The most common problem is adequate sizing of the femoral component, as this is influenced by the underlying principles, “anterior referencing” or “posterior referencing” (Figs. 15.1, 15.2, 15.3, and 15.4), as well as “femur first” or “tibia first” [8]. Femoral size is determined by the anteroposterior dimension, as the reconstruction of the posterior condylar offset is important for reconstructing knee kinematics [9].

Direct correlation has been reported between posterior condylar offset and range of motion [1]. In case of decreased posterior condylar offset, the range of flexion will decrease. If the posterior condylar offset is reduced by 2 mm, the flexion decreases in mean 12.2°.

Although inter- and intraobserver reliability of femoral sizing is very accurate [18], size measurement of the femoral component is highly dependent on the method used: “flexion space balancing”, used in most “tibia-first” instruments, leads to smaller size measurements than “size-matched resection”, used in most “femur-first” instruments [8]. Preoperative varus knees are more sensitive to these differences. In 6.7 % there is a side difference of femoral sizes in the same patient [4].

A flexion or extension error of the distal femoral cut leads to the choice of an up- or downsized femoral component [17]. This fact can also be used in case downsizing is required: additional flexion of the femoral component by 2° leads to 1 mm less resection of the posterior femoral condyles [22] (Fig. 15.5).

There has been broad discussion of whether mediolateral overhang can be avoided by the use of “gender” implants. In general, the ratio between the transepicondylar width and the height of the condyles is constant, but some narrow femora could require narrower implants to avoid mediolateral overhang [19]. On the other hand, despite reduced overhang by using “gender” knee implants, there are no differences in clinical outcome [6].

In the case that the chosen femoral size doesn’t cover the whole mediolateral width, a lateralisation rather than medialisation of the implant is recommended in order to improve patellar tracking (Fig. 15.6).

In general, a change of size in a posterior referenced TKR system means a change of the anterior bone cut (Figs. 15.1 and 15.2).

Oversizing the femoral component in a posterior referenced system means overstuffing of the patellofemoral joint and leads to increased patellofemoral pressure and possibly pain (Fig. 15.1) [10].

Undersizing the femoral component in a posterior referenced system means notching of the anterior cortex and a possible risk of fracture (Figs. 15.2 and 15.7). Minimal anterior notching has no influence on the risk of fracture [20, 7], as long as femoral notching of more than 3 mm with sharp edges at the proximal end of the femoral prostheses is avoided [24].

In general, a change of size in an anterior referenced TKR system means a change of the posterior bone cut (Figs. 15.3 and 15.4).

Oversizing the femoral component in an anterior referenced system means overstuffing of the flexion gap and a reduced ROM [2, 12].

Undersizing the femoral component in an anterior referenced system leads to flexion instability, increased polyethylene (PE) wear and decreased flexion (Fig. 15.8) [1].