I saw a case this week of an 11 year old 35 kg dog that had a 3 month duration of a right forelimb lameness, which seemed to begin after a fall down some stairs. The veterinarian originally believed that the dog had an inflammation of the biceps tendon, a muscle of the forelimb that extends the shoulder and flexes the elbow. The dog became worse, and a follow-up examination 1 month later suggested the same diagnosis, because there was pain when the tendon and muscle were stretched by simultaneously flexing the shoulder and extending the elbow joints. The owner delayed bringing the dog in for another month, but came in when the dog was increasingly getting more painful and lame.
The two main things that come to mind in the picture above are a problem with the radial nerve, the main nerve the causes extension of the carpus or wrist and extension of the elbow joint, and a fracture of the humerus. There was weakness of the muscles, much atrophy of the forelimb muscles, and weak reflexes. There was also swelling at the upper end of the humerus, or arm bone, just beneath the shoulder. Radiographs were made and compared to radiographs made just 1 month before the referral.
Although radiographs made 3 months earlier were normal, and there were only subtle changes in radiographs made 1 month before the dog was seen, the changes in the bone rapidly progressed to the point of fracture. The most likely diagnosis is osteosarcoma, a primary bone tumor seen in the proximal humerus, distal radius, distal femur, and proximal tibia in middle-aged to older large-breed dogs. It may also be seen in younger large breed dogs, as young as 1.5 years. A much less likely possibility is fungal infection. In either case, the bone cannot be repaired and amputation is the only choice to relieve the dog's pain. Median survival time is unfortunately quite short, approximately 2-4 months with amputation alone, and approximately 12 months with amputation and chemotherapy.
The take home message? If things progressively worsen, reconsider the diagnosis and repeat radiographs or other diagnostics. I have seen a number of cases where the bone was quite normal on the original radiographs, and within 2 weeks, much of the bone was destroyed. Osteosarcoma is unfortunately all too common in large and giant breeds of dogs.