Transient elastohydrodynamic lubrication analysis of a novel metal-on-metal hip prosthesis with a non-spherical femoral bearing surface

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 non-spherical bearing surfaces for either acetabular or femoral components. An elastohydrodynamic lubrication model of a novel metal-on-metal hip prosthesis using a non-spherical femoral bearing surface against a spherical cup was solved under loading and motion conditions specified by ISO standard. A full numerical methodology of considering the geometric variation in the rotating non-spherical 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 non-spherical femoral head, Alpharabola, was analysed and compared with those of spherical bearing surfaces and a non-spherical 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 non-spherical 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 but 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 non-spherical head was small, compared with the improvement resulted from the non-spherical geometry

Influence of head size on the development of metallic wear and on the characteristics of carbon layers in metal-on-metal hip joints

Background and purpose Particles originating from the articulating surfaces of hip endoprostheses often induce an inflammatory response, which can be related to implant failure. We therefore analyzed the metal content in capsular tissue from 44 McKee-Farrar metal-on-metal hip prostheses (with 3 different head sizes) and we also analyzed the morphological structure of layers located on articulating surfaces

Bone mineral density of the femoral neck in resurfacing hip arthroplasty: Precision of DXA biased by region of interest and rotation of the hip

Resurfacing total hip arthroplasty (RTHA) may preserve the femoral neck bone stock postoperatively. Bone mineral density (BMD) may be affected by the hip position, which might bias longitudinal studies. We investigated the dependency of BMD precision on type of ROI and hip position

Ethnicity and culture: is it associated with falls?

Diana Anissian,1 Amin Zarghami2 1Student Research Center, 2Department of Neurology, Ayatollah Rohani Hospital, Babol University of Medical Sciences, Babol, IranWe read with great interest in the last issue of Clinical Interventions in Aging the article by Vieira et al, who studied the factors associated with falls among different ethnic groups in community-dwelling older adults and revealed that Afro-Caribbeans had a lower prevalence of falls and that several associations were stronger among this ethnic group.1 On the other hand, those associated factors, including taking medications for anxiety, having incontinence, and age above 75 years, do not seem to be ethnicity-related exclusively, but rather are more attributable to the general population’s lifestyle. Also, they did not discuss the role of ethnicity in falls and differences between ethnic groups

Wear of different materials for total hip replacement under adverse stop-dwell-start in vitro wear simulation conditions

Hip simulation is a common technique for pre-clinical evaluation of wear performance of total hip arthroplasty. Standard techniques replicate kinematics of walking patterns of a typical patient. Attention has focussed in developing simulations of other typical patient daily activities to improve accuracy of wear predictions. A method for simulating stop-dwell-start motion during patient walking and the effect on 36-mm metal-on-metal total hip arthroplasty was previously presented by the authors. This study sought to extend the previous work to look at the effect of these conditions on ceramic-on-ceramic, metal-on-polyethylene and ceramic-on-polyethylene bearings. Two stop-dwell-start protocols were used: one reproducing average patient movement patterns and one examining more severe conditions. For all materials tested, no significant increase in wear was observed under average stop-dwell-start conditions, suggesting the bearing types tested are robust to this type of activity. A significant increase in wear was observed for metal-on-metal, metal-on-polyethylene and ceramic-on-polyethylene bearings under severe stop-dwell-start conditions, this was attributed to depletion of lubricant in the bearing during the dwell period. A greater relative increase in wear was observed for metal-on-metal bearings compared with metal-on-polyethylene and ceramic-on-polyethylene bearings. This may be explained by the contributions of the different lubrication mechanisms in each bearing type. Wear of ceramic-on-ceramic was very low in all tests, suggesting normal measurement variation was masking any effect of the adverse conditions. It was not possible to determine any effect of the different activities. These results emphasise the importance of exploring adverse patient activity simulations. The increase in wear rate associated with an adverse activity such as seen in stop-dwell-start motion, has to be considered in the context of the frequency of the adverse activity cycle relative to other activities such as standard continuous walking, to determine the impact on the total wear in a given time period