Effects of Select Fluids on the Friction of Metal-on-Polyethylene

Lubricants are important factors in the tribology of total joint arthroplasty (TJA) surfaces, which are primarily comprised of a polished metallic or ceramic component articulating on an Ultra-High Molecular Weight Polyethylene (PE) surface. Wear particles released from the PE surface are the primary cause of TJA failure. The human body responds to the foreign, microscale particles by activating a cascade of cytokine responses that ultimately leads to osteolysis and aseptic loosening. Although research in the materials selection and design of TJA components is continually advancing, one of the major intrinsic components that affect the tribological response in joints is overlooked. In particular, the properties and composition of joint fluid directly affect the fluid film and boundary lubrication of artificial prostheses. Since the characteristics of joint fluids are likely to differ from patient to patient as a result of varying disease indications, age, health, gender, and activity level, tribological behavior is also likely to vary significantly. The primary objective of this thesis is to examine the effects of variation in joint fluid composition on tribology. Due to the relative high stresses applied to the knee, the tribological effects related specifically to total knee arthroplasty (TKA) are investigated in detail. Before any joint fluid samples are examined, however, an assay capable of determining appropriate tribological properties is adapted. A unidirectional pin-on-disk (POD) tribometer is therefore selected to measure friction between PE and cobalt-chromium-molybdenum alloy (Co-Cr). Its sufficient precision, short testing time-frame and low cost enables rapid evaluations. Preliminary friction data collected on fluids such as distilled water and bovine serum are used as standards and controls against lubricants in subsequent tests. From this data, the contributions to friction of boundary and fluid-film lubrication in PE on Co-Cr POD systems are discussed. Analysis of these friction properties in conjunction with previously published differences in wear between water and bovine serum leads to a rejection of a hypothesis directly correlating friction and wear. However, since ultimately wear is the important factor in the failure mechanism of TJA, an indirect relationship between friction and wear is investigated and proposed. Friction is then recorded using joint fluids as the lubricant and compared to the standards. Analysis of the joint fluid data demonstrates significance in frictional behavior, indicating that compositional properties affect friction. Moreover, examination of the data reveals large variation in joint fluids. Comparisons of the data to standard lubricants exhibit the potential for large variations in wear among joint fluids. Thesis Supervisor: Myron Spector Title: Senior Lecturer, Department of Mechanical Engineering Professor of Orthopaedic Surgery (Biomaterials), Harvard Medical School