Low Contact Stress Research Articles

Does the grading of chondromalacia patellae influence anterior knee pain following total knee arthroplasty without patellar resurfacing?

The influence of chondromalacia patellae (CMP) on post-operative anterior knee pain (AKP) following total knee arthroplasty (TKA) remains controversial, and few studies have focused on the relationship between them. The purpose of this study was to determine whether different CMP grades affect the incidence of AKP after TKA without patellar resurfacing. We performed a retrospective analysis of prospectively collected data on 290 TKAs with the use of the low contact stress mobile-bearing prosthesis, without patellar resurfacing in 290 patients from February 2009 to January 2013. Patients were assessed by the Outerbridge classification for CMP, visual analog scale for AKP, the Knee Society clinical scoring system of knee score (KS), function score (FS), the patellar score (PS) for clinical function, and patients' satisfaction. The intra-operative grading of CMP: grade I in 30 patients, grade II in 68 patients, grade III in 97 patients, and grade IV in 95 patients. The incidence of AKP at 36-month follow-up was 10.3% (30/290). No statistical difference was detected among the different CMP grades in terms of the incidence of AKP (p=0.995), patients' satisfaction (p=0.832), KS (p=0.228), FS (p=0.713), and PS (p=0.119) at 36-month follow-up. The findings may suggest no relevant influence of CMP grading on the incidence of AKP after TKA without patellar resurfacing.

Tribochemical Wear of Diamond-Like Carbon-Coated Atomic Force Microscope Tips.

Nanoscale wear is a critical issue that limits the performance of tip-based nanomanufacturing and nanometrology processes based on atomic force microscopy (AFM). Yet, a full scientific understanding of nanoscale wear processes remains in its infancy. It is therefore important to quantitatively understand the wear behavior of AFM tips. Tip wear is complex to understand due to adhesive forces and contact stresses that change substantially as the contact geometry evolves due to wear. Here, we present systematic characterization of the wear of commercial Si AFM tips coated with thin diamond-like carbon (DLC) coatings. Wear of DLC was measured as a function of external loading and sliding distance. Transmission electron microscopy imaging, AFM-based adhesion measurements, and tip geometry estimation via inverse imaging were used to assess nanoscale wear and the contact conditions over the course of the wear tests. Gradual wear of DLC with sliding was observed in the experiments, and the tips evolved from initial paraboloidal shapes to flattened geometries. The wear rate is observed to increase with the average contact stress, but does not follow the classical wear law of Archard. A wear model based on the transition state theory, which gives an Arrhenius relationship between wear rate and normal stress, fits the experimental data well for low mean contact stresses (<0.3 GPa), yet it fails to describe the wear at higher stresses. The wear behavior over the full range of stresses is well described by a recently proposed multibond wear model that exhibits a change from Archard-like behavior at high stresses to a transition state theory description at lower stresses.

Effect of out-of-phase loading on fretting fatigue response of Al7075-T6 under cyclic normal loading using a new testing apparatus

In the present study, the effect of phase difference between axial and normal contact loads on fretting behavior of Aluminum alloy 7075-T6 is investigated using a new testing and numerical simulation using finite element method. The results showed that the phase differences ϕ=90° and ϕ=180° improved fretting fatigue life about 8–30% and 12–56%, respectively. The reduction in crack propagation lifetime was about 12–16% and 26–39%, for phase differences of ϕ=90° and ϕ=180°, respectively. The reason for improvement of fretting fatigue life can be attributed to the increase in crack initiation lifetime that happens due to (i) domination of gross slip regime that occurs for both of phase differences ϕ=90° and ϕ=180° compared to partial slip regime that occurs for in-phase condition and (ii) lower contact stresses that occur under out-of-phase loading noting that the fraction of total lifetime spent in crack initiation phase is mainly influenced by the contact stresses.

