Knee Joint Implants Dictate Function and WearThere are no two ways about it: the knee joint is meant to rotate during motion. Studies show that as much as 12 degrees of knee rotation occur during everyday activities. And when total knee replacements (TKRs) don't allow rotation, they may get loose and fail.
Constraint describes the fit where the top and bottom of a new knee joint meet. TKRs must have a balance between joint looseness (low constraint) and reduced motion (high constraint). Too much constraint limits the needed rotation. The design of the implant has a lot to do with how much constraint is present.
Implants have different designs for a reason. For example, some implants are made for patients who still have the posterior cruciate ligament (PCL) in place. The PCL connects the tibia and femur. It is a key stabilizer of the knee. The implant must fit around the PCL, which means that less knee rotation can occur.
Implants with closely matching halves (top half and bottom half fit together well) have more resistance to rotation. TKRs that don't match as well have less resistance and more rotation. Another cause of decreased motion is called box-post impingement. In this case, the side walls of the upper half of the implant come in contact with the central post of the lower half of the implant. The pinching causes increased constraint (less rotation).
In this study, two kinds of TKRs were examined in the lab. The goal was to find out how much rotation is limited by the design of two different knee implants. The bottom half of each implant (called the tibial component) was attached to a special table. During movement, rotation of the top half (the femoral component) over the tibia wasn't stopped.
A repeated load of 450 pounds was applied to each implant. The load was kept steady while rotating the joint. Details of how each implant responded to the forces are reported. The authors determined that rotation of a TKR is a key factor in the function and stability of the joint.
The researchers found that small changes in rotation can more than double the force on the implant, depending on its design. Current implant designs give a trade-off. Implants with less constraint may allow for more knee motion, but this can lead to extra wear that causes the implant to break down earlier. High constraint implants provide better stability, but the forces that happen with knee motion (especially rotation) can take a toll on the spot where the implant is fixed inside the bone. When this happens, the fit of the implant can loosen, leading to problems and the possibility of needing another surgery.
R. Klein, MS, et al. Rotational Constraint in Posterior-Stabilized Total Knee Prostheses. In Clinical Orthopaedics and Related Research. May 2003. Vol. 410. Pp. 82-89.
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