Physiological Loading Research Articles

Analysis of the Feasibility of the OrthoNail Hybrid Intramedullary Implant in the Human Body with Respect to Material Durability

This study focuses on the development and evaluation of the OrthoNail hybrid intramedullary implant for lower limb lengthening in patients requiring significant skeletal reconstruction. The implant addresses the challenges in load-bearing during rehabilitation, providing a robust solution that is capable of supporting physiological loads. Mechanical tests, including axial compression, tension, torsion, and 3,4-point bending, determined the implant’s load capacity and fatigue resistance, while finite element analysis assessed stress distributions in bone tissue and around screw holes during single-leg stance, with boundary conditions derived from Orthoload database data. The OrthoNail implant demonstrated excellent mechanical stability, sustaining torsional loads of up to 19.36 Nm at maximum elongation (80 mm) and 17.16 Nm at zero elongation. Under axial compression, it withstood forces of up to 1400 N, maintaining structural integrity. Fatigue testing revealed resilience under dynamic loading conditions for over 1,000,000 cycles at a load of 500 N, with no mechanical failure or material degradation observed. Stress concentrations near screw holes indicate areas for potential optimization. The findings indicate that the OrthoNail implant demonstrates excellent mechanical stability and is well-suited for clinical application, enabling early full weight-bearing during rehabilitation.

Multiplanar Semicircular New-Generation Implant System Developed for Proximal Femur Periprosthetic Fractures: A Biomechanical Study

Background and Objectives: The study aimed to evaluate a newly designed semicircular implant for the fixation of Vancouver Type B1 periprosthetic femoral fractures (PFFs) in total hip arthroplasty (THA) patients. To determine its strength and clinical applicability, the new implant was compared biomechanically with conventional fixation methods, such as lateral locking plate fixation and a plate combined with cerclage wires. Materials and Methods: Fifteen synthetic femur models were used in this biomechanical study. A Vancouver Type B1 periprosthetic fracture was simulated by osteotomy 5 mm distal to the femoral stem. The models were divided into three groups: Group I (lateral locking plate fixation), Group II (lateral locking plate with cerclage wires), and Group III (new semicircular implant system). All fixation methods were subjected to axial loading, lateral bending, and torsional force testing using an MTS biomechanical testing device. Failure load and displacement were measured to assess stability. Results: The semicircular implant (Group III) demonstrated a significantly higher failure load (778.8 ± 74.089 N) compared to the lateral plate (Group I: 467 ± 68.165 N) and plate with cerclage wires (Group II: 652.4 ± 65.474 N; p < 0.001). The new implant also exhibited superior stability under axial, lateral bending, and torsional forces. The failure load for Group III was more robust, with fractures occurring at the screw level rather than plate or screw detachment. Conclusions: Compared to traditional fixation methods, the newly designed semicircular implant demonstrated superior biomechanical performance in stabilizing Vancouver Type B1 periprosthetic femoral fractures. It withstood higher physiological loads, offered better structural stability, and could be an alternative to existing fixation systems in clinical practice. Further studies, including cadaveric and in vivo trials, are recommended to confirm these results and assess the long-term clinical outcomes.

Topology optimization and biomechanical evaluation of bone plates for tibial bone fractures considering bone healing

ABSTRACT Implant designs highly influence their biomechanical performances when fixed with load-bearing long bone fractures. In this research work, the topology optimisation technique was used to obtain different shapes and designs of the bone plates according to three different loadings, e.g. lateral bending (LB), axial compression (AC), and physiological loads (PL), and solid volume fractions Vf of 30% and 70%. Bi-phasic mechano-regulation algorithm was used to investigate the callus healing for a given bone plate design, and stresses in screws and bone plates were monitored. To further validate the bone plate designs, fatigue analyses using Fe-safe and three-point bending tests were performed using additively manufactured plates. Topology-optimised bone plate PL with Vf 70% showed the maximum bending stiffness (peak load of 138 N and bending stiffness of 29 N/mm) among the optimised bone plates, with the best callus healing normalised stiffness of 0.6 and 0.7 in iterations 21 and 42, respectively. Thus, the bone plates produced using actual loading conditions (PL) outperformed other loading conditions during the biomechanical evaluation of fractured bones.