New Stress-Based Fatigue Life Models for Ball and Roller Bearings

ABSTRACTNew stress-based life models are introduced to define “dynamic stress capacity” in rolling bearings for the first time. The generalized stress capacity equations are formulated, for both point and line contacts, in terms of distinct geometrical and materials parameters while the empirical constants are now material independent. Life equations are first developed for individual rolling element to race contacts and then statistically combined to estimate lives of both races, rolling elements, and, finally, the whole bearings for both ball and roller bearings. An estimate of the empirical constant for the ball bearing equation is derived by regression analysis of available experimental data. The applicable constant for roller bearings is then derived by relating the ball and roller bearing constants to the fundamental subsurface fatigue hypothesis applicable to both point and line contacts. For AISI 52100 bearing steel at room temperature, life predictions with the new stress-based equations are in complete agreement with those currently provided by widely used load-based formulations, where the empirical constant contains the elastic properties of AISI 52100 bearing steel. In addition to these life equations based on the magnitude and depth of maximum orthogonal subsurface shear stress and the volume of material stressed, a new model that eliminates life dependence on the depth of maximum orthogonal shear stress and relates life to only the subsurface maximum shear stress and the stressed volume is presented. Though the predicted life estimates with the currently used and newly introduced life models are comparable in the contact stress range of 2 to 3 GPa, the new model provides significantly higher lives at low contact stresses.

Roles of mirror-like surface finish and DLC coated piston rings on increasing scuffing resistance of cast iron cylinder liners

This study considers two approaches towards increasing the dry scuffing resistance of grey cast iron (CI) cylinder bore (or liner) surfaces, namely by i) decreasing the initial roughness of the CI liner surfaces and ii) by applying a diamond-like carbon (DLC) coating to the top compression piston rings. CI liners were polished to different surface roughness values ranging between 0.08µm and 6.83µm and were subjected to unlubricated reciprocating sliding tests with progressively increasing loads (50–600N) at a constant frequency of 5.0Hz (0.15m/s) using a Cameron-Plint TE77 tribometer. The counterface materials used were CrN coated steel as well as a hydrogenated DLC (H-DLC) coated steel. Microscopic and spectroscopic observations suggested that at low loads a mixed tribolayer comprising of iron oxide (Fe2O3) and graphitic carbon that was formed on wear tracks maintained low wear rates. At higher loads this tribolayer was removed resulting in metallic contact and leading to scuffing. Mirror-polished CI liner contact surfaces (with an average roughness of 0.08µm) provided higher scuffing resistance; the critical scuffing load was 300N compared to 200N for liners with >0.25µm average roughness. The improved scuffing resistance could be attributed to the lower contact stress experienced by the mirror-polished liners with a bearing ratio of 83% compared to medium-finished liners with a bearing ratio of 69%. CrN coated counterface contributed to scuffing of the CI liners at above 300 N load accompanied by a sharp increase in COF (>0.4). The H-DLC coated counterfaces prevented scuffing throughout and maintained a low steady state coefficient of friction (COF) of 0.13 up to 600N load. The low wear of the CI liners run against H-DLC counterfaces was attributed to the formation of the mixed tribolayer on the CI surfaces that remained intact at all tested loads. The results suggest that the use of DLC coated piston rings and a mirror-polish finish of cylinder liners would avert scuffing and provide low COF values while maintaining low wear rates.

A Study of the Tribological Behavior of TiAl-10 wt.%Ag Composite Based on the Contact Stress Evolution

The contact stresses of materials in moving mechanical assemblies are rather important for minimizing friction and decreasing machinery energy dissipation. In this study, the effect of contact stresses on the friction and wear behaviors of TiAl-10 wt.%Ag was studied at 0-240 min using the united methods of experiment research, numerical simulation and theoretical analysis. The results showed that the high contact stresses led to the plastic deformation of TiAl-10 wt.%Ag at 0-50 min. The friction coefficients and wear rates decreased rapidly with the lowering of contact stresses from 1034.28 to 710.52 MPa. At elastic deformation stage (50-240 min), the forming of lubricating film was beneficial to the decrease in friction coefficients and wear rates at 50-150 min, whereas the lower contact stresses caused the increasing of friction coefficients and wear rates. The competition between film lubrication and contact stresses caused the lowering of friction coefficients and wear rates at 50-150 min. At 150-240 min, the lubricating film maintained the equivalent lubricating behaviors, whereas the lowering of contact stresses led to the slight improving of friction coefficients and wear rates. This study was meaningful for optimizing applied loads to realize the excellent contact stress state and tribological behavior of mechanical parts.

