Increase In Contact Area Research Articles

Lithium Metal under Static and Dynamic Mechanical Loading

Macro-scale mechanical testing and finite element analysis of lithium metal in compression have been shown to suggest methods and parameters for producing thin lithium anodes. Consideration of engineering and geometrically corrected stress experiments shows that the increasing contact area dominates the stress increase observed during the compression, not strain hardening, of lithium. Under static loading, the lithium metal stress relaxes, which means there is a speed of deformation (engineering strainrate limit of 6.4×10−5 s−1) where there is no increase in stress during compression. Constant displacement tests show that stress relaxation depends on the initial applied stress and the amount of athermal plastic work within the material. The finite element analysis shows that barrelling during compression and the requirement for high applied stresses to compress lithium with a small height-to-width ratio are friction and geometric effects, respectively. The outcomes of this work are discussed in relation to the diminishing returns of stack pressure, the difficulty in closing voids, and potential methods for designing and producing sub-micron lithium anodes.

Using antocyanins and agar-agar as a basis for smart packaging

The work explored the possibility of using agar-agar-based films containing anthocyanins as smart packaging for fish raw materials. It has been shown that the structural and mechanical characteristics of films are significantly influenced by the amounts of anthocyanins included in their composition. The most suitable structural and mechanical characteristics are those based on agar-agar films containing 30-50% anthocyanins. A study of the influence of the degree of grinding of the product on the change in color of the film showed that an increase in area of contact of fish raw materials with the film leads to a change in its color during the storage of the product in it. The greatest change in film color is observed in the sample using minced fish. The color of the film with pollock fillet wrapped in it does not change after an hour of keeping the product in it. A study of the effect of fat content in fish raw materials on changes in film color showed that an important factor influencing these changes is the shift of active acidity to the alkaline region, which occurs when fish spoils, as well as oxidative processes occurring in fats. The advantage of storing fish fillets in a film containing anthocyanins has been shown. The use of films containing anthocyanins as packaging for fish raw materials allows us to obtain a product with a longer shelf life.

Creep penetration of particles at geomembrane surfaces

Laboratory tests were conducted to measure polyethylene geomembrane contact surface area change with time under sustained loading. The presentation summarizes the test results and findings. A theoretical model for time-dependent contact area is also assessed using the results obtained from the tests conducted. The results show that the contact areas at the geomembrane surface increase with time as anticipated and the rate of increase in contact area decreases with time. The investigations also showed that the time-dependent soil-geomembrane contact area can be predicted using a theoretical model.

ВЫБОР РЕЖИМОВ НАНЕСЕНИЯ ПОКРЫТИЙ ПРИ ГАЛЬВАНОКОНТАКТНОМ МЕТОДЕ

The paper presents studies aimed at studying the movement of the tool when applying dispersion-hardened composite electroplating coatings and its effect on the change in the area of plastic contact. (Research purpose) The research purpose is obtaining analytical dependences that make it possible to perform calculations for frictional contact of friction surfaces when applying dispersion-hardened composite electroplating coatings. (Materials and methods) Conducted research based on the methods of system analysis and the theory of formation of the surface layer. We performed experimental studies using modern standardized techniques on certified equipment. The obtained data were processed using the analytical modeling method, which is aimed at obtaining dependencies linking the desired characteristics with the specified parameters. (Results and discussion) Based on the conducted studies of plastic contact during the application of copper and iron composite coatings, it was concluded that the contact area increases, which requires a shift by a distance equal to 30-40 or more diameters of the contact spots. Consequently, with static friction, the in-crease in the contact area does not exceed 10-15 percent, and it is accompanied by a significant change in the microgeometry of the surfaces. (Conclusions) Theoretically and experimentally proved that the microgeometry of rubbing surfaces is reproduced, the transition from static contact to motion will not cause a significant change in the contact area. These conclusions are confirmed by a change in the convergence of two rough surfaces during sliding. It is possible to perform calculations for friction contact with sufficient accuracy using the formulas obtained for static conditions.