Causes and Prevention Measures of Muscle Strain in Track and Field Athletes

The incidence of muscle strain is mainly due to the athletes' exercise intensity is large, and the symptoms of muscle strain often appear. According to the combination of the data in this paper, the incidence of muscle strain is high due to the high intensity of athletics and the large physiological load, and the muscle strain is still increasing gradually. Muscle strain will not only affect the athletes' training and competition results, the improper handling will affect the recovery of injured muscle function, and serious disability can be left behind. This paper mainly combines the incidence rate, main occurrence items and main causes of muscle strain caused by universities and athletes, as well as athletes' physical fitness, physical condition, training plan and other issues, to prevent and intervene in muscle strain, so as to reduce the probability of muscle strain of athletes and strengthen coaches' training arrangements and training plans. And to further ensure the health of athletes.

Biomechanics of Osseointegration of a Dental Implant in the Mandible Under Shock Wave Therapy: In Silico Study.

The most time-consuming aspect of dental prosthesis installation is the osseointegration of a metal implant with bone tissue. The acceleration of this process may be achieved through the use of extracorporeal shock wave therapy. The objective of this study is to investigate the conditions for osseointegration of the second premolar implant in the mandibular segment through the use of a poroelastic model implemented in the movable cellular automaton method. The mandibular segment under consideration includes a spongy tissue layer, 600 µm in thickness, covered with a cortical layer, 400 µm in thickness, and a gum layer, 400 µm in thickness. Furthermore, the periodontal layers of the roots of the first premolar and first molar were considered, while the implant of the second premolar was situated within a shell of specific tissue that corresponded to the phase of osseointegration. The model was subjected to both physiological loading and shock wave loading across the three main phases of osseointegration. The resulting fields of hydrostatic pressure and interstitial fluid pressure were then subjected to analysis in accordance with the mechanobiological principles. The results obtained have indicated that low-intensity shock wave therapy can accelerate and promote direct osseointegration: 0.05-0.15 mJ/mm2 in the first and second phases and less than 0.05 mJ/mm2 in the third phase. In comparison to physiological loads (when bone tissue regeneration conditions are observed only around the implant distal end), shock waves offer the primary advantage of creating conditions conducive to osseointegration along the entire surface of the implant simultaneously. This can significantly influence the rate of implant integration during the initial osteoinduction phase and, most crucially, during the longest final phase of bone remodeling.

Mechanical Stretch Control of Adipocyte AKT Signaling and the Role of FAK and ROCK Mechanosensors.

Adipose tissue in vivo is physiologically exposed to compound mechanical loading due to bodyweight bearing, posture, and motion. The capability of adipocytes to sense and respond to mechanical loading milieus to influence metabolic functions may provide a new insight into obesity and metabolic diseases such as type 2 diabetes (T2D). Here, we evidenced physiological mechanical loading control of adipocyte insulin signaling cascades. We exposed differentiated 3T3-L1 adipocytes to mechanical stretching and assessed key markers of insulin signaling, AKT activation, and GLUT4 translocation, required for glucose uptake. We showed that cyclic stretch loading at 5% strain and 1 Hz frequency increases AKT phosphorylation and GLUT4 translocation to the plasma membrane by approximately two-fold increases compared to unstretched controls for both markers as assessed by immunoblotting (p < 0.05). These results indicate that cyclic stretching activates insulin signaling and GLUT4 trafficking in adipocytes. In the mechanosensing mechanism study, focal adhesion kinase (FAK) inhibitor (FAK14) and RhoA kinase (ROCK) inhibitor (Y-27632) impaired actin cytoskeleton structural formation and significantly suppressed the stretch induction of AKT phosphorylation in adipocytes (p < 0.001). This suggests the regulatory role of focal adhesion and cytoskeletal mechanosensing in adipocyte insulin signaling under stretch loading. Our finding on the impact of mechanical stretch loading on key insulin signaling effectors in differentiated adipocytes and the mediatory role of focal adhesion and cytoskeleton mechanosensors is the first of its kind to our knowledge. This may suggest a therapeutic potential of mechanical loading cue in improving conditions of obesity and T2D. For instance, cyclic mechanical stretch loading of adipose tissue could be explored as a tool to improve insulin sensitivity in patients with obesity and T2D, and the mediatory mechanosensors such as FAK and ROCK may be targeted to further invigorate stretch-induced insulin signaling activation.

