The prosthetic joint level
The prosthetic joint level is shown in the construct in Figure 6.3. It is at the interface between the femoral component and the polyethylene bearing, the level at which flexion–extension occurs. The operation aims to remove enough bone from the femoral and tibial condyles to create a flexion gap that the constructed implant will just fill. The thickness of the posterior facet of the femoral component varies with its radius from 5.50 mm (extra small) to 7.45 mm (extra large). The thickness of the tibial component is 3 mm which is constant throughout the range of sizes. The thinnest bearing usually considered safe to use (3 mm) has 3.5 mm of polyethelene at its thinnest point. We do however recommend a 4 mm bearing for all but small and extra small femoral components.
In this respect, the thickness of bone removed from the tibia is not critical. If more is removed than the minimum required, the widths of the flexion and extension gaps are increased equally, and their ‘balance’ does not alter. A thicker bearing will be needed to restore stability, but this will affect neither the joint level nor, therefore, the kinematics of the replaced compartment (Fig. 6.3(c)).
Figure 6.3 The prosthetic joint level, the continuous solid line.
The anatomical joint level
For the MCL and cruciate ligaments to resume their isometric function, the prosthesis should be implanted so that its prosthetic joint level coincides, throughout the range of movement, with the old anatomical joint level, i.e. the articular surface of the femoral condyle before it was modified by disease. The relative thicknesses of bone removed from the posterior and the inferior surfaces of the femur is critical because this determines the relative widths of the flexion/extension gaps.
In anteromedial OA, the posterior articular surface of the condyle is preserved (Fig. 6.4(a)). If a layer of cartilage and bone the same thickness as the metal implant is excised from the back of the condyle, the prosthetic and the anatomical joint levels will coincide (Fig. 6.4(b)). This establishes the width of the flexion gap which will not subsequently be changed.
The position of the anatomical joint level anteriorly is lost because all the cartilage, plus an unknown quantity of bone, has been eroded from the inferior surface of the femoral condyle. However, its site can be deduced because the ligaments that once matched it are still intact. As these ligaments are isometric throughout the joint range, the prosthetic joint level will be at the anatomical joint level when the extension gap is the same as the flexion gap (already established). The non-articular surfaces of the femoral component are designed so that removing bone from the inferior surface of the femoral condyle widens the extension gap without altering the flexion gap (Fig. 6.4(b)).
Figure 6.4 The anatomical joint level posteriorly, the continuous solid line.
Effect of using a spherical femoral component
The natural medial femoral condyle is polyradial; its inferior articular surface has a radius of curvature greater than its posterior radius. The prosthetic condyle is circular in the sagittal plane, and so the most anterior part of its articular surface cannot coincide with the old anatomical joint level but is proximal to it. This does not affect the gap measurements described above since the natural condyle is virtually circular between the points of contact at which the extension gap (20°) and flexion gap (110°) are measured and also in higher flexion (Monk et al. 2014) (see Fig. 2.9).
The rationale of balancing the ligaments assumes that, if the prosthetic and the anatomical joint levels coincide at these two points, they will also coincide at all intervening contact points and in higher flexion, maintaining the ligaments at constant tension throughout that range. This approximation will be closest if a femoral component with a radius similar to that of the posterior natural condyle has been chosen.
