Patient-specific finite element analysis of femurs with cemented hip implants

How accurately do finite element models predict the fall impact response of ex vivo specimens augmented by prophylactic intramedullary nailing?

Hip fracture prevention approaches like prophylactic augmentation devices have been proposed to strengthen the femur and prevent hip fracture in a fall scenario. The aim of this study was to validate the finite element model (FEM) of specimens augmented by prophylactic intramedullary nailing in a simulated sideways fall impact against ex vivo experimental data. A dynamic inertia-driven sideways fall simulator was used to test six cadaveric specimens (3 females, 3 males, age 63-83 years) prophylactically implanted with an intramedullary nailing system used to augment the femur. Impact force measurements, pelvic deformation, effective pelvic stiffness, and fracture outcomes were compared between the ex vivo experiments and the FEMs. The FEMs over-predicted the effective pelvic stiffness for most specimens and showed variability in terms of under- and over-predicting peak impact force and pelvis compression depending on the specimen. A significant correlation was found for time to peak impact force when comparing ex vivo and FEM data. No femoral fractures were found in the ex vivo experiments, but two specimens sustained pelvic fractures. These two pelvis fractures were correctly identified by the FEMs, but the FEMs made three additional false-positive fracture identifications. These validation results highlight current limitations of these sideways fall impact models specific to the inclusion of an orthopaedic implant. These FEMs present a conservative strategy for fracture prediction in future applications. Further evaluation of the modelling approaches used for the bone-implant interface is recommended for modelling augmented specimens, alongside the importance of maintaining well-controlled experimental conditions.

Corroboration of coupled musculoskeletal model and finite element predictions with in vivo RSA migration of an uncemented acetabular component

While finite element (FE) models have been used extensively in orthopedic studies, validation of their outcome metrics has been limited to comparison against ex vivo testing. The aim of this study was to validate FE model predictions of the initial cup mechanical environment against patient-matched in vivo measurements of acetabular cup migration using radiostereometric analysis (RSA). Tailored musculoskeletal and FE models were developed using a combination of three-dimensional (3D) motion capture data and clinical computerized tomography (CT) scans for a cohort of eight individuals who underwent primary total hip replacement and were prospectively enrolled in an RSA study. FE models were developed to calculate the mean modulus of cancellous bone, composite peak micromotion (CPM), composite peak strain (CPS) and percentage area of bone ingrowth. The RSA cup migration at 3 months was used to corroborate the FE output metrics. Qualitatively, all FE-predicted metrics followed a similar rank order as the in vivo RSA 3D migration data. The two cases with the lowest predicted CPM (<20 µm), lowest CPS (<0.0041), and high bone modulus (>917 MPa) were confirmed to have the lowest in vivo RSA 3D migration (<0.14 mm). The two cases with the largest predicted CPM (>80 µm), larger CPS (>0.0119) and lowest bone modulus (<472 MPa) were confirmed to have the largest in vivo RSA 3D migration (>0.78 mm). This study enabled the first corroboration between tailored musculoskeletal and FE model predictions with in vivo RSA cup migration. Investigation of additional patient-matched CT, gait, and RSA examinations may allow further development and validation of FE models.

Analysis of modified surface topographies of titanium-based hip implants using finite element method

BACKGROUND

In order to ensure the proper function of the cementless hip implant, the connection between the femoral bone and the implant has to be as strong as possible. According to experimental studies, implants with a rough surface reduce micro-movements between femoral bone and implant, which helps form a stronger connection between them.

OBJECTIVE

The goal of this study was to analyze how half-cylinder surface topographies of different diameter values affect shear stress values and their distribution on the surface of the hip implant and trabecular femoral bone.

METHODS

Nine models with different half-cylinder diameter values (200 μm, 400 μm, and 500 μm) and distances between half-cylinders were created for the analysis using the finite element method. Each model consisted of three layers: implant, trabecular, and cortical femoral bone.

RESULTS

For all three diameter values, the highest shear stress value, for the implant layer, was located after the first half-cylinder on the side where force was defined. For the trabecular bone, the first half-cylinder was under lower amounts of shear stress.

CONCLUSION

If we only consider shear stress values, we can say that models with 400 μm and 500 μm diameter values are a better choice than models with 100 μm diameter values.

