Test configuration
Articular Cartilage
Articular cartilage is the smooth white tissue that covers the ends of bones where they come together to form joints.
About this material
Articular cartilage is the smooth white tissue that covers the ends of bones where they come together to form joints. Healthy cartilage in our joints makes it easier to move. It allows the bones to glide over each other with very little friction and improves the load transfer between them. Articular cartilage can be damaged by injury or normal wear and tear. Its mechanical properties are distributed over each entire articular surface in response to the local mechanical stresses acting inside the joint.
Mechanical testing applications
Mechanical testing of articular cartilage is usually performed ex vivo on entire surfaces or biopsies collected from preclinical studies, tissue banks, total knee replacements or arthroscopic surgical procedures. Mechanical testing is an invaluable outcome measure for the validation of cartilage repair treatments. It is also used in the study of osteoarthritis, prosthesis-induced wear patterns, as well as in the fundamental study of cartilage.
Test configurations (14)
Test configuration
Tension
Test configuration
Unconfined Compression
Test configuration
Indentation (Stiffness)
Test configuration
Friction
Test configuration
3D Thickness Mapping
Test configuration
Torsion
Test configuration
Shear
Test configuration
Impact
Test configuration
Confined Compression
Test configuration
Temperature Control
Test configuration
Positioning (Manual Tilt)
Test configuration
Electromechanical Measurement
Test configuration
Camera Feed
Case studies (9)
Related resources (17)
FAQ
What is Poroelasticity?
Property of an elastic or viscoelastic material with an incompressible free fluid phase. When such a material is put under stress, fluid is released from the solid matrix. The frictional drag generated from the movement of the fluid through the matrix affects the mechanical behavior of such material. These materials are normally tested in a bath of fluid.
Mach-1 Analysis – Poroelastic Model in Unconfined Compression – Parameters
Mach-1 Analysis – Poroelastic Model in Confined Compression – Parameters
Mach-1 Analysis – Poroelastic Model in Confined Compression – Example
Mach-1 Analysis – Poroelastic Model in Unconfined Compression – Cartilage
Mach-1 Analysis – Poroelastic Model in Unconfined Compression – Example
LAB MODULE – Torsion of Disk-Shaped Samples – Student Version
How does the "Interfaces Detection with Needle Probe" feature work?
This tool allows for the computing and visualizing of the surface position (mm) and the interface position (mm) of a “Find Contact” curve (load or force vs. position) obtained from a needle probe measurement (Jurvelin et al., 1995) on a sample (Figure 1). This model is well adapted for the thickness measurement of samples composed of two layers (a soft layer on top and a stiffer layer under), for example: articular surfaces. This analysis can be used in two modes, either automatic or manual. In the automatic mode, the low-pass filter cutoff frequency and the loading limit need to be specified in order to optimize the interface detection for each different sample. A user license is required in order to obtain the results of the interfaces detection with needle probe (Software Add-On, contact Biomomentum to obtain your license).
The needle probe measurement consists of moving the needle probe at a constant speed until it penetrates the surface of the sample and stops inside the interface. The difference between the position of the surface (position where the load starts to increase) and the position of the interface (sharp increase in the load) corresponds to the vertical thickness (Figure 2).
This analysis can be done in two modes, either automatic or manual. The automatic mode allows finding the surface (1) and the interface (2) position through an algorithm.
- In order to find the surface position (1), the algorithm will start from the maximum load and will slowly scan the curve until the load will be equal to the load at the starting point (Figure 3).
- In order to find the interface position (2), the algorithm will first smooth the curve with a low-pass filter (red line) and will then compute the first derivative (green line) and second derivative (blue line) of the curve. The position of the maxima of the second derivative will be considered as the interface position if its position is located before the user-defined loading limit (Figure 4).
Thus, the automatic mode consists of specifying two parameters:
Low-Pass Filter Cutoff Frequency (Hz) – Defines the cutoff frequency of the low-pass filter that will smooth the curve
Loading Limit (load/sec) – The inflection position is located before this loading limit defined by the user. This loading limit varies depending on the sample used; it represents the loading limit at which the user is certain that the needle probe is in the interface.
The manual mode lets the user drag the cursors to the surface the interface positions.
For example, (Figure 5) shows parameters used for the “Find Contact” function in needle probe measurement testing protocol on ovine articular surfaces. Corresponding results are shown in (Figure 1).
Mach-1 Analysis – Interfaces Detection with Needle Probe – Parameters
Mach-1 Analysis – Interfaces Detection with Needle Probe – Needle measurement
Mach-1 Analysis – Interfaces Detection with Needle Probe – Interface Position
Mach-1 Analysis – Interfaces Detection with Needle Probe – Surface Position
Mach-1 Analysis – Interfaces Detection with Needle Probe – Example
How does the "Elastic Model in Indentation" works?
