PATIENT PRESENTATION

A 65-year-old postmenopausal woman presents for prescription refill.

CLINICAL SUSPICION

Osteoporosis

IMAGING MODALITY OF CHOICE

Dual-energy x-ray absorptiometry (DXA)

BACKGROUND

Bone is made up of a variety of cells (eg, osteoblasts, osteoclasts, and osteocytes), an organic matrix (predominantly type I collagen), and an inorganic matrix (predominantly calcium hydroxyapatite). Osteoporosis is the most common metabolic disease of bone. It is due to disturbances in bone remodeling that result in less bone tissue (matrix and mineral) per unit volume. Bone mineral density (BMD), the average concentration of mineral in a defined section of bone, is used to diagnose osteoporosis, estimate the risk of insufficiency fractures, and select and monitor treatment. BMD is estimated by a DXA scanner, a low-dose x-ray tube with two energies. The low-energy and high-energy photons are attenuated differently in bone and soft tissue, allowing BMD to be estimated.

FINDINGS

DXA devices are used to calculate BMD in g/cm² and to compare the values to a reference database. The comparison is quantified as a T-score, which represents the standard deviation by which the BMD of the patient differs from the mean BMD of a young adult reference population of the same ethnicity and sex, and a Z-score, which represents the standard deviation by which the BMD of the patient differs from the mean BMD of an age-matched reference population of the same ethnicity and sex.

According to the World Health Organization, osteoporosis is “a systemic skeletal disease characterized by low bone density and microarchitectural deterioration of bone tissue with a consequent increase in bone fragility” and corresponds to a T-score of −2.5 or lower. Patients with osteopenia (see Box C1.1), defined as T-scores between −1 and −2.5, have less bone density loss than do those with osteoporosis. A T-score higher than −1 is considered normal.

Typically, the lumbar spine and proximal femur are scanned (Fig. C1.1A,B). The forearm (Fig. C1.1C) can be scanned as an alternative to the hip and lumbar spine when these parts cannot be scanned due to artifacts (read further) or when the patient is too heavy for the table. In patients with hyperparathyroidism, the nondominant forearm is scanned in addition to the hip and lumbar spine. This is because BMD loss in hyperparathyroidism is greater in structures with predominantly cortical bone, such as the distal radius.

Interpreting DXA scans requires more than just reading BMD estimates from a computer-generated report. As in other areas of radiology, the images should be assessed for quality and the presence of artifacts related to patient movement and appropriate field-of-view. Various artifacts of can result in a falsely elevated BMD estimate (see Box C1.2 and Fig. C1.2).

In the spine, the vertebral body heights should be assessed and compared with prior DXA studies (when available) to assess for progressive height loss. Finally, the rest of the image should be evaluated for the presence of incidental findings such as calcified leiomyomata, ureteral stones, and avascular necrosis of the femoral head. Figure C1.2 illustrates some common artifacts and incidental findings in DXA.

DIFFERENTIAL DIAGNOSIS

None

REFERENCES AND SUGGESTED READING

PATIENT PRESENTATION

A 55-year-old woman with chronic knee pain that is worse with activity. The patient reports no morning stiffness. Physical examination reveals crepitus, restricted range of motion, and no joint effusion.

CLINICAL SUSPICION

Osteoarthritis (OA, degenerative joint disease)

IMAGING MODALITY OF CHOICE

Radiographs of the knee that include axial and standing flexion views. Although radiography is often the imaging modality of choice for suspected OA, a confident diagnosis of knee OA can often be made clinically without radiographic examination or even with normal radiographs in adults older than 40 years of age who present with usage-related knee pain, only short-lived morning stiffness (< 30 minutes), functional limitation, and one or more typical physical examination findings (see Box C2.1).

When imaging is indicated, radiographs can provide an estimate of cartilage loss as manifested by joint space narrowing and associated abnormalities of the subchondral bone (read further). The preferred method of imaging the knees, the Rosenberg view, is illustrated in Fig. C2.1. Standard supine (non-weight-bearing) anteroposterior (AP) views can underestimate joint space narrowing, a shortcoming that can be addressed by having the patient stand during radiography. The routine standing view, however, may also underestimate joint space narrowing, although for different reasons. For example, because the maximum stresses in the knee occur between 30 and 60 degrees of flexion, standard weight-bearing radiographs taken with an extended knee are not ideal for evaluation of cartilage loss in these areas. The Rosenberg view addresses this and other issues related to radiographic estimation of cartilage loss.

