Surgical education of fracture fixation biomechanics relies mainly on simplified illustrations to distill the essence of the underlying principles. These mostly consist of textbook drawings or hands-on exercises during courses, both with unique advantages such as broad availability and haptics, respectively. Computer simulations are suited to bridge these two approaches; however, the validity of such simulations must be guaranteed to teach the correct aspects. Therefore, the aim of this study was to validate finite element (FE) simulations of bone-plate constructs to be used in surgical education in terms of fracture gap movement and implant surface strain. The validation procedure was conducted in a systematic and hierarchical manner with increasing complexity. First, the material properties of the isolated implant components were determined via four-point bending of the plate and three-point bending of the screw. Second, stiffness of the screw-plate interface was evaluated by means of cantilever bending to determine the properties of the locking mechanism. Third, implant surface strain and fracture gap motion were measured by testing various configurations of entire fixation constructs on artificial bone (Canevasit) in axial compression. The determined properties of the materials and interfaces assessed in these experiments were then implemented into FE models of entire fixation constructs with different fracture width and screw configurations. The FE-predicted implant surface strains and fracture gap motions were compared with the experimental results. The simulated results of the different construct configurations correlated strongly with the experimentally measured fracture gap motions (R. 2. >0.99) and plate surface strains (R. 2. >0.95). In a systematic approach, FE model validation was achieved successfully in terms of fracture gap motion and implant deformation, confirming trustworthiness for surgical education. These validated models are used in a novel online education tool OSapp (. https://osapp.ch/. ) to illustrate and explain the biomechanical principles of fracture fixations in an interactive manner

Osteosynthesis aims to maintain fracture reduction until bone healing occurs, which is not achieved in case of mechanical fixation failure. One form of failure is plastic plate bending due to overloading, occurring in up to 17% of midshaft fracture cases and often necessitating reoperation. This study aimed to replicate in-vivo conditions in a cadaveric experiment and to validate a finite element (FE) simulation to predict plastic plate bending. Six cadaveric bones were used to replicate an established ovine tibial osteotomy model with locking plates in-vitro with two implant materials (titanium, steel) and three fracture gap sizes (30, 60, 80 mm). The constructs were tested monotonically until plastic plate deformation under axial compression. Specimen-specific FE models were created from CT images. Implant material properties were determined using uniaxial tensile testing of dog bone shaped samples. The experimental tests were replicated in the simulations. Stiffness, yield, and maximum loads were compared between the experiment and FE models. Implant material properties (Young's modulus and yield stress) for steel and titanium were 184 GPa and 875 MPa, and 105 GPa and 761 MPa, respectively. Yield and maximum loads of constructs ranged between 469–491 N and 652–683 N, and 759–995 N and 1252–1600 N for steel and titanium fixations, respectively. FE models accurately and quantitatively correctly predicted experimental results for stiffness (R2=0.96), yield (R2=0.97), and ultimate load (R2=0.97). FE simulations accurately predicted plastic plate bending in osteosynthesis constructs. Construct behavior was predominantly driven by the implant itself, highlighting the importance of modelling correct material properties of metal. The validated FE models could predict subject-specific load bearing capacity of osteosyntheses in vivo in preclinical or clinical studies. Acknowledgements: This study was supported by the AO Foundation via the AOTRAUMA Network (Grant No.: AR2021_03)

