Ceramic on Ceramic Bearings in Total Hip Arthoplasty Paradoxes: Friction, Vibrations, Squeaking, Fracture
Seance of wednesday 11 october 2017 (COMMUNICATIONS LIBRES)
DOI number : 10.2699/q32z-zx62/emem.2017.3.011
Abstract
Ceramic on ceramic bearing in Total Hip Arthroplasty (THA) is a favored choice in younger and active patients. Squeaking and fracture are speculated to be related to material properties. How to understand this phenomenon? Firstly we analyze the chemical molecule then the biomechanical and thermo dynamical behavior.
Alumina molecule is Al2O3 pure or Alumina toughened Zirconia. It explains the tribology: hardness and perfect polishing, no chemical and biological reaction, no ion emission, excellent thermal conductivity and lubrication. Adverse effect is: poor ductility and no deformation with friction-vibration, stick-slip phenomenon, squeaking and fracture. Tribology qualities are in balance with fragility and instability: It is the first paradox.
Friction induced vibration system (coupling mode) and wear are limited in standard conditions, meaning concentric sliding with lubrication and a close equilibrium system. But THA is not a steady state mechanical system. Eccentric loading can occur randomly because microseparation between head and insert, impingement, edge loading and tensile stress. Consequences are 40 to 80 higher mechanical stresses than in standard conditions. This mechanical unstable system creates strip wear and vibrations into the new small area contact without lubrication, because shock then stick-slip phenomenon before a concentric loading again. A thermo dynamical analysis focuses on the fluctuation-diffusion phenomenon and the balance between kinetic energy and the opposite energy generated by friction. Here friction is entropy and is to diffuse into the system. When this system is unstable and far from disequilibrium (eccentric loading), wear and vibrations are the larger ways to diffuse. Wear is a surface change then energy could be stocked in the fissures as a stored energy then crack propagation. Whereas vibrations and lock- in frequencies fusion could be diffuse outside and sometimes as squeaking and exteriorized energy like a nonlinear system. This duality between stripe wear as stored energy and vibrations as exteriorized energy is the second paradox.
THA is a global mechanical system with metal connexions between ceramic head and taper of the femoral stem and sometimes an intermediate neck. Ceramic insert is linked into a metal cup. From ceramic on ceramic loading friction far from disequilibrium, vibrations and waves spread to metal connexions and mix themselves with mechanical stress created by movement and realize a fretting-corrosion and crevice -corrosion with metal debris and biological reactions. Stem femoral vibrations can create a squeaking when frequencies are similar to metal frequency vibrations. Thus ceramic on ceramic bearing and friction moment can be compromised by the metal links and their locking mechanisms: it is the third paradox.
Moreover that mechanical system interacts with the biological system, patient activity and behavior. The surgeon operation (positioning and choice) and manufacturing companies are another concern. Finally this complex system should be analyzed in totality.
Alumina molecule is Al2O3 pure or Alumina toughened Zirconia. It explains the tribology: hardness and perfect polishing, no chemical and biological reaction, no ion emission, excellent thermal conductivity and lubrication. Adverse effect is: poor ductility and no deformation with friction-vibration, stick-slip phenomenon, squeaking and fracture. Tribology qualities are in balance with fragility and instability: It is the first paradox.
Friction induced vibration system (coupling mode) and wear are limited in standard conditions, meaning concentric sliding with lubrication and a close equilibrium system. But THA is not a steady state mechanical system. Eccentric loading can occur randomly because microseparation between head and insert, impingement, edge loading and tensile stress. Consequences are 40 to 80 higher mechanical stresses than in standard conditions. This mechanical unstable system creates strip wear and vibrations into the new small area contact without lubrication, because shock then stick-slip phenomenon before a concentric loading again. A thermo dynamical analysis focuses on the fluctuation-diffusion phenomenon and the balance between kinetic energy and the opposite energy generated by friction. Here friction is entropy and is to diffuse into the system. When this system is unstable and far from disequilibrium (eccentric loading), wear and vibrations are the larger ways to diffuse. Wear is a surface change then energy could be stocked in the fissures as a stored energy then crack propagation. Whereas vibrations and lock- in frequencies fusion could be diffuse outside and sometimes as squeaking and exteriorized energy like a nonlinear system. This duality between stripe wear as stored energy and vibrations as exteriorized energy is the second paradox.
THA is a global mechanical system with metal connexions between ceramic head and taper of the femoral stem and sometimes an intermediate neck. Ceramic insert is linked into a metal cup. From ceramic on ceramic loading friction far from disequilibrium, vibrations and waves spread to metal connexions and mix themselves with mechanical stress created by movement and realize a fretting-corrosion and crevice -corrosion with metal debris and biological reactions. Stem femoral vibrations can create a squeaking when frequencies are similar to metal frequency vibrations. Thus ceramic on ceramic bearing and friction moment can be compromised by the metal links and their locking mechanisms: it is the third paradox.
Moreover that mechanical system interacts with the biological system, patient activity and behavior. The surgeon operation (positioning and choice) and manufacturing companies are another concern. Finally this complex system should be analyzed in totality.