Hypothesis 1 - Trauma

The falling off of the bed incident caused traumatic injury to the stifle joint, possibly patellar ligament damage.  This lead to excessive movement of the patella and wear on the joint, ultimately leading to development of osteoarthritis and pain.  The patella could have also been fractured, leading to pain. The femur could also have been fractured and healed incorrectly, causing miss alignment and patellar luxation. The left leg pain during the examination could be contributed to injury during the initial accident or damage to compensation of the right leg pain.

The accident could have also injured the spine leading to joint damage and osteoarthritis. The dog could have compensated for pain in the spine by altering his gait, which could have aggravated and further damaged the stifle joint in the right leg and possible the left leg.