Axial views of the knee should also be obtained to assess the patellofemoral compartment. Two commonly encountered choices for the axial view of the knee are the sunrise (skyline) and Merchant views. Figure C2.2 details the differences. Because of institutional inertia, most axial views of the knee are still labeled as sunrise views. A familiarity with your institution’s protocols is vital to ensure accurate assessment of knee radiographs.

Additional imaging studies are not indicated in patients for whom radiographs are diagnostic of degenerative joint disease unless treatment depends on additional findings, or when symptoms are not explained by the radiographic findings. Magnetic resonance imaging (MRI) can be obtained when radiographs are nondiagnostic or incongruent with clinical findings (eg, suspected internal derangement, insufficiency fracture, or inflammatory arthropathy) or when pain persists despite normal radiographs. As the meniscus also contributes to the joint space, meniscal degeneration and extrusion can account for a significant amount of joint space narrowing seen on radiographs.

FINDINGS

While early joint space narrowing is best assessed radiographically using Rosenberg views, advanced OA can be appreciated on standard AP views. Radiographic findings of OA of the knee (Fig. C2.3A-C) include joint space narrowing, osteophyte formation, subchondral sclerosis and cyst formation, and bone attrition (flattening). Narrowing of the medial compartment is more common and can lead to varus deformity of the knee (“bowlegged”) and widening of the lateral compartment. Mineralized intra-articular bodies can also be appreciated on radiographs. The term “loose body” is avoided by some radiologists because the intra-articular fragments may be adherent to synovial surfaces and therefore not “loose.”

MRI provides a direct assessment of the articular cartilage (Fig. C2.3D-F). Cartilage abnormalities in OA include focal signal changes, thinning, and partial-thickness to full-thickness defects (sometimes with exposure of the subchondral bone). Associated subchondral sclerosis manifests on MRI as thickening of the cortical signal void (T1 and T2 hypointensity). Bone marrow signal abnormalities (T1 hypointensity and T2 hyperintensity) are often focal and seen in the presence of cartilage defects. Subchondral cysts appear as well-defined areas of fluid signal (T1 hypointensity and T2 hyperintensity) that may or may not communicate with a synovial space. The sclerotic margin that is often seen around cysts is more apparent on radiographs.

Attrition, the flattening and depression of the articular surfaces of bones, is associated with severe OA. This is best assessed on MR images by noting deviation from the normally convex surfaces of the medial and lateral femoral condyles, the medial facet of the patella, and the flat to slightly convex surface of the lateral tibial plateau.

MRI is also sensitive for meniscal abnormalities ranging from mucoid degeneration to maceration and extrusion. Mild synovial inflammation and thickening are frequently seen in or near the intercondylar notch, the infrapatellar fat, and the posterior joint. Moderate or large effusions are more frequently seen in patients with pain than in those without pain, and the prevalence of effusions increases with increasing radiographic severity of OA.

Intra-articular bodies may also be present and appear as hypointense structures in the joint capsule or in a Baker cyst.

DIFFERENTIAL DIAGNOSIS

Imaging Differential Diagnosis

Calcium pyrophosphate dehydrate deposition arthropathy (Fig. C2.4) can present as isolated or disproportionate patellofemoral joint space narrowing and should be considered when seen in conjunction with chondrocalcinosis (calcification along fibrocartilage menisci and hyaline cartilage) and calcium deposition in the gastrocnemius tendon origin.

Clinical Differential Diagnosis

OA is the most common cause of chronic atraumatic knee pain in elderly patients. Other considerations for atraumatic knee pain in adults include meniscal and ligament tears, stress fracture (see case 9), transient osteoporosis, and chronic regional pain syndrome. MRI is the imaging modality of choice for evaluation of these conditions.

The presence of severe local inflammation, erythema, and progressive pain unrelated to usage should raise concern for infectious or inflammatory arthropathies.

REFERENCES AND SUGGESTED READING

PATIENT PRESENTATION

A 55-year-old woman with morning stiffness of the hands and swelling at metacarpophalangeal joints.

CLINICAL SUSPICION

Early rheumatoid arthritis

IMAGING MODALITY OF CHOICE

MRI or ultrasound. Radiographs are insensitive for the diagnosis of early rheumatoid arthritis (~70% of patients in the early stages have normal radiographs) but are used to exclude other causes of joint pain (eg, osteoarthritis and chronic calcium pyrophosphate crystal inflammatory arthritis) and to assess structural damage in established cases of rheumatoid arthritis.

MRI and ultrasound are both sensitive for the detection of synovial proliferation, which is one of the earliest detectable changes in rheumatoid arthritis. They can also assess disease activity and are useful in differentiating active disease from remission. Finally, MRI and ultrasound are used to evaluate suspected soft tissue complications of rheumatoid arthritis, such as ruptured tendons.

