title: Two stage fracture of a polyethylene post in a 9-year-old posterior-stabilized knee prosthesis: a case report.
abstract: BACKGROUND
  Several cases of tibial post breakage are reported in the literature. To the best of our knowledge, only three cases of NexGen knee prosthesis (Zimmer, Warsaw, Indiana, USA) tibial post failure have been reported.
  
  
  METHODS
  In November 1999, a 63-year-old Caucasian woman from Italy with a history of symptomatic left knee osteoarthritis underwent a total knee arthroplasty. In March 2008, while rising from a chair, she felt a sudden pain and instability in her left knee. She reported a fracture of the polyethylene post of the tibial insert. No malposition or malalignment of either the femoral or tibial components were identified. The polyethylene tibial insert was studied under light microscopy and scanning electron microscopy. The fracture was also noted to have occurred without any notable polyethylene wear.
  
  
  CONCLUSIONS
  Scanning electron microscopy revealed two different damage patterns that could be explained with a two-stage rupture of our patient's polyethylene post. This could have been caused by a non-optimal ligamentous balancing during first implant surgery. Her knee probably developed a varus instability that weakened the post, and then a posterior anterior stress finally broke the polyethylene.
authors.affiliation: Department of Orthopaedics and Traumatology, University of Insubria, Ospedale di Circolo - Fondazione Macchi, V le Borri 57, 21100 Varese, Italy. fabio.dangelo@uninsubria.it.
authors.forename: Fabio
authors.initials: F
authors.lastname: D'Angelo
authors.forename: Daniele
authors.initials: D
authors.lastname: Marcolli
authors.forename: Paolo
authors.initials: P
authors.lastname: Bulgheroni
authors.forename: Luigi
authors.initials: L
authors.lastname: Murena
authors.forename: Terenzio
authors.initials: T
authors.lastname: Congiu
authors.forename: Paolo
authors.initials: P
authors.lastname: Cherubino