TECHNIQUES AND NORMAL ANATOMY

Radiography

Conventional radiography is the most commonly used imaging technique to evaluate the joints. This technique should always be the first imaging study performed in a patient suspected of having joint problems. Radiography has the following important advantages: It is almost universally available, is relatively inexpensive compared to other imaging studies, and delivers only a small radiation dose to the patient. In all instances, when possible, two images of the joint perpendicular to each other should be performed. These are usually a frontal projection either in the anteroposterior (AP) or posteroanterior (PA) directions and a lateral. In some instances oblique images may also be obtained depending on the preferences of the referring physician or radiologist or the clinical situation. In certain instances it may also be important to obtain images of the joint above and below an injury. Two examples of this scenario are in forearm and lower leg injuries (paired bones), because pathology may exist at either the proximal or distal joint. Because some ionizing radiation is present in radiography, it should be used only when absolutely necessary in pediatric patients and pregnant women.

Historically, radiographic images were printed on film but a gradual conversion to computed radiography is being made in which images are electronically processed and viewed on a computer workscreen. These images can then be transmitted via Ethernet or Internet anywhere.

Conventional Tomography

Conventional tomography is mentioned mainly for historical interest. High radiation dose, relatively poor image resolution, and imaging in only one plane were its major disadvantages. The technique has been almost totally replaced by other imaging tests, especially CT and MR imaging.

Arthrography

Arthrography is a technique in which contrast or contrast and air are injected into the joint. The joint is then imaged using radiography, CT, or MR imaging or a combination of these techniques. The injected contrast may be an iodine-containing water-soluble compound (e.g., Conray), which is then imaged with radiography or CT (Fig. 7–1). Alternatively, a paramagnetic compound (e.g., gadolinium pentazocine) may be injected and imaged with MR technique. MR arthrographic images of the joint may also be performed after intravenous injection of the paramagnetic contrast agent although this technique is not used very often today. MR arthrography is mainly used to evaluate the labra of the glenohumeral joint or hip but is also useful for evaluating the knee for meniscal tears in the patient who has had prior meniscal surgery (Fig. 7–2). CT arthrography and, less commonly, radiographic arthrography are most useful in the patient who cannot undergo MR imaging (Fig. 7–3).

Computed Tomography

Conventional computed tomography, a technique that makes individual axial (transverse) slices of the patient, uses the same ionizing radiation as radiography. CT technique has been vastly improved in the past few years. The recently developed "spiral" CT has major advantages over conventional CT. With the spiral CT technique, axial (transverse) images are acquired much more rapidly with dramatic decreases in radiation dose. For instance, a CT of the chest, abdomen, and pelvis can be performed in about 30 seconds. The CT data are stored in three-dimensional packets that can then be reconstructed and displayed on almost any other plane. The most common images reconstructed from the axial plane are the sagittal and coronal planes (Fig. 7–4).

Magnetic Resonance Imaging

MR imaging has revolutionized the imaging evaluation of almost all body areas but particularly those of the central nervous system and musculoskeletal system. It has tremendous advantages over other imaging modalities in the evaluation of joints because of its excellent soft-tissue contrast, high resolution, and ability to image in every plane. This technique may show pathophysiologic events even before they are seen on x-ray, such as showing the very early changes of avascular necrosis (Fig. 7–5). Because of its exquisite soft-tissue contrast, MR imaging allows radiologists to visualize subtle differences in soft tissues that had never before been seen with other imaging modalities. For example, the subtle contrast between fat and muscle depicted on a radiograph or CT is dramatically highlighted with MR imaging because of their very different chemical compositions (Fig. 7–6). MR imaging can also depict subtle changes within the bone marrow cavity, an area difficult to evaluate with radiography or CT. Finally, this technique allows precise definition of almost each component within or surrounding the joint. Therefore, MR imaging is a tremendous aid to preoperative evaluation of any person who has unexplained joint pain or who has had trauma to a joint. In fact, MR imaging should almost always be performed in patients with these symptoms before any invasive procedure is attempted. The major disadvantages of MR imaging are that some patients with claustrophobia cannot tolerate the small space of the magnet, and patients with metal in their bodies or who have certain heart valves or clips for vascular problems cannot be imaged.

Ultrasonography

Ultrasonography, first developed for use in World War II for detection of submarines, was adopted after the war for use in imaging. High-frequency transmission of sound can be used to evaluate the soft tissues, tendons, ligaments, and even the cartilage of the joint (Fig. 7–7). The ultrasound waves cannot be transmitted through cortical bone so the intramedullary cavity cannot be imaged with this technique. Ultrasound is used much more extensively in Europe than in the United States.

Radionuclide Imaging ("Bone Scans")

Radionuclide imaging uses radioactive materials that are injected intravenously so they can localize in regions of abnormally increased blood flow (hyperemia) or regions of increased osteoblastic or osteolytic changes or heightened metabolic activity. The major uses of bone scanning are in patients suspected of having metastatic disease or infection. Its primary use in suspected joint disease is limited.

Anatomy of the Normal Joint

The typical normal synovial joint consists of at least two articulating bones enclosed in a synovium-lined joint capsule. The apposing bony surfaces are covered by smooth articular cartilage (hyaline cartilage). On radiographs, the normal joint has a separation between the adjacent bones representing the region occupied by the hyaline or articular cartilage, meniscus, and joint fluid (the so-called "articular space") depending on which joint is imaged. These structures are not normally depicted on radiographs because of the limited soft-tissue contrast of the technique unless they are calcified (Fig. 7–8). However, MR imaging exquisitely shows the components of the normal joint (Fig. 7–9).

Joint Disease

The clinical signs and symptoms of joint disease are manifestations of abnormal function such as reduced mobility, hypermobility, and pain. Altered function may be due to pain, discomfort, apprehension, or instability. The wide range of joint abnormalities is summarized below and many of these processes are discussed in the exercises. Any of these signs may occur singly or in combination with any others.

Radiographically, joint disease may be diagnosed by any of the following:

1. Incongruity of the articulating bone as is seen with dislocations, e.g., traumatic dislocation or dislocations caused by arthropathies such as lupus arthritis or rheumatoid arthritis

2. Irregularity of articulating bone ends and margins, as in erosions (e.g., in psoriasis or gout)

3. Increased density or sclerosis of articulating bone ends (also called eburnation), as in osteoarthritis

4. Bony outgrowth (proliferation) at bone ends, known as osteophytes

5. Diffuse reduced density of bone ends, described as juxta-articular or periarticular osteopenia (e.g., rheumatoid arthritis, tuberculous arthritis)

6. Focal areas of loss of medullary bone substance beneath the articular cortex, known as subchondral cysts or geodes(e.g., osteoarthritis, rheumatoid arthritis)

7. Loss of articular space from articular cartilage destruction (e.g., septic arthritis, osteoarthritis)

8. Accumulation of excess joint fluid within the joint (joint effusion) (Excess joint fluid is a common manifestation of joint disorders. The fluid may be synovial fluid, blood, or even pus, depending on the etiology of the joint disease.)

