DOI:10.2214/AJR.06.0921
AJR 2007; 188:1540-1552
© American Roentgen Ray Society
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.
Received July 25, 2006;
accepted after revision November 8, 2006.
Address correspondence to S. E. Anderson
(andersonsembach{at}yahoo.com.au).
CME
This article is available for CME credit. See
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for more information.
FOR YOUR INFORMATION
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Abstract
OBJECTIVE. The purpose of this article is to show the important
radiographic criteria that indicate the two types of femoroacetabular
impingement: pincer and cam impingement. In addition, potential pitfalls in
pelvic imaging concerning femoroacetabular impingement are shown.
CONCLUSION. Femoroacetabular impingement is a major cause for early
"primary" osteoarthritis of the hip. It can easily be recognized
on conventional radiographs of the pelvis and the proximal femur.
Keywords: bone • femoroacetabular impingement • hip • musculoskeletal imaging • orthopedic surgery • radiography
Introduction
Femoroacetabular impingement (previously also called "acetabular rim
syndrome" [1] or
"cervicoacetabular impingement"
[2]) is a major cause of early
osteoarthritis of the hip, especially in young and active patients
[3–6].
It is characterized by an early pathologic contact during hip joint motion
between skeletal prominences of the acetabulum and the femur that limits the
physiologic hip range of motion, typically flexion and internal rotation.
Depending on clinical and radiographic findings, two types of impingement are
distinguished (Fig. 1): Pincer
impingement is the acetabular cause of femoroacetabular impingement and is
characterized by focal or general overcoverage of the femoral head. Cam
impingement is the femoral cause of femoroacetabular impingement and is due to
an aspherical portion of the femoral head–neck junction
(Fig. 2). Most patients (86%)
have a combination of both forms of impingement, which is called "mixed
pincer and cam impingement," with only a minority (14%) having the pure
femoroacetabular impingement forms of either cam or pincer impingement
[7].
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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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During sports activities and activities of daily living, repetitive
microtrauma of these osseous convexities occur. As a consequence of this
recurring irritation, the labrum degenerates
[8] and irreversible chondral
damage occurs that progresses and results in degenerative disease of the hip
joint if the underlying cause of femoroacetabular impingement is not addressed
[9,
10].
In the initial phase of this recently described entity, patients with
femoroacetabular impingement do not have classic radiographic signs of
osteoarthritis such as joint space narrowing, osteophyte formation,
subchondral sclerosis, or cyst formation. Thus, this article will familiarize
radiologists with this pathophysiologic concept and describe the radiographic
findings that are helpful for the correct diagnosis and evaluation before
potential surgical treatment of femoroacetabular impingement. In addition,
potential pitfalls simulating femoroacetabular impingement are discussed, and
some "pearls" for diagnosis are offered.
Clinical Findings
Patients with femoroacetabular impingement are young, usually in their
20s–40s. The estimated prevalence is 10–15%
[11]. Patients present with
groin pain with hip rotation, in the sitting position, or during or after
sports activities. Some patients describe a trochanteric pain radiating in the
lateral thigh. Typically, they are aware of their limited hip mobility long
before symptoms appear. In the clinical examination, patients with
femoroacetabular impingement have a restricted range of motion, particularly
flexion and internal rotation
[3,
8]. A positive impingement sign
is present for anterior femoroacetabular impingement if the forced internal
rotation/adduction in 90° of flexion is reproducibly painful, and for
posterior impingement with painful forced external rotation in full extension
[3,
12]
(Fig. 3). The
"Drehmann's" sign is positive if there is an unavoidable passive
external rotation of the hip while performing a hip flexion
[13].
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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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Conventional Radiographic Imaging Technique
The role of imaging in femoroacetabular impingement is to evaluate the hip
for abnormalities associated with impingement and to exclude arthritis,
avascular necrosis, or other joint problems on radiographs. MRI or MR
arthrography can then be used to confirm or exclude labral tears, cartilage
damage, and other pathologic signs of internal hip derangement if impingement
is suspected. Alternatively, radiography is then usually followed by MRI for
cartilage and labral disorders and a 3D understanding of the bone anatomy.
Standard conventional radiographic imaging for femoroacetabular impingement
includes two radiographs (Fig.
4): an anteroposterior pelvic view and an axial cross-table view
of the proximal femur [3]. An
alternative to the axial view, a Dunn/Rippstein view, preferably in 45° of
flexion, can be obtained to reveal pathomorphologies of the anterior femoral
head–neck junction [14].
