Peak Contact Stress Research Articles

Persistently elevated joint contact stress after periacetabular osteotomy is associated with joint failure at minimum 10-year follow-up.

Periacetabular osteotomy (PAO) is a common treatment for prearthritic hip dysplasia. The goal of this investigation was to determine if computationally assessed hip contact mechanics are associated with joint failure at minimum 10-year follow-up. One hundred patients with hip dysplasia (125 hips) completed patient-reported outcomes an average of 13.8 years (range 10.0-18.0 years) after PAO. 63/125 hips were classified as having failed: 26 converted to total hip arthroplasty (THA) and 37 with significant disability indicated by modified Harris Hip Score (mHHS) ≤ 70. Differences in discrete element analysis-computed contact mechanics were compared between (1) preserved and failed hips, (2) preserved hips and hips that failed by THA, and (3) preserved hips and hips that failed by mHHS ≤ 70. Failed hips had significantly higher preoperative contact stress and exposure metrics (p < 0.001-0.009) than preserved hips. Failed hips also had significantly higher postoperative peak contact stress (p = 0.018), higher mean contact stress (p < 0.001), and smaller contact area (p = 0.044). When assessed based on type of failure, hips that failed by THA had significantly higher postoperative contact stress and exposure metrics than preserved hips (p < 0.001-0.020). In hips that failed by mHHS ≤ 70, mean postoperative contact stress exposure was significantly higher compared to preserved hips (p = 0.043). Despite improved radiographic measures of dysplasia after PAO, pathologic joint contact mechanics can persist and predict treatment failure at minimum 10 years after surgery. Operative and nonoperative techniques specifically intended to reduce harmful contact mechanics in dysplastic hips may have the potential to further improve clinical outcomes after PAO.

Functional outcomes after re-insertion of posterior root tear of medial meniscus using a simple economic technique

Background Root avulsions of the meniscus cause loss of hoop stresses and therefore loss of the meniscal function. This in return is associated with increased peak contact stresses and risk of osteoarthritis. Root repair is therefore crucial to preserve normal biomechanics and kinematics. Many techniques have been used and expensive fancy equipment made the procedure easier, however, cost is of great importance. Aim To demonstrate the results and functional outcomes after root re-insertion using simple economic tools. Patients and methods Between March 2022 and May 2023, 25 patients with root tears were operated upon and followed-up for a mean of 12 months. Patients with acute traumatic tears, degenerative tears with grade 1–2 osteoarthritis, associated genu varum less than 5º and a BMI less than or equal to 35 were included in the study. All patients underwent diagnostic arthroscopy as part of the procedure. Pie-crusting of the medial ligament was performed routinely to facilitate visualization and instrumentation in the medial compartment. Bed preparation was performed using a motorized shaver. This was followed by passing polydioxanone suture (PDS) shuttling sutures through a grey cannula and into the posterior root meniscal tissue, this was then used to shuttle heavy poly-braided nonabsorbable suture material. A tunnel was created through the anterior tibial cortex using the anterior cruciate ligament (ACL) guide. The sutures are then tied over a post (4.5 mm screw). Lysholm, International Knee Documentation Committee (IKDC), and Tegner activity scores were used to evaluate the patients at 6 and 12 months. Results The mean Lysholm score improved from 50.16 ± 12.51 preoperative to 82.08 ± 12.90 postoperative. The mean IKDC score improved from 51.24 ± 11.06 to 84.36 ± 10.18. The mean Tegner activity score improved from 3 ± 1.32 ranging from 1 to 5 to 5.20 ± 1.53. Traumatic tears showed greater improvement as compared with degenerative tears. Conclusion Simple outside techniques can be used to perform adequate reliable root repair. This yields comparable results to other techniques described in the literature.

Failure analysis and pin hole profile optimization design of combined piston for marine diesel engine based on Kriging model and NSGA-Ⅱ algorithm

Amidst the drive for enhanced performance and reliability in marine diesel engines, anti-fatigue design has emerged as a pivotal factor in ensuring safe operation. To investigate the cracking of a steel-topped aluminum-skirted combined piston in a marine diesel engine, thorough analyses were conducted on mechanical properties, chemical composition, and microscopic morphology. Subsequently, a piston thermo-mechanical coupling (TMC) model was established to simulate stress distribution and fatigue strength. Findings revealed that loose defects in the material and deformation mismatch in the pin hole bearing TMC were the primary factors contributing to cracking. Based on this, a multi-objective optimization (MOO) design method for piston pin hole profiles was proposed. Optimal pin hole profile design parameters were determined using the Kriging surrogate model and the second generation non-dominated sorting genetic algorithm (NSGA-II), and the nonlinear relationship between the design variables and the response objectives was obtained. Post-optimization, stress in the piston cracking source region decreased by 40.6%, peak contact pressure and stress in the pin hole decreased by 59.2% and 49.1%, respectively. Furthermore, the minimum fatigue safety coefficient improved from 0.86 to 1.18.

