Abstract
Introduction
We have been developed lateral flare stem and have been using it since 1989. It was custom stem at first. After being experienced, using the same software, off-the-shelf version lateral flare stem (Revelation) was developed in 1996 in the U.S. We could start using it since 2001 in our country. Lateral flare stems are designed to reproduce physiological proximal load transfer lateral side as well as medial side. It was obtained by having bigger and more accurate proximal part with lateral flare. The design is optimized by matching with 3D insertion path.
Using many custom stems including different length and off-the-shelf standard stems, we have come to feel that as for this high proximal fit and load transfer design, it is not necessary to having long distal part and sometimes it is harmful to obtain good proximal load transfer in some situation such as type A (champagne flute) canal. So we have developed short version of the stem. Many makes of the hip stems have included short stems recently. Some aimed to improve easier insertion, some aimed to improve the volume of residual bone quantity. We have aimed to improve proximal fit expecting more proximal and more physiological load transfer to the femur.
Objectives
Our objectives are to comare standard stem and short stem from biomechanical aspect and clinical aspect.
Materials and methods
As for the biomechanical aspect, finite element analyses were done with standard and short stem. As for the clinical aspect, the very last 25 cases of the standard stems; which we have 12 years clinical experience; done at Nagoya City University, the very first 25 cases of the short stems, and the next 25 cases were examined. The distance between stem and cortical bone on medial and lateral side at lateral flare hight of the stem and the bottom of arc deposite coated area. Alignment was assessed by the angle of the stem and canal axis.
Result
By the FEA, small stress point was observed at the tip of the standard stem which pushes canal wall from inside (Fig. 1), which was disappeared at the tip of the short stem. Less micromotion was observed in short stem too. No significant difference was observed in the stem cortical distance. No significant difference of stem alignment was observed between standard stem and all 50 short stems but better alignment (p = 0.07) was seen in the second 25 cases of the short stem than standard stem. Between the first 25 and the second 25 case high difference (p = 0.01) was seen. (Fig. 2)
Discussion
The standard lateral flare stem has very physiological proximal load transfer in most of the cases, sometimes longer distal part could effect to the alignment because of the femoral bending. Short stem could be expected have better alignment being free from femoral bending. On the contrary, distal part could be the insertion guide during the surgery. For the short stem, learning curve exists to realize potentially better alignment.
