Inherent differences in the laxity and stability between the intact knee and total knee replacements

To compare the laxity and stability characteristics between the natural knee and condylar replacements, tests were carried out in a knee simulating machine. The tests consisted of applying compressive forces and then applying cyclic AP force and cyclic torque. The magnitudes were similar to those of functional conditions. For the natural knee, the laxities were only reduced modestly by increases in compressive force, especially internal-external rotation. For a low conformity TKR soft tissue restraint was required under low compression in order to avoid anterior tibial subluxation and internal or external rotation in excess of 20°. As compression was increased, the rapidly increasing effect of the dishing and the frictional effects provided sufficient inherent stability, although on average, soft tissue restraint reduced the laxity by about 30%. A high conformity TKR still required soft tissue restraint at low compression, but as the compressive force was increased, the surfaces reduced the laxity to the point where, the laxities were only a few millimetres and a few degrees, and the soft tissues contributed little. This phenomenon, where the femoral component is constrained to be close to the bottom of the tibial dish, may not be fully recognised at surgery and may result in excessive PCL tensions and contact forces in function, as well as reduced mobility, especially when a deep-dished tibial component was used. Relatively shallow posterior tibial curvature and a steep anterior curvature were concluded to provide the most satisfactory combination of laxity and stability.