Calcium Hydroxyapatite Deposition Disease

Hydroxyapatite deposition disease is a common entity characterized by pain and periarticular deposition of calcium phosphate crystals. The soft tissues around the shoulder are the most common locations for deposits occurring in tendons, joint capsule, ligaments, and bursae. Although this disease process has been assigned multiple names, it is mostly known as calcific bursitis/tendinitis. Middle-aged patients are most commonly affected, and it is slightly more common in men than in women. More than 30% of individuals with calcific deposits about the shoulder are asymptomatic. When clinically apparent, patients present with varying degrees of pain and disability, which may last for weeks or months if left untreated. It is important to note that the finding of hydroxyapatite deposits within the rotator cuff tendons (“calcific tendinitis”) has no relationship with the subsequent development of rotator cuff tears.

Calcific deposits are more easily identified and characterized with radiographs, computed tomography (CT), and ultrasonography (US) than with magnetic resonance (MR). On radiographs, most calcific deposits appear as homogeneous and amorphous densities, ovoid, linear, or triangular in shape, with and without internal trabeculations (Fig. 5-1A). The precise appearance and location varies with the phase of the disease process and specific anatomical structure involved. The supraspinatus is the most frequently affected tendon. US is highly sensitive for detecting even very small calcific deposits and may be used to guide puncture, aspiration, and lavage as therapeutic options. Hyperechoic foci with minimal or no significant posterior shadowing are identified, sometimes as ill defined and fluffy or as discrete, well-defined calcifications that are linear or rounded (Fig. 5-1B). Calcific deposits may be seen on MR as nodular foci of low signal intensity in all pulse sequences (Fig. 5-1C), and may be easier to identify on gradient-echo sequences, as they may induce blooming artifact. Inflammatory changes in surrounding soft tissues may be present and identified as heterogeneous hyperintensity in fluid-sensitive (T2-weighted and short T1 inversion recovery [STIR]) sequences.

Figure 5-1 Calcific bursitis. A, External rotation view of the right shoulder in an adult patient presenting to the emergency room with acute exacerbation of right shoulder pain demonstrates periarticular mineralization (arrow) adjacent to the greater tuberosity of the humerus. B, Oblique longitudinal US image of the shoulder in a different patient also presenting with shoulder pain shows a curvilinear hyperechoic focus (arrow) within the supraspinatus (SS) tendon near its humeral (H) insertion. C, Oblique coronal proton density MR image in the same patient as A shows an amorphous focus of low signal intensity in the subacromial-subdeltoid bursa (arrow) superficial to the SS tendon (asterisk).

Rotator Cuff Abnormalities and Impingement

The concept of extrinsic subacromial impingement was initially described by Neer in 1972 and continues to be a very controversial topic even today. The notion of impingement is based on the anatomical arrangement of the shoulder joint (fixed component) and on the motion of the humeral head relative to the other components of the shoulder joint (dynamic component). In essence, the rotator cuff tendons (primarily the supraspinatus) and muscles, as well as the subacromial-subdeltoid bursa located between the coracoacromial arch and the humeral head, may be impinged with motion of the arm. It is thought that this repetitive microtrauma from friction will lead to tendon degeneration, bursal inflammation, and, ultimately, a tear of the cuff. The “critical zone” is particularly susceptible to this pathophysiological process; this critical zone has been described as a hypovascular area located on the anterior aspect of the supraspinatus tendon, approximately 1 cm from its insertion. Other factors described as potential contributors to the impingement process are narrowing of the subacromial space by enthesophytes arising from the undersurface of the acromion, advanced acromioclavicular joint osteoarthritis with hypertrophic changes, a type III acromion (anteriorly hooked), and the presence of an os acromiale (secondary ossification center at the tip of the acromion that persists after skeletal maturity as a separate bone). However, impingement can occur without any visible anatomic predisposing factor, and the presence of anatomic variations does not necessarily indicate that there is impingement. Thus, the diagnosis of shoulder impingement is made mainly on clinical grounds, rather than on the basis of the radiographs alone. The radiologist should focus on describing, characterizing, and grading rotator cuff disease, as well as on identifying potential contributing sources to the impingement process.

The spectrum of rotator cuff pathology includes tendinopathy or tedinosis, partial- and full-thickness tendon tears, and subacromial-subdeltoid bursitis. Most rotator cuff tears are chronic and occur as the result of repetitive microtrauma. Acute rotator cuff tears are rare but do occur, especially in older patients with preexisting chronic impingement and degenerative tendon changes. It is not unusual, however, for patients with acute or chronic rotator cuff tears to seek care at the emergency room on an urgent basis because of severe or acutely worsening shoulder pain.

Radiographs should always be performed initially for evaluation of shoulder pain. However, they are often not contributory. The presence of gas from vacuum phenomenon in the glenohumeral joint strongly suggests the absence of a full-thickness rotator cuff tear. The rotator cuff tendons cannot be directly seen on radiographs; rather, there are a number of radiographic findings that serve as indirect evidence of cuff pathology and impingement (Fig. 5-2). These include superior subluxation of the humerus with a decreased subacromial space (less than 8 mm) and secondary changes in the humeral head, such as sclerosis, flattening, surface irregularity, and cystic changes. Radiographs may also demonstrate potential anatomic causes of impingement (Fig. 5-3).

Figure 5-2 Chronic rotator cuff tear indirect signs. External rotation view of the shoulder shows superior subluxation of the humeral head with decreased subacromial space to 4 mm (white arrow), sclerosis of the humeral head (asterisk), and surface irregularity at the greater tuberosity (black arrow).

Figure 5-3 Potential anatomic sources of impingement. A, Internal rotation view of the shoulder shows an enthesophyte at the undersurface of the acromion (white arrow) and hypertrophic changes at the acromioclavicular joint (black arrow). B, Y-view of the shoulder reveals a type III (anteriorly hooked) acromion (arrows).

Direct visualization of the rotator cuff tendons is achieved with MR or US. One of these two methods is usually required to accurately diagnose and characterize rotator cuff tears. Selection of the modality depends on availability, individual expertise, and preference of the interpreting radiologist. Both offer relative advantages and disadvantages, which determine preference, and practice trends vary among different world countries. For example, in the United States, MR is used more commonly than US, perhaps because of its faster learning curve and because the method is easily reproducible and less operator dependent than US. On the other hand, US is a great cost-effective alternative in experienced hands and is the first alternative in many institutions throughout the world.

