Fatemeh Malekipour

Shock absorbing ability of articular cartilage and subchondral bone under impact compression.

Despite the important role of subchondral bone in maintaining the integrity of the overlying articular cartilage, little research has focused on measuring its mechanical behavior, particularly under injurious load conditions such as impact compression. In this study, the stiffness and the absorbed energy of subchondral bone were compared to that of its overlying cartilage by applying impact compression to equine cartilage-bone specimens. Deformations of the cartilage and subchondral bone were examined independently within the cartilage-bone unit by analyzing real-time images of cartilage-bone explants. Peak subchondral bone and cartilage stiffness (mean ± SD) were 800.7 ± 250.0 MPa and 119.9 ± 50.8 MPa respectively. The maximum absorbed energy per unit volume of subchondral bone was approximately 4 times lower than that of cartilage. Micro-computed tomography (μCT) images at 9 μm resolution revealed oblique fissures at the cartilage articular surface. At the cartilage-bone interface, micro-cracks as thin as 30 μm in width and micro-fractures of width 200 μm could be seen in the μCT images. The relative energy loss in bone was 76.5 ± 6.8% in specimens with bone fracture and 23.0 ± 20.4% in specimens without bone fracture. Our results indicate that both articular cartilage and subchondral bone absorb shock under impact compression, but the energy absorption of bone is much higher in specimens that fracture. This may spare the overlying cartilage from immediate injury, but is a potential risk for subsequent post-traumatic osteoarthritis (PTOA).

THE EFFECTS OF INTRA-ABDOMINAL PRESSURE ON THE STABILITY AND UNLOADING OF THE SPINE

In spite of earlier experimental and modeling studies, the relative role of the intra-abdominal pressure (IAP) in spine mechanics has remained controversial. This study employs simple analytical and finite element (FE) models of the spine and its surrounding structures to investigate the contribution of IAP to spinal loading and stability. The analytical model includes the abdominal cavity surrounded by muscles, lumbar spine, rib cage and pelvic ring. The intra-abdominal cavity and its surrounding muscles are represented by a thin deformable cylindrical membrane. Muscle activation levels are simulated by changing the Youngs modulus of the membrane in the direction of muscle fibers, yielding IAP values recorded under the partial Valsalva maneuver. In the FE model, the abdominal cavity is cylindrical and filled with a nearly incompressible fluid. The surrounding muscles are modeled as membrane elements with transverse isotropic material properties simulating their fiber orientation. Results indicate a good...

Bone microarchitecture, biomechanical properties, and advanced glycation end-products in the proximal femur of adults with type 2 diabetes

Skeletal fragility is a major complication of type 2 diabetes mellitus (T2D), but there is a poor understanding of mechanisms underlying T2D skeletal fragility. The increased fracture risk has been suggested to result from deteriorated bone microarchitecture or poor bone quality due to accumulation of advanced glycation end-products (AGEs). We conducted a clinical study to determine whether: 1) bone microarchitecture, AGEs, and bone biomechanical properties are altered in T2D bone, 2) bone AGEs are related to bone biomechanical properties, and 3) serum AGE levels reflect those in bone. To do so, we collected serum and proximal femur specimens from T2D (n = 20) and non-diabetic (n = 33) subjects undergoing total hip replacement surgery. A section from the femoral neck was imaged by microcomputed tomography (microCT), tested by cyclic reference point indentation, and quantified for AGE content. A trabecular core taken from the femoral head was imaged by microCT and subjected to uniaxial unconfined compression tests. T2D subjects had greater HbA1c (+23%, p ≤ 0.0001), but no difference in cortical tissue mineral density, cortical porosity, or trabecular microarchitecture compared to non-diabetics. Cyclic reference point indentation revealed that creep indentation distance (+18%, p ≤ 0.05) and indentation distance increase (+20%, p ≤ 0.05) were greater in cortical bone from T2D than in non-diabetics, but no other indentation variables differed. Trabecular bone mechanical properties were similar in both groups, except for yield stress, which tended to be lower in T2D than in non-diabetics. Neither serum pentosidine nor serum total AGEs were different between groups. Cortical, but not trabecular, bone AGEs tended to be higher in T2D subjects (21%, p = 0.09). Serum AGEs and pentosidine were positively correlated with cortical and trabecular bone AGEs. Our study presents new data on biomechanical properties and AGEs in adults with T2D, which are needed to better understand mechanisms contributing to diabetic skeletal fragility.

