The effect of sagittal conformity on knee wear

Durability concerns for knee replacement design have caused increases in tibial–femoral component conformity to lower contact stress. This has lowered stress and improved stability, but may also increase constraint and reduce allowed range of motion. Since a large range of motion is desirable, this study considers the effect of tibial sagittal radius on constraint and wear. The posterior sagittal radius of a given design was relaxed with the purpose of investigating the impact this change has on wear, stress and constraint when subject to the same motions and loading as the original design. Contemporary 10 mm posterior stabilized (PS) components were used, with all dimensions identical except for sagittal radius. The relaxed design doubled the posterior aspect of the sagittal radius, reducing posterior conformity. These were matched with identical femoral components and evaluated for stress, constraint and wear. Wear testing used a displacement controlled knee simulator, with 2667 N (600 lb) peak loading, internal/external (I/E) rotation, passive anterior/posterior (A/P) translation and a maximum flexion angle of 23°. Alpha calf serum (Hyclone Laboratories) was used as a lubricant and wear was determined gravimetrically. Measurements of constraint in the sagittal plane and rotational torque from 0° to 60° of flexion were conducted using an axial-torsional load frame. Stress was determined in two femoral/tibial component alignment positions using solid models (CAD) of components and finite element analysis software (ANSYS). Stress results followed expectations set from literature review. All stresses were higher for the modified geometry with the exception of a 11% decrease in von Mises stress in a rotated and flexed component position. The geometry change caused 67 and 14.5% decrease for rotational stiffness and posterior constraint, respectively, but no change in anterior constraint. This reduction in constraint also lowered wear by 10%. While lower constraint and stiffness are intuitive and increases in stress were expected, the decrease in wear is contrary to the belief that reduced conformity leads to higher wear. Stress analysis studies automatically predicting higher wear should be subject to careful scrutiny.