For the anteroposterior pelvic radiograph, the patient is in the supine
position with the legs 15° internally rotated to compensate for femoral
antetorsion and to provide better visualization of the contour of the lateral
femoral head–neck junction
[15]. The film-focus distance
is 1.2 m; the central beam is directed to the midpoint between a line
connecting both anterosuperior iliac spines and the superior border of the
symphysis (Fig. 4), which can
easily and reproducibly be palpated by the radiology technician
[16,
17]. Accordingly, the
cross-table view of the proximal femur is taken with the leg internally
rotated, with a film-focus distance of 1.2 m, and with the central beam
directed to the inguinal fold
[18]. If these prerequisites
of correct positioning of the patient and accurate radiographic technique are
not fulfilled, the radiographs must be interpreted with caution.
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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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A faux profile of Lequesne and de Sèze
[19] may be used for
quantification of anterior overcoverage but is rarely indicated for
femoroacetabular impingement because it does not show the relationship between
the anterior and the posterior acetabular rims. Rather, it is used to assess
the posteroinferior part of the hip joint to detect the so-called contrecoup
lesions in pincer impingement described later.
To determine accurately the individual pelvic tilt of a patient, a strong
lateral view of the pelvis can be obtained
(Fig. 5B). Correct
interpretation of pelvic tilt is crucial for accurate description and
radiographic assessment of individual hip parameters. A neutral tilt is
defined with a pelvic inclination angle of 60°, which includes a
horizontal line and a line connecting the upper border of the symphysis and
the sacral promontory [20]
(Fig. 5A,
5B,
5C).
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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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The use of gonadal shielding is not particularly recommended for the
initial assessment of the hip because it impedes correct interpretation of the
individual tilt and rotation described later.
Pincer Impingement
Pincer impingement is more common in middle-aged women, occurring at an
average age of 40 years, and can occur with various disorders
(Table 1). Pincer impingement
is the result of overcoverage of the hip and can lead to osteoarthritis
[21]. Pincer impingement is
also the result of a linear contact between the acetabular rim and the femoral
head–neck junction due to general or focal acetabular overcoverage
(Fig. 2). In contrast to cam
impingement, cartilage damage of the acetabular cartilage is restricted in
pincer hips to a small thin strip near the labrum that is more
circumferentially located
[7].
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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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General Acetabular Overcoverage
Normally, general acetabular overcoverage is correlated with the radiologic
depth of the acetabular fossa. A normal hip appears on an anteroposterior
pelvic radiograph with the acetabular fossa line lying laterally to the
ilioischial line (Fig. 6). A
coxa profunda is defined with the floor of the fossa acetabuli touching or
overlapping the ilioischial line medially
(Fig. 7). Protrusio acetabuli
occurs when the femoral head is overlapping the ilioischial line medially
(Fig. 8). Both forms relate to
an increased depth of the acetabuli; however, at this stage no clear
information exists that the two entities are a continuation of each other.
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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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Generally, a deep acetabulum is associated with excessive acetabular
coverage that can be quantified with the lateral center edge angle or the
acetabular index [22]. The
lateral center edge angle is the angle formed by a vertical line and a line
connecting the femoral head center with the lateral edge of the acetabulum. A
normal lateral center edge angle varies between 25° (which defines a
dysplasia) [23] and 39°
(which is an indicator for acetabular overcoverage)
[24]. The acetabular index is
the angle formed by a horizontal line and a line connecting the medial point
of the sclerotic zone with the lateral center of the acetabulum. In hips with
coxa profunda or protrusio acetabuli, the acetabular index (also called
"acetabular roof angle") is typically 0° or even negative.
Another parameter for quantification of femoral coverage is the femoral
head extrusion index, which defines the percentage of femoral head that is
uncovered when a horizontal line is drawn parallel to the interteardrop line
[25]. A normal extrusion index
is less than 25% [26];
however, to our knowledge no study has defined a minimum extrusion.
A pitfall: Formation of a pseudodeep acetabulum can be produced on an
anteroposterior radiograph that is centered over the hip (Fig.
9A,
9B). Because of this centering
error, these radiographs are not useful for reliable diagnosis of a deep
acetabulum.
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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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Focal Acetabular Overcoverage
Focal overcoverage can occur in the anterior or the posterior part of the
acetabulum. Anterior overcoverage is called "cranial acetabular
retroversion" or "anterior focal acetabular retroversion"
and causes anterior femoroacetabular impingement that can be reproduced
clinically with painful flexion and internal rotation. By carefully tracing
the anterior and posterior acetabular rims, different acetabular
configurations can be identified. A normal acetabulum is anteverted and has
the anterior rim line projected medially to the posterior wall line
[16,
27–29]
(Fig. 6). A focal overcoverage
of the anterosuperior acetabulum causes a cranially retroverted acetabulum.