9. Calcification of articular (hyaline) cartilage or fibrocartilage (chondrocalcinosis), or intraarticular soft-tissue calcification such as that seen in scleroderma or polymyositis or dermatomyositis

10. Synovial proliferation or abnormal increase in the synovial lining, such as that seen with pigmented villonodular synovitis (PVNS).

TECHNIQUE SELECTION

Radiography should always be the initial imaging test to evaluate the joints and should only be obtained after the patient has had a thorough history and physical examination and there is a clear indication to obtain the study. Various projections may be used depending on the clinical indication or the situation, but at least two projections perpendicular to each other should be obtained. Often radiographs alone will confirm the clinical diagnosis or the findings will suggest an entirely different one. In many instances, however, it may be necessary to use other more expensive imaging techniques to clarify the x-ray findings or to evaluate significant clinical symptoms or signs or laboratory abnormalities if the radiographs are normal. The following paragraphs discuss the selection of the imaging techniques in a few common clinical scenarios.

Congenital Diseases

If the clinical situation of a suspected congenital anomaly or a pediatric joint abnormality or if a limp is present in a child, an x-ray film of the suspected joint should be obtained first. Because the joint structures are not well mineralized in the child, further evaluation of the joint with ultrasonography or MR imaging will often be required to make a more definitive diagnosis because, as discussed earlier, they are the best techniques to image nonmineralized structures. If MR imaging facilities are not available, the congenital abnormality may be investigated with a combination of conventional radiography, ultrasonography, conventional arthrography, and CT with or without intra-articular contrast injection.

Acute Trauma

In acute trauma, the first line of radiologic investigation is the radiograph. If fractures are identified, further imaging will depend on the needs of the clinician or subspecialist physician as dictated by the clinical situation. Generally, fractures that extend into the joint surface (intra-articular fractures) are treated very aggressively with the aim of reestablishing the integrity of the joint. Intra-articular fractures are frequently treated by operative reduction and internal fixation if the fracture fragments are severely displaced. CT examination of the involved limb and joints may also be performed preoperatively in selected cases for surgical planning. Postoperative CT may also be required to assess the results of surgical intervention. The advantages of CT are that it enables precise assessment of joint reconstitution and also confirms or excludes the presence of any intra-articular fragments that may interfere with proper reduction and healing (see Fig. 7–4).

Subacute and Remote Trauma

If the trauma is subacute or remote, the investigation should also begin with the radiograph. If the joint is normal and there is a strong clinical suspicion of injury, the patient may benefit from a radionuclide bone scan or more appropriately MR imaging because of its superior soft-tissue contrast and resolution. It is, therefore, particularly suited for investigation of the intra-articular and periarticular soft-tissue structures and subchondral bone (see Figs. 7–5 and 7–6).

Nontraumatic Cases

After an initial normal or unrevealing radiographic examination, MR imaging is the next modality of choice. If there is a joint effusion, it may be cost effective to aspirate the fluid and analyze it for metabolic or infectious causes. If there is no obvious effusion at radiography, MR imaging is usually the next best and most cost-effective imaging test to evaluate injury to the intra-articular and periarticular structures of a joint.

EXERCISE 7-1: CONGENITAL JOINT DISORDERS

Clinical Histories:

Case 7-1. This 25-year-old woman had hip pain that had become unbearable. She always walked abnormally and had some discomfort for as long as she could remember. Her pelvis radiograph is shown in Fig. 7–10.

Case 7-2. This child was examined by a pediatrician because of developmental delay. He has not attempted walking and is still crawling at 2 years. Several radiographs were performed. Figure 7–11 is a radiograph of the pelvis.

Case 7-3. This young boy was examined by a pediatrician because "he walks funny," according to his mother. He denies any pain. A film of the tibia and fibula is shown in Fig. 7–12.

Questions:

Radiologic Findings:

7-1. Both hips in the patient in this case (Fig. 7–10) are abnormal. The femoral heads and necks are malformed and dislocated from the acetabula fossae superiorly. The acetabulae are also malformed and oriented more vertically than normal. The patient had bilateral congenital dislocation of the hips that had been ignored by her parents. (A is the correct answer to Question 7-1.) This diagnosis should have been made at birth or shortly thereafter so that corrective therapy could have been instituted.

7-2. In this case (Fig. 7–11), the capital femoral epiphysis on the right (arrow) is laterally displaced and smaller than the left epiphysis (arrowhead). The acetabular fossa on the right side is also malformed and more vertical than the one on the left. The normal development of the acetabulum depends on a normally located femoral head and this is the explanation for this abnormality. These findings are the classical radiographic features of developmental dysplasia of the hip (DDH). When this finding is encountered, MR imaging is the next most appropriate imaging test. (B is the correct answer to Question 7-2.)

7-3. In this case (Fig. 7–12), the calcaneus is deformed (white arrow). The talus is poorly visualized because of its complete dislocation from its normal position below the tibia and there are subluxations at the tibiotalar, talocalcaneal, and talonavicular joints. There is overall frank disorganization of this ankle joint, and clinically there was diffuse soft-tissue swelling around the ankle (not appreciated on this lateral film). There is also a metaphyseal fracture of the proximal tibia with exuberant periosteal/callus formation (black arrow). All of these findings in this patient are caused by chronic repetitive trauma in a patient with congenital insensitivity to pain. (D is the correct answer to Question 7-3.)

Discussion:

Congenital joint disorders are uncommonly encountered but they should be diagnosed as early as possible after birth because delayed diagnosis complicates management. Some of the more common congenital joint disorders include

1. Congenital dislocation of the hips, which is actually a bone dysplasia manifesting as a joint disorder

2. Arthrogryposis multiplex congenita

3. Congenital insensitivity to pain (asymbolia)

Congenital hip dislocation is actually a bone dysplasia. The femoral head is dysplastic and does not provide adequate stimulation for proper development of the acetabulum. Usually the femoral head is displaced laterally out of an unusually shallow (i.e., more vertically oriented) acetabulum (Fig. 7–11). Once the diagnosis of DDH is confirmed, treatment should begin at birth or in the perinatal period to minimize complications.