Rotator cuff tendinopathy (tendon degeneration) is characterized on MR by increased signal within the tendon on low TE sequences (T1 and proton density). The tendon may demonstrate associated focal or diffuse thickening, but this is not a constant finding (Fig. 5-4A). Abnormal signal within the rotator cuff tendons in low TE sequences may be seen in a variety of normal situations, and thus there is need for close clinical correlation: most commonly, magic angle artifact as an area of increased signal at 55 degrees from the main magnetic field, which, on oblique coronal planes, coincides with the supraspinatus “critical zone.” Cuff tendon tears present as disruption (interruption) of fibers and may be either partial or full-thickness in the craniocaudal plane. High-signal fluid is seen separating the disrupted fibers. This fluid may extend from the articular (inferior) surface superiorly in varying degrees to the bursal (superior) surface (Fig. 5-4B). Partial thickness tears affecting the articular surface are more common than isolated bursal surface tears. In full-thickness tears, fluid invariably extends across the tendon (Fig. 5-4C) into the subacromial-subdeltoid bursa. Subacromial-subdeltoid bursitis may occur in isolation or in conjunction with rotator cuff tears.

Figure 5-4 Spectrum of rotator cuff pathology seen on MR. A, Tendinopathy or tendinosis is characterized in this proton-density image as increased signal and thickening (arrow) of the supraspinatus (SS) tendon. B, Partial thickness, articular surface tear of the SS tendon on an oblique coronal STIR image characterized by fluid signal (arrow) between the disrupted articular surface and interstitial fibers and the intact bursal surface fibers (arrowheads).C, Full-thickness tear of the SS tendon seen on an oblique coronal T2-weighted sequence as fluid signal (arrow) extending across the entire thickness of the tendon.

Figure 5-5A shows an intact supraspinatus and its relation to the humeral head and deltoid muscle. The primary or direct signs of full-thickness supraspinatus tear in US include nonvisualization of the tendon and a hypoechoic or anechoic full-thickness defect filling the gap of the torn tendon (Fig. 5-5B). Secondary or indirect signs that are helpful to correlate with the primary signs include sagging of the peribursal fat, cortical irregularity at the greater tuberosity, fluid in the subacromial-subdeltoid bursa, and muscle atrophy. Partial-thickness tears manifest sonographically as focal areas of hypoechoic or anechoic tendon defects involving the bursal or articular surface (Fig. 5-5C). An adequate exam requires evaluation of the extension of the defect on two orthogonal planes to confirm the findings. Tendon degeneration is demonstrated as internal heterogeneous echogenicity.

Figure 5-5 Spectrum of rotator cuff pathology on US. A, Normal longitudinal image of the left shoulder demonstrating an intact supraspinatus (SS) and its relation to the humeral head (H) and deltoid muscle (D). B, Longitudinal US image of the left shoulder with nonvisualization of a fully torn and retracted SS tendon. There is a gap in the expected location of the tendon (arrows). C, Longitudinal US image of the left shoulder demonstrating a partial-thickness articular surface tear of the SS tendon as a focal hypoechoic area at its insertion (arrow).

(Courtesy of Alda Cossi, MD.)

Acromioclavicular Joint Disease (Osteolysis and Osteoarthritis)

The incidence of acromioclavicular joint pathology as a cause of shoulder pain is higher than generally realized. The acromioclavicular joint is often ignored, to the point that it has been termed the “forgotten” or “overlooked” joint. Osteolysis and osteoarthritis are two of the most common causes of shoulder pain arising from the acromioclavicular joint.

Osteolysis

Destruction of bone (osteolysis) may be seen as the result of multiple localized or generalized conditions. Destruction of the distal end of the clavicle is not uncommon and may be idiopathic, post-traumatic, or caused by rheumatoid arthritis, hyperparathyroidism, metastatic disease, multiple myeloma, primary osteolysis syndromes, and infection.

Post-traumatic osteolysis deserves special attention because of its relative frequency as a cause of debilitating pain and painful shoulder motion, and because it very often goes unrecognized. This entity can occur after a single or multiple episodes of minimal or major injury to this region. Most important is the recognition that post-traumatic osteolysis can occur without an obvious acute or known traumatic episode, and in those situations it is thought to be due to repetitive stress, as seen in weightlifters, judo practitioners, and pneumatic tool workers. The pathogenesis is poorly understood, and several theories have been formulated, including a neurologic and/or vascular mechanism, hyperemia, and autonomic phenomenon. More recently, it has been proposed that osteolysis may be the result of a reactive process originating in the synovium or a subchondral fracture.

When advanced, resorption of the distal clavicle may be easily recognized radiographically (Fig. 5-6), with loss of up to 3 cm of bone and widening of the acromioclavicular joint. The radiologist, however, should focus on identifying early signs, as immobilization seems to diminish the amount of bone loss and shorten the natural course of the lytic phase. Early radiographic signs include soft tissue swelling, demineralization, and loss of the subarticular sclerotic cortex at the distal end of the clavicle. MR findings usually precede radiographic findings. Initially, there is periarticular soft tissue swelling/edema, and a bone marrow edema pattern may be evident. The marrow signal abnormality can be limited to the distal end of the clavicle or involve the acromion as well, albeit to a lesser degree. There may be an associated joint effusion, although this finding is variable. The disease process then progresses to bone erosion and frank destruction. Other MR findings include cortical irregularity, subchondral erosion or cystic changes, and a subchondral line suggestive of a subchondral fracture.

Figure 5-6 Post-traumatic osteolysis. A 28-year-old weightlifter with no recognizable acute trauma presented with acute shoulder pain. Anteroposterior radiograph of the right shoulder revealed a widened acromioclavicular joint and resorption of the distal end of the clavicle (arrow).

Glenohumeral Joint Disease (Arthropathy and Adhesive Capsulitis)

The glenohumeral joint may be affected by a multitude of conditions. The discussion in this section is limited to adhesive capsulitis, because of its frequency and complexity, and to rheumatoid arthritis, as this is a very common arthropathy affecting this joint. Only osteoarthritis is more common, and the imaging features of osteoarthritis have already been described for the acromioclavicular joint.

Rheumatoid Arthritis

Rheumatoid arthritis is a symmetric inflammatory arthropathy affecting predominantly the small joints of the hand, wrist, and feet. Involvement of the glenohumeral joint is not infrequent and occurs later in the disease course. Approximately half of patients with rheumatoid arthritis have shoulder symptoms during the first 2 years of their disease.

