Additive manufacturing for metallic spinal implants: A systematic review

Finite element analysis of lattice designed lumbar interbody cage based on the additive manufacturing

Additive manufacturing (AM) methods, which facilitate the production of complex structures with different geometries, have been used in producing interbody cages in recent years. In this study, the effects of Ti6Al4V alloy interbody lattice designed fusion cages between the third and fourth lumbar vertebrae where degenerative disc diseases occur were investigated using the finite element method. Face centered cubic (FCC), body centered cubic (BCC), and diamond structures were selected as the lattice structure suitable for the interbody cage. A kidney shaped interbody lumbar cage was designed. The designated lattice structures were selected by adjusting the cell sizes suitable for the designed geometry, and the mesh configuration was made by the lumbar lattice structure. 400 N Axial force and 7.5 N.m moments were applied to the spine according to lateral bending, flexion, and torsion. 400 N axial force and 7.5 N.m flexion moment is shown high strain and total deformation then lateral bending and torsion on BCC FCC and diamond lattice structured interbody cages. In addition, the effects of lattice structures under high compression forces were investigated by applying 1000 N force to the lattice structures. When von Mises stresses were examined, lower von Mises stress and strains were observed in the BCC structure. However, a lower total deformation was observed in the FCC. Due to the design of the BCC and the diamond structure, it is assumed that bone implant adhesion will increase. In the finite element analysis (FEA), the best results were shown in BCC structures.

Assessment of Additive Manufactured IN 625's Tensile Strength Based on Nonstandard Specimens

The study aimed to evaluate the tensile strength of additively manufactured (AMed) IN 625 using sub-sized test pieces and compare them to standard specimens. Cylindrical round coupons of varying diameters were manufactured along the Z-axis using the laser powder bed fusion technique and subjected to heat treatment. The simulation of the alloy solidification predicted the formation of several intermetallics and carbides under equilibrium conditions (slow cooling), apart from the γ phase (FCC). Sub-sized tensile specimens with different gauge diameters were machined from the coupons and tensile tested at ambient temperature. The results showed that sub-sized specimens exhibited lower tensile and yield strengths compared to standard specimens, but still higher than the minimum requirements of the relevant ASTM standard for AMed IN 625. The lower strength was attributed to the "size effect" of the test specimens. Fracture surfaces of the sub-sized test specimens exhibit a mixed character, combining cleavage and microvoid coalescence, with improved ductility compared to standard test pieces. The study highlights the importance of adapting characterization methods to the particularities of manufactured parts, including reduced thicknesses that make sampling standard-size specimens impractical. It concludes that sub-sized specimens are valuable for quality control and verifying compliance with requirements of AMed IN 625 tensile properties.

A systematic review of follow-up results of additively manufactured customized implants for the pelvic area

INTRODUCTION

While 3D printing of bone models for preoperative planning or customized surgical templating has been successfully implemented, the use of patient-specific additively manufactured (AM) implants is a newer application not yet well established. To fully evaluate the advantages and shortcomings of such implants, their follow-up results need to be evaluated.

AREA COVERED

This systematic review provides a survey of the reported follow-ups on AM implants used for oncologic reconstruction, total hip arthroplasty both primary and revision, acetabular fracture, and sacrum defects.

EXPERT OPINION

The review shows that Titanium alloy (Ti4AL6V) is the most common type of material system used due to its excellent biomechanical properties. Electron beam melting (EBM) is the predominant AM process for manufacturing implants. In almost all cases, porosity at the contact surface is implemented through the design of lattice or porous structures to enhance osseointegration. The follow-up evaluations show promising results, with only a small number of patients suffering from aseptic loosening, wear, or malalignment. The longest reported follow-up length was 120 months for acetabular cages and 96 months for acetabular cups. The AM implants have proven to serve as an excellent option to restore premorbid skeletal anatomy of the pelvis.

Biological and Corrosion Evaluation of In Situ Alloyed NiTi Fabricated through Laser Powder Bed Fusion (LPBF)

In this work, NiTi alloy parts were fabricated using laser powder bed fusion (LBPF) from pre-alloyed NiTi powder and in situ alloyed pure Ni and Ti powders. Comparative research on the corrosive and biological properties of both studied materials was performed. Electrochemical corrosion tests were carried out in phosphate buffered saline at 37 °C, and the degradation rate of the materials was described based on Ni ion release measurements. Cytotoxicity, bacterial growth, and adhesion to the surface of the fabricated coupons were evaluated using L929 cells and spherical Escherichia coli (E. coli) bacteria, respectively. The in situ alloyed NiTi parts exhibit slightly lower corrosion resistance in phosphate buffered saline solution than pre-alloyed NiTi. Moreover, the passive layer formed on in situ alloyed NiTi is weaker than the one formed on the NiTi fabricated from pre-alloyed NiTi powder. Furthermore, in situ alloyed NiTi and NiTi made from pre-alloyed powders have comparable cytotoxicity and biological properties. Overall, the research has shown that nitinol sintered using in situ alloyed pure Ni and Ti is potentially useful for biomedical applications.