The Influence of Talar Displacement on Articular Contact Mechanics: A 3D Finite Element Analysis Study Using Weightbearing Computed Tomography.

Talar displacement is considered the main predictive factor for poor outcomes and the development of post-traumatic osteoarthritis after ankle fractures. Isolated lateral talar translation, as previously studied by Ramsey and Hamilton using carbon powder imprinting, does not fully replicate the multidirectional joint subluxations seen in ankle fractures. The purpose of this study was to analyze the influence of multiple uniplanar talar displacements on tibiotalar contact mechanics utilizing weightbearing computed tomography (WBCT) and finite element analysis (FEA). Nineteen subjects (mean age = 37.6 years) with no history of ankle surgery or injury having undergone WBCT arthrogram (n = 1) and WBCT without arthrogram (n = 18) were included. Segmentation of the WBCT images into 3D simulated models of bone and cartilage was performed. Three-dimensional (3D) multiple uniplanar talar displacements were simulated to investigate the respective influence of various uniaxial displacements (including lateral translation, anteroposterior translation, varus-valgus angulation, and external rotation) on the tibiotalar contact mechanics using FEA. Tibiotalar peak contact stress and contact area were modeled for each displacement and its gradations. Our modeling demonstrated that peak contact stress of the talus and tibia increased, whereas contact area decreased, with incremental displacement in all tested directions. Contact stress maps of the talus and tibia were computed for each displacement demonstrating unique patterns of pressure derangement. One millimeter of lateral translation resulted in 14% increase of peak talar contact pressure and a 3% decrease in contact area. Our model predicted that with lateral talar translation, there is less noticeable change in tibiotalar contact area compared with prior studies whereas external rotation greater than 12 degrees had the largest effect on peak contact stress predictions. Level V, computational simulation study.

Incorporating patient-specific hip orientation from weightbearing computed tomography affects discrete element analysis-computed regional joint contact mechanics in individuals treated with periacetabular osteotomy for hip dysplasia.

Computational models of the hip often omit patient-specific functional orientation when placing imaging-derived bony geometry into anatomic landmark-based coordinate systems for application of joint loading schemes. The purpose of this study was to determine if this omission meaningfully alters computed contact mechanics. Discrete element analysis models were created from non-weightbearing (NWB) clinical CT scans of 10 hip dysplasia patients (11 hips) and oriented in the International Society of Biomechanics (ISB) coordinate system (NWB-ISB). Three additional models were generated for each hip by adding patient-specific stance information obtained via weightbearing CT (WBCT) to each ISB-oriented model: (1) patient-specific sagittal tilt added (WBCT-sagittal), (2) coronal and axial rotation from optical motion capture added to (1; WBCT-combo), and (3) WBCT-derived axial, sagittal, and coronal rotation added to (1; WBCT-original). Identical gait cycle loading was applied to all models for a given hip, and computed contact stress and contact area were compared between model initialization techniques. Addition of sagittal tilt did not significantly change whole-joint peak (p = 0.922) or mean (p = 0.871) contact stress or contact area (p = 0.638). Inclusion of motion-captured coronal and axial rotation (WBCT-combo) decreased peak contact stress (p = 0.014) and slightly increased average contact area (p = 0.071) from WBCT-sagittal models. Including all WBCT-derived rotations (WBCT-original) further reduced computed peak contact stress (p = 0.001) and significantly increased contact area (p = 0.001). Variably significant differences (p = 0.001-1.0) in patient-specific acetabular subregion mechanics indicate the importance of functional orientation incorporation for modeling applications in which local contact mechanics are of interest.