Historically, hip arthrograms were used to define the location of the femoral head in the neonate because the structure is cartilaginous at birth and therefore radiolucent on radiographs. Ultrasonography is an excellent test to assist in the diagnosis of DDH in utero and in the neonatal period because it requires no ionizing radiation. However, the interpretation of ultrasound is very observer dependent. The diagnosis and clarification of DDH can be difficult at times. MR imaging can show the soft-tissue structures, the osseous structures, and the articular cartilage well and is at present the modality of choice in the evaluation of DDH. If patients are not treated or poorly treated they will eventually develop secondary osteoarthritic changes. However, MR imaging is of no diagnostic value at this stage because the advanced nature of the disease can be ascertained just as well from radiographs.

Proximal focal femoral deficiency is a disease of uncertain etiology that is characterized by congenital absence of a segment or all of the proximal third of the femur. The incidence is higher in children of diabetic mothers and there is also an association with congenital hip dysplasia. MR imaging is also the imaging modality of choice in evaluating children with these disorders.

Arthrogryposis multiplex congenita is a noninheritable congenital disease of uncertain etiology characterized by multiple joint contractures. It is believed to be due to neuromuscular events occurring in utero. The joints of the lower limb are almost invariably affected. There may be other associated extraskeletal congenital anomalies.

A neuropathic joint is caused by chronic repetitive trauma in the setting of impaired or absent sensation. The characteristic features of neuropathic joint include soft-tissue swelling, fragmentation of the bony structures, and general disorganization of the joint. Joint effusion is often present. The more common causes of neuropathic joints in the lower extremities include diabetes mellitus and tabes dorsalis (neurosyphilis). Asymbolia or congenital insensitivity to pain, as exhibited in Case 7-3 (Fig. 7–12), is a group of uncommon congenital disorders in which there is a variable degree of loss of pain sensation and is an unusual cause of neuropathic joint. Patients with asymbolia almost always acquire deformities of the extremities after repeated trauma. This diagnosis should be considered in the young patient with multiple healing fractures and without suspicion of child abuse syndrome.

EXERCISE 7-2: JOINT TRAUMA

Clinical Histories:

Case 7-4. A 25-year-old man fell on his outstretched hand. AP and lateral radiographs of his wrist are shown in Fig. 7–13.

Case 7-5. A 30-year-old basketball player fell in practice and Fig. 7–14 shows AP and axillary views of radiographs from his shoulder.

Case 7-6. A 34-year-old anesthesiologist was injured in a basketball game. Figure 7–15 is a sagittal MR image from his knee study.

Case 7-7. A 40-year-old squash player was having shoulder pain and was referred for an MR scan. Figure 7–16 is an oblique coronal MR image from his study.

Questions:

Radiologic Findings:

7-4. In the frontal projection (Fig. 7–13A ), there is disorganization of the carpal arcs. The capitate is no longer articulating with the lunate and partly overlaps the scaphoid (arrow). The scaphoid is elongated on this view but not fractured. On the lateral projection (Fig. 7–13B ), the lunate is still in line with the distal radius but the capitate has been dislocated dorsally (open arrows). Therefore, the patient has a dorsal perilunate dislocation. (B is the correct answer to Question 7-4.)

7-5. The AP radiograph of the shoulder of the basketball player (Fig. 7–14A ) shows inferior displacement of the humeral head out of its normal position within the glenoid. The axillary view of the shoulder (Fig. 7–14B ) shows that the humeral head is dislocated anteriorly in relation to the glenoid, thus representing an anterior shoulder dislocation. This is the classic appearance of an anterior dislocation of the shoulder. (B is the correct answer to Question 7-5.)

7-6. The sagittal MR image shows the advantage of MR imaging in this clinical setting. Figure 7–15 shows a tear of the anterior cruciate ligament (ACL) (arrow). (C is the correct answer to Question 7-6.)

7-7. Figure 7–16 shows a torn supraspinatus tendon (arrow) in the squash player. (C is the correct answer to Question 7-7.)

Discussion:

Dislocation or subluxation: The terms subluxation and dislocation are often used interchangeably. However,subluxation refers to partial loss of congruity between the articulating ends of bones, whereas dislocation denotes complete loss of congruity. Disruption or loss of the integrity of the restraining ligaments around the joint leads to instability and thus permits dislocation to occur. Severe hyperflexion or hyperextension forces often cause traumatic dislocations. Fractures are frequently associated with traumatic dislocations.

CARPAL DISLOCATION

The normal arrangement of the carpal bones of the wrist is seen on the AP view of the wrist (Fig. 7–17A ). Note the three smooth, parallel arcs in the proximal and midcarpal rows. The lateral view of the wrist (Fig. 7–17B ) shows that the radius, lunate, and capitate are in an almost straight line. The two major types of wrist carpal dislocation areperilunate and lunate. In a perilunate dislocation, the lateral film shows that the lunate maintains its normal articulation with the radius and the capitate is displaced dorsally. In a lunate dislocation, the lunate has a triangular shape on the frontal projection (Fig. 7–18A ) and is displaced from its normal articulation, and the radius and capitate maintain a linear relationship (Fig. 7–18B ). Carpal dislocations are usually produced by a fall on the outstretched hand and are more common in young adults. The diagnosis is usually made by radiographic examination, although CT may be used after reduction to evaluate the wrist for joint congruity and for the presence of loose bodies (intra-articular fracture fragments).

SHOULDER DISLOCATION

The two main types of shoulder dislocation are anterior and posterior dislocations. Anterior dislocation, usually caused by falls, is most common and is seen in about 95% of cases. In an anterior dislocation, the humeral head is displaced anteriorly and inferiorly to the scapular glenoid fossa. There are various subtypes of anterior dislocation: subglenoid, subcoracoid, and medial. These subtypes are based on the location of the humeral head relative to the glenoid fossa and coracoid process.

Posterior dislocation is relatively uncommon. It is produced by severe contraction of the muscles of the shoulder girdle, which may occur in electric shock or convulsions. A diagnosis of posterior dislocation in one shoulder should prompt investigation of the other shoulder, since the injury is often bilateral.

If the postreduction radiographs are normal after a single instance of dislocation, there is usually no need for another imaging study in the acute setting. However, if there is a recurrence of dislocation or if the patient remains chronically symptomatic, MR imaging or CT arthrography of the shoulder should be obtained to search for the cause of the dislocations and any associated shoulder abnormalities resulting from the dislocation.

