3 Radiographic Anatomy of The Hip
I. Comprehension of the normal radiographic anatomy of the hip is helpful in the interpretation of hip pathology on radiographs ( Fig. 3.1 ).
Plain Film Radiographs
I. Primary imaging for adult hip. Routine: anteroposterior (AP) and lateral views. Specialized views: frog-leg lateral, anterior and posterior oblique (Judet’s views), false profile, Ferguson’s view (pelvic outlet), and the pelvic inlet view. 1
II. Alignment is evaluated by visualization of symmetry. Important landmarks are as follow: iliopectineal line, ilioischial line of Kohler, Shenton’s line, sourcil, teardrop sign, and acetabular floor. 2
III. Evaluation of plain radiographs:
The pubic symphysis should be in line with the center of the sacrum. The symphyseal joint space should be ≤ 5 mm.
The sacroiliac joint widths should be equal. Normal sacroiliac joint appears as a thin white line. Sclerosis and joint space narrowing characterize sacroiliitis. Arcuate lines should be symmetrical; angular lines indicate sacral fracture.
Iliac crests should both be on the same level.
The obturator foramen should be symmetric bilaterally.
The acetabular walls: the posterior wall should be lateral to the anterior wall. In acetabular retroversion, anterior and posterior walls cross each other. Positive crossover sign indicates presence of femoroacetabular impingement or developmental dysplasia of the hip.
Iliopectineal line: this line extends posteriorly from the sacral promontory, arcuate line, and ends anteriorly at the pectineal line. It divides the pelvis into major (false) and minor (true) pelvis. Disruption indicates anterior column fracture ( Fig. 3.3 ).
Ilioischial line: this line represents the posterior column. On each side, the line is drawn from the medial border of the iliac wing to the medial border of the ischium, ending at the ischial tuberosity ( Fig. 3.3 ). Location of the femoral head medial to the pectineal line indicates acetabular bone loss or possible medial femoral head migration in the acetabulum. 5
The sourcil should be clearly defined. Teardrop sign should be assessed: it marks the convergence of the pubis, the ischium, and the ilium. Asymmetry of the teardrop sign may indicate occult acetabular fracture. 5
Shenton’s line: this line is drawn from the inferomedial neck of the femur to the inferior border of the superior ramus ( Fig. 3.3 ). A discontinuation of this line may indicate femoral neck fracture. 5
The cortex of the femoral head and neck should be smooth and continuous with a normal trabecular pattern. Disruption may indicate a fracture.
The greater and lesser trochanters should be clearly visible and symmetric on both sides.
IV. Pediatric plain radiograph:
Assessment is done on AP and frog-leg lateral views of both pelvis (Figs. 3.4, 3.5 ). The pelvic bones are not fused and appear separated on radiographs. Knowledge of the patient’s age is important in order to differentiate open epiphysis from fractures.
V. Two main features help distinguish male and female pelvis ( Fig. 3.6 ):
The pubic angle is obtuse in females and acute in males.
The iliac crests appear flared in males and broad in females.
VI. Lateral pelvis radiograph ( Fig. 3.7 ):
Cross-table view: this view provides visualization of the anterior and posterior aspects of the femoral neck, and the lateral aspect of the femoral head and the proximal femur.
Femoral head, neck, and shaft; greater and lower trochanters and ischial tuberosity should be visible on the lateral view.
The ischial tuberosity is posterior and aids in orienting the image.
Posteroinferior migration of the femoral head on the lateral view indicates posterior dislocation.
The femoral head and neck should be continuous. The femoral neck appears shorter and discontinuous in displaced fracture.
The gluteal fat stripe is represented by a line parallel to the upper part of neck of the femur. It is formed by fat between the gluteus minimus tendon and the ischiofemoral ligament. In hip joint effusion, the line is directed superiorly.
The iliopsoas fat stripe is represented by a line that runs below the iliopsoas tendon.
The obturator fat stripe is a line that runs parallels to the iliopectineal line and is formed by the pelvic fat adjacent to the obturator internus muscle.