Heat transfer characteristics of solid-liquid interface on nanostructure surface under external electric field

With the size of high-performance electronic device decreasing (down to nanoscale), and the accompanying heat dissipation becomes a big problem due to its extremely high heat generation density. To tackle the ever-demanding heat dissipation requirement, intensive work has been done to develop techniques for chip-level cooling. Among the techniques reported in the literature, liquid cooling appears to be a good candidate for cooling high-performance electronic devices. However, when the device size is reduced to the sub-micro or nanometer level, the thermal resistance on the solid-liquid interface cannot be ignored in the heat transfer process. Usually, the interfacial thermal transport can be enhanced by using nanostructures on the solid surface because of the confinement effect of the fluid molecules filling up the nano-grooves and the increase of the solid-liquid interfacial contact area. However, in the case of weak interfacial couplings, the fluid molecules cannot enter into the nano-grooves and the interfacial thermal transport is suppressed. In the present work, the heat transfer system between two parallel metal plates filled with deionized water is investigated by molecular dynamics simulation. Electronic charges are applied to the upper plate and lower plate to create a uniform electric field that is perpendicular to the surface, and three types of nanostructures with varying size are arranged on the lower plate. It is found that the wetting state at the solid-liquid interface can change from Cassie state into Wenzel state with strength of the electric field increasing. Owing to the transition from the dewetting state to wetting state (from Wenzel to Cassie wetting state), the Kapitza length can be degraded and the solid-liquid interfacial heat transfer can be enhanced. The mechanism of the enhancing hart transfer is discussed based on the calculation of the number density distribution of the water molecules between the two plates. When the charge is further increased, electrofreezing appears, and a solid hydrogen bonding network is formed in the system, resulting in the thermal conductivity increasing to 1.2 W/(m·K) while the thermal conductivity remains almost constant when the electric charge continues to increase.

Friction-reduction effect of the hierarchical surface microstructure of carrion beetle by controlling the real contact area.

The discovery and elucidation of the surface microstructure functions of living organisms are crucial to resolving issues, such as friction. We newly discovered that Necrophila japonica, a type of carrion beetle that lives on the ground surface, exhibited a hierarchical surface microstructure comprising a submicron-sized wrinkle structure on top of a micron-sized microstructure. The surface microstructure of this beetle improved wettability but did not exhibit superhydrophobicity, a well-known function of hierarchical structures, so it was expected to have a different function. By combining the insights in the field of structural mechanics that avoidance of stress concentration by the structural geometry affects deformation with the basic principles of friction, the frictional properties and mechanisms of the hierarchical surface microstructure of carrion beetle were investigated. The measurements of frictional force indicated that the mimicked structure exerted lower frictional forces than flat and single-layer microstructure surfaces. Analysis of finite element method simulations showed that even though the mimicked structure was prone to pressure concentration due to small contact points, the surface contact pressure was reduced more than that of the single-layer structure by hierarchical load dispersion like that of metamaterials. As a result, the suppression of the increase in the real contact area due to deformation suppression contributed to effective friction-reduction. The effective friction-reduction by the hierarchical structure provides not only new insight into the surface microstructure function of various organisms, but also new design for the lubricant-free friction reduction that has been the focus of attention in carbon neutrality and other fields.

Enhancing water hydration in air-cooled proton exchange membrane fuel cell using a staggered tapered slotted flow field

