sitting position, the force is transmitted down the spine to the sacrum and the SI joints and to the ischium by way of the inferior ramus. The bone is very strong in these areas and the anteroposterior (AP) radiograph of the pelvis clearly demonstrates the thick trabecular pattern along these lines of stress. As a result, pelvic fractures more commonly interrupt the ring in areas not involved in weight transmission. A greater force is required to fracture a “weight-bearing” area of the pelvis. In addition, fractures involving the weight-bearing arches are associated with much more pain when stressed than those fractures that do not involve these arches. A good example is the superior ramus fracture. Because this structure is a nonweight-supporting area, it is generally less painful and mechanically stable compared to fractures in weight-supporting portions of the pelvis. A patient with a fracture of the superior pubic ramus may walk into the emergency department (ED), whereas a patient with a fracture through the sacrum will not be able to bear weight without significant pain.
A. Lines of stress in the standing position. B. Lines of stress in the sitting position. Note that in the sitting position the lines go through the ischium.
The concept of the pelvis as an anatomic ring also has important implications for fracture detection. The presence of at least two fractures or one fracture and a dislocation is required to cause a displaced fracture in the ring. Therefore, if a displaced pelvic ring fracture is diagnosed, the clinician should search for a second fracture or joint injury. Single breaks in the pelvic ring are unusual and are usually nondisplaced and occur near or at a joint (SI joint or symphysis pubis).
A displaced fracture of the pelvic ring indicates that there is at least a second fracture or a fracture plus a joint dislocation, most commonly the sacroiliac joint.
Pelvic stability during ambulation is a combined function of ligaments and bones. Anteriorly, the interpubic ligaments join the two pubic bones forming the symphysis pubis. The anterior pelvic structures (symphysis and rami) are responsible for 40% of pelvic ring stability.2 Posteriorly, the SI joint is supported by a series of strong ligaments that are the major stabilizers of the pelvic ring (Fig. 17–3). Disruption of the SI ligaments will alter the normal weight-bearing function of the pelvic ring.
The ligaments surrounding the sacroiliac joint are the strongest in the body.
Disruption of the interpubic ligaments may result in diastasis of the pubic symphysis of up to 2.5 cm. The intact ligaments of the SI joint—specifically the sacrospinous, sacrotuberous, and anterior SI ligaments—limit further opening. If these ligaments are sectioned, the pelvis becomes rotationally unstable and the pelvis will “open like a book.” As long as the posterior ligaments of the SI joint (interosseous SI ligament and posterior SI ligament) remain intact, the hemipelvis will remain vertically stable. Additional injury to the posterior SI ligaments results in a pelvis that is unstable both rotationally and vertically.
The muscles attached to the pelvis serve to support the body in the erect position and to provide mobility to the lower limbs. For the purpose of this text, the essential muscular anatomy concerns only those muscles responsible for avulsion fractures.
The sartorius inserts on the anterior-superior iliac spine.
The rectus femoris inserts on ...
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