The choice between MRI and ultrasound depends on several factors. Ultrasound is relatively inexpensive, readily available, and allows for assessment of multiple joints, but can be time consuming and does not allow for visualization of bone edema or internal bone structure. It also depends on operator experience and radiologist expertise. MRI provides a more global view of a joint and is not operator-dependent, but is expensive, not globally available, requires more time to image a single joint, often requires the use of intravenous contrast, is susceptible to motion artifact, and has lower spatial resolution than ultrasound. The choice often depends on local expertise and the cost and availability of equipment. In most cases, however, MRI will not be cost-effective for monitoring treatment response.

BACKGROUND

Rheumatoid arthritis is an immune-mediated inflammatory disease that results in a symmetric polyarthritis that usually involves the small joints of the hands and feet. Rheumatoid arthritis is the most common of the inflammatory joint diseases, a group that also includes erosive, psoriatic, and reactive arthritis. Early treatment with disease-modifying antirheumatic drugs (DMARDs) such as methotrexate and newer biological agents improves clinical outcomes and reduces the progression of joint damage and disability. Early treatment requires early diagnosis, a task that is beyond the capability of radiography. Indeed, recent guidelines of the American College of Rheumatology and European League Against Rheumatism rely on physical examination findings and laboratory data (eg, autoantibodies such as rheumatoid factor and anti-citrullinated protein antibody) for diagnosis, and suggest the use of MRI or ultrasound if additional evidence of joint activity is needed for confirmation of the clinical findings.

FINDINGS

Recognition of synovitis is the key to early detection of rheumatoid arthritis by MRI and ultrasound (Fig. C3.1). On ultrasound, synovitis appears as abnormal tissue that is nondisplaceable and poorly compressible. It is usually hypoechoic relative to subcutaneous fat but can sometimes be isoechoic or hyperechoic. A joint effusion is frequently present, and can be differentiated from synovitis by being displaceable. Power Doppler ultrasound is more sensitive than color Doppler for detecting synovial inflammation and has been shown to correlate well with clinical disease activity. While it may be difficult to differentiate synovium from the adjacent cartilage on ultrasound, this can be readily achieved on MRI.

On MRI, synovitis appears as thickened synovium with greater than normal contrast enhancement. T2-weighted sequences can sometimes reveal synovitis as areas of signal that is less intense than joint fluid, but the distinction may be difficult.

Ultrasound and MRI are more sensitive than radiographs for detection of bony erosions. Although computed tomography (CT) is even more sensitive, radiation concerns make it less useful for serial monitoring of disease. On ultrasound, an erosion appears as an intra-articular discontinuity of the bone surface that is visible in two perpendicular planes. Ultrasound, however, is limited by its inability to visualize all parts of a joint (eg, the radial and ulnar aspects of all metacarpophalangeal joints) and can lead to incorrect characterization of normal anatomic variants (eg, normal depressions on the dorsal surfaces of the metacarpal heads) as erosions.

On MRI, an erosion appears as a sharply marginated bone lesion in a juxta-articular location that is visible in at least two planes and has a cortical break that is seen in at least one plane. On T1-weighted images, there is loss of the normal low signal intensity of cortical bone and loss of the normal high signal intensity of trabecular bone. The erosion generally contains inflammatory tissue or fluid, both of which are T2 hyperintense on T2-weighted images. In contrast to fluid, the inflammatory tissue will usually enhance.

In the early phase of disease, radiographs are either normal or reveal periarticular soft tissue swelling, a potentially subtle finding that is better appreciated on physical examination. Late radiographic findings of rheumatoid arthritis include periarticular osteopenia, marginal erosions, and joint deformity (Fig. C3.2). Joint deformities include ulnar deviation at the metacarpophalangeal joints; radial deviation of the radiocarpal articulation; swan-neck, boutonnière, and hitchhiker’s thumb deformities; and telescoping of the fingers.

DIFFERENTIAL DIAGNOSIS

Imaging Differential Diagnosis

Psoriatic arthritis, juvenile idiopathic arthritis, and chronic calcium pyrophosphate crystal inflammatory arthritis can produce similar imaging findings and present an imaging diagnostic challenge. Knowledge of the distribution of various arthritides in the hands (Fig. C3.3) can be helpful in some cases. Typical imaging findings in these conditions are shown in Fig. C3.4.

Clinical Differential Diagnosis

Osteoarthritis and other inflammatory arthritides such as psoriatic arthritis and chronic calcium pyrophosphate crystal inflammatory arthritis.

