Lubrication and friction prediction in metal-on-metal hip implants

A Mixed Elasto-Hydrodynamic Lubrication Model for Wear Calculation in Artificial Hip Joints

The aim of this paper was to propose a novel in silico mixed elasto-hydrodynamic lubrication model with the purpose of wear prediction in Total Hip Replacements (THRs). The model considers the progressive wear contribution in the calculation of the meatus filled by the non-Newtonian synovial fluid. The results were referred to the gait cycle kinematics, calculated by using musculoskeletal multibody software, while the loading was assumed by literature in vivo measurements. The simulations allow evaluating the fluid and the contact pressure fields and the acetabular cup wear over the time. The results were obtained considering a Ultra High Molecular Weight PolyEthylene, UHMWPE, cup and were compared with results from the literature, showing a good agreement in terms of total volume wear of the cup.

A novel analytical approach for determining the frictional moments and torques acting on modular femoral components in total hip replacements

A three dimensional analytical approach was developed to determine the frictional moment vector generated by the relative sliding of the head-cup bearing couple of a total hip replacement. The frictional moment projection onto the femoral neck was also determined over the loading cycle. Predicted frictional moments for nine combinations of bearing materials and diameters were in close agreement with existing in vitro data. The analytical method was then applied to simplified gait (lubrication conditions of dry and serum), ISO standard gait and physiological level gait loading cycles. ISO standard gait had a total contact force of about two fold of physiological level gait and there was a corresponding increase in the maximum frictional torque on neck from 0.66×BW%m to 0.88×BW%m. For the ISO standard gait, the maximum frictional torque occurred at the same instance of maximum frictional moment and the maximum contact force. In contrast, for the physiological level gait, the frictional torque did not occur at the same instance as the peak load. This suggests that the neck frictional torque is a function of other parameters, such as angle between neck axis and frictional moment vector, as well as the magnitude of the contact force and frictional moment. The developed methodology was able to predict the maximum magnitude and change of directions of moments and the variation of torque at the head neck interface. The data will be useful for experimental studies assessing the fretting behaviour of the head neck junction, by providing appropriate loading data.

Investigation of Workpiece Positioning Methods for Machining Oil-Pocket on Hip-Implant Spherical Surface

Oil pocket has been reported that it may improve tribology characteristic and thus prolong the lifespan of the joint. In order to implement it on spherical surface, appropriate positioning system is required. This paper reports the investigation of three axes workpiece positioning system in order to machine oil pocket (micro-pits) on hip implant. A conventional linear x-y-z axes configuration (Cartesian coordinate) and two configuration of spherical coordinate (swing-swing and swing-rotate configuration) are applied in simulation. All machined workpiece are investigated in pits distribution, shape, and machined angle. The inspection concludes that spherical method with swing-rotate configuration is the most suitable method for machining oil pocket on spherical surface.

Effect of lubricants on friction in laboratory tests of a total disc replacement device

Some designs of total disc replacement devices have articulating bearing surfaces, and these devices are tested in vitro with a lubricant of diluted calf serum. It is believed that the lubricant found in total disc replacement devices in vivo is interstitial fluid that may have properties between that in Ringer's solution and diluted calf serum. To investigate the effect of lubricants, a set of friction tests were performed on a generic model of a metal against metal ball-and-socket total disc replacement device. Two devices were tested: one with a ball radius of 10 mm and other with a ball radius of 16 mm; each device had a radial clearance of 0.015 mm. A spine simulator was used to measure frictional torque for each device in axial rotation, flexion-extension and lateral bending at frequencies of 0.25-2 Hz, under 1200 N axial load. Each device was tested with two different lubricants: a solution of new born calf serum diluted with deionised water and Ringer's solution. The results showed that the frictional torque generated between the bearing surfaces was significantly higher in Ringer's solution than in diluted calf serum. The use of Ringer's solution as a lubricant provides a stringent test condition to detect possible problems. Diluted calf serum is more likely to provide an environment closer to that in vivo. However, the precise properties of the fluid lubricating a total disc replacement device are not known; hence, tests using diluted calf serum may not necessarily give the same results as those obtained in vivo.

