The pelvic radiograph: lines, arcs and stripes

Pelvic fractures

Pelvic fractures can be simple or complex, and involve any part of the bony pelvis. Unstable fractures of the pelvis in high-energy trauma are classified based on the mechanism of injury: lateral compression (Fig. 3), anteroposterior compression, vertical shear or a combined mechanical-complex fracture pattern (Fig. 4).(4) Low-energy injuries usually result in isolated pelvic fractures such as pubic ramus, acetabular, iliac wing or avulsion fractures.

Fig. 3

Pelvic radiograph shows lateral compression injuries. A laterally directed force causes inward rotation of the left hemipelvis with left superior and inferior pubic rami fractures (white arrows). Posteriorly, an impaction fracture of the ipsilateral sacral ala can be seen, with disruption of the sacral arcuate lines (black arrows). The asterisk (*) indicates that there is an abnormality on the radiograph, which is part of the red dot system or radiographer abnormality detection system in our institution.

Fig. 4

Pelvic radiograph shows complex pelvic fractures with posterior and anterior columns fractures of the right acetabulum, and disruption of the right ilioischial and iliopectineal lines (white arrows); and anterior left acetabular column fracture with discontinuity of the left iliopectineal line (white arrowhead). There is widening of the right sacroiliac joint (black arrow) and minor widening of the pubic symphysis (black arrowhead).

Acetabular fractures

The Judet and Letournel classification is a commonly used classification system for acetabular fractures. It describes the five elementary patterns of acetabular fractures as: anterior wall, anterior column (which commonly disrupts the iliopectineal line, as seen in Figs. 4 and 5), posterior column (which usually leads to interruption of the ilioischial line, as seen in Figs. 4 and 6), posterior wall and transverse acetabular fractures.(5)

Fig. 5

Pelvic radiograph shows disrupted iliopectineal lines on both sides, indicating anterior column fractures of the bilateral acetabula (arrows).

Fig. 6

Pelvic radiograph shows discontinuity of the right ilioischial line (arrows) suggestive of posterior acetabular fracture.

Femoral neck fractures

Femoral neck fractures are common injuries sustained by older patients from falls or younger patients from significant trauma.

Radiographic features for femoral neck fracture include disruption of the Shenton line (Fig. 7), hazy ill-defined sclerosis from impaction, angulated or disrupted bone trabeculae, or asymmetry of the lateral femoral neck or head. The femur is often positioned in flexion and external rotation in femoral neck fracture. Non-displaced femoral neck fractures can be subtle on radiographs and require careful scrutiny to be detected.

Fig. 7

Pelvic radiograph shows a right femoral neck fracture with disruption (black lines) of the Shenton arc or line.

Avulsion injuries

An avulsion injury of the pelvis occurs when an osseous fragment is pulled from its donor bone by forceful contraction of a tendon. This injury is more common in younger individuals.

Radiographic features include the presence of an avulsed bony fragment immediately adjacent to the donor bone in anatomically predictable locations that allude to the tendon attachment responsible for the injury, such as a pubic crest avulsion at the rectus abdominis-aponeurosis attachment disrupting the iliopectineal line (Fig. 8). Other sites include the iliac crest (abdominal wall muscles and tensor fascia lata), ischial tuberosity (common hamstring tendon origin), anterior inferior iliac spine (rectus femoris tendon), anterior superior iliac spine (sartorius tendon), greater trochanter (gluteus minimus and medius tendons, gemelli and piriformis muscles) and lesser trochanter (iliopsoas tendon).

Fig. 8

Pelvic radiograph shows an avulsion fracture of the right pubic crest (arrow), the insertion site of the rectus abdominis muscles, which shows disruption of the iliopectineal line.

Insufficiency fractures

Insufficiency fractures are caused by normal stresses on abnormal bone. Common locations in the pelvis are the sacrum and pubic rami. The radiographic features are vertical fractures through the sacral ala (Fig. 9) parallel to the sacroiliac joints, often associated with a transverse component known as the ‘H-sign’ or ‘Honda sign’, although the latter is often better appreciated on skeletal scintigraphy.(6)

Fig. 9

Pelvic radiograph shows bilateral sacral insufficiency fractures with disruption of bilateral sacral arcuate lines (arrows) in the absence of significant injury.

