surgical treatment of hip arthritis || cross-sectional imaging of the hip

Cross-sectional Imaging of the Hip 19

Cross-sectional Imaging of the Hip

Adam C. Zoga and W. James Malone

Cross-sectional Imaging Modalities 19Injury-specific Imaging 21

Occult Hip Fracture 21Characterization of Known Fracture 21Acetabular Labral Tears 22

Impingement Syndromes 22Muscle Injuries 23Osteonecrosis 25Bursitis 25Infection 25

Arthropathies 27Neoplasm 27Sacroiliac and Lumbosacral Pathology 27Postoperative Patients 27

C H A P T E R O U T L I N E

CROSS-SECTIONAL IMAGING MODALITIES

With rapid technical advances over the last two decades, cross-sectional imaging, most notably CT and MRI, have become integral tools in diagnosis and treatment of muscu-loskeletal disease. Although shoulder and knee MRI have been standard of care for more than a decade, more recently, MRI, MR arthrography, and multidetector CT have played increasingly important roles in diagnosing diseases of the hip. The principal benefit of MRI and multidetector CT over radio-graphs is that they allow for three-dimensional, multiplanar evaluation of the hip joint. Both modalities have strengths and relative weaknesses, and these inherent characteristics typi-cally favor one modality over the other in evaluation of spe-cific pathologic conditions.

A primary advantage of CT is its wide availability and acces-sibility. It is generally a succinct and accurate examination that is commonly used when time and availability are the prime considerations. The more recent advent of multidetector CT allows for the simultaneous acquisition of 4, 16, or 64 thin or overlapping tomographic slices, greatly reducing imaging time, decreasing motion artifact, and markedly improving image resolution compared with the predecessors of multide-tector CT. High-resolution multiplanar reformats can be per-formed days after the scan has been performed. For fine bony detail, multidetector CT offers unparalleled resolution advan-tages compared with MRI or conventional CT. It has no

compatibility issues with metallic prostheses or devices such as pacemakers and protocols using multidetector CT have been designed to allow for supreme resolution at the prosthesis- bone interface. CT exposes the patient to varying degrees of ionizing radiation, however, and higher resolution multide-tector CT studies tend to increase this radiation dose even more. Also, CT is insensitive to soft tissue injuries around the hip, although it can easily detect a hip effusion.

MRI produces excellent tissue contrast compared with the gray-scale images of CT. It allows evaluation of not only the bony integrity of the hip and abnormalities of the surrounding soft tissues, but also the physiologic state of structures, as in bone marrow edema after a traumatic contusion. Furthermore, MRI makes routine contrast discrimination at tissue-tissue interfaces possible, a trait unique to this imaging modality. This means that fibrocartilage and hyaline cartilage structures may be reliably assessed without subjecting the patient to the ionizing radiation required for CT and radiography.

One disadvantage to MRI is that the lengthier MRI exami-nation (typically 30 to 45 minutes) requires the patient to remain motionless for prolonged periods to obtain optimal images. Also, many patients with cardiac pacemakers and shrapnel near the orbits or spinal cord are not candidates for MRI, and true claustrophobia remains an issue with many MRI systems. Nevertheless, mild claustrophobia or general-ized anxiety should not preclude a diagnostic MRI examina-tion. Patients with mild claustrophobia or generalized anxiety should be referred for MRI on newer “open” or “short bore”

C H A P T E R 3

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20 Surgical Treatment of Hip Arthritis

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delay) can be used in similar situations. This method cannot, however, achieve adequate joint distention with indirect arthrography in the absence of a preexisting joint effusion. For this reason, we reserve indirect arthrography of the hip for suspected labral tears when a direct arthro-gram is logistically impractical and for some postoperative indications. The radiologist generally should have a role in deciding which study is most appropriate before imaging, but intra-articular or intravenous contrast administration often requires an order or prescription from the referring clinician.

Although interpretation of cross-sectional imaging studies of the hip might be best left to the radiologist, orthopedists and emergency medicine clinicians frequently find themselves in a setting where they must provide a preliminary interpreta-tion of CT or MRI examinations. With multidetector CT, iden-tifying pathology reliably on a quality study can be easy for someone comfortable with plain x-ray interpretation; getting interpretable images is the most difficult part. All of the infor-mation from the multidetector CT is on one series of axial images, although additional reformatting of this information in coronal and sagittal planes and three-dimensional models can be helpful in confirming pathology. Software applications allowing for accurate three-dimensional reformats are useful in the setting of articular fractures to help quantify the percent-age of surface area involvement (Fig. 3-1).

magnet designs that are tolerated more easily by anxious patients.

