Compressive Peak Strain Research Articles

Ranking of osteogenic potential of physical exercises in postmenopausal women based on femoral neck strains.

The current study aimed to assess the potential of different exercises triggering an osteogenic response at the femoral neck in a group of postmenopausal women. The osteogenic potential was determined by ranking the peak hip contact forces (HCFs) and consequent peak tensile and compressive strains at the superior and inferior part of the femoral neck during activities such as (fast) walking, running and resistance training exercises. Results indicate that fast walking (5–6 km/h) running and hopping induced significantly higher strains at the femoral neck than walking at 4 km/h which is considered a baseline exercise for bone preservation. Exercises with a high fracture risk such as hopping, need to be considered carefully especially in a frail elderly population and may therefore not be suitable as a training exercise. Since superior femoral neck frailness is related to elevated hip fracture risk, exercises such as fast walking (above 5 km/h) and running can be highly recommended to stimulate this particular area. Our results suggest that a training program including fast walking (above 5 km/h) and running exercises may increase or preserve the bone mineral density (BMD) at the femoral neck.

Creep-fatigue interaction in a bimodal 12Cr-ODS steel

The present study reports on the creep-fatigue (CF) behavior of a ferritic bimodal 12Cr-ODS steel. The tests were performed in air at 550°C and 650°C by introducing hold-time at peak tensile, peak compressive and both peak tensile as well as compressive strains of ±0.5%. The symmetrical loops under pure fatigue/continuous cycling (CC) became asymmetrical upon introducing hold-time at either peak tensile or peak compressive strain. On the contrary, hold-time on both sides had no influence regarding symmetry but exhibited minor variation in peak stresses. For the investigated hold-time durations, the increase in hold-time period had a negligible effect on peak stresses. Due to stress relaxation during hold-time, elastic strain gets partially converted into inelastic strain which is relatively lower at 650°C than at 550°C. This eventually led to lower mean stresses; and finally, a relatively lower reduction in lifetime at 650°C than at 550°C. In comparison to similar CC conditions, CF loading resulted in an expeditious and prominent microstructural evolution, which assists in accumulating additional inelastic strain. Nevertheless, microstructural evolution appeared independent of the nature of the applied hold-time waveform which became pronounced with increase in temperature. The dislocation arrangements varied from grain to grain and even within the grains as a consequence of different grains sizes, orientations, and oxide particle distributions. Furthermore, in comparison to CC, an extensive W-enriched Laves phase precipitation/coarsening in the form of continuous or semi-continuous grain boundary networks was realized under CF loading. The damage studies revealed a single surface initiated transgranular crack under CC and multiple surface initiated transgranular cracks under CF loading. The transgranular crack path under CC possessed an intergranular tendency under CF loading which is obviously associated with the lower cyclic life.

Long term creep-fatigue interaction studies on India-specific reduced activation ferritic-martensitic (IN-RAFM) steel

Deterioration in fatigue life occurs due to introduction of hold in peak tensile or peak compressive or in both peak tensile and compressive strains in a triangular waveform. The hold period of a creep fatigue interaction (CFI) cycle denote the uninterrupted service period during which creep damage is predominant. CFI experiments comprising of long hold periods are relatively scarce but are indispensable for the understanding of long term materials behavior under CFI. Keeping this in view, CFI experiments have been performed at 823K on a commercial heat of reduced activation ferritic-martensitic (RAFM) steel using different directions of hold (tensile hold alone, compressive hold alone and simultaneous tensile and compressive holds) for different durations and by employing two strain amplitudes, ±0.25% and ±0.6%. The extent of deterioration in fatigue life in IN-RAFM steel is found to be dependent upon the direction of application of hold. Factors such as oxidation, plastic strain accumulation, mean stresses and stress relaxation during the application of the hold played a significant role in affecting CFI life of RAFM steel. The importance of stress relaxation (SR) in assessing fatigue damage under CFI has been described. A universal relationship between CFI life and SR has been arrived at and which is found to be independent of the type of the waveform, strain amplitude or the duration of hold. Though the relationship between stress relaxation and fatigue life is non-linear, the log–log plot between SR and CFI life value depict an inverse linear relationship. The sum of least squares of the offset from the linear fit, R2 is 0.93 which is a reasonably good approximation. The log–log plot of SR versus CFI life denotes the creep-fatigue interaction line and can be used to predict CFI life for a given amount of SR occurring in the material. The proposed relationship is applicable only for long duration CFI experiments and depicts microstructural creep-fatigue interaction in true sense. Test conditions that do not prominently show the effects of creep-fatigue interaction, do not fall on the CFI interaction line.

