If the ER failed to notice a broken talus bone after a car wreck and months later it is discovered by another dr is this medical malpractice?

It could be a failure to diagnose, but the question is what damages did the misdiagnosis cause?  In other words, did the delay cause there to be a surgery needed, or cause significant pain and suffering in the interim (or afterwards)?  If not, then there are not sufficient damages to pursue the claim.  If the damages are significant, it may be worth pursuing.  More importantly, if you have not yet settled the claim following the car accident, the party responsible for the accident would also be responsible for the broken talus bone.