INTRODUCTION. Mechanical overloading of the knee can occur during activities of daily living such as stair climbing, jogging, etc. In this finite element study we aim to investigate which parameters could detrimentally influence peri-implant bone in the tibial reconstructed knee. Bone quality and patient variables are potential factors influencing knee overloading (Zimmerman 2016). METHODS. Finite element (FE) models of post-mortem retrieved tibial specimens (n=7) from a previous study (Zimmerman 2016) were created using image segmentation (Mimics Materialise v14) of CT scan data (0.6 mm voxel resolution). Tibial tray and polyethylene inserts were recreated from CT data and measurements of the specimens (Solidworks 2015). Specimens with varying implant geometry (keel/pegged) were chosen for this study. A cohesive layer between bone and cement was included to simulate the behavior of the bone–cement interface using experimentally obtained values. The FE models predict plasticity of bone according to Keyak (2005). Models were loaded to 10 body weight (BW) and then reduced to 1 BW to mimic experimental measurements. Axial FE bone strains at 1 BW were compared with experimental Digital Image Correlation (DIC) bone strains on cut sections of the specimens. After validation of the FE models using strain data, models were rotated and translated to the coordinate system defined in Bergmann (2014). Four loading cases were chosen – walking, descending stairs, sitting down and jogging. Element strains were written to file for post-processing. The bone in all FE models was divided into regions of equal thickness (10 mm) for comparison of strains. RESULTS. Results are shown for two specimens at present. Strain-maps of the specimen cut section compare reasonably well with FE cutting-plane strains. The FE models however show some regions of high strain in certain locations which do not correspond with the experimental results (Figure 1). Plasticity predicted by the models at 10 BW is shown in Figure 2. Median bone strains for two loading cases are shown as a function of distance below the tibial tray in Figure 3. This figure shows that specimen 1 is less likely to be overloaded during jogging when compared with specimen 2. Both specimens remain below the 7300 με threshold for compressive yield. DISCUSSION. Using functioning knee replacement tibial specimens, we study which factors influence bone overloading. Validation using DIC strain measurements is challenging due to the large plasticity regions predicted by the material model used here. The present results were obtained using plasticity relationships from Keyak (2005) for the proximal femur. To further improve on our results, plasticity-bone density relationships for the proximal tibia (Keyak 1996) will be included. Proximal tibial bone has been shown to be stiffer than femoral bone (Morgan 2003). Despite these limitations, FE models provide valuable information on the risk of overloading during daily living activities. The study will be expanded to include an analysis of implant geometry, bone quality and other loading cases. For any figures or tables, please contact the authors directly

Introduction. In complex primary and revision total knee replacement (TKR) the operating surgeon may encounter proximal tibial bone defects. The correct management of such defects is fundamental to both the initial stability and long-term survival of the prosthesis. Cement or metal augments have been used to address some such type II unconstrained defects [1]. Aim. The aim of this finite element (FE) study was to analyse the comparative behaviour of cement and metal based augments and quantify the stresses within these different augments and underlying cancellous bone. Materials and methods. A three-dimensional FE model was constructed from a computer tomography (CT) scan of the proximal tibia using SIMPLEWARE v3.2 image processing software. The tibial component of a TKR was implanted with either a block or wedge-shaped augment made of either metal or cement. The model was axially loaded with a force of 3600N and testing was conducted with both evenly and eccentrically distributed loads. Results. Upon loading the FE model, the von-Mises stresses in the cancellous bone underneath the augments were found to be higher with cement based augments in comparison their metal based counterparts. This was evident with both block and wedge-shaped augments. The FE model demonstrated that compressive stresses within the metal based augments were greater than those within the cement based augments. This was evident with both block and wedge designs. Upon even loading the maximum recorded compressive stresses within the metal augments were 5 times less than the endurance limit of the material [3]. However, the maximum recorded compressive stresses within cement augments were only half the endurance limit of the material [4] and upon eccentric loading compressive stresses in excess of the endurance limit were recorded. Discussion. The FE model has demonstrated that cement based augments undergo a greater deformation when loaded and therefore transfer greater loads to the underlying cancellous bone. This is a result of the inherent flexibility of the cement based augment in comparison to the stiffer metal counterparts. The greater transference of load to cancellous bone with cement based augments may reduce the possibility of stress shielding. However, the compressive stresses within cement based augments are too close to the endurance limit of the material and with uneven loading even exceed it. This would imply that cement based augments are more prone to fatigue failure than their metal counterparts. Conclusion. This FE study supports the use of metal based augments over cement based augments in augmented and revision TKR surgery

