Cortical bone failure mechanisms during screw pullout

Impact of NaOH based perfusion-decellularization protocol on mechanical resistance of structural bone allografts

INTRODUCTION

To mitigate the post-operative complication rates associated with massive bone allografts, tissue engineering techniques have been employed to decellularize entire bones through perfusion with a sequence of solvents. Mechanical assessment was performed in order to compare conventional massive bone allografts and perfusion/decellularized massive bone allografts.

MATERIAL AND METHODS

Ten porcine femurs were included. Five were decellularized by perfusion. The remaining 5 were left untreated as the "control" group. Biomechanical testing was conducted on each bone, encompassing five different assessments: screw pull-out, 3-points bending, torsion, compression and Vickers indentation.

RESULTS

Under the experimental conditions of this study, all five destructive tested variables (maximum force until screw pull-out, maximum elongation until screw pull-out, energy to pull out the screw, fracture resistance in flexion and maximum constrain of compression) were statistically significantly superior in the control group. All seven nondestructive variables (Young's modulus in flexion, Young's modulus in shear stress, Young's modulus in compression, Elastic conventional limit in compression, lengthening to rupture in compression, resilience in compression and Vickers Hardness) showed no significant difference.

DISCUSSION

Descriptive statistical results suggest a tendency for the biomechanical characteristics of decellularized bone to decrease compared with the control group. However, statistical inferences demonstrated a slight significant superiority of the control group with destructive mechanical stresses. Nondestructive mechanical tests (within the elastic phase of Young's modulus) were not significantly different.

Finite element modeling of fracture compression by compression plates

Dynamic compression plating is a common type of fracture fixation used to compress between bone fragments. The quality of compression across the fracture is important for postoperative stability and primary bone healing. Compression quality may be affected by surgical variations in plate prebend, screw location, screw torque, fracture gap, and implant material. Computational modeling provides a tool for systematically examining these factors, and for visualizing the mechanisms involved. The purpose of this study was to develop a finite element model of dynamic compression plating that includes screw insertion under torque control, establish model credibility through sensitivity analyses and experimental validation, and use the model to examine the effects of surgical variables on fracture compression and postoperative stability. Model-predicted compressive pressures had good agreement with corresponding synthetic bones experiments under a variety of conditions. Models demonstrated that introducing a 1.5 or 3 mm plate prebend (using a 4.5 mm narrow LCP plate) eliminated gapping at the far cortex, which is consistent with clinical recommendations. However, models also revealed that plate prebend led to sharp decreases in fracture compressive force, exceeding 80% in some cases. A 1.5 mm plate prebend resulted in the most uniform pressures across the fracture. Testing of a simplified model form used in previous computational modeling studies showed large inaccuracies for constructs with plate prebend. This study provides the first experimentally validated computational models of dynamic compression plate fracture fixation, and reveals important effects of plate prebend and fracture gap on fracture compression quality.

Reconstruction with a double-constrained implant design after complex shoulder extra-articular resection

BACKGROUND

Approximately, one-third of patients with tumors of proximal humerus will require an extra-articular resection to achieve oncologic margins. This procedure yields poor functional outcomes with a considerable rate of revisions. Unconstrained implants are prone to instability hindering also function of the elbow and hand, whereas constrained shoulder reconstructions suffer from early aseptic loosening of the glenoid component due to bone overload. The purpose of this study was to develop a constrained implant suitable for extra-articular resection with loss of function in deltoid and rotator cuff, which would provide both stability and passive motion, whilst also decreasing the risk of aseptic loosening of the glenoid component.

METHODS

In cooperation with Czech Technical University in Prague, we devised an implant consisting of two constrained joints in series connected by a dumbbell piece. The biomechanical analysis showed a reduction of load transfer to the glenoid component with a torque of 8.6 Nm capable of generating an 865-N pulling force on bone screw to just 0.07 Nm, hence shielding the glenoid component from undesired forces and decreasing the risk of aseptic loosening. Three patients with extra-articular resection with a total loss of function of both rotator cuff and deltoid muscle received this type of reconstruction. The average follow-up was 16 months.

