Choose the Size of the Femoral Component That Stabilizes the Knee in Flexion

Choose the size of the femoral component that stabilizes the knee in flexion. It is a common and deadly error to measure existing bone and simply fit the corresponding femoral component to it. In almost every case, this will lead to the selection of a femoral component that is too small and an arthroplasty that is unstable in flexion or one in which excessive distal femoral bone must be resected to accommodate an unduly thick articular polyethylene. Undue resection of distal femur results in an unacceptable proximal migration of the joint line.

Ignore the residual bone on the distal femur in this step and visualize the normal bone that was present before any surgery had been performed. Use the size of the failed component, and lateral radiographs of the contralateral knee, if unoperated, to estimate the size of the revision femoral component. The final choice of revision femoral component size will depend upon the an-teroposterior dimension that is necessary to stabilize the knee in flexion. The revision femoral component size will be determined not by residual bone, so much as by the soft tissues, specifically the collateral ligaments.

Stability in flexion is determined not only by the size, but also by the anteroposterior location of the femoral component. Unless the original component was oversized, leaving good posterior condyles for the revision component, fixation will be compromised because bone has been lost from the posterior condyles as a result of the failed knee or the removal of components. That is the purpose of posterior femoral augments. They exist to fill in bone defects and consequently to stabilize the knee in flexion by enabling the surgeon to select an appropriately large femoral component.

Loose Flexion Gap

FIGURE 8.3. The femoral component is sized against the one removed. (A) If the knee was loose in flexion, a large femoral component is selected; or (B) if the original implant was sized correctly, a comparable revision femoral component is chosen.

FIGURE 8.3. The femoral component is sized against the one removed. (A) If the knee was loose in flexion, a large femoral component is selected; or (B) if the original implant was sized correctly, a comparable revision femoral component is chosen.

Without them, we would be forced to use components that were too small.

In the presence of defective bone, due either to defects or soft quality, enhance fixation with medullary stems. These will influence the position of the femoral component and accordingly the stability of the knee in flexion. Stems can create problems. If large canal filling stems are selected, there will be little latitude for adjustment of the component position.6 The component may be positioned in greater varus or valgus, flexion or extension or translated anteriorly or posteriorly, depending on the morphology of the femur. The position of smaller stems that are cemented in the canal (despite the undesirability of methacrylate in the canal) can be manipulated, anterior and posterior to affect the size of the flexion gap.

One situation that may arise when trying to determine the size of the femoral component is a gap mismatch. This important (and unusual) circumstance must be identified in any revision. Simply stated, a knee with an irreconcilable gap mismatch has a capacious flexion gap that, because of soft tissue failure, cannot be balanced to the likely dimensions of the extension gap with conventional releases or selection of the correct femoral component size. When the collateral ligaments, in particular the medial collateral, have stretched, it seems that we cannot find a femoral component large enough to stabilize the knee in flexion without an unacceptably thick polyethylene. The necessary femoral component may be so large that it no longer fits the medial to lateral dimensions of the bone. We have a knee that cannot be stabilized in flexion simply by recreating the anteroposterior dimensions of the femur.

The gap mismatch marks a decision point in the revision requiring a choice between accepting the laxity in flexion and protecting the patient with a constrained component or advancing the collateral ligament on the femur.7 Our preference has been to avoid linked, constrained devices (hinges) in all cases, and to even reconstruct ligaments and use a nonlinked constrained device. With this decision noted, the femoral component size and position established and the tibial insert selected, the difficult work of the arthro-plasty is complete.

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