Medial Meniscus Posterior Root Tears Lead to Changes in Joint Contact Mechanics at Low Flexion Angles During Simulated Gait

Background: Previous biomechanical studies evaluating medial meniscus posterior root tears (MMPRTs) are limited to low loads applied at specified loading angles, which cannot capture the effects of MMPRTs during the multidirectional forces and moments placed across the knee during physiological activities. Purpose: To quantify the effects of MMPRTs on knee joint contact mechanics during simulated gait. Study Design: Controlled laboratory study. Methods: Six human cadaveric knees were mounted on a robotic simulator programmed to apply dynamic forces, moments, and flexion angles to mimic level walking. Twelve cycles of multidirectional and dynamic standard gait input waveforms, normalized to specimen-specific body weight, were applied to the following conditions: (1) native, intact meniscus and (2) MMPRT. Peak contact stress, contact area, and the position of the weighted center of contact across the medial tibial plateau throughout the stance phase of gait were quantified using an electronic sensor placed across the medial tibial plateau. The difference between the intact state and MMPRT condition was calculated for each metric, and then the means and 95% CIs were computed. Results: Despite heterogeneity in knee contact forces, MMPRTs significantly increased peak contact stress by a mean of 2 MPa across 20% to 37% of the simulated gait cycle and significantly decreased the contact area by a mean of 200 mm2 across 16% to 60% of the simulated gait cycle in comparison with the native state. There was no significant difference in the position of the weighted center of contact, in either the anterior-posterior or medial-lateral directions, after MMPRT. Conclusion: MMPRTs led to both a significant increase in peak contact stress and decreased contact areas for a portion of the simulated gait cycle ranging from 20% to 37% of gait, during which time the femur was flexed <15°. Clinical Relevance: Contact mechanics are significantly affected after MMPRTs during early to midstance and at knee flexion angles lower than demonstrated previously. These data provide further biomechanical justification for treating MMPRTs.

The Effects of Residual Femoral Deformity on Computed Contact Mechanics in Patients Treated With In Situ Fixation for Slipped Capital Femoral Epiphysis.

In situ fixation for treatment of slipped capital femoral epiphysis (SCFE) can stabilize the epiphysis and prevent further joint deformation but often leaves residual deformity that may adversely affect intra-articular contact mechanics. The purpose of this study was to investigate the relationship between residual deformity and contact mechanics in the post-SCFE hip. Patient-specific hip models were created for 19 patients with SCFE treated with in situ fixation. For each model, discrete element analysis was used to compute cumulative acetabular and femoral contact stress exposure during a walking gait cycle. Slip severity was evaluated for each patient using the two-dimensional Southwick angle and a novel three-dimensional (3D) assessment of multiplanar femoral deformity (3D slip angle). Of the SCFE cases, 2/7 mild (Southwick angle ≤30 degrees) had peak cumulative femoral exposures equivalent to that of severe (Southwick angle ≥60 degrees) cases. Severe SCFE cases had higher peak ( P = 0.015) and mean ( P = 0.028) femoral contact stress exposure and lower cumulative femoral contact area ( P = 0.003) than mild (Southwick angle ≤30 degrees) SCFE cases. Mean femoral contact stress exposure was also higher in severe SCFE cases than in moderate SCFE cases ( P = 0.027). Acetabular and femoral contact mechanics metrics typically demonstrated stronger correlations with 3D slip angle than two-dimensional Southwick angle. Increased slip severity adversely impacts intra-articular femoral contact mechanics. Contact mechanics metrics demonstrate higher correlations with 3D slip angle, indicating that this novel measurement may better describe global deformity and its relationship to intra-articular mechanics; however, the modest strength of these correlations may also imply that global impingement-generating deformity is not the primary factor driving contact mechanics in the post-SCFE hip. Greater slip severity adversely impacts contact mechanics in the post-SCFE hip. However, focal regions of high contact stress were seen even in mild SCFE deformities, suggesting some type of deformity correction should be considered even for mild slips to alleviate secondary impingement, address focal incongruities, and reduce osteoarthritis development/progression.

