Knee dislocations are uncommon, and prosthetic knee dislocations even rarer.  Some general facts about prosthetic knee dislocations are:

• Posterior dislocations typically occur in the post-operative period and are usually the result of trauma that disrupts the PCL ligament.
• Factors that predispose a person to posterior dislocations are valgus deformity of the knee, malposition or improper selection of prosthetic components, patellar instability, and extensor mechanism dysfunction.
• The mechanism for this dislocation is typically flexion and external rotation of the knee when the lateral side of the knee is too loose.
• Anterior dislocations more commonly occur months to years after surgery and usually are not associated with trauma.
• Many of these dislocations result from loss of integrity of the posterior cruciate ligament, which provides anteroposterior stability of the knee and assists in femoral rollback. This motion is essential for the extensor mechanism of the knee to function.
   

References

Wang CJ, Wang HE. Dislocation of total knee arthroplasty: a report of 6 cases with 2 patterns of instability. Acta Orthop Scand 1997;68:282-285.

Stiehl JB, Komistek RD, Dennis DA, et al. Fluoroscopic analysis of kinematics after posterior-cruciate-retaining knee arthroplasty. J Bone Joint Surg Br 1995;77:884-889.