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33 (); 70-80
doi:
10.1016/j.jor.2022.06.013

Functionally graded additive manufacturing for orthopedic applications

, , , , , ,
a School of Mechanical Engineering, Shri Mata Vaishno Devi University, J&K, India
b Faculty of Technology, University of Sunderland, UK
c School of Engineering, Deakin University, Australia
d School of Science and Technology, Nottingham Trent University, UK

∗Corresponding author: Mir Irfan Ul Haq. haqmechanical@gmail.com

Received: , Accepted: ,
Professor P K Surendran Memorial Education Foundation
Disclaimer:
This article was originally published by Reed Elsevier India Pvt. Ltd. and was migrated to Scientific Scholar after the change of Publisher.

Abstract

Abstract

Additive Manufacturing due to its benefits in developing parts with complex geometries and shapes, has evolved as an alternate manufacturing process to develop implants with desired properties. The structure of human bones being anisotropic in nature is biologically functionally graded i,e. The structure possesses different properties in different directions. Therefore, various orthopedic implants such as knee, hip and other bone plates, if functionally graded can perform better. In this context, the development of functionally graded (FG) parts for orthopedic application with tailored anisotropic properties has become easier through the use of additive manufacturing (AM).

and Rationale: The current paper aims to study the various aspects of additively manufactured FG parts for orthopedic applications. It presents the details of various orthopedic implants such as knee, hip and other bone plates in a structured manner. A systematic literature review is conducted to study the various material and functional aspects of functionally graded parts for orthopedic applications. A section is also dedicated to discuss the mechanical properties of functionally graded parts.

The literature revealed that additive manufacturing can provide lot of opportunities for development of functionally graded orthopedic implants with improved properties and durability. Further, the effect of various FG parameters on the mechanical behavior of these implants needs to be studied in detail. Also, with the advent of various AM technologies, the functional grading can be achieved by various means e.g. density, porosity, microstructure, composition, etc. By varying the AM parameters. However, the current limitations of cost and material biocompatibility prevent the widespread exploitation of AM technologies for various orthopedic applications.

Keywords

Functionally graded parts
Additive manufacturing
Orthopedics
Implants
3D printing
Medical applications
1

1 Introduction

Additive manufacturing (AM) is a layer-by-layer manufacturing technique, which creates the product into the final shape with accurate geometrical dimensions and minimal material wastage.1 At present, AM is being widely used for medical, automotive, aerospace, and marine applications.2-10 In comparison to the conventional manufacturing, AM has the potential for developing complex geometries with ease of customization.11 This application makes it the perfect contender in the medical sector, as the quick customization of implants and tools are very important for most medical (specifically orthopedic and dental) procedures.12,13 In recent years, the advancements in AM have led towards the development of tissues and organs, which will solve the donor shortage problem.14-16

Apart from these mentioned applications, AM is used to print the models for pre-operative surgical preparations that precisely depict the organ on which the surgery is to be performed. This has reduced surgery time and complexity of the procedure.17 In orthopedics, stress shielding of implants is an important challenge faced by orthopedicians that is mainly due to the mismatch of mechanical properties between the implant and the bone, which can lead to implant failure. This issue occurs in conventionally manufactured implants as the properties such as porosity, strength; hardness cannot be patient specific and can become a cause for implant resorption. This problem can be better solved by additive manufacturing due to its control on mechanical and structural properties of implant that can alter the stress concentration, thus preventing stress shielding and failure of implant. Implant failure is also possible by improper torque on the screws and fixtures during the implantation.18 Some fixtures used during orthopedic surgeries are bone screws, intramedullary rods, pins, wires, and spinal fixtures.19

Out of the mentioned fixtures, bone screws are the most commonly used for fixing orthopedic fractures and implants. It is estimated that among the fractures that develop complications after implying fixtures, around 11% of the complications are due to screw-related issues.20 In most cases, the stress shielding effect occurs between the bone and the screw that may cause severe pain and need surgical intervention. This issue has been solved by AM by developing patient-specific fixtures with optimized structure, which help in bone healing. However, the surface roughness associated with AM bone implants and fixtures can hamper bone regeneration, but biomechanical performance is enhanced.21 Apart from the scientific point of view, AM has also proved itself to be cost-friendly. Due to reduced labor and tooling, the cost of implants has also been reduced significantly, indicating that it is the viable option in the present scenario for orthopedic applications.15 The requirements for any orthopedic implant are given in Fig. 1.

Requirements for any orthopedic implants.
Fig. 1 Requirements for any orthopedic implants.

