Smart implants are intended to achieve a longer lifespan in the future through integrated sensory and therapeutic functions. In this way, insufficient osseointegration and loosening of uncemented total hip replacements should be prevented by electrodes attached to the femoral stem, specifically stimulating bone growth to improve fixation and thus avoiding revision. For this function, an energy source is necessary, whereby an internal, autonomous one is desirable. Accordingly, in a previous study, a piezoelectric element was integrated into an uncemented hip stem to convert mechanical energy into electrical energy. However, the electrical output has only been calculated computationally, while in practical applications, energy losses occur during energy storage. Moreover, there is a lack of research on total hip and knee replacements with integrated piezoelectric elements that evaluate electrical power generation during everyday activities other than gait. Therefore, the present study aimed to experimentally determine the generated average electrical power of an uncemented revision hip stem with equipped piezoelectric element during different everyday activities. Furthermore, the storable energy during the most promising activities was evaluated with a standard energy harvesting (SEH) circuit. A simplified test rig was set up for these measurements. The results indicate that jogging and walking, with an average electrical power of 204µW and 75µW, respectively, convert the most energy, followed by stairs up and stairs down. After 20 min of jogging, 20.7 mJ were stored in a 330µF capacitor, while one hour of walking yielded 12.9 mJ in a 470µF capacitor. The energy required for two 1 cm long stimulation electrodes is estimated to range from 0.1 mJ (10µA, 1000 Hz) to 254 mJ (100µA, 20 Hz) for the recommended daily protocol of three 45 min sessions. Accordingly, the harvested energy of 12.9 mJ may be sufficient to support electrical bone stimulation at up to 20µA over 20-1000 Hz. To increase the stored energy, significant energy losses in the SEH circuit due to impedance mismatch must be minimized by improving the energy harvesting circuitry.

The authors wish to make the following corrections to their paper [...]

In the future, self-powered smart hip implants may enhance the longevity of total hip replacements. Uncemented hip stems aim to promote integration within the bone stock through electrical bone stimulation before loosening occurs, necessitating an internal energy source. Piezoelectric energy harvesting presents a viable solution. This study aimed to integrate a piezoelectric element into an uncemented revision hip stem in a manner that maximizes electrical output while preserving mechanical stability of the implant. Finite element (FE) analysis was used to evaluate cavities of varying sizes, based on the integrated piezoelectric element, and different positions in the medial neck area of the hip stem. An FE model with physiological boundary conditions during gait assessed the electrical power output, while a separate static FE model, based on ISO 7206-4, analyzed mechanical stress distribution and fatigue resistance of the implant. The most promising design was selected for endurance testing per ISO 7206-4. For the optimal design, the maximum stress was 315 MPa, with a fatigue life of >109 cycles and a safety factor of 2.01 according to Soderberg criteria. The design successfully passed the ISO 7206-4 endurance test and achieved a calculated average electrical power output of 439 μW during gait. In conclusion, an uncemented revision hip stem can be modified to incorporate a piezoelectric element for energy conversion. Future work will focus on experimental evaluation of the electrical voltage and power generation during physiological activities.

INTRODUCTION:The importance of the assembly procedure on the taper connection strength is evident. However, existent surgical technique guides frequently lack comprehensive and precise instructions in this regard. The aim of our experimental study was to evaluate the influence of the surgical technique guide on the femoral head assembly procedure in surgeons with differing levels of experience in total hip arthroplasty.MATERIALS AND METHODS:Twenty-eight participants, divided into four groups based on their lifetime experience in total hip arthroplasty, conducted a femoral head assembly procedure in a simulated intraoperative environment before and after reviewing the surgical technique guide. Demographic information and the number of hammer blows were documented. Hammer velocity and impaction angle were recorded using an optical motion capturing system, while the impaction force was measured using a dynamic force sensor within the impactor.RESULTS:We observed a high variation in the number of hammer blows, maximum force, and impaction angle. Overall, the number of hammer blows decreased significantly from 3 to 2.2 after reviewing the surgical technique guide. The only significant intragroup difference in the number of hammer blows was observed in the group with no prior experience in total hip arthroplasty. No correlation was found between individual factors (age, weight, height) or experience and the measured parameters (velocity, maximum force and angle).CONCLUSIONS:The present study demonstrated a high variation in the parameters of the femoral head assembly procedure. Consideration of the surgical technique guide was found to be a limited factor among participants with varying levels of experience in total hip arthroplasty. These findings underline the importance of sufficient preoperative training, to standardize the assembly procedure, including impaction force, angle, and use of instruments.