CT arthrography and MR imaging are used to investigate the shoulder for cartilage and soft-tissue injuries resulting from shoulder dislocation. After an anterior dislocation there is frequently associated injury to the anterior glenoid labrum. This is produced by the impaction of the posterior and lateral aspect of the humeral head beneath the anterior and inferior glenoid process. There may also be an accompanying compression fracture of the humeral head, referred to as the Hill-Sachs deformity.

Hip Dislocation

The hip is a relatively stable joint because of the surrounding strong muscles and joint capsule and significant trauma is required for dislocations to occur. A common mechanism causing hip dislocation is the "dashboard injury" where the knee is impacted on the dashboard and the femoral head is driven posteriorly in relation to the acetabulum. The most common dislocating direction of the femur is posterior and when this occurs the femoral head often ends up superior and lateral to the acetabulum. In posterior dislocations, there is almost always an associated fracture of the posterior aspect of the acetabular rim or the femoral head (Fig. 7–19). Anterior hip dislocation is uncommon and is produced by a blow to the hip when the femur is internally rotated and abducted. Central dislocation of the hip usually occurs with direct lateral forces and there is an associated fracture of the quadrilateral plate (medial aspect) of the acetabulum.

KNEE LIGAMENT TEARS

Tears of the ligaments of the knee are commonly seen in athletic individuals. The function of the anterior cruciate ligament (ACL) is to limit the anterior translation of the tibia in relation to the femur. Any sport that requires pivoting and planting of the feet places an enormous stress on the ACL and may cause injury to it. The normal ACL originates on the inner aspect of the lateral femoral condyle and extends anteriorly and slightly obliquely to insert adjacent to the anterior tibial spine. The normal ACL has a fascicular arrangement with individual bands that can easily be seen by MR (Fig. 7–20). The most common mechanism of injury to the ACL is the "clipping" injury with valgus stress. On MR, the injured ACL is diagnosed by high signal intensity within the substance of the ligament (the so-called "pseudomass"). There may also be other associated abnormalities such as bone contusions (usually on the posterolateral aspect of the tibia and the anterolateral aspect of the femur from the transient dislocation that occurs at the time of injury) and medial collateral ligament injury from the valgus stress (Fig. 7–21). There may also be associated meniscal tears, usually vertical tears in the acute setting (Fig. 7–22).

The posterior cruciate ligament (PCL) serves to limit the posterior translation of the tibia in relation to the femur. The PCL is commonly injured in kicking sports such as soccer and is also injured in automobile accidents if the tibia impacts on the dashboard and is translated posteriorly in relation to the femur in the flexed knee.

The normal PCL on MR is a homogeneous structure that originates from the inner aspect of the medial femoral condyle and extends far posteriorly to insert onto the posterior aspect of the tibia (Fig. 7–23). The PCL should easily be seen on all knee MR studies. Tears of the PCL are diagnosed at MR imaging by increased signal within the ligament and swelling (partial tears), and a complete tear of the ligament is diagnosed by complete separation of the ligament at some point along its course (Fig. 7–24). MR imaging is extremely important in the evaluation of the knee of the injured athlete and is used frequently in this setting.

SUPRASPINATUS TENDON TEARS

The supraspinatus, infraspinatus, teres minor, and subscapularis muscles (SITS muscles) comprise the rotator cuff. The shoulder is the most unstable joint in the body and these muscles serve to help stabilize it. The most commonly torn tendon in the shoulder is the supraspinatus, and it usually tears approximately 1 cm proximal to its insertion onto the greater tuberosity of the humeral head. The supraspinatus tendon is easily seen on MR imaging as a low-signal-intensity structure, and tears of the supraspinatus tendon are easily and nicely demonstrated on MR. Fig. 7–16 is a typical example of a supraspinatus tendon tear with fluid within the tendon. The most common causes of supraspinatus tendon tears are aging and impingement. Acute rotator cuff tears are unusual. MR imaging is vital in the preoperative evaluation of the patient suspected of having a rotator cuff tear because the morphology of the tendons, the size of the tear if there is one, and other associated abnormalities of the joint including pathology of the glenoid labrum can be diagnosed with this technique.

ACHILLES TENDON RUPTURE

The injury of achilles tendon rupture occurs most frequently in patients in the fourth and fifth decades of life. Although the injury may occur in any person, individuals who do not exercise regularly (the so-called "weekend warriors") are more susceptible to this tear.

The clinical history and physical findings are often enough to make a diagnosis of Achilles tendon rupture. Radiographic stress views should not be performed in the setting of a suspected Achilles tendon rupture because the stress may actually make the tear worse. Any question of whether the tear is partial or complete should usually be resolved, because the treatment for each of these is different. Moreover, the clinician needs to know the level of injury and how far the tendon fragments are separated. MR imaging is currently the imaging technique of choice to evaluate the Achilles tendon. The whole length of the tendon, including its insertion on the calcaneus, and any associated injuries can be shown in detail. In cases of complete rupture of the tendon, the MR images show separation of the normally low-signal-intensity fibers of the Achilles tendon by high-signal-intensity edema and hemorrhage (Fig. 7–25). The frayed ends of each of the separated fragments may also be seen. In partial tears, areas of intermediate to high signal representing regions of partial disruption are seen within the normally low-signal-intensity tendon and some of the fibers of the tendon remain intact. Ultrasound may also be used to evaluate the Achilles tendon and color Doppler examination may be used to follow the process of revascularization and healing of a partially torn tendon, although musculoskeletal ultrasound is not widely practiced in the United States at this time.

EXERCISE 7-3: JOINT INSTABILITY

Clinical Histories:

Case 7-8. A 45-year-old former baseball pitcher had recurrent dislocations of the shoulder, pain, and inability to reach around to his back pocket. He also feels a click and catching sensation when he moves his arm. Radiographs of his shoulder are normal. He is referred by his orthopedist for MR imaging. A selected axial image from his study is shown in Figure 7–26.

Case 7-9. A 20-year-old football player was examined in the emergency department after being tackled particularly hard. A radiograph was obtained first (Fig. 7–27).

Case 7-10. A little girl was running through her house and tripped on an electrical cord. Intense pain ensued and she was referred for an MR examination. A selected axial image from her MR study is shown in Fig. 7–28.

Case 7-11. A college football player was injured and was thought to have transiently subluxed his knee when he was hit on the lateral side of the leg. His orthopedic sports medicine physician noted medial laxity and referred him for an MR scan. A coronal MR image from his study is shown in Fig. 7–29.