RESULTS

The surgical technique is straightforward. The surgery took 175 min on average with average blood loss of 516 ml. There were no surgical- or implant-related complications. All three patients were pain-free and had a stable shoulder joint after the reconstruction. All had fully functional elbow, wrist, and hand joints. The average Musculoskeletal Tumor Society (MSTS) score was 21/30 (70%). All patients were pleased with the results.

CONCLUSION

The presented innovative implant design has demonstrated to be a promising alternative for reconstruction in these challenging cases.

Decellularized vascularized bone grafts as therapeutic solution for bone reconstruction: A mechanical evaluation

INTRODUCTION

Large bone defects are challenging for surgeons. Available reimplanted bone substitutes can't properly restore optimal function along and long term osteointegration of the bone graft. Bone substitute based on the perfusion-decellularization technique seem to be interesting in order to overcome these limitations. We present here an evaluation of the biomechanics of the bones thus obtained.

MATERIAL AND METHODS

Two decellularization protocols were chosen for this study. One using Sodium Dodecyl Sulfate (SDS) (D1) and one using NaOH and H2O2 (D2). The decellularization was performed on porcine forearms. We then carried out compression, three-point bending, indentation and screw pull-out tests on each sample. Once these tests were completed, we compared the results obtained between the different decellularization protocols and with samples left native.

RESULTS

The difference in the means was similar between the tests performed on bones decellularized with the SDS protocol and native bones for pull-out test: +1.4% (CI95% [-10.5%- 12.4%]) of mean differences when comparing Native vs D1, compression -14.9% (CI95% [-42.7%- 12.5%]), 3-point bending -5.7% (CI95% [-22.5%- 11.1%]) and indentation -10.8% (CI95% [-19.5%- 4.6%]). Bones decellularized with the NaOH protocol showed different results from those obtained with the SDS protocol or native bones during the pull-out screw +40.7% (CI95% [24.3%- 57%]) for Native vs D2 protocol and 3-point bending tests +39.2% (CI95% [13.7%- 64.6%]) for Native vs D2 protocol. The other tests, compression and indentation, gave similar results for all our samples.

CONCLUSION

Vascularized decellularized grafts seem to be an interesting means for bone reconstruction. Our study shows that the decellularization method affects the mechanical results of our specimens. Some methods seem to limit these alterations and could be used in the future for bone decellularization.

Understanding the deformation gradient in Abaqus and key guidelines for anisotropic hyperelastic user material subroutines (UMATs)

This tutorial paper provides a step-by-step guide to developing a comprehensive understanding of the different forms of the deformation gradient used in Abaqus, and outlines a number of key issues that must be considered when developing an Abaqus user defined material subroutine (UMAT) in which the Cauchy stress is computed from the deformation gradient. Firstly, we examine the "classical" forms of global and local deformation gradients. We then show that Abaqus/Standard does not use the classical form of the local deformation gradient when continuum elements are used, and we highlight the important implications for UMAT development. We outline the key steps that must be implemented in developing an anisotropic fibre-reinforced hyperelastic UMAT for use with continuum elements and local orientation systems. We also demonstrate that a classical local deformation gradient is provided by Abaqus/Standard if structural (shell and membrane) elements are used, and by Abaqus/Explicit for all element types. We emphasise, however, that the majority of biomechanical simulations rely on the use of continuum elements with a local coordinate system in Abaqus/Standard, and therefore the development of a hyperelastic UMAT requires an in-depth and precise understanding of the form of the non-classical deformation gradient provided as input by Abaqus. Several worked examples and case studies are provided for each section, so that the details and implications of the form of the deformation gradient can be fully understood. For each worked example in this tutorial paper the source files and code (Abaqus input files, UMATs, and Matlab script files) are provided, allowing the reader to efficiently explore the implications of the form of the deformation gradient in the development of a UMAT.

