Importance of Patella, Quadriceps Forces, and Depthwise Cartilage Structure on Knee Joint Motion and Cartilage Response During Gait.

In finite-element (FE) models of the knee joint, patella is often omitted. We investigated the importance of patella and quadriceps forces on the knee joint motion by creating an FE model of the subject's knee. In addition, depthwise strains and stresses in patellar cartilage with different tissue properties were determined. An FE model was created from subject's magnetic resonance images. Knee rotations, moments, and translational forces during gait were recorded in a motion laboratory and used as an input for the model. Three material models were implemented into the patellar cartilage: (1) homogeneous model, (2) inhomogeneous (arcadelike fibrils), and (3) random fibrils at the superficial zone, mimicking early stages of osteoarthritis (OA). Implementation of patella and quadriceps forces into the model substantially reduced the internal-external femoral rotations (versus without patella). The simulated rotations in the model with the patella matched the measured rotations at its best. In the inhomogeneous model, maximum principal stresses increased substantially in the middle zone of the cartilage. The early OA model showed increased compressive strains in the superficial and middle zones of the cartilage and decreased stresses and fibril strains especially in the middle zone. The results suggest that patella and quadriceps forces should be included in moment- and force-driven FE knee joint models. The results indicate that the middle zone has a major role in resisting shear forces in the patellar cartilage. Also, early degenerative changes in the collagen network substantially affect the cartilage depthwise response in the patella during walking.

Gait alteration strategies for knee osteoarthritis: a comparison of joint loading via generic and patient-specific musculoskeletal model scaling techniques.

Gait modifications and laterally wedged insoles are non-invasive approaches used to treat medial compartment knee osteoarthritis. However, the outcome of these alterations is still a controversial topic. This study investigates how gait alteration techniques may have a unique effect on individual patients; and furthermore, the way we scale our musculoskeletal models to estimate the medial joint contact force may influence knee loading conditions. Five patients with clinical evidence of medial knee osteoarthritis were asked to walk at a normal walking speed over force plates and simultaneously 3D motion was captured during seven conditions (0°-, 5°-, 10°-insoles, shod, toe-in, toe-out, and wide stance). We developed patient-specific musculoskeletal models, using segmentations from magnetic resonance imaging to morph a generic model to patient-specific bone geometries and applied this morphing to estimate muscle insertion sites. Additionally, models were created of these patients using a simple linear scaling method. When examining the patients' medial compartment contact force (peak and impulse) during stance phase, a 'one-size-fits-all' gait alteration aimed to reduce medial knee loading did not exist. Moreover, the different scaling methods lead to differences in medial contact forces; highlighting the importance of further investigation of musculoskeletal modeling methods prior to use in the clinical setting.

Workflow assessing the effect of gait alterations on stresses in the medial tibial cartilage - combined musculoskeletal modelling and finite element analysis.

Knee osteoarthritis (KOA) is most common in the medial tibial compartment. We present a novel method to study the effect of gait modifications and lateral wedge insoles (LWIs) on the stresses in the medial tibial cartilage by combining musculoskeletal (MS) modelling with finite element (FE) analysis. Subject's gait was recorded in a gait laboratory, walking normally, with 5° and 10° LWIs, toes inward ('Toe in'), and toes outward ('Toe out wide'). A full lower extremity MRI and a detailed knee MRI were taken. Bones and most soft tissues were segmented from images, and the generic bone architecture of the MS model was morphed into the segmented bones. The output forces from the MS model were then used as an input in the FE model of the subject's knee. During stance, LWIs failed to reduce medial peak pressures apart from Insole 10° during the second peak. Toe in reduced peak pressures by -11% during the first peak but increased them by 12% during the second. Toe out wide reduced peak pressures by -15% during the first and increased them by 7% during the second. The results show that the work flow can assess the effect of interventions on an individual level. In the future, this method can be applied to patients with KOA.

Optimal graft stiffness and pre-strain restore normal joint motion and cartilage responses in ACL reconstructed knee.