A limitation of MRI and CT is artifact generated by ortho-pedic hardware. Although the “susceptibility artifact” of MRI can be minimized by tailoring the technique, the remaining artifact sometimes precludes optimal evaluation of the area of concern. “Beam hardening” artifact of prostheses in CT was a major problem for years, but multidetector CT pro-tocols have virtually eliminated this problem. At this time, multidetector CT with a metal protocol is the imaging study of choice for indications of periprosthetic lesions, such as component loosening and particle disease.

With both imaging modalities, there are additional con-siderations to keep in mind, such as contrast administra-tion. Contrast-enhanced examinations with intravenously administered contrast agents are typically reserved for evalu-ation for infection, inflammatory arthropathies, neoplasms, and vascular lesions.1-4 Rarely, a contrast-enhanced multide-tector CT scan should be performed over MRI for the afore-mentioned indications. In addition, direct MR arthrography and CT arthrography (which involve direct administration of contrast material into the joint) can be used to better evaluate small intra-articular bodies and cartilaginous struc-tures such as the labrum or articular cartilage. Indirect MR arthrography (intravenous administration of contrast mate-rial, which readily accumulates in the joint after a short

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FIGURE 3-1 A and B, Coronal (A) and sagittal (B) reformatted images from 16 detector row multidetector CT (Philips Medical Systems) show a comminuted and displaced posterior column acetabulum fracture (arrows). C, Coronal oblique three-dimensional reconstruction displays displaced acetabular fragments with an intact hip joint (arrows). D and E, After digital subtraction of the femur from the three-dimensional reconstructions, fracture extension to the articular surface is shown (curved arrow on D) along with the degree of displacement of acetabular rim components (straight arrows on E).


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Characterization of Known FractureComplex fractures such as acetabular fractures, severely com-minuted hip fractures, and hip dislocations (postreduction) are generally best evaluated by multidetector CT due to its superior resolution and multiplanar capabilities. Small bone fragments can easily be missed on MRI, and small degrees of displacement are difficult to quantify. Our policy is to perform coronal, sagittal, and three-dimensional reformatted imaging by multidetector CT in all cases of isolated acetabu-

In most cases, multidetector CT of a bone or joint may be interpreted in a similar fashion to a radiographic series. Dis-placed fractures are often readily visible and practically unmistakable, although the chronicity of some fractures can be more difficult to establish. Arthritis on CT looks similar to arthritis on radiographs. The same can be said for specific radiologic findings; for example, a periosteal reaction in the setting of osteomyelitis can be clearly diagnosed by CT.

Interpretation of MRI sequences can be more daunting. For even the most basic interpretations, each MRI sequence must be categorized as fluid-sensitive or fat-sensitive. Fluid-sensitive sequences include all T2-weighted sequences and short tau inversion recovery (STIR) sequences. On these images, all fluids (including water, blood, and edema) are bright, or hyperintense. On fat-sensitive T1-weighted sequences, fluid is dark, but normal bone marrow is bright. With these images, loss of the normal hyperintense bone marrow signal often leads to identification of pathology. When the interpreter is confident about this categorization of the MRI sequences available, basic and preliminary inter-pretation of pathologies such as fracture and joint effusion is possible for clinicians who have an understanding of the pathologies themselves.5

INJURY-SPECIFIC IMAGING

Occult Hip FractureIn the setting of a radiographic examination that is equivocal for hip fracture or negative for fracture but accompanied by a persistent high clinical suspicion for occult fracture, MRI and multidetector CT can be used for further assessment. In our opinion, which is supported by radiology literature, MRI is the imaging study of choice to exclude occult hip fracture. Even a limited, 15-minute MRI protocol is nearly 100% sensi-tive for occult hip fracture if it is a fluid-sensitive (STIR or T2-weighted fat-suppressed) sequence. In cases of fracture, both of these sequences show hyperintense (bright) bone marrow edema surrounding the fracture site, and an accom-panying T1-weighted sequence can be used for description and classification of the fracture using the hypointense (dark) fracture line (Fig. 3-2).6-9

In difficult cases of subtle nondisplaced fracture in an oste-openic patient, the edema on MRI that alerts the radiologist to fracture is not visible on CT. Similarly, subtle stress frac-tures of the femoral neck, acetabulum, pubic symphysis, and sacrum are common and are best evaluated by MRI for the same reason. Subcapital proximal femur fractures are particu-larly difficult to diagnose on CT and on conventional radio-graphic series. MRI of the hip or of the entire pelvis is the standard of care in these cases when there is discordance between physical examination findings and radiographs or CT, or when CT and radiographic studies are equivocal for fracture. Even so, a multidetector CT examination identifies most hip fractures and is a reasonable option to try, especially when the patient is already undergoing CT scanning as part of a trauma workup. If the CT scan is negative but the clinical suspicion for proximal femoral fracture persists, MRI is indi-cated. In contrast, if even a mediocre-quality MRI examina-tion is negative for hip fracture, there is no acute or subacute hip fracture.10

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FIGURE 3-2 A and B, Coronal STIR (A) and T1-weighted spin echo (B) MR images acquired on a 0.3-T open system (Hitachi Airis II) show extensive bone marrow edema throughout the femoral neck (arrow) diagnostic of a fracture. The hypointense fracture “line” is more subtle, but confirms the diagnosis. These two sequences, and a T2-weighted fast spin echo image not shown, comprise a fast hip fracture protocol that totals 11 minutes of imaging time and is sensitive and specific for fracture, avascular necrosis, effusion, osteoarthritis, and numerous extra-articular pathologies.


