Experimental Validation of an ITAP Numerical Model and the Effect of Implant Stem Stiffness on Bone Strain Energy

A scoring system to evaluate stability of percutaneous osseointegrated implants for transfemoral amputation with validation in the ITAP clinical trial

Percutaneous osseointegrated implants for individuals with lower limb amputation can increase mobility, reduce socket related pain, and improve quality of life. It would be useful to have an evaluation method to assess the interface between bone and implant. We assessed outpatient radiographs from the Intraosseous Transcutaneous Amputation Prosthesis clinical trial using an interface scoring system which summed and weighted equally measures of implant collar cortical ongrowth and radiolucency along the implant stem/bone interface. Radiographs from 12 participants with unilateral transfemoral amputations (10 males, 2 females, mean age = 43.2, SD = 7.4 years) in the clinical trial from cohort I (implanted in 2008/09) or cohort II (implanted in 2013/14) were collated (mean image span = 7.2, SD = 2.4 years), scale normalised, zoned, and measured in a repeatable way. Interface scores were calculated and then compared to clinical outcomes. Explanted participants received the lowest interface scores. A higher ratio of stem to residuum and shorter residuum's produced better interface scores and there was an association (weak correlation) between participants with thin cortices and the lowest interface scores. A tapered, cemented, non curved stem may provide advantageous fixation while stem alignment did not appear critical. In summary, the interface score successfully demonstrated a non-invasive evaluation of percutaneous osseointegrated implants interfaces when applied to the Intraosseous Transcutaneous Amputation Prosthesis clinical trial. The clinical significance of this work is to identify events leading to aseptic or septic implant removal and contribute to clinical guidelines for monitoring rehabilitation, design and surgical fixation choices.

Reliable in vitro method for the evaluation of the primary stability and load transfer of transfemoral prostheses for osseointegrated implantation

Osseointegrated transfemoral prostheses experience aseptic complications with an incidence between 3% and 30%. The main aseptic risks are implant loosening, adverse bone remodeling, and post-operative periprosthetic fractures. Implant loosening can either be due to a lack of initial (primary) stability of the implant, which hinders bone ingrowth and therefore prevents secondary stability, or, in the long-term, to the progressive resorption of the periprosthetic bone. Post-operative periprosthetic fractures are most often caused by stress concentrations. A method to simultaneously evaluate the primary stability and the load transfer is currently missing. Furthermore, the measurement errors are seldom reported in the literature. In this study a method to reliably quantify the bone implant interaction of osseointegrated transfemoral prostheses in terms of primary stability and load transfer was developed, and its precision was quantified. Micromotions between the prosthesis and the host bone and the strains on the cortical bone were measured on five human cadaveric femurs with a typical commercial osseointegrated implant. To detect the primary stability of the implant and the load transfer, cyclic loads were applied, simulating the peak load during gait. Digital Image Correlation was used to measure displacements and bone strains simultaneously throughout the test. Permanent migrations and inducible micromotions were measured (three translations and three rotations), while, on the same specimen, the full-field strain distribution on the bone surface was measured. The repeatability tests showed that the devised method had an intra-specimen variability smaller than 6 μm for the translation, 0.02 degrees for the rotations, and smaller than 60 microstrain for the strain distribution. The inter-specimen variability was larger than the intra-specimen variability due to the natural differences between femurs. Altogether, the measurement uncertainties (intrinsic measurement errors, intra-specimen repeatability and inter-specimen variability) were smaller than critical levels of biomarkers for adverse remodelling and aseptic loosening, thus allowing to discriminate between stable and unstable implants, and to detect critical strain magnitudes in the host bone. In conclusion, this work showed that it is possible to measure the primary stability and the load transfer of an osseointegrated transfemoral prosthesis in a reliable way using a combination of mechanical testing and DIC.

In vitro and in silico methods for the biomechanical assessment of osseointegrated transfemoral prostheses: a systematic review

