I am a little confused. Is his pain medial or lateral. A syndesmosis sprain of any relevance would show as a widening of the ankle mortis on plain x-ray. Distal pain is unlikely to be periosteal in my books. How sever was the old sprain? Could be anything really, bone damage as in a talar dome issue (should be ruled out clinically), small avulsion fracture, syndesmosis injury, anterior implngement due to overactive soleus post sprain etc etc. Best to do a thorough physical examination and provide us more details. thanks richard