A Multi-Station Rolling/Sliding Tribotester for Knee Bearing Materials

Abstract

Total joint replacements traditionally employ ultra high molecular weight polyethylene (UHMWPE) as a bearing material due to its desirable material properties and biocompatibility. Failure of these polyethylene bearings can lead to expensive and risky revision surgery, necessitating a better understanding of UHMWPE’s tribological properties. A six-station rolling/sliding machine was developed to study the behavior of accelerated-aged UHMWPE in cylinder-on-cylinder contact. The normal load and sliding/rolling ratio in the oscillatory contacts can be controlled separately for each test station, as can the liquid test environment. Fatigue tests were run on the machine with UHMWPE versus cobalt-chrome cylinders in a distilled water environment at normal contact pressures of approximately 20 MPa. All specimens failed by subsurface cracking during tribotesting on the machine, and the failures were similar to those that occur in-vivo. The fatigue behavior of the polymer was analyzed to determine its relationship to oxidation and stress state in the rolling/sliding cylinder. At the 20 MPa test load, the number of cycles to fatigue failure by subsurface cracking was inversely proportional to the oxidation level. Analysis of the stress levels through the bulk of the polyethylene specimens and their relationship to the material properties provide insight as to why cracks initiate and propagate subsurface.

Using a surrogate contact pair to evaluate polyethylene wear in prosthetic knee joints

“…Previous researchers using intermediate‐complexity wear tests have generated more abundant damage and spalling, extending to several millimeters of depth, in aged, radiation‐sterilized UHMWPE specimens . The GUR 4150 and GUR 1050 materials examined in all of those previous studies have molecular weights of ∼6 × 10 6 g mol −1 , which is greater than the ∼3.5 × 10 6 g mol −1 molecular weight of the GUR 1020 material in the wear test of the present study .…”

“…Previous researchers using intermediate-complexity wear tests have generated more abundant damage and spalling, extending to several millimeters of depth, in aged, radiation-sterilized UHMWPE specimens. 8,10,12,15,16 The GUR 4150 and GUR 1050 materials examined in all of those previous studies have molecular weights of 6 3 10 6 g mol 21 , which is greater than the 3.5 3 10 6 g mol 21 molecular weight of the GUR 1020 material in the wear test of the present study. 25 Moreover, GUR 4150 is no longer used for orthopedic implants, in part because of suspicions that its processing additive, calcium stearate, may impair fusion of the raw material powder and facilitate fatigue damage.…”

“…This configuration provides the advantage of low equipment complexity but the potential limitation that few total knee prostheses have convex‐on‐flat condyle designs. Van Citters et al developed a multi‐station, rolling‐sliding test rig that articulated convex cylindrical specimens against each other, and they showed that the specimen geometry and loading yielded a contact stress field in the UHMWPE specimen similar to that of convex‐on‐flat condylar contact . To date, none of these approaches has evolved to become a widely adopted and standardized testing practice.…”

“…In addition, several research articles [10][11][12][13][14][15][16][17] have evaluated wear performance of UHMWPE knee implants using approaches that are less complicated than knee simulators and thus are potentially suited to less costly high-cycle, that is, >10 million cycles, durability testing. A few of these have incorporated geometric details of knee condyle design.…”

“…Van Citters et al developed a multi-station, rolling-sliding test rig that articulated convex cylindrical specimens against each other, and they showed that the specimen geometry and loading yielded a contact stress field in the UHMWPE specimen similar to that of convex-on-flat condylar contact. 15,19 To date, none of these approaches has evolved to become a widely adopted and standardized testing practice. Furthermore, primarily qualitative results have been reported on fatigue-evoked subsurface cracking that leads to spalling, which is sometimes called "delamination," and is a common failure mode of tibial inserts.…”