Traumatic Injury
In the history of this foal, the owner mentions that the mare might have stepped on the 
foal.  The acute nature of the swelling, along with the localization to one joint suggests a 
traumatic injury.  Also, the foal is afebrile and severely lame.  We hypothesize that the 
foal was stepped on or kicked and caused a fracture of the sesamoids or other bones in 
the fetlock joint (P1, metacarpal).  A fracture of the sesamoid can be a relatively common 
result of trauma to the fetlock.  If the bone was fractured it could be responsible for the 
acute lameness and effusion that PE demonstrated.  Bone fractures are very painful and 
the foal would not want to bear weight on the right rear limb.  Bone fracture would 
activate pain receptors in subchondral bone and surrounding joint capsule leading to 
inflammatory response and protaglandin releaseÑleading to the soft tissue swelling and 
the severe pain.