Dr. Filigenzi is a Fellow in Musculoskeletal Imaging and Dr. Bredella is an Assistant Radiologist, Department of Radiology, Massachusetts General Hospital, Boston, MA. Dr. Bredella is also an Assistant Professor of Radiology, Harvard Medical School, Boston, MA.
Osteoarthritis (OA) of the hip is a very common problem. While there
are many different etiologies for OA of the hip, often a speciﬁc cause
cannot be determined. Recently, femoroacetabular impingement (FAI) has
been found to cause progressive degenerative changes leading to early OA
of the hip in younger patients.1,2 In FAI, morphologic
abnormalities of the proximal femur, acetabulum, or both cause abnormal
contact between the femur and acetabulum during motion of the hip joint,
especially during ﬂexion and internal rotation. This contact causes
abnormal stress on the acetabular labrum and articular cartilage. The
resulting degeneration, tearing of the labrum, and damage to the
adjacent acetabular cartilage can lead to the development of OA.1-6
of FAI and its relationship to early OA of the hip requires early
diagnosis and treatment to prevent or delay hip degeneration. Magnetic
resonance imaging (MRI) and MR arthrography are excellent imaging
modalities for showing the morphologic abnormalities of the proximal
femur and acetabulum as well as the various changes seen in the hip
secondary to FAI. In this article, the authors describe the 2 different
proposed mechanisms for FAI, imaging of the morphologic abnormalities,
and secondary changes seen around the hip as a result of FAI.
Pathomechanisms of femoroacetabular impingement
normal anatomy of the hip allows for a wide range of motion. The
morphologic abnormalities that predispose a patient to the development
of FAI result in decreased joint clearance between the femoral neck and
acetabulum. This results in the abnormal contact, or impingement,
between the femoral neck and acetabulum, leading to the degenerative
changes in the acetabular labrum and/or adjacent articular cartilage.2,7-9
types of FAI can be distinguished: those secondary to abnormalities of
the proximal femur (cam impingement) and those secondary to acetabular
causes (pincer impingement). Cam impingement is a result of a
morphologically abnormal femoral head impinging against a normal
acetabulum during hip motion.1,3-6,8 Pincer impingement is
caused by contact between the acetabular rim and the femoral neck as a
result of acetabular abnormalities, such as acetabular retroversion
leading to anterior overcoverage of the femoral head.1,8,10-12 While
the 2 types of FAI will be described as 2 separate, distinct entities,
often a combination of factors can lead to the development of FAI.1
Cam impingement results from an
abnormality of the femoral head. These abnormalities include a variety
of different processes, including insufﬁcient femoral head-neck offset,
subtle displacement of the femoral epiphysis, slipped capital femoral
epiphyses (SCFE), or postsurgical or posttraumatic deformities.6,9,13-18
of these changes can be readily identiﬁed on radiographs and MRI. A
decreased offset of the femoral head-neck junction, which is the
difference between the widest diameter of the femoral head and the most
prominent part of the femoral neck, will appear as a lack of the normal
waisting of the femoral head-neck junction. On MRI or radiography, a
prominent lateral extension of the femoral head at the expected step-off
to the adjacent femoral neck can be seen (Figure 1).5,13 This
femoral “bump” is typically located lateral to the physeal scar. The
abnormal ﬂattening or convexity of the usually concave lateral head-neck
junction has been described as a “pistol grip” deformity.19
reduced femoral head-neck junction can be best assessed on MRI using
dedicated oblique axial images that are obtained parallel to the long
axis of the femoral neck. Methods of quantitatively evaluating the
femoral head-neck offset have been described with the “alpha” angle.20 The
alpha angle is calculated on the above-mentioned oblique axial MR
images of the hip. The angle is measured between a line drawn from the
center of the femoral head through the central axis of the femoral neck
and a second line drawn from the center of the femoral head to the point
anteriorly where the distance from the center of the head exceeds the
radius of the subchondral femoral head (Figure 1). It has been proposed
that an alpha angle >55° predisposes a patient to cam FAI.20
In pincer FAI, the acetabulum,
rather than the proximal femur, has a morphologic abnormality that
results in abnormal contact between the femur and acetabulum. As with
cam FAI, several abnormalities can lead to pincer FAI, the common
problem being an increased relative depth of the acetabulum that results
in overcoverage of the femoral head. Abnormalities that can be
associated with pincer FAI include acetabular retroversion, coxa
profunda, and protrusio acetabuli. All of these abnormalities can be
identiﬁed on radiographs and MRI.
