Several electrical fields are known to be present in bone tissue as originally described by Fukada and Yasuda in the year 1957. Intrinsic voltages can derive from bone deformation and reversely lead to mechanical modifications, called the piezoelectric effect. This effect is used in the clinic for the treatment of bone defects by applying electric and magnetic stimulation directly to the bone supplied with an implant such as the electroinductive screw system. Through this system a sinusoidal alternating voltage with a maximum of 700 mV can be applied which leads to an electric field of 5–70 V/m in the surrounding bone. This approach is established for bone healing therapies. Despite the established clinical application of electrical stimulation in bone, the fundamental processes acting during this stimulation are still poorly understood. A better understanding of the influence of electric fields on cells involved in bone formation is important to improve therapy and clinical success. To study the impact of electrical fields on bone cells in vitro, Ti6Al4V electrodes were designed according to the pattern of the ASNIS III s screw for a 6-well system. Osteoblasts were seeded on collagen coated coverslip and placed centred on the bottom of each well. During four weeks the cells were stimulated 3×45 min/d and metabolic and alkaline phosphatase (ALP) activity as well as gene expression of cells were analysed. Furthermore, supernatants were collected and proteins typical for bone remodelling were examined. The electrical stimulation did not exert a significant influence on the metabolic activity and the ALP production in cells over time using these settings. Gene expression of BSP and ALP was upregulated after the first 3 days whereas OPG was increased in the second half after 14 days of electrical stimulation. Moreover, the concentration of the released proteins OPG, IL-6, DKK-1 and OPN increased when cells were cultivated under electrical stimulation. However, no changes could be seen for essential markers, like RANKL, Leptin, BMP-2, IL-1beta and TNF-alpha. Therefore, further studies will be done with osteoblasts and osteoclasts to study bone remodelling processes under the influence of electrical fields more in detail. This study was supported by the German Research Foundation (DFG) JO 1483/1-1.
Persistent patellofemoral (PF) pain is a common postoperative complication after total knee arthroplasty (TKA). In the USA, patella resurfacing is conducted in more than 80% of primary TKAs [1], and is, therefore, an important factor during surgery. Studies have revealed that the position of the patellar component is still controversially discussed [2–4]. However, only a limited number of studies address the biomechanical impact of patellar component malalignment on PF dynamics [2]. Hence, the purpose of our present study was to analyze the effect of patellar component positioning on PF dynamics by means of musculoskeletal multibody simulation in which a detailed knee joint model resembled the loading of an unconstrained cruciate-retaining (CR) total knee replacement (TKR) with dome patella button. Our musculoskeletal multibody model simulation of a dynamic squat motion bases on the SimTK data set (male, 88 years, 66.7 kg) [5] and was implemented in the multibody dynamics software SIMPACK (V9.7, Dassault Systèmes Deutschland GmbH, Gilching, Germany). The model served as a reference for our parameter analyses on the impact on the patellar surfacing, as it resembles an unconstrained CR-TKR (P.F.C. Sigma, DePuy Synthes, Warsaw, IN) while offering the opportunity for experimental validation on the basis of instrumented implant components [5]. Relevant ligaments and muscle structures were considered within the model. Muscle forces were calculated using a variant of the computed muscle control algorithm. PF and tibiofemoral (TF) joints were modeled with six degrees of freedom by implementing a polygon-contact model, enabling roll-glide kinematics. Relative to the reference model, we analyzed six patellar component alignments: superior-inferior position, mediolateral position, patella spin, patella tilt, flexion-extension and thickness. The effect of each configuration was evaluated by taking the root-mean-square error (RMSE) of the PF contact force, patellar shift and patellar tilt with respect to the reference model along knee flexion angle.Introduction
Material and Methods
