Hypothesis one: Trauma


When the dog fell off the bed, she caused trauma to the supporting 
ligaments and tendons of the stifle.  This can lead to inflammation 
and pain in the stifle joint as well as laxity.  This explains the 
patellar luxation and joint dysfunction.  This occurs because 
increased wear and tear overtime and loss of lubrication and smooth 
gliding function. This would explain the medial buttressing on the 
right stifle as the soft tissues try to compensate for the trauma. 
Ultimately this might lead to degenerative bone disease. In light of 
this, the dog would have to compensate for the pain in the right leg 
by overusing the left leg. This could cause strain and pain on the 
left stifle. This pain could be evident by the positive pain 
response on cranial drawer and patellar luxation. However, small 
dogs are not always cooperative to orthopedic exams and her pain 
response could in fact have just been protest.

In addition, by jumping off the bed the dog might have hurt her back 
causing inflammation and pain in the vertebrae.  This inflammtion 
can lead to ostearthritis. This would explain pain on dorsal tail 
flexion. However, again this is a small dog and she may have just 
been protesting.
 
Degenerative bone disease is secondary to some type of trauma which 
may include cranial cruciate ligament rupture, meniscal 
tears, OCD and patellar luxations although the cause might not be 
apparent in all cases. Degenerative bone disease is characterized by 
joint pain, tenderness, limitation of movement, crepitus, occasional 
effusion, and variable degrees of local inflammation, but 
without systemic effects. Pathologically we see irregular 
distributed loss of cartilage, sclerosis of subchondral bone, 
subchondral cysts, osteophytes, increased metaphysal 
blood flow and synovial inflammation.  

Cartilage damage results in release of cytokines that induce 
prostaglandin E and metalloproteinases.  Ultimately we see cartilage 
matrix depletion and initiation of cartilage degradation.  These 
changes are considered inflammatory, although degenerative bone 
disease is not considered to be an inflammatory disease. In 
addition, the joint capsule undergoes hypertrophy and hyperplasia in 
order to compensate and help aid joint stability.  This might lead 
to a decreased range of motion.  Muscles may become weak with 
disuse, and therefor are unable to transmit load across the joint 
increasing to the problem.

Pain is the predominant sign in osteoarthritis.  Pain occurs due to 
stimulation of nociceptors in tissues.  This stimulation can be 
mechanical or chemical.

Some clinical signs include: Altered gait, joint swelling, fibrosis, 
effusion, crepitus, friction created by thickened synovium and joint 
capsule and it moves over cartilage and osteophytes, decreased range 
of motion due to joint capsule fibrosis or pain associated with 
motion, muscle atrophy and pain.