Analysis of 3D strain in the human medial meniscus

The stability of repaired meniscal root can affect postoperative cartilage status following medial meniscus posterior root repair

BACKGROUND

Transtibial pullout repair yields beneficial clinical outcomes in patients with medial meniscus (MM) posterior root tear. However, the relationship between repaired meniscal root healing status and postoperative clinical outcomes remains unclear. We aimed to evaluate changes in articular cartilage damage and clinical scores after pullout repair using two simple stitches (TSS).

METHODS

Thirty-three patients who underwent pullout repair using TSS were assessed. Healing status was assessed by a semi-quantitative second-look arthroscopic scoring system comprising three evaluation criteria (width of bridging tissues, stability of the repaired root, and synovial coverage), 1 year postoperatively. MM medial extrusion (MMME) and cartilage damage were assessed preoperatively and 1 year postoperatively. The medial compartment was divided into 8 zones (A-H) for comparison of preoperative and 1-year postoperative cartilage damage. Clinical outcomes were evaluated using the Knee Injury and Osteoarthritis Outcome score, Lysholm score, International Knee Documentation Committee scores, and visual analogue scale pain score.

RESULTS

Although cartilage damage did not aggravate significantly in most medial compartment areas, MMME progressed at 1 year postoperatively. No statistical differences were observed in cartilage damage between the central-to-medial area of the medial femoral condyle and the medial tibial plateau area at 1 year postoperatively. Regarding semi-quantitative healing scores, the stability score was significantly correlated with the International Cartilage Repair Society grade at 1 year postoperatively. All 1-year and 2-year clinical scores significantly improved compared with the preoperative scores.

CONCLUSION

Regarding TSS repair, stability of repaired meniscal root negatively correlated with cartilage damage in the medial compartment loading area. All 1-year and 2-year clinical scores significantly improved than those of the preoperative scores. Achieving MM stability is crucial for suppressing cartilage degeneration.

LEVEL OF EVIDENCE

IV case series study.

Development of a method to investigate strain distribution across the cartilage-bone interface in guinea pig model of spontaneous osteoarthritis using lab-based contrast enhanced X-ray-computed tomography and digital volume correlation

OBJECTIVE

Strain changes at the cartilage-bone interface play a crucial role in osteoarthritis (OA) development. Contrast-Enhanced X-ray Computed Tomography (CECT) and Digital Volume Correlation (DVC) can measure 3D strain changes at the osteochondral interface. Using lab-based CT systems it is often difficult to visualise soft tissues such as articular cartilage without staining to enhance contrast. Contrast-Enhancing Staining Agents (CESAs), such as Phosphotungstic Acid (PTA) in 70% ethanol, can cause tissue shrinkage and alter tissue mechanics. The aims of this study were, firstly, to assess changes to the mechanical properties of osteochondral tissue after staining with a PTA/PBS solution, and secondly, to visualise articular cartilage during loading and with CECT imaging in order to compare strain across the interface in both healthy and OA joints using DVC.

DESIGN

Nanoindentation was used to assess changes to mechanical properties in articular cartilage and subchondral bone before and after staining. Hindlimbs from Dunkin-Hartley guinea pigs were stained with 1% PTA/PBS at room temperature for 6 days. Two consecutive CECT datasets were acquired for DVC error analysis. In-situ compression with a load corresponding to 2x body weight was applied, the specimen was re-imaged, and DVC was performed between the pre- and post-load tomograms.

RESULTS

Nanoindentation before and after PTA/PBS staining showed similar cartilage stiffness (p < 0.05), however, staining significantly decreased the stiffness of subchondral bone (∼9-fold; p = 0.0012). In severe OA specimens, third principal/compressive (εp3) strain was 141.7% higher and shear strain (γ) was 98.2% higher in tibial articular cartilage compared to non-OA (2 - month) specimens. A 23.1% increase in third principal stain strain and a 54.5% significant increase in the shear (γ) strain (p = 0.0027) was transferred into the mineralised regions of calcified cartilage and subchondral bone in severe OA specimens.

CONCLUSIONS

These results indicate the suitability of PTA in PBS as a contrast agent for the visualisation of cartilage during CECT imaging and allowed DVC computation of strain across the cartilage-bone interface. However, further research is needed to address the reduction in stiffness of subchondral bone after incubation in PBS.

