Nathan C. Nelson Department of Molecular Biomedical Sciences,College of Veterinary Medicine, North CarolinaState University, Raleigh, NC, USA Joint diseases are broadly categorized as degenerative, inflammatory, or neoplastic in origin. Degenerative joint disease, also termed osteoarthritis or osteoarthrosis, is often seen in older patients as an incidental finding that does not cause clinical signs. While degenerative joint disease is typically less debilitating than other joint diseases, it is an ongoing process and can be quite painful when advanced. In comparison, neoplastic or inflammatory joint diseases are much more debilitating and rapidly progressive but less common. Imaging features for these types of joint disease are discussed below. Radiographs are the best initial imaging test when assessing joint disease, due to their low cost and widespread availability. Radiography readily allows assessment of overall joint alignment. Tendons and ligaments are not identifiable on radiographs, as their opacity is the same as regional soft tissues, resulting in effacement of tendon/ligament margins. Joint malalignment implies that the ligaments or tendons that support joints are abnormal. Similarly, articular cartilage is not seen on radiographs, being the same opacity as joint fluid. Subchondral bone is apparent on radiographs, and the presence of subchondral defects implies cartilage damage is present. Periarticular bone proliferation (termed osteophytes) is the cardinal sign of degenerative joint disease and identified on radiography even when small. Ultrasound is most useful when evaluating periarticular structures such as tendons and ligaments, as their highly structured appearance is readily differentiated from surrounding soft tissues. Ultrasound has limited ability to image intraarticular structures as surrounding bones disrupt sound transmission into deeper soft tissues. Ultrasound may be used when an intraarticular or periarticular mass is suspected, to assess regional soft tissue involvement and guide tissue sampling. While magnetic resonance imaging (MRI) is the gold standard for imaging joint and tendon/ligament diseases in people, its use is limited for musculoskeletal imaging in dogs and cats. MRI has more limited availability and the high cost deters some clients. The small size of veterinary patients requires higher image resolution, resulting in longer scan times and more image noise. Coils (MRI equipment that collects signal from the anatomic area of interest) are placed near the joint of interest, but are not specifically designed for veterinary patients, which can cause problems when positioning a patient or attempting to acquire high‐resolution images. Computed tomgraphy (CT) is rarely used in the assessment of musculoskeletal disease in veterinary patients. It is most effective when providing further imaging of equivocal or subtle osseous pathology identified on radiographs (such as suspected fissure fractures or areas of aggressive bone lysis). Evaluation of cartilage and other intraarticular structures requires intraarticular injection of iodinated contrast medium before the scan is performed. Contrast within soft tissues (such as muscle, ligaments, and tendons) is lower than MRI, so CT is rarely used for further evaluation of musculoskeletal soft tissue pathology. Degenerative joint disease is a slowly progressive degenerative disorder of synovial joints, which results in cartilage loss, synovial proliferation, and periarticular bony remodeling [1]. It has many names, including osteoarthrosis, osteoarthritis, and arthritis. The etiology is complex, but degenerative change to joint cartilage is the key underlying component. Degeneration may be primary, as occurs with age, or due to underlying inherited cartilage defects (such as mucopolysaccharidosis). More commonly, degenerative joint disease is a secondary condition caused by conditions that alter joint stability, joint congruity, or acute injury, resulting in secondary damage to the cartilage and eventual cartilage degeneration [2]. Fractures that involve a joint, joint instability from trauma, hip dysplasia, viral infections, and immune‐mediated diseases are some of the many causes of cartilage damage that eventually results in secondary degenerative joint disease [1]. Degenerative joint disease is very common in dogs and cats, and often is undiagnosed or diagnosed on radiography when