It is well-known that wear debris generated by metal-on-metal hip replacements leads to aseptic loosening. This process starts in the local tissue where an inflammatory reaction is induced, followed by an periprosthetic osteolysis. MOM bearings generate particles as well as ions. The influence of both in human bodies is still the subject of debate. For instance hypersensitivity and high blood metal ion levels are under discussion for systemic reactions or pseudotumors around the hip replacement as a local reaction. The exact biopathologic mechanism is still unknown. The aim of this study was to investigate the impact of local injected metal ions and metal particles. We used an established murine inflammation model with Balb/c mice and generated three groups. Group PBS (control group, n=10) got an injection of 50µl 0.1 vol% PBS-suspension, Group MI (Metal-ion, n=10) got an injection of 50µl metal ion suspension at a concentration of 200µg/l and Group MP (Metal-particles, n=10) got an injection of 50µl 0.1 vol% metal particle suspension each in the left knee. After incubation for 7 days the mice were euthanized and the extraction of the left knee ensued. Followed by immunhistochemical treatment with markers of inflammation that implied TNFα, IL-6, IL-1β, CD 45, CD 68, CD 3, we counted the positive cells in the synovial layer in the left knees by light microscopy, subdivided into visual fields 200× magnified. The statistical analysis was done with Kruskal-Wallis test and a post hoc Bonferroni correction.Introduction
Material and Methods
Total knee replacement (TKR) is an established and effective surgical procedure in case of advanced osteoarthritis. However, the rate of satisfied patients amounts only to about 75 %. One common cause for unsatisfied patients is the anterior knee pain, which is partially caused by an increase in patellofemoral contact force and abnormal patellar kinematics. Since the malpositioning of the tibial and the femoral component affects the interplay in the patellofemoral joint and therefore contributes to anterior knee pain, we conducted a computational study on a cruciate-retaining (CR) TKR and analysed the effect of isolated femoral and tibial component malalignments on patellofemoral dynamics during a squat motion. To analyse different implant configurations, a musculoskeletal multibody model was implemented in the software Simpack V9.7 (Simpack AG, Gilching, Germany) from the SimTK data set (Fregly et al.). The musculoskeletal model comprised relevant ligaments with nonlinear force-strain relation according to Wismans and Hill-type muscles spanning the lower extremity. The experimental data were obtained from one male subject, who received an instrumented CR TKR. Muscle forces were calculated using a variant of the computed muscle control algorithm. To enable roll-glide kinematics, both tibio- and patellofemoral joint compartments were modelled with six degrees of freedom by implementing a polygon-contact-model representing the detailed implant surfaces. Tibiofemoral contact forces were predicted and validated using data from experimental squat trials (SimTK). The validated simulation model has been used as reference configuration corresponding to the optimal surgical technique. In the following, implant configurations, i.e. numerous combinations of relative femoral and tibial component alignment were analysed: malposition of the femoral/tibial component in mediolateral (±3 mm) and anterior-posterior (±3 mm) direction.Introduction
Methods
The biological reaction in metallosis and pseudotumor generation after metal on metal total hip arthroplasty or corroding metal implants remains unsettled. Clinically, still lethal cases appear with massive bone loss and metal ions are suspected to be responsible for this inflammatory reaction, solid metal wear particles instead are usually not observed in the common literature. The aim of this study was to compare the biological reactions of metal ions and metal wear particles in a murine
Migration of bone cells and precursor cells to the site of a bone defect can accelerate bone regeneration. Therefore, guidance of these cells by direct current (DC) is an interesting approach to improve implant ingrowth or fracture healing. To allow a better understanding of DC-induced directed migration, a specific stimulation chamber was established and the influence of DC on calcium channel expression in osteoblasts was investigated. Human osteoblasts were isolated from femoral heads of patients undergoing total hip arthroplasty after patient”s consent. The study was approved by the local ethical committee (AZ: 2010–10). Differentiation into osteoblasts was ensured by cultivation in standard cell culture medium enriched with β-glycerophosphate, ascorbic acid and dexamethasone. 2×103 osteoblasts were seeded into custom-made chambers for DC field application. After 12 h DC was applied to chambers via Ag/AgCl electrodes set into separate reservoirs coupled to cell culture area by 2% agarose bridges in order to prevent cytotoxic impact of electrochemical reactions proceeding at the electrodes. Electric fields ranging from 150 to 450 V/m were applied to cells for 7 h. Several cell images were taken over time and used for evaluation of migration direction and speed with ImageJ software. Subsequently, cells were lysed in Trizol for RNA isolation and semiquantitative real-time polymerase chain reaction of voltage-gated calcium channels Cav1.4 and Cav3.2 as well as stretch-activated magnesium and calcium channel TRPM7 was performed.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
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
