Technique

Follow strict aseptic technique, and prepare a small sample site by clipping the coat and scrubbing the space medial or lateral to the patellar ligament with an antiseptic. Analgesia and sedation are indicated and facilitate patient restraint. Wear sterile gloves, and advance a sterile 20- or 22-ga 1- to 1-½-in needle and 3-ml syringe toward the center of the stifle from a point immediately medial or lateral to the ligament. Apply gentle suction to the syringe. Ideally, you should retrieve 1 ml or more of synovial fluid from the joint. Considerably larger volumes can be removed and may provide some temporary pain relief in patients with marked joint effusion.

Divide aliquots of aspirated joint fluid among an EDTA-tube, a red-top clot tube, and two slides (to make air-dried smears). Synovial fluid analysis performed by most commercial laboratories includes evaluating the fluid color, volume, and viscosity and performing a mucin clot test, cell counts, and cytology. If septic arthritis is suspected, an aliquot of joint fluid can be placed in blood culture medium as an enrichment broth for subsequent bacterial culture and sensitivity testing.

Normal joint

Normal joint fluid is clear and colorless to straw-colored with high viscosity and a good mucin clot test result. Obtaining sample volumes in excess of 1 ml may not be possible in some normal joints. The nucleated cell count should range from 0 to 2,900/mm³ with 88% to 100% monocytes and 0% to 12% neutrophils.¹⁷

Acute cranial cruciate ligament rupture

Acute cranial cruciate ligament rupture associated with trauma or high-energy athletic activity may cause hemarthrosis. The joint fluid is typically turbid and homogeneously bloody with decreased viscosity and a fair to poor mucin clot test result. The number of nucleated cells and the differential counts are highly variable. The presence of hemosiderin-laden macrophages and erythrophagocytosis suggests the blood is not from iatrogenic contamination.

Septic arthritis

Joint sepsis can contribute to a great deal of joint pathology, including cranial cruciate ligament rupture. The aspirated joint fluid is turbid and may appear grossly purulent but more often is red- or gray-tinged. The joint fluid viscosity is reduced, and the mucin clot test result is poor to very poor. Nucleated cell counts typically range from 110,000 to more than 267,000 cells/mm³ with 90% to 99% neutrophils and 1% to 10% monocytes. Bacteria may be seen intracellularly or extracellularly. Joint fluid culture is warranted whenever the nucleated cell count is elevated and is predominantly neutrophilic because bacteria are not seen microscopically in many cases of septic arthritis.

Immune-mediated joint disease

Immune-mediated joint disease may also contribute to enzymatic degradation of the cranial cruciate ligament. The joint fluid is typically turbid and may be yellow-tinged or bloody. The fluid viscosity is usually reduced, and the mucin clot test result may be good to poor. The nucleated cell count can range from 3,000 to more than 350,000 cells/mm³. The differential count may vary from predominantly neutrophils to predominantly monocytes.

Degenerative joint disease

Most commonly, findings of the the synovial fluid analysis will be typical of degenerative joint disease. Such fluid is clear and colorless to yellow-tinged. The viscosity is normal to decreased, and the mucin clot test result is good to fair. The nucleated cell count ranges from 0 to 3,500 cells/mm³ with 88% to 100% monocytes and 0% to 12% neutrophils. Cartilage cells and synoviocytes may also be noted.

FINAL DIAGNOSIS OR FURTHER TESTS

Diagnosing cranial cruciate ligament pathology is easy when a supportive history, signalment, gait evaluation, and radiographic appearance are combined with positive results on tibial compression or cranial drawer tests. However, some dogs with cranial cruciate ligament pathology do not have palpable stifle instability. In these dogs, a compatible history and lameness evaluation in conjunction with a palpable medial buttress, pain on full stifle extension, radiographic joint effusion, and synovial fluid indicative of degenerative joint disease support a diagnosis of cranial cruciate ligament pathology even in the absence of palpable stifle instability.

If you suspect a dog has cranial cruciate ligament pathology but the diagnostic findings do not warrant exploratory arthrotomy, consider magnetic resonance imaging (MRI) or arthroscopic evaluation vs. a conservative watchful-waiting approach. With a conservative approach, the dog is reevaluated in four to eight weeks or at the first sign of symptomatic progression, whichever is sooner.