Treatments of Polyethylene Tibial Post Fracture in Posterior Stabilized Knee Prosthesis with Unstable Total Knee Arthroplasty: A Case Series

Background and Clinical Significance: With increasing demand for total knee arthroplasty (TKA), the incidence of post-operative instability has also risen. Although fracture of the polyethylene tibial post in posterior-stabilized (PS) prostheses is relatively uncommon, it should not be overlooked because delayed recognition may lead to severe late sequelae. Case Presentation: Between April 2008 and January 2020, 132 revision TKAs were performed at our institution due to suspected instability with or without component loosening. Among these, three patients were identified as having instability associated with fracture of the polyethylene tibial post in PS implants. All three patients presented late, with a mean interval of 49.66 months (range 34–74) after the index TKA. At presentation, all demonstrated multiplanar global instability, and two showed recurvatum deformity. Pre-operatively, revision surgery was indicated due to progressive global instability. Although tibial post fracture was suspected as a possible underlying cause, the diagnosis could not be definitively established until intraoperative inspection, which confirmed polyethylene tibial post fracture in all cases. Each patient underwent revision TKA using a semi-constrained prosthesis with an increased polyethylene insert height. At final follow-up (12–18 months), all patients achieved complete resolution of clinical instability and demonstrated meaningful improvement in the Hospital for Special Surgery (HSS) and Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) scores. Conclusions: Although rare, polyethylene tibial post fracture should be considered in the differential diagnosis of post-operative knee instability following PS TKA. Early recognition may allow management with isolated polyethylene exchange, whereas delayed diagnosis can result in progressive ligamentous insufficiency and global multiplanar instability, ultimately necessitating revision using a semi-constrained implant.