Chemical Polishing of Additively Manufactured, Porous, Nickel-Titanium Skeletal Fixation Plates.

Nickel-titanium (NiTi) alloys have shown promise for a variety of biomedical applications because of their unique properties of shape memory, superelasticity, and low modulus of elasticity (Young's modulus). Nevertheless, NiTi bulk components cannot be easily machined (e.g., CNC, rolling, grinding, casting, or press molding) due to their thermomechanical sensitivity as well as inherent superelasticity and shape memory. Thus, powder bed fusion (PBF) additive manufacturing has been used to successfully fabricate NiTi medical devices that match the geometric and mechanical needs of a particular patient's condition. However, NiTi PBF fabrication leaves unmelted particles from the source powder adhered to external surfaces, which cause minor dimensional inaccuracy, increase the risk of mechanical failure, and once loose, may irritate or inflame surrounding tissues. Therefore, there is a need to develop a chemical polishing (cleaning) technique to remove unmelted powder from the surfaces of PBF-fabricated implants, especially from inner surfaces that are difficult to access with mechanical polishing tools. This technique is especially useful for highly porous devices printed at high resolution. In this study, a chemical polishing method utilizing HF/HNO3 solution was used to remove loosely attached (i.e., unmelted) powder particles from surfaces of porous, skeletal fixation plates manufactured by PBF AM. It was observed that 7 min of polishing in an HF/HNO3 solution comprising 7.5 HF: 50 HNO3: 42.5 H2O enabled successful removal of all relatively loose and unmelted powder particles. A microcomputed tomography study examination found that the volumetric accuracy of the polished skeletal fixation plates was ±10% compared with the computer-aided design (CAD) model from which it was rendered. This postprocessing chemical polishing protocol is also likely to be useful for removing loose powder, while maintaining CAD model accuracy and mechanical stability for other complexly shaped, porous, three-dimensional (3D), printed NiTi devices.

Biomechanics of Additively Manufactured Metallic Scaffolds-A Review.

This review paper is related to the biomechanics of additively manufactured (AM) metallic scaffolds, in particular titanium alloy Ti6Al4V scaffolds. This is because Ti6Al4V has been identified as an ideal candidate for AM metallic scaffolds. The factors that affect the scaffold technology are the design, the material used to build the scaffold, and the fabrication process. This review paper includes thus a discussion on the design of Ti6A4V scaffolds in relation to how their behavior is affected by their cell shapes and porosities. This is followed by a discussion on the post treatment and mechanical characterization including in-vitro and in-vivo biomechanical studies. A review and discussion are also presented on the ongoing efforts to develop predictive tools to derive the relationships between structure, processing, properties and performance of powder-bed additive manufacturing of metals. This is a challenge when developing process computational models because the problem involves multi-physics and is of multi-scale in nature. Advantages, limitations, and future trends in AM scaffolds are finally discussed. AM is considered at the forefront of Industry 4.0, the fourth industrial revolution. The market of scaffold technology will continue to boom because of the high demand for human tissue repair.

Additively Manufactured NiTi and NiTiHf Alloys: Estimating Service Life in High-Temperature Oxidation.

In this study, the effect of the addition of Hf on the oxidation behavior of NiTi alloy, which was processed using additive manufacturing and casting, is studied. Thermogravimetric analyses (TGA) were performed at the temperature of 500, 800, and 900 °C to assess the isothermal and dynamic oxidation behavior of the Ni50.4Ti29.6Hf20 at.% alloys for 75 h in dry air. After oxidation, X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray spectroscopy were used to analyze the oxide scale formed on the surface of the samples during the high-temperature oxidation. Two stages of oxidation were observed for the NiTiHf samples, an increasing oxidation rate during the early stage of oxidation followed by a lower oxidation rate after approximately 10 h. The isothermal oxidation curves were well matched with a logarithmic rate law in the initial stage and then by parabolic rate law for the next stage. The formation of multi-layered oxide was observed for NiTiHf, which consists of Ti oxide, Hf oxide, and NiTiO3. For the binary alloys, results show that by increasing the temperature, the oxidation rate increased significantly and fitted with parabolic rate law. Activation energy of 175.25 kJ/mol for additively manufactured (AM) NiTi and 60.634 kJ/mol for AM NiTiHf was obtained.

"Effect of Zn content and aging temperature on the in-vitro properties of heat-treated and Ca/P ceramic-coated Mg-0.5%Ca-x%Zn alloys".