This tool allows for the computing and visualizing of the shear modulus (MPa) and the Young’s modulus (MPa) from an indentation test performed using a spherical indenter (Figure 1). This model is well adapted for the mechanical characterization of samples (with critical diameter (width) at least 5 times the indenter diameter) bonded on a flat rigid support (at least 10 times stiffer than the sample), for example: any sheet of material lying on a rigid chamber base (limited sliding at the interface can be assumed) or a cartilage layer attached to a thick sub-chondral bone. A user license is required to obtain the results of the elastic model in indentation (Software Add-On, contact Biomomentum to obtain your license).
Results analysis is performed considering the indentation mechanics of an infinitely wide elastic layer bonded to a rigid half-space which has been developed as a model for the layered geometry of cartilage and subchondral bone (Hayes et al., 1972). (Figure 2) shows a schematic representing the modeled situation.
Many parameters need to be specified in order to calculate the different material properties.
1. Indenter Shape is:
- Spherical
- Plane-Ended Cylindrical
2. Indenter radius in mm.
3. Poisson ratio can be selected from a drop-down list (Figure 3)
4. Position @ contact is – defines the position of the left cursor:
- to be curvefit – will automatically find the position contact which will give the smallest mean squared error (mse)
- the value at t0 – will automatically take the position contact at the start of the curve
- the following value (mm) – the user need to specify the value in the corresponding field next to the drop-down list
- cursor position – the user can drag the left cursor to the wanted position contact
5. Sample thickness is the:
- position @ contact – will automatically find the position at contact (position of the left cursor)
- following value (mm) – the user need to specify the value in the corresponding field next to the drop-down list
6. Curvefit is up to – defines the position of the right cursor:
- peak load – will automatically take the position of the peak load
- delta load (unit) – the user need to specify the value of the desired delta load in the corresponding field next to the drop-down list
- delta position (mm) – the user need to specify the value of the desired delta position in the corresponding field next to the drop-down list
The error on the estimation of the best curve representing the stress relaxation curve is calculated using the root of the mean squares (RMS) method and is displayed under “RMSE”. For example, (Figure 4) shows parameters used for the “Normal Indentation” (Mach-1 Motion) function in an indentation testing protocol on ovine articular surfaces. Corresponding results are shown in (Figure 1).
Spherical Indentation Guide – Mach-1 Mechanical Tester – Biomomentum
Mach-1 Analysis – Elastic Model in Indentation – Parameters
Mach-1 Analysis – Elastic Model in Indentation – Poisson ratio choices
Mach-1 Analysis – Elastic Model in Indentation – Cartilage & Subchondral bone
How does the "Poroelastic Model in Confined Compression" feature works?
This tool allows for the computing and visualizing of the aggregate modulus (MPa), the permeability (mm2/MPa·s) and the mean square error (RMSE) of the fit calculated from a “Stress Relaxation” curve (load or force vs. time) in confined compression (Figure 1) on a disk sample. The curve is fitted to the linear biphasic model (Mow et al., 1980 or Bursać et al., 1999) in order to obtain the mechanical properties of the disk. Sample radius, strain and sample thickness need to be specified.
This model is well adapted for the mechanical characterization of samples containing a liquid fraction and a solid matrix, for example: cartilage or gel samples. A user license is required in order to obtain the results of the poroelastic model in confined compression (Software Add-On, contact Biomomentum to obtain your license).
Mechanical properties can be obtained from the stress relaxation curves obtained under confined compression such as the aggregate modulus (HA) and the hydraulic permeability (k). The aggregate modulus is a measure of stiffness of the sample at equilibrium while the hydraulic permeability is the ability of the fluid to flow through the cartilage matrix.
For example, (Figure 2) shows parameters used for the “Stress Relaxation” function in a confined compression testing protocol on human cartilage disks. Corresponding results are shown in (Figure 1).
This model is well adapted for the mechanical characterization of samples containing a liquid fraction and a solid matrix, for example: cartilage or gel samples. A user license is required in order to obtain the results of the poroelastic model in confined compression (Software Add-On, contact Biomomentum to obtain your license).
Mechanical properties can be obtained from the stress relaxation curves obtained under confined compression such as the aggregate modulus (HA) and the hydraulic permeability (k). The aggregate modulus is a measure of stiffness of the sample at equilibrium while the hydraulic permeability is the ability of the fluid to flow through the cartilage matrix.
For example, (Figure 2) shows parameters used for the “Stress Relaxation” function in a confined compression testing protocol on human cartilage disks. Corresponding results are shown in (Figure 1).
Mach-1 Analysis – Poroelastic Model in Confined Compression – Parameters
Mach-1 Analysis – Poroelastic Model in Confined Compression – Example
How does the "Poroelastic Model in Unconfined Compression" feature works?
This tool allows for the computing and visualizing of the fibril network modulus (MPa), the equilibrium modulus (MPa), the permeability (mm2/MPa·s), and the mean square error (RMSE) of the fit calculated from a “Stress Relaxation” curve (load or force vs. time) in unconfined compression (Figure 1) on a disk or osteochondral core sample. The curve is fitted to the linear fibril-network-reinforced biphasic model (Soulhat et al. 1999) in order to obtain the mechanical properties of the disk. Sample radius, strain, and Poisson’s ratio need to be specified. This model is well adapted for the mechanical description of samples containing a liquid fraction and a fibril network, for example: cartilage or gel samples. A user license is required in order to obtain the results of the poroelastic model in unconfined compression (Software Add-On, contact Biomomentum to obtain your license).