Resultant hip joint forces can currently only be recorded in situ in a laboratory setting using instrumented total hip replacements (THRs) equipped with strain gauges. However, permanent recording is important for monitoring the structural condition of the implant, for therapeutic purposes, for self-reflection, and for research into managing the predicted increasing number of THRs worldwide. Therefore, this study aims to investigate whether a recently proposed THR with an integrated piezoelectric element represents a new possibility for the permanent recording of hip joint forces and the physical activities of the patient. Hip joint forces from nine different daily activities were obtained from the OrthoLoad database and applied to a total hip stem equipped with a piezoelectric element using a uniaxial testing machine. The forces acting on the piezoelectric element were calculated from the generated voltages. The correlation between the calculated forces on the piezoelectric element and the applied forces was investigated, and the regression equations were determined. In addition, the voltage outputs were used to predict the activity with a random forest classifier. The coefficient of determination between the applied maximum forces on the implant and the calculated maximum forces on the piezoelectric element was R2 = 0.97 (p < 0.01). The maximum forces on the THR could be determined via activity-independent determinations with a deviation of 2.49 ± 13.16% and activity-dependent calculation with 0.87 ± 7.28% deviation. The activities could be correctly predicted using the classification model with 95% accuracy. Hence, piezoelectric elements integrated into a total hip stem represent a promising sensor option for the energy-autonomous detection of joint forces and physical activities.

Proximal femur fracture risk depends on subject-specific factors such as bone mineral density and morphological parameters. Here, we aim to analyze the dependency of the femoral strength on sixteen morphological parameters. Therefore, finite-element analyses of 20 human femurs during stumbling and lateral falls on the hip were conducted. Pearson correlation coefficients were calculated and morphological parameters with significant correlations were examined in principal component analysis and linear regression analysis. The dependency of the fracture strength on morphological parameters was more pronounced during lateral falls on the hip compared to stumbling. Significant correlations were observed between the neck shaft angle (r = −0.474), neck diameter (r = 0.507), the true distance between the femoral head center and femoral shaft axis (r = 0.459), and its projected distance on the frontal plane (r = 0.511), greater trochanter height (r = 0.497), and distance between the femoral head center and a plane parallel to the frontal plane containing the projection of the femoral head center to the femoral neck axis (r = 0.669). Principal component analysis was strongly weighted by parameters defining the lever arm during a lateral fall as well as the loaded cross-section in the femoral neck.

Computergestützte Untersuchungsmethoden, wie Finite-Elemente-Analyse (FEA) und muskuloskelettale Mehrkörpersimulation (MKS), sind in der muskuloskelettalen Biomechanik wichtig, denn sie ermöglichen ein besseres Verständnis der Mechanik des Bewegungsapparats sowie die Entwicklung und Evaluation orthopädischer Implantate. Diese Methoden werden zur Analyse klinisch relevanter Fragestellungen in verschiedenen anatomischen Regionen eingesetzt, wie Hüfte, Knie, Schultergelenke und Wirbelsäule. Die präoperative Simulation kann in der Orthopädie die chirurgische Planung verbessern und individuelle Ergebnisse vorhersagen. In diesem Artikel werden die Methoden der FEA und MKS anhand jeweils zweier Beispiele aus der Praxis erläutert und die Aktivitäten des Clusters „Numerische Simulation“ des Forschungsnetzwerks Muskuloskelettale Biomechanik (MSB-NET) näher vorgestellt. Ein Ausblick ordnet die numerische Simulation im Zeitalter der künstlichen Intelligenz ein, und lenkt den Blick auf die Relevanz der Simulation bei der (Neu‑)Zulassung von Implantaten.