Introduction. Achieving an appropriate primary stability after implantation is a prerequisite for the long-term viability of a dental implant. Virtual testing of the bone-implant construct can be performed with finite element (FE) simulation to predict primary stability prior to implantation. In order to be translated to clinical practice, such FE modeling must be based on clinically available imaging methods. The aim of this study was to validate an FE model of dental implant primary stability using cone beam computed tomography (CBCT) with ex vivo mechanical testing. Method. Three cadaveric mandibles (male donors, 87-97 years old) were scanned by CBCT. Twenty-three bone samples were extracted from the bones and conventional dental implants (Ø4.0mm, 9.5mm length) were inserted in each. The implanted specimens were tested under quasi-static bending-compression load (cf. ISO 14801). Sample-specific homogenized FE (hFE) models were created from the CBCT images and meshed with hexahedral elements. A non-linear constitutive model with element-wise density-based material properties was used to simulate bone and the implant was considered rigid. The experimental loading conditions were replicated in the FE model and the ultimate force was evaluated. Result. The experimental ultimate force ranged between 67 N and 789 N. The simulated ultimate force correlated better with the experimental ultimate force (R. 2. =0.71) than the peri-implant bone density (R. 2. =0.30). Conclusion. The developed hFE model was demonstrated to provide stronger prediction of primary stability than peri-implant bone density. Therefore, hFE Simulations based on this clinically available low-radiation imaging modality, is a promising technology that could be used in future as a surgery planning tool to assist the clinician in evaluating the load-bearing capacity of an implantation site. Acknowledgements. Funding: EU's Horizon 2020 grant No: 953128 (I-SMarD). Dental implants: THOMMEN Medical AG

Despite past advances of implant technologies, complication rates of fixations remain high at challenging sites such as the proximal humerus [1]. These may not only be owed to the implant itself but also to dissatisfactory surgical execution of fracture reduction and implant positioning. Therefore, the aim of this study was to quantify the instrumentation accuracy of a highly standardised and guided procedure and its influence on the biomechanical outcome and predicted failure risk. Preoperative planning of osteotomies creating an unstable 3-part fracture and fixation with a locking plate was performed based on CT scans of eight pairs of low-density proximal humerus samples from elderly female donors (85.2±5.4 years). 3D-printed subject-specific guides were used to osteotomise and instrument the samples according to the pre-OP plan. Instrumentation accuracies in terms of screw lengths and orientations were evaluated by comparing post-OP CT scans with the pre-OP plan. The fixation constructs were biomechanically tested until cyclic cut-out failure [2]. Failure risks of the planned and the post-OP configurations were predicted using a validated sample-specific finite element (FE) simulation approach [2] and correlated with the experimental outcomes. Small deviations were found for the instrumented screw trajectories compared to the planned configuration in the proximal-distal (0.3±1.3º) and anterior-posterior directions (-1.7±1.8º), and for screw tip to joint distances (-0.3±1.1 mm). Significantly higher failure risk was predicted for the post-OP compared to the planned configurations (p<0.01) via FE. When incorporating the instrumentation inaccuracies, the biomechanical results could be predicted well with FE (R. 2. =0.70). Despite the high instrumentation accuracy achieved using sophisticated subject-specific 3D-printed guides, even minor deviations from the pre-OP plan significantly increased the FE-predicted risk of failure. This underlines the importance of intraoperative guiding technology [3] in tandem with careful pre-OP planning to assist surgeons to achieve optimal outcomes. Acknowledgements. This study was performed with the assistance of the AO Foundation via the AOTRAUMA Network

Background. Osteoporotic fracture fixation in the proximal humerus remains a critical challenge. While the biomechanical benefits of screw augmentation with bone cement are established, minimising the cement volume may help control any risk of extravasation and reduce surgical procedure time. Previous experimental studies suggest that it may be sufficient to only augment the screws at the sites of the lowest bone quality. However, adequately testing this hypothesis in vitro is not feasible. Methods. This study systematically evaluated the 64 possible strategies for augmenting six screws in the humeral head through finite element simulations to determine the relative biomechanical benefits of each augmentation strategy. Two subjects with varying levels of local bone mineral density were each modeled with a 2-part and 3-part fracture that was stabilised with a PHILOS plate. The biomechanical fixation was evaluated under physiological loads (muscle and joint reaction forces) that correspond to three different motions: 45 degrees abduction, 45 degrees abduction with 45 degrees internal rotation, and 45 degrees flexion. Results. The higher risk cases (low bone quality or 3-part fracture) exhibited greater peri-implant bone strains and derived greater benefits from screw augmentation. When selecting four screws to augment, the biomechanical benefits ranged from a 25% reduction in bone strain to a 59% reduction in bone strain, depending on the choice of screws. Further, the relative benefits of each augmentation strategy varied between patients and under different loading conditions. Correlations between local bone mineral density and benefits of augmentation were not significant. Conclusions. An optimal augmentation strategy is likely patient-specific and a larger cohort, modeled under a variety of conditions, would be required to elucidate any patient-specific factors (e.g. morphology or bone quality) that may dictate the relative benefits of each augmentation strategy