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Although this constellation of findings can be identified with the standard noncontrast hip protocol, we are currently employing a direct arthrographic protocol in the clinical setting when there is suspicion of impingement in order to identify the abnormal morphology and its sequelae. On a direct MR arthrographic study, a triad of findings—an antero-superior labral tear, subjacent articular cartilage defect on the acetabulum, and an abnormal alpha angle on axial oblique images acquired along the femoral neck—has been shown to

lar fracture. In contrast, when proximal femoral fractures are identified, they might be more consistently characterized by MRI. MRI findings of bone marrow edema lend insight into fracture extension and vector of biomechanical force. A sub-capital fracture that was occult on radiographs and CT would be readily identifiable on noncontrast MRI. In subacute frac-tures, MRI is extremely sensitive for early femoral head avas-cular necrosis. Likewise, previously occult femoral neck fractures are easy to distinguish from intertrochanteric frac-tures on MRI by examination of the bone marrow edema pattern. If the size or state of a hematoma is of concern, MRI is the modality of choice, but if the primary objective is to map out the fracture course, CT might be a better tool.

Acetabular Labral TearsThe preferred technique for imaging the acetabular labrum is direct MR arthrography. Labral tears are diagnosed by identifying paramagnetic contrast material (which is white on most MRI sequences) that undermines or outlines the labral defect or extends directly into the labrum substance (which is normally black on MRI sequences) (Fig. 3-3). Smaller, undersurface tears can be differentiated from normal variations such as sublabral recesses (which are cur-rently a subject of controversy in the radiology literature), by their location and by the configuration of the defect. In younger patients with little joint wear and tear, the normal anterior and superior labrum should be sharply defined; it should be triangular and hypointense on all sequences. There is no recess anteriorly, so a defect in the undersurface of the anterior labrum which alters its triangular morphol-ogy should be considered a tear. Signal alteration within the labrum (especially fluid bright defects or findings into which contrast material readily flows) should also raise strong suspicion of a tear.

On noncontrast fluid-sensitive MRI, a paralabral cyst can be the imager’s friend in establishing the presence of a labral tear.11 Even in the absence of a visible labral defect, a mul-tilobulated paralabral cystic structure with a neck extending toward the labrum is indicative of occult labral tear.12-14 Using this criterion alone for establishing the diagnosis of labral tear does not frequently allow for accurate localization of the injury, however; as a result the arthroscopist may encounter difficulties later in portal selection during arthroscopy.15,16

Impingement SyndromesThe radiographic evaluation of the two classic femoroacetabu-lar impingement syndromes (cam type and pincer type) con-tinues to evolve. Cam type is more frequently described and is believed to be a more common cause of the clinical impinge-ment syndrome. Several articles have been published in the radiology journals describing imaging appearances of cam-type femoroacetabular impingement. Although the most widely accepted criteria to date are based on x-ray findings,17 a pattern of MRI findings is emerging as a reliable indicator of cam-type impingement. Capsular hypertrophy, anterior labral injury, and a hyperostotic bump at the anterolateral femoral head/neck junction all have been described in multi-ple series that have investigate the appearance on MRI of cam-type femoroacetabular impingement.18,19

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FIGURE 3-3 A and B, Sagittal (A) and axial (B) T1-weighted spin echo fat-suppressed MR images dedicated to the left hip acquired at 1.5 T (Philips Intera) after direct, intra-articular infusion of dilute gadolinium contrast material (Magnevist; Berlex) show a defect in the undersurface of the anterior acetabular labrum with frank imbibition of contrast material into the labral substance (arrows) diagnostic of a labral tear. Direct MR arthrography is currently the standard of care imaging examination for acetabular labral tears.


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Protocol development for MRI of muscle injury can present numerous issues because the location of the muscle injury is frequently difficult to determine by history and physical examination before imaging. Most frequently, muscle injuries are centered at the myotendinous junctions, so large field of view MRI sequences that cover the articulation (hip, in this case) and the nearest myotendinous junction are often employed. These field-of-view sequences come with a lower reso lution, making accurate description of local pathology challenging.