Conventional radiographs are still important for diagnosis and classification of rheumatoid arthritis. Classically, there is uniform joint space narrowing, periarticular demineralization, subchondral cystic changes, and marginal erosions. Erosions in the shoulder have a predilection for the lateral portion of the humeral head and may resemble a Hill-Sachs deformity (Fig. 5-7). Characteristically, there is lack of productive bone changes. Superior subluxation of the humeral head can also be seen, as chronic rotator cuff tear or cuff atrophy occurs frequently in patients with rheumatoid arthritis. The role of other imaging tests for rheumatoid arthritis is still evolving. US and MR are more sensitive for detection of erosions and soft tissue findings. There may be a role for these modalities in early detection and for evaluation of disease activity or response to therapy. MR can demonstrate erosions earlier than plain radiographs, as well as subchondral cystic changes on both sides of the joint. Erosions are commonly located in the humeral head near the insertion of the rotator cuff tendons. MR can also show common findings not identifiable in radiographs, like joint effusion, signs of synovitis, tears or atrophy of the rotator cuff muscles and tendons, synovial cysts, bursitis, and rice bodies. Routine MR is limited for evaluation of glenohumeral articular cartilage.

Figure 5-7 Rheumatoid arthritis of the glenohumeral joint. Anteroposterior (A) and axillary (B) views of the right shoulder demonstrate uniform joint space narrowing and large humeral head erosions (arrows).

Insufficiency Fractures

Insufficiency fractures are a type of stress fracture that occurs when a usual strength or physiologic force is applied to an abnormal or weakened bone. Most insufficiency fractures are caused by osteoporosis. In the pelvis, subcapital neck fractures (Fig. 5-8) are by far the most common. However, these fractures are usually associated with some degree of trauma. Common locations in the pelvis not usually associated with trauma include the sacrum, pubic rami, and supra-acetabular region. Undisplaced insufficiency fractures are very difficult to diagnose on conventional radiographs. Good radiographic technique and a high index of suspicion, especially when evaluating demineralized bones, are essential. If a stress fracture is suspected clinically and radiographs are not diagnostic, an MR examination should be obtained if available. MR is more accurate than scintigraphy. Additionally, MR may demonstrate coexisting conditions or alternative diagnoses that may influence management.

Figure 5-8 Radiographically occult insufficiency subcapital femoral neck fracture. A 74-year-old man presenting to the emergency room with left hip pain and no history of trauma. Anteroposterior view (A) of the left hip demonstrates mild demineralization of the bones but no evidence of fracture. Coronal T1 image of the left hip (B) revealed an undisplaced subcapital femoral neck fracture characterized by a dark linear band (arrows).

Radiographic findings may be occult or very difficult to detect and depend on the site of the fracture. Most commonly, a sclerotic band or line is evident. This finding is usually subtle and most often the only indication of a stress fracture. Evaluation for symmetry on the anteroposterior view of the pelvis is essential. Other potential findings include a fracture line, cortical disruption, and periosteal reaction. Supra-acetabular insufficiency fractures should be considered in elderly females with hip pain and no history of trauma. Particular attention should be paid to the distinct trabecular pattern of the acetabulum. Dense trabeculae outline a more lucent triangular area immediately above the sclerotic acetabular roof. A band of sclerosis within this triangular lucent region, parallel to the roof of the acetabulum, is characteristic and should be diagnostic of a stress fracture in most cases (Fig. 5-9).

Figure 5-9 Insufficiency fracture, left acetabulum. Elderly male with history of prostate cancer, left hip pain, and no history of trauma. Anteroposterior radiograph of the left hip (A) demonstrates a band of sclerosis (black arrows) parallel and immediately superior to the sclerotic acetabular roof (white arrows). Coronal STIR (B) image of the pelvis reveals a bone marrow edema pattern and a low signal line parallel and just superior to the acetabular roof (arrows).

MR is a highly sensitive and accurate tool for the diagnosis of insufficiency fractures. T1-weighted images may identify the fracture line itself as a serpiginous line of low signal intensity. On T2-weighted images with fat saturation and STIR sequences, the fracture line (high or low signal) may be obscured by the surrounding marrow edema in the acute setting. Linear low-signal intensity on both T1- and T2-weighted sequences located in the supra-acetabular region running parallel to the acetabular roof is the characteristic finding of an insufficiency fracture (see Fig. 5-9).

Bone scintigraphy is also a sensitive modality for the diagnosis of insufficiency fractures. Many times, however, it lacks specificity, demonstrating increased uptake of tracer in the area of fracture.

Transient Bone Marrow Edema and Transient Osteoporosis of the Hip

Transient bone marrow edema describes a painful condition of the hip that eventually resolves without treatment. Transient osteoporosis of the hip is a related condition in which, additionally, there is transient demineralization evident radiographically. Some authors refer to these conditions interchangeably, while others (more purist ones) treat them separately, and reserve the term “osteoporosis” for cases where demineralization is documented. It is unclear whether the high sensitivity of MR for detecting marrow edema pattern enables the diagnosis of the same condition earlier, before demineralization is evident on radiographs. Transient osteoporosis was first described in pregnant women during the third trimester. It is now recognized more commonly in middle-aged men. Complete resolution of symptoms and imaging findings occurs, an average of 6 months after onset. The condition may reappear in the same or other joints after a short interval, and the condition is then called “regional migratory bone marrow edema syndrome” or “regional migratory osteoporosis.” The underlying cause of transient bone marrow edema is unknown. Many theories, including aborted osteonecrosis, synovitis, reflex sympathetic dystrophy, and occult trauma, have been proposed.

Transient bone marrow edema or osteoporosis of the hip remains a diagnosis of exclusion. Differential considerations for bone marrow edema affecting only the femoral side of the hip joint include fractures, osteonecrosis, osteomyelitis, and neoplasms.

Conventional radiographs are normal initially and, over time, show variable degrees of demineralization involving the femoral head and neck regions (Fig. 5-10A). The acetabulum may occasionally be involved, but to a lesser degree. The joint space is preserved. Loss of the subchondral cortex of the femoral head is characteristic. Bone scintigraphy is abnormal before radiography. Increased, often extensive and homogeneous, uptake is evident in the femoral head and neck (Fig. 5-10B). MR shows a bone marrow edema pattern in the head and neck of the femur, sometimes extending into the intertrochanteric region (Fig. 5-10C). Mild involvement of the acetabulum is an inconsistent finding. Heterogeneous low-signal intensity on T1 and high-signal intensity on fluid-sensitive sequences are noted. A joint effusion is frequently present. The surrounding soft tissue and the contralateral side are normal.

Figure 5-10 Transient osteoporosis of the hip. A 55-year-old male with sudden, severe onset of left hip pain. A, Anteroposterior view of the pelvis demonstrates demineralization of the left femoral head and neck. B, Anterior and posterior planar bone scintigraphy images of the pelvis demonstrate increased tracer uptake in the left femoral head and neck (arrows). C, Coronal STIR image of the pelvis reveals a diffuse bone marrow edema pattern involving the left femoral head and neck extending into the intertrochanteric region (arrows).

