1.9 Sacral fractures

1 Introduction

The diagnosis and treatment of sacral fractures remain controversial [1−13]. Most patients suffer from polytrauma; therefore, significant injuries to other body regions generally dominate the primary diagnostic and therapeutic regimen. The typical clinical signs of an injury are usually missing, plain x-rays are difficult to interpret, and neurological deficits are not always obvious on the primary examination. Consequently, these injuries are frequently overlooked, misdiagnosed, or even neglected.

The significance of sacral fractures relates to a high rate of concomitant neurological injuries, as well as their importance as part of posterior pelvic ring instability [6, 14−17]. Several methods for external or internal stabilization of the unstable posterior pelvic ring have been described [13, 18–27], but a specific protocol for sacral fractures as part of this management plan is missing.

2 Anatomy

2.1 Osseous structure

The sacrum ( Fig 1.9-1 ) is a median triangular bone, connecting the two hemipelves to the spinal column. It consists of five fused sacral vertebrae and the rudimentary coccygeal bones. Additionally, it acts as an important stabilizing part of the pelvis by its ligamentous attachments.

The sagittal contour has a slight kyphosis with an average anterior inclination of 47°. Spinal orientation changes from a more horizontal to a vertical shape at the third sacral vertebra.

The ventral pelvic sacral surface forms the posterior wall of the true pelvis with a close relationship to the rectum. Two rows of sacral foramina are present, normally four on each side for the exiting S1–4 anterior nerve roots. The foramina are connected with the central canal via the intervertebral foramina. The anterior foramina are larger in diameter than the corresponding posterior foramina.

**Fig 1.9-1a–c** Osseous anatomy of the sacrum. The sacrum is roughly and irregularly formed and marked with three major vertically orientated crests. On the lateral side the articular surface of the S1 joint is visible. a Anterior view. b Posterior view. c Lateral view.

The thin dorsal cortical surface is rough and marked by three major vertically oriented crests. The rudimentarily visible former spinal processes are fused within the median sacral crest. The smaller intermediate sacral crest is visible medial to the neuroforamina, representing the former facet joints of the sacral spines. The lateral sacral crest is formed by the fused transverse processes. The dorsal rami of the nerve roots exit through the posterior sacral neuroforamina. The sacral hiatus takes up the dural sac at the S1 vertebra.

The lateral surface of the upper three sacral vertebrae forms the kidney-shaped articular surface of the sacroiliac joint, which corresponds to the articular surface of the iliac wing. The upper sacrum is formed by the superior surface of the S1 body. In the midline the S1 vertebral body forms the sacral promontory. Laterally, the sacral ala slopes from posterosuperior to anteroinferior in direct association with the common iliac vessels and lumbosacral trunk.

2.2 Ligaments

Anteriorly, the sacrum is connected to the articular surface of the ilium by the weak anterior sacroiliac ligaments (sacroiliac joint) ( Fig 1.9-2 ). The stronger sacroiliac interosseous ligaments stabilize the sacrum within the pelvic girdle posterior to the sacroiliac joint. The strong posterior sacral ligaments arise from the dorsal sacral interforaminal and lateral surfaces, connecting the sacrum with the iliac bone. The iliolumbar ligaments connect the fifth lumbar transverse process with the posterior part of the iliac crest, whereas the lumbosacral ligaments connect the L5 transverse process with the sacral ala. Thus, displaced fractures of the fifth lumbar transverse process may indicate an unstable posterior pelvic ring lesion. Additionally, the sacrospinous and sacrotuberous ligaments act as pelvic stabilizers by limiting anterior rotation of the sacrum around a horizontal axis.

3 Neurovascular anatomy

The dural sac ends at the level of S2 in 84% of patients. The sacral canal holds the spinal nerves of the sacral and coccygeal plexus, which leave the sacrum through the anterior sacral foramina ( Fig 1.9-3 ). The S1 root exits to join the L4 and L5 roots in front of the sacroiliac joint. The S2–4 roots exit and join the sciatic nerve in front of the piriformis muscle. In addition to the nerve roots forming the sciatic plexus, the pelvic floor also contains pelvic splanchnic nerves, which are mixed parasympathetic nerves that control involuntary sphincter muscle action of the rectum and bladder, and the nervi erigentes, which supply the penis and clitoris, and are important for sexual function.

