Sacral fractures

The sacrum is the keystone of the posterior pelvic ring and the base of the spine. Most sacral fractures are the posterior half of a pelvic ring injury; a few separate the spine from the pelvis (U-type). The closer the fracture runs to the midline, the higher the risk to the sacral nerve roots (Denis zones). In the elderly, insufficiency fractures with no or minimal trauma are common and often missed.

Mechanism

High energy: road traffic, falls from height and suicidal jumps (U-type, "jumper’s fracture"), usually as part of a pelvic ring injury. Low energy: insufficiency fractures in osteoporotic bone, after radiotherapy or long steroid use.

Clinical findings

• Posterior pelvic pain and tenderness; check the whole pelvic ring and treat haemodynamic instability (binder) as for any pelvic injury.
• Examine L5 to S4: ankle dorsiflexion and plantarflexion, perianal sensation, anal tone and squeeze, bulbocavernosus reflex; ask about urinary retention.
• Elderly with low back or buttock pain and normal pelvic X-rays: think of an insufficiency fracture.

Imaging

• AP pelvis, inlet and outlet views.
• CT with coronal and sagittal reformats (standard).
• MRI (STIR) for suspected insufficiency fractures and for nerve root compression.

• AP pelvis with inlet and outlet views (outlet: foramina and vertical displacement); sacral fractures are easily missed on plain films.
• CT with coronal and sagittal reformats for every suspected fracture; a lateral or sagittal view is essential for transverse and U-type fractures.
• MRI (STIR) is the most sensitive for insufficiency fractures (an H-shaped pattern, also seen as the "Honda sign" on bone scan) and for nerve root compression.

Classification

Denis I

Alar zone, lateral to the foramina. Nerve injury in about 6% (L5 root trapped between the ala and the L5 transverse process).

Denis II

Through the foramina. Nerve injury in about 28%, mostly L5 and S1 roots (sciatica).

Denis III

Medial to the foramina, into the canal, including transverse fractures. Nerve injury in more than half: bowel, bladder and sexual dysfunction.

AO/OTA 61

Sacral fractures as part of the ring: 61B incomplete posterior disruption (e.g. buckle with lateral compression), 61C complete posterior disruption.

U-type

Bilateral vertical fractures joined by a transverse fracture (usually S1–S2): the spine and upper central sacrum are separated from the pelvis (spinopelvic dissociation).

Types shown in 3D

Denis I (alar)

Lateral to the foramina. With a lateral compression mechanism the ala buckles and the hemipelvis rotates inwards (with rami fractures). Usually stable; nerve injury is rare.

Denis II (transforaminal)

Through the foramina: with a vertical shear the hemipelvis and the lateral sacrum shift upwards and backwards. L5 and S1 roots at risk; fully threaded (positional) screws if comminuted, to avoid compressing the roots.

Denis III (transverse)

A transverse fracture of the lower sacrum (below the sacroiliac joints) from a fall onto the buttocks: the distal segment flexes forwards. The ring stays stable, but sacral roots S2–S4 (bladder, bowel) are at risk.

U-type (spinopelvic dissociation)

Bilateral vertical transforaminal fractures joined by a transverse fracture at S1–S2: the lumbar spine and the upper central sacrum separate from the pelvis and tip into kyphosis. "Jumper’s fracture"; lumbopelvic fixation.

Insufficiency (H-type)

Bilateral vertical alar fractures joined by a horizontal limb, in osteoporotic bone after little or no trauma. Undisplaced and easily missed on X-ray: MRI (STIR) or CT.

Treatment

Stable (undisplaced alar, lateral compression buckle, insufficiency)

Analgesia and mobilisation with weight bearing as tolerated; osteoporosis work-up and treatment. Persistent pain that prevents mobilisation: iliosacral screws or sacroplasty in selected patients.

Unstable vertical fractures (61C)

Closed or open reduction and iliosacral or transsacral screws, or posterior tension band plating; spinopelvic (lumbopelvic) fixation when the fracture is comminuted or highly unstable. In comminuted transforaminal fractures use fully threaded (positional) screws: compressing them can trap the nerve roots.

U-type and displaced zone III

Lumbopelvic fixation, often combined with iliosacral screws ("triangular osteosynthesis"); decompression of the sacral roots for a deficit with canal compromise.

Aftercare

Protected weight bearing 6–12 weeks after fixation of unstable injuries (earlier after lumbopelvic fixation); early mobilisation after insufficiency fractures.

Complications

• Missed fracture (especially U-type and insufficiency)
• Chronic posterior pelvic pain and malunion
• Persistent bowel, bladder or sexual dysfunction
• Screw malposition (L5 and S1 roots)
• Wound breakdown over posterior implants