Hypothesis 2 - Infection of the stifle joint

The patient is a large active dog that spends a large amount of time outdoors and in the woods.  This would increases the chances of Bonnie suffering a penetrating trauma to the stifle joint resulting in direct bacterial/fungal inoculation of the joint.  It is less likely that the infection would have resulted from hematogenous spread because only one joint is affected and there are no systemic signs of sepsis (ie. Fever).    Although no wound was found, it is possible that it was a small puncture wound and was missed by the owner.   Clinical signs present in this patient that support septic arthritis are firstly that the patient is severely lame on one leg(3/4).   Second, the stifle is swollen, painful, and warm.  And finally, the joint has a reduced range of motion.  

The bacterial/fungal contamination of the joint would lead to inflammation of the joint promoting extravasation of fibrin, leukocytes, clotting factors, protein, and fluid due to the increased leakage of the vessels.  This would cause the joint distention seen in Bonnie's left stifle.  Fibrin deposits on the articular cartilage inhibiting synovial fluid penetration.  The leukocytes phagocytize the bacteria and release enzymes and free radicals, which cause cartilage destruction.  These events lead to a loss of normal synovial fluid integrity, which decreases cartilage nutrition and leads to increased pressure on the cartilage accelerating the degradation of the cartilage.   This may allow the infection to invade subchondral bone causing an osteomyelitis which would cause stimulation of the nociceptors in the subchondral bone contributing to the pain associated with the condition.   The inflammatory mediators released by the damaged chondrocytes, neutrophils, and macrophages would increase stimulation of the nociceptors in the joint capsule and pain.  The distention of the joint capsule (mechanical) from the influx of fluid and inflammatory cells would also significantly contribute to the pain seen in the patient.

	The cranial drawer sign present in the patient is not normally associated with infectious arthritis; however the bacteria could have affected the integrity of the ligaments of the stifle joint.   This could lead to laxity or disruption of the cruciate ligaments and show the cranial drawer sign.  There was no heat noted in the physical exam, which would normally be expected with this condition.