[Non traumatic posterior instability in primary total knee replacement and its revision]

Dislocation after total knee arthroplasty is a rare complication and a difficult problem to address. When the flexion gap is larger than the extension gap and the collateral ligaments are injured, instability and knee arthroplasty dislocation can occur.

We report the case of a patient presenting with a posterior dislocation of a posterior-stabilized prosthesis without trauma. Frank instability in varus stress test and a positive anterior drawer test with tibial internal rotation dismissed the conservative treatment.

A constrained condylar prosthesis was used for the revision. He suffered a similar episode after a month, which demonstrated that the increase in the level of constraint was not enough to correct the severe asymmetric instability in flexion due to the damaged external structures. A rotating-hinge prosthesis was then implanted and the patient reported no additional episodes of instability.

We made an exhaustive review of the literature, analyzed the possible causes that can lead to the tibiofemoral instability after a total knee arthroplasty and described some technical considerations.