Finite Element Analysis of Tyre Contact Interaction Considering Simplified Pavement with Different Aggregate Sizes

This study considered the effect of pavement aggregate grain size on tyre–pavement contact interaction during the late stages of pavement skid resistance. First, hemispherical shells 7, 9, and 13 mm in diameter were used to simulate coarse pavement aggregates. Subsequently, a three-dimensional finite element tyre–pavement contact model developed using ABAQUS was employed to analyse the contact interaction between each simplified pavement type and the tyre under steady–state rolling and braking conditions. Finally, the concept of occlusal depth was proposed and applied to characterise pavement skid resistance. The results showed that under steady–state rolling conditions, the peak contact stress of the simplified pavement increased with the pavement mean texture depth, whereas the contact area decreased. Under steady–state braking conditions, the effect of the contact interaction between the tyre and simplified pavement aggregates was ranked in order of superiority as aggregate grain sizes of 9, 7, and 13 mm, indicating that aggregate grain size did not exhibit any correlation with tyre–pavement contact interaction. Additionally, the squares of linear correlation coefficients between the pavement cumulative occlusal depth and horizontal braking force reached 0.921, 0.941, 0.889, and 0.894 for vehicle speeds of 30, 60, 90, and 120 km/h, respectively, indicating that they could be used to assess pavement skid resistance.

The effect of femoral prosthesis design on patellofemoral contact stresses in total knee arthroplasty: a case–control study with mid-term follow-up minimum 3-year follow-up

BackgroundTo investigate the differences in postoperative patellofemoral pressures and patellar tracking during at least three years of follow-up in patients using three prostheses of different designs in total knee arthroplasty (TKA) without patellar resurfacing.Methods: Radiographic investigationsThe study included 401 patients who had a total of 480 knee prostheses implanted without patellar resurfacing. The prostheses used were Genesis II (external rotation design of femoral prosthesis), Triathlon (design with deep trochlear grooves), and Gemini MK II (deepening of trochlear groove and lateral condylar protrusion that closely follows the anatomical shape). The patients' patellar tracking was assessed by measuring patellar tilt and displacement during postoperative follow-up. Furthermore, postoperative knee function and pain were evaluated through range of motion, Knee Society scores (KSS), and Forgotten Joint Score (FJS) to compare the different groups.Finite element analysisConstructing a finite element model of the knee joint of a normal volunteer after total knee arthroplasty using different prostheses for nonpatellar replacement. The three models' von Mises stress distribution heat map, peak contact pressure, and patellar transverse displacement were compared at 30°, 60°, and 90°, respectively.Results: Radiographic investigationsA total of 456 knees of 384 patients were investigated at a 3-year follow-up after TKA without patellar resurfacing. There were no significant differences in patellar tracking between the three groups. Patients with all three prostheses demonstrated favorable clinical outcomes at 3 years postoperatively, with no statistically significant differences in knee scores (91.9 vs 92.3 vs 91.8) or range of motion (127.9° vs 128.5° vs 127.7°) between the groups. However, there was a significant difference between Genesis II and Gemini MK II in the Forgotten Joint Score (59.7 vs 62.4). Patients with persistent postoperative anterior knee pain were present in all three groups (16 vs 12 vs 10), but the incidence was not significantly different.Finite element analysisThe von Mises stress distribution heat map showed that during flexion, the patellofemoral stresses were mainly concentrated on the lateral side of the prosthesis side, and the contact site gradually shifted downward with increasing flexion angle. At the same time, the peak contact stress of the patellofemoral joint increased with the gradual increase in the flexion angle. Genesis II, with a wider and shallower trochlear groove, showed greater patellofemoral stresses and lateral patellar displacement after TKA without patellar resurfacing. The Gemini MK II with a deeper trochlear groove and slightly protruding lateral condyle is more in line with anatomical design, with smaller patellofemoral joint pressure and better patellar tracking.ConclusionsIn TKA without patellar resurfacing, a prosthesis with a deeper trochlear groove, a slightly higher lateral femoral condyle, and a more anatomically designed knee that better matches the patellar morphology should be a better choice.

The Influence of Multiplanar Talar Displacement on Articular Ankle Loading: A 3D Finite Element Study