While potentially strong enough for load-bearing skeletal reconstruction applications, the corrosion (biodegradation) rate of biocompatible Mg-Zn-Ca-based alloys still presents. The present work reports on the use of heat treatment (strengthening and resorption delaying) and micro arc oxidation (MAO) coating (corrosion delaying) processes which were developed to induce desirable corrosion rates which are essential to maintaining the mechanical integrity of Mg-Zn-Ca-based alloys during the bone healing period. Three Mg-x%Zn-0.5%Ca (wt%) alloys with different levels of Zn content (1.2, 1.6 and 5 wt%) were prepared and heat-treated at different age hardening temperatures (100, 150, 200 and 250 °C). In order to further decrease the corrosion rate and improve the bioactivity, samples of the heat-treated Alloy I (Mg-1.2wt%Zn-0.5wt%Ca) at the optimized age-hardening conditions were successfully coated with a biocompatible composite coating without and with HA/ß-TCP nanoparticles by using an MAO process. The microstructure, morphology and the composition of the heat-treated and coated materials were characterized by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and x-ray diffraction (XRD). Hardness and compression tests were conducted, while a corrosion investigation of heat-treated and coated samples was performed using potentiodynamic polarization (PDP) and a mechanical integrity immersion test. The results confirmed that Zn content and age hardening temperature have significant effects on the mechanical and corrosion properties of heat-treated Mg-Zn-Ca-based alloys. Alloy I, which has 1.2 wt% Zn content and was aged at 200 °C, showed the best combination of corrosion (slowest) and mechanical (highest) properties. The MAO (HA/ß-TCP) composite coating significantly improved corrosion resistance compared to the uncoated heat-treated alloy, with only 11.3% reduction in the compressive strength after 8 weeks of immersion.

Engineering Bone-Implant Materials.

This special issue is dedicated to the simulation as well as experimental studies of biomechanical behavior of biomaterials, especially those that are used for bone implant applications [...].

Application of NiTi in Assistive and Rehabilitation Devices: A Review.

Shape memory alloys (SMAs) have found widespread applications as biomedical devices. Biocompatibility, corrosion resistance, and ductility make these alloys attractive for medical devices such as stents and filters. For these implants, the superelastic property is the primary function of SMAs. Additionally, these alloys, such as NiTi as the prime example, can be used for actuation. Several modes of actuation such as displacement control, force control, and compliance control have been used as harnesses with SMA devices. These two unique properties have opened another application in the form of neurosurgery and robot-assisted surgery devices, as well as controlled assistive and rehabilitation devices. This paper reviews the state of the art of application of SMAs in the latter category where control is applied to harness innovative medical devices. To this end, two major subsets of these devices: prosthesis and orthosis which take the advantage of SMAs in assistive and rehabilitation devices are studied. These devices are further categorized to hand prosthetics, elbow, knee and ankle orthotics. In most of these designs, SMA wires act as artificial muscles to mimic the motion of limbs in the target joints. The evolution of each category is explained, and the specific results of them are reported. The paper also reviews the SMA applications for neurological and neuromuscular rehabilitation. To this end, different categories of rehabilitation devices as a passive and aided exercise for the ankle, knee, and elbow are highlighted. The SMA actuator in these devices can be EMG-controlled to improved patient outcome. In addition to providing a comprehensive overview of the biomedical devices, this paper identifies several possible future directions of SMA related research in the area of assistive and rehabilitation devices.

Achieving superelasticity in additively manufactured NiTi in compression without post-process heat treatment.

Shape memory alloys (SMAs), such as Nitinol (i.e., NiTi), are of great importance in biomedical and engineering applications due to their unique superelasticity and shape memory properties. In recent years, additive manufacturing (AM) processes have been used to produce complex NiTi components, which provide the ability to tailor microstructure and thus the critical properties of the alloys, such as the superelastic behavior and transformation temperatures (TTs), by selection of processing parameters. In biomedical applications, superelasticity in implants play a critical role since it gives the implants bone-like behavior. In this study, a methodology of improving superelasticity in Ni-rich NiTi components without the need for any kind of post-process heat treatments will be revealed. It will be shown that superelasticity with 5.62% strain recovery and 98% recovery ratio can be observed in Ni-rich NiTi after the sample is processed with 250 W laser power, 1250 mm/s scanning speed, and 80 µm hatch spacing without, any post-process heat treatments. This superelasticity in as-fabricated Ni-rich SLM NiTi was not previously possible in the absence of post-process heat treatments. The findings of this study promise the fast, reliable and inexpensive fabrication of complex shaped superelastic NiTi components for many envisioned applications such as patient-specific biomedical implants.