Questions:

Radiologic Findings:

7-8. Figure 7–26 is an axial image. There is a high-signal intensity line through the triangular anterior glenoid labrum representing an anterior glenoid labral tear (C). (C is the correct answer to Question 7-8.) In Fig. 7–26, A is the posterior labrum, B is the biceps tendon, and D is the belly of the subscapularis muscle.

7-9. Figure 7–27 is a lateral radiograph of the knee obtained with a horizontal x-ray beam. There is a joint effusion with a fat-fluid level (arrowheads). This fat-fluid level is called a lipohemarthrosis. It is important to perform horizontal cross table lateral films in the acute trauma setting in order to demonstrate this. Frequently, there is a fracture to account for the presence of fat within the joint because the fat has entered from the bone marrow cavity. (B is the correct answer to Question 7-9.) Fat-fluid levels may also be seen in patients who have acute tears of ligaments, especially the ACL.

7-10. Figure 7–28, the axial image on the little girl injured by tripping over the electrical cord, shows a large joint effusion and high-signal-intensity areas on the medial aspect of the patella and the lateral aspect of the lateral femoral condyle (arrows). These are also in essence a type of "kissing contusion." However, in this instance, the contusions result from impingement of the medial aspect of the patella onto the lateral femoral condyle as the patella dislocates laterally. For this to happen, there has to be a stretch or tear of the medial retinaculum (arrowhead). (C is the correct answer to Question 7-10.) This constellation of findings is typical of transient patellar dislocation-relocation.

7-11. Figure 7–29, a coronal image of the football player injured during a game, shows a complete tear of the medial collateral ligament (MCL) (arrow). (A is the correct answer to Question 7-11.) Injuries to the medial side of the knee occur from lateral trauma (valgus stress). The clinician in this instance noticed the medial joint laxity and suspected an MCL tear.

Discussion

INSTABILITY DISORDERS

Instability disorders are functional joint disorders generally manifested by pain, a feeling of insecurity around the joint, or a sensation of the joint giving way and abnormal motion around the joint. There may be no radiographic evidence of joint abnormality because often only soft-tissue injuries, such as ligamentous or fibrocartilaginous tears, are present. These abnormalities can often only be demonstrated by stress views or MR examinations of the joint in question.

SHOULDER INSTABILITY

The glenohumeral joint is the most inherently unstable ball-in-socket joint in the body. The major stability of the shoulder joint is provided by the joint capsule, the rotator cuff muscles, and the ligaments and tendons that surround it. The glenoid labrum, a fibrocartilaginous structure, contributes to shoulder joint stability by deepening the socket (glenoid labral complex) for this ball (humeral head).

A variety of soft-tissue injuries are associated with the anterior instability syndrome. The most common injuries from anterior glenohumeral dislocation are anteroinferior glenoid labral tears (the Bankart lesion), capsular stripping, Hill-Sachs deformity (compression fracture of the posterolateral aspect of the humeral head), and glenoid labral tears associated with osseous fracture (Figs. 7–30 and 7–31). Tears of the posterior aspect of the glenoid labrum are seen following posterior dislocation (Fig. 7–32). As we discussed in the last section, ruptures or tears of the rotator cuff tendons (SITS) are common causes of shoulder joint dysfunction and instability. The supraspinatus is the most commonly torn tendon in the shoulder. When the subscapularis tendon tears, an associated dislocation of the biceps tendon usually occurs. This results from a tear of the transverse ligament, a fascial extension across the intertubercular sulcus that holds the biceps tendon in place (Fig. 7–33).

The shoulder joint is best evaluated with MR imaging if there is a suspicion of a tendon abnormality. Shoulder arthrography and CT arthrography may also be requested by the orthopedic surgeon and used as alternative investigational tools in patients who have undergone rotator cuff repairs or who have had recent surgery to vital structures with residual ferromagnetic metallic clips. These tests are also used in patients who have aneurysm clips or metallic substances in the body (especially those located in or close to the eyes).

KNEE JOINT INSTABILITY

Stability of the knee, which is a hinge joint, is provided by the muscles and the ligamentous complexes. The most important of these are the anterior and posterior cruciate ligaments and the lateral and medial collateral ligament complexes. The major knee muscles and tendons that cross the joint include the quadriceps muscle and tendon and the patella ligament anteriorly; the biceps femoris tendon, tensor fascia lata, and popliteus muscles laterally; the pes anserinus tendons (sartorius, gracilis, and semitendinosus muscles—"Say grace before tea" is a phrase to help us remember these!) medially; and the gastrocnemius and plantaris muscles posteriorly. Rupture of any of the tendons, ligaments, or muscles compromises stability of the knee joint. All of these structures can be exquisitely demonstrated with MR imaging, which is the best imaging test to evaluate instability in this joint.

EXERCISE 7-4: ARTHRITIDES

Clinical Histories:

Case 7-12. A 75-year-old man presented with bilateral hip pain of several years' duration. Figure 7–34 is a frontal view of his pelvis.

Case 7-13. A 45-year-old woman with generalized body "aches and pains" presented to her rheumatologist. Her erythrocyte sedimentation rate (ESR) was elevated at 70 mm. Figure 7–35 is a frontal view of both of her hands.

Case 7-14. A 59-year-old woman with renal failure and a long history of hemodialysis was seen in the emergency room having experienced 2 days of intense right hip pain and fever. She had a fall 8 days ago. She has substantial limitation of motion in the right hip on physical examination. Figure 7–36 is the frontal view of her right hip.

Case 7-15. A 60-year old man presented to the emergency room with intense foot pain. He was a known ethanol abuser. An oblique view of his right foot is shown in Fig. 7–37.

Case 7-16. A 39-year-old man with a long history of back pain presented with limitation of movement in his neck. His HLA-B27 antigen was elevated. Figure 7–38 is a lateral view of his cervical spine.

Case 7-17. A 58-year-old man with a "rash" on his elbows presented to his rheumatologist with hand pain. Figure 7–39A is a frontal view of his hands and Figure 7–39B is a coned down view of the distal phalanges of his left hand.

Questions:

Radiologic Findings:

7-12. The frontal radiograph of both hips in this case (Fig. 7–34) shows articular space narrowing, sclerosis, and subchondral cyst formation (also called geodes) bilaterally. There is no significant juxta-articular osteopenia. Therefore, the correct answer to Question 7-12 is C and the findings are most compatible with bilateral osteoarthritis (OA) of the hips.

7-13. In this case (Fig. 7–35), the most prominent feature of this lady's hand is soft-tissue calcification and acroosteolysis of the distal tuft. The features are most compatible with a diagnosis of scleroderma. (B is the correct answer to Question 7-13).

