For amputated patients, direct attachment of upper leg prosthesis to the skeletal system by a percutaneous implant is an alternative solution to the traditional socket fixation. Currently available implants, the OPRA system (Integrum AB, Göteborg, Sweden) and the ISP Endo/Exo prosthesis (ESKA Implants AG, Lübeck, Germany) [1-2] allow overcoming common soft tissue problems of conventional socket fixation and provide better control of the prosthetic limb [3], higher mobility and comfort [2, 4]. However, restraining issues such as soft-tissue infections, peri-prosthetic bone fractures [3, 5–8] and considerable bone loss around the stem [9], which might lead to implant's loosening, are present. Finally, a long a residual limb is required for implant fitting.

In order to overcome the limiting biomechanical issues of the current designs, a new concept of the direct intramedullary fixation was developed. The aim was to restore the natural load transfer in the femur and allow implantations in short femur remnants (Figure 1). We hypothesize that the new design will reduce the peri-prosthetic bone failure risk and adverse bone remodeling.

Generic CT-based finite element models of an intact femoral bone and amputated bones implanted with 3 analyzed implants were created for the study. Models were loaded with two loading cases from a normal walking obtained from the experimental measurements with the OPRA device [10-11]. Periprosthetic bone failure risk was evaluated by considering the von Mises stress criterion [12-14]. Subsequently the strain adaptive bone remodeling theory was used to predict long-term changes in bone mineral density (BMD) around the implants. The bone mineral content (BMC) change was measured around implants and the results were visualized in the form of DXA scans.

The OPRA and the ISP implants induced the high stress concentration in the proximal region decreasing in the distal direction to values below physiological levels as compared with the intact bone. The stresses around the new design were more uniformly distributed along the cortex and resembled better the intact case. Consequently, the bone failure risk was reduced as compared to the OPRA and the ISP implants. The adaptive bone remodeling simulations showed high bone resorption around distal parts of the OPRA and the ISP implants in the distal end of the femur (on average −75% ISP to −78% OPRA after 60 months). The bone remodeling simulation did not reveal any bone loss around the new design, but more bone densification was seen (Figure 2). In terms of total bone mineral content (BMC) the OPRA and the ISP implants induced only a short-term bone densification in contrast to the new design, which provoked a steady increase in the BMC over the whole analyzed period (Figure 3).

In conclusion, we have seen that the new design offers much better bone maintenance and lower failure probability than the current osseointegrated trans-femoral prostheses. This positive outcome should encourage further developments of the presented concept, which in our opinion has a potential to considerably improve safety of the rehabilitation with the direct fixation implants and allow treatment of patients with short stumps.

The Osteoprotegerin/RANK/RANKL system has been implicated in the biological cascade of events initiated by particulate wear debris and bacterial infection resulting in periprosthetic bone loss around loosened total hip arthroplasties (THA). Individual responses to such stimuli may be dictated by genetic variation and we have studied the effect of single nucleotide polymorphisms (SNPs) within these genes. We performed a case control study of the Osteoprotegerin, RANK and RANKL genes for possible association with deep sepsis or aseptic loosening. All patients included in the study were Caucasian and had had a cemented Charnley THA and polyethylene acetabular cup. Cases consisted of 91 patients with early aseptic loosening and 71 patients with microbiological evidence at surgery of deep infection. Controls consisted of 150 THAs that were clinically asymptomatic for over 10 years and demonstrated no radiographic features of aseptic loosening. DNA samples from all individuals were genotyped using Taqman allelic discrimination. The A allele (p<0.001) and homozygous genotype A/A (p<0.001) for the OPG-163 SNP were highly associated with aseptic failure. Additionally, the RANK-575 (C/T SNP) T allele (p=0.004) and T/T genotype (p=0.008) frequencies were associated with aseptic failure. No statistically significant relationship was found between aseptic loosening and the OPG- 245 or OPG-1181 SNPs. When the septic group was compared to controls, the frequency of the A allele (p<0.001) and homozygous genotype A/A (p<0.001) for the OPG-163 SNP were statistically significant. No statistically significant relationship was found between septic failure and the OPG- 245, OPG-1181 or RANK-575 SNPs. Aseptic loosening and possibly deep infection of THA may be under genetic influence to candidate susceptibility genes. SNP markers may serve as predictors of implant survival and aid pharmacogenomic prevention of THA failure