Acetabular Implant Finite Element Simulation with Customised Estimate of Bone Properties

The goal of the study is to analyse the strength and stability of a system comprising the pelvis and a customised implant under functional loads using the finite element method. We considered a technique for assessing the elastic properties of bone tissue via computer tomography, constructing finite element models of pelvic bones and a customised endoprosthesis based on the initial geometric models obtained from the National Medical Research Centre for Oncology n.a. N.N. Blokhin (Moscow, Russia). A series of calculations were carried out for the stress-strain state of the biomechanical system during walking, as well as at maximum loads when ascending and descending stairs. The analysis provided conclusions about the strength and stability of the studied device.

Predicting fracture in the proximal humerus using phase field models

Proximal humerus impacted fractures are of clinical concern in the elderly population. Prediction of such fractures by CT-based finite element methods encounters several major obstacles such as heterogeneous mechanical properties and fracture due to compressive strains. We herein propose to investigate a variation of the phase field method (PFM) embedded into the finite cell method (FCM) to simulate impacted humeral fractures in fresh frozen human humeri. The force-strain response, failure loads and the fracture path are compared to experimental observations for validation purposes. The PFM (by means of the regularization parameter ℓ₀) is first calibrated by one experiment and thereafter used for the prediction of the mechanical response of two other human fresh frozen humeri. All humeri are fractured at the surgical neck and strains are monitored by Digital Image Correlation (DIC). Experimental strains in the elastic regime are reproduced with good agreement (R²=0.726), similarly to the validated finite element method (Dahan et al., 2022). The failure pattern and fracture evolution at the surgical neck predicted by the PFM mimic extremely well the experimental observations for all three humeri. The maximum relative error in the computed failure loads is 3.8%. To the best of our knowledge this is the first method that can predict well the experimental compressive failure pattern as well as the force-strain relationship in proximal humerus fractures.

Finite element analysis of a hybrid corrugated hip implant for stability and loading during gait phases

Implants used in total hip replacements (THR) exhibit high failure rates and up to a decade of operational life. These surgical failures could be mainly attributed to the improper positioning, post-surgical stability and loading, of the implants during different phases of the gait. Typically, revised surgeries are suggested within a few years of hip implantation, which requires multiple femoral drilling operations to remove an existing implant, and to install a new implant. The pain and trauma associated with such procedures are also challenging with the existing hip implants. In this work, we designed a novel corrugated hip implant with innovative dimensioning as per ASTM standards, and grooves for directed insertion and removal (using a single femoral drilling and positioning operation). Biocompatible titanium alloy (Ti6Al4V) was chosen as the implant material, and the novel implant was placed into a femur model through a virtual surgery to study its stability and loading during a dynamic gait cycle. A detailed mesh convergence study was conducted to select a computationally accurate finite element (FE) mesh. Tight fit and frictional fit attachment conditions were simulated, and the gait induced displacements and stresses on the implant, cortical and cancellous bone sections were characterized. During walking, the implant encountered the maximum von-Mises stress of 254.97 MPa at the femoral head. The analyses indicated low micro-motions (i.e., approximately 7μm) between the femur and implant, low stresses at the implant and bone within elastic limits, and uniform stress distribution, which unlike existing hip implants, would be indispensable for bone growth and implant stability enhancement, and also for reducing implant wear.

Can neck fractures in proximal humeri be predicted by CT-based FEA?

BACKGROUND

Proximal humeri fractures at anatomical and surgical neck (∼5% and ∼50% incidence respectively) are frequent in elderly population. Yet, neither in-vitro experiments nor CT-based finite element analyses (CTFEA) have investigated these in depth. Herein we enhance (Dahan et al., 2019) (addressing anatomical neck fractures) by more experiments and specimens, accounting for surgical neck fractures and explore CTFEA's prediction of humeri mechanical response and yield force.

METHODS

Four fresh frozen human humeri were tested in a new experimental configuration inducing surgical neck fractures. Digital image correlation (DIC) provided strains and displacements on humeri surfaces and used to validate CTFEA predictions. CTFEA were enhanced herein to improve the accuracy at the proximal neck: A cortical bone mapping (CBM) algorithm was implemented to overcome insufficient scanning resolution, and a new trabecular material mapping was investigated.