The amputee population according to the World-Health-Organization is about 40 million. However, there is a high abandon rate of socket prostheses for the lower limb (25%-57%). The direct connection between the external prosthesis and the patient's bone makes osseointegrated prostheses for transfemoral amputees advantageous (e.g., improvement of the motor control) compared to socket prostheses, which are currently the gold standard. However, similarly to other uncemented prostheses, the osseointegrated ones are at risk of aseptic loosening and adverse bone remodelling caused by stress-shielding. The preclinical assessment of these prostheses has already been evaluated using different methods which did not provide unanimous and comparable evidence. To compare data from different investigations, a clear and detailed overview of the methods used to assess the performance is necessary. In this review 17 studies investigating the primary stability, stress shielding and stress concentration of osseointegrated transfemoral prostheses are examined. Primary stability consists in the biomechanical stability upon implant insertion. Primary stability is assessed measuring extraction force (either with a pull-out or a push-out test) and micromotion at the interface between the implant and the host bone with LVDT (in vitro test) or numerical models. Stress-shielding causes adaptive changes in the bone density around metal implants, and thus in the bone strength and stiffness. Stress-shielding is assessed with strain gauges or numerical models measuring the load transfer and the strain distribution on the surface of the femur, and between the implant and the bone respectively. Stress concentration can lead to the formation of cracks inside the bone, resulting in fractures. The stress concentration is assessed measuring the load transfer and the strain energy density at the interface between the implant and the bone, using numerical models. As a result, a global view and consensus about the methods are missing from all these tests. Indeed, different setup and loading scenario were used in the in vitro test, while different model parameters (e.g., bone properties) were used in the numerical models. Once the preclinical assessment method is established, it would be important to define thresholds and acceptance criteria for each of the possible failure scenarios investigated.

Parametric Design of Hip Implant With Gradient Porous Structure

Patients who has been implanted with hip implant usually undergo revision surgery. The reason is that high stiff implants would cause non-physiological distribution loadings, which is also known as stress shielding, and finally lead to bone loss and aseptic loosening. Titanium implants are widely used in human bone tissues; however, the subsequent elastic modulus mismatch problem has become increasingly serious, and can lead to stress-shielding effects. This study aimed to develop a parametric design methodology of porous titanium alloy hip implant with gradient elastic modulus, and mitigate the stress-shielding effect. Four independent adjustable dimensions of the porous structure were parametrically designed, and the Kriging algorithm was used to establish the mapping relationship between the four adjustable dimensions and the porosity, surface-to-volume ratio, and elastic modulus. Moreover, the equivalent stress on the surface of the femur was optimized by response surface methodology, and the optimal gradient elastic modulus of the implant was obtained. Finally, through the Kriging approximation model and optimization results of the finite element method, the dimensions of each segment of the porous structure that could effectively mitigate the stress-shielding effect were determined. Experimental results demonstrated that the parameterized design method of the porous implant with gradient elastic modulus proposed in this study increased the strain value on the femoral surface by 17.1% on average. Consequently, the stress-shielding effect of the femoral tissue induced by the titanium alloy implant was effectively mitigated.

Improving the Stability of a Hemipelvic Prosthesis Based on Bone Mineral Density Screw Channel and Prosthesis Optimization Design

In pelvic reconstruction surgery, the hemipelvic prosthesis can cause significant changes in stress distribution due to its high stiffness, and its solid structure is not suitable for osseointegration. The purpose of this study was to identify a novel bone mineral density screw channel and design the structure of the prosthesis so as to improve the distribution of stress, promote bone growth, and enhance the biomechanical properties of the prosthesis. The mechanical characteristics of bone mineral density screw and traditional screw were compared by finite element analysis method, and redesigned by topology optimization. The direction of the newly proposed screw channel was the posterolateral entrance of the auricular surface, ending at the contralateral sacral cape. Compared to the original group, the maximum stress of the optimized prosthesis was decreased by 24.39%, the maximum stress of the sacrum in the optimized group was decreased by 27.23%, and the average strain energy density of the sacrum in the optimized group was increased by 8.43%. On the surface of screw and connecting plate, the area with micromotion more than 28 μm is reduced by 12.17%. On the screw surface, the area with micromotion more than 28 μm is reduced by 22.9%. The newly determined screw channel and optimized prosthesis design can effectively improve the biomechanical properties of a prosthesis and the microenvironment of osseointegration. This method can provide a reference for the fixation of prostheses in clinical pelvic reconstruction.

Design of Porous Metal Block Augmentation to Treat Tibial Bone Defects in Total Knee Arthroplasty Based on Topology Optimization

Metal block augmentation, which is used for the treatment of tibial bone defects in total knee arthroplasty, with high stiffness will cause significant alteration in stress distribution, and its solid structure is not suitable for osseointegration. This study aimed to design a porous block to reduce weight, promote bone ingrowth, and improve its biomechanical performance. The metal block augmentation technique was applied to finite element models of tibial bone defects. Minimum compliance topology optimization subject to volume fraction combined with the porous architecture was adopted to redesign the block. Biomechanical changes compared with the original block were analyzed by finite element analysis. The stress distribution of the block and proximal tibia was recorded. The strain energy density of the proximal tibia was obtained. The newly designed block realized 40% weight reduction. The maximum stress in the optimized block decreased by 11.6% when compared with the solid one. The maximum stress of the proximal tibia in the optimized group increased by 18.6%. The stress of the anterior, medial, and posterior parts of the proximal medial tibia in the optimized group was significantly greater than that in the original group (all p < 0.05). The optimized block could effectively improve the biomechanical performance between the block and the bone. The presented method might provide a reference for the design of customized three-dimensional printed prostheses.