A finite element study was carried out to compare the performance of a three-hole locking plate with angled screws to the ‘gold-standard’ four-hole hip plate. Two cases of the three-hole hip plate were examined; (a) three screws and (b) two screws (most proximal and most distal). A 3D model of the proximal femur was constructed from CT scans. A 3D CAD model of the four-hole hip plate was also created. The three-hole hip plate was then created from the four-hole implant in a way that it was possible to switch between all three models by activating/deactivating sections and/or switching material properties. A single common finite element model was generated, and a static analysis of each model variation was then performed in two steps using ABAQUS/standard. In the first, screws were pre-tensioned up to 150N. In the second, loads corresponding to stair climbing were applied. Forces in the screws, permitted to change in the second step, were examined and compared. Maximum principal stresses in the bone were also examined, with a focus on the stresses in the bone at the end of the plate in each model. The highest tensile force was in the proximal screw of the three-hole plate with three screws, followed by the most distal screw in the standard four-hole plate. This suggests that the risk of screw pull-out is highest at the proximal screw of the three-hole hip plate with three screws. A comparison of the forces in the distal screws for all cases shows that the highest tensile force was in the four-hole plate, followed by the three-hole plate with two screws. The lowest was the three-hole plate with three screws, which was in compression at full load. The maximum tensile stresses in the bone at the end of the plate were greatest for the standard four-hole hip plate, followed by the three-hole plate with two screws and then the three-hole plate with three screws. This indicates that the risk of bone fracture at the end of the plate is lowest for the three-hole hip plate with three screws. The risk of bone fracture is significantly lower for the three-hole hip plate, with either two or three screws, compared to the ‘gold-standard’ four-hole hip plate. This is partially offset by a small increase in the risk of screw pull out (in the proximal rather than the distal screw)

Abstract

Abstract

Aim

The aim of this FE study was to analyse the comparative behaviour of cement and metal based augments in TKR and quantify the stresses within these different augments and underlying cancellous bone.

Introduction: Periprosthetic bone resorption following total knee arthroplasty (TKA) is becoming a clinical concern. Decrease in bone quality jeapordises implant fixation, consequently leading to revision surgery. It has been suggested that a reduction in the local stress distribution may cause a decrease in bone mineral density (BMD). Computational bone remodelling has been used previously to predict bone adaptation in total hips. However, little has been reported on its use in TKA remodelling simulations. The aim of this study was to simulate the bone remodelling response of the femur and tibia following TKA, using an adaptive bone remodelling algorithm combined with the finite element (FE) method.

Methods: 3D femur and tibia models were constructed from human cadaveric computed tomography images. Total knee implant geometries were used to reconstruct the knee joint.(RBK, Global Orthopaedic Technology, Australia). Both the femur and the tibia models were loaded at 45% gait cycle for normal walking gait using loads based on Taylor et al. A strain-adaptive remodelling algorithm was used to predict the remodelling behaviour of the femur and tibia following TKA. Analysis was performed using ABAQUS. Virtual DEXA images were generated from the FE models at predetermined time-points, BMD gain and loss were also assessed both quantitatively and qualitatively.

Results: There was an increase and decrease in BMD for the femur and tibia models. BMD loss in the femur was predominantly experienced around the pegs and the distal femoral regions. Femoral BMD gain was displayed around the edges of the bone-implant interface, with higher activity at the anterior-medial and posterior-lateral aspects. BMD gain in the tibia was predominantly at the inferior end of the tibial tray’s keel, with the bone mass tending towards the medial aspect. Some bone gain was displayed on the medial side, surrounding the pegs and at the cortex. There was BMD loss on the lateral aspect of the tibia.