A novel and innovative device to retract rectum during radiation therapy of pelvic tumors.

An effective radiotherapy treatment entails maximizing radiation dose to the tumor while sparing the surrounding and normal tissues. With the advent of SBRT with extreme hypo-fractionation in treating tumors including prostate where ablative dose is delivered in smaller number of fractions, rectum remains a dose-limiting organ and at the risk of rectal toxicity or secondary cancer. The same limitation of rectal toxicity exists for high-dose rate (HDR) treatments of cervical, endometrial, or prostate cancer when creating even a short distance between the anterior rectal wall and field of radiation is ideal in delivering ablative dose to the target. An effective solution to such problem is to physically displace rectum as the organ at risk. This research presents an organ retractor device that is designed to displace the rectum away from the path of radiation beam employing a Nitinol shape memory alloy that is designed for displacing the rectum upon actuation. A control system regulates the motion in a reproducible and safe manner by creating the desirable shape in moving the anterior rectal wall. The study finds the novel organ retractor device to be a promising tool that can be applied in a clinical setting for minimizing dose to the rectum during treatment of pelvic tumors, and creating the potential to deliver an ablative dose to tumor volume or to escalate the dose when needed.

Predicting the Biodegradation of Magnesium Alloy Implants: Modeling, Parameter Identification, and Validation.

Magnesium (Mg) and its alloys can degrade gradually up to complete dissolution in the physiological environment. This property makes these biomaterials appealing for different biomedical applications, such as bone implants. In order to qualify Mg and its alloys for bone implant applications, there is a need to precisely model their degradation (corrosion) behavior in the physiological environment. Therefore, the primary objective develop a model that can be used to predict the corrosion behavior of Mg-based alloys in vitro, while capturing the effect of pitting corrosion. To this end, a customized FORTRAN user material subroutine (or VUMAT) that is compatible with the finite element (FE) solver Abaqus/Explicit (Dassault Systèmes, Waltham, MA, USA) was developed. Using the developed subroutine, a continuum damage mechanism (CDM) FE model was developed to phenomenologically estimate the corrosion rate of a biocompatible Mg⁻Zn⁻Ca alloy. In addition, the mass loss immersion test was conducted to measure mass loss over time by submerging Mg⁻Zn⁻Ca coupons in a glass reactor filled with simulated body fluid (SBF) solution at pH 7.4 and 37 °C. Then, response surface methodology (RSM) was applied to calibrate the corrosion FE model parameters (i.e., Gamma (γ), Psi (ψ), Beta (ß), and kinetic parameter (Ku)). The optimum values for γ, ψ, ß and Ku were found to be 2.74898, 2.60477, 5.1, and 0.1005, respectively. Finally, given the good fit between FE predictions and experimental data, it was concluded that the numerical framework precisely captures the effect of corrosion on the mass loss over time.

Shape memory response of porous NiTi shape memory alloys fabricated by selective laser melting.

Porous NiTi scaffolds display unique bone-like properties including low stiffness and superelastic behavior which makes them promising for biomedical applications. The present article focuses on the techniques to enhance superelasticity of porous NiTi structures. Selective Laser Melting (SLM) method was employed to fabricate the dense and porous (32-58%) NiTi parts. The fabricated samples were subsequently heat-treated (solution annealing + aging at 350 °C for 15 min) and their thermo-mechanical properties were determined as functions of temperature and stress. Additionally, the mechanical behaviors of the samples were simulated and compared to the experimental results. It is shown that SLM NiTi with up to 58% porosity can display shape memory effect with full recovery under 100 MPa nominal stress. Dense SLM NiTi could show almost perfect superelasticity with strain recovery of 5.65 after 6% deformation at body temperatures. The strain recoveries were 3.5, 3.6, and 2.7% for samples with porosity levels of 32%, 45%, and 58%, respectively. Furthermore, it was shown that Young's modulus (i.e., stiffness) of NiTi parts can be tuned by adjusting the porosity levels to match the properties of the bones.

Application of the Superelastic NiTi Spring in Ankle Foot Orthosis (AFO) to Create Normal Ankle Joint Behavior.