Bio-compatibility and wear resistance are the two most critical properties of implant materials.22 In case of long term use of implants in a patient's body, biocompatibility plays a crucial role. Since most of the implant materials consist of metals, assessing their biocompatibility becomes important. Not all metals can be used as implant materials. Titanium and its alloys, stainless steel, magnesium alloys, chromium and cobalt based alloys have proven to exhibit high biocompatibility when used in implants.23 They include materials like 316L stainless steel, Ti–6Al–4V, Ti–6Al–7Nb. In addition to good biocompatibility, Ti–6Al–4V provides excellent corrosion resistance as well owing to its capability to form an oxide layer on its surface upon exposure to oxygen environment, such as atmosphere.24 Along with materials, various lubricants and synovial fluids have also been developed by researches to improve the biocompatibility of various implants.25

The methodology adopted to develop this review paper has been based on searching various research databases by using various keywords related to the scope of the paper. Also, medical journals, reports and magazines have been consulted to compile this paper. The objectives of the paper include a) presenting a brief overview of the literature and basic concepts related to AM and FG b) representing a detailed discussion on how AM technologies can help to produce FG parts c) developing a discussion on how AM can help develop FG implants for orthopedic implants and present various material aspects. d) expressing a detailed literature related to mechanical aspects of FG Implants developed through AM.

1.1

1.1 Brief background of functional grading

The concept of functionally grading of materials involves tailoring of their properties for specific requirements. It is defined as the gradual change in the composition or structure across the volume to attain the required set of properties for specific application areas.26,27 FGMs can be found in nature, bones, tissues of seashells, and plants like bamboo.28-30 It was Naotake who proposed the concept of functionally graded materials based on the observations made of naturally occurring materials or objects like teeth, bone etc.31 These mentioned natural objects or materials exhibit good mechanical or physical properties due to the graded structure along a certain direction that makes them perform better than the other materials.32 FGMs are classified as continuous and discontinuous. In continuous FGMs the parameters like composition, microstructure, and temperature vary along a continuous curve, while in discontinuous FGMs vary in a discreet way (step wise) along the certain direction/length.33 The cellular graded structures in FGM are capable of reducing or distributing the stress concentration in an object subjected to some external loading, Hence, enhancement in the mechanical and other physical properties are marked.34 FGM objects find their application in aerospace, medical, electronics, and energy materials.35-38 They are used for developing high temperature aerospace materials, which can exhibit excellent mechanical properties for high temperature applications. Bio-medical applications of FGMs are evolving at the larger scale from the couple of years. FGMs have been used to develop artificial bone and dental implants with good biomechanical compatibility.39 The manufacturing process for FGM plays an important role in their working and exhibiting the desired properties. The conventional methods for manufacturing of FGM are liquid based (gel casting, centrifugal casting etc.), gas-based methods (CVD, PVD, Thermal Spray etc.) and solid phase powder method (powder metallurgy, spark plasma sintering).35-44 The gas-based methods like CVD require heat, plasma or light as the source of energy. Gases like bromides, hydrides or chlorides are mostly used in gas-based methods. They require a lot of energy and characterized for the emission of toxic gases as the by-product. The liquid-based methods have the capability for mass production of FGM objects. Centrifugal casting is the most explored area of liquid based methods for functional grading but the gradient can be obtained only in the radial direction.45 Similarly, in solid phase methods, the FGM objects show traces of pores, which can degrade the mechanical, thermal, structural properties of objects. Taking these problems in consideration, researchers are exploring FGM with AM and positive results have been observed so far. Fig. 2 gives the pictorial representation of various methods used for FGM.

Methods for developing FGM's.
Fig. 2 Methods for developing FGM's.
1.2

1.2 Additive manufacturing and functional grading

In comparison to conventional manufacturing, Researchers have found that AM is the better way to fabricate FGMs due to its extra-ordinary formability and optimized stress profiles in comparison to conventional manufacturing processes.33 AM provides spatial and temporal control over the properties like microstructure which is not possible with conventional manufacturing processes. The FGM by AM has given rise to new domain called FGAM (Functionally Graded Additively Manufacturing). FGAM has the potential to develop and introduce compositional variation as well as microstructural variation in a material.46 The FGAM is done in three ways; (1) Homogenous composition with gradual variation of other parameters like density, cellular lattices or structures and temperature. (2) Heterogeneous composition of material. (3) The combination of (1) and (2). The FGAM is performed in different stages, which involve modelling of structures at a macro as well as meso scale. Structural modelling entails the design of the full structure and the lattices inside it. Structural modelling is followed by material selection. The FGAM parameters like material density, pattern, temperature, layer thickness are altered with the slicer software available. Before going towards the actual printing, the model undergoes simulation. This provides an initial guess about the properties due to the selected structure or material. Once the structure, material and the grading methods are decided, the printing of FGAM product starts. The product undergoes various types of characterizations for quality and performance tests. At present, FGAM is in its infancy period. No doubt, a lot of research is going on in the field, but very less research is transferred to “Technological Readiness Level”. The reason is very less simulation tools for complex FGAM process. The aim of this review is to provide the recent and important insights of FGAM and orthopedic research. A state of comparison is done between the various FGAM technologies for orthopedic implants, and critical analysis is performed to provide a better option for a specific orthopedic implant or tool. The various variables, which can help to attain the functionality in materials by AM, are shown in Fig. 3.

