Cross-Shear in Metal-on-Polyethylene Articulation of Orthopaedic Implants and its Relationship to Wear.

Wear of polyethylene (UHMWPE) is dependent on cross-shear. The aim of the present study was: 1) to develop a theoretical description of cross-shear, 2) to experimentally determine the relationship between cross-shear motion and UHMWPE wear using a wheel-on-flat apparatus, and 3) to calculate the work it takes to remove a unit volume of wear for the use in advanced computational models of wear. The theoretical description of cross-shear has been based on the previously reported finding that cross-shear is maximal when movement occurs perpendicular to fibril orientation. Here, cross-shear is described with a double-sinusoidal function that uses the angle between fibril orientation and velocity vector as input, and maximum cross-shear occurs at 90° and 270°. In the experimental part of the study, friction and wear of polyethylene were plotted against increasing sliding velocity vector angles, i.e. increasing cross-shear. It was found that wear intensified with increasing cross-shear, and wear depth could be predicted well using the double-sinusoidal function for cross-shear (r2=0.983). The friction data were then used to calculate the work to remove a unit particle by integrating the frictional force over the directional sliding distance. Using the wear volumes, determined for both longitudinal and perpendicular motion directions, the work to remove a unit volume of material was qy = 8.473 × 108 J/mm3 and qx = 1.321 × 108 J/mm3, respectively. Hence, 6.4 times more work was necessary to remove a unit wear volume in the direction of principal motion (i.e. along the molecular fibril orientation) than 90° perpendicular to it. In the future, these findings will be implemented in computational models to assess wear.

In-vivo kinematics of knee prostheses patients during level walking compared with the ISO force-controlled simulator standard.

Differences between wear-scar features of simulator-tested and retrieved tibial total knee replacement (TKR) liners have been reported. This disagreement may result from differences between in-vivo kinematic profiles and those defined by the standard. The purpose of this study was to determine the knee kinematics of a TKR subject group during level walking and to compare them with the motion profiles produced by a wear test conducted according to the force-controlled knee wear testing ISO 14243-1 standard. Ten patients with a posterior cruciate ligament-retaining TKR design were gait tested using the point cluster technique to obtain flexion-extension (FE) rotation, anterior-posterior (AP) translation, and internal-external (IE) rotation motions during a complete cycle of level walking. Motion data were directly compared with the output kinematics from the wear test. The subjects exhibited an FE rotation pattern similar to the output from ISO-14243-1; however, they had higher midstance knee flexion angles. For both AP translation and IE rotation, the standard profiles had significantly smaller total ranges of motion than seen in vivo, with noticeably different patterns of motions. For this particular implant design, significant differences were found in both the pattern and the magnitudes of in-vivo motion during level walking compared with the ISO-14243-1 standard.

Fluid composition impacts standardized testing protocols in ultrahigh molecular weight polyethylene knee wear testing.

Wear of total knee replacements is determined gravimetrically in simulator studies. A mix of bovine serum, distilled water, and additives is intended to replicate the lubrication conditions in vivo. Weight gain due to fluid absorption during testing is corrected using a load soak station. In this study, three sets of ultrahigh molecular weight polyethylene tibial plateau were tested against highly polished titanium condyles. Test 1 was performed in two different institutions on the same simulator according to the standard ISO 14243-1, using two testing lubricants. Test 2 and test 3 repeated both previous test sections. The wear and load soak rates changed significantly with the lubricant. The wear rate decreased from 16.9 to 7.9 mg weight loss per million cycles when switching from fluid A to fluid B. The weight gain of the load soak specimen submersed in fluid A was 6.1 mg after 5 x 10(6) cycles, compared with 31.6 mg for the implant in fluid B after the same time period. Both lubricants were mixed in accordance with ISO 14243 (Implants for surgery - wear of total knee-joint prostheses), suggesting that calf serum should be diluted to 25 +/- 2 per cent with deionized water and a protein mass concentration of not less than 17 g/l. The main differences were the type and amount of additives that chemically stabilize the lubricant throughout the test. The results suggest that wear rates can only be compared if exactly the same testing conditions are applied. An agreement on detailed lubricant specifications is desirable.