Effect of contact stress on the cycle-dependent wear behavior of ceramic restoration

AimCeramic restoration experiences the non-linear wear process during the chewing simulation, which contains running-in, steady and severe wear stages. However since various levels of contact stress may be applied on the occlusal surface during chewing, the cycle-dependent wear behaviors of ceramic crowns may differ. The aim of this study was to investigate the effect of contact stress on the development of wear behavior, as tested in a chewing simulator. Materials and methodsThirty-six anatomical metal-ceramic crowns using Ceramco III as the veneering porcelain were randomly assigned to two groups based on the contact stress applied in the wear testing. Stainless steel balls served as antagonists. The specimens were dynamically loaded in a chewing simulator up to 2.4×106 loading cycles, with additional thermal cycling between 5 and 55℃. For each group, several checkpoints were employed to measure the substance loss of the crowns’ occlusal surfaces and to evaluate the microstructure of the worn areas. ResultsAfter 2.4×106 cycles, the ceramic restorations with lower contact stress demonstrated a long steady wear stage following the running-in, but without the severe wear stage. And a slowly microstructural degradation was observed that the subsurface defect could not be seen until final. With higher contact stress, however, the ceramic restorations experienced a faster transition from running-in to severe wear stage that the steady wear stage nearly disappeared. And an early formation of subsurface defects and the deterioration of microstructure were observed. ConclusionsContact stress is a key factor affecting the wear development of ceramic restoration. The higher contact stress promotes the veneering porcelain to evolve into severe wear stage. In contrast, lower contact stress is prone to keep the veneering porcelain operating in steady wear stage, which delays the arrival of severe wear region.

Different teeth profile shapes of polymer gears and comparison of their performance

This article presents a lifespan testing analysis of polymer gears manufactured by cutting. Compared to injection molding, machine cutting provides higher accuracy of gear geometry. Two different tooth flank geometries were tested; i.e. involute and S-gears. In theory, S-gears have several advantages over involute gears due to the convex/concave contact between the matching flanks. The theoretical tooth flank geometry of S-gears provides more rolling and less sliding between the matching flanks, compared to involute gears. The convex/concave contact leads to lower contact stress, which in combination with less sliding means lower losses due to sliding friction and consequently less heat generated. The goal of our research was to prove that tooth flank geometry affects the lifetime of polymer gears, and to find the mechanisms and quantitative differences in the performance of both analyzed geometries. The gears were tested on specially designed testing equipment, which allows exact adjustment of the central axis distance. Two different material pairs (POM/POM and POM/PA66) of the drive and driven gears were tested. Each test was done at a constant moment load and a constant rotational speed. Several tests were conducted using the same conditions due to repeatability analysis. All the tests were performed till the failure of the gear pair and without lubrication. In lifespan testing, the polymer S-gears showed better performance and longer lifespan than involute polymer gears. (Less)

Clinical Results of Low Contact Stress Mobile Bearing Total Knee Arthroplasty in Patients with Rheumatoid Arthritis –More Than Five Years of Evaluation

Clinical Results of Low Contact Stress Mobile Bearing Total Knee Arthroplasty in Patients with Rheumatoid Arthritis –More Than Five Years of Evaluation

Discussion: “A New Approach for Fatigue Damage Modeling of Subsurface-Initiated Spalling in Large Rolling Contacts” (Walvekar, A. A., Paulson, N., Sadeghi, F., Weinzapfel, N., Correns, M., and Dinkel, M., 2016, ASME J. Tribol. 139(1), p. 011101)

