Wear mechanisms of UHMWPE under physiological conditions
In 2005 one of the giants in implant manufacturing launched the most flawed metal on metal implant. It caused a loss of billions of dollars and more than 8000 patients were forced to a revised surgery because of the high health hazard. At present, polymer on metal implants comprise of 70% of all hip and knee arthroplasty. Increased life expectancy and lower revision surgeries due to wear induced osteolysis are driving development of superior grades of UHMWPE. Since 1970s, the procedure for the development of new orthopaedic bearings initiates with screening of several candidate materials by simple and affordable testing machines such as tribometers. However, in conventional tribometers the unidirectional motion (i.e., linear reciprocation or circular motion) cause hardening of the polymeric chains of UHWMPE along the direction of motion, hereby leading to wear rates two orders of magnitude lower than in vivo conditions. Ducom Biotribometer has overcome this limitation by using an X-Y stage able of multidirectional motion along both axis (see Figure 1). In case of linear reciprocating motion (along X or Y axis), the cross-shear is generated by using the pin rotation feature (up to 2 Hz).
Figure 1. Schematic of one station in Ducom Biotribometer. The X-Y stage can generate linear reciprocation (along X or Y axis), elliptical, rotation, square, and figure 8 wear profiles. The type of motion is the only parameter common to all the stations.
Ducom Biotribometer consists of a single station or up to 6 stations. Each station is equipped with sensors to measure friction and compound wear, a lubricant cup and a heating unit (up to 70°C). The normal load (20 to 400 N) is applied independently on each station and it is fully automated. This allows the generation of physiological gait load profiles (in compliance with the ISO 14242-1 test standard), as well as fixed load profiles (as per ASTM F732 test standard).
Examples of UHMWPE wear mechanisms investigated using Ducom BioTribometer
Crosslinking of UHMWPE
Two different grades of ultra-high-molecular polyethylene (UHMWPE) pins have been tested against Cobalt Chromium Molybdenum (CoCrMo) disks. The load profile was a sine wave from 88 to 186 N over 0.5 million cycles (MC). The motion profile was linear reciprocation with 1 Hz of pin rotation and the test samples were submerged in bovine calf serum at 37°C. Friction and wear were acquired every 0.25 MC.
As shown in Figure 2A, the wear rate of the crosslinked UHMWPE (0.64 mm3 / MC) was almost an order of magnitude lower compared to the uncrosslinked UHMWPE (5.19 mm3 / MC). In fact, crosslinking of polyethylene reduces the mobility of adjacent carbon-carbon chains by reinforcing their bonds, hereby rendering the polyethylene more resistant to wear. Also, severe scratches were observed on both uncrosslinked UHMPWE and its counterbody CoCrMo disk (see Fig. 2B).
Figure 2. (A) Wear rate of UHMWPE (crosslinked) and UHMWPE (uncrosslinked). (B) Wear images of UHMWPE (uncrosslinked) pin and CoCrMo disk after 0.5 million cycles.
Hardness effect of metal substrate on UHMWPE
Standard UHMWPE pins in contact with Ti6Al4V and CoCrMo disks were tested over 1.2 million cycles (MC) using a walking gait load profile (400N as maximum load). Gravimetric weight, friction and wear were acquired every 0.14 MC.
Friction coefficient of UHMWPE_Ti6Al4V was lower than UHMWPE_CoCrMo (see Fig. 3A), possibly due to a thicker oxide layer on Ti6Al4V, while the wear of UHMWPE increased linearly over an increase in the number of cycles up to 1.2 MC (see Fig. 3B). The cumulative wear of UHMWPE_Ti6Al4V and of UHMWPE_CoCrMo was 8.35 mm3/MC and 9 mm3/MC, respectively. The SEM images (see Fig. 4) show that the high wear on UHMWPE_Ti6Al4V had severe fiber pull out (or delamination), a polished surface, deeper cracks and interestingly ripples. There were also more scratches on the Ti6Al4V disk compared to the CoCrMo disk. These surface characteristics usually are observed on retrieved implants or joint simulators. Several clinical studies have proven that entrapped third-body wear particles (including CoCrMo particles) are responsible for scratches on CoCrMo articulating against UHMWPE. Further study is needed to understand the oxide layer formation and corrosion resistance of the metal surfaces using the tribocorrosion setup in Ducom BioTribometer.
Figure 4. Scanning electron microscopy and optical images of UHMWPE pins and metal disks, respectively, after 1.2 MC wear test. The image of unworn UHMWPE as a reference shows the machine marks (inset).
Modified ASTM F732 testing to investigate the friction and wear of UHMWPE
Dynamic loads and heat generated (up to 45°C) during the physical activities like walking and stair climbing can influence wear of UHMPWE. However, the present standard test method ASTM F732 does not include these factors. Here, we modify the ASTM F732 by changing the fixed load into dynamic loads (A), and the effect of temperature on wear of UHMWPE (B). The duration of the test was 0.6 million cycles in both cases.
A. Dynamic load effect
Walking and stair climbing gait load cycles (see Fig. 5) have been reproduced. CoCrMo pins in contact with UHMWPE doped with Vitamin E disks were used as tribopair. Stair climbing shows high run in friction (see Fig. 6A), severe multidirectional scratches, deep grooves and spalling (see Fig. 6B) compared to walking. Gravimetric weight loss after the test was 3.6 mm3/MC and 1.6 mm3/MC for stair climbing and walking, respectively.
Figure 5. Dynamic load profiles for (A) walking and (B) stair climbing gait load cycles. In both profiles, the maximum and minimum loads are 400 N and 40 N, respectively. However, the average load is 172 N and 147 N for walking and stair climbing, respectively.
Figure 6. Friction coefficient (A) and optical microscopy (B) of CoCrMo pin and UHMWPE Vit E disk.
A. Temperature effect
Standard UHMPWE and UHMPWE doped with Vitamin E pins in contact with CoCr disks were tested at 37°C and 45°C during walking gait cycle. As shown in Figure 7, the low friction wear behaviour of UHMWPE doped with vitamin E at 37 °C is not applicable at 45 °C, due to denaturation of Vitamin E at elevated temperatures. Furthermore, large ripples were observed on UHMWPE doped with vitamin E at 45 °C due to low fatigue wear resistance (see Fig.7C).
Figure 7. Friction coefficient of standard UHMWPE and UHMWPE doped with vitamin E pins loaded against CoCrMo disks at 37 °C (A) and 45 °C (B). SEM images of standard UHMWPWE and UHMWPE doped with Vitamin E (C).
Physiological load profiles (e.g., walking, stair climbing, running, etc.) of daily activities and cross shear effect of UHMWPE must be considered in the screening process of new biomaterials. Dynamic load profiles, multidirectional motion, and temperature control features in Ducom Biotribometer offer realistic test platform for simulating in-vivo conditions and reproduce the wear mechanisms and wear rates observed from clinical studies.