REFERENCES AND SUGGESTED READING

PATIENT PRESENTATION

A 50-year-old man with acute-onset pain and swelling of the right first metatarsophalangeal joint. He has had similar episodes in the past that have responded to over-the-counter analgesics.

CLINICAL SUSPICION

Gout

IMAGING MODALITY OF CHOICE

Radiography

BACKGROUND

Gout is the most common crystalline arthropathy. It occurs as a result of longstanding hyperuricemia, which leads to monosodium urate crystal deposition within joints and soft tissues. Precipitation of monosodium urate crystals within the joint space leads to an intense inflammatory reaction, which manifests clinically as gout. Acute gouty arthritis is characterized by excruciating pain and warmth about the joint. The symptoms may be accompanied by fever and leukocytosis and can persist for several days. Gout affects the lower extremities more frequently than the upper extremities and the smaller joints more frequently than the larger joints. The first metatarsophalangeal joint is the most commonly involved joint on first clinical presentation. The distribution is asymmetric, and clinical presentation is typically monoarticular.

FINDINGS

The most common early radiographic feature of gout is soft tissue swelling about the involved joint (Figs. C4.1 and C4.2), which occurs as a result of synovitis, joint capsule distention, and periarticular soft tissue edema. Intermediate-stage and late-stage radiographic features of gout develop 5 to 10 years after onset of clinical symptoms and include:

• Faint soft tissue calcifications about involved joints

• Soft tissue or intraosseous masses representing tophi

• Cortical erosions and irregularity

• Well-defined juxta-articular erosions with sclerotic rims and overhanging margins

• Preservation of bone mineralization and joint space

Secondary degenerative changes with loss of the joint space or even ankylosis may occur at more advanced stages of gout. However, this is rarely seen in the modern era.

Although radiographs are preferred in initial evaluation of gout, MRI (Fig. C4.3A,B) may be performed to detect smaller tophi and earlier-stage disease. Tophi characteristically have low to intermediate signal intensity on both T1- and T2-weighted MR images. Enhancement is typically encountered after administration of intravenous gadolinium due to proliferative synovitis, granulation tissue surrounding the tophus, and inflammatory reaction within adjacent soft tissues.

CT is increasingly used for visualization of tophi (Fig. C4.3C,D). The strength of CT in evaluation of gout lies in its capability to detect subtle bony erosions and to measure attenuation of tophi (150 to 200 Hounsfield units, which is lower than that of calcifications and higher than that of soft tissues). Developments in dual-energy CT currently allow quantification of monosodium urate deposits.

DIFFERENTIAL DIAGNOSIS

Imaging Differential Diagnosis

• Septic arthritis: Microbiological examination and culture of synovial fluid is necessary to exclude septic arthritis, which may coexist with gout.

• Acute calcium pyrophosphate crystal arthritis: Formerly known as pseudogout. Cartilage calcification or chondrocalcinosis is a more dominant feature. Synovial fluid analysis may reveal positively birefringent rhomboid crystals.

• Osteoarthritis: Early loss of joint space and lack of erosions differentiate this entity from gout. In gout, joint space loss tends to occur in later stages of the disease.

• Rheumatoid arthritis: Bilateral, symmetrical joint involvement and juxta-articular osteopenia differentiate this from gout, which tends to be asymmetric and without osteopenia.

Clinical Differential Diagnosis

The main clinical differential consideration is acute calcium pyrophosphate crystal arthritis, formerly known as pseudogout. This is an acute-onset, self-limiting synovitis associated with deposition of calcium pyrophosphate crystals. Patients typically present with severe, rapid-onset joint pain, tenderness, swelling, and overlying erythema. The symptoms peak within 6 to 24 hours.

REFERENCES AND SUGGESTED READING

PATIENT PRESENTATION

A 55-year-old man with diabetes mellitus presents with a swollen foot. There is a skin ulcer along the dorsolateral aspect of the fifth metatarsophalangeal joint.

CLINICAL SUSPICION

Osteomyelitis

IMAGING MODALITY OF CHOICE

Radiographs in conjunction with MRI with intravenous contrast (Fig. C5.1). Radiographs have low sensitivity and specificity, with an overall accuracy of about 50% for diagnosis of osteomye-litis, but should be obtained as an adjunct to MRI for their superior visualization of cortical bone. Contrast is useful for the evaluation of soft-tissue complications such as sinus tract formation, development of associated abscesses, and necrosis, and provides information for operative planning for limited limb resection. Noncontrast CT and three-phase bone scintigraphy can be obtained in cases in which clinical suspicion for infection is low or if MRI is contraindicated.

