In vitro investigation of friction under edge‐loading conditions for ceramic‐on‐ceramic total hip prosthesis

Edge‐loading generates higher wear rates in ceramic‐on‐ceramic total hip prosthesis (THP). To investigate the friction coefficient (FC) in these conditions, three alumina ceramic (Biolox Forte) 32 mm‐diameter components were tested using a hip friction simulator. The cup was positioned with a 75° abduction angle to achieve edge‐loading conditions. The motion was first applied along the edge and then across the edge of the cup. First, tests were conducted under lubricated conditions with 25% bovine serum. Next, to simulate an extremely high contact pressure, the tests were run with the addition of a third body alumina ceramic chip inserted between the edge of the cup and the head. Engineering blue was used to analyze the contact area. Reference values were determined using a 0° cup abduction angle. Edge loading was achieved. The FC increased by three‐ to sixfold when the motion was applied along the edge, and by 70% when the motion was applied across the edge. However, the FC value was still low (about 0.1), which is similar to metal‐on‐metal THP. With the third body alumina ceramic particle inserted, the FC was 26 times higher than in the ideal conditions and intermittent squeaking occurred. High cup abduction angles may generate edge‐loading and an increase in the friction coefficient for ceramic THP. © 2010 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 28:979–985, 2010     

Spectral characterization of squeaking in ceramic‐on‐ceramic total hip arthroplasty: Comparison of in vitro and in vivo values

Squeaking is a cause for concern in total hip arthroplasty (THA). However, the definition of this sound remains vague. Our goals were to characterize spectrally the squeaking in vitro under lubricated conditions and to compare the in vitro frequencies to in vivo frequencies. Four patients, who presented with a squeaking after ceramic‐on‐ceramic THA were investigated. In addition, 3 alumina ceramic 32 mm diameter components were tested in vitro under edge loading conditions using a friction simulator. Tests were conducted under lubricated conditions without and with the addition of a third body alumina particle. In vitro, no squeaking occurred under edge loading conditions. However, with the addition of a third body particle in the contact region, squeaking was obtained; the dominant frequency was 2.6 kHz. In vivo, the main recorded sound frequencies were: 2.3, 2.24, and 2.46 kHz for squeakers during walking and 1.45 kHz for a squeaker rising from a bent position. The in vivo recordings had a dominant frequency similar to, but slightly lower than, the in vitro values. Two types of squeaking were found in vivo and characterized spectrally. The rising squeaker had intermittent squeaking corresponding to a lower fundamental frequency compared to the walking squeakers. This finding suggests two different mechanism of squeaking. © 2011 Orthopaedic Research Society Published by Wiley Periodicals, Inc. J Orthop Res 30:185–189, 2012     

Edge‐loading severity as a function of cup lip radius in metal‐on‐metal total hips—A finite element analysis

While favorable tribological properties and allowance for larger femoral head sizes have made metal‐on‐metal (MoM) bearings an increasingly popular choice for total hip arthroplasty, concerns have mounted regarding adverse reactions to metal wear debris and ions. MoM cups differ from conventional polyethylene cups in terms of edge profile design and reductions from full hemisphericity, suggesting differences in loading at or near the cup edge, especially during subluxation events. Finite element analysis was used to investigate the effects of cup orientation and lip edge curvature on damage propensity for edge or near‐edge loading during subluxation. Increased cup lip radius (resulting in reduced articular arc) had a detrimental effect upon subluxation‐free hip range of motion and upon dislocation resistance. Contact stresses near the cup edge demonstrated complex relationships between edge radius and cup orientation, with peak stresses being influenced by both variables. The tendency for scraping wear at the egress site demonstrated similarly complex dependencies. These data indicate that acetabular cup design is an important determinant of edge and near‐edge loading damage propensity. © 2011 Orthopaedic Research Society Published by Wiley Periodicals, Inc. J Orthop Res 30:169–177, 2012     

