Sacral fractures occur in approximately 30% of pelvic ring fractures, but fewer than 5% occur as isolated injuries. Sacral fractures are commonly associated with pelvic fractures and lumbosacral dislocation with facet injury. Most sacral fractures are caused by indirect forces through the pelvis or lumbar spine, but can be caused by direct trauma.
The sacrum is a triangular shaped bone consisting of 5 fused sacral veterbrae that articulates with the 5th lumbar vertebra superiorly and the coccyx inferiorly. It is part of the pelvic ring and participates in weight transfer from the trunk to the lower extremities. The anterior surface has 4 pairs of sacral foramen where the anterior sacral nerves exit. The posterior surface contains the posterior sacral foramen where the posterior branches of the sacral nerves exit. Ligaments provide stability to the pelvis. The most important are the ligaments on the posterior aspect connecting the sacrum to the innominate bones. These include the sacroiliac ligamentous complex, the sacrotuberous ligament, the sacrospinous ligament, and the lumbosacral ligaments.
One of the most common classification systems of sacral fractures is to use the direction of the fracture line within the sacrum. Vertical fractures can occur in the alae or through the foramina. Oblique fractures can occur anywhere. Transverse fractures are most frequently found between S2 and S3, and less frequently occur at S1 or S2.
The Denis classification divides the sacrum into zones:
- Zone 1 - ala of the sacrum to lateral border of the neural foramen
- Zone 2 - neural foramen
- Zone 3 - central portion of the sacrum and canal
Zone 1 fractures are usually caused by lateral compression of the pelvis, vertical shear fracture, or sacrotuberous avulsions. Zone 2 fractures involve one or more foramen and are usually due to vertical shear. Zone 3 involves the central canal. Although zone 3 fractures are seen less frequently, they have the highest rate of neurological deficits of the three zones.
Presentation/Physical Exam/Radiographic Studes
Sacral injury should be suspected if patient presents with peripelvic pain, posterior sacral bony prominence, or palpable subcutaneous fluid. It is important to do a functional assessment of the lower sacral nerve roots by checking spontaneous and voluntary rectal sphincter contraction, pinprick sensation in dermatomes of S2-S5, and reflexes - perianal wink, bulbocavernosus and cremasteric reflexes. A digital rectal examination and a vaginal exam in women should be conducted to exclude an open fracture. Patients may also report low-back or buttock pain with walking.
AP x-ray of the pelvis provides limited visualization of the sacrum. X-ray of the pelvic inlet shows the sacral spinal canal and superior view of S1. It can be useful in determining anterior or posterior displacement of the SI joint, sacrum, or iliac wing. Pelvic outlet x-ray provides a true AP view of the sacrum. This is useful for determining vertical displacement of the hemipelvis, widened SI joint, discontinuity of the sacral foramina. CT is useful for assessing the sacrum and SI joints. MRI can be useful for imaging of genitourinary or pelvic vascular structures as well as for patients who experience sacral neurological deficits after trauma.
Differential Diagnosis/Associated Injuries
Other injuries to consider include pelvic ring fracture, ligamentous, neurologic, genitourinary and gastrointestinal injuries.
Immediate treatment for sacral fractures consists of general resuscitation measures and temporary reduction of displaced pelvic ring fractures with skeletal traction or external fixation. Pelvic angiography can be used for the treatment of hemorrhage. DVT prophylaxis is important.
Surgical treatment consists of surgical reduction or fixation if fracture is displaced 1 cm. Options include percutaneous iliosacral screws, posterior tension band plating, and transiliac sacral bars. If neurologic injuries are present, open reduction and foraminal decompression are indicated.
SI joint disruption can be fixed by either closed means or by percutaneous iliosacral screw fixation. It is not safe to perform iliosacral screw fixation with persistent fracture displacement. Using longer screws may lead to bending of the fixation. No consensus exists for the indications for open reduction of displaced fractures or the indications for decompression for sacral fracture with nerve root injury.
Neurologic injury is frequently associated with displaced fractures. L5 nerve root damage is common in Zone 2 injuries. In Zone 3 injuries, central canal involvement with cauda equina syndrome is likely. Another complication is pain or malunion.