IMPLANT SELECTION: WHAT’S TO USE & WHEN TO CHOOSE IT

Abstract

The variety of case factors and the myriad choices in prosthesis types and accoutrements make the process of performing the best implantation seem impossible. Furthermore, one can easily become confused while considering all of the options.

A few simple considerations and an empiric method simplify the entire process.

Consider femoral component size. It is essentially never going to be bigger than what was taken out. If nothing else, one has great difficulty closing the wound. Therefore, we are usually at the same size or one size smaller.

We almost always will want to use at least a small to medium sized stem. Many of the times, probably most of the time, we will be using a PS set of components. The tibial size needs to cover the remaining tibial surface as well as possible. This requirement used to be quite challenging; but, today with offset connectors, it is possible to get better coverage without introducing overhang of the tibial baseplate.

Part of the bone preparation becomes easier if one recognises the issues of “stem dedication”. By this I mean the fact that the use of even medium sized stems that are at all “press-fitted” will dictate the varus-valgus position of the component, and, to a major degree, the anterior-posterior position of a femoral component. This fact alone should get us to the use of similarly stemdedicated instrumentation, which can be much easier to use than more traditional measured resection, cutting block-fixed implements.

In fact, the tibial cut, for me, is most easily made after reaming the tibial shaft and then assembling a trial baseplate. The baseplate and stem are then used as a visual cutting guide for the tibial clean up cut.

A primary femoral component is now crudely placed over the existing femur to get a vague assessment of the equality of the flexion versus extension gaps. This point is necessary before one does much distal femoral preparation. It is not appropriate to trim away a lot of bone if the extension space is already equal or large compared to the flexion one.

Limited or “no” preparation of the intercondylar region for the intercondylar “box” can serve to hold a stemmed, trial femoral component distally for better assessment.

Newer stem-dedicated instruments make preparation of the intercondylar region and a match up of the stem to the distal femur much easier.

It is important to appreciate the potential difficulty of handling an enlarged flexion space. An initial thought is to move the femoral component posterior. The first point to appreciate is that one cannot go very far posterior with the first selected size until the trochlear flange begins to sit below the anterior cortical bone. The natural response is to select a larger sized component. Even if we have stems with substantial offset capability we still need to be aware of the next points.

1.) The femoral components’ A-P dimensions only increase by 2 to 4 mm per size change, which is not too much, considering the gap inequality that one is likely to find; 2.) We still have the various problems associated with implanting bigger and bigger components, i.e. medial-lateral overhang and just being too large for closure as well as proper patello-femoral tracking; 3.) and posterior augments or platforms are, by themselves, not an additional factor as they reside within the envelope of the condyles, which we are already thinking about.

A more basic question is why the flexion gap may be so much larger than the extension one. Ordinary bone loss should not really cause this situation. If it exists, one needs to be particularly careful as to whether this inequality exists because of loss of collateral ligament integrity. There may, in extreme cases, be questions as to whether a PS or even CIP (constrained intercondylar prosthesis [a generic term]) can be used safely.

The majority of cases, though, will be handled quite expeditiously with a minimally tedious trial and error method starting from the steps outlined above.