1.3 Pathoanatomy, mechanisms of injury, and classification

1 Introduction

Our knowledge of the pathoanatomy of fractures of the pelvis, the mechanisms, and force vectors that produce these injuries continues to evolve. New developments have led to a better understanding of pelvic ring fractures and with that more useful and effective classification schemes. This chapter reviews the current concepts of the pathoanatomy of fractures of the pelvic ring. It describes the common mechanisms that lead to fractures of the pelvis and reviews the basis of fracture classification for pelvic ring injuries.

2 Pathoanatomy

The osseous and ligamentous tissues of the pelvis form a true ring. For a true ring to be disrupted and displaced, the ring must fail in at least two locations. This fact was not always apparent in reference to pelvic ring fractures. In the past, when plain x-rays were the only imaging available for assessing pelvic fractures, injuries in the anterior portion of the pelvis were often obvious, but coexisting injuries to the posterior portion of the ring often could not be seen. In 1977, Gertzbein and Chenoweth [1] assessed patients with relatively undisplaced pubic rami fractures using technetium bone scans. They found increased uptake of the radiotracer in the posterior elements of the pelvis in all cases. This study confirmed that if the pelvic ring is broken in one area, a second lesion is usually present at another site in the ring. In 1981, further confirmation of the pathoanatomy came from an autopsy study that demonstrated that pelvic fractures with a displaced anterior lesion almost always had an associated posterior pelvic ring fracture [2]. Bucholz [2] reported the autopsy findings of 32 patients, who underwent plain x-rays and postmortem examinations. Group I (14 human anatomical specimens) had no evident posterior lesion on plain x-rays, but all had an undisplaced posterior lesion of some kind: either a vertical fracture of the sacrum, or a disruption of the anterior sacroiliac ligaments. In group II (five human anatomical specimens), x-rays indicated failure of the anterior sacroiliac ligaments but preservation of posterior ligamentous structures, which was confirmed at autopsy. Group III (eleven human anatomical specimens) had radiographic and autopsy evidence of complete posterior ligamentous disruption, allowing a high degree of multiplanar displacement.

As the surgical treatment of these injuries has evolved, and open reductions have become more common, so has our understanding of the different methods of failure of both the posterior and anterior pelvic ring. This cumulative evidence has confirmed the theories of those who initially conceived these injury patterns.

The near universal use of computed tomography (CT) in the workup of patients with pelvic fractures has made the identification of these concomitant anterior and posterior lesions much easier for the clinician. A CT scan almost universally identifies a posterior pelvic injury, even in the lowest energy pelvic fractures [3] ( Fig 1.3-1 ). Occasionally magnetic resonance imaging provided evidence of soft-tissue injury, but the gold standard for determining soft-tissue injury remains displacement on plain x-ray and CT, as well as dynamic examination. It has been established that for ligamentous injuries in the posterior portion of the pelvis, the pelvic floor ligaments (sacrotuberous and sacrospinous) are torn along with the anterior sacroiliac ligaments. In the most severe injury patterns the posterior iliosacral ligaments may be also torn [2, 4].

The key issue is that in almost all pelvic ring fractures, the ring is disrupted in at least two places: an anterior injury and a posterior injury. There may be a few rare injuries when the disruption is isolated to either the anterior or the posterior part of the ring. However, it is always best to assume there are both anterior and posterior lesions. The injury may be stable, but even so, there is usually a failure in both the anterior and posterior aspects of the pelvic ring.

2.1 Anterior injuries

The site of the anterior lesion may be anywhere in the pelvic ring anterior to the acetabulum. Fractures may occur in the pubic rami or in the body of the pubis. A purely ligamentous injury may occur as a diastasis of the symphysis pubis. Injuries in the anterior part of the pelvic ring can combine bone and ligamentous injuries and may occur unilaterally or bilaterally ( Fig 1.3-2 ).

**Fig 1.3-1a–b** a Pelvic ring disruption with clear minimally displaced bilateral superior and inferior rami fractures, but no obvious posterior injury. b A xial CT scan shows compression failure of the left side of the sacrum.

**Fig 1.3-2a–c** Anterior ring injuries can occur through: a The symphysis. b The rami. c A combination of both the symphysis or rami.

2.2 Posterior injuries

Posteriorly, again the lesion may be unilateral or bilateral. It may be an osseous injury, a ligamentous injury, or a combination of both. Ligamentous injuries involve the sacroiliac joint and always comprise a disruption of the anterior sacroiliac ligaments. In completely unstable injuries of the sacroiliac joint, the posterior sacroiliac ligaments are also involved.

