Finite element modeling of the free boundary effect on gyroid additively manufactured samples.

There is a significant need for models that can capture the mechanical behavior of complex porous lattice architectures produced by 3D printing. The free boundary effect is an experimentally observed behavior of lattice architectures including the gyroid triply periodic minimal surface where the number of unit cell repeats has been shown to influence the mechanical performance of the lattice. The purpose of this study is to use finite element modeling to investigate how architecture porosity, unit cell size, and sample size dictate mechanical behavior. Samples with varying porosity and increasing number of unit cells (relative to sample size) were modeled under an axial compressive load to determine the effective modulus. The finite element model captured the free boundary effect and captured experimental trends in the structure's modulus. The findings of this study show that samples with higher porosity are more susceptible to the impact of the free boundary effect and in some samples, the modulus can be 20% smaller in samples with smaller numbers of unit cell repeats within a given sample boundary. The outcomes from this study provide a deeper understanding of the gyroid structure and the implications of design choices including porosity, unit cell size, and overall sample size.

3D Printing Materials and Technologies for Orthopaedic Applications.

SUMMARY: 3D printing technologies have evolved tremendously over the last decade for uses in orthopaedic surgical applications, including being used to manufacture implants for spine, upper extremity, foot and ankle, oncologic, and traumatic reconstructions. Materials used for 3D-printed orthopaedic devices include metals, degradable and nondegradable polymers, and ceramic composites. There are 2 primary advantages for use of 3D printing technologies for orthopaedics: first, the ability to create complex porous lattices that allow for osseointegration and improved implant stability and second, the enablement of complex geometric designs allowing for patient-specific devices based on preoperative imaging. Given continually evolving technology, and the relatively early stage of the materials and 3D printers themselves, the possibilities for continued innovation in orthopaedics are great.

Novel 3D printed lattice structure titanium cages evaluated in an ovine model of interbody fusion.

Background: The use of intervertebral cages within the interbody fusion setting is ubiquitous. Synthetic cages are predominantly manufactured using materials such as Ti and PEEK. With the advent of additive manufacturing techniques, it is now possible to spatially vary complex 3D geometric features within interbody devices, enabling the devices to match the stiffness of native tissue and better promote bony integration. To date, the impact of surface porosity of additively manufactured Ti interbody cages on fusion outcomes has not been investigated. Thus, the objective of this work was to determine the effect of implant endplate surface and implant body architecture of additive manufactured lattice structure titanium interbody cages on bony fusion. Methods: Biomechanical, microcomputed tomography, static and dynamic histomorphometry, and histopathology analyses were performed on twelve functional spine units obtained from six sheep randomly allocated to body lattice or surface lattice groups. Results: Nondestructive kinematic testing, microcomputed tomography analysis, and histomorphometry analyses of the functional spine units revealed positive fusion outcomes in both groups. These data revealed similar results in both groups, with the exception of bone-in-contact analysis, which revealed significantly improved bone-in-contact values in the body lattice group compared to the surface lattice group. Conclusion: Both additively manufactured porous titanium cage designs resulted in increased fusion outcomes as compared to PEEK interbody cage designs as illustrated by the nondestructive kinematic motion testing, static and dynamic histomorphometry, microcomputed tomography, and histopathology analyses. While both cages provided for similar functional outcomes, these data suggest boney contact with an interbody cage may be impacted by the nature of implant porosity adjacent to the vertebral endplates.

Corrosion Resistance of 3D Printed Ti6Al4V Gyroid Lattices with Varying Porosity.

