Diagnostic Plan

1.) Perform lateral and craniocaudal radiographs of the left stifle - Due to the joint effusion we would expect to see the fat pad displaced away from the bone and a fluid density in the joint.  With acute ligament damage to the stifle we would not expect to any specific radiographic changes.  With chronic ligament tears, radiographic signs include osteophyte formation along the trochlear ridge, caudal tibia, and patella.  A thickened joint capsule and subchondral sclerosis may also be evident.  Neoplasia would present a soft tissue mass and periosteal proliferation with or without bone lysis in the epipysis and metaphysis of the bones associated with the joint.  Early signs of joint infection are nonspecific radiographically, but may include a widened joint space and adjacent soft tissue swelling.  Chronic cases of joint infection would show periosteal proliferation, collapsed joint space on stress radiographs, and irregular subchondral bone.  If osteomyelitis is present there would be loss of subchondral bone.

2.) Analyze fluid from joint tap - With partial cruciate tears one would expect to see increased nucleated white cell.  With complete tears, no inflammatory changes would be seen.  With infection of the joint, would see a high WBC (40,000-267,000/ul), protein (>4g/dl), yellow to bloody in color, glucose level half of serum level, turbidity, low viscosity, increased total volume, and possibly bacteria or fungi.  With neoplasia, the joint aspirate may show neoplastic cells with mitotic figures.

3.) Culture/susceptibility joint fluid - With infection you would expect to see the offending organism if present and possible antimicrobials that may be effective in treatment.  The culture would only be performed if the joint fluid analysis indicated an infectious process.

4.) Synovial Biopsy - With infection, this offers a better chance of culturing the organism.  This should only be performed if the joint fluid culture did not reveal the organism and an arthrotomy is performed.  For synovial sarcoma you would expect synovioblastic plump elongated angular or polygonal cell.  These also have a fibroblastic component with spindle shaped cells.  Other neoplasia would have varied appearances consistent with uncontrolled cell growth.  Biopsy should be performed if the radiographs/joint fluid indicate a neoplasm.

5.) Thoracic radiographs (lateral and VD/DV) should be performed if the biopsy indicates a malignant neoplasm to check for pulmonary metastases.

6.) History and Physical/Orthopedic Exam (including sedation) with a cranial drawer movement are diagnostic for cranial cruciate ligament tear/rupture and would not require any further diagnostic tests.

7.) CBC and chemistry - With bony involvement (neoplasia or osteomyelitis) you would expect to see an increased in AST values.  This should also be performed in preparation for surgery to determine a baseline for the patient.  A systemic infection would also be identified by the tests, if present.