BACKGROUND

Foot infections are common in patients with diabetes mellitus and range from superficial infections and cellulitis to deep infections and osteomyelitis. Osteomyelitis (infection of the bone marrow) is the dreaded complication, and delayed diagnosis can increase the risk of amputation. Physical examination can be unreliable because the clinical findings of osteomyelitis may be masked or blunted in patients with diabetes mellitus. In addition, neuropathic (Charcot) arthropathy, which is present in up to 7.5% of patients with diabetes mellitus, can present with erythema, edema, and elevated temperature of the foot and mimic clinical findings of infection. Ulceration, neuropathy, and exposed bone increase the pretest probability for osteomyelitis.

FINDINGS

Radiography has low sensitivity for osteomyelitis, especially early in the course of the disease. After about 2 weeks, radiographs may reveal demineralization, periosteal reaction, and bony destruction, often in proximity to a pedal ulcer.

The primary MRI finding of osteomyelitis in the foot of a diabetic patient is low bone marrow signal on T1-weighted images at the end of a sinus tract. Secondary signs of osteomyelitis on MRI include skin callus, skin ulcer, sinus tract, cellulitis, abscess, foreign object, and periosteal reaction.

DIFFERENTIAL DIAGNOSIS

Imaging Differential Diagnosis

Differential considerations include cellulitis (Fig. C5.2A-D) and neuropathic arthropathy (Fig. C5.2E-H). Cellulitis will present on MRI as soft tissue edema and enhancement. There may be adjacent marrow signal abnormalities; however, marrow T2 hyperintensity without corresponding T1 hypointensity is more likely indicative of osteitis (inflammation of the bone marrow). Osteitis in this setting is a reaction to adjacent soft-tissue infection.

Neuropathic arthropathy can also demonstrate soft tissue edema and bone marrow signal abnormalities. The bone marrow changes in neuropathic joints are periarticular in location, whereas those seen in osteomyelitis are typically diffuse and found adjacent to the ulcer or a sinus tract.

Clinical Differential Diagnosis

Neuropathic joint, fracture, and septic arthritis are clinical considerations. In septic arthritis, MRI will reveal a complex joint effusion in the involved joint, intense and usually thick synovial enhancement, and T2 hyperintensity in the adjacent bone marrow without corresponding T1 hypointensity.

REFERENCES AND SUGGESTED READING

PATIENT PRESENTATION

A 35-year-old man presents to his primary care physician with increasing right knee pain and swelling over the past several weeks. He denies fevers or recent trauma.

CLINICAL SUSPICION

Inflammatory or neoplastic process in the joint

IMAGING MODALITY OF CHOICE

Knee radiographs are preferred in initial evaluation of knee pain. However, magnetic resonance imaging (MRI) is required to assess soft tissue structures.

BACKGROUND

Pigmented villonodular synovitis (PVNS) is a rare disorder characterized by synovial proliferation in the joint, tendon sheath, and bursa. It is believed to be a neoplastic disorder by most authors, whereas others suspect that it is caused by chronic inflammation. PVNS can be classified as localized or diffuse. Distinction between the two is important, since the former can be treated by simple excision with negligible risk of recurrence, whereas the latter requires extensive synovectomy and possibly chemotherapy because of its risk of recurrence.

Symptoms are nonspecific, but primarily consist of painful joint swelling and decreased motion, most commonly in the knee. The underlying effusion is typically hemorrhagic in appearance. Histologically, PVNS is characterized by diffuse hemosiderin staining related to recurrent hemorrhage of the friable villi of proliferative synovium.

FINDINGS

Radiographic findings (Fig. C6.1A) include a large joint effusion or intra-articular soft tissue mass and well-defined erosions with preservation of the joint space.

MRI findings (Fig. C6.1B-D) include a typically large joint effusion, heterogeneous synovial thickening, areas of “blooming artifact” on gradient-echo imaging due to hemosiderin deposition, prominent synovial enhancement, and subtle bony erosions with preservation of the joint space.

DIFFERENTIAL DIAGNOSIS

Imaging Differential Diagnosis

• Synovial hemangioma (Fig. C6.2A,B) can have similar MRI findings due to repetitive intra-articular hemorrhage and synovial hemosiderin deposition.

  Serpentine vessels seen with synovial hemangioma will not be seen with PVNS.

• Synovial chondromatosis (Fig. C6.2C,D): The small, round, ossified bodies of synovial osteochondromatosis are generally apparent on radiographs and allow for easy differentiation from PVNS, whereas the nonossified bodies of synovial chondromatosis can occasionally present as a soft tissue mass on radiographs and MRI.

