2.1 Anterior

The superior and inferior pubic rami act as an anterior strut to maintain the shape and stability of the pelvic ring against physiological and applied forces. Although congenital or traumatic absence of the anterior structures has little effect on pelvic stability in some people, these cases are exceptions. In the acutely injured pelvis, anterior fractures significantly affect pelvic stability. Likewise, injuries to the pubic symphysis, a strong ligamentous and cartilaginous structure that withstands tension (external rotation) and shear forces at the anterior pelvis, contribute to significant instability in some acute injuries. But when injured in isolation, the disrupted pubic symphysis has little effect on pelvic ring stability.

2.2 Posterior

Pelvic stability depends on an intact posterior sacroiliac osseoligamentous complex. This complex is a well-designed biomechanical structure in that it can resist prolonged, powerful forces and can support weight-bearing forces from the spine to the lower extremities during most activities. Most normal forces applied to the pelvis tend to push the sacrum anteriorly and the innominate bones posteriorly. The shapes of the pelvic bones affect the way that they interact. On the inlet view, the sacrum appears wider anteriorly and is held in position by tension in the ligaments, with the strongest ligaments located posteriorly. On the outlet view, the sacrum appears as a classic “keystone” in which applied forces create compression between the bones, contributing to stability of the ring ( Fig 1.2-1 ). This complex interaction between the bones at the sacroiliac joint may have evolved to allow a small amount of motion (shock absorption) but not excessive motion.

The anterior sacroiliac ligaments are flat and strong, allowing them to resist external rotation and shearing forces at the sacroiliac joint. However, with weight bearing, the powerful posterior sacroiliac ligaments resist the majority of the load. The posterior sacroiliac ligamentous structure is similar to a suspension bridge, with the posterior sacroiliac spines acting as the towers, the sacrum as the bridge deck, and the posterior sacroiliac ligaments as the suspending cable ( Fig 1.2-2 ). The posterior complex is further strengthened by the iliolumbar ligaments, which connect the transverse processes of L5 to the posterior iliac crest, and by the fibers of the interosseous ligaments. Professor JCB Grant at the University of Toronto described the posterior sacroiliac ligament as the most powerful ligament in the body. When its function is understood, the reason for its strength becomes obvious.

The posterior sacroiliac ligaments work almost constantly to resist anterior displacement of the sacrum and axial skeleton relative to the innominate bones.

The caudally positioned sacrospinous and sacrotuberous ligaments and the pelvic floor augment pelvic stability. Major disruptions of the pelvic ring are associated with injury to these ligaments and to the pelvic floor.

3.1 Physiological instability: pregnancy

The pelvis expands during pregnancy and childbirth. The hormones of pregnancy are thought to cause significant relaxation of the pelvic ligaments; however, it appears that this effect is relatively small. Bjorkland et al [2] performed ultrasound analysis of the pelvis during delivery, reporting that diastasis of the pubic symphysis during labor and delivery is normally small (approximately 1 mm). Others report that postpartum pain does not correlate with symphyseal distention for these normally small displacements [3].

Although rare, traumatic disruption of the pelvis can occur during labor and delivery. This is reported between 1 in 600 and 1 in 30,000 live births. These typically are partially unstable pelvic injuries in which the posterior sacroiliac ligaments remain intact. The symphysis ruptures, sometimes with an audible crack. The pain can be disabling, although a pelvic binder may provide some symptomatic relief [4]. Symptoms usually, but not always, resolve spontaneously over the course of several months. The dilemma in treating these patients is that the natural history of the injury with and without surgical stabilization is unknown. Also, many women are reluctant to refrain from weight bearing after delivery because they prioritize the care of their baby over the healing of their pelvic injury, and walk on the injury before it has healed. Thus, early healing of ligamentous injuries is compromised regardless of whether surgical stabilization is performed.