We recommend beginning an MRI investigation for sus-pected muscle injury around the pelvis with large field of view (40 cm), fat-suppressed, fluid-sensitive sequences (coronal STIR, axial T2-weighted fast spin echo). A review of these sequences generally allows the imager to localize the pathology. When the precise site of injury is confirmed, smaller field of view anatomy-specific (T1-weighted) and fluid-sensitive (T2-weighted) sequences in all three conven-tional planes can be acquired to accurately assess the

correlate strongly with cam-type femoroacetabular impinge-ment on clinical examination and at surgery (Fig. 3-4).20,21

Muscle InjuriesAs a result of the many muscles that originate and insert around the hip and pelvis, numerous myopathies may be encountered on a routine hip examination, and all are best evaluated by MRI. Fluid-sensitive sequences show location and extent of edema, and so are useful in detecting common injuries that range from tendinosis to muscle strain to com-plete tears (commonly occurring in the gluteal muscles, the hamstrings, the iliopsoas, the quadriceps, and the adductor muscles). T1-weighted images can identify muscle atrophy from chronic injury and diagnose soft tissue hematoma.17 Similarly, the adductor and rectus abdominis tendon origins can be well seen on MRI, making it possible for the radiologist to diagnose pathology in athletic patients with “pubalgia” or “sports hernia” symptoms.

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Alpha angle: 57 degrees

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FIGURE 3-4 MR arthrographic appearance of cam-type femoroacetabular impingement. A and B, Coronal (A) and sagittal (B) T1-weighted spin echo fat-suppressed images acquired at 1.5 T (General Electric Signa, Berlex Magnevist) show an acetabular labral tear at its anterosuperior undersurface (arrows), an osseous prominence at the anterolateral femoral head/neck junction (arrowheads) and an articular cartilage defect at the anterosuperior acetabular rim (curved arrow). C, Axial oblique image acquired along the femoral neck shows an abnormal alpha angle, greater than 55 degrees. D, Coronal image acquired with the patient in a FABER (femoral abduction external rotation) position accentuates the osseous excrescence on the femur and the labral tear.


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severity of the injury. For muscle injuries centered at the myotendinous junction, radiologists have adapted an ortho-pedic classification system based on imaging findings. A grade I strain injury shows a feathery, pennate pattern of muscle edema with no visible disruption of fibers. A grade II partial tear manifests as a fluid-filled gap involving a portion of the muscle, or a partial tear. A grade III injury shows complete disruption of the central tendon with retraction of the tendon and muscle fibers, and a complete, fluid-filled void where the myotendinous junction would normally be (Fig. 3-5).

Avulsion muscle injuries around the pelvis must be inter-preted differently, as radiologists have learned the clinical importance of establishing the exact location of injury. On MRI sequences, periosteal avulsions show a wavy and retracted tendon end with an attached fragment of periosteum that is black on all MRI sequences. Often, a periosteal avulsion can be confirmed on MRI by noting avulsive bone marrow edema at the site of its previous attachment. In contrast, a tendinous tear away from the bony attachment is unlikely to exhibit bone marrow edema. With this injury, it is important to identify and measure the size and length of the torn tendon fragment still attached to the bone.22,23

A final tendinous pathology that one frequently encoun-ters when imaging the hip is hydroxyapatite deposition disease. Sometimes referred to as calcific tendinitis, hydroxyapatite deposition disease is commonly encountered at the gluteus medius insertion on the greater trochanter of the femur, and it can be easily missed when interpreting an MRI examination without the benefit of correlative radio-graphs. On MRI, hydroxyapatite is dark or black on all sequences, and characteristically “blooms” or looks more extensive on gradient echo sequences. The gluteus medius

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FIGURE 3-5 Sagittal T2-weighted fast spin echo fat-suppressed image acquired at 1.5 T (General Electric Signa) shows complete disruption of the semimembranosus, semitendinosus, and biceps femoris origins from the ischial tuberosity with a large, predominately fluid hematoma (arrow). This qualifies as a grade III hamstring tear.

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FIGURE 3-6 A and B, Coronal STIR (A) and axial T2-weighted fast spin echo fat-suppressed (B) images from a 1.5 T system (General Electric Signa) show striking hypointensity at the distal gluteus medius tendon typical for calcium (arrows) surrounded by hyperintense soft tissue edema. C, Axial CT acquisition (Philips) confirms the diagnosis of hydroxyapatite deposition disease at the distal gluteus medius (arrow).

tendon itself is dark, and the hydroxyapatite deposits are easy to overlook. If hydroxyapatite deposition disease is suspected clinically or on the basis of office-based radio-graphs, it is best to alert the radiologist to avoid this poten-tial pitfall (Fig. 3-6).