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 biomechanical evaluation of tendon repair with collagen-based scaffolds in rat model is a common method to determine the functional outcome of the tested material. We introduced a magnetic resonance imaging (MRI) approach to verify the biomechanical test data. In present study different collagen scaffolds for tendon repair were examined. Two collagen test materials: based on bovine stabilized collagen, chemically cross-linked with oriented collagenous fibres (material 1) and based on porcine dermal extracellular matrix, with no cross-linking (material 2) were compared. The animal study was approved by the local review board. Surgery was performed on male Sprague-Dawley rats with a body weight of 400 ± 19 g. Each rat underwent a 5 mm transection of the right Achilles tendon. The M. plantaris tendon was removed. The remaining tendon ends were re-joined with a 5 mm scaffold of either the material 1 or 2. Each scaffold material was sutured into place with two single stiches (Vicryl 4–0, Ethicon) each end. A total of 16 rats (n= 8 each group) were observed for 28 days follow up. The animals were sacrificed and hind limbs were transected proximal to the knee joint. MRI was performed using a 7 Tesla scanner (BioSpec 70/30, Bruker). T2-weighted TurboRARE sequences with an in-plane resolution of 0.12 mm and a slice thickness of 0.7 mm were analysed. All soft and hard tissues were removed from the Achilles tendon-calcaneus-foot complex before biomechanical testing. Subsequently, the specimens were fixed in a materials testing machine (Z1.0, Zwick, Ulm, Germany) for tensile testing. All tendons were preloaded with 1 N and subsequently stretched at a rate of 1 mm/s until complete failure was observed. Non-operated tendons were used as a control (n=4). After 28 postoperative days, MRI demonstrated that four scaffolds (material 1: n=2, material 2: n=2) were slightly dislocated in the proximal part of hind limb. In total five failures of reconstruction could be detected in the tendon repairs (material 1: n=3, material 2: n=2). Tendons augmented with the bovine material 1 showed a maximum tensile load of 57.9 ± 17.9 N and tendons with porcine scaffold material 2 of 63.1 ± 19.5 N. The native tendons demonstrated only slightly higher loads of 76.6 ± 11.6 N. Maximum failure load of the tendon-scaffold construct in both groups did not differ significantly (p < 0.05). Stiffness of the tendons treated with the bovine scaffold (9.9 ± 3.6 N/mm) and with the porcine scaffold (10.7 ± 2.7 N/mm) showed no differences. Stiffness of the native healthy tendon of the contralateral site was significantly higher (20.2 ± 6.6 N/mm, p < 0.05). No differences in the mechanical properties between samples of both scaffold groups could be detected, regardless of whether the repaired tendon defect has failed or the scaffold has been dislocated. The results show that MRI is important as an auxiliary tool to verify the biomechanical outcome of tendon repair in animal models.
Nowadays, biomaterials can be used to maintain or replace several functions of the human body being constricted or lost due to tumors, fractures, injuries as well as chronic diseases, infections or simply aging. Titanium and its alloys, i.e. Ti6Al4V are the most common materials (70 to 80%) used for structural orthopedic implants due to their unique combination of good mechanical properties, corrosion resistance and biocompatibility. Addition of β-stabilizers, e. g. niobium (Nb), can improve the mechanical properties of such titanium alloys further, simultaneously offering excellent biocompatibility. Previous studies concerning biocompatibility analyses with niobium and especially Ti-42Nb specimens are rarely described; none for niobium and Ti-42Nb powders examining human cell viability, collagen and interleukin synthesis. In this
Enhanced micromotions between the implant and surrounding bone can impair osseointegration, resulting in fibrous encapsulation and aseptic loosening of the implant. Since the effect of micromotions on human bone cells is sparsely investigated, an Micromotions ranging from 25 µm to 100 µm were applied as sine or triangle signal with 1 Hz frequency to human osteoblasts seeded on collagen scaffolds. Micromotions were applied for six hours per day over three days. During the micromotions, a static pressure of 527 Pa was exerted on the cells by Ti6Al4V cylinders. Osteoblasts loaded with Ti6Al4V cylinders and unloaded osteoblasts without micromotions served as controls. Subsequently, cell viability, expression of the osteogenic markers collagen type I, alkaline phosphatase, and osteocalcin, as well as gene expression of osteoprotegerin, receptor activator of NF-κB ligand, matrix metalloproteinase-1, and tissue inhibitor of metalloproteinase-1, were investigated.Objectives
Methods