Tensile energy dissipation and mechanical properties of the knee meniscus: relationship with fiber orientation, tissue layer, and water content

Introduction: The knee meniscus distributes and dampens mechanical loads. It is composed of water (∼70%) and a porous fibrous matrix (∼30%) with a central core that is reinforced by circumferential collagen fibers enclosed by mesh-like superficial tibial and femoral layers. Daily loading activities produce mechanical tensile loads which are transferred through and dissipated by the meniscus. Therefore, the objective of this study was to measure how tensile mechanical properties and extent of energy dissipation vary by tension direction, meniscal layer, and water content. Methods: The central regions of porcine meniscal pairs (n = 8) were cut into tensile samples (4.7 mm length, 2.1 mm width, and 0.356 mm thickness) from core, femoral and tibial components. Core samples were prepared parallel (circumferential) and perpendicular (radial) to the fibers. Tensile testing consisted of frequency sweeps (0.01-1Hz) followed by quasi-static loading to failure. Dynamic testing yielded energy dissipation (ED), complex modulus (E*), and phase shift (δ) while quasi-static tests yielded Young's Modulus (E), ultimate tensile strength (UTS), and strain at UTS (ε_UTS). To investigate how ED is influenced by the specific mechanical parameters, linear regressions were performed. Correlations between sample water content (φ_w) and mechanical properties were investigated. A total of 64 samples were evaluated. Results: Dynamic tests showed that increasing loading frequency significantly reduced ED (p < 0.05). Circumferential samples had higher ED, E*, E, and UTS than radial ones (p < 0.001). Stiffness was highly correlated with ED (R² > 0.75, p < 0.01). No differences were found between superficial and circumferential core layers. ED, E*, E, and UTS trended negatively with φ_w (p < 0.05). Discussion: Energy dissipation, stiffness, and strength are highly dependent on loading direction. A significant amount of energy dissipation may be associated with time-dependent reorganization of matrix fibers. This is the first study to analyze the tensile dynamic properties and energy dissipation of the meniscus surface layers. Results provide new insights on the mechanics and function of meniscal tissue.

Strain-Dependent Diffusivity of Small and Large Molecules in Meniscus

Due to lack of full vascularization, the meniscus relies on diffusion through the extracellular matrix to deliver small (e.g., nutrients) and large (e.g., proteins) to resident cells. Under normal physiological conditions, the meniscus undergoes up to 20% compressive strains. While previous studies characterized solute diffusivity in the uncompressed meniscus, to date, little is known about the diffusive transport under physiological strain levels. This information is crucial to fully understand the pathophysiology of the meniscus. The objective of this study was to investigate strain-dependent diffusive properties of the meniscus fibrocartilage. Tissue samples were harvested from the central portion of porcine medial menisci and tested via fluorescence recovery after photobleaching to measure diffusivity of fluorescein (332 Da) and 40 K Da dextran (D40K) under 0%, 10%, and 20% compressive strain. Specifically, average diffusion coefficient and anisotropic ratio, defined as the ratio of the diffusion coefficient in the direction of the tissue collagen fibers to that orthogonal, were determined. For all the experimental conditions investigated, fluorescein diffusivity was statistically faster than that of D40K. Also, for both molecules, diffusion coefficients significantly decreased, up to ∼45%, as the strain increased. In contrast, the anisotropic ratios of both molecules were similar and not affected by the strain applied to the tissue. This suggests that compressive strains used in this study did not alter the diffusive pathways in the meniscus. Our findings provide new knowledge on the transport properties of the meniscus fibrocartilage that can be leveraged to further understand tissue pathophysiology and approaches to tissue restoration.

Experiments and hyperelastic modeling of porcine meniscus show heterogeneity at high strains