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in the femoral head. A subchondral crescent sign can be seen in both conditions as well. These cases may resolve spontane-ously, as in the setting of transient osteoporosis, or progress to cortical collapse and late-stage osteonecrosis. A current theory for this entity is that it is a manifestation of a subchon-dral insufficiency fracture, as is more frequently seen in the medial femoral condyle of the knee (Fig. 3-9). We suggest follow-up noncontrast MRI 3 to 6 weeks after the initial study to monitor resolution or progression of disease, and as a tool in guiding therapy.18,19,25

BursitisA multitude of anatomic bursae exist around the hip, but the iliopsoas and numerous trochanteric bursae are most fre-quently identified as sources of pain and decreased range of motion. MRI with its supreme soft tissue contrast should readily identify fluid-distended bursae on fluid-sensitive sequences. Any organized collection of fluid that lifts the psoas tendon off the anterior hip capsule can be termed iliop-soas bursitis, but distention in the anteroposterior plane may be the best predictor of symptoms (Fig. 3-10).20,21 The diag-nosis of trochanteric bursitis is more complicated because of the six anatomic bursae around the insertions of the gluteus maximus, medius, and minimus tendons around the greater trochanter. A sliver of fluid around the greater trochanter is present in many patients, especially in obese patients, and is likely physiologic. We reserve the term trochanteric bursitis for patients with fluid measuring more than 2 mm in a transverse plane adjacent to the greater trochanter or asymmetric fluid in this location with corresponding unilaterality of symptoms. For patients in whom we are concerned about superimposed septic bursitis, precontrast and postcontrast sequences are acquired.26

InfectionIn addition to septic bursitis, infectious etiologies around the hip involve bone (osteomyelitis), the hip joint (septic arthri-tis), and the surrounding soft tissues (cellulitis, abscess, myositis). Postcontrast MRI and CT can detect cellulitis and

In adolescents, the myotendinous unit may be stronger than the incompletely fused growth plates at tendon origins around the pelvis. Bone marrow edema that exists across a persistent center of transitional cartilage ossification and that is the result either of repeated avulsive forces or a single trauma is a frequent finding on MRI examinations of the teen-aged hip. It is generally referred to in imaging reports as “apophysitis.” After the extensor mechanism of the knee, some of the most frequent locations for apophysitis include the ischial tuberosity, the anterior superior iliac spine, and the anterior inferior iliac spine. Apophysitis can also be seen on MRI examinations of the pelvis or hip. In contrast, this entity is likely to be occult on CT. Findings include hyperintense (bright) signal within the physis on fluid-sensitive sequences and less intense, more poorly defined bright signals on both sides of the growth plate in the periphyseal medullary bone. Additionally on MRI, apophysitis is often bilateral but asym-metric, although symptoms may be unilateral, and imaging of the entire pelvis is recommended (Fig. 3-7).24

OsteonecrosisIntermediate-stage and late-stage osteonecrosis are well depicted with MRI and multidetector CT. MRI is the modality of choice, however, because of its sensitivity in picking up early osteonecrosis (owing to its sensitivity and specificity for staging).18 Not only are the well-known “double line sign” and “crescent sign” of subchondral fracture well seen, but so are the traits of the Federative International Committee on Ana-tomical Terminology (FICAT) radiographic staging, including the presence or absence of cortical collapse, unstable frag-ments, and classic signs of secondary osteoarthritis (Fig. 3-8). When performing MRI for the assessment of potential femoral head osteonecrosis, we recommend combining large field of view coronal and axial images that cover both hips with sagit-tal images dedicated to the hip in question, owing to the great frequency of bilateral disease.

A potential confounder for the diagnosis of acute femoral head osteonecrosis is the entity termed transient osteoporosis of the hip. There is early overlap in imaging findings with these two diagnoses—extensive subchondral bone marrow edema

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FIGURE 3-7 Three sagittal T2-weighted fast spin echo fat-suppressed images of apophysitis acquired at 1.5 T (Philips). A, Avulsive pathology involving the rectus femoris at the anterior inferior iliac spine (arrow) in a 19-year-old female runner with overlying reactive iliopsoas bursitis (arrowhead). B, Similar pathology involving the Sartorius at the anterior superior iliac spine (arrow) in a 23-year-old female runner. C, Fragmentation of the ischial tuberosity apophysis at the hamstring origin (arrows) in a 15-year-old male soccer player.