Osteonecrosis of the Femoral Head

Avascular necrosis can be defined as ischemic necrosis of bone affecting the epyphyseal or subarticular locations. Histopathological findings are identical to bone infarcts, but by convention the term “bone infarct” is reserved for ischemic necrosis occurring in the metaphyseal or diaphyseal locations. Osteonecrosis, a more descriptive term, can be used for any location. Potential causative factors are multiple, and include trauma, medications (especially steroids), alcoholism, pancreatitis, hemoglobinopathies, radiation therapy, dialysis, hypercoagulable states, barotrauma, and storage diseases (such as Gaucher’s disease). Often, however, a specific predisposing factor cannot be identified (idiopathic avascular necrosis). Regardless of the cause, the patterns of bone injury, reactive response, and imaging findings are very similar.

Even though there is no universally satisfactory therapy for early stage disease, diagnosis of this entity before the joint is affected leads to an improved long-term prognosis. The Ficat classification, described in the 1980s, describes five stages based on clinical and radiographic findings (Table 5-1). Additional classification systems incorporating MR findings have also been developed. Imaging plays a pivotal role for the diagnosis, the determination of prognosis, and planning appropriate treatment. The size of the osteonecrosis lesion is an essential parameter for determining prognosis. The presence of bone collapse and joint involvement influences potential treatment options.

Table 5-1 Osteonecrosis: Ficat Classification

Initially, conventional radiographs are normal. In the reparative phase, both lytic and sclerotic areas are identified in the femoral head (Fig. 5-11A). Next, a subchondral fracture may develop. This manifests itself on radiographs as a subchondral crescent lucency (“crescent” sign) and is best seen on the frog leg lateral view (Fig. 5-11B). As the disease progresses, subarticular collapse may follow; this is seen as a loss of the normal rounded contour and flattening of the femoral head (Fig. 5-11D). Ultimately, severe collapse and destruction of the femoral head will lead to secondary osteoarthritis with joint space narrowing, osteophytes, subchondral cyst formation, and involvement of the acetabulum (Fig. 5-11C).

Figure 5-11 Osteonecrosis of the femoral head, radiographic spectrum. A, Anteroposterior radiograph of the pelvis in a patient with left hip pain demonstrates sclerosis of the left femoral head (arrows) and a radiographically normal-appearing right side. MR (not shown) showed findings of osteonecrosis bilaterally (Ficat stage 0 on the right and stage II on the left). B, Frog leg lateral view of the right hip in a different patient showing more advanced osteonecrosis with sclerotic (black arrow) and lytic areas (white arrow) as well as a subchondral crescentic lucency (arrowheads) consistent with a subchondral fracture (Ficat stage III). C, Frog leg lateral view of the right hip on the same patient as in B 6 months later shows more advanced subarticular collapse with new loss of the normal rounded contour of the femoral head (white arrows) as well as joint space narrowing and involvement of the acetabulum (black arrow) (Ficat stage IV). D, Frog leg lateral view of the left hip in a different patient with advanced collapse of the femoral head (black arrows) also shows a subchondral fracture (white arrows) but preservation of the hip joint (Ficat stage III).

MR is more sensitive and specific than radiography and scintigraphy (Fig. 5-12). MR provides an accurate estimation of the size of the lesion and demonstrates early subarticular collapse and secondary osteoarthritis. It is essential that the MR examination include both hip joints, as osteonecrosis is bilateral in more than 50% of cases. The superoanteromedial quadrant of the femoral head is most frequently involved. A focal abnormality is seen outlined by a low-signal intensity margin. This margin represents the reactive interface between necrotic and viable bone and extends to the subchondral bone. The characteristic “double line” sign is seen on T2-weighted images and consists of an inner rim or band of high-signal intensity outlined by an outer low-signal margin. A subchondral fracture appears as a high-signal subchondral crescent on fluid-sensitive sequences. Identification of subchondral collapse, when present, is crucial. Flattening of the articular surface is often seen earlier on the sagittal plane. The MR appearance of late osteonecrosis with severe joint destruction is confusing and may be misinterpreted as neuropathic arthropathy or septic joint. Prior history and prior imaging are of utmost importance for making the differential diagnosis.

Figure 5-12 Osteonecrosis of the femoral head, MRI spectrum. A, T1-weighted coronal image of both hips demonstrates abnormal bone marrow signal in both femoral heads in a distribution pattern consistent with bilateral osteonecrosis. Note focal bone marrow abnormality outlined by a low signal margin (arrows). B, Double line sign (arrows) on an axial T2-weighted image of the left hip. Linear increased signal outlined by a dark signal characteristic of osteonecrosis.

Arthropathies

Different types of arthropathies commonly affect the hip joint and can be the source of debilitating and acute pain. Imaging findings may be very subtle. When evaluating the hip joint for a possible arthropathy, the radiologist must carefully evaluate the width of the joint space. The joint space may be widened if there is a sizable joint effusion; this is an important finding of septic arthropathy, discussed later in this chapter. Most arthropathies cause joint space narrowing. The pattern of joint space narrowing provides clues for classifying the arthropathy and limiting the differential diagnosis. Uniform narrowing is typical of inflammatory arthropathies, whereas a nonuniform joint space narrowing is characteristic of degenerative arthropathy. Depositional arthropathies present with preservation of the joint space, but degenerative changes may ensue as the disease progresses, with consequent narrowing of the joint space. Conventional radiographs are usually sufficient to diagnose and categorize hip arthropathies. MR may allow earlier detection of some articular findings and might be useful for evaluating the soft tissue findings that accompany some arthritides.

Osteoarthritis is by far the most common arthropathy affecting the hip. Nonuniform joint space narrowing occurs most commonly superiorly, with the femoral head migrating superomedially or, more commonly, superolaterally. Buttressing (thickening of bone) of the femoral neck is a characteristic but not pathognomonic finding. The classic findings of osteoarthritis, namely, marginal osteophytes, subchondral cyst formation, and eburnation, can also be found in the hip (Fig. 5-13A). Occasionally, acetabular protrusio can be a result of osteoarthritis. In this condition, medial (rather than superior) migration of the femoral head occurs (Fig. 5-13B). Other causes of protrusio include Paget’s disease, rheumatoid arthritis, osteomalacia, trauma, ankylosing spondylitis, radiation, and infection. A particularly destructive but rare form of osteoarthritis can be seen with a rapid progression of disease, mainly in the elderly. Awareness of rapidly destructive osteoarthritis is important to avoid misdiagnosis as a more aggressive or infectious arthropathy (Fig. 5-13C). In osteoarthritis, MR directly demonstrates loss of articular cartilage earlier and may also identify synovial cysts. MR is particularly useful in the differential diagnosis of osteoarthritis in cases with a confusing clinical presentation and nonspecific radiographic findings.

