Anatomy of the Sacrum

The sacrum is the most caudal fixed segment of the axial skeleton. The sacrum derives its name from the Greek hieron osteon (“sacred bone”). Use of this bone in religious sacrificial rites, and its role in protecting the genitalia, resulted in the word hieron being interpreted as “sacred” or “holy.” Through a direct translation into Latin, the term os sacrum was coined by the Romans and has come down to us unchanged today. The sacrum is a virtually motionless spinal segment that develops from five fused vertebrae. Each vertebra develops from three primary ossification centers, giving rise to the anterior and posterior elements. The fibrocartilaginous disk between the last two sacral vertebrae fuses at about 15 years of age; fusion continues in a cephalad direction until segmental fusion is complete at about 25 years of age. The sacrum sits like a keystone in the posterior arch of the pelvis and distributes load from the axial skeleton to the pelvis through the relatively immobile sacroiliac joints. Stability is enhanced by strong anterior and posterior ligamentous attachments between the sacrum and the pelvis. The sacrum lies at a 40-degree angle from the horizontal at the lumbosacral junction. Thus, axial loads promote rotational stresses that are counteracted by the sacrotuberous and sacrospinous ligaments, which attach opposite the S4 foramina. The first sacral foramen has the largest diameter: the diameter of the foramina in the sacrum decreases from proximal to distal from S1 to S4. The diameter of the nerve root also decreases from one third of the foraminal diameter proximally to one sixth of the foraminal diameter distally; therefore, foraminal entrapment is less likely at the lower sacral levels.

Pathophysiology and Classification of Sacral Fractures

In discussions of stability, the sacrum and pelvis are considered one unit. The pelvic ring is a relatively rigid structure whose disruption requires discontinuity in at least two places. Discontinuity can occur as either a fracture or a ligamentous disruption. Gunterberg and coworkers⁷ performed biomechanical studies of cadaveric specimens loaded to failure to evaluate pelvic stability after major amputation of the sacrum. Specimens resected below S2, sparing the sacroiliac joints, were stable. Specimens resected between S1 and S2 had one third of the sacroiliac joint resected, and stability was reduced 30%. Specimens resected 1 cm below the sacral promontory had half of the sacroiliac joint removed, and the load-to-failure strength was reduced 50%. Overall, the load to failure far exceeded the anticipated physiologic loads.

Bonnin’s original classification divided sacral fractures into six categories³: (1) juxtailiac marginal fractures; (2) fractures involving the S1 or S2 foramen with upward displacement of the lateral mass; (3) fractures through the sacral foramina, which separate the lateral mass from the body of the sacrum; (4) comminuted fractures of the upper sacrum; (5) avulsion fractures of the attachment of the sacrotuberous ligament; and (6) transverse fractures of the sacrum. Bonnin’s classification delineates common fracture types but does not correlate the fracture with the mechanism of injury or aid in clinical evaluation and prognosis. A number of classification systems for sacral fractures have been proposed since Bonnin’s time. The goals of these classification schemes have been to correlate the observed fracture with the mechanism of injury and the clinical findings and to aid in treatment planning. In 1984, Schmidek and coworkers⁶ proposed a classification based on the mechanism of injury and the resulting fracture pattern. They divided sacral fractures into those caused by direct trauma to the sacrum and those resulting from forces applied indirectly to the sacrum. Direct forces include penetrating injuries that result in open fractures, often accompanied by extensive pelvic visceral injuries. Penetrating injuries are usually stable if the sacrum and the sacroiliac joints above the S1 foramina are intact. Direct closed fractures are often caused by a hard fall onto the buttocks, causing low transverse-type fractures near the kyphos of the sacrum. Although such fractures usually occur through the foramina of S4, any of the lower three vertebrae can be involved. The distal fragment is often displaced anteriorly and may perforate the rectum in more severe cases. Because this part of the sacrum is not involved in the transmission of weight, these fractures are typically stable.

Transverse fractures accounted for 5% to 10% of all sacral fractures in the series by Schmidek and colleagues.⁶ Indirect trauma to the sacrum follows injuries to the pelvic ring or lumbar spine. The mechanism, usually a flexion injury from a position of hip flexion with knee extension, causes traumatic spondylolisthesis through the S1 or S2 foramina, with forward displacement of the upper spinal segment. Typically, this type of injury occurs in younger patients before intersegmental ossification is complete. One fourth of transverse sacral fractures resulting from falls also have an associated thoracolumbar burst fracture. Thus, lower extremity motor weakness must prompt the examiner to look for associated spinal injuries. Lumbosacral fracture-dislocations are caused by mechanisms similar to those underlying high transverse sacral fractures, and they usually involve fracture of the S1 facet, with resultant instability.

Most fractures produced by indirect forces are vertical fractures of the sacrum, which almost always occur in conjunction with pelvic fractures. Schmidek and coworkers⁶ classified these fractures into four fracture patterns: (1) lateral mass fractures (extending from the sacral notch through the ventral foramina), (2) juxtaarticular fractures (lateral sacral mass fractures with fragments dissociated from the body of the sacrum), (3) cleaving fractures (vertically oriented fractures from the sacral notch through the sacrococcygeal region), and (4) avulsion fractures (along the convex margin of the sacrum at the attachments of the sacrotuberous and sacrospinous ligaments). Combination fractures, with features of more than one pattern, may also occur. Although this scheme categorizes the different types of sacral fractures, it is cumbersome to remember and apply. In 1988, Denis and colleagues¹ published a series of 236 sacral fractures and proposed a simplified classification scheme that categorizes sacral fractures based on the sacrum’s division into three anatomic zones: zone I (alar region), zone II (foraminal region), and zone III (region of the central sacral canal). A zone II fracture can involve zone I but cannot extend into zone III, whereas a zone III fracture can involve zones I and II. In addition to being simple, this classification is relevant to the biomechanical forces applied to the sacrum and the probability of neural injury, and it aids in choosing among treatment options.

Zone I fractures pass through the ala without damaging the foramina or central canal (Fig. 320-1). They are usually caused by lateral compression of the pelvis during pedestrian accidents in which the posterior sacroiliac ligaments remain intact and a portion of the ala is compressed anteriorly. Zone I fractures are stable by virtue of the intact posterior ligaments. Vertical shear injuries of the pelvis can produce more severe zone I fractures, with superior displacement of the ala and compression of the L5 nerve root between the fracture fragment and the L5 transverse process (Fig. 320-2). Wiltse and colleagues named this mechanism of L5 root injury the traumatic far-out syndrome.⁸ Zone I fractures also include avulsion fractures at the bulbous enlargement of the sacrum adjacent to the S4 foramen, which is the point of attachment of the sacrospinous and sacrotuberous ligaments. A substantial degree of pelvic disruption occurs with avulsion fractures; therefore, the pelvis is often unstable.

FIGURE 320-1 Zone I fracture (Denis classification) through the ala of the sacrum.

FIGURE 320-2 Zone I fracture (Denis classification) through the ala of the sacrum with upward displacement of the fracture fragment, trapping the L5 nerve root against the transverse process of L5 (traumatic far-out syndrome).

Zone II fractures involve one or several sacral foramina, but not the sacral canal (Fig. 320-3