• Gout (Fig. C6.2E,F) and amyloid may have an appearance similar to that of PVNS, with low-intensity nodular lesions on T1- and T2-weighted images. However, no blooming is present on gradient-echo images.

• Hemophilic arthropathy: The proliferative synovium and juxta-articular erosions encountered in hemophilic arthropathy are similar to those seen in PVNS. A history of hemophilia and the presence of epiphyseal and metaphyseal overgrowth on radiographs are helpful in differentiating the two.

Clinical Differential Diagnosis

Internal derangement, septic arthritis, and gout can have similar clinical presentations.

REFERENCES AND SUGGESTED READING

PATIENT PRESENTATION

A 30-year-old man with pain and deformity of the ring finger metacarpal after punching a wall (Fig. C7.1).

CLINICAL SUSPICION

Fracture

IMAGING MODALITY OF CHOICE

Radiographs. Noncontrast CT may be needed for operative planning, and contrast-enhanced CT arteriogram is indicated if there is concern for vascular injury.

FINDINGS

A systematic and standardized approach to fractures will ensure complete evaluation of the patient and proper communication of findings among healthcare professionals. Fractures are classified as open (formerly compound) if a break in the skin and underlying soft tissue communicates directly with the fracture and its hematoma. This is best assessed by direct examination of the patient; however, subcutaneous gas or an obvious soft tissue defect extending to the fracture is a suggestive finding on imaging.

The next step is to describe the location of the fracture (eg, proximal metaphysis of the femur, mid-diaphysis of the humerus). Fractures of the ends of bones must be carefully assessed for extension to the articular surface, since this can have important ramifications for treatment and prognosis. CT is used to fully define the intra-articular extension of fracture planes.

Fracture classification is the next step and involves assessment of the underlying bone, the extent of the fracture, and the plane (or planes) of fracture. A classification scheme is presented in Fig. C7.2.

Next we assess the relationship of the fracture fragments. By convention, we describe the relationship of the distal fragment to the proximal fragment (Fig. C7.3).

DIFFERENTIAL DIAGNOSIS

Imaging Differential Diagnosis

Additional fracture planes that upgrade a simple fracture to a comminuted fracture can sometimes be subtle or occult on radiographs. CT can be helpful in such cases. A careful search must be made to determine if there is an underlying bone lesion, since this will dramatically impact immediate and long-term patient management.

Clinical Differential Diagnosis

The clinical differential diagnosis for fractures varies according to site. Some fractures can mimic soft tissue injuries such as muscle strains or ligament tears.

REFERENCES AND SUGGESTED READING

PATIENT PRESENTATION

A 12-year-old runner with 2 months of pain in her right proximal tibia. She wants to know when she can return to running. Imaging workup is initiated (Fig. C8.1).

CLINICAL SUSPICION

Stress injury

IMAGING MODALITY OF CHOICE

Radiographs are obtained mainly to exclude other causes of pain. In competitive athletes with tight training schedules, MRI can provide prognostic information.

BACKGROUND

Shin splints and tibial fatigue fractures are in the spectrum of soft-tissue and bony abnormalities that develop in response to chronic repetitive stress and can be difficult to differentiate clinically (Fig. C8.2).

Shin splints (also known as medial tibial stress syndrome) refers to pain on the medial aspect of the tibia, often in the mid or distal diaphysis. It is thought to be caused by a combination of traction on the periosteum by the calf muscles and repetitive bending loads across the tibia that result in inflammatory changes of the periosteum followed by periosteal reaction and adaptive changes of the tibial cortex.

Stress fractures occur as the result of repeated loading of bone with forces less than that required for an acute traumatic fracture. Stress fractures due to abnormal activity (eg, excessive running) on normal bone are called fatigue fractures, and stress fractures due to normal activity on abnormal (eg, osteoporotic) bone are called insufficiency fractures.

Recreational runners presenting with symptoms suggestive of a tibial stress injury can be treated conservatively with rest and analgesics, with follow-up imaging if symptoms persist. However, when faced with a competitive runner with tibial pain, the question is often “when can my patient return to full-impact activity?” Fredericson et al. (1995) have proposed an MRI grading system for tibial stress reactions that can be used to plan how long an athlete should rest before returning to full-impact exercise (Table C8.1 and Fig. C8.2).

FINDINGS

Radiographs can be normal in the early stages. Three-phase bone scintigraphy (commonly, technetium Tc-99m methylene diphosphonate [Tc-99m MDP]) is more sensitive than radiography. Early-phase (arterial and blood pool) images are typically normal. Delayed phase images reveal uptake along the tibia that ranges from a small focus of mild cortical uptake to intense and extensive transcortical uptake. However, differentiation between medial tibial stress syndrome and stress fracture can be difficult with scintigraphy. MRI can depict the full spectrum of stress reactions (Table C8.1 and Fig. C8.2).