Predicting wear and blood metal ion levels in metal‐on‐metal hip resurfacing

Suboptimal component position and design are thought to lead to edge wear and raised blood metal ion levels in metal‐on‐metal hip resurfacing (MOM‐HR). These factors are thought to influence the “contact patch to rim distance” (CPRD), and calculation of this distance may improve prediction of wear and blood metal ion levels. We measured blood cobalt and chromium ion levels and the wear rates of the bearing surfaces in 165 MOM‐HR retrieval cases. We then determined the contribution and effect sizes of cup inclination and version angles, component size and design, and CPRD (calculated from case‐specific data) on blood metal ion levels and component wear rates. Acetabular orientation explained between 16.3% and 28.5% of the variation in wear rates and metal ion levels, whereas component size and design explained between 7.3% and 21.8% of the variability. In comparison, CPRD explained up to 67.7% of the variability, significantly greater than any other variable (all p < 0.0001). CPRD is a good predictor of wear and improves our understanding of wear performance and the mechanisms leading to edge loading. © 2013 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 32:167–174, 2014.     

Influence of polyethylene creep behavior on wear in total hip arthroplasty.

After total hip arthroplasty (THA), polyethylene acetabular liner creep occurs quickly and serves to increases head-liner contact area and decrease contact pressures. What effect these early changes in contact mechanics will have on the wear behavior of the articulation remains unclear, and hence, selection or modification of polyethylene materials for optimal creep and wear performance is impossible. The objective of this study was to determine the influence of polyethylene creep behavior on volumetric wear and linear creep and wear penetration during simulated gait loading conditions. A finite element model of THA articulation was developed, and simultaneous numerical creep and wear simulation was performed to 10 million gait cycles with three levels of polyethylene creep behavior. Long-term volumetric wear and penetration were surprisingly unaffected by the polyethylene creep behavior due to the competing decrease in contact pressures coupled with increased contact area. In addition, variation in contact mechanics with the creep levels studied was only noteworthy in the initial postoperative period; after 1 million gait cycles, peak contact pressures and areas were within 13% regardless of the creep material behavior selected. Femoral head size had considerable impact on wear and penetration, while liner thickness primarily affected only early penetration. These results suggest that polyethylene creep behavior plays a major role in early penetration, but has little influence on the more important long-term volumetric wear.     

Thin hard crest on the edge of ceramic acetabular liners accelerates wear in edge loading

Ceramic acetabular liners may exhibit a small, sharp crest—an artifact of discontinuous machining steps—at the junction between the concave spherical surface and the interior edge. On 3 ceramic liners, this crest was found to form a 9° to 11° deviation from tangency. Edge loading wear tests were conducted directly on this crest and on a smoother region of the edge. The crest elicited 2 to 15 times greater volumetric wear on the femoral head. The propensity of the crest to rapidly (<2000 wear cycles) cause elevated wear under low contact force (200 N) suggests that the crest artifact of prevailing machining protocols might be a root cause of stripe wear and squeaking in ceramic acetabular bearings.     

Hip abduction can prevent posterior edge loading of hip replacements

Edge loading causes clinical problems for hard‐on‐hard hip replacements, and edge loading wear scars are present on the majority of retrieved components. We asked the question: are the lines of action of hip joint muscles such that edge loading can occur in a well‐designed, well‐positioned acetabular cup? A musculoskeletal model, based on cadaveric lower limb geometry, was used to calculate for each muscle, in every position within the complete range of motion, whether its contraction would safely pull the femoral head into the cup or contribute to edge loading. The results show that all the muscles that insert into the distal femur, patella, or tibia could cause edge loading of a well‐positioned cup when the hip is in deep flexion. Patients frequently use distally inserting muscles for movements requiring deep hip flexion, such as sit‐to‐stand. Importantly, the results, which are supported by in vivo data and clinical findings, also show that risk of edge loading is dramatically reduced by combining deep hip flexion with hip abduction. Patients, including those with sub‐optimally positioned cups, may be able to reduce the prevalence of edge loading by rising from chairs or stooping with the hip abducted. © 2013 Orthopaedic Research Society Published by Wiley Periodicals, Inc. J Orthop Res 31:1172–1179, 2013

     

The effect of structural parameters of total hip arthroplasty on polyethylene liner wear behavior: A theoretical model analysis

