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Bony Trauma 1: Pelvic Ring 217 14 Bony Trauma 1: Pelvic Ring Philip Hughes CONTENTS 14.1 14.2 14.2.1 14.2.2 14.2.3 14.2.4 14.2.5 14.2.6 14.2.7 14.3 14.3.1 14.3.2 14.3.3 14.3.4 14.3.5 14.3.6 14.4 14.5 Introduction 217 Pelvic Ring Fractures 217 Anatomy 217 Techniques 218 Classification of Pelvic Fractures 218 Force Vector Classification of Pelvic Ring Injury 219 Pelvic Stability 224 Diagnostic Accuracy of Plain Film and Computed Tomography in Identification of Pelvic Fractures 224 Risk Analysis and the Force Vector Classification 224 Acetabular Fractures 224 Acetabular Anatomy 225 Radiographic Anatomy 225 Classification 227 Basic Patterns 227 Complex or Associated Fracture Patterns 229 Relative Accuracy of the AP Radiograph, Oblique Radiographs and Computed Tomography 230 Avulsion Fractures 233 Conclusion 234 References 235 14.1 Introduction Major pelvic ring and acetabular fractures are predominantly high energy injuries and consequently are not infrequently associated with injury to the pelvic viscera and vascular structures. Mortality and morbidity related to these injuries primarily results from haemorrhage, the outcomes have however improved through the use of external fixation devices and other compression devices. Recognition of the type and severity of injuries, particularly those involving the pelvic ring, is essential to the application of corrective forces during external or internal fixation techniques. The pattern and severity of injury also predict the probability of pelvic P. Hughes, MD Consultant Radiologist, X-Ray Department West, Derriford Hospital, Derriford Road, Plymouth, PL6 8DH, UK haemorrhage and visceral injury which can prove influential when assessing the likely site of haemorrhage and the appropriateness of further cross-sectional imaging or operative intervention. Acetabular fractures can be classified into simple and complex patterns which require a thorough understanding of the regional anatomy and the associated radiological correlates. The patterns of fracture determine the operative approach and although predominantly determined by plain film views (AP and Judet obliques) are often supplemented by CT (2D, MPR and 3D surface reconstructions). CT is also required to identify intra-articular fragments that are not usually identifiable on plain films and secondly to assess postoperative alignment of articular surfaces. MR may also be performed following femoral head dislocations or acetabular fracturedislocations where viability of the femoral head is questioned and would alter management. The final group exhibiting a distinctive pattern of pelvic fractures to be considered include avulsion injuries which are encountered predominantly in individuals following sporting activity and are more frequent in the immature skeleton. Stress fractures and pathological fractures of the pelvis are covered in Chaps. 16 and 22, respectively. 14.2 Pelvic Ring Fractures 14.2.1 Anatomy The pelvic ring comprises the sacrum posteriorly and paired innominate bones, each formed by the bony fusion of the ilium, ischium and pubic bones, each having evolved from independent ossification centres. The sacrum and innominate bones meet at the sacroiliac articulations, and the pubic bones at the fibrous symphysis pubis. The integrity of the bony ring is preserved by ligaments, an apprecia- P. Hughes 218 tion of which is essential to the understanding of patterns of injury and the assessment of stability of injured pelvic ring. Anteriorly the symphysis is supported predominantly by the superior symphyseal ligaments (Fig. 14.1a). Posteriorly the sacroiliac joints are stabilised by the anterior and posterior sacroiliac ligaments (Fig. 14.1b). The posterior ligaments are amongst the strongest ligaments in the body, running from the posterior inferior and superior iliac spines to the sacral ridge. The superficial component of the posterior sacroiliac ligament runs inferiorly to blend with the sacrotuberous ligaments. The sacrospinous and sacroiliac ligaments support the pelvic floor and oppose the external rotation of the lilac blade. The iliolumbar ligaments extend from the transverse processes of the lower lumbar vertebrae to the superficial aspect of the anterior sacroiliac ligaments and can avulse transverse processes in association with pelvic fractures. Important arterial structures vulnerable to injury include the superior gluteal artery in the sciatic notch which may be disrupted by shearing forces exerted during sacroiliac joint diastasis. The obturator and pudendal arteries are not uncommonly injured during lateral compression injuries resulting in comminution of the anterior pubic arch. Other commonly injured vessels include the median and lateral sacral, and iliolumbar arteries. Urogenital injuries are also commonly associated with pelvic ring injury consequent upon the close association of the urethra and symphysis and pubic rami and bladder. Anterior compression forces are more commonly responsible for urethral injury, usually affecting the fixed membranous portion of the urethra. 