Aseptic implant loosening after a total joint replacement is partially influenced by material-specific factors when cobalt–chromium alloys are used, including osteolysis induced by wear and corrosion products and stress shielding. Here, we aim to characterize a hybrid material consisting of alumina-toughened zirconia (ATZ) ceramics and additively manufactured Ti-35Nb-6Ta (TiNbTa) alloys, which are joined by a glass solder. The structure of the joint, the static and fatigue shear strength, the influence of accelerated aging, and the cytotoxicity with human osteoblasts are characterized. Furthermore, the biomechanical properties of the functional demonstrators of a femoral component for total knee replacements are evaluated. The TiNbTa-ATZ specimens showed a homogenous joint with statistically distributed micro-pores and a slight accumulation of Al-rich compounds at the glass solder–TiNbTa interface. Shear strengths of 26.4 ± 4.2 MPa and 38.2 ± 14.4 MPa were achieved for the TiNbTa-ATZ and Ti-ATZ specimens, respectively, and they were not significantly affected by the titanium material used, nor by accelerated aging (p = 0.07). All of the specimens survived 107 cycles of shear loading to 10 MPa. Furthermore, the TiNbTa-ATZ did not impair the proliferation and metabolic activity of the human osteoblasts. Functional demonstrators made of TiNbTa-ATZ provided a maximum bearable extension–flexion moment of 40.7 ± 2.2 Nm. The biomechanical and biological properties of TiNbTa-ATZ demonstrate potential applications for endoprosthetic implants.

Although total hip replacements (THR) can be considered one of the most successful implantable medical devices in history, wear remains the ultimate challenge in order to further increase clinical success. Wear assessment on retrieved implants is the most reliable way to perform research into failure mechanisms. Therefor the bearing surface of the explant is measured geometrically by coordinate measuring machine (CMM).Wear determination in geometrical data is carried out in 3 steps: (1) identifying the worn area, (2) reconstructing the pre-wear geometry and (3) quantify wear as the difference between worn area and pre-wear geometry. In previous studies, assumptions to pre-wear geometry had been made for wear determination (step 2) and the worn area was identified by deviations between measured data and assumed form. Thus, the original form of the retrieved endoprostheses, including form deviations due to the manufacturing process and implantation, was not considered which leads to uncertainties in the wear computed.This work introduces a method which allows to identify the wear area without making assumptions to the original form. Instead, the curvature of the bearing surface obtained by simple computations on the measurement data is analysed and the edge of the wear region is recognized by its deviation in curvature.The method is applied to a retrieved Metal-on-Metal prosthetic head and the results are compared to those of the well-known method introduced by Jaeger et al. in 2013. With the new approach the wear region is identified more accurately.

Breast augmentation procedures using silicone implants have become increasingly popular over the past six decades. This article addresses the concerns of patients regarding implant strength by providing clinicians with valuable information in addition to video and pictorial evidence to share, fostering reassurance. The article focuses on the structural integrity and stability of breast implants, which play a critical role in their long-term performance and patient satisfaction. Specifically, it examines the industry standards outlined by the International Organization for Standardization (ISO), with a particular emphasis on ISO14607-2018, which encompasses a range of mechanical and physio-mechanical tests, including the assessment of silicone gel-fill firmness, evaluation of shell integrity, and examination of the impact of environmental conditions on implant performance. Breast implants are not static devices and are subject to aging and fatigue-based degradation. This emphasizes the need for ongoing monitoring and evaluation to ensure the long-term safety and satisfaction of patients. By providing a comprehensive examination of breast implant structure and industry standards, this article equips clinicians with the necessary knowledge to address patient concerns and foster confidence in the safety and longevity of breast augmentation procedures using silicone implants. Level of Evidence III This journal requires that authors assign a level of evidence to each article. For a full description of these Evidence-Based Medicine ratings, please refer to the Table of Contents or the online Instructions to Authors www.springer.com/00266 .

Background: Computational research methods, such as finite element analysis (FEA) and musculoskeletal multi-body simulation (MBS), are important in musculoskeletal biomechanics because they enable a better understanding of the mechanics of the musculoskeletal system, as well as the development and evaluation of orthopaedic implants. These methods are used to analyze clinically relevant issues in various anatomical regions, such as the hip, knee, shoulder joints and spine. Preoperative simulation can improve surgical planning in orthopaedics and predict individual results. Examples from practice: In this article, the methods of FE analysis and MBS are explained using two practical examples, and the activities of the "Numerical Simulation" cluster of the "Musculoskeletal Biomechanics Research Network (MSB-NET)" are presented in more detail. An outlook classifies numerical simulation in the age of artificial intelligence and draws attention to the relevance of simulation in the (re)approval of implants.