Fractures can involve the sacrum or the ilium. Sacral fractures can occur in various patterns, described later in Chapter 1.9, and most commonly are vertical but can be medial, transforaminal, or lateral to the neural foramina. Less commonly, these injuries can be bilateral, and/or have a horizontal fracture line in the sacrum. Iliac fractures can occur anterior to the sacroiliac joint and not involve the joint itself. Sacroiliac fracture dislocations are also common and can involve a variable amount of the ilium posteriorly, or less commonly, the anterior part of the sacrum ( Fig 1.3-3 ).

3 Mechanisms of injury

Typical fracture patterns develop depending on the direction of the force causing the injury. Orthopedic surgeon, George Pennal, first defined the primary force directions and the resulting fracture patterns [4, 5]. He noted that usually forces tend to open the pelvis like a book, collapse it toward the midline, or cause vertical translations. Pennal labels the forces that cause these deformations as anterior posterior compression (APC), lateral compression (LC), and vertical shear (VS). These labels remain a part of modern terminology and are useful in predicting general fracture patterns and concomitant instability. However, the term VS implies only one direction of displacement but when this type of injury occurs it allows the hemipelvis to displace in several directions, so the term “completely unstable” is more representative of the clinical situation. Analysis of a pelvic fracture pattern will provide information about the mechanism of injury. In reality, the actual forces that produce a disrupted pelvis are often complex. High-energy blunt trauma likely results in forces striking the pelvis from multiple directions. Although the fracture patterns give a clue as to the direction of the injurious force, they do not always completely elucidate the mechanism of injury.

**Fig 1.3-3a–d** Posterior ring injuries can occur through: a The sacroiliac joint. b The iliac wing. c The sacrum. d Both osseous and ligamentous structures (here the ilium) and the sacroiliac joint.

The consequences of the various force vectors can be described. It is important to remember that the pelvic ring may fail through the bone, through the ligaments, or through a combination of both. The effect on the pelvis will be the same regardless of whether the failure is in the bone, soft tissue, or through both.

3.1 Anterior posterior compression

The term APC is generally used to describe forces that open the pelvis like a book ( Fig 1.3-4 ). They are sometimes called external rotation forces and may be applied to the pelvis by several mechanisms.

3.1.1 Posterior crush

A direct blow to the posterior superior iliac spine can externally rotate one or both ilia, causing disruption of the anterior part of the pelvic ring and the pelvis to open like a book.

3.1.2 Direct pressure to the anterior superior iliac spine

An anteroposterior force located at the anterior superior iliac spine can externally rotate the ilia, causing disruption of the pubic symphysis or fracture of the pubic rami. With further force, the ligaments of the pelvic floor (sacrotuberous and sacrospinous ligaments) are disrupted along with the anterior sacroiliac ligaments. Further force will sequentially rupture the posterior sacroiliac ligaments. This type of fracture pattern may be exacerbated if it occurs in conjunction with an external rotation through the lower extremity.

In the APC injury pattern, displacement in the vertical plane and/or posterior plane will not occur if the posterior injury is ligamentous and if the posterior sacroiliac ligaments remain intact. The pelvis will be rotationally unstable and will open like a book either unilaterally or bilaterally. When the force overcomes the posterior sacroiliac ligaments the pelvis will also become completely unstable.

**Fig 1.3-4a–b** a A direct blow to the posterior superior iliac spines causes the symphysis pubis to spring open. b External rotation of the femora or direct compression against the anterior superior spines also causes disruption of the symphysis pubis, or equivalent fractures of the public rami.

3.2 Lateral compression

Lateral compression forces generally collapse the pelvis toward the midline ( Fig 1.3-5 ). The force may be applied directly to the iliac crest or to the greater trochanter and may also cause an associated acetabular fracture ( Fig 1.3-6 ).

If the bone is subjected to pure LC, with no shearing element, the posterior soft tissues (the pelvic floor and the sacroiliac ligaments) remain intact and the pelvic ring will maintain some degree of stability. A pure LC force applied to the pelvis does not usually result in vertical instability; however, displacements in the sagittal plane can be substantial. Notably, it is rare that the lateral force is so exactly applied as to cause pure horizontal collapse toward the midline. Most commonly there is an internal rotational component with the anterior pelvis rotating more than the posterior pelvis. There may be a collapse in an inward and upward direction. It is possible to have substantial lower extremity deformity (internal rotation and shortening) ( Fig 1.3-7 ).

The anterior lesion following a LC mechanism may be on the ipsilateral or contralateral side of the posterior injury, or all four rami may be broken. Usually, the rami fractures in this mechanism are relatively horizontally oriented as opposed to a more vertical orientation in injuries caused by other mechanisms ( Fig 1.3-8 ). Alternatively, the symphysis may be disrupted, or there may be a combination of symphysis disruption and rami fractures. The posterior injury may be in the sacrum and can vary from an incomplete anterior sacral impaction (buckle) through to a complete sacral fracture. In high-energy LC injuries of the pelvis, there can be significant comminution in the anterior and even posterior sacrum [3] ( Fig 1.3-9 ).