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effusion (synovitis) and enhancement after contrast adminis-tration further suggest infection, but these findings can also be seen with other pathology. Reactive subchondral marrow is frequently present with a septic hip joint, but, again, this finding alone does not imply infection of the underlying bone. The diagnosis of osteomyelitis should be reserved for MRI examinations that show edema extending beyond the subchondral bone into the medullary cavity on fluid-sensitive images and marrow replacement (hypointensity) on T1- weighted non–fat-suppressed sequences.

abscess by denoting subcutaneous soft tissue enhancement (cellulitis) and rim-enhancing collections (abscess). MRI is the modality of choice because of its sensitivity in detecting find-ings associated with septic hip and osteomyelitis. In the proper clinical setting, an asymmetric hip joint effusion sup-ports the diagnosis of septic hip. An internally complex hip

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FIGURE 3-8 A and B, Coronal STIR (A) and T1-weighted spin echo (B) images from a 1.5-T MRI examination (General Electric Signa) show a typical MRI pattern in acute avascular necrosis of the femoral heads (straight arrows). A, On the STIR image, hyperintense signal in the proximal femoral epiphyses reflects bone marrow edema, and hypointense, crescentic, subchondral lines reflect the margin of the osteonecrosis (curved arrow). Note the hyperintensity within the femoral diaphyses typical for medullary infarction in this patient with sickle cell osteopathy (arrowhead). B, On the higher resolution T1-weighted image, hyperintense signal within the epiphyses remains (arrows), suggesting mummified fat within the osteonecrotic femoral head as demarcated by the hypointense crescent (curved arrow).

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FIGURE 3-9 A and B, Coronal STIR (A) and T1-weighted spin echo (B) images from a 1.5-T MRI examination (General Electric Signa) show extensive bone marrow edema (arrow) without a subchondral crescent in the femoral head of a 60-year-old man with insidious onset of hip pain. The hip joint effusion (arrowhead) and the vague, linear, subchondral line (curved arrow) are suggestive of an insufficiency fracture, as can be seen with transient osteoporosis of the hip, but follow-up with resolution of findings would be necessary to confirm this diagnosis.


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in characterization of the lesion, but it is also sensitive to subtle findings of tumor aggressiveness that are not evident on radiographs, and it provides more accurate staging infor-mation. MRI is without question the modality of choice to diagnose and characterize soft tissue neoplasms, and com-monly the MRI tissue characteristics allow for tumor-specific diagnosis.25 CT can provide additional information, such as identifying subtle matrix calcifications not seen on other modalities. In our opinion, when a tumor has been identified, the patient should have a complete radiologic workup, includ-ing multidetector CT and MRI in addition to the initial radio-graphs. A total body scintigraphic bone scan adds vital information regarding multiplicity of lesions, and is indicated with most malignancies. A bone scan provides the interpret-ing clinician with the most accurate information to aid in diagnosis and staging.28

Sacroiliac and Lumbosacral PathologyCommonly, sacroiliac pathology such as arthropathies or infection, and lumbosacral pathology such as cysts, neuro-mas, and nerve sheath tumors compressing the sciatic nerve, are found incidentally while imaging the hip. In both instances, contrast-enhanced sequences are indicated for optimal evalu-ation. One important neural structure to identify in patients with hip pain and radiculopathic symptoms is the sciatic nerve. Occasionally, one division of the sciatic nerve can take an anomalous course, passing just above the piriformis muscle or between the bellies of the piriformis muscle. In these patients, contraction of the piriformis can cause impingement of the sciatic division involved; this clinical entity is termed piriformis syndrome.

Postoperative PatientsAlmost every type of orthopedic hardware degrades signal in the surrounding tissues on most MRI sequences. Measures can be taken to reduce the susceptibility artifact that makes postoperative MRI of the hip so challenging, but advances in multidetector CT in recent years have entrenched it as the modality of choice for imaging pathologies including pros-thetic loosening, giant cell synovitis, prosthetic failure, and heterotopic ossification.29-31 Exquisite, high-resolution images

MRI can detect infection of the muscles themselves, termed pyomyositis. This entity can be differentiated from simple dependent intramuscular edema and the edema seen with diabetic myonecrosis based on muscle enhancement on post-contrast fat-suppressed T1-weighted images. Muscle edema from denervation can appear similar to infection and should be considered. Although current MRI applications allow for a high sensitivity and specificity for the diagnosis of septic joint and osteomyelitis, joint aspiration remains the gold standard for confirmation because other inflammatory arthropathies can confound the diagnosis.27

ArthropathiesAlthough arthritis remains an important finding, it is rarely the primary diagnostic impetus behind ordering an MRI of the hip. As with osteomyelitis (discussed previously) and bone tumors (discussed subsequently), radiographs remain the workhorse imaging study to support physical examination findings and to guide therapeutic algorithms for most hip arthritis. This is especially true for osteoarthritis. Neverthe-less, MRI of the hip may be the most valuable single imaging modality for atypical arthropathies. On fluid-sensitive and postcontrast images, an asymmetric joint effusion with associ-ated synovitis and pannus serves as an indicator for the pres-ence of an inflammatory arthropathy.24