The diameter of the S1 and S2 nerve roots are about one-third to one-fourth of the diameter of the surrounding foramina, decreasing to one-sixth at the level of S3 and S4. The cross-section of the nerve roots S2–5 amounts to 80%, 60%, 20%, and 15% of the cross-section of the sacroiliac foramen, respectively [14]. Therefore, the S1 or S2 nerve roots are more endangered for injury by fractures involving these neuroforamina than at lower levels. The lumbosacral trunk (L4–5 nerve root) is closely related to the lateral upper sacral alar surface. Therefore, fractures in this area or cranial displacement of the hemipelvis can be associated with a stretch injury of the L5 nerve roots.

**Fig 1.9-2a–b** Ligaments around the sacrum. a Posteriorly, the strong sacroiliac interosseous ligaments stabilize the sacrum within the pelvic girdle. b In contrast, the anterior sacroiliac ligaments are much weaker.

The major pelvic vessels (internal iliac artery and vein) follow the course of the lumbosacral trunk. The median sacral artery and sympathetic component of the autonomic nervous system are closely related to the anterior sacral surface in the area of the promontory.

**Fig 1.9-3** Lumbosacral plexus from anterior.

4 Clinical importance of the sacral anatomy

The sacral foramina, particularly at the S1 and S2 levels tend to weaken the junction between the body of the sacrum and its ala. A significant number of sacral fractures are known to extend through the sacral foramina and up to the groove at the L5 articular process. Because of the sacral roots’ fascial investments, they tend to maintain their relationship with the medial or midline portion of the sacrum during fractures. If there is significant vertical or posterior displacement, the sacral roots that are fixed at the sacral canal and the greater sciatic notch of the pelvis come under tension, causing traction injuries and even nerve root avulsion. Lateral compression injuries of the sacrum involve the sacral foramina. The fracture may cause buckling of bone into the foramen that compresses the roots, leading to neurological injury and/or causalgic pain.

5 Classification

Sacral fractures must be classified into: (1) fracture types not involving the pelvic ring; and (2) fracture patterns involving the ring and normally leading to at least some pelvic ring instability.

Generally, sacral fractures are part of a pelvic ring fracture [28]. According to the AO/OTA Fracture and Dislocation Classification, sacral type A fractures consist of coccyx fractures or sacrococcygeal dislocations (A3.1), nondisplaced transverse fractures of the sacrum below the S2 level not involving the pelvic girdle (A3.2), and displaced transverse fractures of the sacrum below the S2 level not involving the pelvic girdle (A3.3).

Sacral fractures associated with AO/OTA Fracture and Dislocation Classification type B fractures consist of unilateral or bilateral open-book sacral fractures (61-Bl.2 or 61-B3.1), unilateral (B2.1), or bilateral lateral compression injuries (B3.3).

Sacral fractures associated with the AO/OTA Fracture and Dislocation Classification type C fractures are unilateral unstable sacral fractures (Cl.3) or unilateral sacral fractures together with a contralateral posterior type B pelvic ring injury (C2.3). Additionally, bilateral sacral fractures (C3.3) often are suicidal jumper′s fractures with complete disruption of the sacral body from the column vertebral body.

In 1988 Denis et al [14] published a widely used classification based on three zones of the sacrum. The lateral part of the sacrum represents zone I (transalar fractures); the region of the foramen represents zone II (transforaminal fractures); and the region medial to the foramen represents zone III (central fractures) ( Fig 1.9-4 ). The most medial part of the fracture line defines the classification group in fracture lines crossing these zones. After an analysis of 236 sacral fractures, the authors [14] found a close relationship between the anatomical fracture pattern and rate of neurological deficits. The rate of neurological injuries was: zone I, 5.9%; zone II, 28.4%; and zone III, 56.7%. A review [15] of 377 sacral fractures confirmed these results. However, the main prognostic criterion for outcome of a nerve injury related to a sacral fracture was the grade of instability of the pelvic ring injury [15]. Table 1.9.1 describes a more complete sacral fracture classification involving four different types with nine typical fracture lines.

**Table 1.9-1** Descriptive classification of sacral fractures includes transalar, transforaminal, central, and bilateral fractures.

The practical clinical significance of these descriptions is to support the surgeon so that any potential injury or complication is not missed or allowed to occur. The pelvis fracture is first assessed and classified because most sacral fractures are associated with a pelvic ring injury. If a sacral fracture exists in association with a pelvic ring injury, then it is further described using one of the existing pelvic ring classification systems and appended to the pelvic ring description.

5.1 Sacral biomechanics

The load transfer from the lower extremities to the spine takes place through the posterior column of the hip joint to the sacroiliac joint and the sacrum. In the upright stance the body weight is divided into dorsocranial and ventrocranial components that are represented by the structure of the interosseous ligaments [29]. The specific anatomy of the posterior pelvis produces translational and rotational forces to the sacrum.

