Influence of ligament pre-strain on the knee contact forces and contact area - a sensitivity study

status: publishe

Gait Adaptations Relate to Cartilage Lesion Locations in the Tibiofemoral Joint- Preliminary Results

status: accepte

Articular cartilage adjacent to full thickness defects are subjected to excessive shear strains under compressive loading

Objective Full-thickness cartilage defects are commonly found in symptomatic knee patients, and are associated with progressive cartilage degeneration. Although the risk of defect progression to degenerative osteoarthritis is multifactorial, articular cartilage defects change contact mechanics and the mechanical response of tissue adjacent to the defect. The objective of this study was to quantify changes in intra-tissue strain patterns occurring at the defect rim and opposing tissue in an experimental model mimicking in vivo cartilage-on-cartilage contact conditions. Methods Macroscopically intact osteochondral explants with smooth surfaces were harvested form the femoral condyles of 9 months old bovine knees. Two groups were tested; reference group with intact cartilage (n=8) and defect group with a full thickness cylindrical defect (diameter 8 mm) in one cartilage surface from each pair (n=8). The explants with defect articular surface and the opposing intact cartilage were compressed at ?0.33 times body weight (350N) during cycles of 2s loading followed by 1.4s unloading. In plane tissue deformations were measured using displacement encoded imaging with stimulated echoes (DENSE) on a 9.4T MRI scanner. A two-sample t-test was used to assess statistical significance (p<0.05) of differences in maximal Green-Lagrange strains between the defect, opposing surface and intact reference cartilage. Results Strain levels were elevated in the cartilage neighbouring the defect rim and in the opposing articulating surface. Similar to intact cartilage, compressive and tensile strains presented a depth dependent variation. The maximal strains profiles were highest in the superficial zone and decreased with depth for all explants, except for the shear strains in the cartilage opposing the defect which were constant. The maximal tensile strain in the middle and superficial zone were significantly higher for the defect cartilage (3.97±1.99% and 4.52±2.04%) compared to the intact reference (1.91±1.13% and 2.53±1.27%), indicating that the defect edges are bulging towards the defect. The shear strains were significantly higher (?1.5x) throughout cartilage depth of the defect rim compared to the intact reference cartilage. However, in the cartilage opposing the defect, shear strains were significantly lower (?0.5x) compared to the intact cartilage representing less matrix distortion. No significant difference in maximal compressive strains were observed between the opposing intact and defect at all cartilage depths. Conclusions Presence of isolated full thickness cartilage defects will affect the cartilage deformations. Even under pure compressive loading alone, the altered contact mechanics resulted in excessive strains at tissue adjacent to the defect potentially damaging the cartilage and inducing tissue degeneration.status: publishe

Biomechanical properties of bovine knee cartilage under compressive loading: A study at high field MRI (9.4T) using T1, T2 and T1rho relaxometry combined with DENSE-FID

status: publishe

Influence of cartilage defects on the tibiofemoral contact pressure distribution during loading response

Introduction Cartilage defects of the knee often lead to pain and functional disability requiring surgical intervention. Untreated cartilage defects may progress to degenerative arthritis. Although this process is multifactorial, it is known that excessive loading potentially lead to altered biomechanical properties due to tissue degeneration. The aim of this study was to identify the influence of the defect size and location on the magnitude and distribution of tibiofemoral cartilage contact pressure during loading response of gait. Methods In this study a validated musculoskeletal knee model including six degree-of-freedom (dof) tibiofemoral and patellofemoral joints, major knee ligaments and cartilage contact geometry was used. External loads and kinematic data were collected in one healthy subject (female, 31y, 64,2kg) during overground walking at a self-selected speed. An enhanced static optimization routine was used to calculate the muscle forces, patellofemoral kinematics and secondary tibiofemoral kinematics during each frame of the gait cycle by minimizing the weighted sum of muscle activations squared and the elastic foundation contact energy. For each surface element the Young’s modulus, Poisson’s ratio and cartilage thickness could be set using the elastic foundation method. First a uniform Young’s modulus of 10MPa for the cartilage layers was used, assuming constant cartilage thickness of 2mm. Subsequently medial and lateral circular defects of 0.5, 1 and 2cm2 were defined in the tibia and femur cartilage geometry. The workflow was repeated for a Young’s modulus of 1, 2.5, 5, 7.5, 20 and 100MPa representing the biomechanical changes following cartilage injury. Results The defect area, location, and elastic modulus affect the average contact pressure and contact area during loading response. When the Young’s modulus of the defect decreases, the average pressure inside the defect decreases whilst the overall contact area increases. As a result the total tibiofemoral contact force remains constant. An increased defect size results in larger differences in average contact pressure and area. However this is strongly influenced by the location of the defect and its relation to the load bearing area. The largest differences in average contact pressure and area are observed for medial cartilage defects. Furthermore, a slight increase in adduction angle is found for medial cartilage defects for lower Young’s moduli of the defect. The opposite effect is seen for lateral cartilage defects. Conclusions We showed that both defect location and size influence the average contact pressure and contact area during loading response. Difference found between medial and lateral defect location and size can be related to the higher load-bearing area in the medial compared to the lateral compartment. Furthermore, the Young’s modulus settings for bone tissue formation (high) or degenerative changes (low) clearly affects the local load distribution on the articular surface.status: publishe

