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 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.
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