Corrosion of medical implants is a possible failure mode via induced local inflammatory effects, systemic deposition and corrosion related mechanical failure. Cyclic potentiodynamic polarisation (CPP) testing was utilized to evaluate the effect of increased porosity (60% and 80%) and decreased wall thickness in gyroid lattice structures on the electrochemical behaviour of LPBF Ti6Al4V structures. The use of CPP allowed for the landmarks of breakdown potential, resting potential and vertex potential to be analysed, as well as facilitating the construction of Tafel plots and qualitative Goldberg analysis. The results indicated that 60% gyroid samples were most susceptible to the onset of pitting corrosion when compared to 80% gyroid and solid samples. This was shown through decreased breakdown and vertex potentials and were found to correlate to increased lattice surface area to void volume ratio. Tafel plots indicated that despite the earlier onset of pitting corrosion, both gyroid test groups displayed lower rates of corrosion per year, indicating a lower severity of corrosion. This study highlighted inherent tradeoffs between lattice optimisation and corrosion behaviour with a potential parabolic link between void volume, surface area and corrosion being identified. This potential link is supported by 60% gyroid samples having the lowest breakdown potentials, but investigation into other porosity ranges is suggested to support the hypothesis. All 3D printed materials studied here showed breakdown potentials higher than ASTM F2129's suggestion of 800 mV for evaluation within the physiological environment, indicating that under static conditions pitting and crevice corrosion should not initiate within the body.

Effects of 3D printed surface topography and normal force on implant expulsion.

3D printing is a critical method for manufacturing metallic implants as it enables direct fabrication of intricate geometries and porous structures inaccessible to other manufacturing methods. Some common 3D printed porous structures are strut based (e.g. octet truss), triply periodic minimal surfaces (TPMS) (e.g. gyroid) or randomized (e.g. stochastic). When designed to be on the surface of bone interfacing implants, the surface porous region impacts short-term adhesion and friction, ultimately affecting implant stability prior to and during long-term osseointegration. In many orthopedic procedures, expulsion resistance is an essential design requirement, to prevent the risk of the implant migrating from the implantation site. While expulsion tests are universal, they are a poorly understood method to examine the bone-implant interface in determining the performance of an orthopedic implant. In this foundational study, we examine the expulsion behavior of metallic samples in synthetic Sawbone with systematically varied surface topography at increasing applied normal forces. The applied normal force and size of the sample were shown to have the strongest influence on expulsion force followed by surface structure. Compared to a polished sample control, certain 3D printed surface structures are up to 10x more expulsion resistant and should be considered in implants where prevention of implant migration before and during osseointegration is critical. Nonlinear relationships were discovered that reveal "crossover" in expulsion resistance as a function of applied load revealing that the ranking of the relative expulsion resistance of different samples can depend on the normal force selected. This new fundamental understanding has broad implications on both the design and potential standardized regulatory testing of textured orthopedic implants with tailored topologies.

Computational and image processing methods for analysis and automation of anatomical alignment and joint spacing in reconstructive surgery.

PURPOSE: Reconstructive surgeries to treat a number of musculoskeletal conditions, from arthritis to severe trauma, involve implant placement and reconstructive planning components. Anatomically matched 3D-printed implants are becoming increasingly patient-specific; however, the preoperative planning and design process requires several hours of manual effort from highly trained engineers and clinicians. Our work mitigates this problem by proposing algorithms for the automatic re-alignment of unhealthy anatomies, leading to more efficient, affordable, and scalable treatment solutions. METHODS: Our solution combines global alignment techniques such as iterative closest points with novel joint space refinement algorithms. The latter is achieved by a low-dimensional characterization of the joint space, computed from the distribution of the distance between adjacent points in a joint. RESULTS: Experimental validation is presented on real clinical data from human subjects. Compared with ground truth healthy anatomies, our algorithms can reduce misalignment errors by 22% in translation and 19% in rotation for the full foot-and-ankle and 37% in translation and 39% in rotation for the hindfoot only, achieving a performance comparable to expert technicians. CONCLUSION: Our methods and histogram-based metric allow for automatic and unsupervised alignment of anatomies along with techniques for global alignment of complex arrangements such as the foot-and-ankle system, a major step toward a fully automated and data-driven re-positioning, designing, and diagnosing tool.

High-strength, porous additively manufactured implants with optimized mechanical osseointegration.

