Colin B. Carrig, BVSc, PHD; Jacob Rohleder, DVM; Otto I. Lanz, DVM
The number of imaging technologies available to the veterinarian in practice and in referral centers for evaluating lameness has increased in recent years. While routine radiography is not the ideal diagnostic technique to detect musculoskeletal disease, it remains the most available and used diagnostic imaging technique for the evaluation of most bone lesions that cause lameness in small animals. However, conventional radiography does not provide complete diagnostic information, and additional imaging procedures can focus the range of diagnostic possibilities.
Because clinical signs of lameness often precede the appearance of radiographic change, accurately localizing the site of lameness before the radiograph is taken and reviewed will increase the likelihood of detecting subtle lesions.
Evaluation of radiographs of the musculoskeletal system requires systematic evaluation of soft tissues, bones and joints. An assessment of size, shape, location, opacity and, where possible, function is made. High quality radiographs are necessary to define subtle, early changes in both soft tissue and bone. Serial radiographs provide additional information on the dynamic nature of a lesion, and contribute to a correct diagnosis.
The type and pattern of soft tissue abnormality accompanying bone changes will provide important insight into the cause of lameness. Soft tissue changes often precede bone changes and frequently will direct attention to important, early changes in the bone. Increased size of soft tissue structures can be related to fat accumulations (e.g., lipoma), or fluid (e.g., cellulitis, hemorrhage, edema, neoplasm, joint effusion). Decreased size of soft tissue structures can be related to atrophy (disuse, neurogenic).
Perceived skeletal abnormalities can be variations of normal anatomy, so deciding that a change is truly abnormal is necessary before further analysis of a lesion. Comparison radiographs of the opposite limb, especially in the immature patient, can also help determine the significance of an observation.
Changes that occur in bone are a direct reflection of the aggressiveness, virulence, and duration of the disease process, and can also indicate healing and reparative responses of the bone. They are also influenced by external factors such as medical or surgical intervention, or the pre-existence or superimposition of other pathologic processes.
When evaluating bony change, it is important to remember that a considerable amount of bone loss is necessary (possibly up to 50%) before it can be identified on a radiograph, so subtle change can be clinically important and must be evaluated critically. Bone can be affected by disease either systemically or focally. Systemic bone lesions can be characterized by a generalized increase in bone opacity (e.g., osteopetrosis), or a generalized decrease in bone opacity (e.g., hyperparathyroidism).
Focal bone lesions are characterised radiographically by the presence of bone lysis, bone production, or a combination of both of these processes. The pattern by which these processes occur form the basis of classification of bone lesions. When evaluating a bone lesion it is important to decide initially if it is aggressive or nonaggressive. This determination will indicate the probable nature of the disease process, e.g., active or inactive, malignant or benign, neoplastic or infectious, acute or chronic. Overlapping patterns of bone lysis and proliferation are often seen, and this confuses classification of some bone lesions.
Characteristics of a bone lesion that indicate that it is inactive and benign include: 1) minimal clinical signs, unless impinging on adjacent soft tissue structure or involving joint, 2) minor soft tissue swelling, 3) well demarcated lesion with well-defined margins, 4) smooth periosteal surfaces, 5) dense new bone formation, often containing bone trabeculae, 6) abrupt zone of transition between normal and abnormal bone, 7) focal bone lysis or medullary canal expansion with or without cortical thinning, 8) dense zone of endosteal sclerotic bone, 9) intact cortex, sometimes thickened because of consolidation of periosteal new bone, and 10) slow rate of change on follow-up radiographs.
Characteristics of a bone lesion that indicate that it is active and aggressive include: 1) prominent soft tissue swelling--focal or diffuse, 2) moth eaten or permeative bone lysis with poorly defined margins, 3) bone production with ill-defined margins, 4) pathologic fracture, 5) poorly demarcated lesion, 6) gradual zone of transition between normal and abnormal bone, 7) no endosteal sclerotic border, and 8) rapid rate of change on follow-up radiographs.
Based on the identification of these changes, a lesion is classified as being either aggressive (e.g., malignant bone tumor, acute osteomyelitis) or non-aggressive (e.g., benign bone tumor, bone cyst, chronic osteomyelitis, traumatic periostitis).