DIFFERENTIAL DIAGNOSIS

Imaging Differential Diagnosis

Although imaging findings in symptomatic runners are characteristic of various stress injuries, it should be noted that asymptomatic runners can have many of the same imaging findings. Therefore, images cannot provide a diagnosis of a tibial stress reaction without the benefit of clinical information, necessitating the inclusion of the dreaded phrase “clinical correlation recommended” in the report.

Rarely, tibial stress fractures can lead to a more extensive edema pattern and can mimic a neoplastic or infectious process.

Clinical Differential Diagnosis

In addition to medial tibial stress syndrome and fatigue fracture, the clinician must consider pathologic fracture by a neoplasm, exertional compartment syndrome, tendinitis, fascial defect, musculotendinous junction disruption, popliteal artery entrapment, effort-induced venous thrombosis, and nerve entrapment.

REFERENCES AND SUGGESTED READING

PATIENT PRESENTATION

A 65-year-old woman with sudden onset of severe, nontraumatic left knee pain. The patient is otherwise healthy.

CLINICAL SUSPICION

Insufficiency fracture, formerly (and incorrectly) known as spontaneous osteonecrosis of the knee (SONK).

IMAGING MODALITY OF CHOICE

Radiography to start. If radiographs are negative or show a joint effusion, MRI should be obtained.

BACKGROUND

Stress fractures are fractures that occur as the result of repeated loading of bone with forces less than that required for an acute traumatic fracture. Stress fractures due to abnormal activity (eg, excessive running) on normal bone are called fatigue fractures, and stress fractures due to normal activity on abnormal (eg, osteoporotic) bone are called insufficiency fractures. Whereas many stress fractures heal without ever being diagnosed, some may progress to complete fractures and subsequent degenerative changes.

FINDINGS

Initial radiographs may be normal. A careful assessment of the contours of the femoral condyles can reveal a subtle impression along the medial femoral condyle (involvement of the lateral femoral condyle and medial tibial plateau is uncommon, and the lateral tibial plateau is rarely affected). Delayed radiographs (Fig. C9.1A) reveal a subchondral lucency that can progress to flattening and collapse of the articular surface, and finally to degenerative changes.

On MRI (Fig. C9.1B,C), there is extensive bone marrow edema, usually located in the medial femoral condyle extending to the intercondylar notch, out of proportion to degenerative changes that may be present in patients in this age group. When located in the tibia, the bone marrow edema can extend to the tibial tuberosity. A hypointense subchondral crescent or linear focus, either of which indicates the presence of a subchondral fracture, is usually present and is best seen on T1-weighted images. Meniscal tears, particularly large radial meniscal root tears, are often present. Cartilage injuries are also commonly seen in these patients.

The presence of a subchondral fracture and/or articular flattening can suggest progression to collapse of the articular surface and subsequent degenerative changes.

DIFFERENTIAL DIAGNOSIS

Imaging Differential Diagnosis

Bone marrow signal abnormalities (Fig. C9.2) can be seen with osteoarthritis, osteochondral contusion and fracture, osteochondritis dissecans, and bone infarction.

Clinical Differential Diagnosis

Osteoarthritis, fracture, infection, neuropathy, infarction, and tumor (metastases in this age group).

REFERENCES AND SUGGESTED READING

PATIENT PRESENTATION

A 65-year-old woman with sudden-onset right hip pain after getting out of bed this morning.

CLINICAL SUSPICION

Insufficiency fracture of the pelvis

IMAGING MODALITY OF CHOICE

Radiographs followed by MRI or bone scintigraphy (“bone scan”) as needed for diagnosis or CT if needed for operative planning.

BACKGROUND

Stress fractures can be classified as fatigue (abnormal force on normal bone) or insufficiency (normal force on abnormal bone). In elderly persons, insufficiency fractures of the pelvis and proximal femur are usually due to osteoporosis, but other factors can lead to weakening of bone in this patient population, including radiation therapy, rheumatoid arthritis, prolonged corticosteroid therapy, vitamin D deficiency, and renal failure. Patients with pelvic and proximal femoral insufficiency fractures have prolonged hospital stays and increased morbidity and mortality.

FINDINGS

Radiographs may not depict the fracture plane (Fig. C10.1A), especially in the sacrum and ilium, where overlapping bony structures and bowel contents abound. Parasymphyseal and pubic ramus fractures are usually more readily apparent, and radiographic findings may include cortical disruption, sclerotic lines, and a lucent fracture plane.