Air-cooled proton exchange membrane fuel cell (AC-PEMFC) is widely considered as a promising power source for unmanned aerial vehicles (UAVs) due to its merits such as high energy density, short refueling time, and simple auxiliary system. However, the performance of AC-PEMFC is not satisfactory due to the poor membrane hydration caused by the large air supply for heat dissipation demand. This study proposes a staggered tapered slotted flow field (STSF) configuration to address this issue, which has higher contact area between the airflow and the bipolar plate by arranging tapered and slotted sections in the channels along the airflow direction, aiming to enhance the cooling effect and improve the membrane water hydration. Utilizing a three-dimensional (3D) multiphase non-isothermal model verified against experimental data, it was found that the STSF configuration reduces the internal temperature of the cell by about 14.2–28.3 K and increases the water content in the membrane by about 35.1–85.7 % compared with traditional straight channels. In addition, the STSF configuration can enhance mass transfer by inducing cross-flow, reducing concentration losses, which takes more effect for UAVs working at high altitude. Moreover, the slotted sections reduced the volume and weight of the bipolar plates, contributing to an additional power density improvement. Finally, the pressure drop within the flow channels and net power was compared. Due to the increased contact area between the cooling airflow and the bipolar plates, the STSF configuration inevitably results in a higher pressure drop within the channels, but the net power of PEMFC with STSF still increased under severe conditions by 0.080 W.

Investigation of mixing characteristics in a novel SAR micromixer with locally overlapping V-shaped flow channels

Micromixers have been widely used in chemical reactions, drug testing, bio-analysis, and various other fields where achieving effective mixing is crucial for system performance. To enhance mixing efficiency, this study presents a novel micromixer design based on principles of separation diffusion and chaotic convection. It features a double-layer V-shaped channel and mixing chamber structure, along with an innovative approach of locally overlapping flow channels. The results demonstrate that this design induces fluid split and recombination within the flow channel, increasing contact areas between fluids. Furthermore, under the identical conditions, the overlapping of flow channels induces secondary flow separation, further enhancing mixing. Specifically, we observed a 9.39 % improvement in the mixing index when comparing before and after overlapping (when Re = 10). Moreover, when the Reynolds number exceeds 20, the mixing index at the outlet consistently surpasses 99 %, which is superior to most existing micromixers. These novel models significantly improve mixing efficiency at low Reynolds numbers and introduce a promising method for future research.

A CFD modelling for optimizing geometry parameters for improved performance using clean energy geothermal ground-to-air tunnel heat exchangers

In this numerical study, different geometric parameters have been optimized for improved performance using a clean energy geothermal Ground to Air Tunnel Heat Exchanger (GATHE). A commercial computational fluid dynamic code ANSYS Fluent was used, and the code has been validated with the available experimental and analytical results. Three soil samples were used to analyze the thermal properties, each with a thermal conductivity of 0.65, 1.25 and 3.50 Wm−1K−1. The critical soil thickness was found to be 10 times the pipe diameter and the lowest outlet temperature of 300.1 K was achieved for a 60 m pipe with a soil thermal conductivity of 3.50 Wm−1K−1. To further enhance the performance of the system, multiple fin arrays were introduced into the ductwork at 0.5 m, 1.0 m and 2.0 m pitches. For a 10 m long pipe with fins installed every 0.5 m, an outlet temperature of 306.7 K was observed. With 4 fins for temperature variation, performance increased by 19.4 % compared to the case without fins. When the fin spacing is 1 m, the temperature drop increases by 5.6 %, and when the fin spacing is 2 m, the temperature drop increases to 7 %, with an outlet temperature of 306.1 K. Finally, a novel set of multiple blocks in diverging and converging patterns was introduced to restrict airflow and further improve heat dissipation. Compared to a single-fin block, the outlet temperature of the 4-fin block was reduced by 14.52 % due to the increased contact area, thus improving the thermal performance of the heat exchanger system.

In-situ micro-asperity investigation of real contact area formation during sliding with the effects of roughness and normal load considered

