Detection of Bone Erosions

KEY POINTS

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An ultrasound-detected erosion is a cortical break seen in two perpendicular planes.
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Ultrasound is a valid and reliable technique for detecting bone erosions.
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Ultrasound is more sensitive than radiography, but CT and MRI detect more erosions because they do not require an acoustic window.
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In rheumatoid arthritis, ultrasound has most impact in the detection and monitoring of bone erosion in early disease.

Bone erosions are often considered the pathologic hallmark of rheumatoid arthritis, although they are not specific for the disease. They represent a localized destructive process associated with loss of mineralized tissue and a break in the bone cortex. The cause and site of bone erosion is linked to the presence of synovitis and local biomechanical factors. At a cellular level, periarticular damage begins with resorption of mineralized cartilage, followed by more widespread loss of surface cartilage mediated by synovial fibroblasts. Radiographically detected erosions are an important diagnostic criterion for rheumatoid arthritis, and they offer predictive information about future structural damage and poor functional outcomes, as well as providing a means for monitoring disease progression. Radiographically identified erosions in early, undifferentiated arthritis has been shown to be a risk factor for developing persistent arthritis.

Detection of Bone Erosions

For more than a century, plain radiography has been the investigation of choice for detecting and monitoring bone damage. Advantages of radiography include its widespread availability, ease of reading, and ability to assess multiple sites. However, radiographs do not always show changes in the early stages of the disease, with an estimated 30% loss of bone required before damage can be visualized. This means that it has limitations in detecting early disease and changes over time. Other problems with radiography include its use of ionizing radiation (making it less suitable for repeated examinations) and an inability to image soft tissue inflammation. Standard posteroanterior radiographic and additional views, such as an oblique or flexed view of the metacarpophalangeal (MCP) joint in the hand, have demonstrated increased sensitivity for bone damage, but they may not be reproducible and may increase radiation exposure.

Alternative imaging techniques have been sought. Computed tomography (CT) is the obvious choice because it demonstrates bone damage quite well. However, it is limited by ionizing radiation and therefore is not recommended for repeated use. It also has limited value in assessing soft tissues. Its main role in rheumatoid arthritis has been to help validate ultrasound and magnetic resonance imaging (MRI) findings and in proof of concept studies. Ultrasound and MRI can visualize soft tissues and cartilage in addition to bone. MRI has the advantage of being a tomographic tool and being very sensitive. However, it is not readily available in many centers, is time consuming to perform, and is uncomfortable for patients. Extremity MRI may overcome some of these barriers, and it is superior to radiography for erosion detection, but only one joint area can be examined at a time.

Ultrasound Assessment

Ultrasound has several advantages in the assessment of inflammatory arthritis, especially polyarticular disease. It can be used immediately at the point of contact with a patient, it is patient-friendly, and it can be used to assess multiple joints (particularly the hands and feet) at one sitting.

The first use of ultrasound for detecting bone erosions in rheumatoid arthritis was reported by De Flaviis in 1988. In another 5 years, additional reports emerged from other workers, who began to realize the value of ultrasound in this respect.

Routine ultrasound can visualize alterations in the surface of the bone cortex, but not in the underlying bone. It cannot visualize bone marrow edema, which MRI studies suggest predates radiographic bone erosion. An ultrasound-detected erosion therefore is seen as a discontinuity of the cortical surface. To avoid artifacts, the defect must be visualized in at least two perpendicular planes ( Fig. 7-1 ). These points have been incorporated into the definition of an erosion provided by the Outcome Measures in Rheumatoid Arthritis Clinical Trials (OMERACT) group. Scanning in more than one plane can prevent overdiagnosis of erosions. For example, some individuals have a prominent anatomic neck seen on the dorsal aspect of the metacarpal that may appear to be an erosion ( Fig. 7-2 ); however, on the transverse section, no erosions are seen. In children, unfused epiphyseal plates may be confused with erosions, and developing bone may appear to be very irregular ( Fig. 7-3 ).

The position of the erosion helps to determine whether it is related to rheumatoid arthritis. Most erosions in rheumatoid arthritis are found in the periarticular regions of a joint. Because some bones are commonly pitted or irregular (e.g., dorsal aspect of the lunate bone in the wrist, anterior tibia), the threshold for calling a lesion an erosion should be higher. The size of the erosion is important; erosions smaller than 1 mm are less specific markers and more likely to occur in normal patients or those with degenerative disease.

Locations of Erosions

Ultrasound-detected erosions occur at the same sites seen on radiographs. In rheumatoid arthritis, erosions are most commonly seen in the wrist; around the first, second, third, and fifth MCP joints; around the second and third proximal interphalangeal (PIP) joints; and around the first and fifth metatarsophalangeal (MTP) joints, and these areas should serve as the focus for interrogation with ultrasound. In the context of osteoarthritis, erosion detection using ultrasound can be challenging because two adjacent osteophytes may produce a valley between them that may appear to be an erosion. This is sometimes referred to as a pseudo-obstruction. Osteophytes may also have small erosions associated with them. Caution should be used in interpreting findings from joints more susceptible to osteoarthritis changes, such as the wrist, PIP joints, and the first MTP joint.

Within joints themselves, erosions have predilections for certain sites. Erosions are most commonly found on the radial aspect of the second ( Fig. 7-4 ) and third MCP joints ( Fig. 7-5 ) and on the lateral aspect of the fifth MTP joint ( Fig. 7-6 ). In the PIP joints, erosions are most commonly seen on the medial and lateral aspects of the joints ( Fig. 7-7 ). Erosions are more likely to be seen on the dorsal aspect of the metacarpal bone in early disease, but later, they may be seen on the volar aspect and at the phalangeal bases. In the feet, erosions usually are seen on the dorsal aspect before the plantar aspect. In the great toe, erosions are usually seen on the medial aspect of the joint and in the fifth MTP joint on the lateral aspect of the joint. In the knee joint, erosions may be seen around the medial or lateral joint margins. In the elbow, erosions are usually seen around the capitellum ( Fig. 7-8 ) or radial head, whereas in the ankle, erosions are more common at the talonavicular joint than the tibiotalar joint.