Variables for FGAM
Fig. 3 Variables for FGAM
2

2 Materials and functional aspects of orthopedic implants

The use of implants for orthopedic applications by started way back in 1895. Metallic plate was implanted at the fracture site for quick healing.47 Since then, the researchers are constantly working to upgrade the implant technology for orthopedic applications. The era of modern screws for fracture treatment came in 1920 with the introduction of stainless steel as the bone screw material.48 It proved to be corrosion resistant and biocompatible. Apart from stainless steel, Titanium alloys gained for orthopedic implants due to its durability and bio tolerance.49 Nowadays, Austenitic chromium-nickel steel doped with molybdenum is widely used for joint implants.50 Cr–Ni–Mo steels are characterized for low corrosive behavior. In addition, their mechanical properties can be altered according to need by cold working.51 The main problems associated with orthopedic implants is the development of infections, allergies and pain. Also, some life threatening diseases like cancer or tumor can be the cause for implant removal.52 The process of re-surgery for implant removal is hectic and may cause permanent side effects. Therefore, Researchers are working to develop biodegradable implants from metallic alloys, polymers and ceramics.53 Magnesium when alloyed with some material to give better biodegradable implants for orthopedic applications.54 Although the strength of biodegradable implants have less strength than the non-biodegradable materials but the biocompatibility is excellent.55,56 Among the polymeric materials PTFE, PEEK and UHMWPE are used for implants for hip replacements due to good stiffness and strength.57 Ceramics have developed a unique identity for their biocompatibility and porous structure. They are widely used for orthopedic implants with flexible and high load carrying capacity. Some of the important ceramics used for orthopedic applications are: Alumina, Zirconia, Akermanite.58 To avoid the re-surgeries, the selection of perfect non-biodegradable material for orthopedic implants is very important. The researchers are constantly trying to develop the mechanical and biodegradable materials for orthopedic implants, which are job specific and patient specific. This section summarizes the various materials used for different orthopedic implants. The detailed description of mechanical and biocompatibility in implants is provided.

2.1

2.1 Knee implants

The knee consists of four types of bones: the Femur, Patella, Tibial and Fibula.59 Knee replacement is considered one of the remarkable achievements to improve the quality of human life. Around 50,000 knee transplants are done in a year at United States.57 Cobalt-chromium (Co–Cr), SS 316L, NiTi alloy, and titanium (Ti) and its alloys are widely used for knee implants. For every knee implant, it is necessary that the material should have a low modulus of elasticity (15–30 GPa) to eliminate the stress shielding effect.60,61 In addition, the material should be ductile to avoid material failure due to brittle fracture.62 Moreover, the strength and density of the material should be maintained as that of the real bone.63Fig. 1 represents the various bones in knee joint and the femoral component of knee implant.64 Earlier, knee implants were only made up of metals but with the development in the materials, the metals were replaced with alloys, ceramics and polymers 5765. Patients with metal sensitivity can have problems with the metallic implants. In addition, the wear particles of polyethylene can be toxic to human body. To tackle these problems, researchers have developed coated implants with least wear possibility and no metal contacts.66 The knee implants are categorized into four types: total knee replacement, kneecap implant, unicompartmental knee implant and revision knee implant.67 The major components of total knee replacement implants are femoral and tibial components. The femoral part consists of a metal piece, which is attached with the femur end. The tibial component has the plastic spacer and metal piece inserted into the tibia.68 Researchers have given pictorial representation shows the various components of total knee replacement implant.64 The kneecap replacement is the replacement of patella. It is used to replace the worn out knee cap so that pain in the future is avoided.

The wide variety of biomaterials are used for knee implants. With an aim of developing new materials for knee implants, researchers are exploring a wide variety of materials. Apart from that, potential applications for materials with maximum stress shielding capacity, wear resistance and fatigue resistance are designed and explored. Some of the important work related to the knee implants and material observations are given in Table 1.

Table 1 Important work related to knee implant.
Material Material Type Observation Author
PMMA Polymer Also referred as bone cement due to its stability and non-toxicity. It is used as spacer between tibia and femur. 69
UHMWPE Polymer Mostly used as inserts for tibial part. Wear resistance is enhanced by adding small amount of CNTs. 70
CFR-PEEK Polymer Better stress shielding effects and mechanical and tribological properties. 71
Titanium-aluminum-vanadium Alloy High density material used for tibial tray 72
Titanium-niobium nitride Alloy Material has high corrosion resistance and good biocompatibility. 66
Cobalt Chromium Alloy This material is mostly used for femoral component. It possess low stiffness. 67
Co–Cr–Mo Alloy High corrosion resistance and high toughness fatigue. 73
CuAlNi Alloy Good ductility and economical to use. 65
Zirconia Ceramic Used in femoral part. Better corrosion resistance 74
Alumina Ceramic Material has chemical inertness and flexural strength. 74
2.2