3.2 Iatrogenic instability: posterior bone graft

When bone graft is harvested from the posterior iliac crest, instability can result from one of two mechanisms. Aggressive harvesting techniques may compromise the posterior interosseous ligaments, causing pelvic instability. Removal of iliac bone also weakens the pelvic ring so that a stress fracture occurs, and lack of bone in the harvested and fractured area compromises the ability of the stress fracture to heal. Chronic pain from bone graft sites may be due in part to recurrent stress fractures [5].

3.3 Division of specific ligaments

More than 50 years ago, Pennal and Sutherland [6] conducted pioneering studies on the contributions of specific ligaments to pelvic stability. In a film created for the American Academy of Orthopaedic Surgeons in 1961, they demonstrated how selectively cutting specific ligaments affected pelvic stability [6]. The data from these experiments have become the basis for our current understanding of these pelvic ligaments.

Pennal and Sutherland [6] also showed that transection of the pubic symphysis alone allowed motion between the pubic bodies, but the intact posterior structures (ie, the sacrospinous and anterior sacroiliac ligaments) prevented symphyseal diastasis of more than 2.5 cm. However, when the anterior sacroiliac ligaments were sectioned, the pelvis opened like a book with the intact posterior sacroiliac ligaments simulating the binding of the book. Pubic symphyseal diastasis increased without obstruction until the posterior superior iliac spines abutted the sacrum. In this scenario, vertical translation at the sacroiliac joint was not possible due to the intact sacrotuberous and posterior sacroiliac ligaments. When these remaining structures were transected, the entire hemipelvis became unstable.

Later work from Tile′s laboratory at Sunnybrook Health Sciences Centre demonstrated that factors other than the integrity of the ligaments affect pelvic stability. In particular, the method of loading and the order in which ligaments are sacrificed influence the way the pelvis responds to force [7]. In single-leg stance, the anterior pelvis is in compression, and the posterior pelvis is in tension and shear. Thus, cutting the pubic symphysis alone results in little instability to the pelvis when it is loaded in single-leg stance. Transecting the posterior ligaments allows large displacements at the sacroiliac joint. On the contrary, in double-leg stance, the anterior pelvis is in tension, and the posterior pelvis is in relative compression. If the anterior pelvic ligaments are cut, the pelvis opens dramatically. These simple examples illustrate only the contributions of the pubic symphysis and the posterior ligaments. As other ligamentous and soft-tissue contributions to pelvic stability are considered, the details become more complex. The contributions of all these stabilizing structures are equally important in allowing individuals to maintain active lifestyles.

These experiments and others demonstrate that although the posterior ligamentous structures are more important in maintaining pelvic stability, the anterior structures also play a critical role in maintaining pelvic stability. For example, in double-leg stance, approximately 60% of stability comes from the posterior structures and 40% comes from the anterior structures. Understanding these concepts is essential to properly treat patients with completely unstable pelvic injuries. Fixation of the posterior injury alone does not address the entire problem of instability; excessive motion will occur resulting in pain, potential disruption of healing, and a greater chance of malunion [8].

3.4 Division of the pelvic ring structure

The ring-like arrangement of the three pelvic bones contributes strength and stability to the structure. The ring structure also makes a fracture in a single location highly unusual. Gertzbein and Chenoweth [9] obtained bone scans of patients with minimally displaced pubic ramus fractures, noting that all patients had areas of increased activity posteriorly indicative of sacral or posterior iliac fractures. Diagnostic computed tomographic (CT) scans of the pelvis are now commonly obtained in patients with known or suspected pelvic fractures, allowing earlier identification of these “occult” fractures. Small sacral buckle fractures are often detected on CT scans but not on initial plain x-rays. The increasing prevalence of osteopenia in patients makes detection of the second fracture of the pelvic ring even more challenging. Thus, patients with an anterior pelvis fracture also should be assumed to have a concomitant posterior fracture. The presence of the posterior fracture can be proven by CT scan, if needed.