14.2.2 Techniques The AP pelvic radiograph is one of the three basic radiographs performed as part of the ATLS protocol in the setting of major trauma, the other radiographs including views of the cervical spine and chest. The AP views demonstrate the majority of pelvic fractures, excepting intra-articular fragments (Resnik et al. 1992). The pelvic inlet and outlet views supplement the AP view in pelvic ring fractures, the former demonstrating rotation of the pelvis, additional fractures of the pubic rami and compression fractures of the sacral margins while the latter assesses craniocaudal displacement particularly in vertical shear injuries. The widespread use of CT in trauma cases in general and its invariable use in pelvic fractures to assess both severity and requirement for operative fixation have essentially eliminated the requirement for inlet and outlet views. CT technique will vary with the type of scanner used but should include section thicknesses between 2.5–5.0 mm. The mAs can be reduced when the scan is purely performed for the purposes of bony anatomy from the standard around 120 mAs to 70 mAs. 14.2.3 Classification of Pelvic Fractures The classification of pelvic fractures has changed during the last two decades to more accurately reflect the mechanism of injury and quantify the degree of instability. Malgaine, straddle and openbook fractures, used as descriptive terms prior to the 1980s in most standard texts, failed to provide a b Fig. 14.1. a AP view of pelvic ligaments and (b) pelvic inlet perspective demonstrating anterior and posterior sacroiliac ligaments Bony Trauma 1: Pelvic Ring precise detail relating to pelvic injury and did not emphasise the importance of the unseen ligamentous structures. Penall et al. (1980) first described the correlation between the pattern of fracture and the direction of the applied traumatic force. They proposed the forced vector classification of pelvic fractures, identifying anteroposterior compression (AP), lateral compression (LC) and vertical shear as pure bred forces responsible for specific patterns of injury. Tile (1984) subsequently documented the high risk of pelvic haemorrhage particularly in injuries to the posterior pelvis and the advantage of this systematic classification when applying external fixation devices. Young et al. (1986) further refined the classification identifying a constant progression or pattern to pelvic injury within each vector group which was both easily remembered and more importantly accurately reflected the degree of instability based predominantly on the imaging appearances. Later studies also linked probability of pelvic haemorrhage and bladder injury to the pattern of fracture allowing an element of risk stratification to be undertaken in relation to haemodynamically unstable patients with pelvic injury (Ben-Menachem et al. 1991). 14.2.4 Force Vector Classification of Pelvic Ring Injury There are three primary vectors responsible for pelvic injuries, Young et al. (1986) identified an LC pattern in 57% of patients, AP compression in 15% and a vertical shear pattern in 7%. The remainder, 22%, demonstrated hybrid features as a result of oblique or combined multidirectional forces which are referred to as ‘complex’ fractures. 219 ments. The final phase if further force is applied is disruption of the posterior sacroiliac ligaments effectively detaching the innominate bone from the axial skeleton. The extent of posterior pelvic injury allows AP injuries to be stratified into one of three groups reflecting increasing severity and instability. 14.2.4.1.1 AP Type 1 This is the commonest type of AP compression injury, the impact of the trauma is confined to the anterior pubic arch and the posterior ligaments are intact. Radiographs demonstrate either fractures of the pubic rami which characteristically have a vertical orientation (Fig. 14.2) or alternatively disruption and widening of the symphysis. Integrity of the posterior ligaments restricts the symphyseal diastasis to less than 2.5 cm. Compression devices can however re-oppose the margins of a diastased symphysis, caution should therefore be exercised in ruling out injury on the basis of a normal AP radiograph without correlation to the clinical examination. In practice this eventuality occurs rarely. CT scans can occasionally over-estimate the extent of injury of a true type 1 injury by demonstrating minor widening of the anterior component of the sacroiliac joint, which it is postulated, results from stretching rather than disruption of the anterior sacroiliac ligaments (Young et al. 1986). These injuries are essentially stable and require non-operative management. 