Homogenized finite element models can accurately predict screw pull-out in continuum materials, but not in porous materials

BACKGROUND AND OBJECTIVE

Bone screw fixation can be estimated with several test methods such as insertion torque, pull-out, push-in and bending tests. A basic understanding of the relationship between screw fixation and bone microstructure is still lacking. Computational models can help clarify this relationship. The objective of the paper is to evaluate homogenized finite element (hFE) models of bone screw pull-out.

METHODS

Experimental pull-out tests were performed on three materials: two polyurethane (PU) foams having a porous microstructure, and a high density polyethylene (HDPE) which is a continuum material. Forty-five titanium pedicle screws were inserted to 10, 20, and 30 mm in equally sized blocks of all three materials (N = 5/group). Pull-out characteristics i.e. stiffness (S), yield force (F_y), peak pull-out force (F_ult) and displacement at F_ult (d_ult) were measured. hFE models were created replicating the experiments. The screw was modeled as a rigid body and 5 mm axial displacement was applied to the head of the screw. Simulations were performed evaluating two different conditions at the bone-screw interface; once in which the screw fitted the pilot hole exactly ("free-stressed") and once in which interface stresses resulting from the insertion process were taken into account ("pre-stressed").

RESULTS

The simulations representing the pre-stressed condition in HDPE matched the experimental data well; S, F_y, and F_ult differed less than 11%, 2% and 0.5% from the experimental data, respectively, whereas d_ult differed less than 16%. The free-stressed simulations were less accurate, especially stiffness (158% higher than the pre-stressed condition) and d_ult (30% lower than pre-stressed condition) were affected. The simulations representing PU did not match the experiments well. For the 20 mm insertion depth, S, F_y and F_ult differed by more than 104%, 89% and 66%, respectively from the experimental values. Agreement did not improve for 10 and 30 mm insertion depths.

CONCLUSIONS

We found that hFE models can accurately quantify screw pull-out in continuum materials such as HDPE, but not in materials with a porous structure, such as PU. Pre-stresses in the bone induced by the insertion process cannot be neglected and need to be included in the hFE simulations.

Biomechanical models: key considerations in study design

This manuscript summarizes presentations of a symposium on key considerations in design of biomechanical models at the 2019 Basic Science Focus Forum of the Orthopaedic Trauma Association. The first section outlines the most important characteristics of a high-quality biomechanical study. The second section considers choices associated with designing experiments using finite element modeling versus synthetic bones versus human specimens. The third section discusses appropriate selection of experimental protocols and finite element analyses. The fourth section considers the pros and cons of use of biomechanical research for implant design. Finally, the fifth section examines how results from biomechanical studies can be used when clinical evidence is lacking or contradictory. When taken together, these presentations emphasize the critical importance of biomechanical research and the need to carefully consider and optimize models when designing a biomechanical study.

An Equivalent Constitutive Model of Cancellous Bone With Fracture Prediction

To simulate the mechanical and fracture behaviors of cancellous bone in three anatomical directions and to develop an equivalent constitutive model. Microscale extended finite element method (XFEM) models of a cancellous specimen were developed with mechanical behaviors in three anatomical directions. An appropriate abaqus macroscale model replicated the behavior observed in the microscale models. The parameters were defined based on the intermediate bone material properties in the anatomical directions and assigned to an equivalent nonporous specimen of the same size. The equivalent model capability was analyzed by comparing the micro- and macromodels. The hysteresis graphs of the microscale model show that the modulus is the same in loading and unloading; similar to the metal plasticity models. The strength and failure strains in each anatomical direction are higher in compression than in tension. The microscale models exhibited an orthotropic behavior. Appropriate parameters of the cast iron plasticity model were chosen to generate macroscale models that are capable of replicating the observed microscale behavior of cancellous bone. Cancellous bone is an orthotropic material that can be simulated using a cast iron plasticity model. This model is capable of replicating the microscale behavior in finite element (FE) analysis simulations without the need for individual trabecula, leading to a reduction in computational resources without sacrificing model accuracy. Also, XFEM of cancellous bone compared to traditional finite element method proves to be a valuable tool to predict and model the fractures in the bone specimen.