Straight and helical plating with locking plates for proximal humeral shaft fractures- a biomechanical comparison under physiological load conditions.

Helical plating is an established method for treating proximal humeral shaft fractures, mitigating the risk of iatrogenic radial nerve damage. However, biomechanical test data on helical plates under physiological load condition is limited. Hence, the aim of this study was to compare the biomechanical performance of helical and straight PHILOS® Long plates in AO12C2 fractures using static and cyclic implant system testing. Helical and straight PHILOS® Long plates on artificial bone substitutes were tested under physiological axial static (n=6) and cyclic loading (n=12). The axial construct stiffness was the main parameter for comparing the biomechanical performance of the two groups. Mimicking a clinical scenario, the helical deformation was performed consecutively by an experienced surgeon using iron bending tools. The torsional angle was determined computationally from 3D-scanning models afterwards. Helical plating resulted in a significantly reduced axial construct stiffness in all test scenarios compared to conventional straight plating (static testing: p=0.012; cyclic testing: p≤0.010). No failure occurred within the range of physiological loading in both groups. Helical plating favors multidimensional deformation of the test sample in lateral-ventral direction under axial loading, resulting in a reduced axial construct stiffness and in an increased interfragmentary movement. No biomechanical failure is to be expected within physiological load boundaries.

Long-term results of short stem total hip arthroplasty in patients with osteonecrosis of the femoral head.

Short stem total hip arthroplasty (THA) is an alternative to conventional stem, designed to facilitate minimal-invasive surgery, physiological loading and preserve bone stock. However, there is insufficient evidence of the long-term outcomes in patients with osteonecrosis of the femoral head (ONFH). This study aims to analyze the clinical and radiographic results with a minimum follow-up of 10 years. Furthermore, the survival rate of the short stem was calculated. There were 81 patients (101 hips) with the mean age of 45 years (range, 21 to 68) in this study. The Harris Hip Scores (HHS) and Forgotten Joint Scores (FJS) were recorded to evaluate the clinical results. The appearance of osteointegration and radiolucent line were reviewed. The mean follow-up period was 129.3 months (range, 120-154). At the latest follow-up, the mean HHS and FJS were 96.9 ± 5.7 (range, 69-100) and 95 ± 7 (range, 75-100) points respectively. The radiographic changes around the stem showed osteointegration mainly in the proximal part, zones 1 (94.1%), 2 (84.2%), 6 (99%) and 7 (92.1%). One stem underwent revision due to periprosthetic fracture, using conventional stem with plate and screws. Survivorship of the stem with the endpoint for any reason was 99.01% (95% confidence interval [CI], 93.18-99.86%) and for aseptic loosening was 100% at 10 years. Short stem showed promising long-term clinical outcomes in patients with ONFH. The radiographic results demonstrated physiological proximal load transfer.

The role of loading-induced convection versus diffusion on the transport of small molecules into the intervertebral disc.

Limited nutrient transport is hypothesized to be involved in intervertebral disc (IVD) degeneration. It is widely recognized that the dominant mode of transport of small molecules such as glucose is via diffusion, rather than convection. However, recent findings suggest a role for convection-induced by fast (motion-related) and slow (diurnal) dynamic loading in molecular transport of even such small solutes. The aim of this study was to investigate whether fluid exchange induced by simulated physiological loading (composed of both fast cyclic or slower diurnal loading) can influence the molecular transport of a small molecule through the cartilage endplate (CEP) into the nucleus pulposus (NP) of IVDs. The molecular transport of fluorescein through the CEP and into the NP was studied in a bovine CEP/NP explant model and loading was applied by an axial compression bioreactor. The loaded explants (convection and diffusion) were compared to unloaded explants (diffusion alone). In the initial 24h, there were no differences between loaded and unloaded explants, indicating that convection did not enhance molecular transport of small solutes over diffusion alone. Notably, after 48h which corresponds to two complete diurnal cycles of tissue compression, fluid exudation/imbibing and redistribution, the fluorescein concentration was significantly increased in the top and bottom layer of the explant, when compared to the unloaded explant. Slower diurnal cyclic compression of the IVD might enhance the transport of small molecules into the IVD although it could not be discerned whether this was due to diffusion/convection or a combination.