Professor Sadeghi and his coworkers have been developing a method to estimate the rolling contact fatigue life of rolling bearings in recent years using a continuum damage mechanics (CDM) approach. Development of the method has involved progression from a defect-free and inclusion-free model to ones incorporating grain boundary cracks and inclusions.The most recent paper on this topic is authored by Walvekar et al. [1]. In Sec. 2.2, the authors stated “It was hypothesised that failure mechanism for torsional fatigue and rolling contact fatigue are equivalent because the mechanism for damage accumulation is similar for both types of fatigue phenomena.” It is necessary to verify whether such a hypothesis is correct or not but this has not been carried out within the paper.This hypothesis was postulated by Raje and Sadeghi in 2009 in a paper published in the IMechE Journal of Engineering Tribology entitled “Statistical Numerical Modelling of Sub-Surface Initiated Spalling in Bearing Contacts” [2]. In Sec. 3, the authors stated “It is hypothesised that since fatigue damage under rolling contact conditions is caused purely by the action of shear stresses, the mechanism of failure is similar to that prevalent in torsional fatigue. Thus, by knowing the stress-life relationship for a material in torsional fatigue, the fatigue damage parameters can be extracted.”Torsional fatigue data on SAE 52100 bearing steel from Styri were used to provide the shear stress to failure [3], but Styri stated that in about 90% of cases, failures start below the surface, presumably at inclusions or other defects that act as stress concentrating features. However, the model being validated by Raje and Sadeghi is for a material free of defects, voids, or inclusions, while the steel used by Styri would be of 1940 s pedigree but with no statement of its cleanliness.Would the authors comment upon this observation, particularly as, intuitively, the torsional fatigue life of a pristine steel would be expected to be much longer than the one containing defects or inclusions while failures would be surface-initiated?Furthermore, results from the CDM model of pristine steel are compared to L10 bearing lives measured by Lundberg and Palmgren for NU 209 cylindrical roller bearings. The authors present a method in Sec. 4.3 for the conversion of L10 life in shaft revolutions to inner race stress cycles. A comparison of the model results and the bearing lives of Lundberg–Palmgren is then presented. The authors stated that “Stress-life results for this bearing are shown in Fig. 7 and are found to be in good agreement with stress-life results from the current model.”Would the authors comment upon this conclusion, taking into account that the results from the CDM model of pristine steel are being compared to bearings of 1940 s pedigree with no statement of cleanliness?In Sec. 4.2, the authors claim that the model is found to fit within the range of experimental data for cylindrical roller bearings reported by Harris and Barnsby [4]. Reference to this data appears to suggest that shaft revolutions have been plotted as opposed to stress cycles in Fig. 7. Would the authors confirm whether this is the case or not?Intuitively, a defect-free and inclusion-free model should predict lives that are much longer than both the experimental data from Harris–Barnsby and Lundberg–Palmgren. Once defects such as inclusions are introduced into the model, comparisons may then be made with the experimental data. It is necessary to model the size, number, shape, and composition of the inclusions (and other defects, such as carbide stringers) within the steels tested by Harris–Barnsby (1980 s/1990 s pedigree) and Lundberg–Palmgren (1940 s pedigree) in order to carry out such a comparison. Only at this point the claims can be made for validation (or otherwise) of the model. Would the authors comment upon this observation?It also appears that the CDM model has not been validated at lower contact stresses. It may be difficult to validate the model at contact stresses below 2200 MPa (the main region of interest for industrial machines) for the following reasons:Would the authors comment upon validation of the model at lower contact stresses?

