Stress shielding seems to occur at the greater trochanter due to the vast cross-section of the implant. However, the aim of proximal load transfer of the Metha stem seems to be partially achieved. DEXA analysis revealed a concentrated load distribution on the medial portion of the femur, which is an important region to guarantee long-term implant survival.

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In total hip arthroplasty (THA), short stemmed cementless implants are used because they are thought to stimulate physiological bone remodeling and reduce stress shielding. We performed a numerical investigation on bone remodeling after implantation of a specific short stemmed implant using finite element analysis (FEA). Overall bone mass loss was 2.8% in the entire femur. Bone mass decrease was mostly found in the proximal part of the calcar and in the greater trochanter due to the vast cross section of the implant, probably leading to stress shielding. In the diaphysis, no change in the apparent bone density was proven. The assumptions made agreed well with bone remodeling data from THA recipients who underwent dual-energy X-ray absorptiometry. However, the clinical investigation revealed a bone mass increase in the minor trochanter region that was less pronounced in the FEA. Further comparisons to other stem designs must be done to verify if the relative advantages of the investigated implant can be accepted. Keywords: total hip arthroplasty; finite element analysis; short stemmed implant; dual-energy X-ray absorptiometry; bone remodelingThe ''bone preserving strategy'' in total hip arthroplasty (THA) follows the idea of physiological bone remodeling and bony ingrowth of the femoral component. 1 This lead to the development of ''neck preserving'' short stems that are designed to avoid stress shielding and increase periprosthetic bone formation. 2 The Metha 1 neck preserving stem (AESCULAP AG, Tuttlingen, Germany) was introduced in 2004, along with two other short stemmed implants. This implant follows the design of the successful Mayo short stem (Zimmer, Warsaw, IN), 1,3 but is an advancement over the Mayo prosthesis in terms of femoral neck preservation and the inclusion of an osteoinductive coating.Finite element (FE) simulations are widely accepted for estimating bone remodeling after THA. However, only direct bone remodeling data after implantation of the specific stem can validate predictions made from the simulations. Dual-energy X-ray absorptiometry (DEXA) is considered the most reliable tool to address this issue, 4,5 and is useful for evaluating proximal femoral remodeling following cementless THA. 6 The current experiment is part of our collaborative research efforts at the ''German Research Foundation.'' The scope is to develop FE models for different types of implants, so that bone remodeling can be assessed. We introduced the Metha 1 short stem in our clinic in 2005 and have conducted >1,500 procedures with this stem, representing 35% of our cementless THAs in 2010. Here we present the first numerical calculations on bone remodeling after THA with this specific implant.We asked the following research questions: (i) what is the effect of THA with the Metha 1 short stem on femoral bone remodeling?; (ii) can numerical computations be confirmed by DEXA measurement of bone remodeling?; and (iii) what are the differences and can we explain them? MATERIALS AND METHODSThe study was conducted on...

Background: There are several numerical investigations on bone remodelling after total hip arthroplasty (THA) on the basis of the finite element analysis (FEA). For such computations certain boundary conditions have to be defined. The authors chose a maximum of three static load situations, usually taken from the gait cycle because this is the most frequent dynamic activity of a patient after THA.

Aseptic loosening of the prosthesis is still a problem in artificial joint implants. The loosening can be caused by, among other factors, resorption of the bone surrounding the prosthesis owing to stress shielding. In order to find out the influence of the prosthesis type on post-operative stress shielding, a static finite element analysis of a femur provided with the conventional uncemented stem BICONTACT and of one with the femoral neck prosthesis SPIRON was carried out. Strain energy densities and maximal principal strain distributions were calculated and compared with the physiological situation. Here, stress shielding was demonstrated in both periprosthetic femora. To determine the areas of the stress shielding, the bone in each FE model was subdivided into three regions of interest (ROI): proximal, diaphyseal, and distal. The numerical computations show stress shielding in the proximal ROI of both periprosthetic femora. Diaphyseally, the femoral neck prosthesis SPIRON, in contrast to the conventional uncemented long-stem prosthesis BICONTACT, causes no decrease in the strain distribution and thus no stress shielding. Distally, no change in the load distribution of either periprosthetic femur could be found, compared with the physiological situation.

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Purpose The cementless Bicontact® total hip arthroplasty (THA) system (AESCULAP AG, Tuttlingen, Germany) was introduced in 1986/1987 and has been in successful clinical use in an unaltered form up to today. Although good longterm results with the Bicontact® stem have been published, it is questionable whether the implant provides the criteria for a state-of-the-art stem regarding proximal bone stock preservation. The purpose of the study was to monitor the periprosthetic bone mineral density (BMD) in a prospective two-year follow-up dual-energy X-ray absorptiometry (DEXA) study. Methods After power analysis, a consecutive series of 25 patients with unilateral Bicontact® stem implantation was examined clinically and underwent DEXA examinations. Scans of seven regions of interest were taken preoperatively and at one week, six months, and one and two years. Results One patient required stem revision due to a deep infection. The Harris Hip Score increased significantly by 44 points. The most significant bone loss was observed in the calcar region (R7) in the first six months (−19.2 %). It recovered in the following 18 months to −8.5 %. The BMD in the greater trochanter dropped significantly after six months and remained stable at this level. BMD exceeded baseline values in distal regions and even more in the lesser trochanter region after two years. Conclusions We conclude that the Bicontact® stem provides adequate proximal bone stock preservation. We observed some signs of stress shielding at the tip of the stem, which is inevitable to some degree in THA with cementless straight stems. However, in this prospective DEXA investigation, we showed that proximal off-loading does not occur after THA with the Bicontact® system. Thus, we believe that this stem is still a state-of-the-art implant.