Hypothesis one: Degenerative bone disease

Musculoskeletal changes due to trauma are characterized by joint pain, 
tenderness, limitation 
of 
movement, crepitus, occasional effusion, and variables 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 therefore are unable to transmit load across the joint 
increasing to 
the problem.

Joint dysfunction occurs to loss of lubrication, smooth gliding function, 
decreased range 
of motion secondary to a particular change. Nearly all degenerative bone disease 
is 
secondary to some type of trauma, including fracture, cranial cruciate ligament 
rupture, 
meniscal 
tears, OCD and patellar luxations although the cause might not be apparent in 
all cases.

For example, if an animal were to fracture a bone during play or due to other 
physical injury, the homeostasis of the surrounding tissues would be disrupted. 
If the bone were allowed to heal on its own, it might not heal perfectly 
straight leading to abnormal stretching of surrounding structures. This would 
lead to abnormal pressure stress on the joint causing pain and eventually 
degenerative joint or bone disease.

Pain is the predominant sign.  Pain occurs due to stimulation of nociceptors in 
tissues.  
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.