How exactly can computer simulation predict the kinematics and contact status after TKA? Examination in individualized models

BackgroundIt is unknown whether a computer simulation with simple models can estimate individual in vivo knee kinematics, although some complex models have predicted the knee kinematics. The purposes of this study are first, to validate the accuracy of the computer simulation with our developed model during a squatting activity in a weight-bearing deep knee bend and then, to analyze the contact area and the contact stress of the tri-condylar implants for individual patients. MethodsWe compared the anteroposterior (AP) contact positions of medial and lateral condyles calculated by the computer simulation program with the positions measured from the fluoroscopic analysis for three implanted knees. Then the contact area and the stress including the third condyle were calculated individually using finite element (FE) analysis. FindingsThe motion patterns were similar in the simulation program and the fluoroscopic surveillance. Our developed model could nearly estimate the individual in vivo knee kinematics. The mean and maximum differences of the AP contact positions were 1.0mm and 2.5mm, respectively. At 120° of knee flexion, the contact area at the third condyle was wider than the both condyles. The mean maximum contact stress at the third condyle was lower than the both condyles at 90° and 120° of knee flexion. InterpretationIndividual bone models are required to estimate in vivo knee kinematics in our simple model. The tri-condylar implant seems to be safe for deep flexion activities due to the wide contact area and low contact stress.

A knee brace alters patella position in patellofemoral osteoarthritis: a study using weight bearing magnetic resonance imaging

To assess using weight bearing magnetic resonance imaging (MRIs), whether a patellar brace altered patellar position and alignment in patellofemoral joint (PFJ) osteoarthritis (OA). Subjects age 40-70 years old with symptomatic and a radiographic Kellgren-Lawrence (K-L) evidence of PFJOA. Weight bearing knee MRIs with and without a patellar brace were obtained using an upright open 0.25T scanner (G-Scan, Easote Biomedica, Italy). Five aspects of patellar position were measured: mediolateral alignment by the bisect offset index, angulation by patellar tilt, patellar height by patellar height ratio (patellar length/patellar tendon length), lateral patellofemoral (PF) contact area and finally a measurement of PF bony separation of the lateral patellar facet and the adjacent surface on the femoral trochlea (Fig.1). Thirty participants were recruited (mean age 57 SD 27.8; body mass index (BMI) 27.8 SD 4.2); 17 were females. Four patients had non-usable data. Main analysis used paired t tests comparing within subject patellar position with and without brace. For bisect offset index, patellar tilt and patellar height ratio there were no significant differences between the brace and no brace conditions. However, the brace increased lateral facet contact area (P=.04) and decreased lateral PF separation (P=.03). A patellar brace alters patellar position and increases contact area between the patella and femoral trochlea. These changes would lower contact stress at the PFJ. Such changes in patella position in weight bearing provide a possible biomechanical explanation for the success of the PFJ brace in clinical trials on PFJOA.

Posterior Shift of Contact Point between Femoral Component and Polyethylene in the LCS Rotating Platform Implant under Weight Bearing Condition.

PurposeTo analyze the contact mechanics of the femoral component and polyethylene of the Low Contact Stress rotating platform (LCS-RP) in nonweight bearing and weight bearing conditions using full flexion lateral radiographs.Materials and MethodsFrom May 2009 to December 2013, 58 knees in 41 patients diagnosed with osteoarthritis and treated with total knee arthroplasty (TKA) were included in this study. TKA was performed using an LCS-RP knee prosthesis. Full flexion lateral radiographs in both weight bearing and nonweight bearing condition were taken at least one month postoperatively (average, 28.8 months). Translation of femoral component was determined by the contact point between the femoral component and polyethylene. Maximum flexion was measured as the angle between the lines drawn at the midpoint of the femur and tibia.ResultsPosterior shift of the contact point in LCS-RP TKA was observed under weight bearing condition, which resulted in deeper flexion compared to LCS-RP TKA under nonweight bearing condition.ConclusionsIn the LCS-RP TKA, the contact point between the femoral component and polyethylene moved posteriorly under weight bearing condition, and the joint was more congruent and maximum flexion increased with weight bearing.