RESULTS

The new experimental setting induced impacted surgical neck fractures in all humeri. Excellent DIC to CTFEA correlation in strains was obtained at the shaft (slope 0.984, R²=0.99) and a fair agreement (slope 0.807, R²=0.73) at the neck. CBM algorithm had worsened the correlation, whereas the new material mapping had a negligible influence. Yield loads predictions improved considerably when trabecular yielding (maximum principal strain criterion) was considered instead of surface cortical yielding.

DISCUSSION

CTFEA well predicts strains on the shaft and reasonably well on the neck. This enhances former conclusions by past studies conducted using SGs, now also evident by DIC. Yield load prediction for surgical neck fractures (involving crushing of trabecular bone) is predicted better by trabecular failure laws rather than cortex ones. Further FEA studies using trabecular orthotropic constitutive models and failure laws are warrant.

A Critical Review of the Design, Manufacture, and Evaluation of Bone Joint Replacements for Bone Repair

With the change of people’s living habits, bone trauma has become a common clinical disease. A large number of bone joint replacements is performed every year around the world. Bone joint replacement is a major approach for restoring the functionalities of human joints caused by bone traumas or some chronic bone diseases. However, the current bone joint replacement products still cannot meet the increasing demands and there is still room to increase the performance of the current products. The structural design of the implant is crucial because the performance of the implant relies heavily on its geometry and microarchitecture. Bionic design learning from the natural structure is widely used. With the progress of technology, machine learning can be used to optimize the structure of bone implants, which may become the focus of research in the future. In addition, the optimization of the microstructure of bone implants also has an important impact on its performance. The widely used design algorithm for the optimization of bone joint replacements is reviewed in the present study. Regarding the manufacturing of the implant, the emerging additive manufacturing technique provides more room for the design of complex microstructures. The additive manufacturing technique has enabled the production of bone joint replacements with more complex internal structures, which makes the design process more convenient. Numerical modeling plays an important role in the evaluation of the performance of an implant. For example, theoretical and numerical analysis can be carried out by establishing a musculoskeletal model to prepare for the practical use of bone implants. Besides, the in vitro and in vivo testing can provide mechanical properties of bone implants that are more in line with the implant recipient’s situation. In the present study, the progress of the design, manufacture, and evaluation of the orthopedic implant, especially the joint replacement, is critically reviewed.

Which experimental procedures influence the apparent proximal femoral stiffness? A parametric study

Background

Experimental validation is the gold standard for the development of FE predictive models of bone. Employing multiple loading directions could improve this process. To capture the correct directional response of a sample, the effect of all influential parameters should be systematically considered. This study aims to determine the impact of common experimental parameters on the proximal femur’s apparent stiffness.

Methods

To that end, a parametric approach was taken to study the effects of: repetition, pre-loading, re-adjustment, re-fixation, storage, and μCT scanning as random sources of uncertainties, and loading direction as the controlled source of variation in both stand and side-fall configurations. Ten fresh-frozen proximal femoral specimens were prepared and tested with a novel setup in three consecutive sets of experiments. The neutral state and 15-degree abduction and adduction angles in both stance and fall configurations were tested for all samples and parameters. The apparent stiffness of the samples was measured using load-displacement data from the testing machine and validated against marker displacement data tracked by DIC cameras.

Results

Among the sources of uncertainties, only the storage cycle affected the proximal femoral apparent stiffness significantly. The random effects of setup manipulation and intermittent μCT scanning were negligible. The 15^∘ deviation in loading direction had a significant effect comparable in size to that of switching the loading configuration from neutral stance to neutral side-fall.

Conclusion

According to these results, comparisons between the stiffness of the samples under various loading scenarios can be made if there are no storage intervals between the different load cases on the same samples. These outcomes could be used as guidance in defining a highly repeatable and multi-directional experimental validation study protocol.