7-14. In this case (Fig. 7–36), the radiograph of the right hip shows articular space narrowing, bony erosion in the acetabulum and femoral head (arrowheads), and irregular bony sclerosis. The findings are most compatible with septic arthritis. (C is the correct answer for Question 7-14.)

7-15. Figure 7–37 shows marked soft-tissue swelling and erosion of the distal aspect of the first metatarsal as well as erosions involving the proximal aspect of the proximal phalanx. These erosions have "overhanging margins." This radiographic appearance and the location of these changes at the first metatarsal phalangeal joint are characteristic of "podagra" associated with the initial attack of gout. (C is the correct answer to Question 7-15.)

7-16. The lateral view of the cervical spine in Fig. 7–38 shows prominent thin vertically oriented connections between the anterior aspects of the vertebral bodies and fusion of the posterior elements of the spine. The thin vertically oriented ossifications are located anatomically in the outer layers of the annulus fibrosis and represent syndesmophytes that are associated with ankylosing spondylitis. (D is the correct answer to Question 7-16.)

7-17. Figure 7–39 shows soft-tissue swelling at the PIP and DIP joints and "pencil-in-cup" erosions of the distal interphalangeal joint of the second through fifth digits bilaterally. (Fig. 7–39B is a magnified view of Fig. 7–39A .) There is associated periosteal reaction. These findings are most compatible with a diagnosis of psoriasis. (A is the correct answer to Question 7-17.)

Discussion:

Osteoarthrosis (osteoarthritis): The frontal view of the pelvis of Case 7-12 (Fig. 7–34) shows the classic findings of osteoarthritis. These include cartilage loss with resultant articular space narrowing, subchondral cysts, and osteophyte formation. The findings of osteoarthritis are similar regardless of the joint in which they occur. In the knee, the articular space narrowing initially involves the medial compartment (Fig. 7–40), but may progress to involve the lateral and patellofemoral compartments. In the hands, the articular space narrowing typically involves the distal interphalangeal joints. Another classic "target area" of osteoarthritis in the hand is the first carpal metacarpal joint (trapezio-first metacarpal joint). There is a variant of osteoarthritis, erosive osteoarthritis, which can present with a cyclical clinical phase of involvement that mimics rheumatoid arthritis. Patients with erosive osteoarthritis develop central erosions within the joints of the hand in a classic "gull wing" appearance as shown at the proximal interphalangeal joint of the fifth digit in the patient's hand in Fig. 7–41.

CONNECTIVE-TISSUE DISEASES AND SERONEGATIVE SPONDYLOARTHROPATHIES

Generally, a history of polyarticular stiffness suggests a polyarthritis and the initial imaging investigation is a radiographic survey of the affected joint or joints. These images target the most common regions of involvement and usually include films of the hands, wrists, pelvis, knee, feet, and ankles. The findings on these films, coupled with the ESR value, should indicate whether a connective-tissue disorder is the cause of the arthropathy. Often patients will be tested for rheumatoid factor (discussed in the next paragraph). If this test is negative and the patient has symptoms involving the peripheral joints and spine, then a seronegative spondyloarthropathy is considered.

Perhaps the most common and characteristic connective-tissue disease producing arthritis is rheumatoid arthritis (RA). Females (especially middle-aged women) are more commonly affected by this disease than men. RA is thought to be a malfunction of the immune system, and patients with this disorder usually produce a measurable immune complex called rheumatoid factor (RF). They also characteristically have an elevated ESR. The disease often progresses in a symmetrical fashion. The major initial pathologic process in RA is a synovitis, which produces periarticular osteopenia because of the associated hyperemia. Later in the disease, synovial proliferation with pannus formation may then cause erosions in the juxta-articular regions (the so-called "bare areas"). These erosions occur at the cortex where the synovium contacts it and it is not protected by articular cartilage. Subsequently, the disease may progress to secondary degenerative changes and eventually to fibrous or bony ankylosis of the joint. In the wrist, the carpal bones will show osteopenia, carpal crowding, or subluxations. In fact, the ulnar styloid is often one of the first sites of erosions and "pencilling." Juxta-articular osteopenia is seen in the bones of the hands and wrists. Symmetric swelling at the proximal interphalangeal joints of the hand is present, and bone ends show erosions, especially at the metacarpophalangeal and proximal interphalangeal joints (Fig. 7–42).

Systemic lupus erythermatosus is a connective-tissue disorder that can be seen in conjunction with other connective tissue diseases (the so-called "overlap syndrome"). Patients with SLE may show profound osteopenia, including resorption of the tufts, but it does not characteristically result in erosions. The typical appearance is joint instability with multiple subluxations at the wrists and metacarpophalangeal joints. In fact, SLE is the most common cause of a nonerosive subluxing arthropathy (Fig. 7–43). Subluxations also occur in RA, but the subluxations in RA are associated with the "bare area" erosions.

Scleroderma (progressive systemic sclerosis, or PSS) is a disorder characterized by fibrosis and skin thickening. Soft-tissue calcification is a prominent feature of this disorder. The major effects of this disease are not on the joints per se, but are secondary to the diffuse sclerosis with resultant joint stiffness for which the patient may seek treatment initially. About 10% of patients with PSS have synovitis that is indistinguishable from RA at presentation, and many of these patients eventually develop Raynaud's phenomenon. The typical imaging findings are periarticular calcification and resorption of the terminal phalangeal tufts (acroosteolysis) (Fig. 7–35). Scleroderma may also be associated with other connective disorders such as rheumatoid arthritis and SLE in the same individual.

Psoriasis, Reiter's disease, ankylosing spondylitis, and inflammatory bowel disease comprise the major seronegative spondyloarthropathies. Psoriasis is a connective-tissue disorder that primarily affects the skin. However, about 15% of patients with psoriasis develop bone and joint changes and these findings may be the initial manifestations of the disease. Radiographic findings of psoriasis include periosteal reaction (periostitis) and/or focal cortical thickening in the digits. The earliest manifestation of the disease is juxta-articular osteopenia that is less profound than in RA. The disease may then progress to show erosions at the bone ends (marginal erosions). The distribution of these findings in psoriasis is mainly the distal interphalangeal joints, unlike the findings in patients with RA, which are predominately in the proximal interphalangeal joints and carpus. The "pencil-in-cup" erosions seen in Fig. 7–39 are typical of psoriasis. Patients with psoriasis and other seronegative spondyloarthropathies also develop abnormalities of the spine and sacroiliac joints (hence the term spondyloarthropathy).