Figure 5-13 Osteoarthritis. A, Anteroposterior radiograph of the left hip with classic osteoarthritis manifested by nonuniform, superolateral hip joint narrowing, subchondral eburnation (black arrows), marginal osteophytes (arrowhead), and buttressing (white arrows). B, Anteroposterior radiograph of the right hip in a patient with protrusio defined as intrapelvic displacement of the medial wall of the acetabulum (arrows). C, Rapidly destructive osteoarthritis. Lateral frog-leg view of the right hip in a 72-year-old female with severe right hip joint pain (C) demonstrates rapidly progressing destruction as compared to near-normal radiographs 6 months earlier (not shown). Note the destructive changes on both sides of the joint (arrows).

Rheumatoid arthritis can affect the hip joint and causes uniform joint space narrowing. Characteristic radiographic findings include bilateral and symmetric periarticular demineralization, erosions, and lack of proliferative changes (Fig. 5-14). Acetabular protrusion if bilateral and symmetric, should favor the diagnosis of rheumatoid arthritis. Additional findings seen on MR examinations include joint effusion, trochanteric or iliopsoas bursitis, tendon ruptures, pannus, and rice bodies.

Figure 5-14 Rheumatoid arthritis, hip involvement. Anteroposterior radiograph of the pelvis demonstrates bilateral, symmetric, uniform joint space narrowing. There is mild right protrusio (arrowheads), erosions (white arrows), and subchondral cystic changes (black arrows).

The seronegative spondyloarthropathies commonly affect the hip joint. These are a group of joint conditions that include ankylosing spondylitis, reactive arthritis (e.g., Reiter’s syndrome), psoriatic arthritis, arthritis associated with inflammatory bowel disease, and undifferentiated spondyloarthropathies. The joint space appears uniformly narrowed with productive bone changes. Bone mineralization is relatively preserved. Enthesophytes (proliferative bone changes that occur at the insertions of ligaments and tendons) occur commonly. Late-stage complications include acetabular protrusion and ankylosis. Careful attention must be paid to the sacroiliac joints, which often reveal bilateral symmetric involvement in ankylosing spondylitis (Fig. 5-15) and unilateral or bilateral asymmetric involvement in the other spondyloarthropathies.

Figure 5-15 Ankylosing spondylitis. A 62-year-old patient presenting to the emergency room with an intertrochanteric fracture after a fall. Anteroposterior view of the pelvis demonstrates a mildly displaced left intertrochanteric fracture (black arrowheads). Also note the symmetric, uniform joint space narrowing of the hips with large subchondral cystic changes (asterisks), enthesophytes (white arrows), ankylosis of the sacroiliac joints (black arrows), and right acetabular protrusio (white arrowheads).

Other arthropathies that may involve the hip and may occasionally present with acute pain are synovial chondromatosis and pigmented villonodular synovitis (PVNS). These entities are proliferative diseases of the synovium characterized clinically by joint pain and limited motion. Radiographic appearance is similar for both, with pressure erosion usually along the neck of the femur, periarticular demineralization, and late joint space narrowing. Subchondral cystic changes favor PVNS. The intra-articular deposits of synovial osteochondromatosis often calcify, whereas those of PVNS typically do not. MR typically demonstrates the hemosiderin content on PVNS as focal areas of low signal in all imaging sequences. MR findings on synovial chondromatosis are variable depending on the degree of mineralization of the intra-articular deposits, but most commonly reveal intra-articular areas of low to intermediate signal on T1- and very high signal on T2-weighted images with areas of hypointense calcifications. Similar radiographic and MR findings can be expected with involvement of other joints with PVNS or synovial chondromatosis.

Soft Tissue Infection

Cellulitis, an acute suppurative infection of the dermis and subcutaneous tissues, is usually the result of contiguous spread of infection from skin breakdown, and is the initial step in the development of deep soft tissue infections. Findings on radiographs are minimal and nonspecific: increased soft tissue density, soft tissue swelling, and infiltration of the subcutaneous fat. Small radiolucent foci may be seen if gas is present (Fig. 5-16). CT may demonstrate thickening of the skin, subcutaneous tissues, and fascia (Fig. 5-17). US demonstrates increased echogenicity of the involved tissues and anechoic bands traversing the subcutaneous tissues, giving them a cobblestone appearance (Fig. 5-18). Findings on MR are similar, with decreased T1 signal and increased T2 signal in the thickened and infiltrated skin, subcutaneous tissues, and fascia (Fig. 5-19). Enhancement is variable. In clinically confusing cases, the finding of intense enhancement favors the diagnosis of cellulitis over noninfectious causes of subcutaneous edema that otherwise could have the same imaging findings. Three-phase bone scintigraphy demonstrates increased blood flow (initial phase) and blood pool (early phase) activity. Delayed images (third phase) are normal or demonstrate only mild increased uptake of the involved soft tissues.

Figure 5-16 Foot cellulitis. Lateral radiograph of the left foot shows swelling, increased density, and gas (black arrows) within the plantar and dorsal soft tissues. Note an ulcer at the plantar aspect of the forefoot (white arrow).

Figure 5-17 Leg cellulitis CT. Axial image at the level of the proximal tibia demonstrates skin thickening (white arrows), septations, and inflammatory stranding of the subcutaneous fat (black arrows). These findings, however, are nonspecific and may be seen in multiple conditions that cause subcutaneous edema. CT can demonstrate extension to the deeper tissues and formation of organized fluid collections (abscess).

Figure 5-18 Leg cellulitis US. Longitudinal image of the leg shows thickening and increased echogenicity of the subcutaneous tissues (arrows) with intermixed anechoic bands (arrowheads) giving a cobblestone appearance.

Figure 5-19 Leg cellulitis MR. T1, fat-saturated image of the right lower extremity shows decreased signal in the thickened, infiltrated skin and subcutaneous tissues (arrows).