2.2 Hip prosthesis

Hip is one of the important joints in our body as it is subjected to a heavy load of upper body and has the difficult task of joining pelvis with the femur. The head of the femur is smoothly fitted inside the spherical cavity of the acetabulum. The joint is enveloped in a ligament, which is responsible for its stability.75 Osteoarthritis is one of the common problems that is associated with the hip joint in the older age. It is associated with a sharp pain due to the stiffness of the joint. Osteoarthritis causes permanent damage to the hip joint that is irreversible, and joint replacement is the only option. The surgical procedure to address such problem is total hip replacement also called the Total Hip Arthroplasty (THA). The various components involved in total hip replacement are given by Neira et al.76 The history of THA goes back to 1840, the attempt to replace the acetabular and femoral heads with wooden blocks.77 The results of this experiment were not good. The wear debris from the wood caused serious biological issues within the body. To maintain the compatibility issues various biomaterials were applied over the prosthesis, some of them are pig bladder, skin, gold foil etc.78 After a while, these biocompatible materials were replaced with sliver, zinc, rubber and wax.79

The hip replacement surgery has various development phases. In the earlier days of this procedure, the rubber mainly made the femoral. In few years, it was replaced with the ivory nail.77 Similarly, for the femoral cup, glass and bakelite were introduced.80 With the introduction of stainless steel to the domain, the procedure became more popular and success rates were enhanced. The stainless steel material when used with the long-stemming element for femoral neck and femoral head gave good results.77 The modern era for total hip replacement surgery started from 1960's with onset of Low Friction Arthroplasty (LFA). The first model of LFA consisted of stem made up of stainless steel, Cup made up of PTFE and a femoral head with diameter of 22.2 mm fixed with acrylic cement. PTFE showed some inflammatory signs therefore, it was replaced with UHMWPE.81 With the passage of time, new material came into the existence whose biocompatibility are excellent, good mechanical strength and less weight. Some of the advanced materials for THA are in Table 2.

Table 2 Materials used for THA.
Material Material Type Observation Author
PTFE Polymer It showed thermal stability and bio-inertness but wear rate was high. 82 , 83
Glass Fiber Reinforced PTFE Polymer Poor tribological behavior and highly reactive with the skin, hence responsible for developing infections. 84
UHMWPE Polymer UHMWPE were sterilized with gamma radiations, which increased the oxidation and wear resistance. The crosslinking of UHMPE has shown reduction in mechanical properties. 85 , 86
PEEK Polymer Chemical Inertness and biocompatibility are in PEEK based hip implants. The mechanical properties are also relatively good. 87
Cobalt Chromium Molybdenum Alloys Alloy The cobalt chromium alloys are characterized for high stiffness; hence, patients are subjected to pain. 88
Stainless Steel Alloy These alloys show low strength and ductility. Therefore, they have been replaced with CoCrMo Alloys 88
Titanium Alloy Alloy Ti–6Al–4V is considered as the lightweight material with good mechanical properties, which is compatible for femoral stem. However, the problem arises with the vanadium. The vanadium is considered toxic therefore better compositions like Ti–5Al-2.5Fe and Ti–6Al–7Nb, which have better biocompatibility and enhanced modulus of elasticity. 89 , 90 , 91 , 88
Alumina Ceramics It shows good tribological properties, but mechanical properties are not good. The initial level of alumina had problems like low density and weak microstructure. These problems were addressed by the modified version of alumina known as Biolox. 92-94
Zirconia Ceramics Zirconia exhibit excellent mechanical properties and resistance towards crack development. Y-TZP, newly developed zirconia has shown excellent wear properties, toughness, therefore used for THA. 95
Zirconia Toughened Alumina Ceramics With the addition of Zirconia, the toughness of alumina has enhanced. The clinical ZTA that has been used for THA is Biolex Delta. 96
2.3

2.3 Bone plates and other fixtures

Fracture is the most occurring phenomenon with the bones. To fix these fractures, researchers have developed a special kind of fixture known as bone plates that are the most found implants, used for internal fixation of bones.97 For perfect bone plates, it is important for them to exhibit resistance towards various mechanical properties like tension, compression and bending.98 Various surgical procedures that are taken into consideration for the installation of bone plates are open reduction and internal fixation and bridge fixation. Among the three procedures, the bridge fixation method is considered as least destructive methods for plate installation. Rest of the two damage the tissues and blood supply mechanism over that area. In addition, the surgical time for bridge fixation is less as compared to open reduction and internal fixation method.99 To fix the fracture of patients with co-morbidities like cancer, diabetes, special care is to be taken during the surgery like least tissue damage and minimal wound for surgical procedure. Bone plate does not seem to be feasible for these kind of situations therefore intramedullary nails are to be employed for such tasks.100 IN shows excellent torsional load bearing capacity with biomechanical stability.101 They are characterized for steady ambulation and mobilization of joints. Apart from these bone plate and intramedullary nails, there are other fixtures like screws, nails, pin and wires, which are being used in orthopedic surgeries. Some prominent areas of application of these fixtures include provisional fixation and bone traction procedure.102 The material details of these fixtures and plates is given in Table 3.