The Influence Mechanism of Screw Internal Fixation on the Biomechanics of Lateral Malleolus Oblique Fractures.

It remains inconclusive about the stability and optimal fixation scheme of screw internal fixation for lateral malleolus oblique fractures in clinical practice. In this study, the effects of different screw internal fixation methods on the biomechanics of lateral malleolus oblique fractures were investigated. These efforts are expected to lay a theoretical foundation for the selection of internal fixation methods and rehabilitation training regimens in the treatment of lateral malleolus fractures. A healthy ankle joint model and a lateral malleolus fracture internal fixation model were established based on CT data with the aid of some software. Besides, the effects of screw internal fixation modalities on the fracture displacement of fibula fractures, fibula Von Mises stress, and screw Von Mises stress under different physiological conditions and loading conditions were investigated using finite element methods (FEMs) and invitro physical experiments. The double screw vertical fibular axis internal fixation approach had the lowest fracture displacement of fibula fractures and screw Von Mises stress values; while the double screw vertical fracture line internal fixation approach had the lowest fibula Von Mises stress values. Under different physiological conditions, the magnitude of the peak Von Mises stress of the fibula and screw was ranked as plantarflexion 20° > plantarflexion 10° > neutral position > dorsiflexion 10° > dorsiflexion 20°; and the magnitude of the peak displacement of the fibula fracture breaks was ranked as plantarflexion 20° > plantarflexion 10° > neutral position > dorsiflexion 20° > dorsiflexion 10°. The results of invitro physical experiments and finite element analyses were in good agreement, which validated the validity of finite element analyses. The vertical fracture line screw implantation method displays a better load-sharing ability; while the vertical fibular axis screw implantation method exhibits a better ability to prevent axial shortening of the fibula and also reduces the risk of screw fatigue damage. Overall, the double screw achieves better therapeutic effects than the single screw. Given that the ankle joint has high stability in the dorsiflexion position, it is recommended to prioritize dorsiflexion rehabilitation training, rather than dorsiflexion and plantarflexion rehabilitation training with too large angles, in the treatment of lateral malleolus fractures.

The Heart is a Smart Pump: Mechanotransduction Mechanisms of the Frank-Starling Law and the Anrep Effect.

The Frank-Starling law and Anrep effect describe two intrinsic mechanisms that regulate contraction force in the heart. Based on recent advancements and the historical literature, we propose new perspectives and address several critical issues in this review. (a) The Frank-Starling mechanism and Anrep effect are dynamically linked and act synergistically. (b) An open question is how cardiomyocytes sense mechanical load and transduce to biochemical signals (called mechano-chemo-transduction or MCT) to regulate contraction in response to load changes. (c) One research focus is to identify various mechanosensors and decipher their downstream MCT pathways. (d) Innovative experimental techniques engage different mechanosensors that detect different local strain and stress in the cell architecture. (e) Closed-loop MCT feedback in the dynamic excitation-Ca2+ signaling-contraction system enables autoregulation of contraction in response to physiological load changes. ( f ) However, pathological overload such as volume and pressure overload lead to excessive MCT-Ca2+ gain, cardiac remodeling, and heart diseases.

Biology and mechanobiology of the intervertebral disc : Challenges for regenerative therapies

The fibrocartilaginous intervertebral discs between the vertebrae give the spine mobility and flexibility. Age and degeneration contribute to tissue changes that affect the composition and structure of the intervertebral discs and can lead to loss of function and back pain. The intervertebral disc cells are responsible for the formation and maintenance of the tissue and are influenced by the physiological load. Degenerative changes that promote tissue degradation are influenced by cellular responses to overuse, inflammation and nutrient deficiency. The mechanobiology of intervertebral discs deals with the interrelationships of the functional adaptation of this organ down to the cellular and molecular level and contributes to the understanding of the causes and consequences of degeneration. Based on these findings, the challenges for suitable regenerative therapy concepts can be understood.