Introduction. Pedicle screw loosening in posterior instrumentation of thoracolumbar spine occurs up to 60% in osteoporotic patients. These complications may be alleviated using more flexible implant materials and novel designs that could be optimized with reliable computational modeling. This study aimed to develop and validate non-linear homogenized finite element (hFE) simulations to predict pedicle screw toggling. Method. Ten cadaveric vertebral bodies (L1-L5) from two female and three male elderly donors were scanned with high-resolution peripheral quantitative computed tomography (HR-pQCT, Scanco Medical) and instrumented with pedicle screws made of carbon fiber-reinforced polyether-etherketone (CF/PEEK). Sample-specific 3D-printed guides ensured standardized instrumentation, embedding, and loading procedures. The samples were biomechanically tested to failure in a toggling setup using an electrodynamic testing machine (Acumen, MTS) applying a quasi-static cyclic testing protocol of three ramps with exponentially increasing peak (1, 2 and 4 mm) and constant valley displacements. Implant-bone kinematics were assessed with a stereographic 3D motion tracking camera system (Aramis SRX, GOM). hFE models with non-linear, homogenized bone material properties including a strain-based damage criterion were developed based on intact HR-pQCT and instrumented 3D C-arm scans. The experimental loading conditions were imposed, the maximum load per cycle was calculated and compared to the experimental results. HR-pQCT-based bone volume fraction (BV/TV) around the screws was correlated with the experimental peak forces at each displacement level. Result. The nonlinear hFE models accurately (slope = 1.07, intercept = 0.2 N) and precisely (R. 2. = 0.84) predicted the experimental peak forces at each displacement level. BV/TV alone was a weak predictor (R. 2. <0.31). Conclusion. The hFE models enable fast design iterations aiming to reduce the risk of screw loosening in low-density vertebrae. Improved flexible implant designs are expected to contribute to reduced complication rates in osteoporotic patients

There is a critical need for safe innovation in total joint replacements to address the demands of an ageing yet increasingly active population. The development of robust implant designs requires consideration of uncertainties including patient related factors such as bone morphology but also activity related loads and the variability in the surgical procedure itself. Here we present an integrated framework considering these sources of variability and its application to assess the performance of the femoral component of a total hip replacement (THR). The framework offers four key features. To consider variability in bone properties, an automated workflow for establishing statistical shape and intensity models (SSIM) was developed. Here, the inherent relationship between shape and bone density is captured and new meshes of the target bone structures are generated with specific morphology and density distributions. The second key feature is a virtual implantation capability including implant positioning, and bone resection. Implant positioning is performed using automatically identified bone features and flexibly defined rules reflecting surgical variability. Bone resection is performed according to manufacturer guidelines. Virtual implantation then occurs through Boolean operations to remove bone elements contained within the implant's volume. The third feature is the automatic application of loads at muscle attachment points or on the joint contact surfaces defined on the SSIM. The magnitude and orientation of the forces are derived from models of similar morphology for a range of activities from a database of musculoskeletal (MS) loads. The connection to this MS loading model allows the intricate link between morphology and muscle forces to be captured. Importantly, this model of the internal forces provides access to the spectrum of loading conditions across a patient population rather than just typical or average values. The final feature is an environment that allows finite element simulations to be run to assess the mechanics of the bone-implant construct and extract results for e.g. bone strains, interface mechanics and implant stresses. Results are automatically processed and mapped in an anatomically consistent manner and can be further exploited to establish surrogate models for efficient subsequent design optimization. To demonstrate the capability of the framework, it has been applied to the femoral component of a THR. An SSIM was created from 102 segmented femurs capturing the heterogeneous bone density distributions. Cementless femoral stems were positioned such that for the optimal implantation the proximal shaft axis of the femurs coincided with the distal stem axis and the position of the native femoral head centre was restored. Here, the resection did not affect the greater trochanter and the implantations were clinically acceptable for 10000 virtual implantations performed to simulate variability in patient morphology and surgical variation. The MS database was established from musculoskeletal analyses run for a cohort of 17 THR subjects obtaining over 100,000 individual samples of 3D muscle and joint forces. An initial analysis of the mechanical performance in 7 bone-implant constructs showed levels of bone strains and implant stresses in general agreement with the literature