Real contact area is a challenging phenomenon in tribology, where despite established theoretical models, detailed experimental studies are still lacking. This work is focused on the evolution of the real contact area of multi-asperity steel contacts during sliding, by using sub-micron characterisation of entire multi-asperity contact area and simultaneous measurement of the friction force. Steel-sapphire contact was monitored by using an in-situ optical system in real time during 5 mm of unidirectional sliding in surface roughness range Ra = 0.1–1.0 µm and normal loads up to the nominal contact pressure equal to steel yield strength (Y). The results showed that the initial real contact area caused by static loading increased during sliding. At low nominal contact pressure of 0.3Y, the increase in real contact area was the largest: for Ra = 0.1 µm it went from 9.5% of the nominal area under static conditions to 15.0% after sliding, a 56% relative increase, and for Ra = 1.0 µm it went from 5.0% to 11.5% after sliding, a 106% increase. The increase in real contact area was smaller at higher nominal contact pressures of 0.6Y and 1.0Y. At 1.0Y for Ra = 0.1 µm the real contact area went from 18.0% to 21.8% after sliding and for Ra = 1.0 µm it went from 9.4% to 12.9% after sliding. The coefficient of friction increased with roughness at the lowest nominal contact pressure 0.3Y (for Ra = 0.1 µm it was 0.24 and for Ra = 1.0 µm it was 0.29) but became independent of roughness at higher contact pressures (on average 0.18).

Hydraulic Expansion Joint Contact State of Heat Exchanger Based on New Contact Area Measurement Method

The contact state of a seamless internal threaded copper tube and an aluminium foil fin not only affects the heat transfer efficiency of a tube–fin heat exchanger but also seriously affects its service life. In this study, hydraulic expansion technology was used to connect the copper tube with an internal thread with a 7 mm diameter to the fin of the heat exchanger. The influence of the expansion pressure and pressure holding time on the contact state was analysed through experiments and finite element simulation, and the variation law of the two on the contact state was obtained. The contact state was characterised by the contact gap and contact area. In order to obtain the specific contact area value, a new method of measuring the contact area was developed to reveal the variation in contact area between the copper tube and the fin after expansion. The results show that the contact gap decreases with an increase in expansion pressure, while the pressure holding time remains the same. The contact area increases with an increase in expansion pressure, and the rate of increase slows. When the expansion pressure is 18 MPa, the average contact gap is approximately 0.018 mm. When the expansion pressure reaches 16 MPa, the contact area ratio is 91.0%. When the expansion pressure increases to 18 MPa, the contact area ratio only increases by approximately 0.6%. Compared with the influence of the expansion pressure on the increase in contact area, the influence of the pressure holding time on the contact area is lower.

Insight into the chemical transformation and organic release of polyurethane microplastics during chlorination

The ubiquitous occurrence of microplastics in water and wastewater is a growing concern. In this study, the chemical transformation and organic release of virgin and UV-aged thermoplastic polyurethane (TPU) polymers during chlorination were investigated. As compared to virgin TPU polymer, the UV-aged TPU polymer exhibited high chlorine reactivity with noticeable destruction on its surface functional groups after chlorination, which could be ascribed to the UV-induced activation of hard segment of TPU backbone and increased contact area. The concentrations of leached organics increased by 1.6-fold with obviously high abundances of low-molecular-weight components. Additives, monomers, compounds relating to TPU chain extension, and their chlorination byproducts contributed to the increased organic release. Meanwhile, the formation of chloroform, haloacetic acids, trichloroacetaldehyde, and dichloroacetonitrile increased by 3.8-, 1.7-, 4.9-, and 2.4-fold, respectively. Two additives and six chlorination byproducts in leachate from chlorinated UV-aged TPU were predicted as highly toxic, e.g., butyl octyl phthalate, palmitic acid, 2,6-di-tert-butyl-1,4-benzoquinone, and chlorinated aniline. Evaluated by human hepatocarcinoma cells, the 50% lethal concentration factor of organics released from chlorinated UV-aged TPU was approximately 10% of that from its virgin counterpart, indicating a substantially increased level of cytotoxicity. This study highlights that the release of additives and chlorination byproducts from the chemical transformation of UV-aged microplastics during chlorination may be of potentially toxic concern.

