Rheology of Joint Fluid in Total Knee Arthroplasty

Polyethylene wear is a significant clinical problem in total knee arthroplasty (TKA). Although the tribology of joint replacement has consequently become an area of significant research, the effect of joint fluid on lubrication has been largely overlooked in the replaced joint. One factor that affects the tribology of metal on polyethylene articulation in joint prostheses is mechanical properties of the joint fluid. In particular, viscosity and linear viscoelasticity are flow properties that likely contribute to fluid film lubrication in TKA, as they do in the natural knee. The primary objective of this study was to evaluate the flow properties of joint fluids in the context of TKA. Viscosity and linear viscoelasticity were evaluated in joint fluid from patients undergoing TKA, revision of a TKA, and aspiration of joint effusion. The flow properties of two commercially available hyaluronic acid joint supplements were also evaluated. Viscosity was measured over a range of strain rates and fitted to a common model of viscous behavior. Storage and loss moduli were measured for small amplitude oscillatory motion as a function of frequency. Each joint fluid demonstrated a characteristic shear-thinning, but viscosity varied over three orders of magnitude among samples. Both viscoelasticity and viscosity were compromised in many joint fluid samples. None of the samples obtained at revision were as viscous as healthy synovial fluid. In each effusion case, viscosity was at least a factor of six below the lower limit of the normal range. The joint supplements were more viscous than most joint fluid samples, likely based on higher concentration of hyaluronic acid. Normal viscosity correlated to high viscoelasticity (p=0.013). The fluid lubricating replacement knees and joints with effusion differed from healthy joint fluid in flow properties. Further examination of the connection between viscosity and tribology of joint replacement prostheses is warranted. Understanding the nature and performance of joint fluid as a prosthesis lubricant, particularly with respect to variability among patients, can lead to more meaningful prognoses and better therapies. Thesis Supervisor: Myron Spector Title: Senior Lecturer, Department of Mechanical Engineering Professor of Orthopaedic Surgery, Harvard Medical School