Bone scintigraphy (Fig. C10.1B) is sensitive for detection of pelvic insufficiency fractures. The typical butterfly or H-shaped pattern of radiotracer uptake is formed by vertical fractures of the sacral ala joined by a horizontal component. Incomplete H patterns can also be seen, depending on the specific fractures.

CT findings can range from subtle cortical irregularity to irregular sclerosis to obvious fracture planes (Fig. C10.1C). MRI typically reveals bone marrow edema and a fracture plane (Fig. C10.1D,E), although each may be seen in isolation. Associated soft tissue abnormalities are also common, and consist of muscle edema and loss of normal fat planes. Compared with other pelvic and proximal femoral fractures, sacral insufficiency fractures are more likely to present with marrow edema without a clear fracture plane and are less likely to have associated soft tissue changes.

DIFFERENTIAL DIAGNOSIS

Imaging Differential Diagnosis

The fracture plane may not always be visible on MRI, and the bone marrow edema can be similar to that seen with neoplasm and osteomyelitis. CT can help differentiate these entities from fracture. In a fracture, the trabeculae on either side of the fracture plane are intact, and are not destroyed as in tumor or infection.

Clinical Differential Diagnosis

Avascular necrosis of the femoral head, insufficiency fracture elsewhere in the pelvis, and neoplasm with or without pathological fracture can have similar clinical presentations.

REFERENCES AND SUGGESTED READING

PATIENT PRESENTATION

A 25-year-old man with suspected splinter in hand.

CLINICAL SUSPICION

Soft tissue foreign object

IMAGING MODALITY OF CHOICE

In the extremities, ultrasound should be used as the initial imaging modality of choice for detection of foreign objects. High frequency (> 7.5 MHz) linear-array transducers are used. Radiography is limited to visualization of radiopaque foreign objects such as metal and glass.

FINDINGS

All soft tissue foreign objects are hyperechoic on ultrasound and will usually have posterior acoustic shadowing (Fig. C11.1A). A surrounding hypoechoic halo of granulation tissue, edema, or hemorrhage can assist in detection of smaller foreign objects. Ultrasound can also be used to identify complications such as abscess formation and vascular injury.

Plastic and wooden foreign objects can be subtle or invisible on radiographs. Secondary signs of laceration such as a skin defect, soft tissue swelling, and subcutaneous gas should be sought to narrow the area of investigation. Radiography can be used to detect metal and glass. The former is well demonstrated on radiographs (Fig. C11.1B,C). The radiopacity of glass is variable and depends on its density and effective atomic number (ie, its ability to attenuate x-ray beams) compared with those of the surrounding soft tissues (Fig. C11.1D). The lead content of glass is often incorrectly cited as the primary factor determining visibility on radiographs.

MRI does not reliably depict most foreign objects. Metallic objects (Fig. C11.1E) can be detected by noting the artifacts associated with their presence, namely loss of signal, geometric distortion, and failure of fat suppression.

Regardless of the imaging modality used, complications of soft tissue foreign objects should be actively investigated. These include vascular injury, infection, and bone injury.

DIFFERENTIAL DIAGNOSIS

Imaging Differential Diagnosis

In the absence of clinical information, avulsion fragments (Fig. C11.2A) and soft tissue calcifications (Fig. C11.2B) can mimic foreign objects.

Clinical Differential Diagnosis

None

REFERENCES AND SUGGESTED READING

PATIENT PRESENTATION

A 35-year-old man with wrist pain after a fall on an outstretched hand. On physical examination, there is tenderness over the anatomic snuffbox.

CLINICAL SUSPICION

Scaphoid fracture

IMAGING MODALITY OF CHOICE

Strart with 4-view wrist radiography. Standard frontal and lateral radiographs of the wrist are often inadequate for diagnosis of scaphoid fractures, and specialized views are used (Fig. C12.1). The choice of these views, however, varies according to local preferences.

The blood supply of the scaphoid (Fig. C12.2) leaves it susceptible to avascular necrosis and nonunion after fracture. The risk of avascular necrosis varies according to the location of the fracture. Fractures of the middle third of the scaphoid are associated with avascular necrosis in about one-third of cases, whereas fractures of the proximal fifth are associated with avascular necrosis in nearly all cases.

This concern results in a rather aggressive management approach: In patients with normal radiographs in whom a scaphoid fracture is suspected, the wrist is either further assessed with noncontrast MRI or immobilized with a cast, and radiography is repeated in 10 to 14 days. The choice depends on patient factors (age, hand dominance, activity level, insurance), as well as local preferences and availability of equipment.

FINDINGS