Discussion: The adaptive bone remodelling algorithm has shown a good correlation with clinical findings. Reports of clinical and FE studies have shown that for cemented knees, most bone loss occurs at the distal femoral region, especially at the anterior aspect. It has been reported that in the tibia there is generally an over-all decrease in BMD in the proximal tibia and increase below the keel. This is in accordance with our predictions. BMD gain was found to be more predominant on the medial aspect. This may be due to the more medially inclined loading ratio, which affects the stress distribution within the bone. BMD gain in the tibia is shown to follow a path, which starts at the bottom of the keel and tends medially towards the tibial cortex. This illustrates the inherent tendency of load transfer to follow along the stiffest structural path.

Aims: Several designs are available to the surgeon for internal fixation with plates. This study compares the mechanical behaviour of three different designs for screw-plate locking: threaded screw head (Type1), conical joint with titanium insert (Type2) and spherical 3-point wedge-locking (Type3).

Methods: 3D-CAD and FEM models were used to simulate the behaviour of each device component. Two typical loading conditions were considered in order to characterize joint mechanics: screw tension and bending.

Results: The screws performances were compared in terms of stress concentration factors (SCF) with reference to the screw nominal diameter. In particular, Type3 device exhibits a strong influence of screw tightening on SCF. Results can be summarized as follows:

Tension: SCF_T-Type1=4.8; SCF_T-Type2=3.2; SCF_ T-Type3=11.7;

Conclusions: With respect to classic Type1 screw, Type2 device provides a better load distribution while Type3 system offers pivoting capabilities at the expenses of higher stresses acting on the joint.

Background

Use of a baseplate with a smaller diameter in reverse shoulder arthroplasty has been recommended, especially in patients with a small glenoid or insufficient bony stock due to severe glenoid wear. However, effect of a smaller baseplate on stability of the glenoid component has not been evaluated. The purpose of this study was to determine whether a smaller baseplate (25 mm) is beneficial to the initial primary stability of the glenoid component compared to that with a baseplate of a commonly used size (29 mm) by finite element analysis.

Introduction

The number of total knee joint replacements has increased dramatically, from 28,000 in 2004 to over 73,000 in 2008 in the UK. This increase in procedures means that there is a need to assess the performance of an implant design in the general population. For younger, more active patients, cementless tibial fixation is an attractive alternative means of fixation and has been used for over 30 years. However, the clinical results with cementless fixation have been variable, with reports of extensive radiolucent lines, rapid early migration and aseptic loosening [1]. This study investigates the inter-patient variability of bone strain at the implant-bone interface of 130 implanted tibias over a full gait cycle.

Statement of purpose: The aim of this study is to evaluate different designs of unicompartmental knee replacement (UKR) by comparing the peak von Mises and contact stresses in polyethylene (PE) bearings over a step-up activity.

Summary of Methods: A validated finite element (FE) model was used in this study. Three UKR designs were modelled: a spherical femoral component with a spherical PE bearing (fully-congruent), a poly-radial femoral component with a concave PE bearing (semi-congruent), and a spherical femoral component with a flat bearing (non-congruent).

Kinematic data from in-vivo fluoroscopy measurements during a step-up activity was used to determine the relative tibial-femoral position as a function of knee flexion angle for each model. Medial and lateral force distribution was adapted from loads measured in-vivo with an instrumented implant during a step-up activity. The affect that varying the bearing thickness has on the stresses in the bearing was investigated. In addition, varus-valgus mal-alignment was investigated by rotating the femoral component through 10 degrees.

Summary of Results: Only the fully congruent bearing experienced peak von Mises and contact stresses below the PE lower fatigue limit (17MPa) for the step-up activity (fully congruent PE peak contact stress, 5MPa). The highest PE contact stresses were observed for the semi-congruent and non-congruent designs, which experienced approximately 3 times the PE lower fatigue limit. Peak PE von Mises stresses for the semi-congruent and non-congruent designs were similar, peaking at approximately 25MPa. Peak PE von Mises stresses were ameliorated with increased bearing thickness. Varus-valgus mal-alignment had little effect on the peak stresses in the three UKR designs.

Statement of Conclusions: Fully congruent articulating surfaces significantly reduce the peak contact stresses and von Mises stresses in the bearing. The FE model demonstrates that fully congruent bearings as thin as 2.5mm can be used without increasing the contact stresses significantly. Fully congruent designs can use thinner bearings and enable greater bone preservation.

