Introduction

A limping puppy, active working or senior dog - a common presentation in veterinary practice. Sometimes the lameness resolves with rest and supportive therapy, but if it is severe or persistent, a radiographic examination is commonly performed. It is important to be familiar with good radiographic technique, normal anatomy and the most common bone and joint changes in our canine patients. Orthogonal radiographic projections are the baseline for any radiographic study, and oblique or stress views may be added depending on indication. In order to optimize straight patient positioning and reduce pain and stress, sedation is highly recommended for all orthopedic radiographic examinations.

The Basics of Bone Disease

Bones are slow to react to any type of injury, and are limited in the ways they can react. Bone can either be resorbed (lysis or bone atrophy), or they can lay down more bone which is visible radiographically as sclerosis, periosteal reaction, enthesophytosis or osteophytosis. Whenever given enough time and stability, bone is going to try to solidify, bridge and wall off a lesion by adding, smoothing and remodeling new bone. This results in what we radiographically call a non-aggressive lesion; a lesion with smoothly margined, well-mineralized periosteal reaction and sclerosis rather than lysis. If the lesion itself cannot be filled in with bone (for example a subchondral lesion exposed to joint fluid) then the bone will wall it off with a sclerotic rim. Contrary, if a disease process progresses too quickly the osseous reactions of consolidating and remodeling do not have time to fully occur and the result is an irregular periosteal reaction, moth eaten or permeative bone lysis with a poor delineation to normal bone - all characteristics of an aggressive lesion.

Presence of aggressive characteristics of a lesion does not always mean presence of malignant disease, but warrants a more aggressive diagnostic workup.

Diseases of the Growing Puppy

One of the most helpful radiographic studies in a growing puppy with lameness is imaging of the contralateral limb since maturation of growth plates and overall bone remodeling is very variable and often leaves us uncertain if a perceived abnormality is real or a normal finding for this particular puppy. If bilateral disease is present, there is still usually enough asymmetry to detect the changes as pathology.

Hypertrophic osteodystrophy (HOD), also called metaphyseal osteopathy (MO), is a disease of young puppies between 3 and 6 months of age. Typical radiographic findings include a slightly irregular radiolucent line in the metaphysis, running roughly parallel to the growth plate. The growth plate itself is normal. Soft tissue swelling is present regionally. In advanced cases there is amorphous mineralization of the soft tissues adjacent to the epiphysis and metaphysis, mimicking an aggressive periosteal reaction.

Trauma to the distal physes may not be recognized at the time of trauma as it may result in no or only transient lameness. The distal ulnar physis is particularly prone to injury, and premature closure of the growth plate can result. Premature closure of the distal ulnar physis leads to shortening of the ulna and resulting bowing of the radius and carpal valgus. Elbow joint incongruity, increased pressure on the anconeal process, and ununited anconeal process may also occur. Avulsion fractures are mostly seen in juvenile patients due to the softer bone structure and open growth plates. The apophysis of the tibial tuberosity is one of the most common sites of avulsion fracture. The normally very wide growth plate of the tibial apophysis is sometimes mistaken for an avulsion fracture.

Diseases of the Young Adult Dog

Panosteitis is a disease of large breed dogs, predominantly males, between 6 and 24 months of age. German Shepherds are predisposed. The typical findings include focal areas of medullary sclerosis, often along the endosteal surface and in close relationship with the nutrient foramen. Occasionally a subtle periosteal reaction can be seen as well in a later stage of disease.

Osteochondrosis and osteochondritis dissecans (OCD): Rapidly growing large breed dogs are mostly affected, and clinical signs usually develop between 6 and 12 months of age. The most commonly affected sites in the dog include the caudal aspect of the proximal humerus, the medial aspect of the humeral trochlea, the femoral condyles and the trochlear ridges of the talus. The lesions are usually bilateral. Radiographically, flattening of the joint surface or a concave defect with surrounding subchondral sclerosis is seen. The cartilage flap, if present, can only be seen once it is mineralized. Separated, mineralized osteochondral fragments (joint mice) may migrate within the joint capsule and adhere to the synovial lining.

Elbow dysplasia is an umbrella term for three different developmental lesions: osteochondrosis of the medial humeral trochlea, fragmented medial coronoid process and ununited anconeal process. An underlying osteochondritic process has been suspected for the latter two lesions, however, more recently asynchronous growth of radius and ulna as well as proximal ulnar dysplasia with an abnormally shaped ulnar notch have been considered likely causes.

Fragmented medial coronoid process is very difficult to diagnose radiographically. The flexed lateral radiograph allows assessment of the non-articular surface of the anconeal process, where early, small amounts of new bone formation are best seen. Other findings include abnormal shape or poor visibility of the coronoid process and subchondral sclerosis of the trochlear notch.

Ununited anconeal process on the other hand is readily diagnosed radiographically. Flexed lateral radiograph are most helpful. Large breed dogs are more commonly affected since they have a separate center of ossification. Normally this should be fused at 150 days of age. Radiolucent, irregular line separating the anconeal process and the olecranon in dogs older than 150 days is consistent with ununited anconeal process.

Hip dysplasia is another common joint disease that may become clinically apparent at a young age if the disease is severe. Joint laxity in hip dysplasia leads to joint subluxation with subsequent altered stress on the acetabulum and femoral head and development of osteoarthritis. Radiographically, joint subluxation is best seen in an extended hip view where a normal femoral head should be covered by the dorsolateral acetabular rim by at least 50%. Stress views (Penn-Hip or other) allow a more accurate measurement of joint laxity. The sequela of hip dysplasia is osteoarthritis which manifests itself by a radiopaque "Morgan's line", flattening of the femoral head with new bone formation along the femoral neck and remodeling, sclerosis and osteophytosis of the acetabulum.

Diseases of the Older Dog

Osteoarthritis is the number one cause of lameness in the older dog. Many of the above mentioned disease can lead to osteoarthritis eventually. Radiographic signs of osteoarthritis include subchondral sclerosis, osteophyte formation along the joint margins, joint effusion, and sometimes small subchondral cystic lesions. Joint narrowing may be seen in severe cases with complete cartilage erosion, but is difficult to determine radiographically as the dog is not weight-bearing. Cranial cruciate ligament rupture is the most common cause of osteoarthritis in the canine stifle joint.

Bone and joint tumors should always be considered in older lame animals, especially large and giant breed dogs. Primary bone tumors such as osteosarcoma most commonly are monostotic and located in the metaphysis of a long bone. Chondrosarcomas are mostly seen in flat bones such as the scapula and in the ribs. Metastatic bone tumors are often carcinomas; the most common primary tumors are associated with the urogenital tract. Since they are spread hematogenously, they tend to occur in the diaphysis of the long bones, closely associated with the nutrient foramen.

Soft tissue tumors invading bone, usually soft tissue sarcomas such as fibrosarcomas or liposarcomas are not limited to one bone; rather they will invade any bone in the vicinity. Synovial cell sarcoma and histiocytic sarcoma are the most common joint-associated tumors and lead to multifocal lysis of all bones surrounding a joint. Fungal osteomyelitis typically involves young adult large breed dogs in endemic areas. Similar to primary bone tumors the metaphyses are most commonly affected due to their abundant blood supply.

Bacterial osteomyelitis is often caused by direct trauma such as bite wounds, open fractures or surgical sites. Radiographically, osteomyelitis lesions are not typically metaphyseal in origin, and are less aggressive in appearance than bone tumors.

References

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