Category: Trauma; Ankle Introduction/Purpose: Ankle fractures represent an increasingly common injury world-wide and the decision for operative fixation often hinges on the definition of acceptable stability. Mortise malalignment has generally been considered as the main predictive factor leading to poorer outcomes and the development of post-traumatic osteoarthritis. Following pioneering work by Ramsey &amp; Hamilton, 1-2 mm of lateral talar displacement has commonly been regarded as a cutoff value for surgical decision making. However, isolated lateral talar translation (as studied in previous works) does not fully replicate the multidirectional joint subluxation seen in ankle fractures. Furthermore, previous studies have been limited by low fidelity methodologic techniques. Therefore, the aim of this study was to analyze the influence of multiplanar talar displacement on tibiotalar contact mechanics utilizing finite element analysis (FEA). Methods: 19 patients having undergone advanced computed tomography (CT) scanning of the ankle were included. A female patient (age 18 years, weight 60 kg) without preexisting articular ankle pathology underwent a weightbearing CT-arthrogram of the left ankle. Additionally, 18 ankles derived from the contralateral healthy side in patients who underwent bilateral weightbearing CT scans (WBCT ) for evaluation of a contralateral ankle fracture were included (mean age = 38.70 years, mean weight = 80.00 kg, left/right: 9/9). Segmentation of the WBCT images into 3D models of bone and cartilage was performed semi-automatically, while on the non-arthrography WBCT images, personalized cartilage layers were modeled based on a previously validated methodology. 3D multiplanar talar displacement was simulated to investigate their respective influence on the tibiotalar contact mechanics. Tibial peak contact stress, talar peak contact stress and contact area were extracted from the FEA models for each condition. Results: The talar peak contact stress (in MPa), tibial peak contact stress (in MPa) and contact area (in mm²) for each condition are presented in Table 1. Overall, both the mean and peak contact stress for the talus and tibia incrementally increased when the talus was displaced, for all directions. Correspondingly, the contact area incrementally decreased. Contact stress maps of the talus and tibia were computed for each of the conditions, demonstrating unique patterns of pressure derangement. 1 mm of lateral translation resulted in 11% increase in peak contact pressure and a 12% decrease in contact area. External rotation exhibited the greatest influence on contact stress, reaching a peak talar stress of 510% above baseline after 20 degrees of external talar rotation. Conclusion: In this study, we were able to build on the paradigm of prior uniplanar cadaveric studies and provide a multiplanar mortise malalignment model. The use of advanced computational technologies allowed for more precise quantitative evaluation of articular contact mechanics. With lateral talar translation, our model demonstrated less dramatic increases in tibiotalar contact area compared to previous studies, whereas external rotation had the largest effect regarding pathologic joint loading. Clinically, this study provides a novel understanding of the mechanical sequelae of 3D talar displacement and the importance for adequate mortise alignment.

Radiographically successful periacetabular osteotomy does not achieve optimal contact mechanics in dysplastic hips

BackgroundOptimal correction of hip dysplasia via periacetabular osteotomy may reduce osteoarthritis development by reducing damaging contact stress. The objective of this study was to computationally determine if patient-specific acetabular corrections that optimize contact mechanics can improve upon contact mechanics resulting from clinically successful, surgically achieved corrections. MethodsPreoperative and postoperative hip models were retrospectively created from CT scans of 20 dysplasia patients treated with periacetabular osteotomy. A digitally extracted acetabular fragment was computationally rotated in 2-degree increments around anteroposterior and oblique axes to simulate candidate acetabular reorientations. From discrete element analysis of each patient's set of candidate reorientation models, a mechanically optimal reorientation that minimized chronic contact stress exposure and a clinically optimal reorientation that balanced improving mechanics with surgically acceptable acetabular coverage angles was selected. Radiographic coverage, contact area, peak/mean contact stress, and peak/mean chronic exposure were compared between mechanically optimal, clinically optimal, and surgically achieved orientations. FindingsCompared to actual surgical corrections, computationally derived mechanically/clinically optimal reorientations had a median[IQR] 13[4–16]/8[3–12] degrees and 16[6–26]/10[3–16] degrees more lateral and anterior coverage, respectively. Mechanically/clinically optimal reorientations had 212[143–353]/217[111–280] mm2 more contact area and 8.2[5.8–11.1]/6.4[4.5–9.3] MPa lower peak contact stresses than surgical corrections. Chronic metrics demonstrated similar findings (p ≤ 0.003 for all comparisons). InterpretationComputationally selected orientations achieved a greater mechanical improvement than surgically achieved corrections; however, many predicted corrections would be considered acetabular over-coverage. Identifying patient-specific corrections that balance optimizing mechanics with clinical constraints will be necessary to reduce the risk of osteoarthritis progression after periacetabular osteotomy.