MRI also can detect subtle periostitis in young patients with chronic juvenile arthritis. When a single hip is the only joint involved, characteristic MRI findings of ill-defined, masslike, intra-articular deposits with or without secondary erosive bone marrow findings can strongly suggest a primary synovial proliferative process, such as pigmented villonodular synovi-tis. Synovial osteochondromatosis has a similar MRI appear-ance, but manifests as calcific masses on radiographs or CT. Still, there is an overlap of imaging findings in many joint-centered processes including rheumatoid arthritis, amyloid arthropathy, pigmented villonodular synovitis, and infection, and tissue diagnosis is necessary for confirmation of any of these uncommon hip conditions (Fig. 3-11).

NeoplasmMRI is rapidly becoming an integral part of osseous tumor assessment. Not only does MRI provide information that aids

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FIGURE 3-10 A and B, Coronal STIR (A) and sagittal T2-weighted fast spin echo fat-suppressed (B) images from a 1.5-T MRI examination (General Electric Signa) with a large, extra-articular fluid collection anterior to the hip joint (arrows). The signal meets that of fluid, and findings are diagnostic of iliopsoas bursitis.


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of the bone-metal interface are attainable with multidetector CT using metal protocols and software reconstruction algo-rithms, and these images allow for early and accurate diagno-sis of periprosthetic osteolysis and bone loss. Using similar protocols, it is possible to obtain interpretable images of pros-thetic fractures, although radiographs still play a predominant role in this instance. Three-dimensional reconstructions of multidetector CT data have been shown to be useful in accu-rately assessing prosthesis position and version (Fig. 3-12).32

One instance where MRI may still reign superior to multidetector CT in the postoperative patient is in the case of suspected periprosthetic infection. Although CT may show focal and aggressive bony resorption and destruction, MRI with intravenous contrast administration might show enhancement of bone marrow and of fluid collections. If a periprosthetic infection is suspected or if the goal is to assess infection clearing, as in the case of a two-stage total hip revision arthroplasty after a girdlestone procedure, MRI using artifact reduction sequences and multidetector CT may be warranted.33

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FIGURE 3-11 A and B, Coronal T1-weighted spin echo fat-suppressed images acquired at 1.5 T after intravenous administration of gadolinium contrast material (Philips, Berlex Magnevist) show different intra-articular processes. A, There is no bony enhancement, and the complex hip joint effusion contains hemosiderin-laden, hypointense material (arrows), suggesting a primary synovial process in a patient with pigmented villonodular synovitis. B, The complex joint effusion and the articular surfaces and subchondral regions of the bone enhance (arrows), suggesting an inflammatory arthropathy in a patient with a septic hip joint.

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FIGURE 3-12 A and B, Two coronal reformatted images from 16 detector row multidetector CT examinations using a metal protocol (Philips) in patients with hip pain after total hip arthroplasty. A, The prosthesis is in a normal position with a preserved and nicely demonstrated bone-prosthesis interval at the femoral and acetabular components (arrows). B, Regions of intact bone-prosthesis interval (arrow) are directly adjacent to regions of periprosthetic bony resorption at the acetabulum (arrowhead). This patient had loosening of the acetabular component attributed to particle disease. Multidetector CT with a metal protocol is the standard of care imaging test for suspected periprosthetic osteolysis.

References1. Nomikos GC, Murphey MD, Kransdorf MJ, et al: Primary bone tumors

of the lower extremities. Radiol Clin North Am 40:971-990, 2002.2. Lee SK, Suh KJ, Kim YW, et al: Septic arthritis versus transient synovitis

at MR imaging: Preliminary assessment with signal intensity alterations in bone marrow. Radiology 211:459-465, 1999.

3. Huang AB, Schweitzer ME, Hume E, Batte WG: Osteomyelitis of the pelvis/hips in paralyzed patients: accuracy and clinical utility of MRI. J Comput Assist Tomogr 22:437-443, 1998.

4. Czerny C, Krestan C, Imhof H, Trattnig S: Magnetic resonance imaging of the postoperative hip. Top Magn Reson Imaging 10:214-220, 1999.

5. Zoga AC, Morrison WB: Technical considerations in MR imaging of the hip. Magn Reson Imaging Clin N Am 13:617-634, 2005.

6. Haramati N, Staron RB, Barax C, Feldman F: Magnetic resonance imaging of occult fractures of the proximal femur. Skeletal Radiol 23:19-22, 1994.


29

20. Kassarjian A, Yoon LS, Belzile E, et al: Triad of MR arthrographic findings in patients with cam-type femoroacetabular impingement. Radiology 236:588-592, 2005.