Table 3 Materials used for Fixtures and Plates.
Fixture Material Observation Author
Plate 316L Austenitic Steel 316L Austenitic steel have been widely used for manufacturing of plates. The mechanical properties are good; however, the stress shielding comes into play due to its young's modulus that is much higher than the bone, which may slow down the healing process. The other problem related to the biocompatibility, since it contains nickel and chromium, which are considered toxic substances. 103
Plate high-nitrogen nickel-free stainless steel (HNFSS) Biocompatibility is extremely good. The strength high than the conventional 316L Austenitic steel. The bending and compression are also good. 104
Plate Titanium Alloys The widely used Ti–6Al–4V possess high biocompatibility issues due to Al and V and can cause long-term health issues. Also, the wear resistance isn't appreciable. Therefore, Al and V were replaced by β phase materials like Nb, Zr, Mo and Ta, which have better biocompatibility. The Titanium alloys have overall issues with poor wear and fatigue resistance for bone plate applications. 105
Plate Chromium Alloys The chromium alloys are not considered as the perfect material for bone plates due to biocompatibility issues. Two common chromium alloys that are wrought Co–Ni–Cr–Mo alloy and cast Co–Cr–Mo are used for bone plates. They have similar wear resistance but differ in elastic moduli. 105
Bone Screw Titanium Based Bio Materials The biocompatible titanium alloys have been widely used for bone screws. They have low fatigue and wear resistance. 106
Bone Screws Cobalt based alloys The cobalt chrome have excellent mechanical properties. It has been widely used for bone screw development. 107
3

3 Orthopedic implants and additive manufacturing

AM has vital role in the development of orthopedic implants. The ability for mass customization, high speed and accuracy has made it popular for the manufacturing of scaffolds, implants and other fixtures. The feature of patient specific implants and fixture by additive manufacturing have revolutionized the orthopedic sector. The development of strong porous implants with AM has helped in the ingrowth of bone, which is responsible for quick healing and enhance osseo-integration. In addition, the control over the porosity has helped in tibia graft fixation and improved the biomechanical performance bone and screw juncture.108 In the present era, researchers are working for the development of smart implants, which help in early diagnosis of any problem occurring with the implant. They can help in minimizing the threat at an early hour and proper treatment is received.109 The development of smart orthopedic implants with additive manufacturing have almost revolutionized the major problems associated hip prosthesis and knee joints also, more the technology has contributed a lot in bone healing assessment and loosening monitoring.110 The conventional methods of implant manufacturing have lot of irregularities such as manufacturing complex shape and size, the least or no control over the porosity of the implant.111 Since all these parameters, play an important role in bone regeneration and are addressed with the additive manufacturing technologies.

Improved and high quality healthcare is one of the main objectives of any society and new methods are being explored to achieve this. Additive manufacturing has proven to enhance the economic credibility of products by bringing down their final cost. In this context, AM has shown promising results in the field of orthopedics as well.112 AM has always focused on personalized implants for patients and in such cases, economic analysis of the process is very critical. Further, the use of AM in orthopedics at a larger scale is restricted by the high cost of material, technology and skilled manpower related to AM particularly in developing economies. Therefore, the focus should be on making the part affordable so that it can benefit more numbers of patients. Even if the implant manufactured meets all the required service standards, a patient would not feel inclined to use it if it is not affordable. Hence, economic validation is important in such cases. The recent literature citing the importance of AM in orthopedics in given Table 4.

Table 4 Recent advances in AM and orthopedics.
Implant/fixture Material AM Technique Observation
Bone plate136 Tantalum (Ta) coated Ti6Al4V Electron Beam Melting The Tantalum is coated over TiAl4V using CVD process. This bone plate has shown similar elastic moduli as that of cortical bone hence stress shielding is avoided. Ta coated surface enhance the cell proliferation on the scaffolds. In addition to this, osseointegration and osteogenic qualities of scaffolds have enhanced as compared to simple bone plates.
Plastic Liner of Acetabular Component137 PLA Fused Deposition Modelling The surface roughness remains a problem although no inner defects have reported by the radiography.
Acetabular Cups138 Ti6Al4V Electron Beam Manufacturing The EBM manufactured cups were compared with the conventional ones and results showed that first ones were found of cavities while as the later ones were free from cavities.
Multi Material Knee Joint Model139 Agilus30 (FLX935), Tango (FLX930), and Digital ABS (RGD5130). Polyjet Printing This Knee joint is supposed to replace the conventionally manufactured knee joint for educational purposes. Further, the mechanical properties are sufficient to withstand the flexo-extention. The fiber matrix may replace other materials in the future course of time for mimicking soft tissues.
Scaffold Material for cartilage applications.140 PLA Coated with PVA and HLA fibers FDM for printing and Hybrid electrospinning for coating on scaffolds The developed scaffolds are non-toxic and hydrophilic in nature. The overall coating enables increase in the mechanical properties like tensile strength. Further, the failure strain is reduced.