Subject-specific FE models of the human femur predict fracture path and bone strength under single-leg-stance loading

Hip fractures are a major health problem with high socio-economic costs. Subject-specific finite element (FE) models have been suggested to improve the fracture risk assessment, as compared to clinical tools based on areal bone mineral density, by adding an estimate of bone strength. Typically, such FE models are limited to estimate bone strength and possibly the fracture onset, but do not model the fracture process itself. The aim of this study was to use a discrete damage approach to simulate the full fracture process in subject-specific femur models under stance loading conditions. A framework based on the partition of unity finite element method (PUFEM), also known as XFEM, was used. An existing PUFEM framework previously used on a homogeneous generic femur model was extended to include a heterogeneous material description together with a strain-based criterion for crack initiation. The model was tested on two femurs, previously mechanically tested in vitro. Our results illustrate the importance of implementing a subject-specific material distribution to capture the experimental fracture pattern under stance loading. Our models accurately predicted the fracture pattern and bone strength (1% and 5% error) in both investigated femurs. This is the first study to simulate complete fracture paths in subject-specific FE femur models and it demonstrated how discrete damage models can provide a more complete picture of fracture risk by considering both bone strength and fracture toughness in a subject-specific fashion.

Prediction of failure in cancellous bone using extended finite element method

The objective of our study is to develop extended finite element method models of cancellous bone specimens that are capable of accurately predicting the onset and propagation of cracks under mechanical loading. In order to do so, previously published three-point bending test results of a single trabecula were replicated using two different extended finite element method approaches, namely, elastic-plastic-fracture and elastic-fracture that considered different configurations of the elasto-plastic properties of bone from which the best approach to fit the experimental data was identified. The behavior of a single trabecula was then used in 2D extended finite element method models to quantify the strength of the trabecular tissue of the forearm along three perpendicular anatomical axes. The results revealed that the elastic-plastic-fracture model better represented the experimental data in the model of a single trabecula. Considering the 2D trabecular specimens, the elastic fracture model predicted higher strength than the elastic-plastic-fracture model and there was no difference in stiffness between the two models. In general, the specimens exhibited higher failure strain and more ductile behavior in compression than in tension. In addition, strength and stiffness were found to be higher in tension than compression on average. It can be concluded that with proper parameters, extended finite element method is capable of simulating the ductile behavior of cancellous bone. The models are able to quantify the tensile strength of trabecular tissue in the various anatomical directions reporting an increased strength in the longitudinal direction of forearm cancellous bone tissue. Extended finite element method of cancellous bone proves to be a valuable tool to predict the mechanical characteristics of cancellous bones as a function of the microstructure.

Modeling age-related changes in the mechanical behavior of the fracture-fixated human tibia bone during healing

The evolutionary healing phenomenon of fractured tibia bone was investigated by comparing the bio-mechanical response of the human tibia following fracture fixation for two ranges of patient ages, when a body weight pressure (BWP) is applied. Three-dimensional finite element models have been developed by adopting the biomechanical characteristics of cortical and trabecular tibia bones, and considering the time-varying callus properties during the healing process for the two patients. The stress and strain levels generated within the fractured tibia bone by the screw tight fit during the assembly process revealed its dependence on the bone stiffness that degrades with age. They have an impact on primary stability of the implants prior to the osseointegration. The gap capacity to resist and allow a gradual BWP load transfer, through the callus for the tibia bone models, was analyzed. In fact, from 10 weeks after surgery, the callus allowed the BWP transfer for young patients, which guarantees sufficient structure stabilization of the fractured tibia. However, an insufficient load was transferred to the fracture gap for the old patient, even beyond 16 weeks, which delayed the bone consolidation.