Constitutive modeling of the meniscus is critical in areas like knee surgery and tissue engineering. At low strain rates, the meniscus can be described using a hyperelastic model. Calibration of hyperelastic material models of the meniscus is challenging on many fronts due to material variability and friction. In this study, we present a framework to determine the hyperelastic material parameters of porcine meniscus (and similar soft tissues) using no-slip uniaxial compression experiments. Because of the nonhomogeneous deformation in the specimens, a finite element solution is required at each step of the iterative calibration process. We employ a Bayesian calibration approach to account for the inherent material variability and a Bayesian optimization approach to minimize the resulting cost function in the material parameter space. Cylindrical specimens of porcine meniscus from the anterior, middle and posterior regions are tested up to 30% compressive strain and the Yeoh form of hyperelastic strain energy density function is used to describe the material response. The results show that the Yeoh form is able to accurately describe the compressive response of porcine meniscus and that the Bayesian calibration and optimization approaches are able to calibrate the model in a computationally efficient manner while taking into account the inherent material variability. The results also show that the shear modulus or the initial stiffness is roughly uniform across the different areas of the meniscus, but there is significant spatial heterogeneity in the response at high strains. In particular, the middle region is considerably stiffer at high strains. This heterogeneity is important to consider in modeling the response of the meniscus for clinical applications.

Comparison of posterior root remnant cells and horn cells of the medial meniscus

PURPOSE/AIM OF THE STUDY

Previous studies have noted distinctions between medial meniscus posterior root and horn cells. However, the characteristics of root remnant cells have not been explored in detail. The purpose of this study was to evaluate the gene expression levels, proliferation, and resistance to mechanical stress of remnant and horn cells.

MATERIALS AND METHODS

Medial meniscus tissue samples were obtained from patients who underwent total or uni-compartmental knee arthroplasty. Cellular morphology, sry-type HMG box 9, type II collagen, and chondromodulin-I gene expression levels were analyzed. Collagen synthesis was assessed by immunofluorescence staining. Proliferation analysis after 4 h-cyclic tensile strain was performed.

RESULTS

Horn cells displayed triangular morphology, whereas root remnant cells appeared fibroblast-like. sry-type HMG box 9 mRNA expression levels were similar in both cells, but type II collagen and chondromodulin-I mRNA expressions were observed only in horn cells. The ratio of type II collagen-positive cells in horn cells was about 10-fold higher than that in root remnant cells, whereas the ratio of sry-type HMG box 9-positive cells was similar. A significant increase in proliferation was observed in root remnant cells compared to that in horn cells. Further, under cyclic tensile strain, the survival rate was higher in root remnant cells than in horn cells.

CONCLUSIONS

Medial meniscus root remnant cells showed higher proliferation and resistant properties to cyclic tensile strain than horn cells and showed no chondromodulin-I expression. Preserving the medial meniscus posterior root remnant during pullout repair surgery might maintain mechanical stress-resistant tissue and support healing.

Mechanisms of energy dissipation and relationship with tissue composition in human meniscus

OBJECTIVE

The human meniscus is essential in maintaining proper knee joint function. The meniscus absorbs shock, distributes loads, and stabilizes the knee joint to prevent the onset of osteoarthritis. The extent of its shock-absorbing role can be estimated by measuring the energy dissipated by the meniscus during cyclic mechanical loading.

METHODS

Samples were prepared from the central and horn regions of medial and lateral human menisci from 8 donors (both knees for total of 16 samples). Cyclic compression tests at several compression strains and frequencies yielded the energy dissipated per tissue volume. A GEE regression model was used to investigate the effects of compression, meniscal side and region, and water content on energy dissipation in order to account for repeated measures within samples.

RESULTS

Energy dissipation by the meniscus increased with compressive strain from ∼0.1 kJ/m³ (at 10% strain) to ∼10 kJ/m³ (at 20% strain) and decreased with loading frequency. Samples from the anterior region provided the largest energy dissipation when compared to central and posterior samples (P < 0.05). Water content for the 16 meniscal tissues was 77.9 (C.I. 72.0-83.8%) of the total tissue mass. A negative correlation was found between energy dissipation and water content (P < 0.05).

CONCLUSION

The extent of energy dissipated by the meniscus is inversely related to loading frequency and meniscal water content.