Hinge-based Ankle Foot Orthosis (HAFO) is one of the most common non-surgical solutions for the foot drop. In conventional HAFOs, the ankle joint is almost locked, and plantar flexion is restricted due to the high stiffness of the hinge mechanism. This often leads to a rigid walking gate cycle, poor muscle activity, and muscle atrophy. Since the ankle torque-angle loop has a non-linear profile, the use of a superelastic NiTi spring within the hinge, due to its nonlinear behavior, could recreate a close-to-normal stiffness of the normal ankle joint, which, in turn, could create a more natural walk. The focus of this study is to evaluate the performance of a superelastic NiTi spring versus a conventional Stainless Steel spring in a hinge mechanism of a custom-fit HAFO. To this aim, a custom-fit HAFO was fabricated via the fast casting technique. Then, motion analysis was performed for two healthy subjects (Case I and Case II): (i) subjects with bare foot; (ii) subjects wearing a conventional HAFO with no spring; (iii) subjects wearing a conventional Stainless Steel-based HAFO; and (iv) subjects wearing a NiTi spring-based HAFO. The data related to the ankle angle and the amount of moment applied to the ankle during walking were recorded using Cortex software and used for the evaluations. Finally, Finite Element Analysis (FEA) was performed to evaluate the safety of the designed HAFO. The NiTi spring offers a higher range of motion (7.9 versus 4.14 degree) and an increased level of moment (0.55 versus 0.36 N·m/kg). Furthermore, a NiTi spring offers an ankle torque-angle loop closer to that of the healthy subjects.

Fixation Release and the Bone Bandaid: A New Bone Fixation Device Paradigm.

The current gold standard of care for mandibular segmental defeat reconstruction is the use of Ti-6Al-4V immobilization hardware and fibular double barrel graft. This method is often successful immediately at restoring mandible function, however the highly stiff fixation hardware causes stress shielding of the grafted bone and stress concentration in the fixation device over time which can lead to fixation device failure and revision surgery. The purpose of reconstructive surgery could be to create normal stress trajectories in the mandible following engraftment. We investigate the use of a two stage mechanism which separates the immobilization/healing and regenerative phases of mandibular segmental defect treatment. The device includes the use of a very stiff, Ti-6Al-4V, releasable mechanism which assures bone healing. Therefore it could be released once the reconstructed boney tissue and any of its ligamentous attachments have completely healed. Underneath the released Ti-6Al-4V plate would be a pre-loaded nitinol (NiTi) wire-frame apparatus that facilitates the normal stress-strain trajectory through the engrafted bone after the graft is healed in place and the Ti-6Al-4V fixation device has been released. Due to the use of NiTi wires forming a netting that connects vascularized bone and possibly bone chips, bone grafts are also more likely to be incorporate rather than to resorb. We first evaluated a healthy adult mandible during normal mastication to obtain the normal stress-strain distribution. Then, we developed the finite element (FE) model of the mandibular reconstruction (in the M1-3 region) with the proposed fixation device during the healing (locked state) and post-healing (released state) periods. To recreate normal stress trajectory in the reconstructed mandible, we applied the Response Surface Methodology (RMS) to optimize the Bone Bandaid geometry (i.e., wire diameters and location). The results demonstrate that the proposed mechanism immobilizes the grafted bone in the locked state properly since the maximum resultant gap (21.54 micron) between the graft and host mandible surfaces are in the safe region (less than 300 micron). By considering the von Mises criteria for failure, FE analysis together with experimental studies (i.e., compressive and tensile testing on the inferior and superior fixation devices, respectively) confirm that the proposed fixation devices do not fail, showing safety factor of at least 10.3. Based on the Response Surface Methodology (RSM) technique, the optimal parameter values for the wires are achieved (0.65 mm and 1 mm for the superior and inferior wires, respectively) and the required level of preload on each wire are calculated (369.8 N and 229 N for the inferior and superior wires, respectively). The FE results for stress distribution on the reconstructed mandible during the released state closely match that of a healthy mandible.

Finite Element Simulation of NiTi Umbrella-Shaped Implant Used on Femoral Head under Different Loadings.

In this study, an umbrella-shaped device that is used for osteonecrosis treatment is simulated. The femoral head is subjected to various complex loadings as a result of a person's daily movements. Implant devices used in the body are made of shape memory alloy materials because of their remarkable resistance to wear and corrosion, good biocompatibility, and variable mechanical properties. Since this NiTi umbrella-shaped implant is simultaneously under several loadings, a 3-D model of shape memory alloy is utilized to investigate the behavior of the implant under different conditions. Shape memory and pseudo-elasticity behavior of NiTi is analyzed using a numerical model. The simulation is performed within different temperatures and in an isothermal condition with varied and complex loadings. The objective of this study is to evaluate the performance of the device under thermal and multi-axial forces via numerically study. Under tensile loading, the most critical points are on the top part of the implant. It is also shown that changes in temperature have a minor effect on the Von Mises stress. Applied forces and torques have significant influence on the femoral head. Simulations results indicate that the top portion of the umbrella is under the most stress when embedded in the body. Consequently, the middle, curved portion of the umbrella is under the least amount of stress.