Finite element investigation into the use of carbon fibre reinforced PEEK laminated composites for distal radius fracture fixation implants

Carbon fibre reinforced PEEK (CF/PEEK) laminates provide mechanical advantages over homogenous metal osteo-synthesis implants, e.g. radiolucency, fatigue strength and strength to weight ratio. Implants can be designed with custom anisotropic material properties, thus enabling the engineer to tailor the overall stiffness of the implant to the specific loading conditions it will experience in vivo. In the current study a multi-scale computational investigation of idealised distal radius fracture fixation plate (DRP) is conducted. Physiological loading conditions are applied to macro-scale finite element models of DRPs. The mechanical response is compared for several CF/PEEK laminate layups to examine the effect of ply layup design. The importance of ply orientation in laminated DRPs is highlighted. A high number of 0° plies near the outer surfaces results in a greater bending strength while the addition of 45° plies increases the torsional strength of the laminates. Intra-laminar transverse tensile failure is predicted as the primary mode of failure. A micro-mechanical analysis of the CF/PEEK microstructure uncovers the precise mechanism under-lying intra-laminar transverse tensile crack to be debonding of the PEEK matrix from carbon fibres. Plastic strains in the matrix material are not sufficiently high to result in ductile failure of the matrix. The findings of this study demonstrate the significant challenge in the design and optimisation of fibre reinforced laminated composites for orthopaedic applications, highlighting the importance of multi-scale modelling for identification of failure mechanisms.

How a pilot hole size affects osteosynthesis at the screw-bone interface under immediate loading

An inappropriate pilot hole size (PHS) is one of several factors that affects the stiffness of the screw-bone fixation. The present study uses finite element models to investigate the effect of varying the PHS on the biomechanical environment of the screw-bone interface of the fractured bone, after the screw insertion and under the immediate body weight pressure (BWP). Four PHS from 71% up to 85% of the screw external diameter (SED) were considered for analysis. A non linear material behaviour of the bone with ductile damage properties was used in the study. To validate the numerical models, an experimental pull-out test was carried out using a synthetic bone. The results of the insertion process demonstrated that the relatively smaller holes (71% and 75.5% of SED) increased the insertion torque value within the recommended level, caused more bone radial extension deformation and maximized the contact area between the bone threads and the screw, in comparison to the PHS higher than 80% of SED. Under the immediate BWP after osteosynthesis, the stress level exceeds the elastic limit and becomes high enough to initiate the ductile damage of the bone. Also, enlarging PHS from 71% to 75.5% of SED increased the bone microdisplacement at the screw-bone interface from 75 up to 100 μm, and that reduced the stiffness of the fixation.

Understanding the Basics of Computational Models in Orthopaedics: A Nonnumeric Review for Surgeons

Computational models represent more than just finite element analysis, a term that many clinicians may know and globally apply. Over the past 30 years, many published studies have addressed clinically relevant orthopaedic questions with speed and precision by using a wide variety of computational approaches. Given such a wide spectrum of techniques, clinicians often do not have a full understanding of the methods used to create models and therefore do not appreciate the strengths, weaknesses, and potential pitfalls of published results. The short, nonnumeric summaries of the methodologies employed for various computational approaches presented here can help address this issue.

Reasons why dynamic compression plates are inferior to locking plates in osteoporotic bone: a finite element explanation

While locking plate fixation is becoming increasingly popular for complex and osteoporotic fractures, for many indications compression plating remains the standard choice. This study compares the mechanical behaviour of the more recent locking compression plate (LCP) device, with the traditional dynamic compression plates (DCPs) in bone of varying quality using finite element modelling. The bone properties considered include orthotropy, inhomogeneity, cortical thinning and periosteal apposition associated with osteoporosis. The effect of preloads induced by compression plating was included in the models. Two different fracture scenarios were modelled: one with complete reduction and one with a fracture gap. The results show that the preload arising in DCPs results in large principal strains in the bone all around the perimeter of the screw hole, whereas for LCPs large principal strains occur primarily on the side of the screw proximal to the load. The strains within the bone produced by the two screw types are similar in healthy bone with a reduced fracture gap; however, the DCP produces much larger strains in osteoporotic bone. In the presence of a fracture gap, the DCP results in a considerably larger region with high tensile strains and a slightly smaller region with high compressive strains. These findings provide a biomechanical basis for the reported improved performance of locking plates in poorer bone quality.

