Statistical shape modelling of the thoracic spine for the development of pedicle screw insertion guides.

Spinal fixation and fusion are surgical procedures undertaken to restore stability in the spine and restrict painful or degenerative motion. Malpositioning of pedicle screws during these procedures can result in major neurological and vascular damage. Patient-specific surgical guides offer clear benefits, reducing malposition rates by up to 25%. However, they suffer from long lead times and the manufacturing process is dependent on third-party specialists. The development of a standard set of surgical guides may eliminate the issues with the manufacturing process. To evaluate the feasibility of this option, a statistical shape model (SSM) was created and used to analyse the morphological variations of the T4-T6 vertebrae in a population of 90 specimens from the Visible Korean Human dataset (50 females and 40 males). The first three principal components, representing 39.7% of the variance within the population, were analysed. The model showed high variability in the transverse process (~ 4 mm) and spinous process (~ 4 mm) and relatively low variation (< 1 mm) in the vertebral lamina. For a Korean population, a standardised set of surgical guides would likely need to align with the lamina where the variance in the population is lower. It is recommended that this standard set of surgical guides should accommodate pedicle screw diameters of 3.5-6 mm and transverse pedicle screw angles of 3.5°-12.4°.

Additively manufactured lattice structures with controlled transverse isotropy for orthopedic porous implants.

Additively manufactured lattice structures enable the design of tissue scaffolds with tailored mechanical properties, which can be implemented in porous biomaterials. The adaptation of bone to physiological loads results in anisotropic bone tissue properties which are optimized for site-specific loads; therefore, some bone sites are stiffer and stronger along the principal load direction compared to other orientations. In this work, a semi-analytical model was developed for the design of transversely isotropic lattice structures that can mimic the anisotropy characteristics of different types of bone tissue. Several design possibilities were explored, and a particular unit cell, which was best suited for additive manufacturing was further analyzed. The design of the unit cell was parameterized and in-silico analysis was performed via Finite Element Analysis. The structures were manufactured additively in metal and tested under compressive loads in different orientations. Finite element analysis showed good correlation with the semi-analytical model, especially for elastic constants with low relative densities. The anisotropy measured experimentally showed a variable accuracy, highlighting the deviations from designs to additively manufactured parts. Overall, the proposed model enables to exploit the anisotropy of lattice structures to design lighter scaffolds with higher porosity and increased permeability by aligning the scaffold with the principal direction of the load.

Mechanical and morphological properties of additively manufactured SS316L and Ti6Al4V micro-struts as a function of build angle.

Additive manufacturing methods such as laser powder bed fusion (PBF) can produce micro-lattice structures which consist of 'micro-struts', which have properties that differ from the bulk metal and that can vary depending on the orientation of the strut to the build direction (the strut build angle). Characterizing these mechanical and morphological changes would help explain macro-scale lattice behavior. Individual stainless steel (SS316L) and titanium alloy (Ti6Al4V) laser PBF struts were built at 20°, 40°, 70° and 90° to the build platform, with 3 designed diameters and tested in uniaxial tension (n = 5). Micro-CT was used to quantify changes in surface roughness, eccentricity and cross-section. Average elastic modulus was 61.5 GPa and 37.5 GPa for SS316L and Ti6Al4V respectively, less than the bulk material. Yield strength was uniform over build angle for SS316L, but for Ti6Al4V varied from 40% to 98% of the bulk value from 20° to 90° build angles. All lower angle struts had worse morphology, with higher roughness and less circular cross-sections. These data should help inform micro-lattice design, especially in safety critical applications where lower mechanical performance must be compensated for.

Laser powder bed fusion of porous graded structures: A comparison between computational and experimental analysis.

Functionally graded porous structures (FGPSs) are gaining interest in the biomedical sector, specifically for orthopaedic implants. In this study, the compressive behaviour of seven different FGPSs comprised of Face Centred Cubic (FCC) and the Octet truss unit cells (OCT) were analysed. The porosity of the structures were graded in different directions (radially, longitudinally, laterally and longitudinally & radially) by varying the strut diameters or by combining the two types of unit cells. The structures were manufactured by laser power bed fusion and compression tests were performed. Radially and laterally porous graded structures were found to outperform uniform porous structures with an increase in stiffness of 13.7% and 21.1% respectively. The experimental and finite element analysis (FEA) results were in good agreement with differences in elastic modulus of 9.4% and yield strength of 15.8%. A new FEA beam model is proposed in this study to analyse this type of structures with accurate results and the consequent reduction of computational time. The accuracy of the Kelvin-Voight model and the rule of mixtures for predicting the mechanical behaviour of different FGPSs was also investigated. The results demonstrate the adequacy of the analytical models specifically for hybrid structures and for structures with smooth diameter transitions.