The Hip replacement surgery with the involvement of AM started with the development of surgical guides for total hip replacement surgery, specifically for cup replacement.113 The results showed better results as compared with the conventional manufacturing. With the further development in the field, researchers developed 3D printed custom cages for THA. Total haris-hip score and radiography technique analyzed the results, which clearly showed stability in the fixation as compared to the conventional manufacturing.114 The 3D printed acetabular cups, which are the key components in THA have also shown better “time to weight bear” ratios than the conventionally manufactured acetabular cups.115 The feasibility for customization with AM has led to the development of patient specific acetabular components that are suffering with colossal cup damage.116 Apart from acetabular cup, researchers have worked for 3D printed femoral neck and guides which proved efficient than the conventional manufacturing.117 The AM technologies that is mostly associated with acetabular cup manufacturing is Powder bed fusion technique [114–116].118-120 The first 3D printed acetabular component was developed in 2007 and after that, 3D printing technologies emerged as the biggest players in orthopedic applications.121 At present, the maximum pore size of acetabular cup is in between 300 and 900 μm with porosity ranging from 50 to 90%. UK based company called Corin121 designed it.

The Total Knee Replacement surgeries and implants associated with it have undergone a tremendous development with advent of patient specific implants (PSI) with 3D printing. Not implants, 3D printing has also brought revolution in guides and cutting blocks of TKR. Research for the development of patient specific tools as cutting blocks and guides have been reported.122,123 The quality of guides and blocks have improved but they are not cost effective. In the initial stages of AM in TKR, it worsened the condition of implants in patients.124,125 However, with the advancement in domain, it improved the quality and life span of implants.126 The researchers have tried to improve the accuracy of neutral axis in TKR implants by keeping giving the emphasis on PSI's.127 The TKR implants like femoral intramedullary rod showed less drainage but no significant improvement in the surgical time.128 With the development of 3D printed osteotomy guide, the surgical time and blood loss have reduced significantly.129 In addition, the 3D printed Cement less tibial base plate is considered as the success in terms of survival rate and post-surgery complications.130

Bone screws and plates are the integral part of any orthopedic surgery where they are used for fixation. The fixators may be permanent or temporary. The most important part for any screw or plate is its biocompatibility and its strength. The porosity of screws and plates for bone-regeneration and its stability for bone to bone or bone to implant contact is very essential. As the AM applied, the porous screws and plates are obtained which can help in attaining the osseo-integration and vascularization, which is not possible with the conventional manufacturing.21 In addition, the porosity enabled by AM in screws and plated can be a big contributor for bone tibia graft fixation with can improve the biomechanical performance of bone-tendon-screw interface.108 Researchers have observed that 3D printed screws have better vascularization as compared to the conventionally manufactured screws.131 Also, the bonding force in bone and the implant is reported for the 3D printed screws.132 The lattice-printing pattern in AM helps in designing the implants, screws and plates with specific lattice structure, which can be useful in tailoring their mechanical properties. Researchers have designed AM cancellous screws inspired by auxetic lattice structures that provided extra stiffness and strength.133 A similar kind of work was done for pedicles where the AM enabled auxetic structures helped in bone screw fixation by radial expansion that further helped in developing resistance against the pulling out under the tensile force.134 Researchers have also evaluated the effect of infill pattern on the mechanical properties of cancellous screws. The shear strength of cancellous screws for 100% insertion depths is reported for honeycomb lattice while as modulus of toughness is maximum for rectilinear printing patter.135

4

4 Functionally graded orthopedic implants with additive manufacturing

The aim of implementing FGM in orthopedics is to produce implants or fixtures, which have better mechanical and biological properties. The concept of FGM in orthopedics have enhanced the quality of implants. The challenges such as stress shielding and residual stresses on the surfaces of implant and joint are being addressed with FGM via AM. The functionally grading is usually achieved via varying material composition, altering the porosity and microstructure of material in a well-defined manner.141 Researchers have recommended the use of FGM in the domain of orthopedics in order to avoid the use of functionally graded coatings that are more prone to peeling off and chemical instability.142 Moreover, the methods for developing functionally graded coatings, such as plasma spray and chemical vapor deposition, are limited to small parts and consume a lot of energy.35 Since AM is not limited to smaller parts, and because it is feasible for multi-material printing, it has a key role to play in the manufacturing of functionally graded orthopedic implants.143 Porosity is an important parameter for orthopedic implants since bone regeneration depends on it. AM has the capability to develop functionally graded porous structures in which the porosity changes across the volume of the implant. The low porous portion of implant has good mechanical properties and high porous end has better fixation properties; hence, the bone ingrowth is possible.144 AM techniques that are commonly used for FGM implants are powder-based fusion methods, like SLS and SLM. Apart from these methods, DED (Direct Energy Deposition) is also used, but certain limitations like material porosity cannot be controlled.145 The FGM can surely help in better osseointegration and implant stability, which can contribute towards implant stability and hence prevent the implant loosening.

