The use of magnetic resonance (MR) imaging of the hip has been increasing beyond its traditional use in evaluation of fractures and osteonecrosis. The authors review the more established indications for MR of the hip and their pertinent findings, followed by a discussion of other abnormalities that radiologists encounter with increasing frequency.
is an Associate Professor and
is a Fellow in the Department of Radiology, Duke University
Medical Center, Durham, NC.
Traditionally, magnetic resonance (MR) imaging of the hip has
been primarily used for evaluation of fractures and osteonecrosis;
however, there is increasing appreciation of the utility of MR in
diagnosing other musculoskeletal pathology. This article will
review the more established indications for MR of the hip along
with their pertinent findings, followed by a discussion of other
abnormalities that we encounter with increasing frequency.
One of the well-known indications for MR imaging of the hip is
for detection of vascular insults to the femoral head, the most
common of which is avascular necrosis (AVN). In the early stages of
this process, bone scans and radiographs may be normal.
The MR appearance of AVN includes diffuse bone marrow edema, which
becomes more focal in the femoral head with a central area of fatty
marrow surrounded by a serpiginous line of low signal between the
10 o'clock and 2 o'clock positions (Figure 1).
Less commonly, the double-line sign of a low signal area
surrounding an inner line of high signal on T2-weighted images is
seen. The stages of AVN visible on conventional radiography do not
parallel those seen on MR imaging.
MR is recommended when AVN is suspected clinically and the
conventional radiographs are normal, and in patients with known AVN
in one hip to assess for contralateral involvement, which is not an
infrequent occurrence. Treatment includes core decompression,
rotational osteotomy, and free vascularized fibular grafting.
The current etiologic theory of idiopathic transient
osteoporosis of the hip (ITOH) is that it is caused by a vascular
This abnormality has also been called transient painful bone marrow
edema. Although more common in middle-aged men, it also can occur
in pregnant women. Patients present with severe pain but no history
Conventional radiographs may show osteopenia, while MR imaging
shows low T1 signal and high T2 signal in the femoral head
extending to the inter-trochanteric region (Figure 2). There is
often an associated moderate joint effusion. This pattern is
nonspecific and can mimic early AVN and infection but resolves
spontaneously, generally in 6 to 8 months, thus allowing
distinction between AVN and ITOH. A follow-up MR is occasionally
necessary to make the diagnosis.
This process can also migrate to other joints. Treatment is
Stress fractures and insufficiency fractures have been reported
in all bones around the pelvis. Often, the initial radiographs are
normal and, therefore, MR imaging is strongly indicated if there is
suspicion of fracture, as it is sensitive and specific for
identifying stress fractures. These fractures should be diagnosed
in a timely fashion, especially when the femoral neck is involved,
as patients should be protected to prevent progression to complete
fracture, which can occur with continued unlimited weight-bearing.
In such cases, MR imaging shows a linear low signal fracture line
on T1- and T2-weighted images with associated surrounding edema
Sacral stress fractures have recently been described in
long-distance runners. The patient's symptoms mimic that of disk
disease, for which they are often erroneously treated. Findings on
MR examination show low signal on T1-weighted images and high T2
signal paralleling the sacroiliac joint (Figure 4). The treatment
is rest with no further running until symptoms have resolved.
Insufficiency fractures are most common in patients with
osteoporosis and those who have undergone radiation therapy. These
fractures can be radiographically occult. Since the patient's bones
are osteopenic, even incidental trauma can result in a fracture.
Therefore, lack of history of trauma should not dissuade physicians
from considering fracture as a possible explanation for hip pain.
Similarly, since weight-bearing is sometimes tolerated by the
patient when the fracture is nondisplaced, a history of
weight-bearing can be misleading. Bone scintigraphy was formerly
the imaging modality of choice to diagnose radiographically occult
insufficiency fractures, with delayed imaging often necessary in
these osteopenic patients. With MR imaging, the fracture line is
readily visible as linear low signal on T1- and T2-weighted images
with surrounding edema (Figure 5).
The most common locations for insufficiency fractures of the
pelvis are subcapital, intertrochanteric, sacral, supra-acetabular,
and pubic bones; often multiple fractures co-exist.
Supra-acetabular insufficiency fractures are diagnosed by
recognition of a curvilinear low signal fracture line that
parallels the acetabular roof with adjacent edema, while sacral
insufficiency fractures show the low signal fracture line
paralleling the sacroiliac joint (Figures 6 and 7).
Most commonly, the adjacent edema in sacral fractures does not
cross the midline unless the fracture is bilateral. This can mimic
metastatic disease when edema without a distinct fracture line is
present. A search for surrounding soft-tissue extension and bone
destruction supports a pathologic process.
Salter-Harris fractures can occur at the hip and are most common
in adolescence. Slipped capital femoral epiphysis (SCFE) affects
males more often than females and has a higher incidence in
overweight children. Radiographs may be equivocal and MR can be
utilized to show the widened physis with increased signal on
T2-weighted images as well as the medial and posterior displacement
of the femoral head. Early AVN,which affects 15% of children with
this process, can also be detected and can be a result of the
injury to the physis during the Salter injury or as a result of the
Herniation pit is a commonly seen aperture in the cortex of the
femoral neck along its anterior-lateral surface. It is the result
of ingrowth of fibrocartilaginous elements through a perforation in
the cortex. Herniation pits can change in size due to mechanical
forces and can be unilateral or bilateral, but are asymptomatic. MR
shows a well-defined focus of low signal on T1-weighted images and
increased signal on T2-weighted images in the typical anterolateral
MR imaging can clearly demonstrate occult fractures in the
setting of pain, but can also reveal soft-tissue abnormalities
related to the muscles and tendons of the pelvis when conventional
radiography is normal. Muscle strains, hematomas, and tendon tears
can account for pain when no fracture is seen.
