Sacral fractures

1

Describe the important anatomic features and functions of the sacrum.

The sacrum is located at the junction between the pelvis and spinal column and functions as a keystone between the iliac crests. The sacrum is formed by the coalescence of five vertebral segments (S1–S5) and is attached to the caudal lumbar spine and posterior pelvic ring through broad, well-developed ligamentous structures. The sacrum transmits load from the lumbar spine through the sacroiliac joints to the pelvis and distally to the hip joints. Of biomechanical importance is the fact that the main weight transmission of the lumbosacral junction is borne by the S1 segment, while the S2–S5 segments provide minimal additional support. The sacrum also serves as a protective conduit for major neurovascular structures, including the lumbosacral and sacral plexuses, iliac vessels and their bifurcations, and the organs within the lesser pelvis. The kyphotic alignment of the sacrum in the sagittal plane predetermines lumbar lordosis through inclination of its upper sacral endplate relative to the horizon line. Upwards of 10% of patients have a segmentation anomaly of the lumbosacral junction with either an assimilation of the L5 vertebra to the sacrum (sacralization), or an S1 vertebra, which transitions to a lumbar morphology (lumbarization). Failure to recognize such transitional anomalies can have significant clinical implications.

2

What injury mechanisms and patient groups are most commonly associated with sacral fractures?

There are two distinct injury mechanisms and patient groups associated with sacral fractures:

• 1.

  High-energy injury mechanisms: These fractures usually occur in younger, mostly male patients as result of motor vehicle accidents, falls from a height, and crush injuries.

• 2.

  Low-impact, insufficiency or stress fractures: This patient group is generally older and more commonly female. Etiologies include low-energy injury mechanisms such as a ground level fall on the buttocks, insufficiency fractures, and stress fractures. Insufficiency fractures may occur in patients with osteoporosis, other metabolic bone diseases, or previously undiagnosed neoplastic disorders. A sacral stress fracture may arise following instrumented lumbosacral fusions surgery. Another subgroup of patients who develop sacral stress or insufficiency fractures includes endurance sports athletes, especially younger females with amenorrhea.

3

How are patients at risk for sacral fractures evaluated clinically?

Clinical symptoms may be vague or non-specific and diagnosis of sacral fractures is often delayed or missed. Common symptoms include low back and buttock pain. A detailed patient history including mechanism of injury and associated injuries is critical. Patients with sacral insufficiency fractures describe intolerance to being upright, even when sitting, and are much more comfortable in a recumbent position. Physical examination with inspection and palpation of the patient’s backside and thorough examination of neurologic function is important. In patients with high-energy injury mechanisms, clinical inspection, palpation and percussion of the posterior pelvic ring may reveal focal tenderness, ecchymosis, swelling, and discoloration. Crepitus with manipulation of the hip joints may be noted depending upon the type of injury and its acuity. Suspicion of an acute posterior pelvic ring injury usually should prompt rectal examination and inspection of the vaginal vault in female patients (with chaperone present) to assess for occult soft tissue injuries, blood products, and integrity of sphincters. Any disruption of the dorsal integument or rectal or vaginal cavities in the presence of a sacral fracture of any acuity is suspicious for an open fracture, which profoundly changes surgical treatment.

4

Describe the components of a complete neurologic examination for patients with a sacral fracture.

Regardless of the patient’s cognitive status, an evaluation consistent with the Guidelines of the American Spinal Injury Association is performed. Neural structures potentially affected by a sacral fracture include the lumbosacral plexus (L4, L5, and S1 roots on either side) and the sacral plexus (S2–S4 bilaterally), as well as the sacral sympathetic nerves (superior and inferior hypogastric plexus). Injury mechanisms include compression, crush, traction, and transection.

Lumbosacral plexus assessment includes examination of motor, sensory, and reflex function. In addition, pain reproduction with straight leg raises and hip manipulation can help identify nerve root entrapment.