Reiter's disease is a postinfective disorder of the immune system that is characterized by the triad of nongonococcal urethritis, conjunctivitis/iritis, and arthritis. Seen most frequently in male patients, Reiter's disease was originally thought to be caused by Chlamydia, but other organisms, including E. coli and Salmonella organisms, have also been implicated. The imaging findings of Reiter's disease are often indistinguishable from those of psoriatic arthritis except that Reiter's disease most commonly affects the feet and psoriasis most commonly involves the hands. Both diseases show periostitis, erosions, and enthesopathic changes. An enthesis is an area of attachment of a ligament or tendon to bone by the perforating fibers of Sharpey. An enthesopathy is, therefore, an abnormality at this site and is seen on the radiograph as bony excrescences in these areas. A typical example of enthesopathy in Reiter's disease is the bony excrescence on the inferior aspect of the calcaneus, which develops at the site of attachment of the plantar fascia and the short flexors in the foot (Fig. 7–44).

Ankylosing spondylitis (AS) is a rheumatic disease causing arthritis of the spine and sacroiliac joints and can cause inflammation of the eyes, lungs, and heart valve. The typical clinical scenario is intermittent back pain that occurs throughout life. The pain may progress to severe chronic disease attacking the spine, peripheral joints, and other organs resulting in marked loss of motion and deformity over time. The cause of AS is not known, but most of the spondyloarthropathies share a common genetic marker called the HLA-B27 antigen. The disease usually presents in the adolescent and young adult and is most common in Native Americans.

Figure 7–38 shows the typical features of ankylosing spondylitis (AS) in the cervical spine. The thin vertically oriented ossifications connecting the vertebral bodies, syndesmophytes, are anatomically located in the outer layers of the anulus fibrosus. Also typical in this case is the fusion of the posterior elements. In fact, the classic appearance of AS is the "bamboo spine" (Fig. 7–45). This appearance is caused by fusion of all the synovial joints of the spine and predisposes the patient to the development of fractures (insufficiency-type fractures). This insufficiency fracture (also called a pseudoarthrosis) is a well-documented complication of ankylosing spondylitis.

The mainstay of treatment for AS is nonsteroidal anti-inflammatory medication to control pain. However, some patients with severe disease may be given methotrexate.

SEPTIC ARTHRITIS

Septic arthritis is usually bloodborne (hematogenous) and is most commonly monoarticular (involving only one joint at any time). A common cause of septic arthritis in the adult is Staphylococcus aureus, although other infective agents including streptococcus, gonococcus, and other gram-negative organisms may also be encountered. Streptococcus and gram-negative organisms are particularly important in the pediatric age group.

The radiographic examination of the patient in Case 7-14 (Fig. 7–36) provides general anatomic information, helps to determine whether further imaging is necessary, and aids in deciding whether further intervention is appropriate. Her physicians were very worried about septic arthritis in this clinical setting of previous renal transplant (i.e., relatively immunocompromised) and a hip aspiration was requested. Twenty milliliters of bloodstained turbid fluid was aspirated and sent to the microbiology laboratory for Gram stain, culture, and sensitivity studies. The cultures grewStaphylococcus aureus, a common pathogen in septic arthritis.

Figure 7–36 shows the classic radiographic findings of septic arthritis and osteomyelitis. These include articular space narrowing, erosion of bone on both sides of the joint, and sclerosis. An effusion will always be present and can be identified by ultrasound or MR imaging. Septic joints will also be "hot" on bone scan because of the marked hyperemia and bony proliferation.

CRYSTAL DEPOSITION DISEASES

Gout, a disorder most common in middle-aged men, is an inflammatory arthritis caused by abnormal deposition of urates (called tophi) in the soft tissues and cartilage. These deposits cause episodic joint inflammation and are associated with pain and disability. In the earlier stages of the disease, radiographs of the bone and joints may be normal except for soft-tissue swelling and in some instances soft-tissue calcification. The classic initial presentation of gout is podagra, an acute inflammation of the joint, usually the first metatarso–phalangeal joint (Fig. 7–37). At presentation the patient will have severe joint pain and the overlying soft tissues will be swollen and red. With repeated attacks over years (usually at least 15–20 years), bony erosions with "overhanging edges" (or overhanging margins) may develop adjacent to the joint but not within the joint. When the patient is severely incapacitated by pain and not moving the joint, "disuse" osteopenia can be seen at radiography. Occasionally, osteopenia may be due to intraosseous deposition of tophaceous material. The typical areas to screen for gout are the first metatarsophalangeal joint, the heel, the back of the elbow joint (olecranon fossa), and the hands and wrists. Screening for elevated serum levels of uric acid and joint aspiration are probably the best ways to confirm the clinical suspicion of gout. The joint aspirate will show birefringent uric acid crystals in the synovial fluid on polarized light microscopy.

Calcium pyrophosphate dehydrate crystal deposition (CPPD) disease is another common crystal deposition joint disorder. In CPPD disease there is calcification in the fibrocartilage and hyaline articular cartilage (so-called "chondrocalcinosis"). The most common association with chondrocalcinosis is aging although it may also be seen in pseudogout (CPPD disease), gout, ochronosis, hemochromatosis, and hyperparathyroidism. The presence of calcification alone is not diagnostic of CPPD, however. The clinical syndrome of pain from the presence of abnormal cartilage calcification is referred to as the CPPD syndrome, and symptoms may be provoked by various stresses (e.g., surgical procedures). Aspiration of the joint thought to be involved by CPPD and discovery of the typical calcium pyrophosphate crystals confirm the diagnosis.

EXERCISE 7-5: MISCELLANEOUS JOINT DISORDERS

Clinical Histories:

Case 7-18. A 24-year-old male medical student, an avid tennis player, had intermittent joint swelling and minimal knee pain. Lately the pain had worsened and was interfering with his tennis game. Nonsteroidal anti-inflammatory agents were not working well and an orthopedic resident requested that he get a knee x-ray (Fig. 7–46). After looking at the radiograph, the resident scheduled an appointment for the student with an attending surgeon.

Case 7-19. A 20-year-old man complained of a feeling of fullness and gritty sensations in his right shoulder. He has never had a shoulder dislocation, although on several occasions he has been unable to raise the shoulder and has felt a painful "catch" at times. A radiograph of his right shoulder was obtained (Fig. 7–47).

Case 7-20. A 10-year-old boy was referred to an orthopedic surgeon for investigation of a limp. There was no reliable history of trauma. A frontal radiograph of the pelvis is shown and is abnormal (Fig. 7–48).