Numerical simulations of the effect of lateral malleolus fracture malunion on ankle biomechanics: Different offset directions and offsets

IntroductionAnkle fractures account for approximately 10 % of all fractures. Approximately 5–68 % of patients with ankle fractures may suffer from malunion. Besides, suboptimal reduction of fracture fragments can affect the biomechanics of the ankle joint, ultimately leading to damage to the ankle joint. However, there are certain controversies over the conclusion of previous cadaveric studies. MethodsIn this study, a three-dimensional model of the ankle joint was established based on CT image data. In addition, the effects of backward offset (1–2 mm) and outward offset (0.5–1 mm) of the fracture fragment on the contact area, contact pressure, and ligament force of the ankle joint were investigated via the finite element method. Moreover, lateral malleolus fracture malunion in five ankle positions (neutral, 10° dorsiflexion, 10° plantarflexion, 20° dorsiflexion, and 20° plantarflexion) was investigated. ResultsThis model predicted an overall increased contact area in the ankle joint in patients with lateral malleolus fracture malunion compared with the normal ankle joint. The results demonstrated that the outward offset had a more significant effect than the backward one. The larger the dorsiflexion-plantarflexion angle, the more pronounced the effect of malunion. Further, an outward offset can cause the fibula to lose its function. ConclusionPost-traumatic osteoarthritis occurs under the action of unaccustomed cartilage forces due to altered tibial talar joint contact patterns, rather than increased contact pressure reported in previous studies. Malunion leads to an increase or decrease in force on the affected ligament, while the cause of malunion can be envisioned based on a decrease in the force on the ligaments.

Shape-locking in architected materials through 3D printed magnetically activated joints

Shape morphing is the ability of objects to adapt to different shapes and reduce stress concentrations through increased contact area. This is a common trait of natural and engineered objects and has several applications in, among others, soft robotics and orthopedic implants. Shape morphing is achieved through flexible materials or rigid components with either kinematic or compliant joints. An additional step, namely shape locking, is needed for sustained load support. Activation of a locking mechanism can be done with any energy, among which magnetism is one. Here, we present the implementation of a magnetic locking mechanism for kinematically deformable metamaterial structures that maintain shape and support loads upon locking. The structure consists of 3D printed rigid magnetic and non-magnetic components connected by hinges. We created several prototypes of the proposed designs using two additive manufacturing methods (i.e., material extrusion and multi-material jetting) and demonstrated its application in a closed-loop grid for arbitrary shapes. Moreover, we characterized the performance of the prototypes using mechanical tests and multibody kinematic system simulations. This work highlights the viability of the locking concept and provides design considerations for future applications. Further improvement and optimizations are needed for increased efficiency and effectiveness.

Thermal-Assisted Multiscale Patterning of Nonplanar Colloidal Nanostructures for Multi-Modal Anti-Counterfeiting.

Nanotransfer printing of colloidal nanoparticles is a promising technique for the fabrication of functional materials and devices. However, patterning nonplanar nanostructures pose a challenge due to weak adhesion from the extremely small nanostructure-substrate contact area. Here, the study proposes a thermal-assisted nonplanar nanostructure transfer printing (NP-NTP) strategy for multiscale patterning of polystyrene (PS) nanospheres. The printing efficiency is significantly improved from ≈3.1% at low temperatures to ≈97.2% under the glass transition temperature of PS. Additionally, the arrangement of PS nanospheres transitioned from disorder to long-range order. The mechanism of printing efficiency enhancement is the drastic drop of Young's modulus of nanospheres, giving rise to an increased contact area, self-adhesive effect, and inter-particle necking. To demonstrate the versatility of the NP-NTP strategy, it is combined with the intaglio transfer printing technique, and multiple patterns are created at both micro and macro scales at a 4-inch scale with a resolution of ≈2757 pixels per inch (PPI). Furthermore, a multi-modal anti-counterfeiting concept based on structural patterns at hierarchical length scales is proposed, providing a new paradigm of imparting multiscale nanostructure patterning into macroscale functional devices.