Using large femoral heads in total hip arthroplasty (THA) has been widely advocated to improve the function and longevity of the components. However, increasing the head size has been shown to accelerate polyethylene liner wear. Few studies have investigated the effect of other important structural parameters (such as polyethylene liner thickness, metal cup size, head-liner conformity, loading conditions, etc.) on the biomechanical functions of the THAs. In this study, an analytical model was used to evaluate the polyethylene liner wear characteristics of the THAs (defined using a biomechanical wear factor) with various structural parameters of the THAs and loading conditions. For all the THA systems examined in this study, under the same loading conditions, a larger head leads to increasing contact areas, lower contact stresses, and higher biomechanical wear factors. When the head size is fixed, a decrease in the polyethylene liner thickness or a decrease in the head-liner conformity leads to higher peak contact stresses and smaller contact areas and consequently, lower biomechanical wear factors. This study provides a parametric analysis tool for the optimal design/selection of the THA systems and for prediction of early effects of various structural parameters on the biomechanical function (such as contact stresses) and longevity (such as polyethylene liner wear) of the THA systems.     

Squeaking in ceramic‐on‐ceramic hips: No evidence of contribution from the trunnion morse taper

Squeaking in ceramic‐on‐ceramic (CoC) total hip arthroplasty (THA) was investigated with an acoustic monitoring device to distinguish between squeaking emanating from the trunnion morse taper (TMT) connection versus the articular surface. 82 patients with implant problems scheduled for revision were selected and acoustic emissions (AE) with simple movements monitored. Five of these patients with CoC implants underwent surgery and their retrieved implant components were analyzed in vitro. In vivo recordings of all 82 patients found audible squeaks produce frequency content across the entire 0–50 kHz spectrum. Of the five CoC implants the in vivo peak frequency range of flexion/extension motions was 0.8 kHz and comparable to the range of in vitro testing (0.2 kHz). In vitro TMT connection motions were very large comparatively, producing a higher average peak frequency range of 22.5 kHz. All retrieved implants showed evidence of wear at the TMT connection. These findings suggest, the TMT connection does not directly cause audible squeaking. Wear occurring at this junction may contribute to debris which disrupt lubrication, increase friction, and produce noise. This observation supports current evidence suggesting that squeaking is from the bearing surface. Our findings are the first to demonstrate that in CoC THA the recordable noise of a hip squeak does not originate nor have contribution from the TMT connection. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:1793–1798, 2017.

     

Dissociation of Acetabular Polyethylene Liners With a Morse Taper Design

BackgroundPolyethylene liner dissociation is an uncommon complication of hip replacement. Dissociation has been associated with particular acetabular component designs. This study reviewed acetabular liner dissociations in a specific modular cup with a Morse taper locking mechanism that has not been previously reported.MethodsThe senior author performed 655 primary total hip arthroplasties with one particular design of acetabular component using Class A polyethylene liners and metal head articulation. Cases with revision surgery performed for acetabular liner dissociation were reviewed.ResultsSeven of 655 patients with this cup underwent revision surgery for a dissociated liner. Liner dissociation occurred at a mean of 73 months postoperatively. Patients presented with new-onset hip pain or squeaking, 4 of which developed symptoms acutely. Two patients treated with polyethylene liner exchange into the same cup required a second revision surgery for recurrent dissociation.ConclusionPolyethylene liner dissociation is an infrequent but possible complication associated with modular acetabular components using a Morse taper locking. Providers should be vigilant with long-term follow-up of patients with this acetabular system for patient complaints of catching or squeaking. Patients treated for liner dissociation should not have a new liner placed into the same acetabular shell given the risk for further dissociation.     