Thermal Damage on Claystone After Exposure to Different High Temperatures: A Review

Under or after high temperature treatment, the physical and mechanical characteristics of rocks will change obviously. This paper aimed to understand the effect of high temperatures on physical and mechanical properties (such as density, porosity, permeability, thermo physical properties, elastic modulus, compressive strength, and compressive peak strain) of claystone based on a large number of experimental data in reported references. First, the paper introduced some physical and chemical reactions of claystone under high temperature and their impact on production capacity, and then analyzed the changing characteristics of the physical and mechanical parameters of claystone under different temperatures. Finally, the background of engineering application of the heat effect on claystone was listed. The experimental data also indicated that the main reaction of mineral cements of claystone occurred between 300 and 600°C, especially in 400°C–600°C. 300 to 400°C maybe a critical threshold range of the thermal damage of claystone. All of the physical and mechanical parameters showed some deterioration due to the effect of different temperature. Thermal effect was useful for formation and exploitation of oil or natural gas. These results confirm the important link among physical and mechanical properties in response to thermal treatment.

Study on Behavior of RPC Filled Steel Tubular Stub Columns Under Axial Compression

Reactive powder concrete (RPC) with compressive strength higher than 170 MPa was prepared by using ordinary Portland cement, silica fume, ground granulated blast-furnace slag or fly ash. RPC shows high strength, constant Poisson’s ratio, high compressive peak strain but post-peak brittle failure. Based on the RPC mechanical behavior study, investigation of the interaction between steel tubes and core RPC was performed, in view of its effect on the bearing capacity and deformability of RPC filled steel tubular (RPCFT) stub columns subjected to axial loading. RPCFT stub columns have very high ductility and alleviate the RPC disadvantage of brittle failure. However, the confinement effect of steel tubes on RPC is lower than that of normal strength concrete and thus, it can be neglected during the design process for convenience and safety.

Low-Cost Methodology for Skin Strain Measurement of a Flexed Biological Limb.

The purpose of this manuscript is to compute skin strain data from a flexed biological limb, using portable, inexpensive, and easily available resources. We apply and evaluate this approach on a person with bilateral transtibial amputations, imaging left and right residual limbs in extended and flexed knee postures. We map 3-D deformations to a flexed biological limb using freeware and a simple point-and-shoot camera. Mean principal strain, maximum shear strain, as well as lines of maximum, minimum, and nonextension are computed from 3-D digital models to inform directional mappings of the strain field for an unloaded residual limb. Peak tensile strains are ∼0.3 on the anterior surface of the knee in the proximal region of the patella, whereas peak compressive strains are ∼ -0.5 on the posterior surface of the knee. Peak maximum shear strains are ∼0.3 on the posterior surface of the knee. The accuracy and precision of this methodology are assessed for a ground-truth model. The mean point location distance is found to be 0.08 cm, and the overall standard deviation for point location difference vectors is 0.05 cm. This low-cost and mobile methodology may prove critical for applications such as the prosthetic socket interface where whole-limb skin strain data are required from patients in the field outside of traditional, large-scale clinical centers. Such data may inform the design of wearable technologies that directly interface with human skin.

