Findings

Two ventral dorsal views of the pelvis, a lateral view centered on the left stifle joint, a lateral view centered on the left tarsus, metatarsals, and phalanges, and a dorsal plantar view centered on the left tarsus, metatarsals, and phalanges are available for interpretation.
There is subjective decreased muscle thickness within the left pelvic limb when compared to the right consistent with the history of disuse. This is very subtle change. The pelvis and coxofemoral joints are within normal limits.
There is equivocal mild medial displacement of the left patella. Both medial fabella are bipartite. There are osteophytes on the distal margin of the left patella, the left lateral tibial condyle and lateral epicondyles with left femur. There is also slight increased soft tissue opacity within the left stifle joint space. There is subtle sclerosis noted within the distal diaphysis of the left femur.
There is increased thickness of the soft tissues overlying the calcaneal tuberosity. However the common calcaneal tendon is readily visible and appears to be normal in diameter. No abnormalities are noted within the tarsus.
No definitive abnormalities are noted within the metatarsus or phalanges. There is a small skin tag on the medial aspect of the left tarsus that is of doubtful clinical significance.

Conclusions

1. Mild degenerative joint disease of the left stifle joint. There is slight increased soft tissue opacity consistent with very mild effusion.
The left patella is suspected to be slightly medially displaced on the ventral dorsal views of the pelvis as it is summating with the medial trochlear ridge of the femur. This is consistent with the clinical findings of medial patellar luxation but is considered unlikely to be the cause of an acute lameness.

2. Faint sclerosis within the distal diaphysis/metaphysis of the left femur. This is also of questionable clinical significance as there is no evidence of lysis or cortical alteration associated with this. A cluster of growth arrest lines could be considered. Occasionally, this can be seen with trauma or bone infarcts. Osteomyelitis or bone metastasis would be given much lesser consideration given the history.

Recommendations

Unfortunately, the exact cause of the acute lameness is not clear. Acute exacerbation of the mild degenerative joint disease within the left stifle joint is possible. A soft tissue injury could also still be considered. Rest and empirical medical therapy is likely indicated at this time.
Recheck radiographs of the left stifle joint in 4-6 weeks to assess for change in the focal area of sclerosis within the distal diaphysis/metaphysis of the left femur could be considered to determine its clinical significance as an active process would be expected to change over time. A nuclear bone scan could also be considered.