Spontaneous dislocation of a mobile-bearing polyethylene insert after posterior-stabilized rotating platform total knee arthroplasty: a case report.

We describe a dislocation after Scorpio mobile-bearing total knee arthroplasty. This system is a rotating platform posterior-stabilized design and utilizes a single post as part of the metal tibial tray. Only one locking ring inside the socket of the polyethylene insert secures a tight connection with the post. Spontaneous dislocation between the polyethylene insert and the metal tray occurred at 22 months post surgery while rising from the supine position with slight knee flexion. Operative findings revealed failure of the locking ring and the original insert was replaced with a thicker insert. Our case and a duplicated saw bone model demonstrated that failure of the locking system resulted in the dislocation of the insert.