Tensile fatigue strength and endurance limit of human meniscus

The knee menisci are prone to mechanical fatigue injury from the cyclic tensile stresses that are generated during daily joint loading. Here we characterize the tensile fatigue behavior of human medial meniscus and investigate the effect of aging on fatigue strength. Test specimens were excised from the medial meniscus of young (under 40 years) and older (over 65 years) fresh-frozen cadaver knees. Cyclic uniaxial tensile loads were applied parallel to the primary circumferential fibers at 70%, 50%, 40%, or 30% of the predicted ultimate tensile strength (UTS) until failure occurred or one million cycles was reached. Equations for fatigue strength (S-N curve) and the probability of fatigue failure (unreliability curves) were created from the measured number of cycles to failure. The mean number of cycles to failure at 70%, 50%, 40%, and 30% of UTS were estimated to be approximately 500, 40000, 340000, and 3 million cycles, respectively. The endurance limit, defined as the tensile stress that can be safely applied for the average lifetime of use (250 million cycles), was estimated to be 10% of UTS (∼1.0 MPa). When cyclic tensile stresses exceeded 30% of UTS (∼3.0 MPa), the probability of fatigue failure rapidly increased. While older menisci were generally weaker and more susceptible to fatigue failures at high-magnitude tensile stresses, both young and older age groups had similar fatigue resistance at low-magnitude tensile stresses. In addition, we found that fatigue failures occurred after the dynamic modulus decreased during cyclic loading by approximately 20%. This experimental study has quantified fundamental fatigue properties that are essential to properly predict and prevent injury in meniscus and other soft fibrous tissues.

Multiscale Strain Transfer in Cartilage

The transfer of stress and strain signals between the extracellular matrix (ECM) and cells is crucial for biochemical and biomechanical cues that are required for tissue morphogenesis, differentiation, growth, and homeostasis. In cartilage tissue, the heterogeneity in spatial variation of ECM molecules leads to a depth-dependent non-uniform strain transfer and alters the magnitude of forces sensed by cells in articular and fibrocartilage, influencing chondrocyte metabolism and biochemical response. It is not fully established how these nonuniform forces ultimately influence cartilage health, maintenance, and integrity. To comprehend tissue remodelling in health and disease, it is fundamental to investigate how these forces, the ECM, and cells interrelate. However, not much is known about the relationship between applied mechanical stimulus and resulting spatial variations in magnitude and sense of mechanical stimuli within the chondrocyte’s microenvironment. Investigating multiscale strain transfer and hierarchical structure-function relationships in cartilage is key to unravelling how cells receive signals and how they are transformed into biosynthetic responses. Therefore, this article first reviews different cartilage types and chondrocyte mechanosensing. Following this, multiscale strain transfer through cartilage tissue and the involvement of individual ECM components are discussed. Finally, insights to further understand multiscale strain transfer in cartilage are outlined.

Anisotropy and inhomogeneity of permeability and fibrous network response in the pars intermedia of the human lateral meniscus

Within the human tibiofemoral joint, meniscus plays a key role due to its peculiar time-dependent mechanical characteristics, inhomogeneous structure and compositional features. To better understand the pathophysiological mechanisms underlying this essential component, it is mandatory to analyze in depth the relationship between its structure and the function it performs in the joint. Accordingly, the aim of this study was to evaluate the behavior of both solid and fluid phases of human meniscus in response to compressive loads, by integrating mechanical assessment and histological analysis. Cubic specimens were harvested from seven knee lateral menisci, specifically from anterior horn, pars intermedia and posterior horn; unconfined compressive tests were then performed according to three main loading directions (i.e., radial, circumferential and vertical). Fibril modulus, matrix modulus and hydraulic permeability of the tissue were thence estimated through a fibril-network-reinforced biphasic model. Tissue porosity and collagen fibers arrangement were assessed through histology for each region and related to the loading directions adopted during mechanical tests. Regional and strain-dependent constitutive parameters were finally proposed for the human lateral meniscus, suggesting an isotropic behavior of both the horns, and a transversely isotropic response of the pars intermedia. Furthermore, the histological findings supported the evidences highlighted by the compressive tests. Indeed, this study provided novel insights concerning the functional behavior of human menisci by integrating mechanical and histological characterizations and thus highlighting the key role of this component in knee contact mechanics and presenting fundamental information that can be used in the development of tissue-engineered substitutes. STATEMENT OF SIGNIFICANCE: This work presents an integration to the approaches currently used to model the mechanical behavior of the meniscal tissue. This study assessed in detail the regional and directional contributions of both the meniscal solid and fluid phases during compressive response, providing also complementary histological evidence. Within this updated perspective, both knee computational modeling and meniscal tissue engineering can be improved to have an effective impact on the clinical practice.

