Research Day

Posterior Dislocation of a Total Knee Arthroplasty with a Polyethylene Tibial Post Fracture

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Introduction: The incidence of Total Knee Arthroplasty (TKA) dislocation is rare and that in combination with a fracture of the polyethylene tibial post is even rarer. The literature cites an incidence of 0.15-0.5% of TKA dislocation and a 0.51-1.2% of polyethylene tibial post fracture. The purpose of this case report is to examine why certain TKA designs are more susceptible to polyethylene tibial post fractures and dislocations. We then review the treatments for this problem. Case: We present a rare case of a 70-year-old male patient who presented to the ED with a posterior dislocation of a right TKA. He was closed reduced in the ED under conscious sedation and placed into a knee immobilizer. The patient continued to have right knee instability and pain while weight bearing in a knee immobilizer following the reduction. Revision of the right TKA was performed where it was discovered that the tip of the posterior stabilized (PS) polyethylene tibial post was fractured off. The fractured polyethylene insert was replaced with a super stabilized insert that also had an increase in size restoring stability to the knee. Discussion: The implant used in our patient's initial procedure was the Journey BCS I PS TKA. The design of this implant has been aimed at achieving more natural knee kinematics and deeper knee flexion. However, this design could lead to higher dislocation rates. At greater degrees of knee flexion the femoral CAM rises higher on the polyethylene tibial post and also results in increased stretch on the soft tissues. The Journey I BCS implant also has a low jump distance due to its design making it more susceptible to dislocation. Polyethylene tibial post wear is another well-known complication that can lead to instability and dislocation in PS TKA implants. The Journey I BCS allows earlier femoral cam and tibial post engagement resulting in more natural knee kinematics, but it results in greater contact forces on the tibial post. While these designs attempt to compensate for the higher contact forces by spreading them out over a larger contact area it can still result in a higher risk for tibial post wear and fracture. Of the 30 tibial post fractures in the recent literature 20 of them were treated with an exchange of the polyethylene tibial insert, five were treated with revision of all components, one was removed arthroscopically, and four did not report treatment. Only one exchange failed.

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