In total hip arthroplasty, press-fit anchorage is one of the most common fixation methods for acetabular cups and mostly ensures sufficient primary stability. Nevertheless, implants may fail due to aseptic loosening over time, especially when the surrounding bone is affected by stress-shielding. The use of acetabular cups made of isoelastic materials might help to avoid stress-shielding and osteolysis. The aim of the present numerical study was to determine whether a modular acetabular cup with a shell made of polyetheretherketone (PEEK) may be an alternative to conventional titanium shells (Ti6Al4V). For this purpose, a 3D finite element analysis was performed, in which the implantation of modular acetabular cups into an artificial bone stock using shells made of either PEEK or Ti6Al4V, was simulated with respect to stresses and deformations within the implants. The implantation of a modular cup, consisting of a shell made of PEEK or Ti6Al4V and an insert made of either ceramic or polyethylene (PE), into a bone cavity made of polyurethane foam (20 pcf), was analysed by 3D finite element simulation. A two-point clamping cavity was chosen to represent a worst-case situation in terms of shell deformation. Five materials were considered; with Ti6Al4V and ceramic being defined as linear elastic and PE and PEEK as plastic materials. The artificial bone stock was simulated as a crushable foam. Contacts were generated between the cavity and shell (μ = 0.5) and between the shell and insert (μ = 0.16). In total, the FE models consisted of 45,282 linear hexahedron elements and the implantation process was simulated in four steps: 1. Displacement driven insertion of the cup; 2. Relief of the cup; 3. Displacement driven placement of the insert; 4. Load driven insertion of the insert (maximum push-in force of 500 N). The FE model was evaluated with respect to the radial deformations of the shell and insert as well as the principal stresses in case of the ceramic inserts. The model was experimentally validated via comparison of nominal strains of the titanium shells.Introduction
Methods
Modern acetabular cups require a convenient bone stock for sufficient cup fixation. Thereby, fixation stability is influenced by the chosen interference fit of the acetabular cup, the cup surface structure, circularity of the reamed acetabulum and by the acetabular bone quality. The ideal implantation situation of the cup is commonly compromised by joint dysplasia and acetabular bone defects. The aim of the present experimental study was to characterise implant fixation of primary acetabular cups in case of definite acetabular cavity defects. For the experimental determination bone substitute blocks (100 × 100 × 50 mm) made of polymethacrylimide (PMI) foam with a density of 7 pcf were used. The created acetabular defect situations were derived from the defect classification according to Paprosky. The defect geometries in the PMI foam blocks were realised by a CNC drilling machine. Thereby the defects are described in the dorso-ventral direction by the angle α and in medio-lateral direction by the angle β (given as angle combination α/β) related to the centre of rotation of the reamed cavity. For the lever-out tests the defect types IIb and IIIa (each with different α and β angles) were considered and compared to the intact fixation situation. Therefore, a macrostructured titanium cup (Allofit, Zimmer GmbH, Wintherthur, Switzerland) with an outer diameter of 56 mm were displacement-controlled (v = 20 mm/min) pushed into the 2 mm diametric under reamed PMI-foam cavities. Three cups were inserted until the cup overhang pursuant to surgical technique was reached. Subsequently the cups were displacement-controlled (v = 20 mm/min) levered out via a rod which was screwed into the implant pole by perpendicular displacement (Uaxial) of the rod in direction of the defect aperture. The lever-out moments were calculated by multiplying the first occurring force maximum (Fmax) with the effective lever arm length (llever), whereby moments caused by the deadweight of the rod were considered. Primary stability was defined by the first maximum lever-out moment.Introduction
Materials and Methods
Despite decades of clinical research in artificial joints and underlying failure mechanisms, systematical and reproducible identification of reasons for complications in total knee replacements (TKR) remains difficult. Due to the complex dynamic interaction of implant system and biological situs, malfunction eventually leading to failure is multifactorial and remains not fully understood. The aim of present study was to evaluate different TKR designs and positions with regard to joint kinematics and stability under dynamic conditions by using a robot-based hardware-in-the-loop (HiL) setup. An industrial 6-axis robot with 6-axis force-torque sensor mounted into its end-effector moved and loaded real, commercially available TKR (bicondylar, cruciate-retaining) that were in virtual interaction with a subject-specific computational multibody model representing the anatomical situs of the knee joint while performing passive seated deep knee flexion. The subject-specific musculoskeletal multibody model (MMB) included rigid bones of the lower right extremity. Bone and cartilage geometries were reconstructed from MRT/ CT data sets preserving anatomical landmarks and allowing for the calculation of inertial properties. M. quadriceps femoris was modeled as single passive tensile force elements. Knee ligaments were modelled as elastic spring elements with a nonlinear force-displacement characteristic. Providing the flexion angle, the robot moved and loaded the mounted femoral implant component with respect to the tibial component while being in continuous interaction with the MMB. Several influencing parameters like implant position (internal/external rotation, varus/valgus alignment) and design (fixed vs. mobile bearing, tibia-insert height) as well as ligament insufficiency and joint loading on joint kinematics and stability was systematically analysed.Introduction
