Dynamic measurements of lubrication film thickness of UHMWPE contacts for total joint replacements

A new contact sensing technology previously developed in the Biotribology Laboratory at Clemson University was further studied, evaluated, and characterized to extend its use to the measurement of lubricating film thickness. First, the laboratory's force-controlled knee joint simulator was used while dynamic contact pressure measurements under both dry and lubricated conditions were made using the sensor technology employed in two different artificial knee implant geometries. Each implant was machined by the manufacturer from custom blocks of ultra high molecular weight polyethylene (UHMWPE) containing a grid of discrete sensing regions. The difference between the dry and lubricated contact areas measured at different phases of the gait cycle for each implant suggested that the dynamic lubrication thickness might also be able to be quantified by the sensing technology. To gain insight on this, simplified contacts of metal on UHMWPE were studied with the sensing technology being employed in the UHMWPE side of the contacts. First, the UHMWPE sensor's outputs were studied under static, lubricated conditions while the surface separation was directly controlled. The insights gained during the static testing were used to develop a more representative contact that was then characterized under hydrodynamic conditions. The experimental contact model was designed to mimic a single sensing point of the knee sensors used earlier in this study. It consisted of a UHMWPE sensor pin with a spherical tip sliding on a flat stainless steel counterpart with an implant-grade finish. Hydrodynamic motions were applied to the contact with the laboratory's custom-designed multi-axis pin on disk wear testing machine with friction measuring capabilities. To relate the sensor pin's output to the mode of lubrication, a Stribeck curve was experimentally developed and was used to determine the lambda (lambda) values specific to the UHMWPE on metal sliding point contact. It was found that the boundary lubrication regime existed for lambda < 1, mixed lubrication was present for 1 < lambda 3.5. Calibration equations relating the sensor's output to the film thickness were obtained using simple linear reciprocating motion, and it was found that in the boundary lubricated regime, the sensor's output was linearly related to the film thickness. It was also determined that for mixed and fluid film lubrication, the sensor's output was linear on a log-log scale to the film thickness; thus, there was a power-law relationship. Finally, the calibration equations were used to measure the lubricating film thickness of the UHMWPE contact in a clinically relevant, cross-path motion complete with sliding speeds relevant to the phases of gait where lubricating films can potentially exist for artificial knee joints. Two different loads were applied to the contact for these measurements. For the lightest load, mixed lubrication and HL were measured, and the film thickness varied from 2mum to over 10mum. With the higher load, the film thickness was seen to fall to 1mum for a small portion of the cycle, showing that the contact experienced the full range of lubrication modes from boundary to full hydrodynamic lubrication.*.;*This dissertation is a compound document (contains both a paper copy and a CD as part of the dissertation). The CD requires the following system requirements: Windows MediaPlayer or RealPlayer.