4.1 Anteroposterior compression/external rotation

Anteroposterior compression tends to open the pelvis like a book. The most common mechanism of injury is direct pressure to the anterior ilia, which may occur when a patient is struck from the front by a truck. This force causes external rotation of the ilia until the pubic symphysis fails in tension. Continued application of the force results in failure of one or both anterior sacroiliac ligaments, which may be followed by disruption of the posterior sacroiliac ligaments.

Another common mechanism is external rotation through the femur. This may occur when a motorcyclist strikes a stationary object with the medial thigh. The femur acts as a lever and disrupts the symphysis anteriorly, which may disrupt the anterior and posterior sacroiliac ligaments on one or both sides.

Although it may seem counterintuitive, a direct posterior blow to the pelvis may disrupt the pubic symphysis in tension. A posterior force applied to the posterior superior iliac spines externally rotates the ilia, which may occur, for example, when a patient has fallen directly posteriorly from an elevated hunting stand.

In each of these examples, the injury force may result in partially or completely unstable pelvic injuries, depending on whether the posterior sacroiliac ligaments are disrupted. When the posterior sacroiliac ligaments are intact, the pelvis may open like a book but vertical displacement of the pelvis will not be possible.

4.2 Lateral compression

Lateral compression forces push one or both sides of the pelvis toward the midline, causing the pelvis to collapse. The force may be applied to the iliac crest, the greater trochanter, or both. Transverse acetabular fractures often are associated with these injuries. A purely lateral compression force to the posterior pelvis usually results in failure of the bone in compression—typically a sacral ala fracture. The posterior ligaments remain intact if the force is purely lateral compression. The anterior structures of the pelvic ring fail in shear, most commonly as bilateral rami fractures, although unilateral injuries and pubic symphyseal disruptions may occur alone or in combination.

Although the most common posterior injury attributed to a lateral compression force is a buckle fracture of one or both sacral ala, the sacrum sometimes will compress anteriorly and the posterior sacroiliac ligaments and the interosseous sacroiliac ligaments will fail in tension, resulting in a completely unstable pelvic injury. This may be due in part to the fact that the applied force is not purely lateral compression.

Even when the posterior sacroiliac ligaments remain intact, a significant lateral compression injury that is not reduced and stabilized can result in inward and upward rotation of the involved hemipelvis, with an apparent leg-length discrepancy from the pelvic asymmetry.

Basic patterns of injury often reflect the mechanical properties of the body parts. For instance, in young healthy people, the bone of the anterior sacrum may be strong enough to resist fracture when a lateral compression force is applied, whereas the posterior sacroiliac ligaments will fail in tension. This example is analogous to lateral forces to the knee, which in elderly patients will cause depression fractures of the lateral tibial plateau but in young athletes may cause serious medial collateral and cruciate ligament disruptions without significant fracture. Thus, the consequences to the pelvis of a lateral compression force may differ, depending on the relative strengths of the bone and ligamentous structures.

4.3 Shear mechanism

A shearing force crosses perpendicular to the main trabecular pattern of the posterior pelvic complex in the sagittal plane and usually is directed posteriorly or superiorly. Unlike lateral compression forces, which generally cause impaction of the cancellous bone without disruption of the ligaments, shearing injuries generally cause marked displacement of the bone and gross disruption of the soft-tissue structures. With continued application of shearing forces, the soft tissues fail, resulting in an unstable pelvic ring injury. Instability is present anteriorly and posteriorly, and the amount of possible displacement will have no finite endpoint.

Severe vertical shear injuries can result in massive disruptions of all stabilizing structures, resulting in open pelvic injuries or traumatic hemipelvectomies [10–21]. A common mechanism of injury is a fall from a height in which the patient strikes a structure with one side of the lower body. The inferior aspects of the pubic symphysis and the sacroiliac joints are loaded first, with successive disruption of the ligaments, followed by ripping of the pelvic floor muscles and ligaments including the iliopsoas.