Optimization of porous titanium alloy scaffolds designed for orthopedic implants requires balancing mechanical properties and osseointegrative performance. The tradeoff between scaffold porosity and the stiffness/strength must be optimized towards the goal to improve long term load sharing while simultaneously promoting osseointegration. Osseointegration into porous titanium implants covering a wide range of porosity (0%-90%) and manufactured by laser powder bed fusion (LPBF) was evaluated with an established ovine cortical and cancellous defect model. Direct apposition and remodeling of woven bone was observed at the implant surface, as well as bone formation within the interstices of the pores. A linear relationship was observed between the porosity and benchtop mechanical properties of the scaffolds, while a non-linear relationship was observed between porosity and the ex vivo cortical bone-implant interfacial shear strength. Our study supports the hypothesis of porosity dependent performance tradeoffs, and establishes generalized relationships between porosity and performance for design of topological optimized implants for osseointegration. These results are widely applicable for orthopedic implant design for arthroplasty components, arthrodesis devices such as spinal interbody fusion implants, and patient matched implants for treatment of large bone defects.

Effect of surface topography on in vitro osteoblast function and mechanical performance of 3D printed titanium.

Critical-sized defects remain a significant challenge in orthopaedics. 3D printed scaffolds are a promising treatment but are still limited due to inconsistent osseous integration. The goal of the study is to understand how changing the surface roughness of 3D printed titanium either by surface treatment or artificially printing rough topography impacts the mechanical and biological properties of 3D printed titanium. Titanium tensile samples and discs were printed via laser powder bed fusion. Roughness was manipulated by post-processing printed samples or by directly printing rough features. Experimental groups in order of increasing surface roughness were Polished, Blasted, As Built, Sprouts, and Rough Sprouts. Tensile behavior of samples showed reduced strength with increasing surface roughness. MC3T3 pre-osteoblasts were seeded on discs and analyzed for cellular proliferation, differentiation, and matrix deposition at 0, 2, and 4 weeks. Printing roughness diminished mechanical properties such as tensile strength and ductility without clear benefit to cell growth. Roughness features were printed on mesoscale, unlike samples in literature in which roughness on microscale demonstrated an increase in cell activity. The data suggest that printing artificial roughness on titanium scaffold is not an effective strategy to promote osseous integration.

Functional repair of critically sized femoral defects treated with bioinspired titanium gyroid-sheet scaffolds.

Despite the innate ability for bone to remodel and repair, its regeneration has a limit. In these cases of critically sized bone defects (CSBD), the bone deficit must be repaired using reconstructive techniques that support immediate load bearing and encourage bone bridging across the defect. High-strength porous titanium implants offer a solution for treatment of CSBD in which the scaffold can support physiological loads, provide a matrix to guide ingrowth, and carry graft materials and/or biologics. Fabrication of titanium meta-materials via additive manufacturing (AM) has unlocked the potential to modulate mechanical and biological performance to achieve a combination of properties previously unachievable. Meta-material scaffolds with topology based on triply periodic minimal surfaces (TPMS) have gained increasing interest for use in biomedical applications due to their bioinspired nature. Despite enthusiasm for TPMS-based titanium scaffolds due to their high strength to stiffness ratio, high permeability, and curvature similar to trabecular bone, there is little preclinical evidence to support their in vivo response in bone. The present study sought to evaluate the performance of gyroid-sheet titanium scaffolds produced via AM to repair a critically size femoral cortical bone defect in rats. Empty gyroid-sheet scaffolds were shown to repair segmental defects with up to 38% of torsional strength and 54% torsional stiffness of the intact femur (control) at 12-weeks. Gyroid-sheet scaffolds carrying recombinant bone morphogenic protein-2 demonstrated bridging bone growth across the length of the defect, with torsional strength and stiffness superior to that of the intact controls.

High-Strength Hydrogel Attachment through Nanofibrous Reinforcement.