Material & methods
The purpose of this study was to experimentally evaluate impingement and dislocation of total hip replacements while performing dynamic movements under physiological-like conditions. Therefore, a hardware-in-the-loop setup has been developed, in which a physical hip prosthesis actuated by an industrial robot interacts with an in situ-like environment mimicked by a musculoskeletal multibody simulation-model of the lower extremity. The multibody model of the musculoskeletal system comprised rigid bone segments of the lower right extremity, which were mutually linked by ideal joints, and a trunk. All bone geometries were reconstructed from a computed tomography set preserving anatomical landmarks. Inertia properties were identified based on anthropometric data and by correlating bone density to Hounsfield units. Relevant muscles were modeled as Hill-type elements, passive forces due to capsular tissue have been neglected. Motion data were captured from a healthy subject performing dislocation-associated movements and were fed to the musculoskeletal multibody model. Subsequently, the robot moved and loaded a commercially available total hip prosthesis and closed the loop by feeding the physical contact information back to the simulation model. In this manner, a comprehensive parameter study analyzing the impact of implant position and design, joint loading, soft tissue damage and bone resection was implemented.Introduction
Methods
The influence of the bone mineral density (BMD) on the mechanical behavior of bones can be examined using computer tomography (CT) data and finite element (FE) simulations, because the BMD correlates with the Hounsfield scale (HU) of the CT data. Therefor the material mapping strategy, which is required to assign the HU values to the FE mesh, is of crucial importance. In this study a nodal mapping strategy was analyzed concerning its sensitivity towards FE mesh parameters and an averaging of HU values from the area around the respective nodes. The FE simulation is based on CT data of a human proximal femur. Once the bone shape was reconstructed, the resulting model was meshed with quadratic tetrahedral elements in ABAQUS/CAE and all nodes were assigned an HU value from the CT data by using the respective node coordinates. In this process, the mesh density, the threshold, which could be used to exclude connective tissue and fat from the material mapping process, the considered volume around the nodes and the method of averaging were varied. The material assignment was realized by an HU value dependent, linear elastic material definition. The femur model was clamped at the level of the isthmus and a displacement of 0.5 mm was applied at the femoral head. The evaluation was based on the resulting reaction forces.Introduction
Method
Current implant designs and materials provide a high grade of quality and safety, but aseptic implant loosening is still the main reason for total hip revision. Highly cross-linked polyethylene (HX-PE) is used successfully in total hip replacements (THR) since several years. The good wear properties lead to a reduction of wear debris and may contribute to a longer survival time of the THRs. Furthermore, thin HX-PE liner allows the use of larger femoral heads associated with a decreased risk of dislocation and an improved range of motion. However, the cross-linking process is associated with a loss of mechanical properties of the polyethylene material which compromise the use of thin HX-PE liner in terms of high stress situations. The aim of the present study was the experimental wear analysis of HX-PE liner under steep acetabular cup position. Furthermore, a finite element analysis (FEA) was performed in order to calculate the stress within the HX-PE material in case of steep cup position under physiological loading. Experimental wear testing was performed for 5 Mio load cycles, using highly cross-linked polyethylene (HX-PE) acetabular liner combined with 44 mm ceramic femoral heads at a standard position of the acetabular cup (30° inclination) according to ISO 14242 as well as at 60° cup inclination. The wall thickness of the HX-PE liner was 3.8 mm. A hip wear simulator, according to ISO 14242 (EndoLab GmbH, Rosenheim, Germany), was used and wear was determined gravimetrically. Moreover, finite element models of the THR system at standard and steep cup position was created by Abaqus/CAE (Dessault Systemes Providence, USA). Using the finite element software Abaqus (Dessault Systemes Providence, USA) the total hip implants were physiologically loaded with maximum force of the gait cycle (3.0 kN). Thereby, the stresses within the HX-PE material were analysed. The average gravimetrical wear rates of the HX-PE liners at standard implant position (30°) and 60° cup inclination showed small wear amounts of 3.15 ± 0.32 mg and 1.92 ± 1.00 mg per million cycles, respectively. The FEA revealed a clear increase of stresses at the HX-PE liner with respect to steep cup position (von Mises stress of 8.78 MPa) compared to ISO standard implant position (von Mises stress of 5.70 MPa). The wear simulator tests could not demonstrate significant differences of gravimetrical wear amount of HX-PE liners under steep hip cup position compared to standard implant position. The small contact surface between the femoral head and the SX-PE liner during the wear testing may lead to the low wear rate of the misaligned acetabluar cup. Moreover, the FEA showed that the effect of a misaligned acetabular cup on the stresses within the polyethylene liner can be critical. Although an increase of wear could not be detected a steeper acetabular cup position using thin HX-PE liners should be avoided due to higher stresses preventing implant failure in clinical application.