Mechanical and shape memory properties of porous Ni50.1Ti49.9 alloys manufactured by selective laser melting.

Near equiatomic NiTi shape memory alloys were fabricated in dense and designed porous forms by Selective Laser Melting (SLM) and their mechanical and shape memory properties were systematically characterized. Particularly, the effects of pore morphology on their mechanical responses were investigated. Dense and porous NiTi alloys exhibited good shape memory effect with a recoverable strain of about 5% and functional stability after eight cycles of compression. The stiffness and residual plastic strain of porous NiTi were found to depend highly on the pore shape and the level of porosity. Since porous NiTi structures have lower elastic modulus and density than dense NiTi with still good shape memory properties, they are promising materials for lightweight structures, energy absorbers, and biomedical implants.

Microstructural, mechanical and corrosion characteristics of heat-treated Mg-1.2Zn-0.5Ca (wt%) alloy for use as resorbable bone fixation material.

Mg-Zn-Ca alloys have grabbed most of the recent attention in research attempting to develop an Mg alloy for bone fixation devices due to their superior biocompatibility. However, early resorption and insufficient strength remain the main problems that hinder their use. Heat treatment has previously been thoroughly studied as a post-shaping process, especially after the fabrication of complex parts (e.g. porous structures) by 3D-printing or powder metallurgy. In this work, the effect of heat treatment on Mg-1.2Zn-0.5Ca (wt%) alloy's microstructural, mechanical and corrosion properties was studied. The surface morphology of samples was characterized by optical microscopy, scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and x-ray diffraction (XRD). Hardness, compression and tensile tests were conducted, while the in vitro corrosion characteristics of the prepared samples were determined using potentiodynamic polarization (PDP) and immersion tests. It was found that increasing the age hardening duration up to 2-5h increased the heat-treated Mg-1.2Zn-0.5Ca alloy's mechanical properties. Further increase in the age hardening duration did not result in further enhancement in mechanical properties. Similarly, heat treatment significantly altered the Mg-1.2Zn-0.5Ca alloy's in vitro corrosion properties. The corrosion rate of the Mg-1.2Zn-0.5Ca alloy after the heat treatment process was reduced to half of that for the as-cast alloy. XRD results showed the formation of biocompatible agglomerations of hydroxyapatite (HA) and magnesium hydroxide (Mg(OH)2) on the corroded surface of the heat-treated Mg-1.2Zn-0.5Ca alloy samples. The performed heat treatment process had a significant effect on both mechanical and corrosion properties of the prepared Mg-1.2Zn-0.5Ca alloy. The age hardening duration which caused the greatest increase in mechanical and the most slowed corrosion rate for Mg-1.2Zn-0.5Ca alloy material was between 2 and 5h.

Rotary mechanical circulatory support systems.

A detailed survey of the current trends and recent advances in rotary mechanical circulatory support systems is presented in this paper. Rather than clinical reports, the focus is on technological aspects of these rehabilitating devices as a reference for engineers and biomedical researchers. Existing trends in flow regimes, flow control, and bearing mechanisms are summarized. System specifications and applications of the most prominent continuous-flow ventricular assistive devices are provided. Based on the flow regime, pumps are categorized as axial flow, centrifugal flow, and mixed flow. Unique characteristics of each system are unveiled through an examination of the structure, bearing mechanism, impeller design, flow rate, and biocompatibility. A discussion on the current limitations is provided to invite more studies and further improvements.

Resorbable bone fixation alloys, forming, and post-fabrication treatments.

Metallic alloys have been introduced as biodegradable metals for various biomedical applications over the last decade owing to their gradual corrosion in the body, biocompatibility and superior strength compared to biodegradable polymers. Mg alloys possess advantageous properties that make them the most extensively studied biodegradable metallic material for orthopedic applications such as their low density, modulus of elasticity, close to that of the bone, and resorbability. Early resorption (i.e., <3months) and relatively inadequate strength are the main challenges that hinder the use of Mg alloys for bone fixation applications. The development of resorbable Mg-based bone fixation hardware with superior mechanical and corrosion performance requires a thorough understanding of the physical and mechanical properties of Mg alloys. This paper discusses the characteristics of successful Mg-based skeletal fixation hardware and the possible ways to improve its properties using different methods such as mechanical and heat treatment processes. We also review the most recent work pertaining to Mg alloys and surface coatings. To this end, this paper covers (i) the properties and development of Mg alloys and coatings with an emphasis on the Mg-Zn-Ca-based alloys; (ii) Mg alloys fabrication techniques; and (iii) strategies towards achieving Mg-based, resorbable, skeletal fixation devices.