Metal on metal bearings are used especially in hip resurfacing. On the one hand, small bone preserving implants can be used. On the other hand recent studies found a variety of local and systemic side effects, for instance the appearance of pseudotumors, that are explained by pathologic biological reaction of the metal wear debris. The detailed mechanisms are still not understood until now. Thus it was the aim of this study to investigate the local reaction of metal wear particles and metal ions in a murine model. The hypothesis was that mainly metal ions provoke adverse histopathological reactions in vivo. Three groups, each with 10 Balb / c mice were generated. Group A: injection of a 50 µl metal ion suspension at a concentration of 200 µg / l in the left knee. Group B: injection of a 50 µl 0,1 vol% metal particle suspension into the left knee joint. Group C (control group): injection of a 50 µl of 0,1 vol% PBS-suspension in the left knee. Incubation for 7 days, followed by euthanasia of the animals by intracardiac pentobarbital. The left and right knee, the lungs, kidneys, liver and spleen were removed. Histologic paraffin sections in 2 microns thickness were made, followed by HE (overview staining) and Movat (Pentachrom staining) staining. The histologic analysis was a done by a light microscopic evaluation of the subdivided visual fields at 200× magnification.Introduction
Material and 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.
The higher resisting torque against dislocation and the large range of motion due to the enlarged effective head diameter substantiate the use of eccentric dual-mobility cups in case of total hip joint instability [1,2]. As a result of force-dependent self-centering mechanism, an increased movement of the intermediate-component can be expected whose effect on wear propagation is unknown so far. Currently available hip joint simulators are only able to vary the load by the absolute value and not by the direction of resulting force. Therefore, the uniaxial force transmission may lead to a unique and stable alignment of the intermediate-component during testing. The purpose of this numerical study was to evaluate relative movements of the intermediate-component during daily life activities with respect to wear propagation. The numerical analysis was based on a standard dual-mobility system consisting of a polished metallic cup, a UHMWPE intermediate-component (40 mm outer diameter) with an eccentric offset of 2 mm and a 28 mm ceramic femoral head [Fig. 1]. The relative motion of the intermediate-component was affected by the geometrically generated self-centering torque (TC) and the friction torque for inner (TFi) and outer (TFo) articulation around the centre of rotation Z1[Fig. 2]. In order to consider lubrication conditions the lambda ratio was estimated for different daily life activities [3], including the calculation of composite roughness and minimum film thickness for a ball-on-plate configuration. The friction torque was related to the product of load (Introduction:
Method:
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
At present, wear investigations of total hip replacement (THR) are performed in accordance with the ISO standard 14242, which is based on empirically determined relative motion data and exclusively describes the gait cycle. However, besides continuous walking, a number of additional activities characterize the movement sequences in everyday life and influence the wear rates as well as the size and shape of wear debris. Disagreements of in vitro and in vivo wear mechanisms seemed to be a result of differences between in vitro and in vivo kinematics and dynamics. This requires an optimization of the current test procedures and parameters. Hence, the aim of the present study was to evaluate most frequent activities of daily living, based on available in vivo data, in order to generate parameter sets according to loading and rotational movements close to the physiological situation. For the generation of angular patterns, time-dependent three-dimensional trajectories of reference points were used from the HIP98 database of Bergmann. The data set was evaluated and interpolated using analytical techniques to simulate consecutive smooth motion cycles in hip wear simulators or further test devices. The calculated relative joint movement was expressed by an ordered set of three elementary rotations and was complemented with three force components of the joint contact force to generate kinematically and dynamically consistent parameter sets. The obtained sets included the activities walking, knee bending, stair climbing and a combined load case of sitting down and standing up for an averaged patient. Generated slide tracks, created by the use of the angular patterns, demonstrated differences according to the kinematics between selected daily life activities and those established for the ISO standard 14242. In particular, for the relative flexion-extension rotational movement, routine activities showed significant higher ranges of motion. Additionally, the depicted force pattern underlined that the prevailing force component varied considerably between different activities. These deviations in range of motion and joint forces could be attributed to disagreements between in vitro and in vivo results of THR wear testing. The Integration of frequent activities of daily living in the in vivo test protocol could be realized by means of the sequential arrangement of the four investigated activities.