Rolling Contact Fatigue Tests of Ceramics by Various Methods: Comparison of Suitability to the Evaluation of Silicon Nitrides

AbstractResults of rolling contact fatigue (RCF) tests by balls-on-rod (BOR) and two-roller methods have been compared to those by the balls-on-flat (BOF) method by using three different silicon nitride grades. Two were bearing grade (BG) and one was a general purpose (GP) grade. For a GP material, there was not much difference between the results of BOR and those of BOF, and the observations of a spalled region by BOR suggested similar spalling behavior to BOF. Meanwhile, for BG materials, most tests were terminated without spalling, but grooves, whose likely cause is inferred to be insufficient lubrication, were formed. As for the lifetime in BOR, however, the results for BG materials were much longer than those of the GP, demonstrating that the BOR result is qualitatively consistent with that of BOF, where instead most specimens failed in RCF by spalling or peeling. In the two-roller method, tests of a limited number of specimens were operated at high slip ratios of 10 % during longer cycles under low contact stress, and wear was predominant, suggesting the potential of providing the information on wear behavior rather than RCF. The results suggest that the two-roller method could differentiate the BG materials in terms of wear behavior. Considering these results, both the BOF and the BOR are applicable to distinguish BG and GP materials in terms of fatigue resistance. In particular, the BOF is judged to be more suitable for comparison of the RCF behavior of BG materials, which are commonly tested under higher contact stresses.

Positive-Drive Cam Mechanisms With a Translating Follower Having Dual Concave Faces

This article proposes a cam mechanism with a translating follower that has dual concave faces, and also demonstrates how to design such a mechanism. This is a positive-drive cam mechanism because the dual concave faces of the follower can simultaneously contact the cam at any instant. The contact forces and the contact stresses of the mechanism are analyzed to illustrate the nature and the capability of reducing contact stress of this novel follower. This cam mechanism is found to have lower contact stress over the constant-breadth and the constant-diameter cam mechanisms.

Resultados clínicos y funcionales de la artroplastia total de rodilla Low Contact Stress (LCS®) con un mínimo de 5 años de seguimiento

IntroductionRotating-platform knee prosthesis were design to reduce contact stress of the implants and to increase the durability of implants. ObjectiveTo report the clinical outcomes of a consecutive series of patients who underwent primary rotating-platform knee arthroplasty, with a mean follow-up of 94 months. Material and methodFrom 2003 to 2010, 75 total knee replacements were performed with Low Contact Stress (LCS®) rotating-platform prosthesis. Painful knee osteoarthritis was the main diagnosis. Mean follow-up was 94 (60-118) months. Preoperatively, 64 knees had varus angulation (mean: 169°) and eleven knees had valgus angulation (mean: 187.8°). Patients were evaluated with the Hospital for Special Surgery (HSS) score at 1, 12, 36 and 60 months postoperatively. ResultsTwo intraoperative partial patella tendon avulsions (2.6%) were diagnosed. Six knees showed loosening of the tibial component (8%) and four (5.3%) required revision. Two chronic infections (2.6%) were successfully managed. No polyethylene dislocation (spin-out) was observed. The mean preoperative HSS score was 44.6 points, which improved to 86.9 points at the end of the follow-up (P<0.001). ConclusionOur study showed good survivorship and functional results of rotating-platform knee replacement at a mean follow-up of 94 months, similar to results of conventional fixed-bearing TKA. Level of evidenceIV.

Design of Cam Mechanism with Negative Radius Roller-Follower

Cam mechanisms with a negative radius roller-follower allow the design of simple machines capable of exerting remarkably high forces. Furthermore, the concave-convex nature of the contact surface between the cam and the follower results in a low contact stress and the ready formation of a lubricating layer. This paper presents a simple method for the design and analysis of a cam mechanism with a negative radius roller-follower. In the proposed approach, conjugate surface theory is employed to derive a kinematic model of the cam mechanism. Analytical expressions for the pressure angle and principal curvatures of the cam profile are then derived. The validity of the proposed design methodology is demonstrated by machining a cam mechanism having a negative radius rollerfollower with a radius of 100 mm.

