Master Techniques in Orthopedic Surgery Knee CH21

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21 Managing Bone Loss with Metaphyseal Cones

Prosthesis Selection ●● Beginning on the tibial side, the trial component, sized earlier in the case, that adequately covers the proximal surface is selected and the appropriate rotation set. ●● The appropriate diameter stem extension, sized earlier in the case, is selected and attached to the trial. ●● The trial component with the appropriate diameter and length stem extension is then impacted through the real metaphyseal cone (Figure 21-17). ●● Axial alignment is then verified with a drop rod and adjustments are made as required. ●● Once alignment has been optimized, the extent of any remaining bone defects is assessed and block or wedge augments are selected. The trial is removed, the augments attached and then reimpacted and checked. ●● Preparation of the femur begins by identifying the surgical transepicondylar (TEA) axis. ●● The femoral component size and stem are then selected. As with the tibia, long diaphyseal engaging stem trials are used initially to optimize alignment. ●● The box cut is then performed by inserting the jig attached to the previously determined stem extension. Rotation is set along the surgical TEA, the jig pinned in place, and the cut performed. ●● The trial femoral component is assembled with distal augments to set the preliminary distal joint line approximately 25 to 30 mm distal to the epicondyles. It is impacted through the real femoral cone that has been inserted, and the distal and posterior bone gaps are evaluated (Figure 21-18). ●● The trial is removed and standard augments distally and posteriorly are optimized. ●● A tibial polyethylene trial that tensions the flexion gap is then inserted. ●● The knee is then extended and the extension gap is evaluated. ●● If the knee has a residual flexion contracture, the distal augments are reduced and the femoral component moved more proximally. Alternatively, the next bigger femoral component can be selected to better fill the flexion gap, if it can be accommodated by the medial–lateral dimension of the femur. This will allow a thinner polyethylene insert to be used. ●● If the knee hyperextends, additional distal augments are added onto the distal femur which better fills the extension gap. ●● When either the medial or lateral collateral ligament is nonfunctional, or in cases where a gross mis- match exists between the flexion and extension gap that cannot be managed by adjusting the position or size of the femoral component, a more constrained condylar prosthesis or hinged device must be used.

FIGURE 21-17. The trial tibial component with 5-mm medial and lateral augments and a short cemented stem trial is passed through the already placed real tibial cone.

FIGURE 21-18. The trial femoral component with appropriate augments and a mid-length cemented stem trial is passed through the already placed real femoral cone.