Computational biomechanics of the human knee joint – Role of collagen fibrils networks

In the present computational study, a poroelastic fibril-reinforced finite element model of cartilage was initially developed in which an appropriate membrane element was used to represent horizontal collagen fibrils. Volume fraction contents and the strain-dependent material properties for the pure collagen fibrils were explicitly accounted in the formulation. This approach, as compared to earlier ones, is more meaningful in not lumping various fibril properties (i.e. volume fraction and nonlinear stress-strain curve) into one stiffness term which risks losing its physical interpretation and significance. In this composite model, in accordance with tissue structure, the matrix and fibrils membrane network experienced dissimilar stresses despite identical strains in the fibre directions. Different unconfined compression and indentation case studies were performed to determine the distinct role of membrane collagen fibrils in nonlinear poroelastic mechanics of articular cartilage. By individual adjustments of the collagen volume fraction and collagen mechanical properties, the model allowed for the alterations in the fibrils network structure of the tissue that can simulate damage processes and repair attempts.

The primary orientation of collagen fibrils alters along the cartilage depth; being horizontal in the superficial zone, random in the transitional zone, and vertical in the deep zone. In a subsequent axisymmetric poroelastic model of cartilage, the three fibrillar networks at superficial, transitional and vertical zones based on the layerwise composite structure of cartilage were introduced. Membrane elements were used for horizontal superficial and vertical deep fibrils with appropriate formulation and volume content. Brick elements were used for transitional zone to consider random distribution of fibrils in this region. Therefore, at each increment of load and iteration of solution, continuum elements that take the principal strain directions as the material principal axes simulate reorientation of fibrils with tension-only stiffness. Under both relaxation and creep indentation loading conditions, it was shown that deep vertical fibrils play an important role in mechanics of articular cartilage by increasing the stiffness of the tissue and protecting the solid matrix against large distortions. This role, however, disappeared both with time in post-transient period and at loading rates slower than those expected in physiological activities such as walking.

Our foregoing axisymmetric model studies subsequently served as a foundation to extensively improve an existing 3-D model of the entire knee joint towards a novel one that considered, for the first time, the anisotropic nature of tibial and femoral cartilage layers in addition to menisci. Collagen fibrils networks in cartilage and menisci of knee joints change in content and structure from a region to another yielding highly nonlinear and nonhomogeneous tissues. While resisting tension, they influence global joint response as well as local strains particularly at short-term periods in activities such as walking and running. To investigate the role of fibrils networks in knee joint mechanics and in particular cartilage response in compression, a novel model of the tibiofemoral joint was developed that incorporated the cartilage and meniscus fibrils networks as well as depth-dependent properties in cartilage. The joint response at full extension under up to 2000 N compression was investigated for a number of conditions simulating the absence in cartilage of deep vertical or superficial fibrils networks as well good agreement with reported experimental measurements. Role of deep vertical collagen fibrils, and to a lesser extent, horizontal superficial and random transitional fibrils networks in protecting the solid matrix from deleterious strains and in augmenting tissue stiffness were demonstrated. Any treatment modality attempting to repair or regenerate cartilage defects involving partial or full thickness osteochondral grafts should account for the crucial role of collagen fibrils networks and the demanding mechanical environment of the tissue. Alterations following ACL reconstruction attempts simulated by changes in prestrain and material properties as well as partial meniscectomy significantly changed the load distribution in the knee joint. Adequate considerations of the new mechanical environment of the joint are crucial for an improved assessment of the likelihood of success in various treatment attempts. (Abstract shortened by UMI.)