Necrotizing fasciitis is a rare but very aggressive and often fatal condition characterized by necrosis of subcutaneous and deep fascial tissues. Patients with underlying conditions leading to decreased immunity, such as the elderly, those with HIV infection and leukemia, drug abusers, alcoholics, and those taking immunosuppressive medication, are all at increased risk of developing this lethal disease. The infection is most commonly polymicrobial, with both aerobic and anaerobic organisms. The disease is a surgical emergency requiring fasciotomy and extensive débridement of the necrotic tissue. Rapid diagnosis and prompt surgical intervention are essential. Cellulitis has clinical and imaging characteristics similar to those of necrotizing fasciitis, but the treatment is not surgical. The clinical dilemma always lies between acting rapidly and waiting for imaging test results that may or may not be helpful. If imaging cannot be done expeditiously, delaying surgical intervention is not justified. Radiographic and sonographic findings are similar to those of cellulitis, but with more severe involvement (Fig. 5-20). Presence of soft tissue gas on radiographs is an ominous sign. Severe asymmetric thickening, with air and fluid collections, is the hallmark of necrotizing fasciitis on CT. However, this constellation of findings occurs inconsistently. CT often demonstrates nonspecific thickening and enhancement of the superficial and deep fascial layers. MR images show thickening, high T2 signal, and abnormal enhancement in the subcutaneous tissues and deep fascial planes. However, when necrosis is established, only minimal or peripheral enhancement surrounding the area of necrosis may be present. MR overestimates the extent of deep fascial involvement as compared with findings at the time of surgery. The absence of deep fascial involvement on MR virtually excludes the diagnosis of necrotizing fasciitis.

Figure 5-20 Forearm necrotizing fasciitis. Lateral radiograph of the elbow demonstrates nonspecific soft tissue swelling and increased soft tissue density (arrows). Necrotizing fasciitis was found at surgery.

An abscess is defined as a localized collection of pus (necrotic tissue, inflammatory debris, and bacteria). An earlier stage in the development of an abscess, before liquefaction and organization ensue, is called a phlegmon. An abscess can occur anywhere in the soft tissues and, when located in a skeletal muscle (which is relatively resistant to infection), the term pyomyositis is used. Radiographs may be noncontributory, showing only diffuse or focal increased soft tissue density, focal prominence of the affected soft tissues, and displacement of fat pads. The sonographic appearance of abscesses is variable. Most commonly, a complex hypoechoic, predominantly fluid-containing mass with increased through-transmission is identified. The margins of the mass may be well or ill defined, depending on the stage of evolution. Internal septations and amorphous internal echoes are additional common findings (Fig. 5-21). Often, dynamic evaluation of the area with gentle compression is necessary to reveal the liquid nature of the contents. Color or power Doppler demonstrates absent internal flow and hyperemia of the wall and adjacent tissues. CT reveals an organized low attenuation fluid collection with an enhancing wall of variable thickness (Fig. 5-22), internal septations, and high-attenuation internal foci. Surrounding edema or findings of cellulitis are common. MR depicts abscesses and adjacent soft tissue changes to greater advantage (Fig. 5-23). Focal fluid collections demonstrating low or intermediate signal on T1- and high signal on T2-weighted sequences surrounded by reticulated, edematous soft tissue are characteristic. Intravenous contrast increases the conspicuity of the lesion with intense peripheral (wall) enhancement.

Figure 5-21 Arm abscess US. Single longitudinal US image of the arm just superior to the antecubital fossa in a patient with soft tissue infection demonstrates a complex, hypoechoic, predominantly cystic fluid collection with increased through-transmission internal septations (white arrow) and dependent debris (black arrow).

Figure 5-22 CT image of a posterior thigh abscess. Axial image of a contrast-enhanced CT scan of the thigh shows a well-defined low-attenuation fluid collection (white arrow) with a thick enhancing wall (arrowheads) and an internal focus of high attenuation (black arrow).

Figure 5-23 Subcutaneous abscess at the posterolateral ankle. T1 (A), fat-suppessed T2 (B), and fat-suppressed, post-contrast T1 (C) axial images of the ankle demonstrate a focal complex fluid collection with heterogeneous foci of low T1 and high T2 signal (A and B,arrows). Note the peripheral enhancement (C,arrows) and adjacent edematous soft tissues (A,arrowheads).

Infectious Arthritis

Infectious arthritis is a common clinical problem with devastating consequences if not diagnosed and treated early. It is classified as pyogenic (septic) or nonpyogenic. Pyogenic causes are divided into gonococcal and nongonococcal. Staphylococcus and streptococcus species are the most common pathogens causing nongonococcal septic arthritis. Nonpyogenic pathogens include mycobacteria, fungi, and viruses. Synovial involvement occurs first and may be the result of hematogenous spread, spread from a contiguous focus, direct implantation, or postsurgical contamination. Septic arthritis is monoarticular in the majority of cases and can affect any joint. The prevalence of the specific joint affected depends on the mechanism of infection and the patient population studied. Risk factors include advanced age, diabetes, underlying rheumatoid arthritis, recent joint surgery, presence of soft tissue infection, intravenous drug abuse, infection with HIV, and other immunosuppression states. Although the imaging findings of septic arthritis are discussed in this section, it should always be kept in mind that the most rapid and definitive test for making a diagnosis of septic arthritis is arthrocentesis and microbiological evaluation of the aspirated joint fluid.

Early findings on conventional radiographs include periarticular demineralization, joint widening/effusion, and soft tissue swelling. Later, there may be joint space narrowing, periosteal reaction, erosions (both marginal and central), destruction of the subchondral bone, subluxations and dislocations, and, ultimately, ankylosis (Figs. 5-24 and 5-25). Intra-articular gas is a rare finding. The diagnosis of septic joint superimposed on known rheumatoid arthritis or other inflammatory arthropathy is very challenging. Infection should be suspected if there is widening of the joint space along with rapid articular destruction and significant soft tissue asymmetry.

Figure 5-24 Septic arthritis of the left shoulder and humeral head osteomyelitis in a 36-year-old intravenous drug–addicted male patient presenting to the emergency room with left shoulder pain. External rotation (A) and internal rotation (B) anteroposterior radiographs of the left shoulder show cortical irregularity and destruction of the subchondral bone (white arrows) as well as well-demarcated focal osteopenia (arrowheads). Incidental calcific bursitis is also noted (black arrows). Total body anterior bone scintigram (C) and anterior total body gallium scan (D) demonstrate abnormal increased uptake in the left shoulder region (arrows). Note that the extent of involvement is much greater in the gallium scan, suggesting additional involvement of the soft tissues.

Figure 5-25 Osteomyelitis. Anteroposterior radiograph in a diabetic patient with osteomyelitis of the left fifth metatarsal head and septic arthritis of the metatarsophalangeal joint demonstrates focal soft tissue swelling (white arrow), focal demineralization (black arrow), frank bone destruction, and minimal periosteal reaction (white arrowheads). Note widening of the metatarsophalangeal joint and marginal erosions at both sides of the joint (black arrowheads).

Sonography is very sensitive for demonstrating a joint effusion and an excellent tool for aspiration guidance. Synovial thickening is seen consistently in infectious and inflammatory arthropathies. Bone scintigraphy demonstrates increased blood flow, prominent blood pool, and increased delayed activity in the distribution of the affected joint. The use of gallium citrate as a marker of inflammation has been greatly replaced by imaging with labeled leukocytes, either with indium 111 or technetium 99m hexamethylpropylene-amine-oxime. Increased tracer uptake of these agents in the joint improves specificity for the diagnosis of infectious arthritis (see Fig. 5-24).