Self-tapping ability of carbon fibre reinforced polyetheretherketone suture anchors

An experimental and computational investigation of the self-tapping ability of carbon fibre reinforced polyetheretherketone (CFR-PEEK) has been conducted. Six CFR-PEEK suture anchor designs were investigated using PEEK-OPTIMA® Reinforced, a medical grade of CFR-PEEK. Experimental tests were conducted to investigate the maximum axial force and torque required for self-taping insertion of each anchor design. Additional experimental tests were conducted for some anchor designs using pilot holes. Computational simulations were conducted to determine the maximum stress in each anchor design at various stages of insertion. Simulations also were performed to investigate the effect of wall thickness in the anchor head. The maximum axial force required to insert a self-tapping CFR-PEEK suture anchor did not exceed 150 N for any anchor design. The maximum torque required to insert a self-tapping CFR-PEEK suture anchor did not exceed 0.8 Nm. Computational simulations reveal significant stress concentrations in the region of the anchor tip, demonstrating that a re-design of the tip geometry should be performed to avoid fracture during self-tapping, as observed in the experimental component of this study. This study demonstrates the ability of PEEK-OPTIMA Reinforced suture anchors to self-tap polyurethane foam bone analogue. This provides motivation to further investigate the self-tapping ability of CFR-PEEK suture anchors in animal/cadaveric bone. An optimised design for CFR-PEEK suture anchors offers the advantages of radiolucency, and mechanical properties similar to bone with the ability to self-tap. This may have positive implications for reducing surgery times and the associated costs with the procedure.

Are plasticity models required to predict relative risk of lag screw cut-out in finite element models of trochanteric fracture fixation?

Using a finite element model of unstable trochanteric fracture stabilized with a sliding hip screw, the benefits of two plasticity-based formulations, Drucker-Prager and crushable foam, were evaluated and compared to the commonly used linear elastic model of trabecular bone in order to predict the relative risk of lag screw cut-out for five distinct load cases. The crushable foam plasticity formulation leads to a much greater strain localization, in comparison to the other two models, with large plastic strains in a localized region. The plastic zone predicted with Drucker-Prager is relatively more diffuse. Linear elasticity associated with a minimum principal strain criterion provides the smallest volume of elements susceptible to yielding for all loading modes. The region likely to undergo plastic deformation, as predicted by the linear elastic model, is similar to that obtained from plasticity-based formulations, which indicates that this simple criterion provides an adequate estimate of the risk of cut-out.

Evaluation of a new approach for modelling the screw–bone interface in a locking plate fixation: A corroboration study

Computational modelling of the screw–bone interface in fracture fixation constructs is challenging. While incorporating screw threads would be a more realistic representation of the physics, this approach can be computationally expensive. Several studies have instead suppressed the threads and modelled the screw shaft with fixed conditions assumed at the screw–bone interface. This study assessed the sensitivity of the computational results to modelling approaches at the screw–bone interface. A new approach for modelling this interface was proposed, and it was tested on two locking screw designs in a diaphyseal bridge plating configuration. Computational models of locked plating and far cortical locking constructs were generated and compared to in vitro models described in prior literature to corroborate the outcomes. The new approach led to closer agreement between the computational and the experimental stiffness data, while the fixed approach led to overestimation of the stiffness predictions. Using the new approach, the pattern of load distribution and the magnitude of the axial forces, experienced by each screw, were compared between the locked plating and far cortical locking constructs. The computational models suggested that under more severe loading conditions, far cortical locking screws might be under higher risk of screw pull-out than the locking screws. The proposed approach for modelling the screw–bone interface can be applied to any fixation involved application of screws.