Transient Elastohydrodynamic Lubrication Analysis of a Novel Metal-On-Metal Hip Prosthesis with a Non-Spherical Femoral Bearing Surface

Abstract

Effective lubrication performance of metal-on-metal hip implants only requires optimum conformity within the main loaded area, while it is advantageous to increase the clearance in the equatorial region. Such a varying clearance can be achieved by using nonspherical bearing surfaces for either acetabular or femoral components. An elastohydrodynamic lubrication model of a novel metal-on-metal hip prosthesis using a nonspherical femoral bearing surface against a spherical cup was solved under ISO standard specified dynamic loading and motion conditions. A full numerical methodology of considering the geometric variation in the rotating nonspherical head in elastohydrodynamic lubrication solution was presented, which is applicable to all non-spherical head designs. The lubrication performance of a hip prosthesis using a specific nonspherical femoral head, Alpharabola, was analyzed and compared with those of spherical bearing surfaces and nonspherical Alpharabola cup investigated in previous studies. The sensitivity of the lubrication performance to the anteversion angle of the Alpharabola head was also investigated. Results showed that the nonspherical head introduced a large squeeze film action and also led to a large variation in clearance within the loaded area. With the same equatorial clearance, the lubrication performance of the metal-on-metal hip prosthesis using an Alpharabola head was better than that of the conventional spherical bearings while worse than that of the metal-on-metal hip prosthesis using an Alpharabola cup. The reduction in the lubrication performance caused by the initial anteversion angle of the nonspherical head was small, compared with the improvement resulted from the nonspherical geometry.

Predictive wear modeling of the articulating metal-on-metal hip replacements

“…Compared with the ideal spherical bearing geometry, the nonspherical geometries of bearing surfaces, which may be a result of wear, affect the gap between the cup and head surfaces (clearances) and consequently affect lubrication and wear. The change of bearing geometry during wear resulting in a decreased contact pressure was also observed in the EHL studies of hip implants, such that the peak pressure was reduced and pressure was distributed more evenly on the bearing surface.…”

“…Such studies have shown that the fluid-filled lubricating gap is at nanometer size-an extremely small gap when compared to typical engineering bearing problems, where the fluid-filled gap is typically in a range of 1-100 mm. 14,15 Compared with the ideal spherical bearing geometry, the nonspherical geometries of bearing surfaces, which may be a result of wear, affect the gap between the cup and head surfaces (clearances) and consequently affect lubrication [16][17][18] and wear. The change of bearing geometry during wear resulting in a decreased contact pressure was also observed in the EHL studies of hip implants, 19 such that the peak pressure was reduced and pressure was distributed more evenly on the bearing surface.…”

Contact mechanics and lubrication analyses of ceramic-on-metal total hip replacements

“…Such a conforming 1 geometry reduces the pressure gradient at the inlet, resulting in the reduction in the 2 Poiseuille flow and an increase in the Couette flow. As a result, more lubricant is 3 allowed to flow into the loaded area to improve lubrication [35]. Therefore, a decrease 4 in radial clearance from 100 to 30 µm caused the increase of film thickness (Figures 9 5 b and 11).…”

Transient viscoelastic lubrication analyses of UHMWPE hip replacements

“…In order to facilitate the numerical process and improve the stability, the governing equations were non-dimensionalized. A multi-grid method [20,21] was used to solve the Reynolds equation. On each grid level, the finite difference method was used to approximate the Reynolds equation.…”

“…The central difference was applied to the left hand side of the equation and the backward difference to the right hand terms. The detailed numerical approach was similar to that of the linear elastic model in the previous publications [20,21].…”