Experimental Study on Bearing Behavior and Soil Squeezing of Jacked Pile in Stiff Clay

In order to study the bearing behavior and soil-squeezing of jacked piles in stiff clay, two groups of pile penetration tests were performed, with a rough pile that can reproduce the quick-shear behavior of the pile–soil interface, i.e., group 1 in stiffer clay, and group 2 in softer clay for comparison. For each group, the adjacent pile was additionally penetrated at different pile spacings to study the soil-squeezing effect on an adjacent pile. The results show that the penetration resistance increased rapidly at the beginning and then increased at a lower rate. This is because the resistance at the pile end increased rapidly at the beginning and then kept stable with fluctuations, whereas the resistance at the pile side continually increased due to the increasing contact area. Therefore, the ratio of the resistance at the pile end to the total penetration resistance exhibited a softening behavior, which first increased to a peak and then gradually decreased. In addition, there was soil-squeezing stress and soil-squeezing displacement in the ground and adjacent piles due to pile penetration. In stiffer clay, the soil-squeezing stress was larger than that in softer clay due to the higher strength, whereas the soil-squeezing displacement was smaller than that in softer clay due to the low compressibility. In addition, the nonlinear equation form y = ae−bx can be employed to describe the effect of pile spacing on the vertical flotation, horizontal deviation, and pile strain of the adjacent pile.

Impact and adhesion mechanics of block copolymer micro-particles with a silicon substrate

Deformation of a two-phase block copolymer (BCP) during high velocity impacts is studied experimentally and theoretically with an aim to use this material in cold spray (CS) additive manufacturing. Micron scale (10–20 μm) spherical particles of polystyrene-block-polydimethylsiloxane (PS-b-PDMS) are impacted on a silicon substrate by using a laser-induced projectile impact test (LIPIT) setup with impact velocities in the range of 50–600 m/s. Experiments indicate that polymer particles adhere to the substrate when their impact velocities fall within the range of 140–500 m/s. A constitutive model that accounts for the effects of both strain rate and temperature on the mechanical behavior of such materials is developed. A critical energy release rate function which depends on the surface temperature and rate of separation is formulated and used in a cohesive zone model (CZM) to model bonding of the BCPs on the substrate. The model parameters are calibrated by comparing the deformed and computed deformed particle shapes and coefficient of restitution values of the rebounding particles. Simulations show that the particles experience ultra-high strain rates (>104 s−1), large deformation, and temperature elevation due to plastic dissipation and interfacial friction. The outer rim of the contact interface is predicted to experience temperature levels above the glass transition temperature of the PS-domain of the BCP. Bonding is correlated with increase of contact area, plastic dissipation and temperature rise in the interface.

Four-Leg Sustained Dynamic Compression Staple and Screw Generates Superior Lapidus Construct as Compared to Plate and Screw: A Biomechanical Study