Accurate detection of implant loosening is crucial for early intervention in total hip replacements, but current imaging methods lack sensitivity and specificity. Vibration methods, already successful in dentistry, represent a promising approach. In order to detect loosening of the total hip replacement, excitation and measurement should be performed intracorporeally to minimize the influence of soft tissue on damping of the signals. However, only implants with a single sensor intracorporeally integrated into the implant for detecting vibrations have been presented in the literature. Considering different mode shapes, the sensor’s position on the implant is assumed to influence the signals. In the work at hand, the influence of the position of the sensor on the recording of the vibrations on the implant was investigated. For this purpose, a simplified test setup was created with a titanium rod implanted in a cylinder of artificial cancellous bone. Mechanical stimulation via an exciter attached to the rod was recorded by three accelerometers at varying positions along the titanium rod. Three states of peri-implant loosening within the bone stock were simulated by extracting the bone material around the titanium rod, and different markers were analyzed to distinguish between these states of loosening. In addition, a modal analysis was performed using the finite element method to analyze the mode shapes. Distinct differences in the signals recorded by the acceleration sensors within defects highlight the influence of sensor position on mode detection and natural frequencies. Thus, using multiple sensors could be advantageous in accurately detecting all modes and determining the implant loosening state more precisely.

Background: In total knee arthroplasty, femoral components with coated or oxidized surfaces are commonly used as alternative to CoCrMo in metal sensitive patients. Data on the in vivo behaviour of different coating types is, however, rare. Aim of the study was the investigation of coating stability with respect to implant and patient specific parameters.Methods: Coating thickness and coating thickness reduction, respectively, was measured at 37 retrieved femoral components with TiNbN, TiN, ZrN or oxidized zirconium (OxZr) surface by the crater grinding method. The results were correlated to surface type, manufacturer, time in vivo of the implant, patient body weight and patient activity.Results: Mean coating thickness reduction in the overall retrieval collection was 0.6 & mu;m & PLUSMN; 0.8 & mu;m. There was no correlation between coating thickness reduction and coating type, time in vivo, patient body weight, and patient activity. If grouped according to manufacturers, implants of one manufacturer showed an increased coating thickness reduction. 10 of 37 retrievals exhibited coating abrasion with exposure of the underlying alloy. TiNbN coatings showed the highest incidence (9/17) of coating abrasion. No coating breakthrough was observed in the ZrN or OxZr surfaces.Conclusion: Our results indicate that TiNbN coatings should be optimized to improve their wear resistance in the long-term.

Total hip joint replacement (THR) is clinically well-established. In this context, the resulting range of motion (ROM) is crucial for patient satisfaction when performing joint movements. However, the ROM for THR with different bone preserving strategies (short hip stem and hip resurfacing) raises the question of whether the ROM is comparable with conventional hip stems. Therefore, this computer-based study aimed to investigate the ROM and type of impingement for different implant systems. An established framework with computer-aided design 3D models based on magnetic resonance imaging data of 19 patients with hip osteoarthritis was used to analyse the ROM for three different implant systems (conventional hip stem vs. short hip stem vs. hip resurfacing) during typical joint movements. Our results revealed that all three designs led to mean maximum flexion higher than 110 degrees. However, hip resurfacing showed less ROM (-5% against conventional and -6% against short hip stem). No significant differences were observed between the conventional and short hip stem during maximum flexion and internal rotation. Contrarily, a significant difference was detected between the conventional hip stem and hip resurfacing during internal rotation (p = 0.003). The ROM of the hip resurfacing was lower than the conventional and short hip stem during all three movements. Furthermore, hip resurfacing shifted the impingement type to implant-to-bone impingement compared with the other implant designs. The calculated ROMs of the implant systems achieved physiological levels during maximum flexion and internal rotation. However, bone impingement was more likely during internal rotation with increasing bone preservation. Despite the larger head diameter of hip resurfacing, the ROM examined was substantially lower than that of conventional and short hip stem.