The repair of a cartilage lesion with a hydrogel requires a method for long-term fixation of the hydrogel in the defect site. Attachment of a hydrogel to a base that allows for integration with bone can enable long-term fixation of the hydrogel, but current methods of forming bonds to hydrogels have less than a tenth of the shear strength of the osteochondral junction. This communication describes a new method, nanofiber-enhanced sticking (NEST), for bonding a hydrogel to a base with an adhesive shear strength three times larger than the state-of-the-art. An example of NEST is described in which a nanofibrous bacterial cellulose sheet is bonded to a porous base with a hydroxyapatite-forming cement followed by infiltration of the nanofibrous sheet with hydrogel-forming polymeric materials. This approach creates a mineralized nanofiber bond that mimics the structure of the osteochondral junction, in which collagen nanofibers extend from cartilage into a mineralized region that anchors cartilage to bone.

Reinforcement and Fatigue of a Bioinspired Mineral-Organic Bioresorbable Bone Adhesive.

Bioresorbable bone adhesives may provide remarkable clinical solutions in areas ranging from fixation and osseointegration of permanent implants to the direct healing and fusion of bones without permanent fixation hardware. Mechanical properties of bone adhesives are critical for their successful application in vivo. Reinforcement of a tetracalcium phosphate-phosphoserine bone adhesive is investigated using three degradable reinforcement strategies: poly(lactic-co-glycolic) (PLGA) fibers, PLGA sutures, and chitosan lactate. All three approaches lead to higher compressive strengths of the material and better fatigue performance. Reinforcement with PLGA fibers and chitosan lactate results in a 100% probability of survival of samples at 20 MPa maximum compressive stress level, which is almost ten times higher compared to compressive loads observed in the intervertebral discs of the spine in vivo. High adhesive shear strength of 5.1 MPa is achieved for fiber-reinforced bone adhesive by tuning the surface architecture of titanium samples. Finally, biological and biomechanical performance of the fiber-reinforced adhesive is evaluated in a rabbit distal femur osteotomy model, showing the potential of the bone adhesive for clinical use.

3D printing of high-strength, porous, elastomeric structures to promote tissue integration of implants.

Despite advances in biomaterials research, there is no ideal device for replacing weight-bearing soft tissues like menisci or intervertebral discs due to poor integration with tissues and mechanical property mismatch. Designing an implant with a soft and porous tissue-contacting structure using a material conducive to cell attachment and growth could potentially address these limitations. Polycarbonate urethane (PCU) is a soft and tough biocompatible material that can be 3D printed into porous structures with controlled pore sizes. Porous biomaterials of appropriate chemistries can support cell proliferation and tissue ingrowth, but their optimal design parameters remain unclear. To investigate this, porous PCU structures were 3D-printed in a crosshatch pattern with a range of in-plane pore sizes (0 to 800 µm) forming fully interconnected porous networks. Printed porous structures had ultimate tensile strengths ranging from 1.9 to 11.6 MPa, strains to failure ranging from 300 to 486%, Young's moduli ranging from 0.85 to 12.42 MPa, and porosity ranging from 13 to 71%. These porous networks can be loaded with hydrogels, such as collagen gels, to provide additional biological support for cells. Bare PCU structures and collagen-hydrogel-filled porous PCU support robust NIH/3T3 fibroblast cell line proliferation over 14 days for all pore sizes. Results highlight PCU's potential in the development of tissue-integrating medical implants.

From Patient to Procedure: The Process of Creating a Custom 3D-Printed Medical Device for Foot and Ankle Pathology.

Three-dimensional (3D) printing technology has advanced greatly over the past decade and is being used extensively throughout the field of medicine. Several orthopaedic surgery specialties have demonstrated that 3D printing technology can improve patient care and physician education. Foot and ankle pathology can be complex as the 3D anatomy can be challenging to appreciate. Deformity can occur in several planes simultaneously and bone defects either from previous surgery or trauma can further complicate surgical correction. Three-dimensional printing technology provides an avenue to tackle the challenges associated with complex foot and ankle pathology. A basic understanding of how these implants are designed and made is important for surgeons as this technology is becoming more widespread and the clinical applications continue to grow within foot and ankle surgery.Levels of Evidence: Level V.

Compressive anisotropy of sheet and strut based porous Ti-6Al-4V scaffolds.