imaging the patient for another reason. Regardless of the underlying cause, joint diseases result in increased synovial fluid and synovial proliferation. Radiography cannot differentiate synovial fluid from synovial proliferation as both are similarly soft tissue opaque. Increased fluid/synovium causes soft tissue thickening which is concentrically centered on the affected joint. This appearance is termed intracapsular swelling. When mild, intracapsular swelling remains immediately adjacent to the joint margin on radiographs (Figure 8.1). As swelling progresses, the joint capsule distends and swelling may extend further along the margin of adjacent bones (Figure 8.2). Degenerative joint disease usually causes mild intracapsular swelling, whereas inflammatory and neoplastic joint diseases cause more severe swelling. FIGURE 8.1 Lateral radiograph of mild tarsocrural osteoarthrosis. There is mild intracapsular swelling (arrows) seen dorsal and plantar to the tarsocrural joint and periarticular osteophytes (arrowheads). FIGURE 8.2 Lateral radiograph of severe tarsocrural effusion. There is severe intracapsular swelling (arrows) seen dorsal and plantar to the tarsocrural joint. An osteophyte is seen dorsal to the proximal intertarsal joint (arrowhead). When structures outside the joint capsule (such as ligaments, subcutaneous structures, and muscles) are diseased, the soft tissue swelling tends to extend more proximally and distally on the limb, rather than being immediately at the level of the joint. This “extracapsular swelling” is usually eccentrically located, with the swelling being more severe in one area of the limb, centered on the most diseased tissue (Figure 8.3). Extracapsular swelling is rare in degenerative joint disease, and identification of significant extracapsular swelling should raise concern for some other soft tissue disease or concurrent injury to extracapsular structures. FIGURE 8.3 Lateral (A) and DP (B) radiographs showing an extracapsular mass in the area of the tarsus (arrowheads). Notice the soft tissue swelling is not centered on the joint and is eccentric to the lateral aspect of the distal crus. Osteophytes are bony proliferations along the periarticular margins of synovial joints (Figures 8.1 and 8.2). They are immediately adjacent to the joint margin, which distinguishes them from bony proliferation at the origin or insertion of tendons or ligaments (termed enthesophytes; see below). Osteophytes are the earliest radiographic evidence of degenerative joint disease, as direct visualization of articular cartilage is not possible on radiographs. An enthesophyte is a bony proliferation at the origin or insertion on bone of a tendon, ligament, or fascial plane. Enthesophytes may form as a consequence of degenerative joint disease, as the degenerative process of the joint may involve regional soft tissues. They are often a result of a previous traumatic episode to a soft tissue attachment (Figure 8.4), but tendons or ligaments undergoing primary degenerative processes may also form enthesophytes (Figure 8.5). FIGURE 8.4 Immediate postoperative radiograph (A) of a femoral fracture, with recheck radiographs (B) 2 months later. There is a large enthesophyte that extends from the caudal margin of the femur (arrow), that was not present initially. Bony proliferation in this area is common after trauma, due to strain or avulsion of muscular attachments. FIGURE 8.5 Lateral projection of the stifle showing moderate osteoarthrosis with an enthesophyte at the insertion of the cranial cruciate ligament (arrow). Increased fluid in the stifle joint is seen (arrowheads). There is partial or complete loss of the articular cartilage in cases of advanced degenerative joint disease. With weight bearing, cartilage loss allows narrowing of the joint space and if cartilage loss is complete, bone‐on‐bone contact occurs. In a nonweight‐bearing patient, loss of downward pressure on the joint allows it to open to its normal width. For this reason, joint space narrowing is rarely recognized on radiography of dogs and cats even in severe joint degeneration, as most musculoskeletal radiographs are acquired in a nonweight‐bearing position. More common is artifactual joint narrowing due to obliquity of the projection relative to the joint or flexion of a joint, allowing adjacent bones to summate (Figure 8.6). FIGURE 8.6 Craniocaudal projections of a stifle with flexion (A) and full extension (B). Notice when the stifle is flexed, there is overlap of the tibia and femur (arrow), resulting in artifactual joint narrowing. When appropriately extended, the joint space is normal and osteophytes are more apparent (arrowhead). Loss of cartilage during the degenerative process results in decreased cartilage cushioning and increased forces on the adjacent subchondral bone. As a result, the subchondral bone remodels to become more opaque (Figure 8.7). FIGURE 8.7 Lateral (A) and caudocranial (B) stifle radiographs showing severe stifle joint degeneration. There is subchondral sclerosis (arrow) and peripheral osteophytosis (arrowheads). Resorption of subchondral bone occurs with more advanced degenerative joint disease and results in an irregular appearance to the articular margin of the subchondral bone. This resorption is uncommon, and when present is often very mild. If severe subchondral resorption is present or if this resorption occurs along the nonarticular peripheral margin of a joint, suspicion should be elevated for either septic arthritis or synovial neoplasia (see below). Cyst‐like lesions may occur as part of the degenerative process in a joint. This is particularly common in the intercondylar region of the stifle in patients with chronic cranial cruciate ligament ruptures, but other joints may be affected (Figure 8.8). As opposed to erosive lesions that tend to be poorly defined with an irregular shape, cyst‐like lesions have well‐defined margins and a more rounded shape. FIGURE 8.8 Lateral (A) and caudocranial (B) radiographs of mild stifle joint degeneration. Osteophytes (arrows) are present. There are also rounded cyst‐like lesions within the intercondylar space (arrowheads). The ventrodorsal, extended leg radiograph is best suited for evaluation of coxofemoral degeneration. Lateral pelvic radiographs rarely contribute significantly to screening for coxofemoral degeneration, but are useful in diagnosing dorsal femoral head luxation due to severe dysplasia. In dogs with hip dysplasia (see Chapter 7), coxofemoral laxity eventually results in cartilage damage and degenerative joint disease. In younger dogs presenting with moderate to severe coxofemoral degenerative joint disease, the underlying cause of these changes is attributable to hip dysplasia. Subluxation of the joint is confirmatory of dysplasia, and is recognized by widening of the medial aspect of the joint relative to the lateral aspect of the joint (Figure 8.9). FIGURE 8.9 VD projection of the pelvis showing moderate subluxation of both femoral heads (arrows), osteophytes along the femoral neck resulting in thickening (arrowheads), and osteophytes at the area of joint capsule insertion. One of the earliest signs of coxofemoral degeneration is a row of osteophytes on the femoral neck, sometimes termed a Morgan’s line (Figures 8.9 and 8.10). This line may be subtle and seen in the absence of other degenerative changes, but is associated with progressive coxofemoral osteoarthritis [3]. FIGURE 8.10 VD projection of the pelvis showing mild coxofemoral osteoarthrosis, indicated by thin enthesophytes at the area of joint capsule insertion (arrowheads). There is also mineralization at the insertion of the gluteal musculature (arrows). Continued osteophyte formation causes progressive femoral neck thickening in the dysplastic patient (Figures 8.11 and 8.12). The acetabulum will become shallow due to osteophyte formation but also due to lack of femoral head contact, allowing acetabular remodeling to occur. The femoral head typically should have 50% or more coverage by the acetabulum, but is decreased in cases of dysplasia. FIGURE 8.11 Severe coxofemoral osteoarthrosis characterized by osteophyte formation on the acetabulum (arrows) and femoral necks (arrowheads). FIGURE 8.12 Severe coxofemoral osteoarthrosis characterized by osteophyte formation on the acetabulum (arrows) and femoral necks (arrowheads). The acetabulae are shallow with decreased coverage of the femoral heads. Elbow osteoarthritis in dogs is often caused by elbow dysplasia (see Chapter 7), particularly if degeneration is recognized in a young dog. The anconeal process is the earliest site of osteophyte formation, recognized by proliferation on the proximal margin of the anconeal process on the flexed lateral elbow projection. Early osteophyte formation also occurs on the medial coronoid process, best seen on