Design of three-stage sealing structure and investigation of sealing performance for 7000 fracturing plunger pump

As a key component of the drilling mud pump, the plunger seal often has problems such as seal failure in reconstruction of low-permeability sandstone reservoirs and the development of unconventional reservoirs, which, in turn, leads to low efficiency of drilling mud pumps and even great potential safety hazard. Based on this, this paper designs a three-stage sealing device at the hydraulic head for the 7000 fracturing plunger pump, and analyzes the sealing performance of the system. The effects of different working loads of the hydraulic head and axial pre-tightening force on the contact force of the seal ring and the stress distribution in the seal ring are explored, and the sealing effect of the three-stage seal of the fracturing plunger pump is confirmed. The results show that the peak stress and the peak contact pressure of the three rubber rings increases with the increasing of the working load of the hydraulic head; the peak stress and the peak contact stress of the three rubber rings during the installation process increase with the increase of the installation pre-tightening force. But the equivalent stress peak and the contact stress peak are directly related the fluid load under the action of the working fluid load; the maximum contact force of the rubber during the sealing process of the system is greater than that of the system working medium under different working fluid pressures. With internal pressure, the three sealing rubber rings can play a sealing role to varying degrees in sequence, and increasing the total life of the sealing body.

Design Optimisation of Bi-Cruciate Retaining Total Knee Arthroplasty (TKA) Prosthesis via Taguchi Methods

Total knee replacement has become a viable option for treating severe knee arthritis. The demand for more kinematically functional implants that better replicate natural knee kinematics led to the development of total knee arthroplasty (TKA), including bi-cruciate-retaining (BCR) TKA. However, optimised design parameters of BCR TKA knee implants that can help achieve a long-term prosthetic survival rate remain unknown. Therefore, this study aimed to investigate the effect of the design parameters of BCR TKA knee implants on the mechanics of knee joints and optimise and individualise the knee implant design parameters using the Taguchi method incorporating finite element analysis. Herein, experimental factors and levels were selected and nine finite element models of BCR TKA knee implants were developed to optimise the design of the following parameters: the curvature ratio on the sagittal plane, curvature ratio on the coronal plane, and tibial slope. In addition, finite element analysis was used to determine the effect of the design parameters on the peak contact stress on ultra-high-molecular-weight polyethylene (UHMWPE) and its deformation. Consequently, among the three parameters that affect the peak contact stress and its deformation, the curvature ratio on the sagittal plane had the greatest effect (range = 10.96), followed by the curvature ratio on the coronal plane (range = 3.54), and the tibial slope (range = 2.56). The optimal design parameters for the BCR TKA knee implant were a curvature ratio of 1.5 on both the sagittal and coronal planes and a tibial slope of 5°. Under these conditions, the peak contact stress and deformation were 25.80 MPa and 0.0835 mm, respectively. The optimisation method based on finite element analysis and the Taguchi method can produce one of the highest-performing BCR TKA knee implant designs, thereby reducing the peak contact stress and deformation. This method sheds fresh light on the development of the BCR TKA knee implant as well as biomechanical decision-making to implant the TKA prosthesis correctly.

Biomechanical comparison of the effect of bilateral sagittal split ramus osteotomy with or without Le Fort I osteotomy on the temporomandibular joints of the patients with maxillofacial deformities under centric occlusion

Mandibular deformities negatively affect the daily activities of the patients and may cause temporomandibular disorders (TMD). Bilateral sagittal split ramus osteotomy (BSSRO) and Le Fort I osteotomy are effective treatments to correct the mandibular deformities. The aim of this study was to investigate and compare the effects of the BSSRO with or without Le Fort I on the stress distributions of the temporomandibular joints (TMJs) of the patients with mandibular deformities under centric occlusion based on finite element (FE) method. Preoperative and postoperative cone-beam computed tomography (CBCT) images of twenty-four patients diagnosed with mandibular prognathism, including ten patients with BSSRO and another 14 patients with bimaxillary osteotomy (BSSRO with Le Fort I), were used to construct maxillofacial models. Ten asymptomatic individuals were also performed CBCT scanning and defined as the control group. In addition, the muscle forces and boundary conditions corresponding to centric occlusions were applied on each model. For the preoperative groups with both the BSSRO and bimaxillary osteotomies, the average peak contact stresses of the TMJs were both greater than those of the control group. After the surgeries, the contact stresses of the discs and temporal bones of both groups considerably decreased. However, the contact stresses on the condyles slightly increased after BSSRO but decreased after bimaxillary osteotomy. The TMJs of the patients with maxillofacial deformities suffered abnormal tensile and compressive stresses compared with the asymptomatic subjects under centric occlusion. Both of the BSSRO and bimaxillary osteotomy could improve the risk stress distributions of the TMJs.