Functionally graded ortho-implants by means of porosity grading have been widely explored by researchers. Both numerical as well as experimental investigations have been performed. Researchers have developed functionally graded acetabular cups with SLM technology.146 Here the porosity was graded in the octet-based lattice structure. The researchers also proposed increasing the strut diameter form outer to inner surface of the cup. It was analyzed that the model could sustain maximum stresses that occur in the day-to-day life. Researchers have also used the combination of porosity and material grading with the help of a DED-based manufacturing system for acetabular cups.147 The results showed improved bone ingrowth due to the porous titanium alloy at the mating end. In addition, the wear rates were reduced due to cobalt alloy mating with the metallic part. This design also enhances the implant stability. Researchers have also addressed the porosity grading in axial as well as radial directions. Octahedron lattice structure was used for Co–Cr alloy with SLM process. This method of grading reduced stress-shielding effects, which in turn improved the implant stability.148 Researchers have also worked on functionally graded 3D-printed femoral heads with polycaprolactone (PCL) and b-tricalcium phosphate (b-TCP). The grading in porosity affected the mechanical properties like compressive modulus. The scaffolds of lower porosity exhibited high compressive strength.149 The control over the porosity and material concentration proved an efficient method for treating early stage osteonecrosis of femoral bone. Further, in the thirst for proper methodology for functionally graded additive manufacturing, researchers worked on scaffold manufacturing with dense in and dense out samples of Ti6Al4V for scaffold application.150 SLM for BCC lattice structure manufacturing is used. The results of graded samples are compared with uniform lattices of strut diameters of 0.4, 0.6, and 0.8 mm. The compressive stress of FG samples was twice that of uniform samples. In addition, deformation in uniform samples was more abrupt than in FG samples. FG by means of varying the material concentration also has significant effect of the mechanical properties of implants. Researchers observed the effect of grading the volume percentage in Invar 36/TiC composite with varying concentrations of TiC from 0 to 50% with laser based manufacturing at the printing speed of 20 mm/s. As the TiC % increases, mechanical properties like hardness and tensile strength also increases.151

With the advancement in the fields of FGM and orthopedics, researchers came to know that to achieve the mechanical properties and fracture behavior of real bone, one must not rely on the porosity and material gradient. There are other parameters also, like strain distribution with multi-material, various hierarchical designs of printing patterns and basic unit cell of implant, which must be taken into the consideration. Mathematical models like Voxel approximation have been widely used by researchers. In order to achieve the proper combination for hard-soft (bone type) material, researchers used voxel-by-voxel multi-material 3D printing manufacturing using brick and mortar arrangement for higher fracture energies.152 A new domain for cell modification optimization for scaffold application is emerging. In context of this idea, researchers worked on developing optimum cell that have better mechanical and biological importance. A modified face-centered cubic cell is proposed with spherical pores for scaffold applications.153 The stiffness is tailored with porosities of different percentages and optimized cell shape and size. This model is examined with FEA and 3D printed for mechanical testing. The cell ingrowth and elastic modulus were enhanced as compared to the orthogonal cylindrical struts.

New mathematical approach (TPMS) helps in balancing the porosity in the overall volume of scaffolds by just changing the formula of TPMS. It helps in attaining the smooth transition in porosity-based FGM scaffolds.154 It is also responsible for enhancing the higher energy absorption and reducing the shear failure among the porosity graded scaffolds.155 In addition, a TPMS-based sigmoid function enabled smooth transition of properties in a FG porous material.156 Researchers have also developed a semi-empirical formula that can be used to predict the mechanical behavior of TPMS based laser-manufacturing systems for scaffold applications.157 New studies in the domain of FGAM and orthopedic involve topological optimization of the lattices and methods of FGM.158–160 The FGAM enhances the microstructure and mechanical characteristics, which could be considered important for orthopedic applications. Also, the SEM analysis of various FGM composites have good layering and smooth transition from one material to other which may reduce brittle fractures for FGAM orthopedic implants.161 In addition, the introduction of artificial intelligence and machine learning techniques can provide optimal grading methods of material, temperature, porosity, microstructure etc. For better mechanical and biological properties. The mechanical behavior of FGM biomaterials for orthopedic applications is given in the section below:

