Hypothesis 2 - Neoplasia

Due to the signalment and history of our patient neoplasia is certainly a rule-out for this hind limb lameness. Several different primary and metastatic tumors are known to affect the stifle joint. Primary bone tumors include osteosarcoma, hemangiosarcoma (primary or secondary), and synovial cell sarcoma. Secondary bone tumors could mestastasize from other locations, including mammary, lung, and thyroid carcinomas. 

Synovial sarcomas are uncommon in the dog, but when seen are most often seen in middle-aged medium to large breed dogs. The most commonly affected joints are the stifle and elbow, and arise from primitive mesenchymal precursor cells outside the synovial membrane. They grow slowly at first, and are first noticeable near a joint as a homogenous soft tissue mass. Radiographs would confirm the presence of a soft tissue mass, and parts may be calcified due to mineral deposition. As they are likely to invade adjacent bone, bone involvement appears as a spiculated periosteal formation, and ragged erosion of the cortical bone adjacent to the tumor. It has the ability to metastasize, but this is only seen in half the reported patients. 

Osteosarcoma is the most common primary bone tumor in canines. It most commonly affects dogs at 1.5 and 8-10 years of age, and is often found in large and giant breed dogs. It affects the forelimb most often, with the proximal humerus and distal radius presenting as the two most commonly affected sites. It has also been shown to affect the hindlimb. Initially, the dog will present with lameness due to pain followed by palpable swelling and visible enlargement of the affected area one to two weeks later. Limitation of motion of the affected joint will be noted. Clinical signs associated with osteosarcoma arise acutely, and the early metastatic spread is common. Therefore this tumor fits the clinical signs seen in our patient (acute onset). Radiographic signs will include focal soft tissue swelling and possible cortical and medullary bone lysis of the long bones associated with the joint. Joint involvement typically occurs later in the disease progress, and our patient may not have shown early clinical signs of bone neoplasia. 

Hemangiosarcoma is a tumor that most commonly involves the proximal and distal ends of long bones, and when seen is reported with increased incidence in large breed dogs. When found, pain and lameness are seen in conjunction with radiographic evidence of bony destruction.

Clinical signs associated with secondary bone tumors are similar to the aforementioned primary bone tumors, and include pain, swelling, lameness, and loss of function of the affected area. The majority of the secondary bone tumors are epithelial in origin and spread hematogenously to the cancellous bone of the proximal epiphysis and metaphysis. Lesions in these areas could certainly affect joint function and cause lameness.

As many of the primary and secondary bone tumors have metastatic potential, thoracic radiographs are mandatory to assess patient diagnosis and prognosis. Diagnosis is obtained through punch biopsies of bone lesions associated with the affected joint. Radiography is used as a guide to select the biopsy site.