Porous metallic scaffolds show promise in orthopedic applications due to favorable mechanical and biological properties. In vivo stress conditions on orthopedic implants are complex, often including multiaxial loading across off axis orientations. In this study, unit cell orientation was rotated in the XZ plane of a strut-based architecture, Diamond Crystal, and two sheet-based, triply periodic minimal surface (TPMS) architectures, Schwartz D and Gyroid. Sheet-based architectures exhibited higher peak compressive strength, yield strength and strain at peak stress than the strut-based architecture. All three topologies demonstrated an orientational dependence in mechanical properties. There was a greater degree of anisotropy (49%) in strut-based architecture than in either TPMS architectures (18-21%). These results support the superior strength and advantageous isotropic mechanical properties of sheet-based TPMS architectures relative to strut-based architectures, as well as highlighting the importance of considering anisotropic properties of lattice scaffolds for use in tissue engineering.

Fatigue behavior of As-built selective laser melted titanium scaffolds with sheet-based gyroid microarchitecture for bone tissue engineering.

Selective laser melting (SLM) has enabled the production of porous titanium structures with biological and mechanical properties that mimic bone for orthopedic applications. These porous structures have a reduced effective stiffness which leads to improved mechanotransduction between the implant and bone. Triply periodic minimal surfaces (TMPS), specifically the sheet-based gyroid structures, have improved compressive fatigue resistance due lack of stress concentrations. Sheet-based gyroid microarchitectures also have high surface area, permeability, and zero mean curvature. This study examines the effects of the gyroid microarchitectural design in parallel with SLM parameters on structure and function of as-built titanium alloy (Ti6Al4V ELI) scaffolds. Scaffold design was varied by varying unit cell size and wall thickness to produce scaffolds with porosity within the range of trabecular bone (50-90%). Manufacturer's default and refined laser parameters were used to examine the effect of input energy density on mechanical properties. Scaffolds exhibited a stretching-dominated deformation behavior under both compressive and tensile loading, and porosity dependent stiffness and strength. Internal void defects were observed within the walls of the gyroids structure, serving as sites for crack initiation leading to failure. Refinement of laser parameters resulted in increased compressive and tensile fatigue behavior, particularly for thicker walled gyroid microarchitectures, while thinner walls showed no significant change. The observed properties of as-built gyroid sheet microarchitectures indicates that these structures have potential for use in bone engineering applications. Furthermore, these results highlight the importance of parallel design and processing optimization for complex sheet-based porous structures produced via SLM. STATEMENT OF SIGNIFICANCE: Selective laser melting (SLM) is an additive manufacturing technology which produces complex porous scaffolds for orthopedic applications. Titanium alloy scaffolds with novel sheet-based gyroid microarchitectures were produced via SLM and evaluated for mechanical performance including fatigue behavior. Gyroid structures are function based topologies have been hypothesized to be promising for tissue engineering scaffolds due to the high surface area to volume ratio, zero mean curvature, and high permeability. This paper presents the effects of scaffold design and processing parameters in parallel, a novel study in the field on bone tissue scaffolds produced via additive manufacturing. Additionally, the comparison of compressive and tensile behavior of scaffolds presented is important in characterizing behavior and failure mechanisms of porous metals which undergo complex loading in orthopedic applications.

The effect of surface topography and porosity on the tensile fatigue of 3D printed Ti-6Al-4V fabricated by selective laser melting.

