Hypothesis #1:  Trauma-induced fracture

Ramon was hit by a car Ğ the force of the impact may have been sufficient enough to 
snap his femur and his tibia.  This would lead to considerable pain, inflammation 
and swelling, as well as the difficulty walking noted in the physical exam.  When a 
bone is broken traumatically, inflammatory mediators such as cytokines, 
leukotrienes, and prostaglandins are released.  Prostaglandins cause the pain 
threshold on the nociceptors to be lowered, creating increased pain response.  

Because the dog is only 17  months old, his bones may not be fully ossified yet.  Also, 
Ramon is on an inferior diet, which could have caused him to grow too quickly, 
leading to improper endochondral ossification, causing bone weakness that could 
predispose him to fractures.