CoCrMo metal-on-metal hip replacements

After the rapid growth in the use of CoCrMo metal-on-metal hip replacements since the second generation was introduced circa 1990, metal-on-metal hip replacements have experienced a sharp decline in the last two years due to biocompatibility issues related to wear and corrosion products. Despite some excellent clinical results, the release of wear and corrosion debris and the adverse response of local tissues have been of great concern. There are many unknowns regarding how CoCrMo metal bearings interact with the human body. This perspective article is intended to outline some recent progresses in understanding wear and corrosion of metal-on-metal hip replacement both in vivo and in vitro. The materials, mechanical deformation, corrosion, wear-assisted corrosion, and wear products will be discussed. Possible adverse health effects caused by wear products will be briefly addressed, as well as some of the many open questions such as the detailed chemistry of corrosion, tribochemical reactions and the formation of graphitic layers. Nowadays we design almost routinely for high performance materials and lubricants for automobiles; humans are at least as important. It is worth remembering that a hip implant is often the difference between walking and leading a relatively normal life, and a wheelchair.

Tribological property of the cobalt—chromium femoral head with different regions of wear in total hip arthroplasty

The tribological properties of engineering and biological materials have been investigated at microscale levels through the calculation of the surface roughness and frictional coefficient using atomic force microscopy (AFM). Although a number of previous studies have reported the frictional coefficients of diverse bearing materials in total hip arthroplasty (THA), the relationship between the surface roughness and frictional coefficient of bearing materials of THA have not been reported, and furthermore, the tribological properties for different wear regions of a cobalt-chromium (Co—Cr) femoral head have not been well identified. Therefore, the objective of this study is to investigate the relationships between the surface roughness, frictional coefficient, and hardness for both the main-wear and the least-wear regions of a Co—Cr femoral head 10 years after THA. The average Vickers hardness of the Co—Cr femoral head was 380.7 ± 11.3 HV. With the scanned area of 25 μm×25 μm through AFM, the frictional coefficients of the main-wear and the least-wear regions were 0.229 ± 0.054 and 0.243 ± 0.059, respectively, and showed no statistical differences between these two regions ( p = 0.449). However, differences in the surface roughness ( Rq) between the main-wear region ( Rq = 96.5 ± 26.2 nm) and the least-wear region ( Rq = 17.7 ± 4.2 nm) were statistically significant ( p<0.0001). The results of the current study suggest that the frictional property of the Co—Cr femoral head is not significantly correlated with its surface roughness, and also provide guidelines for improving the surface characteristics of metallic implant materials.

The epidemiology of bearing surface usage in total hip arthroplasty in the United States

BACKGROUND

Hard-on-hard bearings offer the potential to improve the survivorship of total hip arthroplasty implants. However, the specific indications for the use of these advanced technologies remain controversial. The purpose of this study was to characterize the epidemiology of bearing surface utilization in total hip arthroplasty in the United States with respect to patient, hospital, geographic, and payer characteristics.

METHODS

The Nationwide Inpatient Sample database was used to analyze bearing type and demographic characteristics associated with 112,095 primary total hip arthroplasties performed in the United States between October 1, 2005, and December 31, 2006. The prevalence of each type of total hip arthroplasty bearing was calculated for population subgroups as a function of age, sex, census region, payer class, and hospital type.

RESULTS

The most commonly reported bearing was metal-on-polyethylene (51%) followed by metal-on-metal (35%) and ceramic-on-ceramic (14%). Metal-on-polyethylene bearings were most commonly reported in female Medicare patients who were sixty-five to seventy-four years old, while metal-on-metal and ceramic-on-ceramic bearings were most commonly reported in privately insured male patients who were less than sixty-five years old. Thirty-three percent of patients over sixty-five years old had a hard-on-hard bearing reported. There was substantial regional variation in bearing usage; the highest prevalence of metal-on-polyethylene bearings was reported in the Northeast and at nonteaching hospitals, and the highest prevalence of metal-on-metal bearings was reported in the South and at teaching hospitals.

CONCLUSIONS

The usage of total hip arthroplasty bearings varies considerably by patient characteristics, hospital type, and geographic location throughout the United States. Despite uncertain advantages in older patients, hard-on-hard bearings are commonly used in patients over the age of sixty-five years. Further study is necessary to define the appropriate indications for these advanced technologies in total hip arthroplasty.