Case 7-21. A 35-year-old man with episodic right hip pain that began 4 to 6 months ago presented with a dull aching pain now in both hips. A frontal radiograph of both hips was obtained (Fig. 7–49).

Questions:

Radiologic Findings:

7-18. The AP view of the right knee (Fig. 7–46) shows an ovoid bony fragment on the inner aspect of the medial femoral condyle (arrow) separated from the femur by a lucency (arrowheads). This appearance is diagnostic of OCD of the knee. (D is the correct answer to Question 7-18.)

7-19. The radiograph of the right shoulder in Fig. 7–47 shows multiple rounded calcific bodies overlying the proximal humerus and glenoid process of the scapula. The distribution of these is within the joint and axillary recess (arrows). This appearance is classic for SOC. (B is the correct answer to Question 7-19.)

7-20. The radiograph of the pelvis in Fig. 7–48 shows collapse of the right capital femoral epiphysis, which is broad and short, and forms an acute angle with the shaft of the femur. The femoral head is displaced laterally and is not completely covered by the mildly deformed acetabulum. The left hip is normal. The findings are characteristic of the late changes in Legg-Calvé-Perthes disease. (C is the correct answer to Question 7-20.)

7-21. The radiograph of both hips in Fig. 7–49 demonstrates that the right femoral head is no longer smooth and spherical (loss of spherocity), and this is due to the presence of subchondral collapse in the superiolateral aspect (arrow). The left femoral head is still spherical but shows sclerosis. Also note the marginal osteophytes arising from the inferior and medial aspect of the left femoral head (arrowhead). The acetabuli are normal and these radiographic features are typical of avascular necrosis (osteonecrosis) of the femoral head. (D is the correct answer to Question 7-21.)

Discussion:

Osteochondritis dissecans (OCD) is a bone disorder that produces joint symptoms because of the intra-articular location of the abnormality. OCD, as classically demonstrated in Fig. 7–46, is seen on the radiograph as a semicircular focus of bone and overlying cartilage separated from the convex articular surface of the native bone by a lucency. The etiology is uncertain but current opinion favors repetitive microtrauma and vascular insult to the subchondral bone. Almost any joint may be affected, but the knee (distal femur), ankle (dome of the talus), and elbow (capitellum) joints are the most commonly involved sites. The disease is slightly more common in active young men but is increasingly being encountered in young women because they are more actively involved in athletics today. In the knee, OCD most commonly involves the non-weight-bearing aspect of the medial femoral condyle (i.e., the inner aspect and area shown in Fig. 7–46) and the lateral femoral condyle. MR imaging is the most appropriate modality to stage the lesion, predict whether the fragment may become separated entirely, and plan definitive treatment. CT or CT arthrography is an alternate test to use in patients who cannot undergo MR imaging.

Synovial osteochondromatosis (SOC) is a joint abnormality characterized by the presence of cartilaginous and osseous loose bodies within the synovial cavity in the joint. The exact cause is not known, but "primary" SOC is thought to be caused by synovial metaplasia and "secondary" OCD is assumed to be due to fractures of osteophytes or articular cartilage that shed into the joint cavity. Radiography may show only a joint effusion if the intrasynovial fragments are not ossified. These intra-articular bodies can be seen if they are calcified (Fig. 7–47, arrows). MR imaging is the best modality to use to show both ossified and nonossified intra-articular fragments and to evaluate the other soft-tissue structures around the joint.

Pigmented villonodular synovitis is a condition of unknown etiology characterized by hyperplasia or excessive villous proliferation of the synovium. This condition may occur in a single joint (localized form) or involve multiple joints (diffuse form). Thought to be caused by hemorrhage, PVNS shows hemosiderin-laden macrophages within the synovium best appreciated by gross examination. Radiographs often show a joint effusion with preservation of the articular space and normal bone mineral density. The later stages of the disease result in erosions on both sides of the joint. Joint aspiration yields dark brown fluid ("chocolate" effusion) due to the presence of the hemosiderin-laden macrophages. MR imaging is an excellent preoperative test to evaluate PVNS because the pigmented material (hemosiderin) shows low signal intensity on both the short TE (T₁-weighted) and the long TE (T₂-weighted) MR sequences. In fact, this finding is a very specific appearance for this disease. Heberden's node is a disfigurement of the interphalangeal joints as a result of severe osteoarthritis. Initially, it is due to soft-tissue inflammatory changes and is subsequently due to bony changes at the distal interphalangeal joints. It is more commonly seen in female patients.

Osteonecrosis can occur in any bone and is associated with a variety of disorders, including sickle cell hemoglobinopathy, Gaucher's disease, SLE, pancreatitis, alcoholism, steroid treatment, and barotrauma. When the process occurs at a bone end, it is known as avascular necrosis; when it occurs in the metaphysis of the bone, it is commonly referred to as a bone infarct. Eponyms have been used to designate osteonecrosis in certain sites. For example, Perthes' disease (Legg-Calvé-Perthes disease) is the eponym used to refer to idiopathic osteonecrosis of the femoral head occurring in a child as shown in Case 7-20 (Fig. 7–48). Other common eponyms include Freiberg's infraction (avascular necrosis of the head of the second or third metatarsal), Köhler's disease (tarsal navicular),Panner's disease (capitellum of the humerus), and Kienböck's disease (carpal lunate). The exact mechanism of the development of osteonecrosis is unknown, although bone-marrow edema after thrombosis and occlusion of the osseous capillaries and end arterioles are believed to be primarily responsible.

Radiography is a rather insensitive diagnostic tool for evaluating the early manifestations of osteonecrosis. Osteonecrosis does cause zones of increased sclerosis and osteolysis in the affected bone that can be appreciated on radiographs. However, these changes are late and early treatment cannot be performed at this stage (Fig. 7–49). If the disease is not diagnosed and treated early, the affected bone may go through a phase of subchondral collapse and become deformed. Subsequently, secondary osteoarthrosis will develop in the affected joint as a complication of neglected or poorly treated disease. Traditionally, nuclear medicine bone scanning has been used in this setting, but today MR imaging is the most sensitive available modality for the early diagnosis of this disease (Fig. 7–50).

Hemochromatosis is a rare disorder of iron metabolism, in which iron is deposited in the skin, parenchymal organs, and articular cartilage. This predisposes the joint to degenerative disease. Arthritis due to hemochromatosis is characterized by loss of joint space and formation of peculiar hooked osteophytes especially at metacarpal heads.

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