Differences between the root and horn cells of the human medial meniscus from the osteoarthritic knee in cellular characteristics and responses to mechanical stress

BACKGROUND

Many histological, mechanical, and clinical studies have been performed on the medial meniscus posterior root attachment, as it often tears in patients with osteoarthritic knee. Medial meniscal root repair is recommended in clinical situations; however, to date, no studies have examined the differences between meniscal root and horn cells. The aim of this study was, therefore, to investigate the morphology, reaction to cyclic tensile strain, and gene expression levels of medial meniscal root and horn cells.

METHODS

Meniscal samples were obtained from the medial knee compartments of 10 patients with osteoarthritis who underwent total knee arthroplasty. Root and horn cells were cultured in Dulbecco's modified Eagle's medium without enzymes. The morphology, distribution, and proliferation of medial meniscal root and horn cells, as well as the gene and protein expression levels of Sry-type HMG box 9 and type II collagen, were determined after cyclic tensile strain treatment.

RESULTS

Horn cells had a triangular morphology, whereas root cells were fibroblast-like. The number of horn cells positive for Sry-type HMG box 9 and type II collagen was considerably higher than that of root cells. Although root and horn cells showed similar levels of proliferation after 48, 72, or 96 h of culture, more horn cells than root cells were lost following a 2-h treatment with 5% and 10% cyclic tensile. Sry-type HMG box 9 and α1(II) collagen mRNA expression levels were significantly enhanced in both cells after 2- and 4-h cyclic tensile strain (5%) treatment.

CONCLUSIONS

Medial meniscal root and horn cells have distinct morphologies, reactions to mechanical stress, and cellular phenotypes. Our results suggest that physiological tensile strain is important to activate extracellular matrix production in horn cells.

Degeneration Affects Three-Dimensional Strains in Human Menisci: In situ MRI Acquisition Combined With Image Registration

Degenerative changes of menisci contribute to the evolution of osteoarthritis in the knee joint, because they alter the load transmission to the adjacent articular cartilage. Identifying alterations in the strain response of meniscal tissue under compression that are associated with progressive degeneration may uncover links between biomechanical function and meniscal degeneration. Therefore, the goal of this study was to investigate how degeneration effects the three-dimensional (3D; axial, circumferential, radial) strain in different anatomical regions of human menisci (anterior and posterior root attachment; anterior and posterior horn; pars intermedia) under simulated compression. Magnetic resonance imaging (MRI) was performed to acquire image sequences of 12 mild and 12 severe degenerated knee joints under unloaded and loaded [25%, 50% and 100% body weight (BW)] conditions using a customized loading device. Medial and lateral menisci as well as their root attachments were manually segmented. Intensity-based rigid and non-rigid image registration were performed to obtain 3D deformation fields under the respective load levels. Finally, the 3D voxels were transformed into hexahedral finite-element models and direction-dependent local strain distributions were determined. The axial compressive strain in menisci and meniscal root attachments significantly increased on average from 3.1% in mild degenerated joints to 7.3% in severe degenerated knees at 100% BW (p ≤ 0.021). In severe degenerated knee joints, the menisci displayed a mean circumferential strain of 0.45% (mild: 0.35%) and a mean radial strain of 0.41% (mild: 0.37%) at a load level of 100% BW. No significant changes were observed in the circumferential or radial directions between mild and severe degenerated knee joints for all load levels (p > 0.05). In conclusion, high-resolution MRI was successfully combined with image registration to investigate spatial strain distributions of the meniscus and its attachments in response to compression. The results of the current study highlight that the compressive integrity of the meniscus decreases with progressing tissue degeneration, whereas the tensile properties are maintained.

Bovine Meniscus Middle Zone Tissue: Measurement of Collagen Fibril Behavior During Compression

BACKGROUND

Type I collagen is the major component of the extracellular matrix of the knee's meniscus and plays a central role in that joint's biomechanical properties. Repair and reconstruction of tissue damage often requires a scaffold to assist the body to rebuild. The middle zone of bovine meniscus is a material that may be useful for the preparation of extracellular matrix scaffolds.

METHODS

Here, synchrotron-based small-angle X-ray scattering (SAXS) patterns of bovine meniscus were collected during unconfined compression. Collagen fibril orientation, D-spacing, compression distance and force were measured.