In small animals, the differentiation of malignant bone tumors and acute osteomyelitis can be difficult. Both can produce a wide spectrum of destructive and productive bony changes. Many of their radiographic features can be common to both disease processes, especially in the early stages. This means that a diagnosis based solely on the radiographic appearance will often be erroneous. Consideration of all supporting clinical data (history, signalment e.g., gender, size and breed, clinical signs and physical examination, site of lesion, local or systemic, monostotic or polyostotic, geographic location,), and additional laboratory data (biopsy and histological examinations, serology where fungal disease is suspected and thoracic radiographs), and information on previous treatments, all influence the ranking of possible diagnoses following interpretation of the radiograph.
Examples of clinical associations that illustrate this approach include: age--developmental diseases usually seen in immature patients; size--primary bone tumors in giant breed dogs; avascular necrosis of the femoral head in toy breed dogs; breed--panosteitis most common in German shepherd and Bassett hound breeds; site of lesion--hypertrophic osteodystrophy affects the metaphysis; systemic disease--fungal osteomyelitis can affect multiple bones, lymph nodes and lungs; monostotic or polyostotic--primary bone tumors usually monostotic; metastatic neoplasms usually polyostotic.
Once a list of rule-outs is established, specific clinical and radiographic changes can be identified to support a diagnosis, e.g., clinical signs and radiographic findings that support a diagnosis of primary bone tumor include: giant breed or large breed dogs, average age 7 years, but many occur at 1-2 years, solitary, monostotic lesion, located in metaphyseal region of long bones (proximal humerus, distal radius and ulna, distal femur, proximal/distal tibia, proximal ulna), soft tissue mass, aggressive bony changes with periosteal new bone formation, usually no involvement of adjacent joint, pathologic fracture, may be associated with metallic implants or complicated fracture healing.
Other imaging modalities that can be used to supplement findings on survey radiographs include arthrography, ultrasound, computed tomography and magnetic resonance and nuclear scintigraphy.
Arthrography, the injection of aqueous, iodinated contrast material into the joint space, will allow evaluation of structures normally not seen on the survey radiograph, including articular cartilage and joint capsule. Abnormalities detected by this technique include cartilage fissuring and fragmentation, joint mice, synovitis, synovial proliferation and joint capsule tears.
Routine ultrasound evaluation of bone lesions is limited because of the inability of the sound beam to penetrate bony tissues. Information on the extra-osseous component of a lesion, however, can be obtained. When defects in the cortex of a bone occurs, e.g., with primary bone tumor, evaluation of the intra-osseous structures becomes possible. The selection of sites for, and guidance of, percutaneous needle biopsies is a useful application of ultrasound. When successful, it provides a method for the rapid confirmation of the nature of a bony lesion. Ultrasound is valuable for assessing joint disease, joint effusion, joint capsule thickening, cartilage defects and tendons. Abscesses, foreign bodies, hematomas and soft tissue tumors can also be imaged.
Computed tomography enables definition of soft tissue and bone changes that are not detected by conventional radiography because it provides excellent contrast resolution, cross sectional display, and the ability to measure specific attenuation values. Subtle new bone formations and bone lysis are better identified on CT images when compared to conventional radiography because of better physical density discrimination, the ability to manipulate the gray scale of the digital image, and the elimination of superimposed structures.
Magnetic resonance imaging permits direct visualization of all bone, soft tissue and joint components simultaneously. It is characterized by great inherent contrast, excellent special resolution and exquisite anatomic display. Limb-sparing techniques for treating primary bone tumors have been developed as an alternative to amputation in animals with pre-existing orthopedic or neurological disease. These involve local tumor removal with wide margins and replacement of the resection with a fresh-frozen allograft. To minimize local recurrence, precise determination of the length of bone involvement by the tumor is necessary. Comparisons have been made between radiography, computed tomography, MRI, and scintigraphy to determine their respective efficacy in accurately defining tumor length. MRI is the method of choice in both animal and human patients, however, conventional radiographs also provide close estimations. Scintigraphy and computed tomography generally overestimate the length of bone affected by a tumor.