**Fig 1.3-5a–c** a A lateral compressive force directed against the iliac crest causes the hemipelvis to rotate internally, crushing the anterior sacrum and displacing the anterior pubic rami. b Lateral compression injury also may be caused by a force against the greater trochanter. In that situation the femoral head acts as a battering ram, dividing the pubic rami as shown, often through the anterior column of the acetabulum. The ipsilateral sacroiliac complex is also crushed in this injury. c Lateral compressive forces are directed parallel to the trabeculae of the sacroiliac joint, therefore causing impaction of the bone. If the force is purely lateral compression, the pelvic floor—including its major ligaments, the sacrospinous and sacrotuberous—remains intact, thus retaining pelvic ring vertical stability. The stability is maintained by the pelvic floor even if the posterior ligamentous structures rupture. In most cases of lateral compressive force the posterior structures remain intact.

**Fig 1.3-6** Pelvic ring disruption with concomitant transverse acetabulum fracture.

**Fig 1.3-7a–b** High-energy lateral compression injury has shortening, and internal rotation, visible both on the x-ray and clinical examination. a X-ray. b Clinical examination.

**Fig 1.3-8a–b** a Compression injuries cause more oblique rami fractures. b Vertical shear injuries cause more vertical rami fractures.

**Fig 1.3-9a–e** Variable severity of posterior sacral compression failure. a Anterior buckle fracture. b Simple anterior fracture. c Simple posterior fracture. d Comminuted anterior fracture (simple posterior fracture also visible on this cut). e Comminuted posterior fracture (comminuted anterior fracture also visible on this cut).

In some injuries the anterior sacrum may be crushed; the posterior ligamentous structures, including the posterior sacroiliac ligaments, may be ruptured. The posterior lesion is, in some cases, confined to the ilium just anterior to the sacroiliac joint and sometimes is a fracture dislocation of the sacroiliac joint with the fracture in the ilium being of varying size, producing the so-called crescent fracture ( Fig 1.3-10 ).

In younger people with strong cancellous bone, the posterior injury may be a major disruption of the posterior ligaments ( Fig 1.3-11 ). The same force direction in a person with weaker bone may produce a compression fracture in the posterior ring.

3.3 Completely unstable injury (vertical shear)

A shearing force is one that crosses perpendicular to the main trabecular pattern of the pelvis in a vertical or posterior direction ( Fig 1.3-12 ). Shearing forces cause marked displacement of the bone and gross disruption of the soft-tissue structures. Complete displacement of the disrupted hemipelvis occurs proximally (vertically) and posteriorly. In the extreme case, the entire hemipelvis may be avulsed, resulting in traumatic hemipelvectomy ( Fig 1.3-13 ).

In clinical practice, the mechanism of injury is not always purely APC, LC, or VS, but commonly is a combination of these force directions.

**Fig 1.3-10a–b** Lateral compression injury with failure through the ilium (crescent fracture). a Plain x-ray. b A computed tomographic scan.

**Fig 1.3-11** In some instances, a lateral compressive force may rupture the posterior ligamentous structures, especially in younger patients, where the bone sacroiliac complex is extremely strong. Therefore, the internal rotational force will tend to rupture the sacroiliac ligaments posteriorly rather than fracture the strong bone. However, in such cases pelvic stability may be maintained by the intact pelvic floor ligaments.

**Fig 1.3-12a–b** A shearing force (arrows) crosses perpendicular to the main trabecular pattern of the posterior pelvic complex in the vertical plane. These forces cause marked displacement of bone and gross disruption of the soft tissues, including the pelvic floor, resulting in major pelvic instability.

**Fig 1.3-13a–b** Outlet and inlet views. a Vertical displacement on the outlet view. b Posterior displacement on the inlet view, typical of a vertical shear injury.

4 Classification

4.1 Defining injury types

To define an injury, a surgeon must carefully assess both the patient and injury, which includes the degree of displacement, stability of the pelvic ring, direction of force, condition of the soft tissues, anatomy of the injury, and associated injuries.

Overall, classification systems should perform various functions and be useful for both clinicians and researchers. In both clinical care and in research, a good classification system is a tool for communication. A fracture classification system should have good intraobserver and interobserver reliability. There should be clear clinical relevance in terms of treatment, prognosis, or risk of complication. Ideally, a classification system is all inclusive and mutually exclusive (a given fracture should belong in only one category). Finally a classification system is logical, understandable, and relatively simple [6].

Many systems have been used for classification of pelvic ring fractures. The main themes of pelvic ring fracture classification have concentrated on anatomy, mechanism of injury, or stability.

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1.3 Pathoanatomy, mechanisms of injury, and classification