How does asymmetric tibial insert affect tibiofemoral kinematics and contact stresses in total knee Arthroplasty?

BackgroundAsymmetric tibial insert design is expected to restore normal knee kinematics better than symmetric design. A tri-condylar implant has asymmetric and symmetric tibial inserts with a ball-and-socket joint to replace the post-cam mechanism. The purpose of this study was to compare the knee kinematics of the two designs and to measure tibiofemoral contact stresses, including that of the ball-and-socket joint. MethodsUsing a computer simulation, the anteroposterior position and axial rotation of the femoral component were simulated during a weight-bearing deep knee bend for six validated models. Contact forces were simultaneously simulated in the medial, lateral, and ball-and-socket compartments. The relative position and the magnitude and direction of each contact force were applied to aforce/displacement control knee simulator. The contact stresses were measured individually using a pressure sensor. ResultsThe asymmetric tibial insert demonstrated a more posterior position of the femoral component in the lateral compartment during the entire range of motion and greater external rotation of the femoral component, compared to the symmetrical tibial insert. The mean peak contact stress of the medial and lateral compartments was < 9 Mpa, with no significant differences between the two designs except at 0°. The contact stress of the ball-and-socket joint was < 5 MPa. ConclusionsAsymmetry of the tibial insert shows significant kinematic difference and has little influence on the peak contact stress, which is considerably lower than the yield strength of polyethylene. The asymmetric tibial insert can lead to clinical benefits owing to its kinematic and kinetic properties.

ACL transection results in a posterior shift and increased velocity of contact on the medial tibial plateau

Our objective was to quantify the effect of ACL transection on dynamic knee joint contact force distributions during simulated gait. Given the prevalence of medial compartment osteoarthritis in un-reconstructed ACL ruptured knees, we hypothesized that changes in contact mechanics after ACL transection would be most prevalent in the medial compartment. Twelve human cadaveric knees were tested using a dynamic knee gait simulator which was programmed to mimic a clinical Lachman exam and gait. An electronic pressure sensor was placed on the medial and lateral tibial plateaus under the menisci to quantify dynamic contact forces before and after ACL transection. Tibial translations and rotations, medial and lateral plateau peak contact stress, and position and velocity of the Weighted Center of Contact (WCoC) were computed. After ACL transection, the tibia translated more anteriorly in the Lachman examination and at heel strike during gait. Changes in contact mechanics across the medial tibial plateau during simulated gait were: an increase in the velocity of WCoC and a posterior shift in the WCoC, both of which occurred at heel strike; increased peak contact forces in the posterior-peripheral quadrant of the tibial plateau at 45% of the gait cycle; and an additional posterior shift in WCoC from 25 to 55% of the gait cycle. The only change in contact mechanics in the lateral plateau was a decrease in WCoC velocity in late stance. This data is suggested to further the study of biomechanical pathways (biomechanical biomarkers) in the relationship between altered knee contact mechanics and chondrocyte metabolic responses after ACL transection.

Computational biomechanics of human knee joint in stair ascent: Muscle-ligament-contact forces and comparison with level walking.

About a third of knee joint disorders originate from the patellofemoral (PF) site that makes stair ascent a difficult activity for patients. A detailed finite element model of the knee joint is coupled to a lower extremity musculoskeletal model to simulate the stance phase of stair ascent. It is driven by the mean of measurements on the hip-knee-ankle moments-angles as well as ground reaction forces reported in healthy individuals. Predicted muscle activities compare well to the recorded electromyography data. Peak forces in quadriceps (3.87 BW, body weight, at 20% instance in our 607 N subject), medial hamstrings (0.77 BW at 20%), and gastrocnemii (1.21 BW at 80%) are estimated. Due to much greater flexion angles-moments in the first half of stance, large PF contact forces (peak of 3.1 BW at 20% stance) and stresses (peak of 4.83 MPa at 20% stance) are estimated that exceed their peaks in level walking by fourfold and twofold, respectively. Compared with level walking, ACL forces diminish in the first half of stance but substantially increase later in the second half (peak of 0.76 BW at 75% stance). Under nearly similar contact forces at 20% of stance, the contact stress on the tibiofemoral (TF) medial plateau reaches a peak (9.68 MPa) twice that on the PF joint suggesting the vulnerability of both joints. Compared with walking, stair ascent increases peak ACL force and both peak TF and PF contact stresses. Reductions in the knee flexion moment and/or angle appear as a viable strategy to mitigate internal loads and pain.

