Commonest dangerous complication of posterior dislocation of knee is -
Popliteal artery injury
Sciatic nerve injury
Ischaemia of lower leg compartment
Femoral artery injury
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.