Total Hip Replacement (THR) is one of the most successful operations in all of medicine, however surgeons just rely on their experience and expertise when choosing between cemented or cementless stem, without having any quantitative guidelines. The aim of this project is to provide clinicians with some tools to support in their decision making. A novel method based on bone mineral density (BMD) measurements and assessments was developed 1) to estimate the periprosthetic fracture risk (FR) while press-fitting cementless stem; 2) to evaluate post-operative bone remodeling in terms of BMD changes after primary THR. Data for 5 out of over 70 patients (already involved in a previous study. 1. ) that underwent primary THA in Iceland were selected for developing novel methods to assess intra-operative FR and bone mineral density (BMD) changes after the operation. For each patient three CT images were acquired (Philips Brilliance 64 Spiral-CT, 120 kVp, slice thickness: 1 mm, slice increment: 0.5 mm): pre-op, 24 hours and 1 year post-operative. Pre-op CT scan was used to create 3D finite element model (Materialise Mimics) of the proximal femur. The material properties were assigned based on Hounsfield Units. Different strategies were analyzed for simulating the press-fitting operation, developing what has already been done in prior study. 1. In the finite element simulation (Ansys Workbench), a pressure (related to the implant hammering force of 9.25 kN. 2. ) was applied around the femur's hollow for the stem and the distribution of maximum principal elastic strain over the bone was calculated. Assuming a critical failure value. 3. of 7300 με, the percentage of fractured elements was calculated (i.e. FR). Post 24 hours and Post 1 year CT images were co-registrated and compared (Materialise Mimics) in order to assess BMD changes. Successively, volumes of bone lost and bone gained were calculated and represented in a 3D model. Age and gender should not be taken as unique indicators to choose between implants typologies, since also three dimensional BMD distribution along with volume of cortical bone influence the risk of periprosthetic fractures. Highest FR values were experienced in the calcar-femorale zone and in similar location on the posterior side. BMD loss volume fractions after 1 year were usually higher than BMD gain ones. Consistently with prior studies. 4. , BMD loss was mainly concentrated around the proximal end (lesser trochanter area, outer bone). If present, BMD gain occurred at the distal end (below stem's tip) or proximally (lesser trochanter area, interface contact with the stem). The use of clinical data for BMD assessments serves as an important tool to develop a quantitative method which will support surgeons in their decisions, guiding them to the optimal implant for the patient. Knowing the risk of fracture if choosing a cementless stem and being aware of how the bone will remodel around the stem in one year's time can eventually lead to reduction in revisions and increased quality of life for the patient. Further work will target analysis of a larger cohort of patients and validate FE models

