Page 84 - Demo

Chapter 382UHMWPE is a type of linear unbranched PE with a high molecular weight (>1×106 amu) and high degree of crystallinity, which can be found in various forms.(60) The polymer is relatively cheap to process and has many advantageous properties, such as high stiffness and high impact strength, low coefficient of friction, good impact load damping capability, and good resistance to body fluids.(38) Throughout 5 decades of use, further improvements have been made, resulting in the current high grade cross-linked UHMWPEs, which have significantly better wear resistance and lower wear rates and coefficients of friction, compared with other polymers, such as high density PE, PMMA, and PTFE.(38,61) Because the material is used as an insert between load-bearing surfaces, the type of material and finish of the counter-element and the environment have a defining influence on the wear and friction of UHMWPE. When the opposing material has a smoother surface, the abrasive wear will be lower than when the opposing material is rougher. Body fluids that surround the implant help create an elastohydrodynamic lubrication between the two surfaces. The amount of pressure and kinematics also have a significant influence on the amount of wear to which UHMWPE is subjected.(60) (Table 2). Important to note is that, while the amount of wear between metal-on-UHMWPE implants is greater than with metalon-metal implants using CoCrMo alloys, potential problems due to wear can be prevented by increasing the thickness of the articulating surface. As such, the UHMWPE fossa of the Biomet TMJ prosthesis has a minimal thickness of 4 mm.(8) One problem that current stock TMJ implant devices using UHMWPE can still encounter is that of creep (also known as cold flow), which is a form of slow permanent deformation resulting from long-term exposure to loading. This phenomenon has been well documented in orthopedic hip TJR and was also observed in stock TMJ TJR with flange screw fixation.(60) Deformation of the UHMWPE components risks diminishing the fit of the prosthesis part, resulting in increased micromotions and potentially eventual device failure.(38) Nikolas de Meurechy NW.indd 82 05-06-2024 10:14