The nonhomogeneous composite model (i.e. linear fibril-network-reinforced biphasic model) is an extension of the previously developed isotropic homogeneous biphasic model, and was achieved by reinforcing the latter with a network of nonlinear fibrils distributed homogeneously in the r, θ, z directions and which resist extension only (Figure 2). The isotropic matrix is described by the drained elastic constants Em (Young’s modulus), vm (Poisson’s ratio) and a hydraulic permeability k. The fibril network is described by the Young’s modulus of individual fibrils ef which, combined with a fibril volume fraction χ (fibrils volume/ total-volume), results in an equivalent tensile modulus of the fibril network, Ef = 1/3 χ ef (since 1/3 of the fibrils are oriented in each of three mutually orthogonal directions).
For example, (Figure 3) shows parameters used for the “Stress Relaxation” function in an unconfined compression testing protocol on ovine cartilage disks. Corresponding results are shown in (Figure 1).
The nonhomogeneous composite model (i.e. linear fibril-network-reinforced biphasic model) is an extension of the previously developed isotropic homogeneous biphasic model, and was achieved by reinforcing the latter with a network of nonlinear fibrils distributed homogeneously in the r, θ, z directions and which resist extension only (Figure 2). The isotropic matrix is described by the drained elastic constants Em (Young’s modulus), vm (Poisson’s ratio) and a hydraulic permeability k. The fibril network is described by the Young’s modulus of individual fibrils ef which, combined with a fibril volume fraction χ (fibrils volume/ total-volume), results in an equivalent tensile modulus of the fibril network, Ef = 1/3 χ ef (since 1/3 of the fibrils are oriented in each of three mutually orthogonal directions).
For example, (Figure 3) shows parameters used for the “Stress Relaxation” function in an unconfined compression testing protocol on ovine cartilage disks. Corresponding results are shown in (Figure 1).
Mach-1 Analysis – Poroelastic Model in Unconfined Compression – Parameters
Mach-1 Analysis – Poroelastic Model in Unconfined Compression – Cartilage
Mach-1 Analysis – Poroelastic Model in Unconfined Compression – Example
How the Arthro-BST Quantitative Parameter (QP) is calculated?
The quantitative parameter (QP) of the Arthro-BST corresponds to the number of microelectrodes in contact with cartilage (voltage below -2 mV) when the sum of their electric potentials reaches -100 mV.
What is an aggregate modulus?
The aggregate modulus (Ha) corresponds to the stiffness of the material at equilibrium when fluid has ceased flowing through it in a confined compression test configuration. The aggregate modulus can be calculated from Young's modulus (E) and the Poisson ratio (v) using the formula Ha=E(1-v)/[(1+v)(1-2v)].
How does the creep function works?
The Creep function moves a stage until a predefined load variation (Load Target (relative to initial load)) is reached and maintains this loading through real-time closed-loop PID adjustment of the stage position. This function can be used, for example, to study the deformation of a sample under constant loading. Note that tuning a PID requires a reasonable amount of closed-loop system understanding. A set of PID parameters optimized for a particular experimental configuration will probably be inappropriate for a different configuration and could even create system instabilities (oscillations) that can result in damage to the system.
Parameters:
STAGE AXIS - On multi-axis systems, the user must specify which axis will perform the “Creep”.
DIRECTION - Specify if the stage has to start moving in the positive or negative direction. With a positive displacement, a vertical stage will move down (compression), a horizontal stage will move right, and a rotation stage will move clockwise. Enter a negative value for stage displacements in opposite directions.
STAGE VELOCITY, MM/S - Specify the speed at which the stage will move. Refer to the specifications of your Mach-1™ model to learn about the velocity range allowed. In Mach-1 Motion, the speed is always specified in absolute value terms.
RELATIVE LOAD TARGET, GF OR N - Specify the relative amount of load to be applied and maintained on the sample. When a value is entered, the stage will initiate its movement until a change in load of that magnitude is reached. The value for Relative Load Target is specified in absolute value terms.
CREEP TIME, SEC - This is the time period during which the relative loading will be maintained. Note that after this period, the final stage position will be maintained.
P GAIN - This is the Proportional gain of the PID control loop. This is the main parameter that drives the feedback loop. Our experiments revealed that a good starting point for the optimization of this parameter is given by P = 0.001 (and I = 0), then raising P in logarithmic increments (0.01, 0.1, 1) until a satisfactory amplitude in reached. The higher the P value, the faster the error will be corrected, however if the P is made too large, the error can go negative (actual load > desired load) which may start a process where the stage starts to oscillate between speeding up and slowing down. This oscillation can result in the motor control system going unstable. A simple way to eliminate this instability would be to decrease P until the oscillation stops.
I GAIN - This is the Integral gain of the PID control loop. Integral control sums the error from each position update then multiplies the sum by the Integral gain, I. Our experiments revealed that for most creep tests, best results are achieved with I = 0. If a residual error exists while holding a load, the I value can also be incrementally increased until the error disappears.