RESULTS

The collagen fibrils in middle zone meniscal fibrocartilage become more highly oriented perpendicular to the direction of compression. The D-spacing also increases, from 65.0 to 66.3 nm with compression up to 0.43 MPa, representing a 1.8% elongation of collagen fibrils perpendicular to the compression.

CONCLUSION

The elasticity of the collagen fibrils under tension along their length when the meniscus is compressed, therefore, contributes to the overall elastic response of the meniscus only under loads that exceed those likely to be experienced physiologically.

Personalized Fiber-Reinforcement Networks for Meniscus Reconstruction

The menisci are fibrocartilaginous tissues that are crucial to the load-sharing and stability of the knee, and when injured, these properties are compromised. Meniscus replacement scaffolds have utilized the circumferential alignment of fibers to recapitulate the microstructure of the native meniscus; however, specific consideration of size, shape, and morphology has been largely overlooked. The purpose of this study was to personalize the fiber-reinforcement network of a meniscus reconstruction scaffold. Human cadaveric menisci were measured for a host of tissue (length, width) and subtissue (regional widths, root locations) properties, which all showed considerable variability between donors. Next, the asymmetrical fiber network was optimized to minimize the error between the dimensions of measured menisci and predicted fiber networks, providing a 51.0% decrease (p = 0.0091) in root-mean-square (RMS) error. Finally, a separate set of human cadaveric knees was obtained, and donor-specific fiber-reinforced scaffolds were fabricated. Under cyclic loading for load-distribution analysis, in situ implantation of personalized scaffolds following total meniscectomy restored contact area (253.0 mm2 to 488.9 mm2, p = 0.0060) and decreased contact stress (1.96 MPa to 1.03 MPa, p = 0.0025) to near-native values (597.4 mm2 and 0.83 MPa). Clinical use of personalized meniscus devices that restore physiologic contact stress distributions may prevent the development of post-traumatic osteoarthritis following meniscal injury.

Insertion of Small Diameter Radiopaque Tracking Beads into the Anterior Cruciate Ligament Results in Repeatable Strain Measurement Without Affecting the Material Properties

The characterization of ligamentous soft tissue properties is limited by a lack of measurement methods capable of minimally-invasively quantifying regional strain. Previous implementations of radiographic imaging and tissue-embedded radiopaque markers demonstrated promising regional strain measurements, but found error associated with non-repeatable bead positions within the tissue after load application. No study has investigated the effects of cyclic loading on the strains measured within the tissue. The purpose of this study was to quantify the effect of joint loading on strain measurement using radiopaque markers and micro-computed tomography imaging. Six cadaveric porcine femur-anterior cruciate ligament-tibia complexes were instrumented with small diameter (0.8 mm) zirconium dioxide marker beads. The compound was imaged at 10 N then at 100 N of anterior force using micro-computed tomography. The bead positions in the images were used to calculate tissue strain between the 10 and 100 N anterior joint load conditions. Up to 100 intermediate joint cycles were applied, then images were acquired again at 10 and 100 N anterior force. No statistically significant difference was found between the strains measured before and after intermediate cycling (p > 0.05). This indicates that tissue loading did not introduce statistically significant changes to strains measured in tissue tested following this methodology.

Brain Strain from Motion of Sparse Markers

Brain strain secondary to head impact or inertial loading is closely associated with pathologic observations in the brain. The only experimental brain strain dataset under loadings close to traumatic levels was calculated by imposing the experimentally measured motion of markers embedded in the brain to an auxiliary model formed by triad elements (Hardy et al., 2007). However, fidelity of the calculated strain as well as the suitability of using triad elements for three-dimensional (3D) strain estimation remains to be verified. Therefore, this study proposes to use tetrahedron elements as a new approach to estimate the brain strain. Fidelity of this newly-proposed approach along with the previous triad-based approach is evaluated with the aid of three independently-developed finite element (FE) head models by numerically replicating the experimental impacts and strain estimation procedures. Strain in the preselected brain elements obtained from the whole head simulation exhibits good correlation with its tetra estimation and exceeds its triad estimation, indicating that the tetra approach more accurately estimates the strain in the preselected region. The newly calculated brain strain curves using tetra elements provide better approximations for the 3D experimental brain deformation and can be used for strain validation of FE models of human head.