The most common muscle strain at the pelvis involves the
hamstrings, which originate from the ischial tuberosity. The
quadriceps and adductor muscles can also be injured but less
frequently. MRI findings in muscle strain show muscle enlargement
on T1 with loss of normal intervening fat
and possible hemorrhage, while T2-weighted images show a feathery
interstitial increased signal from edema and hemorrhage with
perifascial edema (Figure 8).
Complete tears are readily diagnosed with MR imaging by noting the
free edge of the tendon.
Gluteus medius tendon tears
Gluteus medius tendon tears have also been referred to as
greater trochanter pain syndrome, as patients complain of chronic
pain around the hip or groin, which can mimic intra-articular hip
pathology. This entity is most often seen in older women, but can
be seen in men also. This is associated with tears or tendinopathy
of the gluteus medius or minimus and their associated muscles. MR
imaging shows high signal in the muscles on T2-weighted images with
increased signal in the tendons, which may be discontinuous or
avulsed from the greater trochanter. Signal is isointense to
musculature on T1-weighted images and is difficult to appreciate as
The sciatic nerve is intimately associated with the piriformis
muscle and usually lies just anterior to the muscle. Variations do
exist and the nerve can split through the muscle, or the nerve can
split, with only a portion going through the muscle and a portion
remaining superficial to the muscle belly. Because of these
variations, compression, hypertrophy, or injury to the piriformis
muscle can cause irritation of the sciatic nerve, mimicking
radicular symptoms from disc pathology. MR can show asymmetry in
muscle size, without abnormal signal in the muscle itself unless
there has been trauma, in which case high signal on T2-weighted
images may be seen due to edema and hematoma.
The largest bursa in the body is the iliopsoas bursa, which is
located anterior to the hip joint and adjacent to the femoral
vessels and iliopsoas muscle. It is not seen unless it is distended
by fluid, in which case it is identified as a well-defined area of
low signal on T1-weighted images and increased on T2-weighted
images immediately anterior to the iliopsoas muscle. After
administration of gadolinium, the fluid does not enhance but the
bursal lining does, thus differentiating this from a solid
soft-tissue mass, which would enhance diffusely. If bursae are
distended, they can cause pain, usually in patients with rheumatoid
or osteoarthritis. This bursa can communicate with the hip joint in
15% of patients.
Greater trochanter bursitis can also cause hip pain, usually
laterally, and is secondary to repetitive hip flexion. It can mimic
gluteus medius tendon tears clinically, which occur in a similar
population. It is often associated with gluteus medius
tendinopathy. MR imaging shows isointense fluid to muscle along the
greater trochanter, which is increased in signal on T2-weighted
images (Figure 9). Bursitis is treated with anti-inflammatory
medications or direct steroid injection into the bursa.
The acetabular labrum is a rim of fibrocartilaginous tissue
around the acetabulum that deepens the acetabular fossa and
provides additional coverage of the femoral head. The labrum is
normally triangular on axial and coronal MR images, is low in
signal on all pulse sequences, and is thicker at the
posterosuperior portion and thinner at the anteroinferior
Symptoms of labral tears include clicking, persistent pain, and
decreased range of motion. Etiologies include a single traumatic
event, chronic stress associated with athletic participation, or
developmental dysplasia of the hip. Most tears occur along the
anterior-superior labrum with chondral defects associated in 30% of
MR imaging with intra-articular administration of gadolinium and
a smaller field of view to visualize only the hip in question,
rather than the entire pelvis, is the most sensitive method of
imaging labral tears. Findings include linear or diffuse high
signal within the labrum, deformity of contour, or complete
detachment (Figures 10 through 12). A paralabral cyst may also be
visible and is a strong indicator of an underlying tear (Figure
Pitfalls to be avoided in diagnosing labral tears include
undercutting of the acetabular cartilage that extends medially deep
to the labrum. In addition, the iliopsoas tendon courses anteriorly
to the labrum and can mimic a tear (Figure 14).
Surgery is generally indicated for symptomatic labral
Osteochondral defects of the femoral head are yet another entity
that can cause hip pain in the setting of normal plain radiographs.
These are more common in athletes and there may or may not be a
history of a specific traumatic event. MR imaging shows a
wedge-shaped signal abnormality along the medial femoral head from
the 10 o'clock to 11 o'clock positions (Figures 15 and 16). This
area is smaller than that seen for AVN, and does not show
serpiginous low signal as is seen with AVN. Several of these
patients have also been shown to have delamination injuries at
The more well-recognized etiologies of hip pain, such as
vascular abnormalities and fractures, are often radiographically
occult. Other potential sources of hip pain, such as labral
pathology, muscle and tendon injuries, inflamed bursa, and
osteochondral lesions, are diagnosed exclusively with MR imaging.
Therefore, MR imaging is the modality of choice for evaluating
pathology in and around the hip and pelvis. A patient with hip pain
and negative plain films should undergo MR imaging to assess the
source of pain, as the lesions presented are radiographically
occult and management of the patient can be affected.