The high risk and the associated high mortality of secondary, contralateral hip fractures [1,2] could justify internal, invasive prophylactic reinforcement of the osteoporotic proximal femur to avoid these injuries in case of a low energy fall. Previous studies have demonstrated high potential of augmentation approaches [3,4,5], but to date there has no ideal solution been found. The development of optimized reinforcement strategies can be aided with validated computer simulation tools that can be used to evaluate new ideas. A validated non-linear finite element (FE) simulation tool was used here to predict the yield and fracture load of twelve osteoporotic or osteopenic proximal femora in sideways fall based on high resolution CT images. Various augmentation strategies using bone cement or novel metal implants were developed, optimized and virtually performed on the bone models. The relative strengthening compared to the non-augmented state was evaluated using case-specific FE analyses. Strengthening effect of the cement-based augmentation was linearly proportional to cement volume and was significantly affected by cement location. With the clinically acceptable 12.6 ± 1.2 ml volume and optimized location of the cement cloud, compared to the non-augmented state, 71 ± 26% (42 – 134%) and 217 ± 166% (83 – 509%) increase in yield force and energy was reached, respectively. These were significantly higher than previously published experimental results using the “central” cement location [5], which could be well predicted by our FE models. The optimized metal implant could provide even higher strengthening effect: 140 ± 39% (76 – 194%) increase in yield force and +357 ± 177% (132 – 691%) increase in yield energy. However, for metal implants, a higher risk of subcapital fractures was indicated. For both cement and metal, the originally weaker bones were strengthened exponentially more compared to the stronger ones. The ideal solution for prophylactic augmentation should provide an appropriate balance between the requirements of being clinically feasible, ethically acceptable and mechanically sufficient. Even with the optimized location, the cement-based approach may not provide enough strengthening effect and adequate reproducibility of the identified optimal cement cloud position may not be achieved clinically. While the metal implant based strategy appears to be able to deliver the required strengthening effect, the ethical acceptance of this more invasive option is questionable. Further development is therefore required to identify the ideal, clinically relevant augmentation strategy. This may involve new cement materials, less invasive metal implants, or a combination of both. The FE simulation approach presented here could help to screen the potential ideas and highlight promising candidates for experimental evaluation

Low back pain (LBP) is the leading cause of disability worldwide, interfering with an individual's quality of life and work performance. Understanding the degeneration mechanism of the intervertebral disc (IVD), one of the key triggers of LBP, is hence of great interest. Disc degeneration can be mimicked in animal studies using the injection of enzymatic digestion, needle puncture, stab injury, or mechanical over-loading [1]. However, the detailed response of the artificial degenerated disc using needle puncture under physiological dynamic loading in diurnal activities has not yet been analyzed using FE-models. To fill the gap in literature, this study investigates the role of needle puncture injury on the biomechanical response of IVD using a combination of Finite Element (FE) simulations and in-vitro lumbar spine sheep experiments. 16 lumbar motion segments (LMS) were dissected from juvenile sheep lumbar spines. The harvested LMSs were assigned equally to two groups (control group with no incision and an injured group punctured with a 16-gauge needle). All specimens were mounted in a homemade chamber filled with saline solution and underwent a stress-relaxation test using a mechanical testing apparatus (Zwick/Roell, Ulm-Germany). A validated inverse poroelastic FE methodology [2] in conjunction with in-vitro experiments were used to find the elastic modulus and permeability. Subsequently, specimen-specific FE models for the 16 discs were simulated based on daily dynamic physiological activity (i.e., 8h rest followed by a 16h loading phase under compressive loads of 350 N and 1000 N, respectively). The results of the individual FE models were well fitted with the in-vitro stress-relaxation experiments, with an average error of 7.48 (±2.24)%. The results of the simulations demonstrated that the variation of axial displacement in the control discs was significantly higher than the injured ones (P=0.037). At the end of day, the intradiscal pressure (IDP) was slightly higher in the control group (P=0.061) although the maximum axial stress in the annulus fibrosus (AF) was significantly higher in the injured group (P=0.028). The total fluid loss after 24h was significantly higher in the control group (p<0.001). We found that needle puncture can decrease the strain range, IDP, and fluid loss in an IVD, although it increases the axial stress. We therefore hypothesize that the fissures, clefts or tears produced by needle puncture alter the saturation time for disc deformation and pore pressure. The collapsed disc structure hinders the fluid flow capability; hence, the total fluid loss decreases for the injured discs, inhibiting the transportation of nutrients. Higher stresses in the AF were observed for the injured group in alignment with previous studies [3]. It is therefore concluded that the needle puncture injury methodology can be effectively used to mimic the degeneration mechanism in animal models. It is a convenient, reproducible, and cost-effective technique. Future work includes exploring degenerated disks induced by needle puncture to investigate potential regenerative therapeutics