Due to the increasing number of periprosthetic femoral fractures (PFF), the optimisation of implant design gains importance. For the presented research a validated, subject specific finite element model of a human femur with an inlying total hip stem was used to compare the influence of different geometrical implant parameters on the development of PFF. The heterogeneous bone tissue was modelled on the basis of computed tomography scans. A ductile damage model with element deletion was applied to simulate bone fracture in a load case re-enacting a stumbling scenario. The results were compared in terms of fracture load, subsidence and fracture pattern to analyse the influence of friction at the implant-bone interface, implant size and stem length. The results showed that higher friction coefficients lead to an increase of fracture load. Also, the usage of an oversized implant has a negligible effect while an undersized implant reduces the fracture load by 48.9% for the investigated femur. Lastly, a higher fracture load was reached with an elongated stem, but the bending and change in fracture path indicate a more distal force transmission and subsequent stress shielding in the proximal femur.

The authors wish to make the following corrections to their paper [...].

Instrumented implants are a promising approach to further improve the clinical outcome of total hip arthroplasties. For the integrated sensors or active functions, an electrical power supply is required. Energy harvesting concepts can provide autonomous power with unlimited lifetime and are independent from external equipment. However, those systems occupy space within the mechanically loaded total hip replacement and can decrease the life span due to fatigue failure in the altered implant. We previously presented a piezoelectric energy harvesting system for an energy-autonomous instrumented total hip stem that notably changes the original implant geometry. The aim of this study was to investigate the remaining structural fatigue failure strength of the metallic femoral implant component in a worst-case scenario. Therefore, the modified hip stem was tested under load conditions based on ISO 7206-4:2010. The required five million cycles were completed twice by all samples (n = 3). Additionally applied cycles with incrementally increased load levels up to 4.7 kN did not induce implant failure. In total, 18 million cycles were endured, outperforming the requirements of the ISO standard. Supplementary finite element analysis was conducted to determine stress distribution within the implant. A high stress concentration was found in the region of modification. The stress level showed an increase compared to the previously evaluated physiological loading situation and was close to the fatigue data from the literature. The stress concentration factor compared to the original geometry amounted to 2.56. The assessed stress level in accordance with the experimental fatigue testing can serve as a maximum reference value for further implant design modifications and optimisations.

Instrumented implants can improve the clinical outcome of total hip replacements (THRs). To overcome the drawbacks of external energy supply and batteries, energy harvesting is a promising approach to power energy-autonomous implants. Therefore, we recently presented a new piezoelectric-based energy harvesting concept for THRs. In this study, the performance of the proposed energy harvesting system was numerically and experimentally investigated. First, we numerically reproduced our previous results for the physiologically based loading situation in a simplified setup. Thereafter, this configuration was experimentally realised by the implantation of a functional model of the energy harvesting concept into an artificial bone segment. Additionally, the piezoelectric element alone was investigated to analyse the predictive power of the numerical model. We measured the generated voltage for a load profile for walking and calculated the power output. The maximum power for the directly loaded piezoelectric element and the functional model were 28.6 and 10.2 µW, respectively. Numerically, 72.7 µW was calculated. The curve progressions were qualitatively in good accordance with the numerical data. The deviations were explained by sensitivity analysis and model simplifications, e.g., material data or lower acting force levels by malalignment and differences between virtual and experimental implantation. The findings verify the feasibility of the proposed energy harvesting concept and form the basis for design optimisations with increased power output.

Energy harvesting is a promising approach to power novel instrumented implants that have passive sensory functions or actuators for therapeutic measures. We recently proposed a new piezoelectric concept for energy harvesting in total hip replacements. The mechanical implant safety and the feasibility of power generation were numerically demonstrated. However, the power output for the chosen piezoelectric element was low. Therefore, we investigated in the present study different geometry variants for an increased power output for in vivo applications. Using the same finite element model, we focused on new, customised piezoelectric element geometries to optimally exploit the available space for integration of the energy harvesting system, while maintaining the mechanical safety of the implant. The result of our iterative design study was an increased power output from 29.8 to 729.9 µW. This amount is sufficient for low-power electronics.