Metallic Fixation of Mandibular Segmental Defects: Graft Immobilization and Orofacial Functional Maintenance.

The aim of this study is to investigate the behavior of the healthy mandible under maximum molar bite force to demonstrate the problems associated with the current standard of care procedures for mandibular segmental defect reconstruction (ie, use of Ti-6Al-4V hardware and either a single- or double-barrel fibular graft). With current Ti-6Al-4V mandibular reconstruction hardware, there is a significant stiffness mismatch among the hardware, graft, and the remaining host anatomy. How the distribution of mechanical forces through the mandible is altered after a segmental bone loss and reconstruction is incompletely understood. METHODS: We studied a healthy adult mandible for stress, strain, and reaction force distribution during normal mastication. Stress distribution of this model was then used to study problems encountered after mandibular segmental defect reconstructive surgery. We model the use of both single- and double-barrel fibular grafts to repair the loss of the left M1-3 containing segment of the mandible. These simulations were done using 2 sets of plates with different thicknesses. RESULTS: We found that the stiffness mismatching between the fixation hardware and the graft and host bone causes stress shielding of that bone and stress concentrations in the fixation hardware and screws. These effects are expected, especially during the bone healing period. However, long term, this abnormal stress-strain distribution may lead to either the hardware's failure due to stress concentration or graft failure due to bone resorption as a result of stress shielding. We found that the stress-strain distribution is more normal with a double-barrel fibular graft. Additionally, we found that thinner fixation plates can reduce stress shielding. CONCLUSION: The proposed model can be used to evaluate the performance and optimization of the fixation device.

Finite Element Simulation and Additive Manufacturing of Stiffness-Matched NiTi Fixation Hardware for Mandibular Reconstruction Surgery.

Process parameters and post-processing heat treatment techniques have been developed to produce both shape memory and superelastic NiTi using Additive Manufacturing. By introducing engineered porosity, the stiffness of NiTi can be tuned to the level closely matching cortical bone. Using additively manufactured porous superelastic NiTi, we have proposed the use of patient-specific, stiffness-matched fixation hardware, for mandible skeletal reconstructive surgery. Currently, Ti-6Al-4V is the most commonly used material for skeletal fixation devices. Although this material offers more than sufficient strength for immobilization during the bone healing process, the high stiffness of Ti-6Al-4V implants can cause stress shielding. In this paper, we present a study of mandibular reconstruction that uses a dry cadaver mandible to validate our geometric and biomechanical design and fabrication (i.e., 3D printing) of NiTi skeletal fixation hardware. Based on the reference-dried mandible, we have developed a Finite Element model to evaluate the performance of the proposed fixation. Our results show a closer-to-normal stress distribution and an enhanced contact pressure at the bone graft interface than would be in the case with Ti-6Al-4V off-the-shelf fixation hardware. The porous fixation plates used in this study were fabricated by selective laser melting.

State of the art review of knee-ankle-foot orthoses.

Knee-ankle-foot orthoses (KAFOs) are used to assist in ambulation. The purpose of this paper is to review existing KAFO designs which can be grouped into passive KAFOs, stance control (SC) KAFOs, and dynamic KAFOs. The conventional passive KAFOs do not provide any active control for knee motions. SCKAFOs lock the knee joint during the stance phase and allow free rotations during the swing phase. Some SCKAFOs switch between the stance and swing phases using body posture, while others use some kind of a control system to perform this switch. Finally, dynamic KAFOs control the knee joint during both stance and swing phases. Four dynamic systems are identified in the literature that use pneumatics, linear springs, hydraulics, and torsional rods made of superelastic alloys to control the knee joint during the gait cycle. However, only the two systems that use linear springs and torsional rods can reproduce the normal knee stiffness pattern which has two distinct characteristics: a soft stiffness during the swing phase and a hard stiffness during the stance phase. This review indicates that there is a need to conduct research regarding new KAFO designs that duplicate normal knee function during the whole gait cycle.