This movement leads to a tightening of the dorsal sacroiliac ligaments under load, whereby the iliac bones are compressed and the sacrum is stabilized [30–32]. The resulting torsion of the sacrum (nutation) is limited by the strong pelvic floor ligaments (sacrotuberous and sacrospinous) with the inserting muscles (gluteus maximus muscle, piriformis muscle) ( Fig 1.9-5 ) [33, 34].

Analysis indicates that motion of the sacroiliac joint is complex and cannot be described with a single horizontal axis. An approximated transverse rotational axis can be projected posteriorly to the sacroiliac joint between the first and the second sacral bodies with a moving position under load [35–40].

**Fig 1.9-4a–c** Classification of sacral fractures according to Denis et al [14] is based on three zones with transalar (zone I), transforaminal (zone II), and central (zone III) fractures.

In summary, a combined range of motion with 2–12° of rotation and 2–26 mm of translation is to be expected [37, 41]. The amount of movement depends on age and gender. In women, the range of motion is larger and increases during pregnancy. Mobility decreases with increasing age. Men who are older than 50 years have an observed ankylosis rate of 75% [42].

**Fig 1.9-5a–b** Sacral biomechanics. Axial load leads to a tightening of the dorsal sacroiliac ligaments, thereby the iliac bones are compressed and the sacrum is stabilized. The resulting torsional movement of the sacrum is limited by the sacrotuberous and sacrospinous ligaments.

6 Incidence and injury mechanism

In 1847 Malgaigne [43] reported only one isolated sacral fracture in an analysis of 2,358 fractures observed in all body regions. Wakeley [44] and Noland and Conwell [45] reported a 4% and 2.4% incidence of sacral fractures after pelvic fractures, respectively ( Table 1.9-2 ). The first detailed analysis of sacral fractures that included a classification was published in 1945 by Bonnin [46]. It estimated a 45.5% incidence after pelvic fractures. In 1972 Huittinen and Slätis [47] reported 14% sacral fractures in a series of 407 pelvic fractures. In 1988 Denis et al [14] analyzed 776 pelvic fractures, describing an incidence of 30.4% sacral fractures. The Hannover experience and the results of the German Multicenter Pelvic Study Group found a rate of 18–30% sacral fractures [48].

**Table 1.9-2** Incidence of sacral fractures.
Source	Year	No. of patients	Sacral fracture, %
Wakeley [44]	1929	100	4.0
Noland et al [45]	1933	60	11.7
Medelman*	1937	50	44.0
Furey†	1942	94	74.0
Bonnin [46]	1945	44	45.5
Huittinen H Slätis [47]	1972	407	14.0
Laasonen‡	1977	156	33.3
Melton 3rd**	1981	204	13.7
Denis et al [14]	1988	776	30.4
Gibbons et al [6]	1990	253	17.4
Isler [16]	1990	193	69.4
Pohlemann et al [15]	1992	1,350	27.8
German Pelvic study	1998	1,076	17.3
Total		5,409	22.8
* Medelman J. Fractures of the sacrum: their incidence in fracture of the pelvis. Am J Roentgenol Radium Ther Nucl Med. 1939;42:100–107. †Furey W. Fractures of the pelvis with special reference to associated fractures of the sacrum. Am J Roentgenol. 1942;47:89–96. ‡ Laasonen EM. Missed sacral fractures. Ann Clin Res. 1977 Apr;9(2):84–87. ** Melton LJ 3rd, Sampson JM, Morrey BF, et al. Epidemiologic features of pelvic fractures. Clin Orthop Relat Res . 1981 Mar–Apr(155):43–47.

Several injury mechanisms are accompanied by sacral fractures. High-energy trauma after a motor vehicle injury or fall from a great height is the predominant injury mechanisms resulting in a sacral fracture.

Although normally a significant correlation between the injury mechanism and the severity of the pelvic fracture is present in pelvic ring fractures, there is no correlation between the sacral fracture pattern and mechanism of injury in sacral fractures. Only the rare transverse sacral fracture is usually associated with a simple fall. Additionally, a lateral impact is associated with acetabular or sacral fracture ( Fig 1.9-6 ), whereas frontal collisions tend to lead to acetabular fractures. Rare mechanisms of injury are indirect dislocation mechanisms, such as severe hyperflexion injuries (suicidal jumper′s fracture) [49].