the craniocaudal projection. Enthesopathy of the medial and lateral humeral epicondyles, at the origin of the flexor and extensor muscles of the antebrachium, is often seen concurrently with elbow degenerative joint disease. Most cases of stifle degenerative joint disease in dogs are attributable to degeneration of the cranial cruciate ligament. Enthesopathy of the cranial cruciate ligament insertion on the cranial intercondyloid region of the tibia is common (Figures 8.5 and 8.13). Osteophytosis is typically seen along the apex and base of the patella, proximal margin of the trochlear ridges, medial and lateral tibial condyles, and the fabella (Figure 8.14). FIGURE 8.13 Lateral (A) and caudocranial (B) readiographs of mild stifle osteoarthrosis with increased fluid in the stifle joint (arrows) and an enthesophyte at the insertion of the cranial cruciate ligament (closed arrowhead). There is artifactual narrowing of the medial stifle joint (open arrowhead). FIGURE 8.14 Lateral (A) and caudocranial (B) radiographs of severe stifle osteoarthritis due to cranial cruciate ligament rupture. There is severe osteophyte formation (arrowheads) along the margins of the stifle. The tibia is cranially subluxated relative to the femur due to the cranial cruciate rupture. Mild joint effusion is easily recognized in the stifle because of the infrapatellar fat pad. In a normal stifle, this infrapatellar fat pad should extend caudally to the cranial margin of the tibial plateau (Figure 8.15). With mild increases in joint fluid, small strands of fluid extend more cranially in the joint and as synovial effusion becomes worse, the infrapatellar fat pad appears compressed cranially. Even mild amounts of joint effusion should alert the clinician to underlying joint disease. FIGURE 8.15 Lateral (A) and caudocranial (B) projections of a normal stifle. The arrowhead indicates the normal soft tissue within the stifle. Definitive clinical diagnosis of cranial cruciate rupture is through the detection of cranial drawer or positive tibial thrust on physical examination, but cranial displacement of the tibia relative to the femoral condyles may be seen radiographically in dogs with complete cranial cruciate rupture (Figure 8.16). When the stifle is positioned so that the diaphysis of the femur is perpendicular to the diaphysis of the tibia, the femoral condyles should align directly with the intercondylar eminences of the tibia. If cranial cruciate rupture is present, the intercondylar eminences are positioned more cranially. Dogs with partial cruciate rupture will maintain a normal orientation of the femur and tibia. Extension of the stifle masks the subluxation (Figure 8.17). FIGURE 8.16 A normal stifle (A) compared to a stifle with cranial cruciate rupture and joint subluxation (B). In the normal stifle, the femoral condyle (black outline) is lined up with the tibial eminences (gray outline). In the patient with subluxation, the tibial eminences are cranially displaced relative to the femoral condyle. FIGURE 8.17 Radiographs of a patient with cranial cruciate ligament rupture, but the stifle is more extended than in Figure 8.16. The tibial eminences (gray outline) are cranially displaced relative to the femoral condyle (black outline), but the subluxation is less severe due to the extended stifle positioning. The tibial eminences are smaller than the patient in Figure 8.16. Degenerative joint disease is common in the metacarpo/metatarsophalangeal and interphalangeal joints of dogs, and often seen concurrently with regional collateral ligament enthesopathy (Figure 8.18). These degenerative changes may be radiographically severe, but rarely result in clinically significant lameness (Figure 8.19). Degenerative changes to the manus/pes are rare in cats. FIGURE 8.18 Dorsopalmar projection of the manus showing mild osteoarthrosis of the fourth metacarpophalangeal joint. Osteophytes (arrowhead) are seen at periarticular margins. Enthesophytes are seen at the area of collateral ligament insertion (arrow).
CHAPTER 8
Imaging of Joint and TendonDiseases
Introduction
Choice of Modality
Degenerative Joint Disease
Radiographic Findings of Degenerative Joint Disease
Swelling
Osteophytes
Enthesophytes
Joint Collapse
Subchondral Sclerosis
Subchondral Resorption, Cysts
Degenerative Joint Disease of Specific Joints
Coxofemoral Joint
Elbow
Stifle
Phalanges
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