This is defined with the anterior rim line being lateral to the posterior rim
in the cranial part of the acetabulum and crossing the latter in the distal
part of the acetabulum. This
figure-8 configuration is
called the "cross-over" sign
(Fig. 10).
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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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To distinguish between a too-prominent anterior wall and a deficient
posterior wall, the posterior wall must be depicted in more detail. Therefore,
the "posterior wall" sign was introduced as an indicator for a
prominent posterior wall. This can cause posterior impingement with
reproducible pain in hip extension and external rotation
(Fig. 3). In a normal hip, the
visible outline of the posterior rim descends approximately through the center
point of the femoral head (Fig.
6). If the posterior line lies laterally to the femoral center, a
more prominent posterior wall is present
(Fig. 11). In contrast, a
deficient posterior wall has the posterior rim medial to the femoral head
center. A deficient posterior wall is often correlated with acetabular
retroversion or dysplasia
[27]; an excessive posterior
wall can often be seen in hips with coxa profunda or protrusio acetabuli but
can also occur as an isolated entity. Acetabular retroversion can also be
caused by acetabular reorientation procedures if the configuration of the
acetabular rims is not taken into consideration
[30,
31].
This persistent abutment in the anterior part of the joint can lead to a
slight subluxation posteroinferiorly. The increased pressure between the
posteroinferior acetabulum and the posteromedial aspect of the femoral head
can cause chondral damage to the posteroinferior part of the acetabulum as a
contrecoup lesion, which occurs in approximately one third of pincer cases
[3,
7,
32]. The resulting loss of
joint space can be visualized on a faux profile and is a bad prognostic sign
(Fig. 12).
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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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Regarding pitfalls, in certain hips, distinguishing between the two lines
of the acetabular rim is difficult. As a helpful guideline, the posterior rim
line can always be readily identified when starting from the inferior edge of
the acetabulum.
An anteroposterior radiograph centered over the hip is not usable for
reliable diagnosis of acetabular retroversion. This projection will imply a
discrepancy in the appearance of the acetabular rim compared with a standard
pelvic radiograph, on which the anterior rim will be displayed more
prominently because it lies closer to the X-ray beam source
[17,
29]. Therefore, acetabular
version is generally overestimated when interpreting an anteroposterior
radiograph centered over the hip. In addition, a cross-over sign can even be
missed if only an anteroposterior radiograph of the hip is available (Fig.
13A,
13B).
The appearance of acetabular morphology depends on the individual pelvic
orientation, which can vary considerably in terms of tilt and rotation
[33]. Increased pelvic tilt or
a rotation to the ipsilateral hip leads to a more pronounced retroversion sign
and vice versa [16,
17,
34,
35] (Fig.
14A,
14B,
14C). A neutral pelvic
rotation is defined as the tip of the coccyx pointing toward the midpoint of
the superior aspect of the symphysis pubis. As a general rule, a neutral
pelvic tilt is defined as the distance of 3.2 cm between the upper border of
the symphysis and the midportion of the sacrococcygeal joint for men, and 4.7
cm for women [16]. With the
help of one additional lateral radiograph, the radiographs of extensively
rotated or tilted pelves can be calculated back with recently developed
software Hip2Norm (University of Bern, Switzerland) to ensure a
tilt and rotation independent of anatomically based interpretation of the
acetabular morphologic configuration
[17] (Fig.
5A,
5B,
5C). If obtained, the lateral
pelvic view must be taken after the anteroposterior projection without motion
of the patient and with the central beam directed to the upper tip of the
greater trochanter (Fig.
4).
In addition to acetabular pathomorphologies, pincer impingement can also be
caused by excessive hip motion in patients in whom no obvious acetabular
disorder is present. It occurs typically in hypermobile young women (e.g.,
ballet dancers).
Cam Impingement
Cam impingement is more common in young men, occurring at an average age of
32 years. Cam impingement is the femoral cause of femoroacetabular impingement
and is caused by an aspherical shape of the femoral head where the
nonspherical portion is jammed into the acetabulum as a result of several
known causes or idiopathically
[6,
36,
37]
(Fig. 2 and
Table 1). These osseous bumps
lead to a decreased femoral head–neck offset, which is defined by the
distance between the widest diameter of the femoral head and the most
prominent part of the femoral neck (Fig.
15A,
15B,
15C). The recurrent irritation
leads to an abrasion of the acetabular cartilage or its avulsion from the
subchondral bone [38]. The
cartilage area involved in cam impingement is much larger than pure pincer
impingement and may be associated with large areas of cartilage delamination
or fissuring. However, in both mechanisms, although there is significant and
irreversible prearthritic damage of the cartilage, there is no joint space
narrowing because only the quality of the cartilage, and not its diameter, is
impaired in the early stage of the disease.