Slipped upper femoral epiphysis

SUFE represents a Type 1 Salter-Harris fracture, resulting in posteromedial slippage of the femoral head. It is most common in adolescents, and boys are more commonly affected.(7) Risk factors include obesity and endocrine disorders such as hypothyroidism, hyperparathyroidism and renal osteodystrophy.

SUFE can be evaluated on the anteroposterior view of the pelvis (Fig. 10a). Dedicated frog-leg lateral view of the hip is usually performed for confirmation of the diagnosis (Fig. 10b). Radiographic features include physeal widening, apparent reduction of epiphyseal thickness and Trethowan’s sign (Fig. 10c). In the chronic state, the physis becomes sclerotic and the metaphysis is widened, giving rise to coxa magna. Avascular necrosis of the femoral head can occur in about 15% of patients.(8)

Fig. 10

(a) Anteroposterior pelvic radiograph shows slipped upper femoral epiphysis on the right. (b) Frog-leg lateral radiograph confirms slipped upper femoral epiphysis with widening of the physis (black arrow), with the direction of the slippage shown (white arrows). (c) Radiograph shows that the line of Klein (white line) running along the lateral margin of the femoral neck does not cross the femoral head epiphysis.

Developmental dysplasia of the hip

This aberrant development of the hip joint is typically diagnosed in the neonatal period by clinical examination and then usually confirmed with ultrasonography. Radiography can be used for assessment after substantial ossification has occurred.

Radiographic features include delay in ossification, the upper femoral epiphysis being displaced from the inferomedial quadrant (based on Hilgenreiner and Perkin’s lines; Fig. 11) and discontinuation of the Shenton line due to superolateral migration of the proximal femur (Fig. 12).

Fig. 11

Pelvic radiograph shows developmental dysplasia of the hip on the left where the femoral epiphysis (white asterisk) lies in the superolateral quadrant (lateral to Perkin’s line, black; and above Hilgenreiner’s line, white). It should normally be in the inferomedial quadrant (black asterisk).

Fig. 12

Pelvic radiograph shows bilateral dysplastic hips with disruption of the Shenton arcs (black lines).

Femoroacetabular impingement

Femoroacetabular impingement occurs due to certain abnormal morphology at the femoral head-neck junction or acetabulum, resulting in impingement of the labrum or surrounding musculotendinous structures. There are two types: the ‘cam’ type, which is focal bone infilling/prominence at the anterior femoral head-neck region, giving a pistol grip deformity (Fig. 13); and the ‘pincer’ type, which is related to over-coverage of the femoral head anteriorly due to either a retroverted acetabulum (Fig. 14) or coxa profunda (Fig. 15).(9) In acetabular retroversion, the superior portion of the acetabular posterior wall projects medial to the superior portion of the acetabular anterior wall, creating a crossover or ‘figure of eight’ sign.(10) The line that demarcates the posterior wall runs medial to, rather than at or lateral to, the centre of the femoral head. In coxa profunda, the medial acetabular wall is at or medial to the ilioischial line.

Fig. 13

Pelvic radiographs show focal bony prominence at the lateral aspect of the femoral head-neck regions compatible with cam lesions (arrows).

Fig. 14

(a & b) Pelvic radiographs show an example of bilateral acetabular retroversion. The crossover sign is seen in (b), where the posterior wall (black line) of the acetabulum is seen to cross over and project medial to the anterior wall (white line) of the acetabulum.

Fig. 15

(a & b) Pelvic radiographs show an example of bilateral osteoarthritic hips with coxa profunda. In (b), the medial acetabular wall (black line) projects medial to the ilioischial line (white dashed line).

Achondroplasia

Achondroplasia is the commonest form of short-limbed dwarfism, which can be inherited as an autosomal dominant or sporadic gene mutation and affects endochondral ossification.(11) Radiographic features (Fig. 16) include horizontal acetabular roof, small sciatic notches, small square-shaped iliac wings (tombstone), champagne glass-type pelvic inlet, small trident pelvis and metaphyseal flaring.(12)

Fig. 16

Pelvic radiograph shows the characteristic findings of achondroplasia: the roof of acetabula appears flat, the greater sciatic notches are small and the femoral metaphyses are widened.