Introduction

Radiographs are the mainstay of the initial evaluation of the orthopaedic small animal patient. If needed, advanced imaging may be performed for a definitive diagnosis.¹

Principles of Radiographic Evaluation²

Usually, radiographs in at least 2 planes of the area of interest are needed. Additional views and comparison radiographs may be necessary for full evaluation. Soft tissues are assessed for swelling, atrophy, superficial defects, presence of foreign material, mineralization, or areas of decreased opacity indicating fat or gas inclusions. Evaluation of bony structures includes assessment of anatomic conformation and alignment, periosteal and endosteal surface, cortex and medulla for evidence of areas of increased opacity (e.g., new bone proliferation) or areas of decreased opacity (e.g., lysis). Evidence of generalized increased or decreased bone opacity should also be noted. In young animals, growth plates are evaluated for evidence of premature closure or signs of infection. Joint evaluation should include joint space width, joint surfaces and the insertion site of the joint capsule. Soft tissue swelling in the vicinity of a joint should be classified as extracapsular or intracapsular which is typically accomplished by visualization of displacement of local fat planes. Periarticular osteophytosis and joint effusion/capsular thickening is indicative of degenerative joint disease (DJD) which is a common sequela to many different joint diseases.

Bone and Joint Diseases in Young Animals^3,4

General: Osteochondrosis/osteochondrosis dissecans (OC/OCD) is a developmental anomaly which predominantly effects young large breed dogs. It can affect various joints and is recognized by focal flattening of the joint surface with associated subchondral sclerosis and with or without the presence of an isolated flap. Intracapsular soft tissue swelling and DJD may also be observed.

Diseases of the forelimb: The most common disorders of the scapulohumeral joint in dogs are OC/OCD and shoulder dysplasia which predominantly affects Dachshunds and manifests as flattening of the glenoid cavity, deformity of the humeral head, and often subluxation and DJD. Elbow dysplasia is common in large breed dogs and encompasses the disease complexes OC/OCD of the medial humeral condyle, fragmentation of the medial coronoid process (FCP)/medial coronoid process disease and ununited anconeal process (UAP). Radiographic findings with FCP include elbow joint incongruity, sclerosis of the ulnar trochlar notch, blunting of the medial coronoid process, and DJD. An isolated fragment of the medial coronoid process may be seen. Radiographic findings with UAP include separation of the anconeal process from parent bone. Additional findings include joint incongruity and DJD. Incomplete ossification of the humeral condyle predominantly affects Cocker Spaniels and certain hunting breeds. Radiographically, a lucent line may be visible separating the lateral from the medial aspect of the humeral condyle on a craniocaudal view. This condition often remains undiagnosed until patients present with acute distal humeral fracture after minor trauma. Angular limb deformity is a common developmental anomaly affecting the antebrachium especially in chondro dystrophic dogs due to premature physeal closure of the distal ulna. Radiographic findings include cranial curvature of the radius, shortening of the ulna, carpus valgus deformity and elbow joint incongruity with secondary DJD. Congenital elbow luxation is rare and may be difficult to distinguish radiographically from a traumatic elbow luxation sustained at young age. Valgus or varus deformity of the carpus is the most common carpal anomaly in young dogs. Incomplete ossification of the intermedioradial carpal bone is rarely of clinical significance although it predisposes this bone for pathologic fracture after minor trauma.

Diseases of the hindlimb: Hip dysplasia is a very common condition especially in large breed dogs. Radiographically, incongruity/subluxation of one or both coxofemoral joints are evident. Secondary DJD is very common. Legg-Calvé-Perthes disease affects small breed dogs. Radiographically, there is widening of the coxofemoral joint, and the femoral head has irregular margins and smaller size compared to normal.

A femoral head and neck osteoarthropathy resulting in spontaneous femoral capital physeal fractures and originally described in young overweight cats is occasionally encountered in dogs and may be difficult to distinguish from traumatic femoral capital physeal fractures. Diseases of the stifle joint in young dogs include OC/OCD of the lateral and less commonly the medial femoral condyle, patella luxation and a variation of Osgood -Schlatter-Syndrome seen in people. Patella luxation is most commonly seen in small breed dogs. Radiographically, the patella may or may not be luxated medially. Additional findings include varus deformity of the stifle joint and intracapsular soft tissue swelling in case of concurrent cranial cruciate ligamentous injury. In large breed dogs, lateral patella luxation is more common. Osgood-Schlatter-Syndrome in people is a developmental anomaly in which the tibial crest or tuberosity separates from parent bone. This is occasionally seen in dogs.

