Fracture of a polyethylene post in a 9-year-old posterior-stabilized knee prosthesis: light microscopy and SEM evaluation

Objective Aim of this study is to evaluate the surface of the TKA polyethylene liner, harvested after the breakage of the post nine years after the implant in a 63 years old female (BMI 39) after an hyperextension trauma. Methods During the revision we harvested sample of the periprosthetic tissue which was prepared for the light microscopy evaluation. The samples were stained using both haematoxylineosin and Von Kossa. The PE liner was prepared for the Scanning Electron Microscopy. Results The SEM evaluation revealed two different damage patterns considering the medial part and the lateral aspect of the sample. The medial part presented a fracture line laminated in front and smooth behind and with the tear lines with a medio-lateral and anterior posterior orientation. The lateral part presented a sharp fracture line that ends anteriorly with a laminated tear paralleled to the anterior edge of the polyethylene insert, and which implies that this area could be the terminal failure area of the fractured post. The medial part of the fracture edge appears to be smooth and with a different orientation of the fracture lines. Conclusions These features could be explained with a ‘‘two stage’’ rupture of the polyethylene post. This could have been caused by a non-optimal ligamentous balance that weakened the post, which was finally broken by a postero-anterior stress.