14.2.4.1.2 AP Type 2 These comprise anterior arch disruption as described above with additional diastasis of the anterior aspect 14.2.4.1 Anteroposterior Compression Injuries These injuries are commonly the result of head on road traffic accidents or compressive forces applied in the AP plain. The effect of this force is to externally rotate the pelvis, the posterior margin of the sacroiliac joint acting as the pivot. This force will initially result in fractures of the pubic rami or disruption of the symphysis and symphyseal ligaments. Progressive force will further externally rotate the pelvis disrupting the sacrotuberous, sacrospinous and anterior sacroiliac liga- Fig. 14.2. AP type 1 injury characterised by vertical fracture line in inferior pubic ramus typical of AP compression injury P. Hughes 220 of the sacroiliac joint space commonly referred to as an “open book” injury or “sprung pelvis”(Fig. 14.3). Sacroiliac diastasis is more accurately assessed by CT than plain film (Fig. 14.4). These injuries exhibit partial instability being stable to lateral compressive forces (internal rotation) but unstable to AP compressive forces (external rotation). 14.2.4.1.3 AP Type 3 This pattern of injury result in total sacroiliac joint disruption (Fig. 14.5). Features described in the less severe types 1 and 2 injuries are present but in addition the sacroiliac joint is widely diastased posteriorly as well as anteriorly due to the posterior sacroiliac ligament rupture (Fig. 14.6). The hemipelvis is unstable to all directions of force, and usually requires operative stabilisation. Variants on the type three pattern include preservation of the sacroiliac joint integrity at the expense of sacral or iliac fracture (Fig. 14.7). Complications of AP compression injuries include bladder rupture, usually intra-peritoneal type, which requires cystography for confirmation (Fig. 14.8) and vascular injury, particularly affecting the superior gluteal artery due to shear forces in the sciatic notch. the symphysis is disrupted and overlaps. Three types of LC fracture are recognised. 14.2.4.2.1 LC Type 1 This represents the least severe injury pattern and is sustained by lateral force applied over the posterior pelvis causing internal rotation of the innominate bone which pivots on the anterior margin of the sacroiliac joint (Fig. 14.9). Radiographic features include pubic rami fractures, which are oblique, segmental (Fig. 14.10), frequently comminuted and rarely overlapping (Fig. 14.11) in contrast to the vertical fractures of AP compression injuries. Compression fractures of the anterior margin of the sacrum 14.2.4.2 Lateral Compression Injuries The commonest pattern of pelvic injury is discussed in the review of Young et al. (1986). Most patients with this mechanism of injury demonstrate pubic rami fractures. Exceptions are encountered when Fig. 14.4. CT scan demonstrating AP type 2 injury (openbook). Diastasis of the anterior part of the left sacroiliac hinged on its posterior margin as the posterior sacroiliac ligament remains intact Fig. 14.3. AP type 2 injury Fig. 14.5. AP type 3 injury Bony Trauma 1: Pelvic Ring 221 a b Fig. 14.6a,b. a AP type 3 injury comprising wide diastasis of the symphysis (> 2.5 cm) and diastased sacroiliac joint (black arrows). b CT demonstrating AP type 3 injury, wide diastasis throughout right sacroiliac joint, anterior and posterior sacroiliac ligaments are disrupted Fig. 14.7. AP type 3 variant. Symphyseal diastasis, intact sacroiliac joints but midline sacral fracture (arrow) Fig. 14.9. LC type 1 Fig. 14.8. Cystogram demonstrating intraperitoneal bladder rupture. The compression device has reduced the pelvic diastasis, pelvic instability cannot be excluded by a normal radiograph Fig. 14.10. LC type 1 injury demonstrating oblique (black arrow) and buckle fracture (white arrow) indicative of lateral compression P. Hughes 222 Fig. 14.11. LC type 1 injury overlapping pubic rami Fig. 14.12. CT demonstrating LC type 1 injury, compression fracture of the anterior sacral margin (white arrow) are better demonstrated by CT than plain films (Fig. 14.12) (Resnik et al. 1992). These injuries have little resultant instability and do not require operative management. 14.2.4.2.2 LC Type 2 The lateral compressive force in type 2 injuries is usually applied more anteriorly (Fig. 14.13). The pubic rami injuries are as described for type 1 but as the pelvis internally rotates pivoting on the anterior margin of the sacroiliac joint the posterior sacroiliac ligaments are disrupted. An alternative outcome if the strong posterior ligaments remain intact is for the ilium to fracture. This latter pattern is referred to as a type 2a injury (Fig. 14.14) as it was the first recognised but in reality the posterior sacroiliac joint diastasis, type 2b injury (Fig. 14.15), is the more commonly encountered pattern. 