Equine subchondral bone failure threshold under impact compression applied through articular cartilage.

Subchondral bone microdamage due to high-impact loading is a key factor leading to post-traumatic knee osteoarthritis. A quantified assessment of the mechanical characteristics of subchondral bone at the tissue-level is essential to study the mechanism of impact-induced microdamage. We combined mechanical impact testing of equine cartilage-bone with µCT image-based finite element models (μFEM) of each specimen to determine subchondral bone (including calcified cartilage: CCSB) elastic tissue modulus and local stresses and strains associated with micro-fractures within the CCSB tissue. The material properties of each specimen-specific μFEM were iteratively adjusted to match the FE-predicted stress-strain curves with experimental results. Isotropic homogeneous material properties for both uncalcified cartilage (UC) and CCSB were assumed. UC large-deformation was simulated using hyperelastic material properties. Final UC shear and CCSB tissue elastic modulus of G=38±20MPa and E(t)=3.3±0.7GPa were achieved after fit procedure. The results suggested that initial failure in CCSB occurred at local tensile and compressive stresses of 29.47±5.34 MPa and 64.3±21.3MPa, and tensile and compressive strains of 1.12±0.06% and 1.99±0.41%, respectively. Tissue-level material properties can be used in finite element modeling of diarthrodial joints under impact loading, and also in designing artificial cartilage-bone to replace the damaged tissue in the joint. Results can provide an estimate for the threshold of initial failure in subchondral bone tissue due to an impact compression transmitted through the overlying articular cartilage.

Restrained tibial rotation may prevent ACL injury during landing at different flexion angles

BACKGROUND Internal tibial rotation is a risk factor for anterior cruciate ligament (ACL) injury. The effect of restraining tibial rotation (RTR) to prevent ACL injury during single-leg landing is not well understood. We aimed to investigate the effect of impact load and RTR on ACL injury with respect to flexion angle. We hypothesized that RTR could protect the knee from ACL injury compared to free tibial rotation (FTR) regardless of flexion angle and create a safety zone to protect the ACL. METHODS Thirty porcine specimens were potted in a rig manufactured to replicate single-leg landing maneuvers. A mechanical testing machine was used to apply external forces in the direction of the tibial long axis. A 3D displacement sensor measured anterior tibial translation (ATT). The specimens were divided into 3 groups of 10 specimens and tested at flexion angles of 22 ± 1°, 37 ± 1° and 52 ± 1° (five RTR and five FTR) through a consecutive range of actuator displacements until ACL failure. After dissection, damage to the joint was visually recorded. Two-way ANOVA were utilized in order to compare compressive forces, torques and A/P displacements with respect to flexion angle. RESULTS The largest difference between peak axial compressive forces (~3.4 kN) causing ACL injury between RTR and FTR was reported at a flexion angle of 22°. Tibial torques with RTR was in the same range and < 20 Nm at the instance and just before ACL failure, compared to a significant reduction when cartilage/bone damage (no ACL failure) was reported. Isolated ACL injuries were observed in ten of the 15 FTR specimens. Injuries to bone and cartilage were more common with RTR. CONCLUSIONS RTR increases the threshold for ACL injury by elevating the compressive impact load required at lower flexion angles. These findings may contribute to neuromuscular training programs or brace designs used to avoid excessive internal/external tibial rotation. Caution must be exercised as bone/cartilage damage may result.