7 Patient selection and indications

7.1 Patient selection

Besides the standard clinical examination of patients, radiographic diagnostics includes at least an AP view of the pelvis [8, 49–51]. The diagnosis of a sacral fracture can be evaluated in 88–94% of cases with the standard AP pelvis; however, the amount and direction of displacement cannot be judged. Therefore, if an injury to the posterior pelvic ring is suspected, then the standard pelvic AP x-ray should be complemented by inlet and outlet views and computed tomographic (CT) scan ( Fig 1.9-7 ), as described in Chapter 1.5 [52]. The inlet view allows analysis of an AP displacement and rotation, whereas the outlet view shows craniocaudal displacements and superior rotation. Important markers that may indicate the presence of sacral fractures are the interruptions of the arcuate lines of the sacrum, a fractured transverse process of L5, and avulsion fractures of the sacrospinous and sacrotuberous ligaments ( Fig 1.9-8 ) [8, 49]. The true lateral view of the sacrum is indicated in displaced transverse fractures as well as in suicidal jumper′s fractures for analyzing the amount of displacement in the sagittal plane.

The CT scan provides definitive information to detect even minor injuries to the sacrum [49, 50, 52]. In many cases of sacral fractures only the detailed analysis of the CT scan allows the division of the posterior pelvic injuries into the rotational group (AO/OTA Fracture and Dislocation Classification type B, partial posterior stability) and the translational group (AO/OTA Fracture and Dislocation Classification type C, no posterior stability) [53]. The CT scan allows the analysis of the detailed fracture pattern, the detection of fragments compromising neurological structures, and zones of fragmentation. The size of the sacral fragments can be evaluated for planning of the internal fixation. A CT scan, especially with frontal and sagittal reformations, thus is recommended in every sacral fracture. The 3-D CT reconstruction gives a summary of the radiographic diagnostics.

The diagnosis has to be completed by neurological examination as early as possible. The quality of clinical examination often is limited in the early phase because of the polytrauma situation. Thus, the risk of neurological damage can be predicted only indirectly by the grade of pelvic instability, fracture pattern, and nature of the fracture line (fragmentation, fragments close to nerve roots) [15].

**Fig 1.9-6a–d** a Trauma mechanism and sacral fracture. b Typical lateral compression injury mechanism. The reconstruction of the car deformation shows the extended impression zone near the front seat. **c–d** The x-ray and computed tomographic scan reveal the complete transforaminal compression fracture of the sacrum (type C). (From: Tscherne H, Pohlemann T, eds. *Unfall/chirurgie Becken und Acetabulum.* Heidelberg: Springer-Verlag, 1998, with permission.)

**Fig 1.9-7a–f** Radiological diagnostics in suspected sacral fractures. The pelvic AP x-ray (a) is useful in the polytrauma situation but added views are needed. The inlet view (b), outlet view (c), and 2-D computed tomographic (CT) scan (d) are the standard views. Frontal and sagittal reformations (e) and 3-D CT scan (f) are desirable (see Chapter 1.5).

**Fig 1.9-8** Typical signs indicating a sacral fracture, a fractured transverse process of L5 (top arrow); interruptions of the arcuate lines of the sacrum (middle large and two smaller arrows); and avulsion fracture of the sacrospinous and/or sacrotuberous ligaments (bottom arrow).

Concomitant nerve injuries are critical to detect because of their frequency and resultant functional disabilities. The nerve damage usually is caused by direct compression by fragments or results from overstretching. Therefore, nerve structures can be damaged to a greater or lesser extent than the severity of the fracture. Generally, all muscles and skin areas below the knee are innervated by the L4–S2 nerve roots and injury can be diagnosed by clinical examination. The damage of S3–5 nerve roots needs special attention. Clinically, they present as disturbances of the genitourinary system and sexual function or as sensory disturbances in the perineal region. The role of a rectal examination, in particular active control of the sphincter muscle, is extremely important as it may inform the surgeon of injury to these nerve roots.

In large series of sacral fractures [14, 15], the incidence of accompanying nerve damage ranged from 21–60%, depending on the sacral fracture type. The analyses of our own series showed some differences from Denis et al observations [14]. The rate of neurological damage was primarily related to the amount of pelvic ring instability (AO/OTA Fracture and Dislocation Classification of pelvic fractures) [28], and secondarily to the specific fracture pattern in the sacrum (zones I–III) [15]. Stable type A pelvic fractures generally did not show neurological failures, whereas in rotationally unstable type B fractures nerve damage was observed in < 10% in each sacral fracture zone (Denis I, II, and III). Completely unstable (type C) fractures showed the highest rates of additional nerve injury, with an increase of 32.6% in zone I (lateral), 42.9% in zone II (foraminal), and 63.6% in zone III (central). Bilateral sacral fractures presented the highest rates of nerve damage.