Wear resistance of the biocompatible phospholipid polymer‐grafted highly cross‐linked polyethylene liner against larger femoral head

The use of larger femoral heads to prevent the dislocation of artificial hip joints has recently become more common. However, concerns about the subsequent use of thinner polyethylene liners and their effects on wear rate have arisen. Previously, we prepared and evaluated the biological and mechanical effects of a novel highly cross‐linked polyethylene (CLPE) liner with a nanometer‐scaled graft layer of poly(2‐methacryloyloxyethyl phosphorylcholine) (PMPC). Our findings showed that the PMPC‐grafted particles were biologically inert and caused no subsequent bone resorptive responses and that the PMPC‐grafting markedly decreased wear in a hip joint simulator. However, the metal or ceramic femoral heads used in this previous study had a diameter of 26 mm. Here, we investigated the wear‐resistance of the PMPC‐grafted CLPE liner with a 40‐mm femoral head during 10 × 10⁶ cycles of loading in the hip joint simulator. The results provide preliminary evidence that the grafting markedly decreased gravimetric wear rate and the volume of wear particles, even when coupled with larger femoral heads. Thus, we believe the PMPC‐grafting will prolong artificial hip joint longevity both by preventing aseptic loosening and by improving the stability of articular surface. © 2015 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 33:1103–1110, 2015.     

Gait alterations can reduce the risk of edge loading

Following metal‐on‐metal hip arthroplasty, edge loading (i.e., loading near the edge of a prosthesis cup) can increase wear and lead to early revision. The position and coverage angle of the prosthesis cup influence the risk of edge loading. This study investigates the effect of altered gait patterns, more specific hip, and pelvis kinematics, on the orientation of hip contact force and the consequent risk of antero‐superior edge loading using muscle driven simulations of gait. With a cup orientation of 25° anteversion and 50° inclination and a coverage angle of 168°, many gait patterns presented risk of edge loading. Specifically at terminal double support, 189 out of 405 gait patterns indicated a risk of edge loading. At this time instant, the high hip contact forces and the proximity of the hip contact force to the edge of the cup indicated the likelihood of the occurrence of edge loading. Although the cup position contributed most to edge loading, altering kinematics considerably influenced the risk of edge loading. Increased hip abduction, resulting in decreasing hip contact force magnitude, and decreased hip extension, resulting in decreased risk on edge loading, are gait strategies that could prevent edge loading. © 2015 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 34:1069–1076, 2016.     

Assessment of polyethylene wear in total hip replacement

The three-dimensional technique is a method for the measurement of polyethylene wear in patients with total hip joint replacement. Application of image processing technology allows automation of point selection from digital images of radiographs scanned into the computer. Validation of image processing modifications reveals a three-fold increase in accuracy and a 40-fold increase in reproducibility compared with manual input of points from a digitizer during bench testing. A review of three-dimensional technique application to clinical patients gives information on the factors that influence polyethylene wear. Increasing age, activity level, femoral head size, decreasing polyethylene thickness, and insertion of total hip prostheses without cement all increase polyethylene wear. Restoration of femoral offset during total hip replacement seems to decreases polyethylene wear. No apparent difference in polyethylene wear rate could be found between two groups of patients, one group had a stainless steel-polyethylene articulation and the other had a ceramic-polyethylene articulation. Measurement of the serial polyethylene wear of individual patients reveals a high rate of femoral head penetration during the first 2 years after total hip replacement using metal-backed acetabular components inserted without cement. Interpretation of this femoral head penetration as true polyethylene wear may be erroneous, however, because creep of the polyethylene and acetabular liner movement within its metal shell cannot be measured.     

Dual poly liner mobility optimizes wear and stability in THA: opposes

Total hip arthroplasty (THA) is an effective intervention for the treatment of arthrosis with excellent survivorship. Nonetheless, dislocation and osteolysis remain significant complications. A dual-mobility acetabular component has been advocated to improve stability and wear. Stability is imparted by increasing the effective femoral head size, which allows a larger range of motion (ROM) before neck-socket impingement occurs. Increasing ROM, however, introduces an additional problem of bony impingement of the trochanter against the pelvis. Consequently, there is little improvement in ROM for heads >36 to 40 mm. A 0.4% incidence of instability at the larger articulation has been reported in primary THA using the dual-mobility liner, which is equivalent to reports for conventional THA. The dual-mobility liner has introduced the unique complication of intraprosthetic dislocation, where the femoral head dissociates at the smaller articulation as a result of polyethylene wear. An incidence of intraprosthetic dislocation of 3.6% has been reported, which far exceeds dislocation of conventional arthroplasty. The dual-mobility liner is a monoblock acetabular component without the capacity for augmented bony fixation. Inability to achieve primary stability has been reported as high as 18% and therefore its usefulness in revision THA is questionable. Proponents of the dual-mobility liner cite improved wear characteristics over conventional THA; however, few studies support this proposition. Retrieval studies have reported that the dual-mobility liner does not avoid wear or osteolysis. Theoretically, it is inconceivable that wear would be diminished with an additional articulation with a huge surface area, where the differential hardness has been reversed to a soft-on-hard bearing.     

