The femoral drill guide
The femoral drill guide, femoral saw guide, and concave spherical mill are all available in five sizes to match the diameters of the five sizes of femoral component. With the knee flexed, the drill guide is inserted with its upper foot touching the intact cartilage on the posterior surface of the medial femoral condyle and its face touching the eburnated bone on the inferior surface of the condyle (Fig. 6.8(a)). The correct alignment of the drill guide is achieved by linking it to an intramedullary rod. A drill (with a fixed collar) passes through the guide to make a hole in the bone. The axis of the hole is the same distance from the foot of the instrument as the radius of the chosen femoral component, and the depth of the hole is the same as the length of the shafts of the spigots
Figure 6.8 Level of posterior femoral saw-cut.
Femoral saw guide
The guide is positioned in the drill hole and directs the saw to remove from the femoral condyle a slice of bone and cartilage the same thickness as the metal of the posterior part of the femoral implant. This restores the level of the joint line in flexion (see Fig. 6.4).
The concave spherical mill
The mill is used to remove bone incrementally from the inferior surface of the femoral condyle, simultaneously shaping the bone to match the inner surface of the femoral implant. The mill rotates about a spigot inserted into the drill hole already made in the condyle.
Figure 6.9 Concave spherical mill used to mill the inferior surface of the condyle. Milling is complete when the shaft of the spigot is seen, through the window, to touch the stop.
The spigots (Fig. 6.9) are provided in a range numbered 0 to 7 depending on the thickness of the collar that acts as a stop to the mill. As the thickness of the collar decreases in 1 mm steps from spigot 0 to spigot 7, the amount of bone removed increases similarly.
The lengths of the shafts on both sides of the collar are the same, and are constant throughout the range, so each spigot can be used either way around. The depth of the drill hole in the condyle is the same as the length of the shafts (see Fig. 6.8(a)), and so the spigots register at two sites, the worn surface of the condyle and the bottom of the drill hole (Fig. 6.10(a)).
The 0 spigot (the one with the thickest collar) is always used first (Fig. 6.10(b)). The mill then shapes the surface of the bone (Fig. 6.10(c)) so that a trial femoral component can be inserted (Fig. 6.10(d)). This establishes (as the spigot’s number suggests) a zero point from which subsequent measurements are made. Because the contour of the arthritic condyle has been flattened by loss of bone and cartilage, the bone removed is peripheral and mostly anterior; no bone is removed centrally (Figs. 6.10(b) and 6.10(c)). After milling with the zero spigot, the articular surface of the trial femoral component lies about 4 mm distal to the ebur nated surface of the bone (Fig. 6.10(d)) so that the extension gap is smaller than the flexion gap, an essential requirement for the next step.
First gap measurement
With the tibial template and the trial femoral component in place (Fig. 6.11(a)), the flexion gap (with the knee flexed to about 110°) is measured with a gap gauge (say, 4 mm). The joint is then extended (to 20° flexion) and the extension gap is measured (say, 1 mm). The difference between the flexion gap and the extension gap (4 mm – 1 mm = 3 mm) gives the thickness of bone to be milled from the inferior surface of the femur to make the gaps equal. The 3 spigot (with a collar 3 mm thinner than the 0 spigot) is inserted into the drill hole and the second milling is completed, having removed 3 mm of bone (Figs. 6.11(b) and 6.11(c)).
Figure 6.10 First milling.
Second gap measurement
The tibial template and the trial femoral component are reinserted and the gaps are again measured; they are usually found to be the same (Fig. 6.11(d)).
Occasionally, the second measurement shows that the extension gap remains narrower than the flexion gap, and that more bone must be milled away to achieve balance. To remove a further 1 mm, the 4 spigot (with a collar 1 mm thinner than the 3 spigot) is inserted and a third milling is performed. As Figure 6.11(c) shows, the small ring of bone under the collar of the spigot escapes the second milling and has to be removed to allow the femoral component to seat. This robs the spigot of one of its points of reference (Fig 6.10(a)). However, if a third milling has to be undertaken, the spigots continue to function as before by registering off the bottom of the drill hole. Figure 6.12 shows that, in this case, the temptation to hammer the spigot into the hole until the collar touches the bone must be resisted.
