The aim of this study was to compare the primary stability and strain distribution of a calcar loading (Metha) and a shortened tapered stem (Fitmore). We found a high and almost equivalent primary stability for both stem designs. Furthermore, our results demonstrated reduced proximal stress-shielding with a tendency to more physiological load transfer for the Metha stem, although a stress-shielding effect was present for both stems. The results are in agreement with our hypothesis.
That a shorter stem design, in endoprosthetic treatments of the hip with good bone quality, does not negatively affect primary stability is well examined in biomechanical studies [2, 3, 32]. However, it must be noted that results of different in-vitro studies are not reliably comparable due to different test protocols and test preparations . For this reason, comparative studies are necessary, particularly to minimise the impact of anatomical variations .
Due to the metaphyseal anchorage concept of shorter stem designs, the comparison with a conventional stem model has shown a more favourable rotational stability for a type 2 and a type 4 short stem in two separate studies with an almost identical test-setup [2, 3]. Similar results of the in-vitro primary stability were obtained in a study comparing two type 2 short stems with a thrust plate prosthesis, for which good long-term data already existed . In this three-dimensional test setup using composite femora and a load application of up to 1700 N, reversible micromotions at all measuring points were likewise less than 150 µm. This is also supported by available excellent clinical mid-term results of these implant designs [13, 21, 34]. Yamako et al. reported controversial observations of an experimental study, which examined shorter variants of an anatomical stem . This in-vitro study using composite bones with a similar maximum loading of 1600 N showed increasing micromotions in the axial and rotational directions for shortened variants of the conventional model (120 mm) by 40 mm and 70 mm, respectively. This investigation indicated that simply shortening a conventional implant may not necessarily achieve the required stability. Multipoint cortical fixation, particularly within the cortical neck level, supported by cancellous bone compaction of the type 2 stem, as well as the tapered-wedge design with a large coronary diameter of the proximal implant third of the type 4 stem are design features that already have shown sufficient primary stability in earlier in-vitro studies [2, 3, 14, 30].
The measured proximal stress reduction of the Fitmore stem corresponds to previous measurements of this stem model of our research group . In this in-vitro study, the comparison with a type 2 stem (Mayo, Zimmer, Warsaw, Indiana) showed no significant difference in strain distribution. By contrast, the comparison with a conventional straight stem (CLS Spotorno, Zimmer, Warsaw, Indiana) showed a more physiological load transfer for the Fitmore stem. This was also supported by clinical trials measuring periprosthetic bone density changes [16, 27]. Interestingly, Maier et al. reported a high rate of postoperative cortical hypertrophy mainly in zones 3 and 5 according to Gruen for this implant model in a single centre study without negatively affecting clinical outcome . The authors proposed a reduced primary stability for this phenomenon. However, these hypotheses were countered by an associated, comparative biomechanical in-vitro study using synthetic bones and a dynamic axial load of up to 4000 N with a proven conventional straight stem . Primary stability in the present study did not support these hypotheses either. Another hypothesis for this observation would be a more distal load transmission and consequent bone remodelling. However, findings of a dual-energy X-ray absorptiometry (DEXA) study over 5 years of this stem model counter this consideration . In this investigation, the Fitmore stem showed no corresponding change in bone mineral density (BMD) of the distal third of the implant and less proximal stress-shielding compared to the CLS straight stem. The current study also showed no relevant difference in the change of the cortical surface strain in this area compared to the Metha stem. For both the CLS and the Metha stem used as references in these studies, no frequent occurrence of cortical hypertrophies was described. However, the stress-shielding effect around the proximal third of the implant was less pronounced for the Metha stem in the present study.
A biomechanical study using synthetic bones showed that the resection height appears to have a considerable influence on the load transmission of the Metha stem in the proximal femur . The deeper the resection, the more physiological were the strain patterns in this investigation. This aspect was not considered in the present study. However, in this implant group, the resection height directly determines the stem position, so that this effect is directly related to the individual anatomy to be reconstructed. Furthermore, a lower resection height negatively influences the rotational stability of a femoral implant . In contrast to these findings, a finite element model investigating the design-specific effect of stress-shielding of shorter femoral stems depending on the resection height showed no difference in behaviour in the proximal femur . In our study, a preferably calcar-sparing resection height was chosen according to the preoperative planning of the implant position in reconstructing individual anatomy.
The influence of the offset variant used on load transmission in the proximal femur represents another essential aspect. A biomechanical comparison of different offset variants of a modular conventional straight stem demonstrated only minor changes on overall femoral load transmission , whereas varus implant positioning resulted in a significant increase in the distal load transfer . Relevant changes in the proximal third of the implant, particularly in the calcar region, were not found in this biomechanical investigation. However, a transfer of these results to shorter femoral stems appears to be limited. For the Metha stem with a small CCD-angle and correspondingly larger offset reconstruction, a higher load transmission in the calcar region and only minor changes in the distal third were observed in a biomechanical study using synthetic bones . In the present study, both implants displayed a decrease in the cortical surface strain of the calcar region, which was less pronounced for the Metha stem. We used the offset variant according to the preoperative planning, taking into consideration the native femoral offset. However, in the present study, a high offset stem variant was used with only one specimen and not included for the cortical strain measurement. Furthermore, an increased load transmission in the calcar region by implantation of a high offset stem in a usually normal configured synthetic bone can be expected and probably does not correspond to the situation in vivo. Corresponding to our results, a biomechanical study using synthetic bones and a Metha stem with a CCD-angle of 135° demonstrated a reduction of the cortical surface strain in the calcar region . Furthermore, this is in agreement with findings of a DXA study involving 25 patients after total hip arthroplasty (THA) with a Metha stem, which showed a decrease of BMD in the region of the calcar of approximately 13% after two years .
This study has some limitations. First and importantly, this experimental model was intended to simulate conditions that occur during the first postoperative weeks and might underestimate individual activity levels. Torsional and bending moments were generated by simulating a single-leg stand and a load application, which corresponds to normal gait and weight. We are aware that in-vitro studies are only partially able to reflect conditions in vivo. In particular, the influence of muscle force on implant behaviour in the femoral bone was not considered in this test setup. Nonetheless, this offers the advantage of high reproducibility [4, 7]. We only used strain gauges positioned in the frontal plane. Particularly in the case of a pronounced implant tilt in the sagittal plane, which can occur with a high osteotomy and native femoral torsion, load transmission of the distal implant third may be underestimated by the chosen strain gauge positions. We are aware that gauge position R0 depends on the osteotomy level and, therefore, the measurement results are not directly comparable. Nevertheless, we have chosen this positioning to gain information at the level of the calcar osteotomy, which could best detect a proximal load introduction. Cortical strain measurement does not take into consideration adaptive bone remodelling. Nevertheless, clinical studies investigating changes in bone mineral density after THA are consistent with in-vitro findings for several stem designs [16, 22, 25]. Nonetheless, regarding statistical analyses, the power of the analyses is restricted by the small number of cases. For this reason, only a descriptive analysis was made for cortical strain measurements.
In conclusion, the current study demonstrated that both short stems achieved high primary stability, with micromotions well below the critical threshold above which osseointegration may be disturbed. Both stems could not avoid proximal stress-shielding. The design concept of the Metha stem with a comparatively large proximal rectangular cross-section, which provides for a cortical implant contact at the osteotomy level, ensures a more favourable load transmission with regard to the effect of stress-shielding. Clinical studies need to evaluate whether the design concepts differ in terms of the long-term performance.