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Balancing the Stresses in Knee Joint Implants

A total knee replacement can help people with painful knees walk and do other activities with greater ease. But walking can put uneven forces on the new joint implant, causing wear and tear. The joint replacement can even fail altogether.

One major factor on implant wear and tear is the way the joint is placed in the knee. Lining up the implant with the bones and soft tissue is very important. Poor alignment is probably the number one cause of uneven wear.

Designing and making a long-lasting implant is the focus of current research. Researchers at the Orthopedic Research Labs in Cleveland, Ohio, studied four total knee implants. Only the bottom half of the joint was studied. This half of the implant is inserted into the lower leg bone (tibia). The scientists were interested in stress on the tibial implant during the toe-off phase of walking.

The authors found that implants with matching edges limit contact stress during walking. However, with this design, rotation is limited. As a result, the torque, or twist, is increased where the implant meets the bone. The chance that the joint implant will loosen increased with this type of implant.

Less conforming implants allow for more rotation. However, they showed increased stress on the edges of the implant. The stress was even worse if the muscles weren't in balance. Surgeons attempt to balance the muscles and soft tissues during surgery because uneven pulling of the muscles is enough to cause uneven wear and tear.

The results of this study give design engineers information for implant improvement. Managing contact stress is an important key to solving the problem of damage patterns to the tibial insert.

References: Edward A. Morra, MSME, et al. The Effects of External Torque on Polyethylene Tibial Insert Damage Patterns. In Clinical Orthopaedics and Related Research. May 2003. Vol. 410. Pp. 90-100.

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