The Design and In Vivo Testing of a Locally Stiffness-Matched Porous Scaffold.

An increasing volume of work supports utilising the mechanobiology of bone for bone ingrowth into a porous scaffold. However, typically during in vivo testing of implants, the mechanical properties of the bone being replaced are not quantified. Consequently there remains inconsistencies in the literature regarding 'optimum' pore size and porosity for bone ingrowth. It is also difficult to compare ingrowth results between studies and to translate in vivo animal testing to human subjects without understanding the mechanical environment. This study presents a clinically applicable approach to determining local bone mechanical properties and design of a scaffold with similar properties. The performance of the scaffold was investigated in vivo in an ovine model. The density, modulus and strength of trabecular bone from the medial femoral condyle from ovine bones was characterised and power-law relationships were established. A porous titanium scaffold, intended to maintain bone mechanical homeostasis, was additively manufactured and implanted into the medial femoral condyle of 6 ewes. The stiffness of the scaffold varied throughout the heterogeneous structure and matched the stiffness variation of bone at the surgical site. Bone ingrowth into the scaffold was 10.73±2.97% after 6 weeks. Fine woven bone, in the interior of the scaffold, and intense formations of more developed woven bone overlaid with lamellar bone at the implant periphery were observed. The workflow presented will allow future in vivo testing to test specific bone strains on bone ingrowth in response to a scaffold and allow for better translation from in vivo testing to commercial implants.

Individual response variations in scaffold-guided bone regeneration are determined by independent strain- and injury-induced mechanisms.

This study explored the regenerative osteogenic response in the distal femur of sheep using scaffolds having stiffness values within, and above and below, the range of trabecular bone apparent modulus. Scaffolds 3D-printed from stiff titanium and compliant polyamide were implanted into a cylindrical metaphyseal defect 15 × 15 mm. After six weeks, bone ingrowth varied between 7 and 21% of the scaffold pore volume and this was generally inversely proportional to scaffold stiffness. The individual reparative response considerably varied among the animals, which could be divided into weak and strong responders. Notably, bone regeneration specifically within the interior of the scaffold was inversely proportional to scaffold stiffness and was strain-driven in strongly-responding animals. Conversely, bone regeneration at the periphery of the defect was injury-driven and equal in all scaffolds and in all strongly- and weakly-responding animals. The observation of the strain-driven response in some, but not all, animals highlights that scaffold compliance is desirable for triggering host bone regeneration, but scaffold permanence is important for the load-bearing, structural role of the bone-replacing device. Indeed, scaffolds may benefit from being nonresorbable and mechanically reliable for those unforeseeable cases of weakly responding recipients.

Additive manufactured push-fit implant fixation with screw-strength pull out.

Additive manufacturing offers exciting new possibilities for improving long-term metallic implant fixation in bone through enabling open porous structures for bony ingrowth. The aim of this research was to investigate how the technology could also improve initial fixation, a precursor to successful long-term fixation. A new barbed fixation mechanism, relying on flexible struts was proposed and manufactured as a push-fit peg. The technology was optimized using a synthetic bone model and compared with conventional press-fit peg controls tested over a range of interference fits. Optimum designs, achieving maximum pull-out force, were subsequently tested in a cadaveric femoral condyle model. The barbed fixation surface provided more than double the pull-out force for less than a third of the insertion force compared to the best performing conventional press-fit peg (p < 0.001). Indeed, it provided screw-strength pull out from a push-fit device (1,124 ± 146 N). This step change in implant fixation potential offers new capabilities for low profile, minimally invasive implant design, while providing new options to simplify surgery, allowing for one-piece push-fit components with high levels of initial stability. © 2017 The Authors. Journal of Orthopaedic Research® Published by Wiley Periodicals, Inc. on behalf of the Orthopaedic Research Society. J Orthop Res 36:1508-1518, 2018.