Mechanics and Growth of Articular Cartilage Around a Localized Metal Implant

Sammanfattning: Articular cartilage is a specialized connective soft tissue that resides on the ends of long-bones, and transfers the load smoothly between the bones in diarthrodial joints by providing almost frictionless, wear resistant sliding surfaces during joint articulation. Focal chondral or osteochondral defects in articular cartilage are common and show limited capacity for biological repair. Furthermore, changes in the bio-mechanical forces at the defect site may make the tissue more susceptible to continued degeneration. Alternatively, a contoured focal resurfacing metal implant can be used to treat such full-thickness cartilage defects. Physiological and biomechanical studies on animal models with metal implant have shown good clinical outcomes. However, the mechanical behavior of cartilage surrounding the implant has remained largely unanswered with respect to the joint function.First, we developed a simple 3-dimensional finite element model by approximating one of the condyles of a sheep knee joint and parametrically studied the effects of shape, size and positioning of the implant on the mechanical behavior of the cartilage surrounding the implant. The mechanical sealing effect due to the wedge shape of the implant was studied. We also simulated the time dependent behavior of the cartilage surrounding the implant. In the second part, we developed a more sophisticated model accounting for biological growth aspects of the cartilage around the implant together with the in vivo mechanical response of the cartilage in an intact human knee joint. An axisymmetric representation of a human knee condyle including both cartilage layers, meniscus and tibia was considered. A cartilage growth finite element model incorporating dynamic loading from walking, which drives the growth stimulation in the cartilage, was developed. Two individually growing constituents in the solid matrix of cartilage together with the biphasic contacts in the joint were considered in the growth model. From our simulations it is evident that the cartilage near the implant was more stimstimulated, whence the defect edge of the cartilage was growing onto the implant.The models developed in the present work are simulation tools and have a potential, in relevant aspects, to predict the physiological behavior of the cartilage surrounding the metal implant.