Introduction: Cemented hip resurfacing component orientation may, in part, be associated with femoral neck fracture. Orientation offset may be introduced due to the cement setting prior to achieving a completely seated component. Varus/valgus orientation error may occur due to surgical error or poor instrumentation design. We modeled a number of different orientations and investigated bone mineral density change using the finite element method.

Methods: CT scans were used to reconstruct the femoral geometry and create a finite element model. The boundary conditions applied were hip muscle forces at the 45% position of the gait cycle. Two models were created, a preoperative (reference) and a postoperative (reconstructed) model. The post operative model was reconstructed using the Birmingham Hip Replacement (BHR). Implant offsets and varus/valgus orientations were analysed. The bone mineral density (BMD) changes at nine positions along the superior and inferior aspects of the alignment stem were analyzed.

Results: Results suggest bone loss decreases with increasing offset distances. Femoral offset distance is defined as the perpendicular distance from the center line of the femoral shaft to the center of the femoral head. Greater femoral stem offsets increases the abductor moment arm and this decreases the abductor force need for walking as well as the overall articulating reactive force at the articulating surface. As the BHR orientation deviates away from the an extreme valgus to a more varus position, the volume of bone that will decrease in BMD increases.

Discussion: There is minimal difference between the 1mm and 3mm offsets and their respective bone remodeling volumes. The 5mm offset has a larger bone volume where the BMD will increase; this is due to the larger moment applied to the proximal femur and is not an advisable surgical position as there may be a large density gradient at the mouth of the resurfacing component and could predispose the femoral neck to fracture. There is also not a lot of difference in bone remodeling volume between the extreme valgus, 5° and 10° cases. However, the extreme valgus case does present a “notching” risk. The objective of this study was to implement a consistent theoretical adaptive bone remodelling rule that may, in part, give an understanding as to how a femoral resurfacing component’s orientation would influence and simulate BMD changes in the proximal femur.

Hip resurfacing arthroplasty (HRA) is increasingly carried out as an alternative to total hip arthroplasty (THA) in young patients. During the procedure, a metal stem on the retrosurface of the HRA is inserted into the femoral head to ensure the implant is located centrally with respect to the femoral neck. It has been suggested that the stem may interfere with bone loading. In light of this, the current study employed finite element (FE) models to investigate the change in the HRA-implanted bone mechanics as a result of removing the stem. FE models of a cadaveric femur pre- and post-HRA surgery were analysed to determine changes in bone stress/ strain. The implanted models simulated geometry for a cemented HRA with and without a non-cemented stem (HRA-Stem and HRA-NoStem, respectively) and included more accurate multiple material parameters to simulate the non-homogeneous material distribution in the femoral bone. The models included loading conditions simulating an instant at 10% of the gait cycle. Bone stresses/strains in the femoral head and neck of the implanted models were compared with the intact condition to assess the change in bone mechanics. Changes in cement mantle stresses between the HRA-Stem and HRA-NoStem models were also compared. When comparing similar volumes of bone in the femoral neck, both HRA models showed a similar variation in stress from the intact condition and bone stresses were low in comparison to the ultimate strength of cortical bone. There was less change in peak strain energy in the femoral head of the HRA-NoStem model than the HRA-Stem model. Cement mantle stresses in the HRA-NoStem model were slightly higher than for the HRA-Stem model and the peak compressive stress was close to the fatigue limit for bone cement. These preliminary results suggest that the bone loading is more normal without the stem. However, there are increased cement mantle stresses

Effectiveness and long term stability of hip resurfacing and total hip arthroplasty for osteoarthritis patients are still debated nowadays. Several clinical and biomechanical issues have to be considered, including pain relief, return to function, femoral neck fractures, impingement and prosthesis loosening. Normally, patients with hip arthroplasties are facing gait adaptation and at risk of fall. Sudden impact loading and twisting during sideway falls may lead to femoral fractures and joint failures. The purposes of this study are (i) to investigate the stress behavior of hip resurfacing and total hip arthroplasty, and (ii) to predict pattern of femoral fractures during sideway falls and twisting configurations.