Biomechanical analysis of immediately loaded implants according to the “All-on-Four” concept

PurposeThe purpose of this study was to investigate the biomechanical behavior of immediately loaded implants in an edentulous mandible according to the “All-on-Four” concept. MethodsA 3D-finite element model of an edentulous mandible was constructed. Four implants were placed between the bilateral mental foramen according to “All-on-Four” concept. A framework made of titanium or acrylic resin between the bilateral first molars was modeled. Immediate loading and a delayed loading protocol were simulated. A vertical load of 200N was applied at the cantilever or on the abutments region of the distal implants, simulating the absence of a cantilever. ResultsThe peak principal compressive strains in the immediate loading models resulted in 24.0–35.8% and 26.4–39.0% increases compared with the delayed loading models under non-cantilever loading and cantilever loading, respectively. The loading position greatly affected the principal compressive and tensile strain values. The peak principal compressive strains in non-cantilever loading resulted in a 45.3–52.6% reduction compared with those in cantilever loading. The framework material did not influence the peak compressive and tensile strain. The maximum micromotion at the bone–implant interface in the immediate loading models was 7.5–14.4μm. ConclusionsMandibular fixed full-arch prostheses without cantilevers may result in a favorable reduction of the peri-implant bone strain during the healing period, compared with cantilevers. The maximum micromotion was within the acceptable limits for uneventful implant osseointegration in the immediate loading models. Framework material did not play an important role in reducing the peri-implant bone strain and micromotion at the bone–implant interface.

Study on Mechanical Properties and Constitutive Equation of Hybrid Fiber Reinforced Cementitious Composites Under Static Loading

The Quasi-static mechanical properties of hybrid fiber (steel fiber and Polyvinyl alcohol (PVA) fiber) reinforced cementitious composites (HFRCC(SP)) were investigated by compressive and tensile experiments. The compressive strength, peak strain, elastic modulus and tensile strength are studied as compared with that of engineered cementitious composite (ECC). Study results indicate that steel fibers can improve the compressive and tensile strength of HFRCC(SP) but the peak strain of HFRCC(SP) decreases. The formulas modified based on codes are proposed to calculate compressive peak strain, elastic modulus and tensile strength. The relationship between tensile strain at peak load and tensile strength of HFRCC with different volume fractions of polyethylene fiber and steel fiber were studied and the tensile stress-strain relation was presented. The parameters k1 and k2 of constitutive formulas for fiber reinforced high strength concrete presented by Mansur are modified to describe the stress-strain curve of HFRCC(SP), the modified formulas show good agreement with the experimental results.

A semi-empirical model for peak strain prediction of buried X80 steel pipelines under compression and bending at strike-slip fault crossings

The strike-slip fault is a main type of permanent ground deformation (PGD) faced by long distance gas pipelines. Based on the non-linear finite element method, a numerical model for buried pipelines at strike-slip fault crossings under compression combined with bending was proposed at first. The model, with the advantages of reduced time consumption and high precision, was proven to be reasonable by comparing the numerical results with previous researchers' experiments and numerical results. The peak compressive strain of X80 steel pipelines subjected to strike-slip fault displacement under compression combined with bending was studied using the FE model. According to the investigation of the Second West to East Gas Pipeline Project, suitable ranges of all parameters, including the pipe diameter, wall thickness, soil properties, fault displacement and crossing angle, were obtained. The influence of these parameters on the peak compressive strain was discussed in detail. A regression equation for predicting the peak compressive strain of X80 steel pipelines was derived based on approximately 800 numerical results and regression analysis, and the applicable range of the formula was given. 15 true design cases of the Second West to East Pipeline Project in China were investigated to demonstrate the accuracy and applicability of the proposed methodology by comparing the predicted peak compressive strain results with the FEM results. It was shown that the proposed semi-empirical model predicts the peak compressive strain with good accuracy much less time consumption. It is thus applicable for the strain-based and reliability-based design of X80 steel pipelines subjected to strike-slip fault displacement.