Articular cartilage deformation determined in intact and defect osteochondral plugs by displacement-encoded MRI

status: publishe

Cartilage defect location and stiffness predispose the tibiofemoral joint to aberrant loading conditions during stance phase of gait

Objectives The current study quantified the influence of cartilage defect location on the tibiofemoral load distribution during gait. Furthermore, changes in local mechanical stiffness representative for matrix damage or bone ingrowth were investigated. This may provide insights in the mechanical factors contributing to cartilage degeneration in the presence of an articular cartilage defect. Methods The load distribution following cartilage defects was calculated using a musculoskeletal model that included tibiofemoral and patellofemoral joints with 6 degrees-of-freedom. Circular cartilage defects of 100 mm2 were created at different locations in the tibiofemoral contact geometry. By assigning different mechanical properties to these defect locations, softening and hardening of the tissue were evaluated. Results Results indicate that cartilage defects located at the load-bearing area only affect the load distribution of the involved compartment. Cartilage defects in the central part of the tibia plateau and anterior-central part of the medial femoral condyle present the largest influence on load distribution. Softening at the defect location results in overloading, i.e., increased contact pressure and compressive strains, of the surrounding tissue. In contrast, inside the defect, the contact pressure decreases and the compressive strain increases. Hardening at the defect location presents the opposite results in load distribution compared to softening. Sensitivity analysis reveals that the surrounding contact pressure, contact force and compressive strain alter significantly when the elastic modulus is below 7 MPa or above 18 MPa. Conclusion Alterations in local mechanical behavior within the high load bearing area resulted in aberrant loading conditions, thereby potentially affecting the homeostatic balance not only at the defect but also at the tissue surrounding and opposing the defect. Especially, cartilage softening predisposes the tissue to loads that may contribute to accelerated risk of cartilage degeneration and the initiation or progression towards osteoarthritis of the whole compartment.ISSN:1932-620

Gait adaptations following medial condyle cartilage lesions reduce condyle loading – Preliminary results

status: publishe

Knee Cartilage Thickness, T1ρ and T2 Relaxation Time Are Related to Articular Cartilage Loading in Healthy Adults

Cartilage is responsive to the loading imposed during cyclic routine activities. However, the local relation between cartilage in terms of thickness distribution and biochemical composition and the local contact pressure during walking has not been established. The objective of this study was to evaluate the relation between cartilage thickness, proteoglycan and collagen concentration in the knee joint and knee loading in terms of contact forces and pressure during walking. 3D gait analysis and MRI (3D-FSE, T1ρ relaxation time and T2 relaxation time sequence) of fifteen healthy subjects were acquired. Experimental gait data was processed using musculoskeletal modeling to calculate the contact forces, impulses and pressure distribution in the tibiofemoral joint. Correlates to local cartilage thickness and mean T1ρ and T2 relaxation times of the weight-bearing area of the femoral condyles were examined. Local thickness was significantly correlated with local pressure: medial thickness was correlated with medial condyle contact pressure and contact force, and lateral condyle thickness was correlated with lateral condyle contact pressure and contact force during stance. Furthermore, average T1ρ and T2 relaxation time correlated significantly with the peak contact forces and impulses. Increased T1ρ relaxation time correlated with increased shear loading, decreased T1ρ and T2 relaxation time correlated with increased compressive forces and pressures. Thicker cartilage was correlated with higher condylar loading during walking, suggesting that cartilage thickness is increased in those areas experiencing higher loading during a cyclic activity such as gait. Furthermore, the proteoglycan and collagen concentration and orientation derived from T1ρ and T2 relaxation measures were related to loading.status: publishe