Sacral plexus evaluation is more limited in its direct clinical findings. It is important to evaluate light touch and pinprick sensation in the S2–S4 dermatomes on either side along the buttocks, the perineum, and include the perianal and genital area. External bladder sensation and internal bladder wall sensation, as applied through a Foley catheter, for instance, usually implies at least partial integrity of the upper sacral plexus. Integrity of perianal sensation on either side and presence of a baseline anal sphincter tone, as well as strong voluntary anal contraction, are helpful to identify integrity of the lower sacral plexus. Presence of a bulbocavernosus reflex is a key determinant in identifying the conclusion of spinal shock following spinal cord injury. A complete rectal examination should document:

• 1.

  Presence of spontaneous anal sphincter tone

• 2.

  Maximum voluntary anal sphincter contractility

• 3.

  Perianal sensation to light touch and pinprick

• 4.

  Presence of anal wink and bulbocavernosus reflex

In a postacute setting, postvoid residuals on bladder ultrasound scan can be used as a follow-up test for patients with a neurogenic bladder to assess for reinnervation. A bladder scan and establishing postvoid residuals can be very helpful in the diagnosis and long-term follow-up of patients with a neurogenic bladder.

5

What imaging tests are used to assess sacral fractures?

The basic radiographic assessment starts with an anteroposterior (AP) pelvis radiograph . It is important to realize that the naturally curved kyphotic shape of the sacrum partially obscures visualization of the entire sacrum on the AP pelvic radiograph. Subtle details, such as disruption of the fine cortical lines of the sacrum and especially its foramina, aid as a screening tool. Abnormal pelvic projections, such as seeing a pelvic inlet view on a standard AP pelvic radiograph are other important pointers toward detection of a major sacral fracture. Suspicion based on radiographic findings, clinical findings, or injury mechanism should prompt ordering of a computed tomography (CT) scan, which remains the single most useful test for diagnosis of a sacral fracture. More specific plain radiographs that can reveal posterior pelvic ring and sacral injuries include pelvic inlet and outlet views and a lateral sacral radiograph. While these are helpful, initial radiographic studies for evaluation of sacral alignment, the three-dimensional complexities of sacral and posterior pelvic ring pathology require assessment with CT, including sagittal and coronal reformatted images. In cases associated with bleeding in the region of the sacrum and sciatic notch, CT-angiography is an essential tool for assessment of vascular injury. Magnetic resonance imaging (MRI) is not routinely necessary but is helpful for diagnosis of sacral insufficiency or stress fractures, metastatic disease, and to evaluate unclear neurologic injuries. MRI neurography can localize known root or plexus injury in a subacute setting. For diagnosis of insufficiency fractures and as screening tool for neoplastic and infectious diseases, technetium bone scans with single-photon emission CT (SPECT) images can reveal abnormal osseous activity. This test, however, has little or no role in the assessment of acute injuries.

6

What other diagnostic tests play a role in the evaluation of patients with sacral injuries?

There are a number of tests that can help delineate neurologic injuries and also measure recovery:

• Electromyogram (EMG) of L5 and S1 innervated muscles (lumbosacral plexus function)

• Anal sphincter EMG (S2–S4 function)

• Somatosensory-evoked potentials (SSEPs) of tibial and peroneal nerves

• Pudendal sensory-evoked potentials (pudendal SEPs)

• Cystometrography (CMG) in combination with cystoscopy

• Direct visualization of urethral sphincter function during cystoscopy

• Postvoid residuals on bladder ultrasound scan

Use of conventional EMG is limited to assessment of the L5 and S1 roots and usually requires waiting for at least 2–3 weeks before injury-related changes are detectable. Anal sphincter EMG and CMG-EMG can diagnose lower sacral root damage but are usually not useful in the immediate postinjury period. CMG-EMG has been used as a follow-up study for patients with neurogenic bladder and may demonstrate bladder reinnervation. Assessment of postvoid residuals on a bladder ultrasound scan is the most commonly used test to determine sacral plexus recovery in an outpatient setting.