Incidence

Spontaneous, pathologic fractures of the sacrum are now well known, but were only first described as recently as 1982 by Lourie. Such injuries are often termed as “insufficiency fractures,” indicating that the bone strength is insufficient to withstand normal mechanical and physiologic forces. The incidence of sacral insufficiency fractures (SIFs) is substantially less than that of osteoporotic fractures involving the lumbar and thoracic spine; however, in a published series evaluating elderly patients with acute onset of low-back pain and negative radiographs, bone scintigraphy diagnosed 102 sacral fractures over a 2-year period at a single institution. In a second study, using retrospective review of 1017 consecutive bone scans in patients older than 70 years, SIFs were identified in 194 patients (19%). Weber and colleagues reported an incidence of 1.8% of SIFs (n = 20, total = 1015) in a female population, older than 55 years, admitted to a single department with low-back pain. In an active, single practice setting that treated approximately 2000 patients with osteoporotic vertebral fractures, 80 patients with sacral insufficiency fractures were identified and treated, which was a relative incidence of at least 4%.

Reported data for incidence of localized sacral fractures from tumor destruction are limited at this time.

Causes of Sacral Fractures

Sacral fatigue or stress fractures occasionally occur in long-distance runners. Sacral pathologic (insufficiency or tumor related) fractures may occur in patients with fragile or abnormal bone owing to osteoporosis, disorders of calcium metabolism, osseous metastatic disease, and prior radiation therapy. Postmenopausal women with osteoporosis have been reported to be the most common group to suffer from sacral insufficiency fractures.

Diagnosis

Sacral fractures present clinically with pain. Pain localizes mainly to the sacral and buttock region, but may be referred to the low lumbar spine. Pain is usually acute in onset and exacerbated by weight bearing, which is typically relieved by rest or the supine position. Referred pain to the hips or groin is common. For osteoporotic fractures, there can be concurrent fractures of the ischial and pubic rami, or coexistent vertebral compression fractures. There is usually no related neurologic abnormality. Physical examination may show sacral tenderness on lateral compression, and there is restricted mobility that is indistinguishable from sacroiliac arthropathy and other spinal and pelvic pathologic conditions.

Conventional plain radiographs are insensitive, only occasionally showing sclerosis of the sacrum or an actual fracture line. Computed tomography (CT) offers greater sensitivity ( Fig. 1 ); however, acute, nondisplaced fractures without sclerosis may be difficult to visualize. Bone scintigraphy is reliable, but with less anatomic detail compared with CT. Bone scintigraphy demonstrates increased tracer activity in the areas of the fracture ( Fig. 2 ). Ambiguity on bone scan in this region is common because of bowel and bladder activity or uptake around the sacroiliac (SI) joint caused by degenerative change. Careful attention to findings on the posterior nuclear images is therefore helpful when evaluating the sacral region.

( A ) Computed tomography (CT) scan. The sacral fracture is indicated by the arrows. In this figure it is detectable but less obvious than in B . Some fractures may present with only minor discontinuities at the bone margins. ( B ) CT scan. Arrows indicate the bilateral alar fractures of the sacrum. These fractures are much more obvious than in A .

Bone scintigraphy of pelvis, posterior view. Abnormal activity in the sacrum caused by a sacral insufficiency fracture. The arrows point to the abnormal uptake in the lateral margins of the sacrum.

Magnetic resonance (MR) imaging is sensitive and specific in demonstrating sacral fractures. Marrow edema is visible as decreased signal on the T1-weighted images. T2-weighted short tau inversion recovery (STIR) or fat-saturated T2-weighted images show increased signal related to edema ( Fig. 3 ). Axial or coronal images show the fractures in 2 complementary planes. Although sacral insufficiency fractures demonstrate enhancement after intravenous contrast agent administration, this often is not required because the imaging findings on the unenhanced MR imaging examination are fairly characteristic. In a patient with tumor history, sacral fractures may be misinterpreted as metastatic disease, which sometimes can be differentiated with diffusion-weighted images.

( A ) Coronal T1 magnetic resonance (MR) image. Arrows point to the unilateral sacral fracture. The fracture line is dark on T1 imaging. ( B ) Axial T1 MR image. Arrows indicate the unilateral sacral fracture. ( C ) Inversion recovery MR image. Arrows point to the sacral fracture. The actual fracture line is less distinct compared with A and B . However, the marrow signal abnormality associated with the edema is much more obvious with the fat suppression.

Sacral Fracture Types

The classic appearance of a sacral insufficiency fracture on skeletal scintigraphy is an H-shaped uptake pattern ( Fig. 4 ). This pattern results from fractures of the sacral ala bilaterally with a horizontal fracture component that extends through the central sacrum (usually at about the S2 level). The fracture may present with only the vertical component on one or both sides of the sacrum. The horizontal fracture may also accompany a single unilateral component. The complexity of the fracture makes the diagnosis difficult based solely on the bone scan, thus anatomic confirmation with CT or MR imaging is always preferred.

The usual location of the fracture lines in the sacrum with a typical H-shaped pattern. Lateral fracture lines usually extend between the sacroiliac joints and the neural foramina. These lines can extend into the foramina or, rarely, the spinal canal. The central component usually traverses the transmembrane segment S2 region.

Functional Outcome of Sacral Fractures

Functional outcome data are limited for sacral insufficiency fractures with few reported series. In a series of 60 hospitalized patients (mean age of 83 years) with pelvic insufficiency fractures, of which 16 involved the sacrum, the average hospital stay was 45 days. After 1 year, only 36.6% had the same level of self-sufficiency as before the fracture and 25% were discharged to institutions with a 1-year mortality of 14.3%. Less bleak outcomes have also been reported. In a series of 20 patients with sacral insufficiency fractures (mean age of 74 years), 17 patients had resolution of local pain and 3 patients required analgesics for pain control. In another series of 20 hospitalized patients with sacral insufficiency fractures (mean age 79), with analgesics and physical therapy, pain resolved for all 20 patients at 9 weeks. In patients with sacral fractures associated with tumor, functional outcomes data are more limited because these are patients with shorter life expectancy directly related to the tumor.