Is it a lateral luxation??? If so it is a congenital deffect very little risk it is from trauma. No you do not have to sedate a dog to check if it has LP, basically the vet really only needs to tap and bend the joint to check for luxation...
http://www.offa.org/patluxinfo.html
That website has excellent LP information, and we have been through two LP surgeries so if you have any questins please feel free to ask.