Higher strains in the inner region of the meniscus indicate a potential source for degeneration

Complex structural properties of menisci can be characterized in part by their inhomogeneous strain response under compression. This pilot study explored the feasibility to quantify characteristic strain distributions on meniscus cross-sections subjected to static compression using electronic speckle pattern interferometry (ESPI). Cross-sectional specimens of 5-mm thickness were harvested from eight human menisci. After application of 20% pre-strain, strain maps in response to 10μm compression were captured with ESPI. The 10μm compression induced an aggregate strain of nominally 0.14% and resulted in highly non-uniform strain distributions. Local compressive strain captured by ESPI ranged from 0.03% to 0.7%. The highest strain was in the central region of meniscus cross-sections, and the lowest magnitude of strain was at the femoral surface of the meniscus. After stratifying for age, peak compressive strain in older menisci (71±6 years, n=4) was 0.33%±0.09, compared to 0.25%±0.06 in younger menisci (34±9 years, n=4).In conclusion, this study captured for the first time continuous strain distribution maps over entire meniscus cross-sections. The non-uniform strain distributions demonstrated inhomogeneous structural properties. Age-related differences in characteristic strain distributions likely represent degenerative changes. As such, ESPI provides a novel strategy of further characterize meniscal function and degeneration.

The sensitivity of cartilage contact pressures in the knee joint to the size and shape of an anatomically shaped meniscal implant

Since meniscal geometry affects the cartilage contact pressures, it is essential to carefully define the geometry of the synthetic meniscal implant that we developed. Recently, six independent modes of size- and shape-related geometry variation were identified through 3D statistical shape modeling (SSM) of the medial meniscus. However, this model did not provide information on the functional importance of these geometry characteristics. Therefore, in this study finite element simulations were performed to determine the influence of anatomically-based meniscal implant size and shape variations on knee cartilage contact pressures.Finite element simulations of the knee joint were performed for a total medial meniscectomy, an allograft, the average implant geometry, six implant sizes and ten shape variations. The geometries of the allograft and all implant variations were based on the meniscus SSM. Cartilage contact pressures and implant tensile strains were evaluated in full extension under 1200N of axial compression.The average implant induced cartilage peak pressures intermediate between the allograft and meniscectomy and also reduced the cartilage area subjected to pressures >5MPa compared to the meniscectomy. The smaller implant sizes resulted in lower cartilage peak pressures and compressive strains than the allograft, yet high implant tensile strains were observed. Shape modes 2, 3 and 6 affected the cartilage contact stresses but to a lesser extent than the size variations. Shape modes 4 and 5 did not result in changes of the cartilage stress levels.The present study indicates that cartilage contact mechanics are more sensitive to implant size than to implant shape. Down-sizing the implant resulted in more favorable contact mechanics, but caused excessive material strains. Further evaluations are necessary to balance cartilage contact pressures and material strains to ensure cartilage protection and longevity of the implant.

Biomechanical evaluation of subcrestal dental implants with different bone anchorages

This study evaluated the biomechanical influence of apical bone anchorage on a single subcrestal dental implant using three-dimensional finite element analysis (FEA). Four different bone anchorage designs were simulated on a posterior maxillary segment using one implant with platform switching and internal Morse taper connection as follows: 2 mm subcrestal placement with (SW) or without (SO) the implant apex engaged into the cortical bone or position at bone level with anchorage only in the crestal cortical (BO) bone or with bicortical fixation (BW). Each implant received a premolar crown, and all models were loaded with 200 N to simulate centric and eccentric occlusion. The peak tensile and compressive stress and strain were calculated at the crestal cortical, trabecular, and apical cortical bone. The vertical and horizontal implant displacements were measured at the platform level. FEA indicated that subcrestal placement (SW and SO) created lower stress and strain in the crestal cortical bone compared with crestal placement (BO and BW models). The SW model exhibited lesser vertical and horizontal implant micromovement compared with the SO and BO models under eccentric loading; however, stress and strain were higher in the apical cortical bone. The BW model exhibited the lowest implant displacement. These results indicate that subcrestal placement decreases the stress in the crestal cortical bone of dental implants, regardless of apical anchorage; however, apical cortical anchorage can be effective in limiting implant displacement. Further studies are required to evaluate the effects of possible remodeling around the apex on the success of subcrestal implants.

