Hypothesis III: Infectious

Septic Arthritis
Septic Arthritis is associated with the hematogenous distribution of microorganisms, 
commonly associated with omphalophlebitis (inflammation of the umbilical veins) or by 
direct contamination because of trauma or non-sterile technique.  Joint infection is caused 
by the inoculation of the synovial membrane or synovial fluid with bacteria.  This incites 
and inflammatory reaction that provides an environment for established infection 
(organisms binding to tissues).  The anatomy of the synovial capillary tufts favor the 
entrapment and colonization of bacteria in the synovial lining.  The resulting 
inflammatory cascade results in the loss of proteoglycan and collagen from the articular 
cartlidge.  The loss of cartliage with mechanical trauma leads to rapid, irreversible, and 
progressive cartilage destruction (osteoarthritis).  Periartiular fibrosis often results that 
may limit the range of motion and compromise synovial membrane function and cartlidge 
nutrition.

Possible Etiologies
Hematogenous septic arthritis is more common in neonates, where portals of entry can be 
associated with the umbilicus (oophalophlebitis), respiratory tract, GI tract, or 
intrauterine.  Usually multiple joints are affected, and failure of passive transfer is an 
important predisposing factor.  Common isolates from cytology include Actinobacillus 
sp., E. coli, Salmonella sp., and Streptococcus sp.
Septic arthritis due to trauma is typically found in older animals.  Minimal soft tissue 
swelling over the joint effected will be noted.  Common isolates from cytology would 
include Pseudomonas, Proteus, and Staphylococcus.

Radiographic Signs
Radiographic signs of early septic arthritis are periarticular soft tissue swelling and 
distention of the joint.  Bony changes include subchrondral bone lysis and periosteal 
proliferation at the joint margins.  Cartlidge space may appear increased at areas of 
subchondral bone lysis.