The rational for cemented stem design

Aim: The purpose of this study was to compare cemented anatomic stems with cemented straight stems regarding cement mantle and stem orientation in the medullary canal. Methods: In a cadaver study, 10 anatomic SP II stems and 10 MEM straight stems were implanted in paired human femora using a standardised modern cementing technique. In one group the femoral canal was prepared using conventional broaches, in the other group diamond hollow-cutters were used. Standardised horizontal cuts were made and evaluated regarding stem orientation and cement mantle using digitised image analysis. Results: All SP II stems in the "diamond group" showed good stem centralisation without cement mantle defects. In the "broach group" all diaphyseal cancellous bone had been removed and only 3/5 stems were well orientated. No reproducible stem centralisation was achieved with the straight stems. All stems showed an oblique orientation from antero-proximally to postero-distally with direct stem to bone contact. The variation from the optimal stem alignment along the canal axis showed a mean deviation of 3.1 mm in the anatomic stem group and 10 mm in the straight stem group. The canal preparation using broaches showed frequent fractures of the cancellous bone and debris interposition despite the use of jet-lavage. Most of the cancellous bone in the diaphysis had been destroyed. Conclusions: Anatomic stems show a better stem centralisation and a more even cement mantle than straight MEM stems. The use of high volume straight stems carries a significant risk of producing cement mantle defects. Diamond instruments are less traumatic than broaches for femoral canal preservation and help to preserve diaphyseal cancellous bone, which improves stem self-centralisation.