Characterization of cancellous and cortical bone strain in the in vivo mouse tibial loading model using microCT-based finite element analysis

The in vivo mouse tibial loading model has been increasingly used to understand the mechanisms governing the mechanobiological responses of cancellous and cortical bone tissues to physical stimuli. Accurate characterization of the strain environment throughout the tibia is fundamental in relating localized mechanobiological processes to specific strain stimuli in the skeleton. MicroCT-based finite element analysis, together with diaphyseal strain gauge measures, was conducted to quantify the strain field in the tibiae of 16-wk-old female C57Bl/6 mice during in vivo dynamic compressive loading. Despite a strong correlation between the experimentally-measured and computationally-modeled strains at the gauge site, no correlations existed between the strain at the gauge site and the peak strains in the proximal cancellous and midshaft cortical bone, indicating the limitations of using a single diaphyseal strain gauge to estimate strain in the entire tibia. The peak compressive and tensile principal strain magnitudes in the proximal cancellous bone were 10% and 34% lower than those in the midshaft cortical bone. Sensitivity analyses showed that modeling bone tissue as a heterogeneous material had a strong effect on cancellous strain characterization while cortical strain and whole-bone stiffness were primarily affected by the presence of the fibula and the proximal boundary conditions. These results show that microCT-based finite element analysis combined with strain gauge measures provides detailed resolution of the tissue-level strain in both the cancellous and cortical bones of the mouse tibia during in vivo compression loading, which is necessary for interpreting localized patterns of modeling/remodeling and, potentially, gene and protein expression in skeletal mechanobiology studies.

Experimental and finite element analysis of strains induced by axial tibial compression in young-adult and old female C57Bl/6 mice

Axial compression of the mouse tibia is used to study strain-adaptive bone (re)modeling. In some studies, comparisons between mice of different ages are of interest. We characterized the tibial deformation and force–strain relationships in female C57Bl/6 mice at 5-, 12- and 22-months age. A three-gauge experimental method was used to determine the strain distribution at the mid-diaphysis, while specimen-specific finite element analysis was used to examine strain distribution along the tibial length. The peak strains in the tibial mid-diaphyseal cross-section are compressive and occur at the postero-lateral apex. The magnitudes of these peak compressive strains are 1.5 to 2 times those on the opposite, antero-medial face (a site often used for strain gauge placement). For example, −10N force applied to a 5-months old mouse engenders a peak compressive strain of −2800µε and a tensile strain on the antero-medial face of +1450µε. The orientation of the neutral axis at the mid-diaphysis did not differ with age (p=0.46), indicating a similar deformation mode in young and old tibiae. On the other hand, from 5- to 22-months there is a 25% reduction in cortical thickness and moment of inertia (p<0.05), resulting in significantly greater tibial strain magnitudes in older mice for equivalent applied force (p<0.05). We conclude that comparisons of tibial loading responses in young-adult and old C57Bl/6 tibiae are facilitated by similar deformation pattern across ages, but that modest adjustment of force levels is required to engender matching peak strains.

Correlation of muscle function and bone strain in the hindlimb of the river cooter turtle (Pseudemys concinna)

During terrestrial locomotion, limb muscles must generate mechanical work and stabilize joints against the ground reaction force. These demands can require high force production that imposes substantial loads on limb bones. To better understand how muscle contractile function influences patterns of bone loading in terrestrial locomotion, and refine force platform equilibrium models used to estimate limb bone safety factors, we correlated in vivo recordings of femoral strain with muscle activation and strain in a major propulsive hindlimb muscle, flexor tibialis internus (FTI), of a species with a published model of hindlimb force production (river cooter turtles, Pseudemys concinna). Electromyography (EMG) recordings indicate FTI activity prior to footfall that continues through approximately 50% of the stance phase. Large EMG bursts occur just after footfall when the muscle has reached its maximum length and is beginning to actively shorten, concurrent with increasing compressive strain on the anterior femur. The FTI muscle shortens through 35% of stance, with mean fascicle shortening strains reaching 14.0 ± 5.4% resting length (L0 ). At the time of peak compressive strains on the femur, the muscle fascicles remain active, but fascicles typically lengthen until mid-stance as the knee extends. Influenced by the activity of the dorsal knee extensor femorotibialis, the FTI muscle continues to passively lengthen simultaneously with knee extension and a shift to tensile axial strain on the anterior femur at approximately 40% of stance. The near coincidence in timing of peak compressive bone strain and peak muscle shortening (5.4 ± 4.1% stance) indicates a close correlation between the action of the hip extensor/knee flexor, FTI, and femoral loading in the cooter hindlimb. In the context of equilibrium models of limb bone loading, these results may help explain differences in safety factor estimates observed between previous force platform and in vivo strain analyses in cooters.

