The original (Phase 1) femoral component had a single oblique peg with faceted surfaces because the femoral condyle was prepared with a saw. The Phase 2, and subsequently the Phase 3, had a single peg parallel to the flat posterior surface, which allowed incremental milling (see Fig. 1.3). These components were designed to be implanted in neutral flexion/extension. When the cementless femoral component was designed, it included a second peg for more stable fixation and an anterior extension of about 17° (see Fig. 7.20(a) & (b)). A two-peg cemented femoral component of similar design to the cementless was introduced at the same time. The anterior extension meant that the femoral component could be implanted 10° to 20° flexed. It was also advantageous for cementless components as, in high flexion, the joint contact force was less likely to loosen the component. The anterior extension also ensures that, for both cemented and cementless components, high stresses in the bearing caused by contact with the posterior/proximal edge of the femoral component in high flexion are less likely. The components with the anterior extension do not fit within the milled surface until anterior bone has been removed to prevent impingement. This is advantageous as it ensures that anterior bone is removed. However it means that, for initial balancing, a one-peg trial component is needed.
The Microplasty method reliably implants the femoral components about 10° flexed. If surgeons wish to implant components more flexed, a 15° Microplasty adaptor is available. This is used by some surgeons in Asia whose patients require full flexion. Studies in these patients have not shown any clinical advantage of flexing the component 15° or more. However, we have seen a dislocated bearing which had a transverse groove caused by end loading retrieved from a patient with full flexion in whom the femoral component was not flexed 15°. Therefore there may be an advantage, in the long term, of flexing the component 15° in patients with full flexion.
As the cemented two-peg component was introduced in the USA at the same time as Microplasty and as it was felt there was no need to flex the component in the USA more than 10°, the two-peg component without the full anterior extension was used. This component, which has a 7° extension to support the second peg, fits within the milled surface. Therefore, a trial component based on the definitive component can be used for balancing (see Fig. 7.20).