21. Pfirrmann CW, Mengiardi B, Dora C, et al: Cam and pincer femoroa-cetabular impingement: Characteristic MR arthrographic findings in 50 patients. Radiology 240:778-785, 2006.

22. Shabshin N, Rosenberg ZS, Cavalcanti CF: MR imaging of iliopsoas musculotendinous injuries. Magn Reson Imaging Clin N Am 13:705-716, 2005.

23. Koulouris G, Connell D: Hamstring muscle complex, an imaging review. Radiographics 25:571-586, 2005.

24. Nelson EN, Kassarjian A, Palmer WE: MR imaging of sports-related groin pain. Magn Reson Imaging Clin N Am 13:727-742, 2005.

25. Yamamoto T, Nakashima Y, Shuto T, et al: Subchondral insufficiency fracture of the femoral head in younger adults. Skeletal Radiol 36(Suppl):S38-S42, 2006.

26. Ficat RP: Idiopathic bone necrosis of the femoral head early diagnosis and treatment. J Bone Joint Surg Br 67:3-9, 1985.

27. Meislin R, Abeles A: MR imaging of hip infection and inflammation. Magn Reson Imaging Clin N Am 13:635-640, 2005.

28. Bancroft LW, Peterson JJ, Kransdorf MJ: MR imaging of tumors and tumor-like lesions of the hip. Magn Reson Imaging Clin N Am 13:757-774, 2005.

29. Imhof H, Mang T: Advances in musculoskeletal radiology: Multidetector computed tomography. Orthop Clin North Am 37:287-298, 2006.

30. Borrelli J Jr, Ricci WM, Steger-May K, et al: Postoperative radiographic assessment of acetabular fractures: A comparison of plain radiographs and CT scans. J Orthop Trauma 19:299-304, 2005.

31. Buckwalter KA, Farber JM: Application of multidetector CT in skeletal trauma. Semin Musculoskelet Radiol 8:147-156, 2004.

32. Wines AP, McNicol D: Computed tomography measurement of the accu-racy of component version in total hip arthroplasty. J Arthroplasty 21:696-701, 2006.

33. Walde TA, Weiland DE, Leung SB, et al: Comparison of CT, MRI, and radiographs in assessing pelvic osteolysis: A cadaveric study. Clin Orthop Relat Res 437:138-144, 2005.

7. Oka M, Monu JU: Prevalence and patterns of occult hip fractures and mimics revealed by MRI. AJR Am J Roentgenol 182:283-288, 2004.

8. Pandey R, McNally E, Ali A, Bulstrode C: The role of MRI in the diagnosis of occult hip fractures. Injury 29:61-63, 1998.

9. Bogost GA, Lizerbram EK, Crues JV 3rd: MR imaging in evaluation of suspected hip fracture: Frequency of unsuspected bone and soft-tissue injury. Radiology 197:263-267, 1995.

10. Lubovsky O, Liebergall M, Mattan Y, et al: Early diagnosis of occult hip fractures MRI versus CT scan. Injury 36:788-792, 2005.

11. Schnarkowski P, Steinbach LS, Tirman PF, et al: Magnetic resonance imaging of labral cysts of the hip. Skeletal Radiol 25:733-737, 1996.

12. McCarthy JC, Noble PC, Schuck MR, et al: The Otto E. Aufranc Award: The role of labral lesions to development of early degenerative hip disease. Clin Orthop Relat Res 393:25-37, 2001.

13. Czerny C, Hofmann S, Urban M, et al: MR arthrography of the adult acetabular capsular-labral complex: Correlation with surgery and anatomy. AJR Am J Roentgenol 173:345-349, 1999.

14. Leunig M, Werlen S, Ungersbock A, et al: Evaluation of the acetabu-lar labrum by MR arthrography. J Bone Joint Surg Br 79:230-234, 1997.

15. Toomayan GA, Holman WR, Major NM, et al: Sensitivity of MR arthrog-raphy in the evaluation of acetabular labral tears. AJR Am J Roentgenol 186:449-453, 2006.

16. Chan YS, Lien LC, Hsu HL, et al: Evaluating hip labral tears using mag-netic resonance arthrography: A prospective study comparing hip arthroscopy and magnetic resonance arthrography diagnosis. Arthros-copy 21:1250, 2005.

17. Leunig M, Podeszwa D, Beck M, et al: Magnetic resonance arthrography of labral disorders in hips with dysplasia and impingement. Clin Orthop Relat Res 418:74-80, 2004.

18. Jager M, Wild A, Westhoff B, Krauspe R: Femoroacetabular impingement caused by a femoral osseous head-neck bump deformity: Clinical, radio-logical, and experimental results. J Orthop Sci 9:256-263, 2004.

19. Notzli 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 84:556-560, 2002.

surgical treatment of hip arthritis || cross-sectional imaging of the hip

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