Femoroacetabular Impingement: Radiographic Diagnosis—What the Radiologist Should Know
Moritz Tannast1,
Klaus A. Siebenrock1 and
Suzanne E. Anderson2,3
1 Department of Orthopaedic Surgery, Inselspital, University of Bern,
Switzerland.
2 Department of Diagnostic, Pediatric and Interventional Radiology, Inselspital,
University of Bern, Switzerland.
3 Present address: Royal Melbourne Hospital, University of Melbourne, Melbourne,
Australia.
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Fig. 2 —Normal configuration of hip with sufficient joint clearance
allows unrestricted range of motion (top). In pincer impingement,
excessive acetabular overcoverage leads to early linear contact between
femoral head–neck junction and acetabular rim, resulting in labrum
degeneration and significant cartilage damage. Posteroinferior portion of
joint is damaged (contrecoup) due to subtle subluxations (center). In
cam impingement, aspherical portion of femoral head–neck junction is
jammed into acetabulum (bottom).
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Fig. 3 —Clinical tests to assess femoroacetabular impingement.
Anterior impingement sign (left) is positive, with painful forced
internal rotation in 90° of flexion. In extreme forms, there is
unavoidable passive external rotation of hip during hip flexion
("Drehmann's" sign, center). "Posterior
impingement" sign is positive when there is painful forced external
rotation in maximal extension (right).
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Fig. 4 —Correct setting for anteroposterior and strong lateral
(left) pelvic radiography. Cross-table axial radiograph of hip
(right) is needed to visualize anatomy of anterior femoral
head–neck junction, which is not visible on anteroposterior pelvic
radiograph.
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Fig. 5A —27-year-old woman. Bilateral "cross-over" sign is
visible on this anteroposterior pelvic radiograph that is analyzed with
specifically developed software Hip2Norm (University of Bern,
Switzerland) for tilt and rotation correction of parameters of pelvic
radiographs [17].
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Fig. 6 —Schematic (left) and radiographic (right)
appearances of normal hip (detailed view of anteroposterior pelvic radiograph)
in 35-year-old man. Acetabular fossa (F) is lateral to ilioischial line (IIL).
Acetabular index (AI) is positive, and femoral head (H) is not entirely
covered by acetabulum (E). Projected anterior wall (AW) lies medially to
posterior wall (PW), which typically runs more or less through center of
femoral head. Extrusion index (E / [A + E]) is approximately 25%. Lateral
center edge (LCE) angle is 25–39°. Epiphyseal scar lies in femoral
head circle (arrows). A = covered portion of femoral head, E =
uncovered portion of femoral head.
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Fig. 7 —Schematic (left) and radiographic (right)
presentations of coxa profunda (detailed view of anteroposterior pelvic
radiograph) in 29-year-old woman. Acetabular fossa (F) is touching or
overlapping ilioischial line (IIL). Femoral head (H) is more covered,
resulting in decreased femoral head extrusion index (E / [A + E]), neutral
acetabular index (AI'), and increased lateral center edge (LCE') angle. A' =
covered portion of the femoral head, E' = uncovered portion of the femoral
head.
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Fig. 8 —Schematic (left) and radiographic (right)
presentations of protrusio acetabuli (detailed view of anteroposterior pelvic
radiograph) in 42-year-old woman. Femoral head line (H) is crossing
ilioischial line (IIL). As a consequence, femoral head extrusion index (E / [A
+ E]) is zero or even negative, acetabular index (AI") is negative, and
lateral center edge (LCE") angle increases. F = acetabular fossa.
A" = covered portion of femoral head, E" = uncovered portion of
femoral head.
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Fig. 9A —Influence of direction of center of X-ray beam on appearance
of acetabular depth in 22-year-old man. Arrows show herniation pit caused by
cam type of femoroacetabular impingement. IIL = ilioischial line, AW =
anterior wall, PW = posterior wall, F = fossa. Section of anteroposterior
pelvic radiograph shows regular acetabular configuration with acetabular fossa
lying lateral to ilioischial line.
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Fig. 9B —Influence of direction of center of X-ray beam on appearance
of acetabular depth in 22-year-old man. Arrows show herniation pit caused by
cam type of femoroacetabular impingement. IIL = ilioischial line, AW =
anterior wall, PW = posterior wall, F = fossa. Hip radiograph centered over
hip shows apparent coxa profunda. In addition, version of acetabulum seems to
be larger with anterior wall being projected more medially.
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Fig. 10 —Schematic (left) and radiographic (right)
presentations of focal anterior overcoverage of hip in 29-year-old woman.
Acetabular retroversion is defined as anterior wall (AW) being more lateral
than posterior wall (PW), whereas in normal hip anterior wall lies more
medially. This cranial acetabular retroversion can also be described by
figure-8 configuration.
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Fig. 13A —Retroversion sign can be missed if central X-ray beam is not
directed correctly. In this cadaveric pelvis with wire marking acetabular
rims, cranial acetabular retroversion is visible on left side on
anteroposterior pelvic radiograph. Center of X-ray beam is marked with
radiopaque marker.
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Fig. 14A —Influence of individual pelvic orientation on appearance of
acetabular rim. Normal acetabular configuration is shown in this cadaveric
pelvis with wire marking acetabular rims. a = vertical distance between upper
border of symphysis and sacrococcygeal joint.
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Fig. 14B —Influence of individual pelvic orientation on appearance of
acetabular rim. Increased pelvic tilt (visible on increased distance between
symphysis and sacrococcygeal joint, a') leads to apparent retroversion of
acetabular rim on both sides. Arrows indicate apparent bilateral retroversion
due to increased pelvic tilt.
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Fig. 14C —Influence of individual pelvic orientation on appearance of
acetabular rim. Rotation to right (with consecutive increased horizontal
distance between middle of symphysis and sacrococcygeal joint, b (horizontal
distance between mid of symphysis and mid of sacrococcygeal joint) leads to
apparent retroversion of right hip and to pronounced anteversion of left hip.
Arrow indicates creation of apparent retroversion on right side due to
rotation on right.
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Fig. 16A —Secondary radiographic signs of femoroacetabular impingement.
Recurrent impingement can lead to ossification of labral basis (white
arrow) and to osseous apposition of acetabular rim, which is visible as
double contour (black arrows) in 45-year-old woman.
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Fig. 16B —Secondary radiographic signs of femoroacetabular impingement.
Because of abnormal stress in impinging hips, prominent acetabular bone
fragment can even be separated from adjacent bone margin (os acetabuli,
arrow) in 36-year-old man with pistol-grip deformity.
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Fig. 17A —Pincer hips in 37-year-old woman. In pincer hips,
corresponding linear indentation often occurs on femoral side (black
arrows) with reactive cortical thickening (white arrows), which
can be seen on conventional radiograph (A) and on MR arthrogram with
intraarticular contrast agent (B).
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Fig. 17B —Pincer hips in 37-year-old woman. In pincer hips,
corresponding linear indentation often occurs on femoral side (black
arrows) with reactive cortical thickening (white arrows), which
can be seen on conventional radiograph (A) and on MR arthrogram with
intraarticular contrast agent (B).
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Copyright © 2007 by the American Roentgen Ray Society.
<50°) in 32-year-old man.