Reporting checklist for verification and validation of finite element analysis in orthopedic and trauma biomechanics

Finite element analysis (FEA) has become a fundamental tool for biomechanical investigations in the last decades. Despite several existing initiatives and guidelines for reporting on research methods and results, there are still numerous issues that arise when using computational models in biomechanical investigations. According to our knowledge, these problems and controversies lie mainly in the verification and validation (V&V) process as well as in the set-up and evaluation of FEA. This work aims to introduce a checklist including a report form defining recommendations for FEA in the field of Orthopedic and Trauma (O&T) biomechanics. Therefore, a checklist was elaborated which summarizes and explains the crucial methodologies for the V&V process. In addition, a report form has been developed which contains the most important steps for reporting future FEA. An example of the report form is shown, and a template is provided, which can be used as a uniform basis for future documentation. The future application of the presented report form will show whether serious errors in biomechanical investigations using FEA can be minimized by this checklist. Finally, the credibility of the FEA in the clinical area and the scientific exchange in the community regarding reproducibility and exchangeability can be improved.

Influence of a metaphyseal sleeve on the stress-strain state of a bone-tumor implant system in the distal femur: an experimental and finite element analysis

Background

Aseptic loosening of distal femoral tumor implants significantly correlates with the resection length. We designed a new “sleeve” that is specially engaged in the metaphysis at least 5 cm proximal to the knee joint line to preserve as much bone stock as possible. This study investigates the influence of a metaphyseal sleeve on the stress-strain state of a bone tumor implant system in the distal femur.

Methods

Cortex strains in intact and implanted femurs were predicted with finite element (FE) models. Moreover strains were experimentally measured in a cadaveric femur with and without a sleeve and stem under an axial compressive load of 1000 N. The FE models, which were validated by linear regression, were used to investigate the maximal von Mises stress and the implanted-to-intact (ITI) ratios of strain in the femur with single-legged stance loading under immediate postoperative and osseointegration conditions.

Results

Good agreement was noted between the experimental measurements and numerical predictions of the femoral strains (coefficient of determination (R²) ≥ 0.95; root-mean-square error (RMSE%) ≈ 10%). The ITI ratios for the metaphysis were between 13 and 28% and between 10 and 21% under the immediate postoperative and osseointegration conditions, respectively, while the ITI ratios for the posterior and lateral cortices around the tip of the stem were 110% and 119% under the immediate-postoperative condition, respectively, and 114% and 101% under the osseointegration condition, respectively. The maximal von Mises stresses for the implanted femur were 113.8 MPa and 43.41 MPa under the immediate postoperative and osseointegration conditions, which were 284% and 47% higher than those in the intact femur (29.6 MPa), respectively.

Conclusions

This study reveals that a metaphyseal sleeve may cause stress shielding relative to the intact femur, especially in the distal metaphysis. Stress concentrations might mainly occur in the posterior cortex around the tip of the stem. However, stress concentrations may not be accompanied by periprosthetic fracture under the single-legged stance condition.

Comparative analysis of the biomechanical behavior of two different design metaphyseal-fitting short stems using digital image correlation

BACKGROUND

The progressive evolution in hip replacement research is directed to follow the principles of bone and soft tissue sparing surgery. Regarding hip implants, a renewed interest has been raised towards short uncemented femoral implants. A heterogeneous group of short stems have been designed with the aim to approximate initial, post-implantation bone strain to the preoperative levels in order to minimize the effects of stress shielding. This study aims to investigate the biomechanical properties of two distinctly designed femoral implants, the TRI-LOCK Bone Preservation Stem, a shortened conventional stem and the Minima S Femoral Stem, an even shorter and anatomically shaped stem, based on experiments and numerical simulations. Furthermore, finite element models of implant-bone constructs should be evaluated for their validity against mechanical tests wherever it is possible. In this work, the validation was performed via a direct comparison of the FE calculated strain fields with their experimental equivalents obtained using the digital image correlation technique.

RESULTS

Design differences between Trilock BPS and Minima S femoral stems conditioned different strain pattern distributions. A distally shifting load distribution pattern as a result of implant insertion and also an obvious decrease of strain in the medial proximal aspect of the femur was noted for both stems. Strain changes induced after the implantation of the Trilock BPS stem at the lateral surface were greater compared to the non-implanted femur response, as opposed to those exhibited by the Minima S stem. Linear correlation analyses revealed a reasonable agreement between the numerical and experimental data in the majority of cases.

CONCLUSION

The study findings support the use of DIC technique as a preclinical evaluation tool of the biomechanical behavior induced by different implants and also identify its potential for experimental FE model validation. Furthermore, a proximal stress-shielding effect was noted after the implantation of both short-stem designs. Design-specific variations in short stems were sufficient to produce dissimilar biomechanical behaviors, although their clinical implication must be investigated through comparative clinical studies.