Cam impingement can be caused by an osseous bump on the femoral
head–neck junction or by a retroverted femoral neck or head. Osseous
bumps are typically located either in the lateral (so-called pistol grip, seen
on an anteroposterior pelvic radiograph
[Fig. 15A]) or in the
anterosuperior (seen on an axial cross-table view of the proximal femur [Figs.
15B and
15C]) portion of the femoral
head–neck junction (Figs.
15B and
15C). A pistol-grip deformity
is characterized on radiographs by flattening of the usually concave surface
of the lateral aspect of the femoral head due to an abnormal extension of the
more horizontally oriented femoral epiphysis
[39–42]
(Fig. 15A,
15B,
15C).
Cam impingement is usually caused by a primary osseous variant of the
head–neck junction that is considered to be caused by a growth
abnormality of the capital femoral epiphysis
[42], but it can also be the
result of several known causes, such as a subclinical slipped capital femoral
epiphysis
[43–45]
or Legg-Calvé-Perthes disease
[4,
46], or it can occur after
femoral neck fractures [2,
47]; it may also be idiopathic
(Table 1).
Quantification of the amount of asphericity can be accomplished by the
angle 
, the femoral offset, or the offset ratio
[37]. Angle is the
angle between the femoral neck axis and a line connecting the head center with
the point of beginning asphericity of the head–neck contour (Fig.
15A,
15B,
15C). It can be measured on
radiographs. An angle exceeding 50° is an indicator of an abnormally
shaped femoral head–neck contour.
Another parameter for quantification of cam impingement is the anterior
offset, which is defined as the difference in radius between the anterior
femoral head and the anterior femoral neck on a cross-table axial view of the
proximal femur (Fig. 15A,
15B,
15C). In asymptomatic hips,
the anterior offset is 11.6 ± 0.7 mm; hips with cam impingement have a
decreased anterior offset of 7.2 ± 0.7 mm
[18]. As a general rule for
clinical practice, an anterior offset less than 10 mm is a strong indicator
for cam impingement. In addition, the so-called offset ratio can be
calculated, which is defined as the ratio between the anterior offset and the
diameter of the head. The offset ratio is 0.21 ± 0.03 in asymptomatic
patients and 0.13 ± 0.05 in hips with cam impingement.
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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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Another cause for cam impingement is femoral retrotorsion, which can occur
as a primary entity [48] or
posttraumatically after healed femoral neck fractures
[47]. Femoral retrotorsion can
be calculated reliably only on CT scans involving the proximal and distal
parts of the femur [49]. In
addition, a coxa vara (defined by a centrum collum diaphyseal angle [CCD] of
less than 125°) has been recognized as a cause of cam impingement
[50].
A pitfall: In the initial phase of the disease, these entities are anatomic
abnormalities and do not represent classic osteophytes. Classic osteophytes
occur in an advanced stage of the disease when the cartilage damage already
has taken place. Osteophyte formation can lead to a worsening of
femoroacetabular impingement, an increase of the overcoverage for pincer hips,
or a further loss of femoral head–neck offset. Through careful
evaluation of the radiographs, the original acetabular rim can be identified.
Occasionally, on the femoral side at the head–neck junction, a linear
indentation may be observed in hips with pincer impingement and a cortical
thickening (Fig. 17A,
17B). In the end stage of
pincer impingement, posteroinferior cartilage abrasion occurs, which is the
result of the contrecoup lesion during subtle subluxation of the femoral head.
This bad prognostic sign is best seen on a faux profile of the hip or, if
available, on MRI (Fig.
12).
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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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Secondary Radiographic Changes in Hips
Unrecognized femoroacetabular impingement leads to recurrent irritation of
the acetabular labrum, which is the first structure involved and which is seen
in both types of impingement. It leads to a reactive ossification,
particularly of the labral basis
[8] (Fig.
16A,
16B). In an advanced stage of
the disease, additional reactive bone apposition at the osseous rim leads to
further deepening of the acetabulum, thereby increasing the impingement
problem, which can also be seen as a double contour of the acetabular rim
(Fig. 16A,
16B). Because of the abnormal
stress in impinging hips, the prominent acetabular bone fragment can even be
separated from the adjacent bone margin. This os acetabulum is an acetabular
rim fracture, presumed to be a stress or impingement fracture, resulting from
a constant jamming of the femoral head against the acetabulum
[27] (Fig.
16A,
16B).