Clinically applied methods of assessing implant fixation and implant loosening are of sub-optimal precision, leading to the risk of unsecure indication of revision surgery and late recognition of bone defects. Loosening diagnosis involving measuring the eigenfrequencies of implants has its roots in the field of dentistry. The changing of the eigenfrequencies of the implant-bone-system due to the loosening state can be measured as vibrations or structure-borne sound. In research, vibrometry was studied using an external shaker to excite the femur-stem-system of total hip replacements and to measure the resulting frequencies by integrated accelerometers or by ultrasound. Since proper excitation of implant components seems a major challenge in vibrometry, we developed a non-invasive method of internal excitation creating an acoustic source directly inside the implant. In the concept proposed for clinical use, an oscillator is integrated in the implant, e.g. the femoral stem of a total hip replacement. The oscillator consists of a magnetic or magnetisable spherical body which is fixed on a flat steel spring and is excited electromagnetically by a coil placed outside the patient. The oscillator impinges inside the implant and excites this to vibrate in its eigenfrequency. The excitation within the bending modes of the implant leads to a sound emission to the surrounding bone and soft tissue. The sound waves are detected by an acoustic sensor which is applied on the patient's skin. Differences in the signal generated result from varying level of implant fixation. The sensor principle was tested in porcine foreleg specimens with a custom-made implant. Influence of the measurement location at the porcine skin and different levels of fixation were investigated (press-fit, slight loosening, advanced loosening) and compared to the pull-out strength of the implant. Evaluation of different parameters, especially the frequency spectrum resulted in differences of up to 12% for the comparison between press-fit and slight loosening, and 30% between press-fit and advanced loosening. A significant correlation between the measured frequency and the pull-out strength for different levels of fixation was found. Based on these findings, an animal study with sensor-equipped bone implants was initiated using a rabbit model. The implants comprised an octagonal cross-section and were implanted into a circular drill hole at the distal femur. Thereby, definite gaps were realized between bone and implant initially. After implantation, the bone growth around the implant started and the gaps were successively closed over postoperative period. Consequently, since the tests had been started with a loose implant followed by its bony integration, a reverse loosening situation was simulated. In weekly measurements of the eigenfrequencies using the excitation and sensor system, the acoustic signals were followed up. Finally, after periods of 4 and 12 weeks after implantation, the animals were sacrificed and pull-out tests of the implants were performed to measure the implant fixation. The measured implant fixation strengths at the endpoint of each animal trial were correlated with the acoustic signals recorded.
Dislocation of total hip replacements (THRs) remains a severe complication after total hip arthroplasty. However, the contribution of influencing factors, such as implant positioning and soft tissue tension, is still not well understood due to the multi-factorial nature of the dislocation process. In order to systematically evaluate influencing factors on THR stability, our novel approach is to extract the anatomical environment of the implant into a musculoskeletal model. Within a hardware-in-the-loop (HiL) simulation the model provides hip joint angles and forces for a physical setup consisting of a compliant support and a robot which accordingly moves and loads the real implant components [2]. The purpose of this work was to validate the HiL test system against experimental data derived from one patient. The musculoskeletal model includes all segments of the right leg with a simplified trunk. Bone segments were reconstructed from a human computed tomography dataset. The segments were mutually linked in the multibody software SIMPACK (v8.9, Simpack AG, Gilching, Germany) by ideal joints starting from the ground-fixed foot. Furthermore, inertia properties were incorporated based on anthropometric data. Inverse dynamics was used to obtain muscle forces. Thus, optimization techniques were implemented to resolve the distribution problem of muscle forces whereas muscles were assumed to act along straight lines. For validation purposes the model was scaled to one patient with an instrumented THR [1]. Averaged kinematic measurements were used to obtain joint angles for a knee-bending motion. Then, the model was exported into real-time capable machine code and embedded into the HiL environment. Real implant components of a standard THR were attached to the endeffector of the robot and the compliant support. Finally, the HiL simulation was carried out simulating knee-bending. Experimentally measured hip joint forces from the patient [1] were used to validate the HiL simulation.Introduction
Methods
Due to the commercial launch of newly developed ceramic-on-metal (COM) bearings, we compared the deformation and stresses in the liner with ceramic-on-ceramic (COC), metal-on-metal (MOM) as well as ceramic-on-polyethylene (COP) bearings using a finite-element (FE)-model, analyzing a variety of head size and implant position. Liner deformation in terms of change in inner diameter as well as peak stresses were evaluated. The FE-model consisting of a commercial THR, the proximal femur and a section of the hemipelvis was created based on our previously published approach. Static load and muscle forces were applied according to the maximum load during gait. Polyethylene was modelled using a nonlinear definition with isotropic hardening, cobalt-chromium was modelled elastic-plastic and ceramic was modelled linear-elastic. Validity of the model was checked using an experimental setup with artificial bone and strain gauges located at the rim of the liner. Implant material (COM vs. COC vs. MOM vs. COP), head size (28 mm vs. 36 mm) and cup position (45° inclination/15° anteversion vs. 60° incl./0° antev.) were varied.Introduction
Methods