Stiffness and energy dissipation across the superficial and deeper third metacarpal subchondral bone in Thoroughbred racehorses under high-rate compression

Subchondral bone injury due to high magnitude and repetition of compressive loading is common in humans and athletic animals such as Thoroughbred racehorses. Repeated loading of the joint surface may alter the subchondral bone microstructure and initiate microdamage in the bone adjacent to the articular cartilage. Understanding the relationship between microdamage, microstructure and mechanical properties of the subchondral bone adjacent to the articular cartilage is, therefore, essential in understanding the mechanism of subchondral bone injury. In this study, we used high-resolution µCT scanning, a digital image-based strain measurement technique, and mechanical testing to evaluate the three-dimensional pre-existing microcracks, bone volume fraction (BVF) and bone mineral density (BMD), and mechanical properties (stiffness and hysteresis) of subchondral bone (n = 10) from the distopalmar aspect of the third metacarpal (MC3) condyles of Thoroughbred racehorses under high-rate compression. We specifically compared the properties of two regions of interest in the subchondral bone: the 2 mm superficial subchondral bone (SSB) and its underlying 2 mm deep subchondral bone (DSB). The DSB region was 3.0 ± 1.2 times stiffer than its overlying SSB, yet it dissipated much less energy compared to the SSB. There was no correlation between structural properties (BVF and BMD) and mechanical properties (stiffness and energy loss), except for BMD and energy loss in SSB. The lower stiffness of the most superficial subchondral bone in the distal metacarpal condyles may protect the overlying cartilage and the underlying subchondral bone from damage under the high-rate compression experienced during galloping. However, repeated high-rate loading over time has the potential to inhibit bone turnover and induce bone fatigue, consistent with the high prevalence of subchondral bone injury and fractures in athletic humans and racehorses.

Shock absorbing ability in healthy and damaged cartilage-bone under high-rate compression

Articular cartilage is a soft tissue that distributes the loads in joints and transfers the compressive load to the underlying bone. At high rate and magnitudes of mechanical loading, cartilage and subchondral bone together are susceptible to damage. In addition, any disruption to the cartilages structure, caused by injury, trauma or disorder such as osteoarthritis (OA), can alter the mechanism of load transfer from the cartilage to the underlying bone. Changes in the cartilage structure can also alter the ability of cartilage-bone to absorb and dissipate the impact energy. To investigate the effects of cartilage degradation on cartilage-bone shock absorption ability, the top 50% of the cartilage thickness was removed (modified cartilage) to mimic the cartilage thickness reduction in Grade III cartilage lesion and the remaining cartilage-bone unit (modified cartilage-bone) was compressed at high-rate (4% strain at 5 Hz). High-speed camera and microscope were used to capture microscopic deformation, and digital image correlation technique (DIC) employed to quantify the deformation of cartilage and bone. The mechanical properties (i.e. stiffness, strain, absorbed and dissipated energies) of cartilage and bone were calculated before and after the removal of the top 50% of the cartilage thickness, consisting of both the superficial tangential zone (STZ) and part of the middle zone of the cartilage. The results showed a significant degradation in the mechanical properties of the cartilage-bone unit after the removal of the top 50% cartilage thickness. The stiffness of the modified cartilage reduced significantly (by ~39%) and energy absorption in underlying bone increased by 32%, which can make the bone more vulnerable to damage in the modified cartilage-bone unit. In addition, the energy dissipation in the modified cartilage-bone unit was also increased by approximately 14%. These changes in mechanical properties suggest a crucial role of the STZ and middle zone (within the top 50% cartilage thickness) in protecting the underlying bone from the severe compressive impact loading. Results also indicated that under physiological contact stress of 7 MPa, strain in damaged cartilage was increased by 3.22% without affecting the mechanical behaviour of the underlying bone.

Mathematical modelling of bone adaptation of the metacarpal subchondral bone in racehorses

In Thoroughbred racehorses, fractures of the distal limb are commonly catastrophic. Most of these fractures occur due to the accumulation of fatigue damage from repetitive loading, as evidenced by microdamage at the predilection sites for fracture. Adaptation of the bone in response to training loads is important for fatigue resistance. In order to better understand the mechanism of subchondral bone adaptation to its loading environment, we utilised a square root function defining the relationship between bone volume fraction

Contributions of the Soleus and Gastrocnemius muscles to the anterior cruciate ligament loading during single-leg landing

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