Additionally, the type of neurological deficit depends on the specific pattern of the fracture line. Unilateral vertical fractures may involve the sacral roots of one side and preserve normal bladder and bowel function unless the S1 root is involved. Minimal sensory loss usually is difficult to detect.

Transforaminal displacement, which often is indicated by a fracture of the L5 transverse process, can lead to injuries to the L5 root (“far-out syndrome,” usually leading to a foot drop). The neurological disturbances in the central fracture pattern include bowel, bladder, and sexual dysfunction. The amount of dysfunction depends on the involvement of the different sacral roots. The preservation of at least one of the two S2 and S3 roots might result in the absence of functional incontinence. Even if the malfunction is discrete and often stays undiagnosed in the initial and early evaluation, it can lead to a significant reduction of the patient′s quality of life.

The following observations were made in a recent analysis of 600 sacral fractures in the Hannover series [53]:

Patients with coccyx fractures, sacrococcygeal dislocations, or nondisplaced transverse fractures of the sacrum below S2 level showed no nerve injuries.
Two of seven patients with displaced transverse sacral fractures below S2 level had sacral fracture–related nerve injuries. Both required open reduction and sacral nerve root decompression, with incomplete recovery of their nerve deficit.
Four of 307 patients with a type B fracture had a sacral fracture—related lesion of the lumbosacral plexus; all occurred after a transforaminal fracture.
Sacral fracture—related nerve injury was present in 36.5% of neurologically evaluated patients after type C sacral fractures.
The incidence of a nerve deficit was 11.1% after transalar, 28.9% after transforaminal, and 53.8% after central fractures in type C injuries.
Bilateral type C fractures had the highest incidence of nerve deficits (79.2%).
There was a significant correlation of sacral fracture displacement and the presence of a lumbosacral plexus injury in type C fractures (threshold: 5 mm, 22% versus 38%).
A fractured L5 transverse process in type C fractures was a significant indicator for a lumbosacral plexus lesion (25% versus 39%).

7.2 Indications

The indication for operative or nonoperative treatment is based on the grade of instability of the pelvis, the fracture displacement, and the probable additional sacral fracture–related nerve injury.

Type A fractures of the sacrum (eg, transverse fractures below the S2 level or injuries of the coccyx) are predominantly treated nonoperatively because they are normally nondisplaced fracture types. Indications for operative treatment are severe displacement disturbing the rectum or anal region, and lesions of the lower sacral nerve roots in displaced transverse fractures. Nonoperative treatment consists of pain-dependent weight bearing and analgesics.

Specific stabilization of the sacrum usually is not performed if the sacral fracture represents the posterior part of a rotational injury (type B), most frequently a lateral compression injury. A distracting external fixator sometimes is indicated in the presence of a narrowing of the plane of the pelvic brim by lateral compression or internal rotation. For the anteroposterior/open book injury, the anatomy of the pelvic ring is sufficiently restored by anterior internal (eg, symphysis plate) or external fixation. A decompression of the central canal is indicated in the rare case of concomitant neurological injury caused by nerve root compression from fragments. The specific fracture pattern can be analyzed clearly on CT scan (see Chapter 1.8.4).

Internal fixation of the sacrum is indicated if the sacral fracture represents the posterior part of a completely unstable injury (type C pelvic instability). A minimally displaced sacral fracture line associated with a fracture of the L5 transverse process and/or an avulsion of the sacrotuberous or sacrospinous ligaments is highly suspicious for type C pelvic instability. The risk of a secondary displacement of the fracture under functional treatment is high if a complete anteroposterior sacral fracture line exits through Denis zones II and III injuries because no posterior pelvic ligaments are crossing this specific anatomical region. Unstable type C fractures of the sacrum should be posteriorly stabilized by open or closed techniques, especially when combined with lesions of the lumbosacral plexus or fragments compromising the lumbosacral nerve roots. Nerve root decompression becomes mandatory because early results show at least a partial recovery (see Chapter 1.8.6) [54].

A special fracture type is the “suicidal jumper′s fracture,” with a U- or H-shaped fracture of the upper sacrum and transverse fracture line, normally at the level of S1/2, without an anterior pelvic ring lesion in most cases. These fracture types generally show an anterior rotational deformity with the first sacral spine anterior to the lower sacrum and have a high incidence of additional lumbosacral plexus lesions. Therefore, reduction and nerve root decompression may be indicated.

Poor general health and severe osteoporosis of the pelvis are contraindications for open operative treatment.

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1.9 Sacral fractures