Category: Ankle; Basic Sciences/Biologics Introduction/Purpose: Lapidus procedures are frequently used to treat moderate to severe hallux valgus in the setting of first tarsometatarsal instability or arthritis. The push for early weightbearing can lead to nonunion rates reaching 12%. A growing number of surgeons are utilizing orthopedic staples manufactured from NiTiNOL in foot and ankle arthrodesis procedures. These staples are inserted in an “open expanded” state which enables application of sustained dynamic compression (SDC) between bony segments, which can persist even when challenged with joint settling / bony resorption. The biomechanical performance of these devices has not been quantified in the setting of Lapidus procedures. This study determined the optimum SDC staple construct during Lapidus procedures, and to compare their performance to traditional plate and screw construct in an established model. Methods: Lapidus procedures were performed on synthetic bone replicates (Sawbones) using three SDC staple constructs and one static device construct: (1) dorsal four-leg inline SDC staple, (2) dorsal four-leg inline SDC staple with medial two-leg SDC staple (3) dorsal four-leg inline SDC staple with plantar medial-distal to lateral-proximal cannulated screw and (4) dorsomedial plate and distal dorsal to proximal plantar cannulated screw (FigA). Samples underwent four-point bending with calibrated servo- hydraulic machine (MTS). Max load and construct stiffness was calculated. Interfragmentary contact force, area, and pressure was measured with pressure sensor (Tekscan) and joint plantar gap measurements were completed with 3D motion capture system (Motion Analysis). Testing consisted of stepwise loading to 1, 2, and 3mm of crosshead displacement. For brevity, only data from 3mm displacement test are included herein. Data were analyzed with one-way ANOVA with Dunnett’s post hoc test (N=5/group). Results: The four-leg inline staple + two-leg staple and the four-leg inline staple + screw groups exhibited a 2.5x (p=0.03) and 7.5x (p <.001) increase in joint contact force as compared to the plate + screw group, respectively (FigB). The four-leg inline staple + two-leg staple and the four-leg inline staple + screw groups demonstrated a 3.5x (p=0.002) and 6.6x (p <.001) increase in joint contact area as compared to the plate + screw group, respectively (FigC). The four-leg inline staple + screw group exhibited a 65.1% reduction in plantar gap as compared to the plate + screw group (p=.039, FigD). Conclusion: The four-leg inline staple + screw group exhibited the optimal Lapidus construct biomechanical properties (greatest joint contact force, greatest joint contact area, and least plantar gap). The biomechanical testing performed herein was undertaken to simulate loading akin to early post-operative weight bearing. These data highlight the ability of the four-leg SDC NiTiNOL staple in combination with a static screw to provide meaningfully increased joint compression, uniformity of joint compression, and decreased construct motion as compared to the standard plate + screw construct that is frequently used clinically.

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 & 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.

Weight-Bearing Computed Tomography Evaluation of Acute SER4a Ankle Fractures

Category: Ankle; Trauma Introduction/Purpose: Recent studies have reported cadaveric and early clinical data regarding Weber B supination-external- rotation (SER)4a ankle fractures. SER4a ankle fractures have discordance of the medial clear space (MCS) between weight-bearing (WB) and gravity stress (GS) radiographs, showing instability on the GS view which normalizes with WB. Unfortunately, evaluation of the medial clear space only provides a two-dimensional evaluation of a three-dimensional structure. Prior cadaveric research has shown significant decreases in the tibiotalar joint contact area with only 1 mm of displacement of the talus. The purpose of this study is to utilize weight-bearing computed tomography (WBCT) to evaluate differences in the tibiotalar joint contact area and rotational alignment in patients with acute SER4a ankle fractures compared to their contralateral uninjured ankle. Methods: Adult patients presenting within ten days of injury with an isolated Weber B fibula fracture and a normal MCS ( < 5 mm) on injury radiographs were evaluated with weight-bearing and gravity stress radiographs. Patients determined to have isolated Weber B SER4a ankle fractures (GS positive but WB negative radiographs) and no history of injury to the contralateral ankle were included. The ability to place equal weight on both extremities was confirmed by standing on side-by-side scales. WBCT scans were obtained of both the healthy and injured ankles. The images were evaluated using Disior Bonelogic (Paragon28, Helsinki, Finland) software to calculate the tibiotalar joint contact area, axial tibiotalar rotation, and talocrural angle. An a priori power analysis was performed with six patients needed. Results: Five out of ten patients have been enrolled in the study to date (3 females, 2 males, average age 52.4). Preliminary data show an average 4.3% (standard deviation (SD) 4.9) increase in tibiotalar joint contact area between the injured and uninjured extremity. The average difference in axial tibiotalar angle is 3.6 (SD 6.9) degrees of internal rotation of the injured ankle. The talocrural in the injured and uninjured extremity have an average difference of 0.062 (SD 1.8) degrees. Conclusion: This study provides the first in vivo evaluation of the joint contact area changes seen in SER4 ankle fractures. Although limited in sample size, the results demonstrate that SER4a ankle fractures do not have decreased tibiotalar joint contact area relative to the healthy, uninjured side. The trend towards increased surface area in the injured ankles is hypothesized to be a result of the increased reliance on joint congruency with injury to the fibula and deltoid.