Anterior cruciate ligament (ACL) rupture leads to abnormal loading of the knee joint and increases the risk of osteoarthritis. It is unclear how different ACL reconstruction techniques affect knee joint motion and mechanics. As the in vivo measurement of knee joint loading is not possible, we used finite element analysis to assess the outcome of ACL reconstruction techniques. Effects of different ACL reconstruction techniques on knee joint mechanics were studied using six models during gait; with 1) healthy ACL, 2) ACL rupture, 3) single bundle ACL reconstruction, 4) double bundle ACL reconstruction, 5) weakened (softer) single bundle reconstruction and 6) single bundle reconstruction with less pre-strain. Early in the gait, the ACL rupture caused substantially increased tibial translation in the anterior direction as well as a smaller but increased lateral translation and internal tibial rotation. ACL rupture substantially reduced average stresses and strains, while local peak stresses and strains could be either increased or decreased. Single bundle and double bundle reconstructions restored joint motion close to normal levels. However, cartilage strains and stresses were elevated during the entire gait cycle. Models with modulated graft stiffness and pre-strain restored the joint motion and cartilage stresses and strains close to the normal, healthy levels. Results suggest that rather than the choice of reconstruction technique, stiffness and pre-strain of the ACL reconstruction affect the motion and mechanics of the operated knee. We suggest that an optimal choice of graft properties might help restore normal knee joint function and cartilage responses, thus, minimizing the risk of osteoarthritis.

Deformation of articular cartilage during static loading of a knee joint--experimental and finite element analysis.

Novel conical beam CT-scanners offer high resolution imaging of knee structures with i.a. contrast media, even under weight bearing. With this new technology, we aimed to determine cartilage strains and meniscal movement in a human knee at 0, 1, 5, and 30 min of standing and compare them to the subject-specific 3D finite element (FE) model. The FE model of the volunteer׳s knee, based on the geometry obtained from magnetic resonance images, was created to simulate the creep. The effects of collagen fibril network stiffness, nonfibrillar matrix modulus, permeability and fluid flow boundary conditions on the creep response in cartilage were investigated. In the experiment, 80% of the maximum strain in cartilage developed immediately, after which the cartilage continued to deform slowly until the 30 min time point. Cartilage strains and meniscus movement obtained from the FE model matched adequately with the experimentally measured values. Reducing the fibril network stiffness increased the mean strains substantially, while the creep rate was primarily influenced by an increase in the nonfibrillar matrix modulus. Changing the initial permeability and preventing fluid flow through noncontacting surfaces had a negligible effect on cartilage strains. The present results improve understanding of the mechanisms controlling articular cartilage strains and meniscal movements in a knee joint under physiological static loading. Ultimately a validated model could be used as a noninvasive diagnostic tool to locate cartilage areas at risk for degeneration.

Importance of depth-wise distribution of collagen and proteoglycans in articular cartilage--a 3D finite element study of stresses and strains in human knee joint.

Proteoglycans and collagen fibrils are distributed inhomogeneously throughout the depth of articular cartilage, providing the tissue with its unique depth-dependent properties and directly influencing local tissue deformations and stresses in in vitro/in situ. The aim of this study was to investigate the importance of the proteoglycan and collagen distributions for cartilage stresses and strains resulting from dynamic joint loading (i.e., a simulated gait cycle) and mechanical equilibrium in a knee joint. A 3D finite element model of a human knee joint including femoral and tibial cartilages and menisci was created. In order to characterize the effects of collagen orientation, collagen distribution and proteoglycan distribution on knee joint stresses and strains, five fibril-reinforced poroviscoelastic models with different depth-wise tissue structure were created. For each model strains and stresses were evaluated at four different depths in the medial tibial compartment during a gait cycle (simulating walking) and at mechanical equilibrium (simulating standing). The model with arcade-like collagen fibril architecture predicted substantially lower stresses than the homogeneous model, especially during dynamic joint loading. The depth-wise proteoglycan gradient caused a substantial increase in stresses and axial strains in the superficial layer, and reduced stresses and strains in the deep layer under static loading. The effect of fibril volume density distribution was minor during both dynamic joint loading and at mechanical equilibrium. The present study emphasizes the importance of the arcade-like collagen fibril orientation for cartilage function in a human knee joint. However, we suggest that, for practical reasons, a constant fibril volume density may be used in 3D models of knee joints, whereas a realistic depth-wise proteoglycan distribution should be applied when simulating the cartilage response during mechanical equilibrium.