CT is rarely used for imaging patients with suspected joint infection, except for patients with orthopedic hardware. All findings described in conventional radiographs may be seen on CT, easier and earlier (see Fig. 5-24). Additionally, synovial thickening may be identified. Synovial and periartcular soft tissue enhancement is variable.

MR is highly sensitive and more specific than other imaging modalities for the detection of septic arthritis. MR easily demonstrates even a small joint effusion, which is often the first sign of an infected joint. However, an effusion does not necessarily indicate the presence of infection. Other MR findings include synovial thickening and perisynovial edema, seen as periarticular areas of increased signal on fluid-sensitive sequences. Focal abnormal bone marrow signal in the adjacent bone is not diagnostic of superimposed osteomyelitis, as it may be a reactive change. Superimposed osteomyelitis is favored when the bone marrow signal abnormality is diffuse and is seen mostly as decreased signal on T1-weighted images. After intravenous administration of gadolinium chelates, there is synovial and perisynovial soft tissue enhancement. Intense synovial enhancement is not a normal finding in a healthy joint.

Acute Osteomyelitis

A proper use of pertinent terms is important when discussing imaging of osteomyelitis. Osteomyelitis indicates infectious involvement of the bone marrow. Infective osteitis is infection of the bone cortex and infective periostitis infection of the periosteum. These entities are often seen together, as the infectious process may extend outward from the marrow or inward from the soft tissues. As with infectious arthritis, multiple mechanisms can cause the inoculation of an infectious agent into the bone. Hematogenous spread is rarely the source of acute primary osteomyelitis in the adult appendicular skeleton. More commonly, osteomyelitis in the adult occurs from direct spread or surgery. Contiguous spread from ulcerations is most common in patients with foot ulcers (such as the diabetic foot), patients who suffered a spinal cord injury, and bedridden patients.

The overall sensitivity of conventional radiographs for early osteomyelitis is poor. Appearance of radiographic findings may be delayed for weeks after the initial infection. The earliest sign is usually swelling of the deep soft tissues. Early radiographic findings in the bone itself include focal demineralization and periosteal reaction. However, periosteal reaction may be absent in small bones such as those of the feet. Later, cortical lucency and frank bone destruction occur (see Fig. 5-25). Sonography is limited for the diagnosis of acute osteomyelitis. Deep soft tissue swelling, adjacent to the bone, may be seen, but this finding is nonspecific. A fluid collection immediately adjacent to the bone in the proper clinical setting is highly suggestive of osteomyelitis, but is rarely observed. CT is helpful in selected cases of acute osteomyelitis, although osseous findings may be detected to greater advantage and earlier than in radiographs. These include focal demineralization of the affected bone, cortical destruction, periosteal reaction, and hyperattenuation of the bone marrow. Soft tissue findings associated with osteomyelitis can also be demonstrated.

Three-phase bone scintigraphy is a highly sensitive test for the diagnosis of osteomyelitis. There is increased focal or regional uptake in the initial blood flow and early blood pool images. Focal increased activity in the affected area of the bone is noted on delayed images. A normal bone scan excludes the diagnosis of osteomyelitis, unless there is severe underlying vascular disease. Unfortunately, the specificity of bone scintigraphy is low, as many other conditions, such as trauma, prior surgery, or underlying arthropathy, can present with similarly abnormal findings. Labeled white blood cell scintigraphy or gallium scan may be performed in conjunction with bone scan to increase specificity. Focal accumulation of radiotracer paralleling bone scan findings is consistent with osteomyelitis.

MR is the imaging test of choice that should be obtained whenever osteomyelitis is suspected and radiographs show normal findings. Furthermore, MR may be indicated even in the presence of abnormal radiographs for evaluation of extent of infection and for planning potential surgical interventions. MR may also be helpful in limiting the differential diagnosis of patients with complex radiographic abnormalities, or whenever the presence of underlying bone pathology limits the specificity of plain radiographs. MR findings of osteomyelitis can be divided into those affecting the osseous structures and those affecting the soft tissues.

Soft tissue findings are almost invariably seen in patients with osteomyelitis. Soft tissue findings include ulcers, sinus tracts, cellulitis, and abscess formation (Fig. 5-26). The MR features of most of these conditions have already been described in this chapter. An ulcer presents on MR as a cutaneous and soft tissue defect with granulation tissue at its base, which usually enhances avidly after administration of gadolinium chelates. Sinus tracts may extend outward from the bone to the superficial soft tissues and skin or inward from an ulcer toward the bone. Sinus tracts appear as linear areas of increased signal on fluid-sensitive sequences, but they are much more easily identified after contrast administration as parallel linear areas of enhancement in a “tram–track” pattern.

Figure 5-26 Osteomyelitis. Short axis T1-weighted (A) and STIR (B) MR images of the forefoot reveal a periosteal abscess (arrows) at the fourth metatarsal head (MT) as a heterogeneous low T1 and high STIR signal fluid collection. Note the replacement of the metatarsal (MT) marrow signal consistent with osteomyelitis.

Osseous findings include abnormal bone marrow signal and enhancement, cortical interruption or destruction, and periosteal reaction. A focus demonstrating low signal intensity on T1-weighted, high signal intensity on T2-weighted or STIR sequences, and enhancement is highly consistent with the diagnosis of osteomyelitis. However, these findings are not completely specific and should be analyzed in the context of the clinical presentation, underlying osseous or articular pathology, and soft tissue involvement. The negative predictive value of a normal STIR sequence is very high, approaching 100%. Periosteal reaction appears as a thin linear edema-like pattern and enhancement paralleling or surrounding the bone cortex.

Foreign Bodies

Puncture wounds and suspected retained foreign bodies (FBs) are a common cause of emergency room visits. Retained FBs are frequently overlooked initially, leading to inflammatory and infectious complications that are often severe. Therefore, prompt detection and removal are imperative. Precise localization of the object is extremely useful as this may minimize the extent of surgical dissection and shorten surgical time. A high clinical suspicion is always necessary, especially in patients with neuropathy who may be unaware of a prior puncture and present with a soft tissue infection (Fig. 5-27A and B). Wood, glass, and metal account for the vast majority of retained FBs encountered. On conventional radiographs, metal is almost always visible. Glass is seen radiographically in more than 90% of cases. On the other hand, wood is identified in only a minority of patients (approximately 15% or less).