Potential risk of excising the femoral insertion of the popliteus tendon during primary total knee arthroplasty: a biometric study

BackgroundDuring total knee arthroplasty (TKA), excision of the popliteus tendon leads to extensive static gaps and reduced mobility. The purpose of this study was to determine the positional relationship between the femoral insertion of the popliteus tendon and the bone cutting lines of various TKA systems. MethodsThis study included 21 cadaveric right femurs presenting no macroscopic deformity. The lateral image of the femur and the template of the femoral component were overlaid to determine the preservation/excision of the popliteus tendon insertion. TKA systems used were Genesis II, NexGen, low contact stress (LCS), PFC Σ, Scorpio, and Vanguard. The knees in which the insertion was preserved in all implants or excised in at least one implant were classified into intact or the high-risk groups, respectively. ResultsThe popliteus tendon was preserved in all specimens with the LCS system. In contrast, the popliteus tendon insertion was excised in ≥1/3 of the specimens with the other systems. The anteroposterior diameter was significantly larger in the intact group than that in the high-risk group (58.1±4.5mm vs. 53.7±2.7mm; p=0.018). The high-risk group included more knees from female cadavers than the intact group (70 vs. 9%; p=0.008). ConclusionDuring primary TKA, the femoral insertion of the popliteus tendon could be inevitably excised, regardless of technical problems. We demonstrate that the unique design of the LCS system preserves the popliteus tendon insertion. In addition, small knees and females may be risk factors for excision of the insertion.

Mobility of the rotating platform in low contact stress knee arthroplasty is durable.

The mobile bearing or rotating platform (RP) in total knee arthroplasty (TKA) is originally part of a low contact stress (LCS) concept, with bearing undersurface mobility compensating higher bearing upper-surface congruency. The in vivo range of axial femorotibial rotation in RP knees has been the subject of many studies, but always involving the performance of relatively low demanding task conditions. Hardly any study has addressed the maintenance of this rotation over time. Two consecutive series of patients with LCS RP knees were studied in a cross-sectional study of 1- and 5-year follow-up. They were assessed using optoelectronic movement analysis during gait and the performance of a sit-to-walk (STW) task with and without turning steps. A mean range of rotation (SD) was found in the 1-year group of 13.4° (3.7) during gait, 17.8° (6.8) during STW straight, and 17.9° (6.9) during STW with turning. The range in the 5-year group was 11.2° (6.0) during gait, 18.5° (8.7) during STW straight, and 18.3° (8.3) during STW with turning. A so-called paradoxical axial rotation pattern during gait and STW straight occurred in both groups in a normal prevalence. The amount and pattern of rotation in a LCS RP knee does not become impaired between 1 and 5years postoperatively. The theoretical benefit of RP TKA has not been proven in any clinical study so far, and studies with suitable long-term follow-up need to prove whether this mobility also leads to improved prosthesis survival. However, our findings support the functioning of the rotating platform at a basal science level and illustrate the need for the use of more complex tasks in kinematic studies. Therapeutic study, Level III.

Transgranular liquation cracking of grains in the semi-solid state.

Grain refinement via semi-solid deformation is desired to obtain superior mechanical properties of cast components. Using quantitative in situ synchrotron X-ray tomographic microscopy, we show an additional mechanism for the reduction of grain size, via liquation assisted transgranular cracking of semi-solid globular microstructures. Here we perform localized indentation of Al-15wt.%Cu globular microstructures, with an average grain size of ∼480 μm, at 555 °C (74% solid fraction). Although transgranular fracture has been observed in brittle materials, our results show transgranular fracture can also occur in metallic alloys in semi-solid state. This transgranular liquation cracking (TLC) occurs at very low contact stresses (between 1.1 and 38 MPa). With increasing strain, TLC continues to refine the size of the microstructure until the grain distribution reaches log-normal packing. The results demonstrate that this refinement, previously attributed to fragmentation of secondary arms by melt-shearing, is also controlled by an additional TLC mechanism.