Design and Experiment of Nondestructive Post-Harvest Device for Tomatoes

This paper proposed a whole process tomato harvester with a nondestructive post-harvest collection operation mode, which was aimed to solve the high damage rate problem during the automatic greenhouse tomato harvesting process. The post-harvest device mainly included the net bag mechanism, the conveying and collecting mechanism, whose structure and materials were carefully designed to satisfy the nondestructive collection principle. Numerical simulation was done to evaluate the damage under three working conditions, which showed that the peak contact stress of tomatoes was 0.107 MPa, 0.098 MPa, and 0.11 MPa, respectively, all smaller than the damage stress of tomato peel tissue. In the postharvest prototype experiment, the degree of mechanical damage based on the shelf life of tomatoes during the color turning stage and red ripening stage was used as the evaluation index. Results showed that when tomatoes were dropped from the 60 mm higher position than the net bag mechanism, and the speed of the conveyor belt was 9 r min−1, the degree of mechanical damage at the color turning stage and red ripening stage was 1.9% and 9.5%, respectively. The harvest time of greenhouse tomatoes was always around the color turning stage, thus the proposed device can well meet the agricultural requirements.

Effect of modeling femoral version and head-neck offset correction on computed contact mechanics in dysplastic hips treated with periacetabular osteotomy

While correction of dysplastic acetabular deformity has been a focus of both clinical treatment and research, concurrent femoral deformities have only more recently received serious attention. The purpose of this study was to determine how including abnormalities in femoral head-neck offset and femoral version alter computationally derived contact stresses in patients with combined dysplasia and femoroacetabular impingement (FAI). Hip models with patient-specific bony anatomy were created from preoperative and postoperative CT scans of 20 hips treated with periacetabular osteotomy and femoral osteochondroplasty. To simulate performing only a PAO, a third model was created combining each patient’s postoperative pelvis and preoperative femur geometry. These three models were initialized with the femur in two starting orientations: (1) standardized template orientation, and (2) using patient-specific anatomic landmarks. Hip contact stresses were computed in all 6 model sets during an average dysplastic gait cycle, an average FAI gait cycle, and an average stand-to-sit activity using discrete element analysis. No significant differences in peak contact stress (p = 0.190 to 1), mean contact stress (p = 0.273 to 1), or mean contact area (p = 0.050 to 1) were identified during any loading activity based on femoral alignment technique or inclusion of femoral osteochondroplasty. These findings suggest that presence of abnormal femoral version and/or head-neck offset deformities are not themselves predominant factors in intra-articular contact mechanics during gait and stand-to-sit activities. Inclusion of modified movement patterns caused by these femoral deformities may be necessary for models to adequately capture the mechanical effects of these clinically recognized risk factors for negative outcomes.

The Effect of Pore Structure on Impact Behavior of Concrete Hollow Brick, Autoclaved Aerated Concrete and Foamed Concrete

Porous concrete is an energy absorption material, which has been widely used in civil engineering, traffic engineering and disaster reduction engineering. However, the effect of pore structure on the impact behavior of the porous concrete is lacked. In this study, a series of drop-weight impact tests were carried out on three typical types of porous concrete, i.e., concrete hollow brick (CHB), autoclaved aerated concrete (AAC) and foamed concrete (FC), to investigate the effect of pore structures on their impact behavior. For comparison, static load tests were also conducted as references. According to the damage to the samples, the developments of impact force, strain, contact stress–strain relationship and absorbed energy during drop-weight during the impact test were measured and analyzed. The results show that the ratio between the peak impact stress and compressive strength of CHB was 0.44, while that of AAC and FC increased to about 0.6, indicating that the small and uniform pore structure in AAC and FC had a higher resistance against impact load than the hollow cavity of CHB. In addition, the elastic recovery strain in AAC increased by about 0.2% and its strain at peak contact stress increased by about 160% for a comparison of CHB, implying that a small open pore structure could enhance ductility. Besides, the peak contact stress of FC was close to that of AAC during impact loading, while the strain at peak contact stress of FC increased by about 36% compared with AAC, revealing that the closed-pore structure could further enhance the deformation potential. Correspondingly, the energy absorption rates of CHB, AAC and FC were 85.9 kJ/s, 54.4 kJ/s and 49.7 kJ/s, respectively, where AAC decreased by about 58% compared with CHB, and FC decreased by about 10% compared with AAC.