Implanted Magnets to Off-Load the Varus Knee

Principal Investigators: David G. Lewallen, M.D. and Darryl E. Barnes, M.D.
Project Coordinator: Kenton R. Kaufman, Ph.D. — kaufman.kenton@mayo.edu

Figure 17: Pressure across each plateau, illustrating off-loading obtained at 68kg axial load with implanted magnets.

During normal daily activity forces two to four times body weight cross the knee joint. Two-thirds of this load passes through the medial compartment. In a varus knee, greater loads are concentrated in the medial joint compartment. This misalignment can accelerate the development of symptomatic degenerative changes in the varus knee. Methods to "off-load" the medial joint compartment have served as the main treatment in the management of symptomatic varus knee arthritis.

Rare earth magnets create low friction surfaces in non-medical industries. Magnetic-field joint surfaces have theoretical benefits that make them intriguing alternatives to conventional joint surfaces: unaltered joint space, no third body wear, unlikely osteolysis, minimal bone stock loss. In this study, two neodymium magnets were implanted in cadaver knees to test that magnets can off-load the medial knee compartment under physiologic load.

The presence of a repelling force across the medial compartment shifted both force and area to the lateral compartment (Figure 17). Total compartment joint pressure peak compartment load and total compartment weight-bearing area decreased significantly in the medial joint compartment. At the same time, significant increases were observed in lateral compartment pressure, load, and area.

The presence of a repelling magnetic force across the medial compartment in a ligament constrained human knee predictably shifted both force and weight bearing contact area to the lateral joint compartment - effectively "off-loading" the medial compartment of the knee. Magnets represent an exciting new material that may offer novel solutions to a number of orthopedic challenges. Future study is needed to address anisotropic magnetic domains, corrosion resistance, biologic fixation, passive an active joint constraint, magnetic shielding, and global safety issues.