14.2.4.2.3 LC Type 3 This pattern of injury often referred to as the “windswept” pelvis (Fig. 14.16), results from internal rotation on the side of impact and external rotation on the other, and is often the result of a roll-over injury. The associated ligamentous injury and radiographic features combine lateral compression injuries on one side and AP compression on the other, as described in the preceding text. Recognition of lateral compression injuries is important as external fixation devices and other methods of stabilisation tend to exert internal compressive forces that could exacerbate deformity and Fig. 14.13. LC type 2 increase the risk of progressive haemorrhage in this group. 14.2.4.3 Vertical Shear Vertical shear injuries are usually the result of a fall or jump from a great height but loads transmitted through the axial skeleton from impacts to the head and shoulders can have identical consequences. The injury is typically unilateral comprising symphyseal diastasis or anterior arch fracture and posterior disruption of the sacroiliac joint with cephalad displacement of the pelvis on the side of impact (Fig. 14.17). Variants include disruption of the sacroiliac joint opposite to the side of impact or fracture of the sacrum. Vertical shear injuries are invariably severe in that all ligaments are disrupted, the pelvis being totally unstable. There are no subcategories in this Bony Trauma 1: Pelvic Ring 223 Fig. 14.15. CT demonstrating avulsion fracture of the posterior ilium by the posterior sacroiliac ligament (LC type 2b injury) a b Fig. 14.14a,b. Pelvic radiograph (a) and CT scan (b) demonstrating LC type 2a injury. Oblique superior ramus fracture and iliac blade fracture on plain film (white and black arrows, respectively). CT demonstrates intact sacroiliac joint and fractured ilium injury type. Radiographs demonstrate ipsilateral or contralateral pubic rami fractures, which have a vertical orientation similar to that described in AP compression injuries. The sacroiliac joint is also disrupted but the main differentiating feature from AP injuries is cephalad displacement of the pelvis on the side of impact. Careful attention to the relative positions of the sacral arcuate lines and lower border of the sacroiliac joint is a good guide to malalignment. a b Fig. 14.16a,b. LC type 3 injury: Windswept pelvis. LC injury on side of impact (a) and AP injury on the “roll-over” side (b) 14.2.4.4 Complex Injuries Complex patterns are not uncommon and when reviewed the majority will demonstrate a predominate pattern usually an LC type. Recognition of the complexity is important as external fixation devices and operative intervention will have to apply the appropriate corrective forces. P. Hughes 224 Fig. 14.17. Vertical shear pattern of injury. Disrupted symphysis and sacroiliac joint (black arrows), lines drawn through sacral foramen and symphysis highlight the extent of cephalad displacement on the side of impact 14.2.5 Pelvic Stability Stability depends on integrity of the bony ring and supporting ligaments. Tile (1984) demonstrated that in AP compression disruption of the symphysis and its ligaments will allow up to 2.5 cm of diastasis. Widening of the symphysis by more than 2.5 cm is only achieved by disruption of the sacrotuberous, sacrospinous and anterior sacroiliac ligaments. Total pelvic instability only results if the posterior sacroiliac ligaments are also disrupted. It can be appreciated therefore that stability or more precisely instability of the pelvis represents a spectrum dependent on the extent of disruption of the bony ring and ligaments. A sequential graded pattern of instability also applies to lateral compression injuries 14.2.6 Diagnostic Accuracy of Plain Film and Computed Tomography in Identification of Pelvic Fractures Considerable variation exists in the accuracy of plain radiographic evaluation of pelvic fractures. A 6-year retrospective review identified that plain films failed to diagnose 29% of sacroiliac joint disruptions, 34% of vertical shear injuries, 57% of sacral lip fractures and 35% of sacral fractures (Montana et al. 1986). Computed tomography (CT) was used as the gold standard and considerably improved diag- nostic accuracy. When the films were re-reviewed by this group applying the force vector classification, with particular attention to sacral alignment and detail, their accuracy increased, the vertical shear injuries benefited most, accuracy of identification increasing to 93%. Resnik et al. (1992) prospectively evaluated a similar number of patients with pelvic fractures presenting over an 8-month period. In all, 160 fractures were identified in total with CT, of these only 9% were not identified prospectively. This group included sacroiliac joint diastasis, sacral lip fractures, iliac and pubic rami fractures, but all were subtle and none altered the management