Prediction of collagen orientation in articular cartilage by a collagen remodeling algorithm

Tissue engineering is a promising method to treat damaged cartilage. So far it has not been possible to create tissue-engineered cartilage with an appropriate structural organization. It is envisaged that cartilage tissue engineering will significantly benefit from knowledge of how the collagen fiber orientation is directed by mechanical conditions. The goal of the present study is to evaluate whether a collagen remodeling algorithm based on mechanical loading can be corroborated by the collagen orientation in healthy cartilage.

According to the remodeling algorithm, collagen fibrils align with a preferred fibril direction, situated between the positive principal strain directions. The remodeling algorithm was implemented in an axisymmetric finite element model of the knee joint. Loading as a result of typical daily activities was represented in three different phases: rest, standing and gait.

In the center of the tibial plateau the collagen fibrils run perpendicular to the subchondral bone. Just below the articular surface they bend over to merge with the articular surface. Halfway between the center and the periphery, the collagen fibrils bend over earlier, resulting in a thicker superficial and transitional zones. Near the periphery fibrils in the deep zone run perpendicular to the articular surface and slowly bend over to angles of -45 degrees and +45 degrees with the articular surface.

The collagen structure as predicted with the collagen remodeling algorithm corresponds very well with the collagen structure in healthy knee joints. This remodeling algorithm is therefore considered to be a valuable tool for developing loading protocols for tissue engineering of articular cartilage.