Mismatched wear couple zirconium oxide and aluminum oxide in total hip arthroplasty

A patient complained about a squeaking noise in his total hip arthroplasty. Clinical evaluation revealed good function, and there were no signs of loosening on the radiograph. Physiotherapy did not alter this phenomenon, and ultimately a revision was performed 42 months after the first surgery. The analysis of the retrievals revealed that a zirconium oxide ceramic head had been paired with a monolithic alumina ceramic cup. The cup showed large deviations from an ideal sphere but minor wear signs. The head exhibited heavy local damage in the articulation zone. This damage might have been caused by the observed unsatisfactory fit between cup and ball, resulting in high stress concentrations and increased wear of the zirconium head. The characteristics of the zirconium and aluminum ceramics pairing might have worsened the process. The combination of implants used in this retrieved wear couple was never approved. To prevent such problems, components of different manufacturers should never be mixed and matched unless explicitly stated.     

Center index method—an alternative for wear measurements with radiostereometry (RSA)

Radiostereometry (RSA) is considered to be the most precise and accurate method for wear‐measurements in total hip replacement. Post‐operative stereoradiographs has so far been necessary for wear measurement. Hence, the use of RSA has been limited to studies planned for RSA measurements. We compared a new RSA method for wear measurements that does not require previous radiographs with conventional RSA. Instead of comparing present stereoradiographs with post‐operative ones, we developed a method for calculating the post‐operative position of the center of the femoral head on the present examination and using this as the index measurement. We compared this alternative method to conventional RSA in 27 hips in an ongoing RSA study. We found a high degree of agreement between the methods for both mean proximal (1.19 mm vs. 1.14 mm) and mean 3D wear (1.52 mm vs. 1.44 mm) after 10 years. Intraclass correlation coefficients (ICC) were 0.958 and 0.955, respectively (p < 0.001 for both ICCs). The results were also within the limits of agreement when plotted subject‐by‐subject in a Bland–Altman plot. Our alternative method for wear measurements with RSA offers comparable results to conventional RSA measurements. It allows precise wear measurements without previous radiological examinations. © 2012 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 31: 480–484, 2013     

人工全髋聚乙烯假体磨损及其生物学效应

目的 探讨人工关节聚乙烯磨损与假体松动的关系。方法 采用光镜与电镜对15例人工髋关节翻修病例的聚乙烯假体臼杯内表面及假体周围软组织作形态学观察。进而在动物实验中观察微小聚乙烯磨损颗粒促使骨吸收性细胞因子增加，植入物周围骨溶解以及植入物-骨整合强度下降的生物效应。结果 聚乙烯假体在人工关节松动形成时均明显磨损，聚乙烯磨损颗粒诱发机体单核-巨噬细胞增殖并分泌骨吸收性细胞因子。结论 聚乙烯磨损与假体松动密度相关。其中生物因素是引起假体松动的主要途径。     

Influence of Acetabular Cup Rim Design on the Contact Stress During Edge Loading in Ceramic-on-Ceramic Hip Prostheses

The purpose of the study was to investigate the contact stresses in 3 different acetabular cup rim designs (new, worn, chamfer) during edge loading, after microseparation of ceramic on ceramic hip prostheses. A 3-dimensional finite element analysis was conducted for a 28-mm diameter alumina ceramic bearing with a radial clearance of 40 μm using a normal load of 2500 N under edge loading. At a separation distance of 250 μm, the maximum tensile stress in the “new” design was of similar magnitude to the flexural strength of the alumina material that supports the localized breakdown (stripe wear) of the acetabular cup surface observed clinically. Introducing a 2.5-mm radius chamfer should reduce the maximum tensile stress in the region of 60%.