The mechanisms of injury described here are simplifications of forces applied in a single, pure direction. In reality, the direction is rarely so pure. Thus, unusual patterns of injury will be seen and can result from combinations of injury patterns.

4.4 Effect of these force patterns on soft tissue and viscera

The traumatic forces that injure the bony pelvis also cause damage to the contents of the pelvis, specifically the arteries, viscera, veins, nerves, and muscles. Forces of external rotation or shear tend to tear these soft-tissue structures of the pelvis. These forces result in traction injuries to the vessels, viscera, and nerves, with consequent avulsion injuries or visceral tearing [1].

Lateral compression injuries can cause visceral damage by bony penetration. The inward rotation of a hemipelvis during a lateral compression injury may drive a sharp bony spike from the superior pubic ramus into the bladder, or the consequent increased intravesicular pressure from the force may cause bladder rupture. Posterior compression of the sacrum can result in transforaminal sacral fractures (Denis type II) that cause direct compression of sacral nerve roots.

5.1 Review of stability

Stable pelvis fractures (type A) do not displace significantly with physiological loading. Patients with these injuries often require protective weight bearing and limited activity until significant fracture healing has occurred. With proper protection from forces, such as weight bearing, these fractures heal properly without operative stabilization.

Partially unstable injuries (type B) are characterized by significant rotational instability of the pelvic ring, with sufficient residual ligamentous stability to prevent vertical displacement of the posterior pelvis. These injuries, especially open book (type B1) fractures, benefit from operative stabilization of the anterior pelvic ring. If satisfactory reduction and stabilization of the anterior pelvic ring is achieved, both the anterior and posterior pelvic injuries will remain reduced until the injury has healed.

Vertically unstable (type C) pelvic injuries are characterized by disruption of both the anterior and the posterior pelvic ring and require more complex fixation to maintain reduction. Stabilization of the anterior pelvis can be achieved with internal or external fixation, but stabilization of the posterior pelvis requires internal fixation. Several techniques can be used to achieve pelvic stability but understanding the biomechanics of the injury and having experience with a variety of fixation techniques is essential to select the best treatment for a particular patient [22].

5.2 Evaluation of biomechanical studies of pelvic fixation

Numerous biomechanical studies have evaluated methods of stabilizing the unstable pelvis. Surgeons must understand the seemingly small, but significant, differences in design among studies. These differences may limit the valid extrapolation of in vitro mechanical data to the treatment of patients in vivo. Furthermore, comparison of results from studies with different designs may not be valid.

Studies may differ by type of bone or bone composite used, the injury pattern created for testing, the means by which load is applied to the pelvis, and the means by which displacement is measured. Each factor should influence data interpretation.

5.3 Type of bone used

Most pelvic studies use bone from anatomical specimen for testing, which is commonly obtained from elderly persons who donate their bodies to medical science. Thus, the bone tends to be osteoporotic and weaker than that typical of trauma patients. The strength of osteoporotic bone obtained from osteoporotic anatomical specimen also varies widely. Consequently, the strength or stability of fixation achieved with such bone is highly variable, making it more difficult to demonstrate that an observed difference in fixation strength or stability is statistically significant.

Conversely, pelvic bone obtained from organ donors often comes from younger patients who have healthier bone. These specimens correlate better with bone of trauma patients. With these studies the reverse caveat is true: high fixation strength obtained in vitro with strong bone from anatomical specimen must not be extrapolated to the clinical situation for surgically treating an elderly, osteoporotic person with an unstable pelvic fracture.

When used for mechanical testing, composite bones or sawbones have the advantage that they are less expensive and less variable in measured strength and stiffness. Thus, demonstrating that an observed difference in fixation strength or stability is statistically significant becomes easier. The way that artificial bone models fail can be dissimilar from the way that human bone fails. Thus, use of artificial bone can be an acceptable means by which to measure stiffness (ie, ratio of applied load to displacement in response to that load) of a fixation construct, but it is a less reliable means by which to compare fixation strength (ie, load at which failure occurs).