Restoration of physiologic loading after engineered disc implantation mitigates immobilization-induced facet joint and paraspinal muscle degeneration.

Intervertebral disc degeneration is commonly associated with back and neck pain, and standard surgical treatments do not restore spine function. Replacement of the degenerative disc with a living, tissue-engineered construct has the potential to restore normal structure and function to the spine. Toward this goal, our group developed endplate-modified disc-like angle-ply structures (eDAPS) that recapitulate the native structure and function of the disc. While our initial large animal studies utilized rigid internal fixation of the eDAPS implanted level to ensure retention of the eDAPS, chronic immobilization does not restore full function and is detrimental to the spinal motion segment. The purpose of this study was to utilize a goat cervical disc replacement model coupled with finite element modeling of goat cervical motion segments to investigate the effects of remobilization (removal of fixation) on the eDAPS, the facet joints and the adjacent paraspinal muscle. Our results demonstrated that chronic immobilization caused notable degeneration of the facet joints and paraspinal muscles adjacent to eDAPS implants. Remobilization improved eDAPS composition and integration and mitigated, but did not fully reverse, facet joint osteoarthritis and paraspinal muscle atrophy and fibrosis. Finite element modeling revealed that these changes were likely due to reduced range of motion and reduced facet loading, highlighting the importance of maintaining normal spine biomechanical function with any tissue engineered disc replacement. STATEMENT OF SIGNIFICANCE: : Back and neck pain are ubiquitous in modern society, and the gold standard surgical treatment of spinal fusion limits patient function. This study advances our understanding of the response of the spinal motion segment to tissue engineered disc replacement with provisional fixation in a large animal model, further advancing the clinical translation of this technology.

Do we need new standards in dental implant testing?

Abstract The ISO 14801 standard test is repeatedly criticized for not reflecting the actual conditions and loads realistically enough to which dental implants are exposed (e. g. during chewing). The aim of this paper is to provide a brief review of the test parameters of the ISO standard and to compare them critically with the parameters that prevail in vivo. The research revealed significant simplifications of the test conditions, wich can partly be justified with better reproducibility and comparability. Nevertheless, the realization remains that the test deviates considerably from real physiological loading conditions and should be adapted accordingly or supplemented by further tests.

Analysis and Imaging of Osteocytes.

Osteocytes are the bone cells that are thought to respond to mechanical strains and fluid flow shear stress (FFSS) by activating various biological pathways in a process known as mechanotransduction. Confocal image-derived models of osteocyte networks are a valuable tool for conducting Computational Fluid Dynamics(CFD) analysis to evaluate shear stresses on the osteocyte membrane, which cannot be determined by direct measurement. Computational modeling using these high-resolution images of the microstructural architecture of bone was used to numerically simulate the mechanical loading exerted on bone and understand the load-induced stimulation of osteocytes. This study elaborates on the methods to develop 3D single osteocyte models using confocal microscope images of the Lacunar-Canalicular Network(LCN) to perform CFD analysis utilizing various computational modeling software. Prior to confocal microscopy, the mouse bones are sectioned and stained with Fluorescein isothiocyanate (FITC) dye to label the LCN. At 100x resolution, Z-stack images are collected using a confocal microscope and imported into MIMICS software (3D image-based processing software) to construct a surface model of the LCN and osteocyte-dendritic processes. These surfaces are then subtracted using a Boolean operation in 3-Matic software (3D data optimization software) to model the lacunar fluidic space around the osteocyte cell body and canalicular space around the dendrites containing lacunocanalicular fluid. 3D volumetric fluid geometry is imported into ANSYS software (simulation software) for CFD analysis. ANSYS CFX (CFD software) is used to apply physiological loading on the bone as fluid pressure, and the wall shear stresses on the osteocytes and dendritic processes are determined. The morphology of the LCN affects the shear stress values sensed by the osteocyte cell membrane and cell processes. Therefore, the details of how confocal image-based models are developed can be valuable in understanding osteocyte mechanosensation and can lay the groundwork for future studies in this area.