Development and Assessment of a Microcomputed Tomography Compatible Five Degrees-of-Freedom Knee Joint Motion Simulator

Currently available knee joint kinematic tracking systems fail to nondestructively capture the subtle variation in joint and soft tissue kinematics that occur in native, injured, and reconstructed joint states. Microcomputed tomography (CT) imaging has the potential as a noninvasive, high-resolution kinematic tracking system, but no dynamic simulators exist to take advantage of this. The purpose of this work was to develop and assess a novel micro-CT compatible knee joint simulator to quantify the knee joint's kinematic and kinetic response to clinically (e.g., pivot shift test) and functionally (e.g., gait) relevant loading. The simulator applies closed-loop, load control over four degrees-of-freedom (DOF) (internal/external rotation, varus/valgus rotation, anterior/posterior translation, and compression/distraction), and static control over a fifth degree-of-freedom (flexion/extension). Simulator accuracy (e.g., load error) and repeatability (e.g., coefficient of variation) were assessed with a cylindrical rubber tubing structure and a human cadaveric knee joint by applying clinically and functionally relevant loads along all active axes. Micro-CT images acquired of the joint at a loaded state were then used to calculate joint kinematics. The simulator loaded both the rubber tubing and the cadaveric specimen to within 0.1% of the load target, with an intertrial coefficient of variation below 0.1% for all clinically relevant loading protocols. The resultant kinematics calculated from the acquired images agreed with previously published values, and produced errors of 1.66 mm, 0.90 mm, 4.41 deg, and 1.60 deg with respect to anterior translation, compression, internal rotation, and valgus rotation, respectively. All images were free of artifacts and showed knee joint displacements in response to clinically and functionally loading with isotropic CT image voxel spacing of 0.15 mm. The results of this study demonstrate that the joint-motion simulator is capable of applying accurate, clinically and functionally relevant loads to cadaveric knee joints, concurrent with micro-CT imaging. Nondestructive tracking of bony landmarks allows for the precise calculation of joint kinematics with less error than traditional optical tracking systems.

The effect of unloader knee braces on medial meniscal strain

BACKGROUND:

A medial meniscal tear is a common knee injury, especially following an anterior cruciate ligament injury. Decreasing the compressive force on the medial meniscus during dynamic activities using an unloader knee brace could reduce meniscal strain, effectively reducing injury risk and/or severity.

OBJECTIVES:

To investigate the efficacy of two unloader knee braces on medial meniscus strain during dynamic activities in intact & deficient anterior cruciate ligament states.

STUDY DESIGN:

Combined in vivo/in vitro study.

METHODS:

In vivo knee kinematics and muscle force profiles from a healthy individual performing single/doubleleg squats and walking motions were simulated on 10 cadaveric specimens using a dynamic knee simulator system. Simulations were performed on knees in unbraced and braced scenarios, with and without the anterior cruciate ligament. Anterior and posterior medial meniscal strains were measured.

RESULTS:

Two different braces each showed a significant reduction in the posteromedial meniscal strain ( p ⩽ 0.01) in an intact anterior cruciate ligament state. Neither brace mirrored this result for the anteromedial strain ( p > 0.05). In the deficient anterior cruciate ligament state, the braces had no significant effect on strain ( p > 0.05).

CONCLUSION:

Two unloader knee braces effectively reduced strain in the medial meniscus with an intact anterior cruciate ligament during dynamic activities. Neither brace made a significant reduction in strain for anterior cruciate ligament-deficient knees.

CLINICAL RELEVANCE

Unloader knee braces could be used to reduce the medial meniscus strain following meniscal surgery and during rehabilitation in patients with an isolated medial meniscus injury. However, these braces cannot be recommended for this purpose in patients with an anterior cruciate ligament deficiency.

Experimental mechanical strain measurement of tissues

Strain, an important biomechanical factor, occurs at different scales from molecules and cells to tissues and organs in physiological conditions. Under mechanical strain, the strength of tissues and their micro- and nanocomponents, the structure, proliferation, differentiation and apoptosis of cells and even the cytokines expressed by cells probably shift. Thus, the measurement of mechanical strain (i.e., relative displacement or deformation) is critical to understand functional changes in tissues, and to elucidate basic relationships between mechanical loading and tissue response. In the last decades, a great number of methods have been developed and applied to measure the deformations and mechanical strains in tissues comprising bone, tendon, ligament, muscle and brain as well as blood vessels. In this article, we have reviewed the mechanical strain measurement from six aspects: electro-based, light-based, ultrasound-based, magnetic resonance-based and computed tomography-based techniques, and the texture correlation-based image processing method. The review may help solving the problems of experimental and mechanical strain measurement of tissues under different measurement environments.