Radiographic ﬁndings of acetabular retroversion include the crossover or “ﬁgure-8” sign and the posterior wall sign.21,22 The
crossover sign is caused by the anterior wall of the superior aspect of
the acetabulum projecting lateral to the posterior wall of the superior
aspect of the acetabulum (Figure 2).21,22 The line that
demarcates the posterior wall passes medial to the center point of the
femoral head, rather than following the normal passage at or lateral to
the center of the femoral head.21,22
acetabuli and acetabular retroversion (a posteriorly oriented
acetabulum) can be seen on oblique axial MRI of the hip. The acetabular
retroversion will result in an increase in coverage of the anterior
aspect of the femoral head.22 Acetabular depth can be
quantiﬁed on the same oblique axial images that are used to calculate
the alpha angle. The depth is calculated by drawing a line connecting
the anterior and posterior acetabular rims and a parallel line that
passes through the center of the femoral head. The distance between
these 2 lines is the acetabular depth, with the value being positive if
the center of the femoral head is lateral to the acetabular rim.23 As
would be expected, the acetabulum is signiﬁcantly deeper in patients
with pincer FAI than in patients in cam FAI, in which the depth is
normal.23 Future research may recognize a speciﬁc value for
the acetabular depth that puts a patient at increased risk for the
development of pincer FAI.
Imaging ﬁndings of femoroacetabular impingement
ﬁndings of changes secondary to FAI within the acetabulum can be seen
involving the acetabular bone, labrum, and articular cartilage. Recent
research by James et al24 suggests that a majority of the
abnormalities are located peripherally at the labral-chondral transition
zone. The articular cartilage can show a variety of changes. Early
damage is manifested by areas of signal hyperintensity that represent
areas of chondral softening. Eventually, this softening progresses from
articular cartilage ﬁssures to chondral fragmentation and resultant
full-thickness chondral defects (Figures 3 and 4). These changes are
classically described as appearing in the antero-superior aspect of the
acetabulum. However, recent research suggests that patients with cam FAI
tend to have articular cartilage lesions in the antero-superior portion
of the acetabulum, while patients with pincer FAI tend to have
cartilage lesions in the posteroinferior aspect of the acetabulum.23
chondral abnormalities are present, subsequent changes can also be seen
in the adjacent subchondral bone. This includes the development of
subchondral edema, sclerosis, and cysts (Figures 3 through 5).
Fragmentation of the anterior acetabular margin is frequently seen
(Figure 4). In fact, an os acetabuli is often identiﬁed in patients with
Changes in the acetabular labrum include
degenerative signal manifested as increased signal in the normally dark
labrum on ﬂuid-sensitive sequences as well as labral tears and complete
labral detachments. Associated paralabral cysts can also be seen
(Figures 6 and 7).7,26,27
Changes secondary to FAI are
not just limited to the acetabulum but can also be seen in the proximal
femur. Such abnormalities include the development of subchondral edema
and juxta-articular ﬁbrocystic changes within the proximal femur,
usually at the site of the dysplastic femoral bump (Figure 8). The
femoral-sided ﬁbrocystic changes, including the “Pitt’s Pit,” likely
represent the point of impingement on the femur and do not represent a
normal variation of synovial herniation, given their relatively high
prevalence in patients with FAI and their location at the anatomic site
Additional ﬁndings as a result
of FAI can also be seen in the soft tissues around the femur and
acetabulum, usually as a result of secondary inﬂammation related to the
FAI. There may be thickening of the joint capsule, often with an
associated synovitis, and thickening of the iliofemoral ligament
Femoroacetabular impingement is a relatively
common disorder that can result in the development of OA in the younger
patient population. This article addressed the various morphologic
abnormalities that predispose a patient to the development of FAI and
detailed the changes that can be seen around the hip as a result of FAI.
MRI and MR arthrography are accurate, noninvasive methods for the
evaluation of both the structural abnormalities and the pathologic
changes associated with FAI. Correct diagnosis of this disease process
is essential to prevent or delay the progression of OA.
- Ganz R, Parvizi J, Beck M, et
al. Femoroacetabular impingement: A cause for osteoarthritis of the
hip. Clin Orthop Relat Res. 2003;417:112-120.
- Tanzer M, Noiseux
N. Osseous abnormalities and early osteoarthritis: The role of hip
impingement. Clin Orthop Relat Res. 2004;429:170-177.
- Beck M,
Leunig M, Parvizi J, et al. Anterior femoroacetabular impingement: Part
II. Midterm results of surgical treatment. Clin Orthop Relat Res. 2004;
- Ito K, Leunig M, Ganz R. Histopathologic features of
the acetabular labrum in femoroacetabular impingement. Clin Orthop
Relat Res. 2004;429:262-271.
- Ito K, Minka MA 2nd, Leunig M, et
al. Femoroacetabular impingement and the cam-effect. A MRI-based
quantitative anatomical study of the femoral head-neck offset. J Bone
Joint Surg Br. 2001;83:171-176.