The association of different types of stress, and stress accumulation with low back pain in call-center workers - a cross-sectional observational study

BackgroundLow back pain (LBP) is a common health complaint and a prominent factor in the development of LBP among the working population is stress. Mostly, stress is addressed as a general problem, which is why LBP prevention programs are often imprecise. Accordingly, a closer look at the association between specific stress types and the development of LBP is necessary. Therefore, this paper aims (1) to identify the stress types most closely associated with LBP; (2) to examine the relationship between stress accumulation and LBP.Methodsn = 100 call-center workers were approached for participation. Stress levels and LBP were assessed with questionnaires (TICS, ERI, CPG, BPI) and hair cortisol levels were measured (ELISA-KIT, 3-months period). Mann-Whitney U tests were used to identify stress types most closely associated with LBP. Further, ANCOVA analysis was conducted to determine the association of the number of experienced stress types with LBP intensity and impairment.ResultsFinally, data from n = 68 participants (mean age: 43.2 (± 12.8) years; 62% female) were used for presented analysis. Participants, who were affected by work-related stress showed higher pain severity (excessive demands at work: 23.6 ± 21.8 vs. 42.4 ± 25.0 (p = 0.005)) and more impairment (excessive demands at work: 13.7 ± 17.6 vs. 28.7 ± 22.3 (p = 0.003); work overload: 15.4 ± 20.4 vs. 26.3 ± 17.4 (p = 0.009)) than their less affected colleagues. Other stress types (e.g. Effort, Reward) showed no significant association with LBP. Furthermore, participants who experienced two or more of the most associated stress types simultaneously suffered from stronger pain and more impairment (p < 0.01).ConclusionsThe results suggest that it is essential to divide and evaluate stress in specific domains. Furthermore, the accumulation of different stress types and the resulting physiological load should be taken into account when designing prevention and intervention programs. Results may be of high relevance for the development of LBP prevention programs for people within a predominantly sitting working context.

Three-dimensional kinematic analysis of the cervical spine following posterior atlantoaxial fusion under physiological loading: An in vivo study

BackgroundThis study aimed to analyze the three-dimensional cervical motion characteristics in patients who underwent posterior atlantoaxial fusion surgeries using cone beam computed tomography and 3D3D registration technology. MethodsThe study selected 20 patients who underwent posterior atlantoaxial fusion surgery and 20 healthy people as the control group. All subjects underwent cone beam computed tomography scans of the occipital and cervical spine in 7 different functional positions, then 3D3D registration of Occipital-C7 was performed at each functional position to calculate the motion characteristics of each segment. The ranges of motion of the entire cervical spine and each segment were obtained in each functional position. FindingsIn the experimental group, ranges of motion of C1-C7 in flexion-extension and left-right twisting were significantly lower compared to controls (41.9° ± 13.8° vs. 56.6° ± 11.6°, 29.3° ± 9.6° vs. 91.2° ± 13.7°, respectively, P < 0.05). In the occipital-atlas segment, range of motion in flexion-extension was significantly smaller in the experimental group than controls (10.7° ± 3.2° vs. 19.4° ± 4.2°, P < 0.001), but it was larger in twisting (5.3° ± 4.2° vs. 2.1° ± 1.8°, P < 0.05). The twisting range of motion of C2-C3 was 4.7° ± 2.0° in the experimental group and 3.1° ± 1.6° in the control group (P < 0.05). Additionally, the alteration in ranges of motion during flexion-extension was primarily characterized by less extension. InterpretationThe posterior atlantoaxial fusion surgery induced biomechanical changes in the cervical spine. Following the procedure, the movement of C1-C7 during flexion-extension and twisting was significantly lower, with varying degrees of impact on adjacent and lower cervical segments. Moreover, the surgery had a greater effect on cervical extension than flexion.

Design, Evaluation, and Implementation of a Novel Magnetic Resonance Imaging-Compatible Physiologic Loading Simulator for Ex-Vivo Joints.