Additive manufacturing (3D printing) is emerging as a key manufacturing technique in medical devices. Selective laser melted (SLM) Ti-6Al-4V implants with interconnected porosity have become widespread in orthopedic applications where porous structures encourage bony ingrowth and the stiffness of the implant can be tuned to reduce stress shielding. The SLM technique allows high resolution control over design, including the ability to introduce porosity with spatial variations in pore size, shape, and connectivity. This study investigates the effect of construct design and surface treatment on tensile fatigue behavior of 3D printed Ti-6Al-4V. Samples were designed as solid, solid with an additional surface porous layer, or fully porous, while surface treatments included commercially available rotopolishing and SILC cleaning. All groups were evaluated for surface roughness and tested in tension to failure under monotonic and cyclic loading profiles. Surface treatments were shown to reduce surface roughness for all sample geometries. However, only fatigue behavior of solid samples was improved for treated as compared to non-treated surfaces Irrespective of surface treatment and resulting surface roughness, the fatigue strength of 3D printed samples containing bulk or surface porosity was approximately 10% of the ultimate tensile strength of identical 3D printed porous material. This study highlights the relative effect of surface treatment in solid and porous printed samples and the inherent decrease in fatigue properties of 3D printed porous samples designed for osseointegration.

Bioinspired Mineral-Organic Bioresorbable Bone Adhesive.

Bioresorbable bone adhesives have potential to revolutionize the clinical treatment of the human skeletal system, ranging from the fixation and osteointegration of permanent implants to the direct healing and fusion of bones without permanent fixation hardware. Despite an unmet need, there are currently no bone adhesives in clinical use that provide a strong enough bond to wet bone while possessing good osteointegration and bioresorbability. Inspired by the sandcastle worm that creates a protective tubular shell around its body using a proteinaceous adhesive, a novel bone adhesive is introduced, based on tetracalcium phosphate and phosphoserine, that cures in minutes in an aqueous environment and provides high bone-to-bone adhesive strength. The new material is measured to be 10 times more adhesive than bioresorbable calcium phosphate cement and 7.5 times more adhesive than non-resorbable poly(methyl methacrylate) bone cement, both of which are standard of care in the clinic today. The bone adhesive also demonstrates chemical adhesion to titanium approximately twice that of its adhesion to bone, unlocking the potential for adherence to metallic implants during surrounding bony incorporation. Finally, the bone adhesive is shown to demonstrate osteointegration and bioresorbability over a 52-week period in a critically sized distal femur defect in rabbits.

Design and Structure-Function Characterization of 3D Printed Synthetic Porous Biomaterials for Tissue Engineering.

3D printing is now adopted for use in a variety of industries and functions. In biomedical engineering, 3D printing has prevailed over more traditional manufacturing methods in tissue engineering due to its high degree of control over both macro- and microarchitecture of porous tissue scaffolds. However, with the improved flexibility in design come new challenges in characterizing the structure-function relationships between various architectures and both mechanical and biological properties in an assortment of clinical applications. Presently, the field of tissue engineering lacks a comprehensive body of literature that is capable of drawing meaningful relationships between the designed structure and resulting function of 3D printed porous biomaterial scaffolds. This work first discusses the role of design on 3D printed porous scaffold function and then reviews characterization of these structure-function relationships for 3D printed synthetic metallic, polymeric, and ceramic biomaterials.

Thermo-mechanical behavior and structure of melt blown shape-memory polyurethane nonwovens.

New processing methods for shape-memory polymers allow for tailoring material properties for numerous applications. Shape-memory nonwovens have been previously electrospun, but melt blow processing has yet to be evaluated. In order to determine the process parameters affecting shape-memory behavior, this study examined the effect of air pressure and collector speed on the mechanical behavior and shape-recovery of shape-memory polyurethane nonwovens. Mechanical behavior was measured by dynamic mechanical analysis and tensile testing, and shape-recovery was measured by unconstrained and constrained recovery. Microstructure changes throughout the shape-memory cycle were also investigated by micro-computed tomography. It was found that increasing collector speed increases elastic modulus, ultimate strength and recovery stress of the nonwoven, but collector speed does not affect the failure strain or unconstrained recovery. Increasing air pressure decreases the failure strain and increases rubbery modulus and unconstrained recovery, but air pressure does not influence recovery stress. It was also found that during the shape-memory cycle, the connectivity density of the fibers upon recovery does not fully return to the initial values, accounting for the incomplete shape-recovery seen in shape-memory nonwovens. With these parameter to property relationships identified, shape-memory nonwovens can be more easily manufactured and tailored for specific applications.