Dislocation of Polyethylene Insert

 

Dislocation of a polyethylene insert Mechanism of Dislocation (DPI in TKA) Proposed Cause

Atraumatic nature	No clear cause identified
Traumatic mechanism (insert dislocation)	Repeated posteriorly directed compressive force on the polyethylene insert from the femur
Biomechanical basis for dislocation	During knee flexion, femoral tibial contact moves posteriorly, leading to anterior lift-off of the insert
Possible causes of anterior lift-off	- Failure to secure the insert at surgery <br> - Overt locking mechanism failure <br> - Repeated stress causing wear on the locking mechanism and eventual failure
Uneven load distribution	Ligament laxity leading to point bearing of the lateral femoral condyle onto the posterolateral corner of the polyethylene insert, leading to accelerated wear and failure of the insert's retaining lip
Impingement on soft tissues or osseous structures	Impingement of the insert on soft tissues or osseous structures can lead to DPI; a case with a large posterior osteophyte was observed, suggesting it may be responsible for dislocation

Ahmed I, Murray J. Dislocation of a polyethylene insert in an infected knee joint after a Triathlon total knee arthroplasty. J Surg Case Rep. 2020 Aug 14;2020(8):rjaa287.

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Complete dislocation of the polyethylene insert in fixed-bearing total knee arthroplasty.

※ Risk Factors of Polyethylene Insert Dislocation

• Varus alignment may impact long-term survival. 
• Anterior tilt of tibial baseplate or mismatch between tibial and femoral components increases risk
• None of the known risk factors observed in the 3 cases at the institution. (J Arthroplasty. 2011 Feb;26(2):339.e1-4)

 

The dislocated polyethylene insert can cause significant damage to the patellar tendon.