The causes of insert backside wear in total knee arthroplasty.

Wear of the insert backside occurs ostensibly because of micromotion at the undersurface articulation that occurs with loading. When a cyclic axial load was applied to contemporary knee implants, all inserts tested moved 2 to 25 microm in the shear plane relative to the metal backing suggesting that undersurface motion may be inevitable. Variables that increase the forces between the insert and metal backing can worsen relative micromotion and backside wear. Forces at the undersurface articulation, created during physiologic loading, are influenced by insert type, articular design, and surgical technique. Increasing articular insert constraint can cause forces at the main articulation to be resisted and transferred to this and the other interfaces. Designs with a cam post mechanism that force rollback at a certain flexion angle create a significant force in this shear plane. Inserts with highly conforming articular geometries can have a similar affect if used to inhibit anteroposterior or mediolateral motion of the femur on the tibial insert. Component alignment and position, and ligament balance also may influence backside wear as suggested by the great variability of wear patterns seen on like insert retrievals and by kinematic differences observed in fluoroscopic studies of the same implant design. Only by understanding these potential causes of backside motion and subsequent wear, can backside wear be mitigated.