Does screw–bone interface modelling matter in finite element analyses?

The effect of screw–bone interface modelling strategies was evaluated in the setting of a tibial mid-shaft fracture stabilised using locking plates. Three interface models were examined: fully bonded interface; screw with sliding contact with bone; and screw with sliding contact with bone in an undersized pilot hole. For the simulation of the last interface condition we used a novel thermal expansion approach to generate the pre-stress that the bone would be exposed to during screw insertion. The study finds that the global load-deformation response is not influenced by the interface modelling approach employed; the deformation varied by less than 1% between different interaction models. However, interface modelling is found to have a considerable impact on the local stress–strain environment within the bone in the vicinity of the screws. Frictional and tied representations did not have significantly different peak strain values (<5% difference); the frictional interface had higher peak compressive strains while the tied interface had higher tensile strains. The undersized pilot hole simulation produced the largest strains. The peak minimum principal strains for the frictional interface were 26% of those for the undersized pilot hole simulation at a load of 770 N. It is concluded that the commonly used tie constraint can be used effectively when the only interest is the global load-deformation behaviour. Different contact interface models, however, alter the mechanical response around screw holes leading to different predictions for screw loosening, bone damage and stress shielding.

Experimental testing and analytical modeling of CFRP-confined large circular RC columns subjected to cyclic axial compression

A physically informed analytical model which describes the cyclic axial stress–strain behavior of reinforced concrete (RC) columns confined with fiber-reinforced polymer (FRP) composite wraps is necessary for the non-linear analysis and seismic design of such columns. Although extensive studies have been conducted on the monotonic axial behavior of small sized FRP-confined unreinforced concrete columns, there is a lack of research on FRP-confined RC columns and also their cyclic axial response. As a consequence, there is a lack of cyclic axial stress–strain models which can enable seismic response to be reliably simulated by numerical means. Due to such knowledge gaps, this study reports the results of thirty CFRP-confined large-scale unreinforced and RC circular columns subjected to monotonic and cyclic axial compression loading. The test results indicate that the overall shape of the stress–strain curves, the peak compressive stress and strain at failure, the unloading/reloading paths, and also the plastic strain of CFRP-confined concrete are influenced by the CFRP wrap and internal hoop steel reinforcement. On the basis of the experimental results, a cyclic stress–strain model for CFRP-confined circular RC columns is then proposed. The proposed model consists of three main components, namely (i) a monotonically ascending portion to describe the envelope curve, (ii) a polynomial equation to describe unloading paths, and (iii) a straight line to describe reloading paths. The influence of internal hoop reinforcement on the monotonic model, as well as the unloading and reloading responses is also considered. The accuracy of the proposed model is validated with the test data reported in this paper as well as other test results of relevance extracted from the open literature.

