Abstract
Stems provide short- and long-term stability to the femoral and tibial components. Poorer epiphyseal and metaphyseal bone quality will require sharing or offloading the femoral and tibial component interfaces with a stem. One needs to use stem technique most appropriate for each individual case because of variable anatomy and bone loss situations. The conflict with trying to obtain stability via the stem is that most stems are cylindrical but femoral and tibial metaphyseal/diaphyseal areas are conical in shape. Viable stem options include fully cemented short and long stems, uncemented long stems, offset uncemented stems, and a hybrid application of a cemented proximal end of longer uncemented diaphyseal engaging stems.
Stems are not without their risk. The more the load is transferred to the cortex, the greater the risk of proximal interface stress shielding. A long uncemented stem has similar stress shielding as a short cemented stem. Long diaphyseal engaging stems that are cemented or uncemented have the potential to have end of stem pain, especially if more diaphyseal reaming is done to obtain greater cortical contact. A conical shaped long stem can provide more stability than a long cylindrical stem and avoid diaphyseal reaming. Use of long stems may create difficulty in placement of the tibial and femoral components in an optimal position. If the femoral or tibial components do not allow an offset stem insertion, using a long offset stem or short cemented stem is preferred. The amount of metaphyseal bone loss will drive the choice of stem used. Short cemented stems will not have good stability in poor metaphyseal bone without getting the cement out to the cortex. Long cemented stems provide satisfactory survivorship, however, most surgeons avoid cementing long stems due to the difficulty of removal, if a subsequent revision is required. If the metaphyseal bone is excellent, use of a short cemented stem or long uncemented stem can be expected to have good results. Long fully uncemented stems must have independent stability to be effective, or should be proximally cemented as a hybrid technique. Cases with AOI type IIb and III tibial and femoral defects are best managed with use of metaphyseal cones with short cemented stems or long hybrid straight or offset stems. Some studies also suggest that if the cone is very stable, no stem may be required.
My preference is to use a short cemented stem or hybrid conical stem in patients with good metaphyseal bone. If significant metaphyseal bone loss is present, I will use a porous cone with either a short cemented stem, hybrid cylindrical or offset stem depending on the primary stability of the cone and whether the femoral or tibial component can be placed in an optimal position in patients with good metaphyseal bone.
