Commonest dangerous complication of posterior dislocation of knee is -
Dislocation of the knee

This rare injury results from severe violence to the knee so that all of its supporting ligaments are torn.
It is a major damage to the joint, and is often associated with injury to the popliteal artery.
Treatment is by reduction followed by immobilization in a cylinder cast.
Popliteal artery injury
Sciatic nerve injury
Ischaemia of lower leg compartment
Femoral artery injury