4.1

4.1 Mechanical aspects of functionally graded additively manufactured orthopedic implants

Large bone fractures and deep injuries have remained a problem for orthopedics. Replacement of bones with new additively manufactured scaffold is the alternative to this problem, but to fulfill stress shielding criteria and biocompatibility, one has to develop PSI's for effective scaffolds, which have good mechanical load bearing capability and biocompatibility. All these innovations take place to avoid re-surgeries and make life easier for patients. In context of this, researchers have worked on scaffolds with biomaterials with porous cellular structures that exhibit good mechanical behavior throughout the structure.161-163 A lot of work has been done to investigate the mechanical behavior of the AM manufactured cellular structures.164,165 Researchers are in search of new methods for maintaining the balance between porosity (for bone ingrowth) and mechanical properties (compressive strength). Therefore, to meet these demands, functionally graded porous biomaterials are proposed, where the combination of controlled and graded porosity helps in meeting the demands of bone-in growth and high stress regions for bearing the high loads.166,167,40 Hence, to develop a model for understanding the correlation between the mechanical properties and design for porous functionally graded structures are of great importance. In this regard, researchers worked on tuning the morphological parameters like porosity and pore size by altering the strut diameter of the lattice cell.168 The variation in strut diameter can be multi or uni-directional.169 Here the relationship between the biomechanical response and strut diameter of the lattice is observed.170 Other researchers investigated the impact of post heat treatments and lattice transition methods on the mechanical behavior of FGAM porous biomaterials.169,150 The results showed better strength and high-energy absorption with systematic distribution of porosity over the implants. Further, researchers started working on developing the hybrid grading method for bio implants where different types of unit cells were used in a single part.171,172 In this case, porosity, cell size and pore size are altered separately with more flexible and smooth transitions of mechanical properties. It is also observed by researchers that varying the strut diameter is the best option among all the parameters for enhancing the functionality (biomechanical) in FGAM implants for orthopedic applications.173

5

5 Limitation of additive manufacturing for orthopedic applications

AM is currently growing at faster pace in the domain of orthopedics. Researchers are putting their efforts to develop efficient and reliable patient specific implants using additive manufacturing.174-176 However, there are certain limitations associated with the mechanical strength of implants and scaffolds due to the layer-by-layer manufacturing.177 The conventional additive manufacturing process have poor bionic issues, which can lead to the necrosis.177 Therefore, to avoid such issues, researchers are looking for more sophisticated AM technologies. The use of AM for orthopedics needs expertise over the use of proper material for implant or scaffold development. The generation of free radicals from the photopolymerizable resins, which are uncured, may become the source of cancer due to the presence of free radicals.178 Moreover, it needs skilled operators that must have the ability to check the bed level, infill parameters and material fed to the nozzle.179 The time factor is also the important limitation of AM for orthopedic implants. In most of the cases, it takes around 24 h to print any standard implant. In addition, the availability of materials required for bio printing is also limited. Due to all these limitations AM has been widely used in surgical preparations and least used for development of patient specific artificial bones.

6

6 Conclusions

In this paper, we have discussed the importance of FGMs with AM for orthopedics. FGM has enabled implants with better mechanical properties. The control over the porosity and material concentration makes it a perfect candidate for scaffolds and orthopedic surgical tools. The implementation of FGM has brought a revolution in the orthopedic sector. The thermal stresses among the implants have been lowered to a large extent. In addition, the wear rate among the FGM materials is also lower than the conventionally manufactured materials; this advantage helps in implants to avoid post-surgery complications. FGM parts possess better strength and the deformity among the parts is very small. The FGM with AM has extreme control over microstructure, thus different lattices are used for patient specific implants and tools, so not only small parts but also implants of higher dimensions are possible with AM.180 The paper shall help orthopedic experts and AM scientists to work further to exploit the potential of AM technologies in developing implants and parts for orthopedic applications.

7

7 Future recommendations

AM has opened a window for grading the parts in all the directions. Apart from these advantages, there are some challenges associated with it. The FGM is associated with high material processing costs. The energy optimization for FGAM must be a research area for future course of time. The surface quality of FGAM orthopedic implants is always a problem. Due to the accuracy factor associated with it, it is not possible to manufacture FGAM implants in a large quantity. Hence, bulk manufacturing is not possible for FGAM implants in the present situation. The numerical methods for FGAMs are the future research areas, which include developing mathematical model for FGMs and simulation for various AM processes.181 Also, researchers need to put their focus on process optimization for developing perfect orthopedic implants. In-depth research is required for cost cutting, since FGAM implants are expensive. Moreover, research into its reliability for high stress concentration areas is necessary.182

Funding

Not Applicable.

Author contribution

Writing Original Draft: SR and AM Conceptualization, Review and Supervision: AR, Conceptualization, Review and Supervision MIUH, Conceptualization and Review: NN, Conceptualization and Review: AZ, Conceptualization and Review: MB.

Informed consent

Not Applicable.

Institutional ethical committee approval

Not Applicable.

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