INTRODUCTION. Patellofemoral joint (PFJ) replacement is a successful treatment option for isolated patellofemoral osteoarthritis. With this approach only the involved joint compartment is replaced and the femoro-tibial joint remains intact. Minimizing periprosthetic bone loss, which may occur due to the stress shielding effect of the femoral component, is important to insure long-term outcomes. The objective of this study was to investigate, using finite element analyses, the effects of patellofemoral replacement on the expected stress distribution of the distal femur eventually leading to changes in bone density. METHODS. MRI images of a healthy knee were acquired, segmented and reconstructed into a 3D physiological model of the bony and cartilaginous geometries of distal femur and patella with patellar tendon and insertion of the quadriceps tendon. This model was modified to include PFJ replacements with either a Journey PFJ or a Richards II PFJ prosthesis, and a Genesis II TKA (Smith&Nephew, Memphis, TN). The prosthetic components were incorporated in the intact model based on the manufacturer's instructions or previously described surgical techniques (Figure 1). Cortical bone was modeled with orthotropic properties, while homogeneous linear isotropic elasticity was assumed for trabecular bone, cartilage, cement and femoral components materials. The patellar tendon was given Neo-Hookean behavior. UHMWPE patellar buttons for all designs were assigned non-linear elasto-plastic material. The simulated motion consisted of a 10 second loaded squat, starting from 0° until a flexion angle of 120° matching experimental kinematics tests performed in previous in-vitro analysis on physiological cadaveric legs [1-2]. The patella model was constrained fixing the distal part of the patellar ligament and applying a quadriceps force distributed on the quadriceps insertion on the proximal surface of the patella. During the dynamic simulation the average Von Mises stress was calculated in two regions of interest (ROI) defined in the femoral bone: one anterior and one proximal. The location of the ROIs was defined to fit the same regions as used in a previous bone mineral density analysis following patellofemoral arthroplasty (height 1cm, length 1cm). RESULTS AND DISCUSSION. Overall, the average bone stresses in both ROIs increased with flexion angle. Maximal stresses during squat were reached at 90° flexion angle, (2.8–3.8 MPa for the anterior ROI and 1.4–1.6 MPa for the proximal ROI). Mean stresses in the proximal ROI were similar for both PFJ designs and the physiological model, and slightly lower for the TKA. Between 80° and 120°, anterior ROI bone stresses for Journey PFJ design were comparable to the physiological knee, while reduced by almost 25% for the other designs (Figure 1). These results suggest a different stress-shielding behavior depending on design geometry and material properties. CONCLUSIONS. This study evaluated periprosthetic bone stress distributions of different patellofemoral replacements. The numerical analyses of physiological and replaced knee models predicted a decrease in stress behind the anterior flange of the femoral component for some designs. This reduction was dependent on prosthesis design geometry and materials properties

INTRODUCTION. Bone resorption around hip stems, in particular periprosthetic bone loss, is a common observation post-operatively. A number of factors influence the amount of bone loss over time and the mechanical environment following total hip replacement (THR) is important; conventional long stem prostheses have been shown to transfer loads distally, resulting in bone loss of the proximal femur. More conservative, short stems have been recently introduced to attempt to better replicate the physiological load distribution in the femur. The aim of this study was to evaluate the bone mineral density (BMD) change over time, in a femur implanted with either a short or a long stem. METHODS. Finite element models of two implants, a short (Minihip, Corin, UK) and long (Metafix, Corin, UK) hip stem were used to simulate bone remodeling under a physiological load condition (stair climbing). The magnitudes and directions of the muscle forces and joint reaction force were obtained from Heller et al (2001, 2005). An unimplanted femur was also simulated. A strain-adaptive remodelling theory (Scannel & Prendergast 2009) was utilised to simulate remodelling in the bone after virtual implantation. COMSOL Multiphysics software was used for the analysis. The strain component of the remodelling stimulus was strain energy density per unit mass. This was calculated in the continuum model from the strain energy density, and apparent density. Bone mass was adapted using a site-specific approach in an attempt to return the local remodelling stimulus to the equilibrium stimulus level (calculated from the unimplanted femur). The minimal inhibitory signal proposed by Frost (1964), was included in the model and described by a ‘lazy zone’, where no bone remodelling occurred. The three dimensional geometry of the femur was constructed from computed tomography data of the donor (female, 44 years old, right side). Elemental bone properties were assigned from the Hounsfield Unit values of the CT scans. The elastic modulus of the bone was assumed to be isotropic and was determined using a relationship to the apparent bone density (Frost 1964, Rho 1995). The Poisson's ratio for the bone regions varied between 0.2 and 0.32 depending on the apparent density of the bone (Stulpner 1997). The period of implantation analysed was 2 years. The muscle forces and joint contact loads applied were ramped linearly from zero to full load over a period of two weeks, representing the estimated post operative rest period of a patient. RESULTS AND DISCUSSION. The overall percentage BMD change observed for Gruen zones 1 through to 7, were −14%, +4%, +40%, +12%, +4%, 0%, 12% respectively at 2 years for the Minihip. The corresponding overall percentage BMD change observed for Gruen zones 1 through to 7 for the Metafix were −8%, −2%, 18%, 26%, +12%, −9%, −42% respectively (Figure 1,2). CONCLUSIONS. Considerably more bone resorption occurs in Gruen zone 7 with the long stem. Long stem designs distrupt the mechanical environment more than short stems, and lead to a greater bone mineral reduction over time