To ensure the correct size, position the tibial template with its posterior margin flush with the posterior tibial cortex (Fig. 7.25(a)). This is facilitated by pushing the template too far back and passing the universal removal hook [14] over the posterior cortex of the tibia.
Pull the hook and thus the template forward until it is flush with the posterior cortex. The tibial template should be flush with the medial cortex (ignoring osteophytes) or overhanging slightly. If it overhangs by 2 mm or more use a smaller size tibial template. The front of the tibial template should also be within 3 mm of the front of the tibia. If it is not, redo the vertical cut so that a larger size template can be used. Note where the anterior margin of the template is.
Figure 7.25(a)
Force the tibial template laterally against the vertical cut and hammer the tibial template nail [6] into place, ideally in the posterior hole. Hold the nail throughout sawing to prevent movement of the template.
Introduce the keel cut saw into the front of the slot and saw until it has sunk to its shoulder (Fig. 7.25(b)). The saw blade is lifted up and down as it is advanced posteriorly. Confirm the cut is complete by holding the pin and feeling the saw hit the front and back of the keel slot. Once the saw cuts are complete, remove the tibial template, and wash the cut surfaces.
Figure 7.25(b)
Cemented/cementless differences
Select the appropriate keel cut saw. The standard Microplasty instruments are designed for the cementless saw and this should be used whether cemented or cementless fixation is used. In some countries, such as the USA, where cementless fixation is not yet available, Microplasty cemented templates are used with cemented keel cut saws.
If the cemented keel cut saw has been used, after removing the tibial template, excavate the groove to the correct depth by scooping out the bone with the blade of the cemented tibial groove cutter [18], taking care not to damage the anterior and posterior cortices (Fig. 7.26). The safest way to prepare the back of the groove is to feel the posterior cortex with the tibial groove cutter and then move it anteriorly by 5 mm before pushing down and bringing forward to empty the groove.
Figure 7.26
If the cementless keel cut saw has been used, the slot should have been accurately cut so the groove cutter should not be needed for cementless or cemented fixation. However some surgeons who use cemented fixation prefer to widen the slot with the cemented tibial groove cutter. The slot will then be the same width as the slot in long term studies.
Insert the trial tibial component [7] and tap with the tibial impactor [12] until fully seated (Fig. 7.27).
Figure 7.27
Ensure that the component is flush with the bone and that the posterior margin of the component is flush with the back of the tibia. If the component does not seat fully remove it and clean the keel slot out with the appropriate tibial groove cutter. The cementless groove cutter is designed to be used through the Microplasty template whereas the cemented groove cutter should be used without the template.
Use only the small toffee hammer [34] to avoid the risk of plateau fracture.
