Experimental and mathematical studies of the mobility and stability of the normal knee show that each requires interaction between the articular surfaces and the ligaments.
The joint achieves its mobility because the surfaces can roll and slide on each other while the ligaments rotate about their origins and insertions on the bones. The articular surfaces keep some fibres of the ligaments just tight over the range of movement, while the ligaments keep the surfaces just in contact. These interactions require no tissue deformation and give the joint its range of unresisted motion, leading to the coupling of axial rotation to flexion angle and to femoral rollback.
The joint achieves its stability because the articular surfaces resist indentation under compressive forces and the ligaments resist elongation under tensile forces. As load increases, the indentations and elongations continue and the contact areas adjust until the ligament forces balance the applied loads and muscle forces. This mechanism gives the joint its characteristic laxity, requiring rapidly increasing applied loads to move the bones from their passive positions.
In activity, motion occurs under load and the mechanisms which control mobility and stability are combined.
All these features of natural mobility and stability can be restored to a satisfactory extent by retaining all ligaments and implanting fully unconstrained prosthetic components which allow but do not resist the movements required by the soft tissues. Polyethylene wear can be minimised by the use of fully conforming mobile meniscal bearings. Exact reproduction of the shapes of the natural articular surfaces is not required. The balance between restoration of function and resistance to wear is achieved by the use of spherical and flat articular surfaces on the metal components with an interposed fully conforming meniscal bearing to give a ligament compatible arthroplasty.
