In this FEA study, significant differences in cortical stresses were found in the proximal femur when analyzing short (SHA, Metha) and standard (THA, CLS) stem hip arthroplasty. Both stems induced a clearly reduced cortical stress at the proximal femur, however, SHA was able to realize better proximal and metaphyseal stress transfer compared to THA, indicating a more physiological femoral loading.
A considerable reduction of cortical stress was found in the proximal femur after SHA and THA, which is a well-known phenomenon reported by multiple studies [11, 16–18]. In the native bone load transfer occurs via the subchondral bone and is transferred distally . Insertion of any implant into the femoral cavity subsequently changes this pattern and bypasses the load via the implant to the distal femoral bone . This phenomenon of unloading the proximal femur by shielded it from stress is known as stress-shielding, which is prone to cause periprosthetic bone loss contributing to aseptic loosing or periprosthetic fractures [7, 20].
It is well known that the proximal femur is the most affected region , the reason why new stem designs aim for a more proximal loading. Short hip stems lately represent an alternative to conventional stem and although long-term studies are pending, short and mid-term results are promising . Two main advantages have emerged: the preservation of soft tissue and bone stock  as well as the assumption of an improved femoral load transfer .
The results of this study strengthened this assumption as the FE analyses demonstrated that SHA succeeds to realize better proximal and metaphyseal load transfer. Still, the very proximal region shows less cortical stress compared to the native bone, but SHA offered clearly higher cortical stress transfer in the proximal regions compared to standard THA.
This matches well with clinical studies demonstrating that SHA can provide an improved bone remodeling compared to standard THA [9, 25]. In accordance to our FE analysis, a dual-energy x-ray absorptiometry (DXA) study by Lerch et al found a clear reduction of bone mineral density (BMD) in the greater trochanteric region, while the metaphyseal BMD increased . These results have similarly been reported by others [26–28] and are in line with our FEA results with the highest mean and peak cortical stress recorded metaphyseal. No relevant change in femoral BMD was noted in the most distal region  confirming our findings of a more proximal load transfer compared to THA. For standard THA, the proximal region showed clearly less cortical stress compared to SHA and the highest peak stresses were observed distally, arguing for a more distal shift of the load transfer than SHA.
These findings are confirmed by a systematic review of clinical DXA studies by Yan et al. who reported on bone remodeling for SHA. Despite SHA could not completely avoid bone loss in the trochanteric region. SHA designs featuring a proximal anchorage showed a more balanced and reduced remodeling compared to standard THA . Likewise, a meta-analysis of randomized controlled trials reported on a superior bone remodeling for SHA with similar survival rates and clinical outcomes compared to THA .
In vitro studies also confirmed these clinical observations, reporting that the stress reduction in the proximal femur was less in SHA than in standard THA [10, 11, 30]. Gronewold et al demonstrated by measuring strain pattern in synthetic femora that SHA reached a much closer strain pattern proximally compared to standard THA . Bieger et al measured strain pattern and micromotions with a biomechanical setup and showed that the SHA stem could realize a better strain pattern proximally, but could not completely avoid stress shielding in Gruen zones 1 and 7 .
Other FEA studies evaluating stress and bone remodeling in SHA found similar results [19, 31, 32]. Lerch et al described a BMD reduction for a short stemmed femoral implant in the trochanteric region, a metaphyseal load, but no adverse effects distally which is in accordance to our results . Razfar et al evaluated the stress changes in the proximal humerus after short, stemless and standard shoulder implants . Their findings for the humerus correspond very well with ours for the hip and they concluded that stress shielding cannot completely be avoided, but may be reduced through the use of shorter implants .
Nevertheless, comparison with other FEA studies must be interpreted cautiously due to the diversities in the simulating approaches, bone-implant interface, loading conditions and specimens [12, 14]. Besides, frictional face-to-face contact and frictionless node-to-node contact are used to describe the bone-implant interface [12, 14], while in this study the bone-implant interface was bonded.
Further limitations have to be discussed and are closely linked to the FEA design and method applied. Firstly, the inner bone surface shared the same interface with the outer implant surface. This was different with the experimental study, the latter allowed contact between the bone and implant as well as slide and penetration, which might have an effect on the cortical stress distribution pattern after implantation. Second, there were only two specific implanted FE models, and subjected to simple loading configurations without muscle forces. Third, the material properties designed in the FEA might not exactly represent the actual properties in experimental study, because only the compressive moduli were used in the FEA, regardless of longitudinal and transverse tensile moduli. Fourth, the load transfer pattern was estimated according to the mean and peak cortical von Mises stress in divided regions, regardless of the direction of stress, such as compressive or tensile stress. Last, the stress distribution patterns were estimated by FEA without being validated experimentally. Nevertheless, the results match well with current clinical data especially remodeling of the BMD around both implants .