Quantitative magnetic resonance imaging (qMRI), in combination with mechanical testing, offers potential to investigate how loading (e.g., from daily physical exercise) is related to joint and tissue function. However, current testing devices compatible with magnetic resonance imaging (MRI) are often limited to uniaxial compression, often applying low loads, or loading individual tissues (instead of multiple), while more complex simulators do not facilitate MRI. Hence, in this work, we designed, built and tested (N = 1) an MRI-compatible multi-axial load-control system, which enables scanning cadaveric joints (healthy or pathologic) loaded to physiologically relevant levels. Testing involved estimating and validating physiologic loading conditions before implementing them experimentally on cadaver knees to simulate and image gait loading (stance and swing). The resulting design consisted of a portable loading device featuring pneumatic actuators to reach a combined loading scenario, including axial compression (≤2.5 kN), shear (≤1 kN), bending (≤30 N·m) and muscle tension. Initial laboratory testing was carried out; specifically, the device was instrumented with force and pressure sensors to evaluate loading and contact response repeatability in one cadaver knee specimen. This loading system was able to simulate healthy or pathologic gait with reasonable repeatability (e.g., 1.23-2.91% coefficient of variation for axial compression), comparable to current state-of-the-art simulators, leading to generally consistent contact responses. Contact measurements demonstrated a tibiofemoral to patellofemoral load transfer with knee flexion and large contact pressures concentrated over small sites between the femoral cartilage and menisci, agreeing with experimental studies and numerical simulations in the literature.

Axis screw parallel to the sagittal plane versus traditional pedicle screw in the treatment of atlantoaxial fixation: a finite element study.

This study aims to conduct a finite element analysis (FEA) to assess the bio-mechanical properties of C2 sagittal-parallel pedicle screw (PPS) in fixation for atlantoaxial instability, thereby providing a theoretical foundation for its clinical application. A total of 5 intact C1-2 finite element models were established. Based on this, instability models were developed and two different fixation methods were applied for each model: C1 lateral mass screw (LMS) combined with C2 sagittal-parallel pedicle screw (C1LMS + C2PPS), and C1 lateral mass screw combined with C2 traditional pedicle screw (C1LMS + C2PS). Under a physiological load of 40N, a pure moment of 1.5 Nm was used to simulate movements of the cervical spine in flexion, extension, lateral bending, and axial rotation. The von Mises stress of implants and the segment range of motion (ROM) were analyzed and compared statistically. The intact model was validated and showed good consistency with other studies in terms of range of motion (ROM). In flexion and extension, the C1LMS + C2PPS resulted in lower segment ROM (12.4% and 6.3% decrease) and stress concentration (15.9% decrease in flexion) compared to C1LMS + C2PS. However, in lateral bending and axial rotation, the C1LMS + C2PPS exhibited higher segment ROM (42.9% and 5.9% increase) and stress concentration (8.7% and 21.4% increase) compared to C1LMS + C2PS. Both methods were safe and stable for the fixation of atlantoaxial instability. Compared to C1LMS + C2PS, the use of C1LMS + C2PPS may provide better stability and a lower risk of implant failure in flexion and extension. Clinically, it is feasible to utilize the C2 sagittal-parallel pedicle screw in fixing atlantoaxial instability.

Origins of synergy in multilipid lubrication

Lipid bilayers, ubiquitous in living systems, form lubricious boundary layers in aqueous media, with broad relevance for biolubrication, especially in mechanically stressed environments such as articular cartilage in joints, as well as for modifying material interfacial properties. Model studies have revealed efficient lubricity by single-component lipid bilayers; synovial joints, however (e.g. hips and knees), comprise over a hundred different lipids, raising the question of whether this is natural redundancy or whether it confers any lubrication benefits. Here, we examine lubrication by progressively more complex mixtures of lipids representative of those in joints, using a surface forces balance at physiologically relevant salt concentrations and pressures. We find that different combinations of such lipids differ very significantly in the robustness of the bilayers to hemifusion under physiological loads (when lubrication breaks down), indicating a clear lubrication synergy afforded by multiple lipid types in the bilayers. Insight into the origins of this synergy is provided by detailed molecular dynamics simulations of potential profiles for the formation of stalks, the essential precursors of hemifusion, between bilayers of the different lipid mixtures used in the experiments. These reveal how bilayer hemifusion-and thus lubrication breakdown-depends on the detailed lipid bilayer composition, through the corresponding separation into domains that are better able to resist stalk formation. Our results shed light on the role of lipid-type proliferation in biolubrication synergy, point to improved treatment modalities for common joint diseases such as osteoarthritis, and indicate how lipid-based interfacial modification in a materials context may be optimized.