Numerical simulations on periprosthetic bone remodeling: a systematic review

BACKGROUND AND OBJECTIVE

The aim of the present study was to review the literature concerning the analysis of periprosthetic bone remodeling through finite element (FE) simulation.

METHODS

A systematic review was conducted on 9 databases, taking into account a ten-year time period (from 2009 until 2020). The inclusion criteria were: articles published in English, publication date after 2009, full text articles, articles containing the keywords both in the abstract and in the title. The articles were classified through the following parameters: dimensionality of the simulation, modelling of the bone-prosthesis interface, output parameters, type of simulated prosthesis, bone remodeling algorithm.

RESULTS

Sixty-seven articles were included in the study. Femur and tooth were the most evaluated bone segment (respectively 41.8% and 29.9%). The 55.2% of the evaluated articles used a bonded bone-prosthesis interface, 73% used 3D simulations, 67.2% of the articles (45 articles) evaluate the bone remodeling by the bone density variation. At last, 59.7% of the articles employed algorithms based on a specific remodeling function.

CONCLUSIONS

Increasing interest in the bone remodeling FE analysis in different bone segments emerged from the review, and heterogeneous solutions were adopted. An optimal balance between computational cost and accuracy is needed to accurately simulate the bone remodeling phenomenon in the post-operative period.

Use of a dentin bonding agent for the fixation of strain gauges on bone

Capturing deformation of bone using strain gauges is still a common approach in biomechanics. So far, the bone surfaces had to be sanded and cleaned using various solvents prior to attaching the sensors using cyanoacrylate. This work proposes a novel approach employing common bonding procedures to enamel and dentin as used in restorative dentistry. Bovine ribs were obtained and osteotomies were created following clinical protocols for dental implant insertion. A unidirectional strain gauge was attached to the outer surface of the bone specimens using the steps of acid etching with phosphoric acid, application of a three-bottle dentin adhesive and light curing. Emerging strains were then captured during implant insertion as well as during static loading of the implants in a universal testing machine. The method described here allowed for the reliable fixation of strain gauges on unaltered bone surfaces. Both, compressive and tensile strains emerging in the surrounding of the dental implants could be captured.

The effect of functionally graded materials on bone remodeling around osseointegrated trans-femoral prostheses

Osseointegrated trans-femoral fixations have been used as alternatives for conventional sockets in recent years. Despite numerous advantages, the dissimilarity of the mechanical properties between bone and implant has led to issues in periprosthetic bone adaptation. This study aims to address these issues by proposing fixations made of functionally graded materials (FGMs). The computational study of bone remodeling was performed by linking a bone remodeling algorithm to the finite element analysis. The 3D model of the femur was created by computerized tomography (CT) scan images, and a Titanium fixture, along with nine Titanium/Hydroxyapatite FGM fixtures, were modeled. The analyses revealed evident advantages for the FGM fixtures over the conventionally used Titanium fixtures. Furthermore, it was shown that the gradation direction considerably affects the bone adaptation procedure. The results showed that using a radial FGM with low-stiffness material in the outer layer and less metal composition significantly improves the bone remodeling behavior.

Anisotropic electrospun honeycomb polycaprolactone scaffolds: Elaboration, morphological and mechanical properties

Tissue engineering technology requires porous scaffolds, based on biomaterials, which have to mimic as closely as possible the morphological and anisotropic mechanical properties of the native tissue to substitute. Anisotropic fibrous scaffolds fabricated by template-assisted electrospinning are investigated in this study. Fibers of electrospun Polycaprolactone (PCL) were successfully arranged spatially into honeycomb structures by using well-shaped 3D micro-architected metal collectors. Fibrous scaffolds present 2 × 4 mm² wide elementary patterns with low and high fiber density areas. Distinct regions of the honeycomb patterns were analyzed through SEM images revealing different fiber diameters with specific fiber orientation depending on the regions of interest. Tensile test experiments were carried out with an optical observation of the local deformation at the pattern scale, allowing the determination and analysis, at small and large deformation, of the axial and transverse local strains. The honeycomb patterned mats showed significantly different mechanical properties along the two orthogonal directions probing an anisotropic ratio of 4.2. Stress relaxation test was performed on scaffolds at 15% of strain. This measurement pointed out the low contribution of the viscosity of about 20% in the mechanical response of the scaffold. An orthotropic linear elastic model was consequently proposed to characterize the anisotropic behavior of the produced patterned membranes. This new versatile method to produce architected porous materials, adjustable to several polymers and structures, will provide appealing benefits for soft regenerative medicine application and the development of custom-made scaffolds.