Conditions for thrust faulting in a glacier

Dipping, arcuate bands of debris‐rich ice outcropping near the margins of glaciers are often interpreted as thrust faults, assumed to originate in zones of longitudinal compression. Identification of thrusts is typically based either on the geometry and sedimentology of the debris bands or on the crystal fabric of surrounding ice, but the physical processes necessary to generate thrusts are rarely evaluated. Herein, we combine a numerical model of compressive ice flow near a glacier margin with theoretical stress and strain rate criteria for ice fracture and stress criteria for frictional slip to determine the conditions necessary for thrust faulting in glaciers. This model is applied to two different glaciological settings where longitudinal compression has been documented: (1) the transition between warm‐based and cold‐based ice near the terminus of Storglaciären, Sweden, and (2) the downglacier extent of the 1983 surge front of Variegated Glacier where surging ice encountered stagnant ice. Simulations representing the margin of Storglaciären indicate that peak compressive strain rates are six orders of magnitude too small to induce fracture, whereas at Variegated Glacier, strain rates were an order of magnitude too small for compressive fracture. In both groups of simulations, preexisting fractures governed by Coulomb friction are susceptible to slip if they span the ice thickness, are oriented close to the optimal fracture angle, and, in the case of Storglaciären, are subject to water pressures that are a large fraction of ice overburden pressure. Variations about the optimal fracture orientation, low or zero water pressure, high sliding friction coefficient, and limited vertical or lateral fracture extent each tend to suppress thrusting.

Osteochondral Grafting: Effect of Graft Alignment, Material Properties, and Articular Geometry

Osteochondral grafting for cartilage lesions is an attractive surgical procedure; however, the clinical results have not always been successful. Surgical recommendations differ with respect to donor site and graft placement technique. No clear biomechanical analysis of these surgical options has been reported. We hypothesized that differences in graft placement, graft biomechanical properties, and graft topography affect cartilage stresses and strains. A finite element model of articular cartilage and meniscus in a normal knee was constructed. The model was used to analyze the magnitude and the distribution of contact stresses, von Mises stresses, and compressive strains in the intact knee, after creation of an 8-mm diameter osteochondral defect, and after osteochondral grafting of the defect. The effects of graft placement, articular surface topography, and biomechanical properties were evaluated. The osteochondral defect generated minimal changes in peak contact stress (3.6 MPa) relative to the intact condition (3.4 MPa) but significantly increased peak von Mises stress (by 110%) and peak compressive strain (by 63%). A perfectly matched graft restored stresses and strains to near intact conditions. Leaving the graft proud by 0.5 mm generated the greatest increase in local stresses (peak contact stresses = 6.7 MPa). Reducing graft stiffness and curvature of articular surface had lesser effects on local stresses. Graft alignment, graft biomechanical properties, and graft topography all affected cartilage stresses and strains. Contact stresses, von Mises stresses, and compressive strains are biomechanical markers for potential tissue damage and cell death. Leaving the graft proud tends to jeopardize the graft by increasing the stresses and strains on the graft. From a biomechanical perspective, the ideal surgical procedure is a perfectly aligned graft with reasonably matched articular cartilage surface from a lower load-bearing region of the knee.

Two-dimensional strain fields on the cross-section of the human patellofemoral joint under physiological loading

The objective of this study was to provide a detailed experimental assessment of the two-dimensional cartilage strain distribution on the cross-section of the human patellofemoral joint (PFJ) subjected to physiological load magnitudes and rates. The medial side of six human PFJs sectioned along their mid-sagittal plane was loaded up to the equivalent of two body weights on a whole joint, and strain measurements obtained from digital image correlation are reported at 0.5 s. Normal strains tangential to the articular surface and shear strains in the plane of the cross-section showed consistent patterns among all specimens, whereas normal strains perpendicular to the articular surface exhibited some variability that may be attributed to subject-specific variations in material properties through the depth of the articular layers. Elevated tensile and compressive principal normal strains were observed near the articular surface, around the center of the contact region, with additional locations of elevated compressive strains occurring at the bone–cartilage interface. Under an average contact stress of ∼3.3 MPa, the peak compressive principal normal strains for the patella and femur averaged −0.158±0.072 and −0.118±0.051, respectively, magnitudes that are significantly greater than the relative changes in cartilage thickness, −0.090±0.030 and −0.072±0.038 ( p<0.005). These experimental results provide a detailed description of the manner by which human PFJ articular layers deform in situ under physiological load conditions.