What’s the Diagnosis – Case 148
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Findings The patient has undergone a prior PCL substituting or posterior stabilizing (PS) total knee arthroplasty as seen by the prominent box or femoral cam. There is no difference on the lateral flexion radiographs. Subsequent MRI shows no marrow edema to indicate fracture, no loosening (circumferential tissue would be present at the component/prosthesis/cement interfaces), and no findings of infection (aggressive synovial response, extracaspular edema, prominent nodes) are present. What is seen is a truncation of the tibial polyethylene component. A small, low signal, geometric structure is present in the anterior aspect of the knee joint. As compared to most prior cases I am going to show the normal architecture of a PS knee for those that are not used to looking at it, particularly on MRI. Pointed out in particular will be the polyethylene post which sits in the femoral cam or box.
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Diagnosis: Diagnosis:Fractured polyethylene post of TKA A rare complication and one that the surgeon was alert to in this case particularly given the patient’s history and physical exam. As in this case there is often not a high energy traumatic event but a gradual wearing of the tibial post with a low grade injury causing the final displacement (the low signal, geometric focus is a portion of the displaced post). The tibial post fits into the metal cam or box of the femur to help allow for greater rollback of the femur and a greater degree of knee flexion. The post also helps prevent a posterior displacement of the femur during this maneuver. The keys in this case are one, assistance from the referring surgeon in terms of what to look for but also understanding what the normal architecture of the prosthesis should be. Once that normal architecture is distorted it alerts the radiologist to seek out the specific abnormality. This case also illustrates the marked utility of advanced protocols tailored around metal to allow accurate diagnoses. Not seen in this case but with a hyperflexion x-ray, a posterior displacement of the tibia can be seen. This patient underwent subsequent revision without any complication.
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References Five cases of tibial post fracture in posterior stabilized total knee arthroplasty using Prolong highly cross-linked polyethylene. Owen J Diamond, Lisa Howard, Bassam Masri. Knee. 2018 Aug;25(4):657662.doi: 10.1016/j.knee.2018.05.005. Epub 2018 Jun 6. Fracture of a polyethylene tibial post in a Scorpio posterior-stabilized knee prosthesis. Hong Chul Lim, Ji Hoon Bae, Jin Ho Hwang, Seung Joo Kim, Ji Yeol Yoon. Clin Orthop Surg. 2009 Jun;1(2):118-21. doi: 10.4055/cios.2009.1.2.118. Nontraumatic fracture of the polyethylene tibial post in a bi-cruciate stabilized total knee prosthesis. Daan Renson, Wouter Lenaerts, Johan Feyen. Arthroplast Today. 2019 Jul 23;5(3):284-287. doi: 10.1016/j.artd.2019.06.001.
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