Computed tomography (CT) based images of a 54-year old male were used in developing a 3D femoral model. The femur model was designed to be inhomogeneous material as defined by Hounsfield Unit of the CT images. CAD data of hip arthroplasties were imported and aligned to represent RHA and THA femur modelas shown in Fig.1. Prosthesis stem is modeled as Ti-6Al-4V material while femoral ball as Alumina properties. Meanwhile, RHA implant is assigned as Co-Cr-Mo material. Four types of loading and boundary conditions were assigned to demonstrate different falling (FC) and twisting (TC) configurations (see Fig.2). Finite element analysis combined with a damage mechanics model was then performed to predict bone fractures in both arthroplasty models. Different loading magnitudes up to 4BW were applied to extrapolate the fracture patterns.

Prediction of femoral fracture for RHA and THA femurs are discussed in corresponding to maximum principal stress and damage formation criterion. The load bearing strain was set to 3000micron, the physiological bone loading that leads to bone formation. The test strength was wet to 80% of the yield strength determined from the CT images. Different locations of fracture are predicted in each configuration due to different loading direction and boundary conditions as shown in Fig.3. For falling configurations, fractures were projected at trochanteric region for intact and RHA femur, while THA femurs experience fracture at inner proximal region of bone. Differs to twisting configurations, both arthroplasties were predicted to fracture at the distal end of femurs.

Osteosynthesis of high-energy metaphyseal proximal tibia fractures is still challenging, especially in patients with severe soft tissue injuries and/or short stature. Although the use of external fixators is the traditional treatment of choice for open comminuted fractures, patients' acceptance is low due to the high profile and therefore the physical burden of the devices. Recently, clinical case reports have shown that supercutaneous locked plating used as definite external fixation could be an efficient alternative. Therefore, the aim of this study was to evaluate the effect of implant configuration on stability and interfragmentary motions of unstable proximal tibia fractures fixed by means of externalized locked plating. Based on a right tibia CT scan of a 48 years-old male donor, a finite element model of an unstable proximal tibia fracture was developed to compare the stability of one internal and two different externalized plate fixations. A 2-cm osteotomy gap, located 5 cm distally to the articular surface and replicating an AO/OTA 41-C2.2 fracture, was virtually fixed with a medial stainless steel LISS-DF plate. Three implant configurations (IC) with different plate elevations were modelled and virtually tested biomechanically: IC-1 with 2-mm elevation (internal locked plate fixation), IC-2 with 22-mm elevation (externalized locked plate fixation with thin soft tissue simulation) and IC-3 with 32-mm elevation (externalized locked plate fixation with thick soft tissue simulation). Axial loads of 25 kg (partial weightbearing) and 80 kg (full weightbearing) were applied to the proximal tibia end and distributed at a ratio of 80%/20% on the medial/lateral condyles. A hinge joint was simulated at the distal end of the tibia. Parameters of interest were construct stiffness, as well as interfragmentary motion and longitudinal strain at the most lateral aspect of the fracture. Construct stiffness was 655 N/mm (IC-1), 197 N/mm (IC-2) and 128 N/mm (IC-3). Interfragmentary motions under partial weightbearing were 0.31 mm (IC-1), 1.09 mm (IC-2) and 1.74 mm (IC-3), whereas under full weightbearing they were 0.97 mm (IC-1), 3.50 mm (IC-2) and 5.56 mm (IC-3). The corresponding longitudinal strains at the fracture site under partial weightbearing were 1.55% (IC-1), 5.45% (IC-2) and 8.70% (IC-3).

From virtual biomechanics point of view, externalized locked plating of unstable proximal tibia fractures with simulated thin and thick soft tissue environment seems to ensure favorable conditions for callus formation with longitudinal strains at the fracture site not exceeding 10%, thus providing appropriate relative stability for secondary bone healing under partial weightbearing during the early postoperative phase.

Introduction: Pre-operative coronal curve flexibility assessment is of key importance in the surgical planning process for scoliosis correction. The fulcrum bending radiograph is one flexibility assessment technique which has been shown to be highly predictive of potential curve correction using posterior surgery, however little is known about the extent to which soft tissue structures govern spinal flexibility. The aim of this study was to explore how the mechanical properties of spinal ligaments and intervertebral discs affect coronal curve flexibility in the fulcrum bending test. To this end a biomechanical analysis of a scoliotic thoracolumbar spine and ribcage was carried out using a three dimensional finite element model.