Strain shielding for cemented hip implants

BACKGROUND

Long-term survival of hip implants is of increasing relevance due to the rising life expectancy. The biomechanical effect of strain shielding as a result of implant insertion may lead to bone resorption, thus increasing risk for implant loosening and periprosthetic fractures. Patient-specific quantification of strain shielding could assist orthopedic surgeons in choosing the biomechanically most appropriate prosthesis.

METHODS

Validated quantitative CT-based finite element models of five femurs in intact and implanted states were considered to propose a systematic algorithm for strain shielding quantification. Three different strain measures were investigated and the most appropriate measure for strain shielding quantification is recommended. It is used to demonstrate a practical femur-specific implant selection among three common designs.

FINDINGS

Strain shielding measures demonstrated similar trends in all Gruen zones except zone 1, where the volumetric strain measure differed from von-Mises and maximum principal strains. The volumetric strain measure is in better agreement with clinical bone resorption records. It is also consistent with the biological mechanism of bone remodeling so it is recommended for strain shielding quantification. Applying the strain shielding algorithm on three different implants for a specific femur suggests that the collared design is preferable. Such quantitative biomechanical input is valuable for practical patient specific implant selection.

INTERPRETATION

Volumetric strain should be considered for strain shielding examination. The presented methodology may potentially enable patient-specific pre-operative strain shielding evaluation so to minimize strain shielding. It should be further used in a longitudinal study so to correlate between strain shielding predictions and clinical bone resorption.

Patient-Specific Bone Multiscale Modelling, Fracture Simulation and Risk Analysis-A Survey

This paper provides a starting point for researchers and practitioners from biology, medicine, physics and engineering who can benefit from an up-to-date literature survey on patient-specific bone fracture modelling, simulation and risk analysis. This survey hints at a framework for devising realistic patient-specific bone fracture simulations. This paper has 18 sections: Section 1 presents the main interested parties; Section 2 explains the organzation of the text; Section 3 motivates further work on patient-specific bone fracture simulation; Section 4 motivates this survey; Section 5 concerns the collection of bibliographical references; Section 6 motivates the physico-mathematical approach to bone fracture; Section 7 presents the modelling of bone as a continuum; Section 8 categorizes the surveyed literature into a continuum mechanics framework; Section 9 concerns the computational modelling of bone geometry; Section 10 concerns the estimation of bone mechanical properties; Section 11 concerns the selection of boundary conditions representative of bone trauma; Section 12 concerns bone fracture simulation; Section 13 presents the multiscale structure of bone; Section 14 concerns the multiscale mathematical modelling of bone; Section 15 concerns the experimental validation of bone fracture simulations; Section 16 concerns bone fracture risk assessment. Lastly, glossaries for symbols, acronyms, and physico-mathematical terms are provided.

The Effect of External Fixator Configurations on the Dynamic Compression Load: An Experimental and Numerical Study

(1) Objective: External fixation systems are commonly used by surgeons to ensure stabilization and consolidation of bone fractures, especially in patients who are at high risk for systematic complications. Both rigid and elastic external fixations are important in the fracture healing process. This study aims to evaluate the behavior of the Orthofix Limb Reconstruction System (LRS)® in the dynamic compression mode. (2) Methods: Experimental and numerical setups were developed using a simplified model of a human tibia which consisted of a nylon bar with a diameter of 30 mm. The bone callus was included in both setups by means of a load cell-based system, which consisted of two carbon epoxy laminated composite plates with a final stiffness of 220 N/mm. The system was evaluated experimentally and numerically, considering different numbers of pins and comparing distances between the external fixator frame and the bone, achieving a good correlation between experimental and numerical results. (3) Results: The results identified and quantified the percental load transferred to the fracture and its sensibility to the distance between the external fixator and bone. Additionally, LRS locking stiffness was evaluated which resulted from the clamp-rail clearances. The results show that the blocking effects of the free clamp movement are directly related to the fixator configuration and are responsible for changes in the amount of load that crosses the bone callus. (4) Conclusions: From the biomechanical point of view, the results suggest that the average bending span of Schanz pins and the weights of the patients should be included into clinical studies of external fixators comparisons purpose.