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In 2001, Mother had her left knee joint replaced. There was quite a fuss about this new LCS system she was getting. Now just six years later, the joint has fractured. How does this happen?

LCS stands for Low Contact Stress mobile-bearing knee replacement. It has a unique design that has shown long-lasting wear capacity. A mobile-bearing total knee replacement (TKR) is different from the standard TKR.

It allows for a small amount of rotation in the joint as the knee bends and straightens. This is needed in order to mimic normal motion. It helps prevent uneven wear on the joint.

The mobile-bearing TKR also puts less stress on the polyethylene (plastic) liner. In all TKRs, the lower portion of the prosthesis is attached to the tibia (lower leg bone).

In many TKRs, the lower portion of the implant is fixed and doesn't move. This puts extra stress on the polyethylene (plastic) liner or spacer. With the LCS, the spacer between the tibia and femur (thigh bone) rotates with the femoral (upper) part of the TKR. The reduced stress results in less wear and fewer fractures.

If there are bony or soft tissue imbalances that are not corrected when the TKR goes in, then uneven wear or increased loads can lead to fracture of the liner. Even mild changes in the angle of the joint (called varus or valgus) can increase the risk of subluxation, dislocation, or fracture.

With the LCS, bearing fractures occur most often when the knee is bending while the bearing is subluxed posteriorly. This means the liner slips backwards. It moves just enough that the contact points for the implant and liner change. The result can be stress that eventually fractures the plastic spacer.

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