Congenital dysostosis and other hereditary, devel­opmental, nutritional and hormonal abnormalities of bony and cartilage development: A large number of diseases falls under these categories. Examples include polydactyly, syndactyly, amelia/hemimelia/phocomelia, ectrodactyly, mucopolysaccharidoses, pituitary dwarfism, congenital hypothyroidism, rickets, hyperparathyroidism, osteochondrodysplasias, multiple cartilaginous exostoses, and retained cartilage cores. These diseases are rare in veterinary patients.

Other bone diseases in young animals: Panosteitis is most common in large breed dogs. Radiographs may be normal or may show focal or multifocal areas of poorly defined increased opacity within the medullary cavities of the long bones. In advanced stages, periosteal reaction may also be present. Hypertrophic osteodystrophy is a disease of undetermined etiology which results in development of lucent zones within the metaphysis of long bones paralleling the physis. These lucent zones are of­ ten bordered by sclerosis. In advanced stages, extensive periosteal reaction and potentially growth abnormalities may be observed.

Acquired Bone and Joint Diseases^3,4

Bacterial osteomyelitis in young animals may be caused by haematogenous spread, while it is most commonly due to direct inoculation (e.g., penetrating wound) in older dogs. Radiographic findings include soft tissue swelling, irregular/ immature and often columnar periosteal proliferation and commonly lysis. Mycotic osteomyelitis (e.g., coccidiomycosis) develops secondary to a systemic infection and hematogenous spread and is often polyostotic. Radiographically, lesions manifest as aggressive, mixed lytic and proliferative, and can give rise to confusion with bone tumours. Joint involvement is possible with bot h bacterial and mycotic disease (septic arthritis/mycotic arthritis). Unlike with the infectious form, non-infectious/immune-mediated arthritides usually involve multiple joints. In the nonerosive form intracapsular soft tissue swelling is the only radiographic abnormality. In the erosive form (e.g., rheumatoid arthritis) intracapsular soft tissue swelling is followed by subchondral lysis progressing to extensive erosion with destruction of joint surfaces, collapse of joint spaces, and, in severe cases, (sub)luxation of the affected joint. Neoplasia of the skeletal system is common in dogs. A primary bone tumor (e.g., osteosarcoma) results in an aggressive focal lesion, commonly found in the metaphysis of the long bones (proximal humerus, distal radius, distal femur and proximal tibia).

Metastases to bone may be seen in the appendicular or axial skeleton, tend to occur in the vicinity of nutrient foramina, may be small and easily overlooked. Other tumours affecting bone include round cell tumours (e.g., multiple myeloma) and soft tissue neoplasms extending into bone. Radiographic findings in these conditions are variable. Joint associated tumours may also be encountered with histiocytic sarcoma, myxomas and synovial cell sarcomas being most common. Radiographically, severe intracapsular soft tissue swelling and multifocal lytic changes of adjacent bony structures are noted.

Hypertrophic osteopathy results in extensive columnar periosteal proliferation of the long bones extending from the distal limbs proximally. This condition develops secondary to intrathoracic or less commonly intra-abdominal mass lesions. Trauma to the musculoskeletal system is a frequent cause of lameness in veterinary patients, and traumatic lesions (e.g., fractures) are usually a fairly straightforward diagnosis. Chronic soft tissue lesions can result in radiographic changes especially over time.

One example is cranial cruciate ligament rupture which is common in large breed dogs and is typically recognized based on intracapsular soft tissue swelling of the stifle joint with displacement of the infrapatellar fat-pad and periarticular osteophytosis. Unlike in people, cranial cruciate ligamentous injury in dogs is often partial and may or may not result in cranial subluxation of the tibia. Similarly, chronic tendinopathy of the bicipital tendon or flexor tendons of the distal forelimb can result in radiographic soft tissue mineralization and degenerative changes to the proximal and distal humerus, respectively.

References

1.  Barr FJ, Kirberger RM. BSAVA Manual of Canine and Feline Musculoskeletal Imaging. Gloucester: BSAVA; 2006.

2.  Thrall DE et al. Section III: the appendicular skeleton: canine, feline, and equine. In: Thrall DE, ed. Textbook of Veterinary Diagnostic Radiology. 6th ed. St. Louis, MO: Elsevier Saunders; 2013.

3.  Johnson KA, Watson ADJ. Skeletal diseases. In: Ettinger SJ, Feldman EC, eds. Textbook of Veterinary Internal Medicine. 5th ed. Philadelphia, PA: WB Saunders; 2000.

4.  Pedersen NC, Morgan JP, Vasseur PB. Joint diseases of dogs and cats. In: Ettinger SJ, Feldman EC, eds. Textbook of Veterinary Internal Medicine. 5th ed. Philadelphia, PA: WB Saunders; 2000.