decision. Acetabular fractures were also evaluated, 80% of intra-articular fractures could not be identified on plain film indicating the essential requirement of CT in this subset of patients. These studies identify firstly the importance of an understandable system of classification as an adjunct to improving performance and secondly the benefits of regular exposure to pelvic trauma in the latter study, which improves familiarity with injury pattern and subtle signs associated with pelvic trauma. Plain films will always remain the initial assessment in the emergency room, and should allow most fractures to be appreciated. CT is essential preoperatively and should also be considered earlier in the diagnostic work-up if there are clinical doubts or if trauma exposure and expertise is limited. 14.2.7 Risk Analysis and the Force Vector Classification Ben-Menachem (1991) analysed the outcomes of patients with pelvic trauma. In type 1 injuries due to either lateral or AP compression the risk of severe haemorrhage was less than 5%. Conversely the risk of severe haemorrhage in the AP type 3 injury was 53%, 60% in LC type 3, 75% in vertical shear and 56% in complex injuries. This probability data, whilst not an absolute, enables an informed judgement on the likelihood of pelvic haemorrhage as an alternative to other visceral injury. 14.3 Acetabular Fractures Acetabular injuries have complex fracture lines and in order to accurately describe these injuries Bony Trauma 1: Pelvic Ring 225 according to the classification described by Judet et al. (1964) and Letournel (1980), a comprehensive understanding of the three-dimensional acetabular anatomy is required. It is inadequate to report an acetabular injury as “complex fracture as shown” as an accurate description using the aforementioned classification determines the requirement for surgery and the operative approach. 14.3.1 Acetabular Anatomy The acetabulum comprises two columns (posterior and anterior) and two walls (posterior and anterior) which are connected to the axial skeleton by the sciatic buttress (Fig. 14.18). The anterior column is long and comprises the superior pubic ramus continuing cephalad into the iliac blade. The posterior column is shorter and more vertical extending cephalad from the ischial tuberosity into the ilium. greater sciatic notch. It defines the anterior part of the pelvis which includes the anterior column, disruption of this line as will be discussed can result from fractures other than anterior column injury. The ilioischial line runs vertically from the greater sciatic notch past the cotyloid recess through the ischial tuberosity and comprises the posterior supportive structures of the acetabulum including the posterior column. The anterior wall crosses the acetabulum obliquely and is less substantial and more medially positioned than the posterior wall which is lateral and more vertically orientated. The obturator ring if intact or not breached at two points excludes the 14.3.2 Radiographic Anatomy Several important lines are identifiable on the anteroposterior radiograph, these include the iliopectineal (iliopubic) line, the ilioischial line and the margins of the anterior and posterior walls of the acetabulum (Fig. 14.19). The integrity of the obturator ring is also an important factor in fracture classification. The iliopectineal line runs along the superior margin of the superior pubic ramus towards the a b Fig. 14.19. Radiographic lines essential to identification and classification of acetabular fractures. Iliopectineal (iliopubic) line (white arrows), ilioischial line (black arrows), posterior acetabular wall (black arrowhead), anterior acetabular wall (white arrowhead) and obturator ring circled c Fig. 14.18a–c. Acetabular (column) anatomy. Pink shaded area represents short posterior column (a), anterior column shaded blue (b) and enclosing roof, anterior and posterior walls supported between the columns (c) P. Hughes 226 a b c d Fig. 14.20a–d. Serial CT sections through the acetabulum, pink shading representing posterior column and blue the anterior column a b c d e f g h i j Fig. 14.21a–k. Elementary and complex patterns of acetabular fracture. Elementary group: (a) posterior wall; (b) anterior wall; (c) posterior column; (d) anterior column; (e) transverse. Complex group: (f) posterior column and posterior wall; (g) both columns; (h) transverse and posterior wall; (i) T-shaped; (j) anterior column and posterior hemi-transverse possibility of a column fracture irrespective of disruption to the iliopectineal or ilioischial lines. Oblique radiographic views (Judet pair) are often requested to gain additional detail. These views are referred to as the iliac oblique (IO) view which demonstrates the ilium en face and the obturator oblique (OO) view. The IO view improves evaluation of the anterior wall, posterior column and blade of the