Figure 5-27 Foreign body, right hand. Adult diabetic patient presenting to the emergency room with a red and swollen right hand after working in his garden. Anteroposterior radiograph at initial presentation (A) shows soft tissue swelling and diffuse increased density in the region of the thenar eminence (arrow). Anteroposterior radiograph of the hand 8 weeks later (B), obtained after a second visit to the emergency room for worsening swelling and erythema, demonstrates periosteal reaction (white arrows) and sclerosis (black arrows) of the second metacarpal bone. No radio-opaque foreign body was identified. Transverse US image (C) shows a linear hyperechoic foreign body (arrows) adjacent to the second metacarpal (MC) with posterior shadowing and a surrounding hypoechoic rim (arrowheads). Axial STIR image (D) of the right hand demonstrates the foreign body (arrow) as a low-signal structure surrounded by a rim of high signal, adjacent to the second metacarpal bone. At surgery, a thorn was extracted.

Ultrasound is highly sensitive and specific in the detection of retained FBs. US is rapid, inexpensive, accessible, and lacks ionizing radiation. It should be the modality of choice for radiographically occult FBs in the superficial soft tissues. All FBs are hyperechoic with acoustic posterior shadowing (see Fig. 5-27C). The degree of acoustic shadowing is variable and depends on the surface characteristics rather than the composition of the FB. Flat, smooth surfaces often encountered in metal and glass produce reverberation artifact or “dirty” shadowing, whereas irregular surfaces usually produce “clean” shadowing. Sonographic detection may be enhanced by the presence of a hypoechoic rim surrounding the FB. This rim may be seen after at least 24 hours, when an inflammatory reaction has developed. US also allows examination of the surrounding tissues for infectious complications and associated soft tissue injuries.

MR is rarely used in the acute setting for the detection of foreign bodies. More commonly, FBs may be detected incidentally on MR scans performed for the work-up of musculoskeletal infection. Foreign bodies exhibit low signal intensity on all pulse sequences and may show magnetic susceptibility artifact. Conspicuity on MR depends on size, location, and composition. Small FBs are difficult to distinguish from adjacent low-signal structures such as tendons, scars, and calcifications. Foreign objects may incite an inflammatory reaction, seen as surrounding areas of low signal intensity on T1- and high signal intensity on T2-weighted images. MR is highly accurate for identifying infectious musculoskeletal complications (see Fig. 5-27D). Not infrequently, small FBs may become chronically embedded within the soft tissues and form foreign body granulomas. These lesions generate variable degrees of inflammatory response but often present with little or no T2 signal changes. They should be suspected when there is an area of magnetic susceptibility or signal void with peripheral enhancement.

Infected Orthopedic Hardware

There have been important improvements in orthopedic surgical technique and preoperative and postoperative care leading to a considerable decrease in overall postsurgical infections. The frequency of hardware infections has been reduced from nearly 10% in the early years to somewhere between 0.5% and 2% currently. Nonetheless, postoperative infection continues to be an important cause of morbidity and mortality. Prompt detection and localization of hardware infection are essential for appropriate patient management and to avoid even more serious consequences.

Approximately one third of arthroplasty-related infections occur within the first 3 months after the operation. The remainder occur beyond this time period and are considered late infections. Late infections are more indolent and can be difficult to diagnose. Differentiating between late infection and other causes of hardware failure is challenging clinically, radiographically, and histopathologically. In most (but not all) cases, the diagnosis of hardware infection is made by isolating organisms from the fluid or tissues around the hardware. Occasionally, organisms are not found around hardware later proven to be infected. Pain, increased white cell count, elevated C-reactive protein, and increased erythrocyte sedimentation rate are often present, but lack specificity in the postoperative period. Clinicians must utilize a combination of clinical, laboratory, and imaging data to confirm or exclude the diagnosis of hardware infection. Imaging plays an important role and helps guide the clinical management of these patients.

Radiographs should be performed initially for suspected hardware infection. Most often, there are no radiographic findings suggestive of infection especially in the patients with acute presentations. Radiographic findings include periosteal reaction, osteolysis, cortical irregularity, erosions, and frank bone destruction. Chronic infections can produce areas of bone sclerosis. Additionally, radiographs may serve to suggest or exclude other causes of postoperative pain and hardware failure. The temporal evolution is extremely useful when prior radiographs are available. Rapid development of the previously discussed radiographic abnormalities should be viewed as highly suspicious for infection. Cross-sectional imaging studies such as CT and MR can also help when evaluating potentially infected hardware. Unfortunately, image quality with both modalities is degraded by the presence of metallic hardware. With the advent of multidetector CT, the severity of beam hardening artifact from metal has been reduced. CT exquisitely depicts osseous findings of hardware infection: periosteal reaction, cortical erosion, cortical tracts, osseous fistulae, bony sequestrae, and areas of osteolysis. MR depicts the soft tissue findings to greater advantage.

Radionuclide imaging plays an important role in the evaluation of orthopedic hardware infection and is used in many centers as the second line of imaging after radiographs. Multiple radionuclides have been used for the detection of musculoskeletal infection and all show areas of increased tracer activity in the infected tissue, albeit via different mechanisms of uptake. Nuclear scintigraphy studies currently being used for the detection of musculoskeletal infection include technetium 99m methylene diphosphonate (MDP), radionuclide-labeled white blood cells (WBCs), and gallium-67 citrate imaging. More recently, F18-fluorodeoxyglucose positron emission tomography (FDG PET) has emerged as an alternative for imaging infection and inflammation.

Technetium 99m MDP localizes in areas of increased bone turnover. The sensitivity of bone scintigraphy is very high, and the lack of accumulation virtually excludes bone infection. The overall specificity for bone infection is low, and it is even worse in the postoperative period when bone uptake related to the surgical procedure itself can be seen to variable degrees within the first year, which is also when most infections occur. Additionally, uptake may be abnormally high in multiple noninfectious postoperative complications.

Radionuclide-labeled WBCs can be performed using indium (In)-111 or technetium 99m hexamethylpropyleneamine. These tracers accumulate in areas of neutrophil-mediated inflammation and infection. The sensitivity is similar to and the specificity is better than that of bone scintigraphy. Labeled WBC scans are more useful for acute hardware infections than for chronic, low-grade cases. In the past, gallium-67 citrate was a popular agent used to image infection, although the mechanism of tracer localization in infection is poorly understood. In view of the lower accuracy, unfavorable imaging characteristics, and long wait prior to imaging, gallium has been largely replaced by other agents. Although initial reports suggested that FDG PET could differentiate between infection and other causes of hardware failure (specifically aseptic loosening), more recent data are less convincing. Advantages of FDG PET over other modalities include faster imaging times, higher spatial resolution, and increased sensitivity for low-grade infections. Cost and availability also limit its utilization.