- Jäger M, Wild A, Westhoff B,
Krauspe R. Femoroacetabular impingement caused by a femoral osseous
head-neck bump deformity: Clinical, radiological, and experimental
results. J Orthop Sci. 2004;9:256-263.
- Leunig M, Podeszwa D,
Beck M, et al. Magnetic resonance arthrography of labral disorders in
hips with dysplasia and impingement. Clin Orthop Relat
- Lavigne M, Parvizi J, Beck M, et al.
Anterior femoroacetabular impingement: Part I. Techniques of joint
preserving surgery. Clin Orthop Relat Res. 2004;418:61-66.
M, Leunig M, Clarke E, Ganz R. Femoroacetabular impingement as a factor
in the development of nonunion of the femoral neck: A report of three
cases. J Orthop Trauma. 2004;18:425-430.
- Reynolds D, Lucas J, Klaue K. Retroversion of the acetabulum. A cause of hip pain. J Bone Joint Surg Br. 1999;81:281-288.
SJ, Bryant JT, Ganz R, Ito K. The acetabular labrum seal: A poroelastic
ﬁnite element model. Clin Biomech (Bristol, Avon). 2000;15:463-468.
SJ, Bryant JT, Ganz R, Ito K. An in vitro investigation of the
acetabular labral seal in hip joint mechanics. J Biomech. 2003;36:171-178.
- Leunig M, Beck M, Woo A, et al. Acetabular rim degeneration: A
constant ﬁnding in the aged hip. Clin OrthopRelat Res. 2003;413:201-207.
- Eijer H, Myers SR, Ganz R. Anterior femoro-acetabular impingement after femoral neck fractures. J Orthop Trauma. 2001;15:475-481.
K, Durnin CW, Ganz R. The acetabular rim syndrome. A clinical
presentation of dysplasia of the hip. J Bone Joint Surg
- Leunig M, Casillas MM, Hamlet M, et al.
Slipped capital femoral epiphysis: Early mechanical damage to the
acetabular cartilage by a prominent femoral metaphysis. Acta Orthop
Scand. 2000;71: 370-375.
- Myers SR, Eijer H, Ganz R. Anterior
femoro-acetabular impingement after periacetabular osteotomy. Clin
OrthopRelat Res. 1999;363:93-99.
- Pitto RP, Klaue K, Ganz R,
Ceppatelli S. Acetabular rim pathology secondary to congenital hip
dysplasia in the adult. A radiographic study. Chir Organi
- Stulberg SD, Cordell, LD, Harris WH, et
al. Unrecognized childhood hip disease: A major cause of idiopathic
osteoarthritis of the hip. Proceedings of the Third Open Scientiﬁc
Meeting of the Hip society. The Hip. St Louis, MO: CV Mosby; 1975,
- Nötzli HP, Wyss TF, Stoecklin CH, et al. The contour
of the femoral head-neck junction as a predictor for the risk of
anterior impingement. J Bone Joint Surg Br. 2002;84:556-560.
KA, Schoeniger R, Ganz R. Anterior femoroacetabular impingement due to
acetabular retroversion. Treatment with periacetabular osteotomy. J Bone
Joint Surg Am. 2003;85-A(2): 278-286.
- Reynolds D, Lucas J,
Klaue K. Retroversion of the acetabulum. A cause of hip pain. J Bone
Joint Surg Br. 1999;81:281-288. Comment in: J Bone Joint Surg
- Pﬁrrmann CW, Mengiardi B, Dora C, et al. Cam and pincer
femoroacetabular impingement: Characteristic MR arthrographic ﬁndings in
50 patients. Radiology. 2006;240:778-785.
- Erratum in: Radiology. 2007;244:626. Comment in Radiology. 2007;244:625-626; author reply 626.
- James SL, Ali K, Malara F, et al. MRI ﬁndings of femoroacetabular impingement. AJR Am J Roentgenol. 2006;187:1412-1419.
A, Yoon LS, Belzile E, et al. Triad of MR arthrographic ﬁndings in
patients with cam-type femoroacetabular impingement. Radiology. 2005;
- Stoller DW, Tirman PFJ, Bredella MA.
Femoroacetabular impingement. In: Diagnostic Imaging: Orthopaedics. Salt
Lake City, UT: Amirsys; 2004.
- Sadro C. Current concepts in magnetic resonance imaging of the adult hip and pelvis. Semin Roentgenol. 2000;35:231-248.
M, Beck M, Kalhor M, et al. Fibrocystic changes at anterosuperior
femoral neck: Prevalence in hips with femoroacetabular impingement.