3D strain in native medial meniscus is comparable to medial meniscus allograft transplant

PURPOSE

Injury or degeneration of the meniscus has been associated with the development of osteoarthritis of the knee joint. Meniscal allograft transplant (MAT) has been shown to reduce pain and restore function in patients who remain symptomatic following meniscectomy. The purpose of this study is to evaluate and compare the three-dimensional (3D) strain in native medial menisci compared to allograft-transplanted medial menisci in both the loaded and unloaded states.

METHODS

Ten human cadaveric knees underwent medial MAT, utilizing soft-tissue anterior and posterior root fixation via transosseous sutures tied over an anterolateral proximal tibial cortical bone bridge. The joint was imaged first in the non-loaded state, then was positioned at 5° of flexion and loaded to 1× body weight (650 ± 160 N) during MR image acquisition. Anatomical landmarks were chosen from each image to create a tibial coordinate system, which were then input into a custom-written program (Matlab R2014a) to calculate the 3D strain from the unloaded and loaded marker positions. Six independent strains were obtained: three principal strains and three shearing strains.

RESULTS

No statistically significant difference was found between the middle and posterior strains in the native knee compared to the meniscus allograft. This would suggest that soft-tissue fixation of meniscal allografts results in similar time zero principal and shear strains in comparison to the native meniscus.

CONCLUSION

These results suggest that time zero MAT performs in a similar manner to the native meniscus. Optimizing MAT strain behavior may lead to potential improvements in its chondroprotective effect.

Short cracks in knee meniscus tissue cause strain concentrations, but do not reduce ultimate stress, in single-cycle uniaxial tension

Tears are central to knee meniscus pathology and, from a mechanical perspective, are crack-like defects (cracks). In many materials, cracks create stress concentrations that cause progressive local rupture and reduce effective strength. It is currently unknown if cracks in meniscus have these consequences; if they do, this would have repercussions for management of meniscus pathology. The objective of this study was to determine if a short crack in meniscus tissue, which mimics a preclinical meniscus tear, (a) causes crack growth and reduces effective strength, (b) creates a near-tip strain concentration and (c) creates unloaded regions on either side of the crack. Specimens with and without cracks were tested in uniaxial tension and compared in terms of macroscopic stress-strain curves and digital image correlation strain fields. The strain fields were used as an indicator of stress concentrations and unloaded regions. Effective strength was found to be insensitive to the presence of a crack (potential effect < 0.86 s.d.; β = 0.2), but significant strain concentrations, which have the potential to lead to long-term accumulation of tissue or cell damage, were observed near the crack tip.

Fatigue life of bovine meniscus under longitudinal and transverse tensile loading

The knee meniscus is composed of a fibrous extracellular matrix that is subjected to large and repeated loads. Consequently, the meniscus is frequently torn, and a potential mechanism for failure is fatigue. The objective of this study was to measure the fatigue life of bovine meniscus when applying cyclic tensile loads either longitudinal or transverse to the principal fiber direction. Fatigue experiments consisted of cyclic loads to 60%, 70%, 80% or 90% of the predicted ultimate tensile strength until failure occurred or 20,000 cycles was reached. The fatigue data in each group was fit with a Weibull distribution to generate plots of stress level vs. cycles to failure (S-N curve). Results showed that loading transverse to the principal fiber direction gave a two-fold increase in failure strain, a three-fold increase in creep, and a nearly four-fold increase in cycles to failure (not significant), compared to loading longitudinal to the principal fiber direction. The S-N curves had strong negative correlations between the stress level and the mean cycles to failure for both loading directions, where the slope of the transverse S-N curve was 11% less than the longitudinal S-N curve (longitudinal: S=108-5.9ln(N); transverse: S=112-5.2ln(N)). Collectively, these results suggest that the non-fibrillar matrix is more resistant to fatigue failure than the collagen fibers. Results from this study are relevant to understanding the etiology of atraumatic radial and horizontal meniscal tears, and can be utilized by research groups that are working to develop meniscus implants with fatigue properties that mimic healthy tissue.