J Orthop Sci (2009) 14:68–75 DOI 10.1007/s00776-008-1287-6 Original article Knee model of hydrodynamic lubrication during the gait cycle and the influence of prosthetic joint conformity ANTONIO PASCAU 1 , BLANCA GUARDIA 1 , JOSÉ ANTONIO PUERTOLAS 2 , and ENRIQUE GÓMEZ-BARRENA 3 1 Fluid Mechanics Area-LITEC, Centro Politécnico Superior, Universidad de Zaragoza-CSIC, Zaragoza, Spain 2 Department of Materials Science and Technology, Centro Politécnico Superior, Universidad de Zaragoza, Zaragoza, Spain 3 Servicio de Cirugía Ortopédica y Traumatología, Fundación Jiménez Díaz, Universidad Autónoma de Madrid, Madrid 28040, Spain Abstract Background. The influence of the total joint components’ elastic deformation on lubrication is generally accepted, but little is known about the influence of joint conformity under hydrodynamic lubrication based on fluid film interposition. The aim of this study was to evaluate induced pressure and stresses in the knee under fluid film lubrication during the stance phase of walking under various joint conformity conditions. Methods. A theoretical two-dimensional (2D) geometric model of knee prosthesis contact, with Dirichlet boundary conditions at both edges, and with a conformity index (CI) of 0, 0.3, 0.5, 0.6, 0.7, 0.8, 0.9, 0.92, 0.94, 0.96, 0.98, 0.99, 0.995, and 1.0, was used to calculate the spatiotemporal lubricant flow on a synovial fluid rheological model. With the instantaneous load as a source term, the Reynolds lubrication equation was subsequently solved following a finite volume approach in two dimensions and three dimensions. Results. Conformity strongly influenced the peak pressure, from 47 MPa with CI = 0 to 1.4 MPa with CI = 1, with a definite behavior change from CI = 0.96. The role of hydrodynamic lubrication was restricted to early steps of the stance phase. With CI < 0.96, there was a smooth maximum pressure decrease with increasing CI. In contrast, the maximum pres- sure fell abruptly with conformity > 0.96. Conclusion. The present model suggested the limited modify- ing effect of hydrodynamic lubrication in total knee replace- ment systems. However, its role during the early stance phase, coupled with high conformity, helps significantly to decrease compressive stresses on the polyethylene, fostering the benefi- cial effect of high conformity in a mixed lubrication regime. This beneficial effect may also be of great interest in total knee replacement systems based on materials with less deformation. Introduction The role of lubrication has been rarely considered in total knee replacement (TKR), despite its well-known influence on joint tribology. Lubrication modes in TKR are utterly different from those of natural knee joints. 1 As the negligible porosity found in the artificial articu- lating surfaces is not suspected to allow for weeping lubrication, artificial joint lubrication modes include elastohydrodynamic and hydrodynamic modes, depend- ing on the magnitude of the stresses and boundary regimes 2,3 ; and load capacity mostly relies on the pres- sure field induced by the squeezed fluid velocity caused by the approaching parts. Whereas a total hip replacement (THR) design is based on significant conformity, the TKR design shows large variations in conformity that significantly affect contact stresses in the joint. 2,4,5 Lubrication about the hip has prompted design modifications, and its influence has been related to good performance of present hips, including the tribological challenge of metal-on-metal designs. 6 Hydrodynamic lubrication about the hip 3 has been shown to affect only 10% of the walking cycle. However, the influence of lubrication about the knee is less clear, and the particular contribution of hydrody- namic lubrication is unknown. The role of lubrication is also relevant due to the multifactorial origin of ultra-high-molecular-weight polyethylene (UHMWPE) wear in artificial joints. In knee implants, this has been related to material pro- perties, polyethylene thickness, component design, and alignment; contact stresses and load; and clinical vari- ables such as implantation time. 4,7–10 Not only is increas- ing femorotibial conformity associated with UHMWPE contact stress reduction in the knee, 10 but the effect of a load increase is higher in nonconforming joints. Correspondingly, retrieval studies 11,12 have shown less delamination in more conforming femorotibial pairs, whereas less conforming implants were associated with Offprint requests to: E. Gómez-Barrena Received: July 24, 2007 / Accepted: September 22, 2008