Hypothesis 2b
Degenerative changes leading to stifle joint pain

Right hind limb lameness could be due to degenerative changes in the stifle joint.  A previous trauma and increased weight could have caused damage to the joint leading to degenerative changes.  The trauma of falling off the bed leads to abnormal forces on cartilage.  This results in damage to cartilage matrix and chondrocytes.   Proteoglycans from the damaged matrix are released into the joint space causing swelling of the cartilage leading to a loss of tensile strength and further damage and fissures and clefts on articular surface.  There will be an inablilty to distribute normal forces on the joint.   Abnormal stress is put on the subchondral bone causing it to thicken this puts extra stress on the remaining cartilage eventually the bone remodels and osteophytes are formed.  These changes become self perpetuating.  Pain is due to stimulation of nocieptors in fibrous joint capsule and in subchondral bone by inflammatory mediators which causes a dull aching pain.  Or pain can be due to mechanical stimulation which leads to acute pain. 

Radiographic changes expected:
With a standard medio-lateral and a caudo-cranial view of stifle joint:

Increased joint fluid seen on lateral view (displacement of fat pad away from joint)

Periarticular osteophytes on medial and lateral femoral trochlear ridges and distal border of the patella.

Subchondral bone sclerosis and marrowing of joint space, flattening of the articular surfaces

Enthesiopytes on cranial intercondylar area of tibia near the site of attatchement of cranial cruciate ligament.