Methods: CT-derived spinal anatomy for a 14 year old female adolescent idiopathic scoliosis patient was used to develop the 3D finite element model. Physiological loading conditions representing the gravitational body weight forces acting on the spine when the patient lies on their side over the fulcrum bolster were simulated. Initial mechanical properties for the spinal soft tissues were derived from existing literature. In six separate analyses, the disc collagen fibre and ligament stiffness values were reduced by 10%, 25% and 40% respectively, and the effects of reduced tissue stiffness on fulcrum flexibility were assessed by comparison with the initial model. Finally, the effect of discectomy on fulcrum flexibility was simulated for thoracic levels T5 to T12.

Results: Reducing disc collagen fibre stiffness resulted in a greater change in segmental rotations in the fulcrum bending test than reducing ligament stiffness. However, reductions of up to 40% in disc collagen fibre stiffness and ligament stiffness produced no clinically measurable increase in fulcrum flexibility (increase of 1.2%). By contrast, following removal of the discs, the simulated fulcrum flexibility increased by more than 80% compared to the initial case.

Discussion: Disc collagen fibre and ligament stiffness both have minimal influence on scoliotic curve flexibility. However, discectomy simulation shows that the intervertebral discs are of critical importance in determining spinal flexibility.

Summary Statement

Simulated increases in body weight led to increased displacement, von Mises stress, and contact pressure in finite element models of the extended and flexed knee. Contact shifted to locations of typical medial osteoarthritis lesions in the extended knee models.

Introduction: High ion release along with bone resorption at the bone/implant interface is still a problem, leading to pain, poor function and the possibility of bone fracture. Treatment of a loose implant is not easy and can lead to less than satisfactory revision surgery. The reason for ion release, loosening or periprosthetic fracture of an implant is multifactorial. One factor for ion release that has been reported is inclination angle. Another can be the version angle of the implant and subjecting it to an abnormal loading environment. Few studies have been reported in the literature on hip resurfacing performance based on implant orientation. More studies are required into investigating the use of this predictive technique in orthopaedics to investigate the bearing behaviour and potential ion release due to implant surgical positioning. In this study we modeled a number of different version angles and investigated the contact area, stress and wear characteristics using the finite element method.

Methods: CT scans were used to reconstruct the part of the femur and pelvic geometry. A 3D finite element mesh was created using PATRAN (MSC Software, Santa Ana, CA). The femur loading was taken at peak load position of the gait cycle. The loading was applied to the femur and pelvis was fixed. Material properties were applied using the Hounsfield units from the CT file. Two models were generated, a preoperative and a postoperative state model. The post operative model was reconstructed using the Birmingham Hip Replacement (BHR) system (Smith & Nephew Inc, Memphis, TN). The BHR acetabular cup was oriented at different anteversion angles (5°, 30° & 45° to the saggital plane) to investigate the contact mechanics between the head and cup. Serum ion levels were taken from 12 patients and the change in ion levels over the first 12 month period were analysed statistical to investigate the correlation with anteversion angle. Radiographs from the same patients were analysed to determine the cup anteversion angle using image analysis and edge matching techniques.

Results: The contact areas increased with increasing anteversion angle, 137.3, 165.3 and 169.9mm2 respectively. As a consequence, the contact pressure decreased. The change in ion levels for the patients over the first 12 month period correlated significantly (p< .05) with the anteversion angle using Pearson’s r test.

Discussion: Statistical analysis showed a good Pearson’s correlation of anteversion angle to a change in serum ion levels, 0.867 and 0.734 with p values of 0.001and 0.012 respectively. Acetabular version angle appears to be, at the least, important in determining serum metal ion levels and in evaluating causes of metallosis, the influence of anteversion angle needs to be considered when using metal on metal bearing technology when placing the cup in the acetabulum.

Introduction

Lateralizing the center of rotation (COR) of reverse total shoulder arthroplasty (rTSA) has the potential to increase functional outcomes of the procedure, namely adduction range of motion (ROM). However, increased torque at the bone-implant interface as a result of lateralization may provoke early implant loosening, especially in situations where two, rather than four, fixation screws are used. The aim of this study was to utilize finite element (FE) models to investigate the effects of lateralization and the number of fixation screws on micromotion and adduction ROM.

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.