Hips with femoroacetabular impingement have a significantly higher
prevalence of herniation pits, which are thought to be benign and incidental
and the cause of which was not clearly understood
[51]. Herniation pits are
radiolucencies surrounded by a sclerotic margin that are typically located in
the anterior proximal superior quadrant of the femoral neck and occur in some
33% of patients; they range in size from 3 to 15 mm (mean, 5 mm)
[52]. This previously
described location corresponds well to the typical location where the
femoroacetabular impingement occurs. Therefore, hips with these juxtaarticular
cysts should be considered a joint at risk for femoroacetabular impingement
rather than one with a benign lesion, but herniation pits are not always
associated with symptomatic impingement.
General Pearls and Pitfalls of Femoroacetabular Impingement Imaging
Systemic disorders with hip joint involvement may superficially mimic
femoroacetabular impingement; these include ankylosing spondylitis, diffuse
idiopathic skeletal hyperostosis (DISH), and congenital hip dysplasia.
However, these are usually easy to distinguish from systemic disorders with
hip joint involvement by reviewing the sacroiliac joints that will be fused or
pathologic with ankylosing spondylitis and other seronegative
spondyloarthropathies and the spine for anterior longitudinal ligament
calcification with DISH. Congenital hip dysplasia presenting in adulthood is
characterized by a lack of acetabular coverage and by lateral proximal femoral
head subluxation and is more commonly associated with large labral ganglion
[53].
Rarely, patients with femoroacetabular impingement may have additional
disorders such as hydroxyapatite deposition in the acetabular labrum; however,
this calcification has usually resolved on follow-up radiographs at 6 weeks.
More commonly, in the younger adolescent age group, there may be associated
enthesopathy of the greater trochanter associated with gluteal tendon overuse,
as evident by bone spurring of the greater trochanter.
Femoroacetabular impingement is often bilateral but may present
asynchronously. Although symptomatic presentation may be delayed on one side,
reviewing both hip joints is recommended. In patients with typical
femoroacetabular impingement, radiographic features may be asymptomatic as a
result of lack of activity or of being at an early stage in the development of
femoroacetabular impingement. Although there are characteristic imaging
findings of femoroacetabular impingement, at this stage of knowledge, the gold
standard remains the patient's pain and not the imaging findings alone.
However, because the prognosis of the hip joint is significantly better if the
intraarticular impingement is eliminated as early as possible, surgical
reconstruction of the hip joint is recommended as soon as the first symptoms
occur [4,
38].
A suboptimal or faulty radiographic technique of the pelvis and hip joint
may over- or underestimate or falsely diagnose femoroacetabular impingement.
In addition to first reviewing for overall symmetry of the hip joints on the
frontal radiograph, in a busy clinical setting with no strong lateral view
obtained, brief review of the location of the sacrococcygeal joint in relation
to the superior aspect of the symphysis pubis is helpful. If the
sacrococcygeal joint is within approximately 3.2 cm of the symphysis for men
or 4.7 cm for women, then the pelvic tilt should be largely neutral.
Suboptimal technique may be minimized by obtaining radiographs as described in
this article or by using a computer-assisted program that corrects for
malpositioning [17] (Fig.
5A,
5B,
5C). Accurate angles,
measurements, and ratios are also possible with such a tool, as is accurate
preoperative planning.
Another way of reviewing the radiographs for femoroacetabular impingement
in a busy clinical setting is to use the PACS tool to prescribe a circle,
beginning centered on the central point of the femoral head, and to enlarge
this circle until the femoral head expansion is met. If there is any bone
beyond this circle, then cam impingement is likely. These circles can be drawn
on both the frontal and axial radiographs (Fig.
15A,
15B,
15C).
Treatment of Femoroacetabular Impingement
Surgical treatment of femoroacetabular impingement focuses on improving the
clearance for hip motion and alleviation of femoral abutment against the
acetabular rim. This includes basically the surgical resection of the
impinging cause, by trimming the acetabular rim or the femoral head–neck
offset either via a surgical hip dislocation
[3,
12,
54] or arthroscopically
[55], or, rarely, by the
reorientation of a retroverted acetabulum via a reversed periacetabular
osteotomy [28]. Mid-term
results from these procedures are promising
[4,
38].
Conclusion
In conclusion, two main forms of femoroacetabular impingement—pincer
and cam—occur in young active individuals presenting with hip pain,
although most patients will have a combination of both impingement types. The
radiographic technique and typical findings have been presented. MRI and MR
arthrography are important for further evaluation of the osseous and
soft-tissue abnormalities of impingement; these will be presented in a future
article.
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Abstract
Introduction
