Bone provides a living system with essential rigidity. Accidents, infections, bone tumors, and other diseases make bone repair and regeneration a substantial challenge in orthopedics. Major bone reconstruction methods usually use autografts or allografts with good biocompatibility to enhance bone union. However, these bone grafts suffer from numerous limitations, making synthetic alternatives a fascinating option. Calcium phosphates act as synthetic bone graft substitutes because they are osteoconductive and exhibit the possibility of biodegradation.
Osteoconduction is the ability of bone-forming cells to invade the scaffold that is formed by the osteoconductive material and gradually replace it with new bone. However, calcium phosphates also carry the risk of implant failure due to insufficient porosity. Dense biomaterials represent only an interfacial connection with host tissue and may cause encapsulation of the implant by fibrous tissue. Thereby, dense biomaterials may hinder the degradation process and are therefore prone to microbial adhesion and the development of infections.
Osteogenic implants should mimic the bone structure, morphology, and function of human bone to optimize integration with surrounding native tissue. Notably, these factors vary dependent on bone type. In trabecular bone, the morphology is porous ranging between 50-90% porosity and pore sizes on the order of one millimeter in diameter. In cortical bone, the structure is more solid with a porosity ranging between 3-12%. However, the porosity also depends on age, nutritional state, activity, and disease status, like other mechanical properties, such as strength . For these reasons, there is an evident demand for fabrication procedures for porous materials that can control pore size and distribution, porosity, and the mechanical properties of implants for medical applications.
The impact of pore size on bone formation
So why is the porosity so important in the fabrication of medical implants? Porosity is defined as the percentage of void space in a solid, and it is a morphological property independent of the material. In bone tissue formation, pores are necessary because they allow vascularization as well as migration and proliferation of osteoblasts, osteoclasts, and mesenchymal cells. Therefore, the internal architecture and porosity of the implant is critical for the full restoration of the functionality of bones. Furthermore, a porous surface improves the mechanical interlocking and thereby stability between the surrounding natural bone and the implant.
Notably, porosity is linked to the pore size though they are not the same thing. Macroporosity is defined as pore sizes over 50 micrometers (μm), while microporosity is defined as pore sizes below 10 μm. Macroporosity has a strong impact on osteogenic outcome. However, microporosity and pore wall roughness play an essential part as well, as microporosity increases the surface area of the scaffold, which is believed to contribute to higher bone-inducing protein adsorption and ion change. Surface roughness improves attachment, proliferation, and differentiation of bone forming cells.
Studies have shown that, in vivo, the minimum pore size required to regenerate mineralized bone is approximately 100 micrometers. Large pores (100-200 μm) facilitate significant bone ingrowth, whereas smaller pores (75-100 μm) result in ingrowth of unmineralized osteoid tissue. Mini pores (10-75 μm) are only penetrated by fibrous tissue .
Furthermore, studies have demonstrated that osteogenesis seems to be optimal when the macropore diameter is greater than 400 micrometers due to cell size and migration and vascularization requirements . Moreover, one study found that the osteogenesis process is optimal when macropore diameter is 1000 micrometers . The same pattern is confirmed to be true with respect to the degree of porosity, where higher porosity leads to improved osteogenesis. Recognizing the importance of porosity, 3D engineering technologies are increasingly used to develop optimal scaffolds that support a rapid and strong bone regeneration.
Particle3D – the developer of a patented 3D printing technology to produce natural, patient specific, and resorbable bone implants – pays special attention to the size and interconnectivity of the implant pores through 3D engineering. The company endeavors to continuously refine its implants to mimic natural bone topography with a controlled distribution of heterogeneous micro and macropores.
3D printing enables bone implant customization with regard to both shape and structure
Newly published pre-clinical data on the performance of a novel form of porous ceramic bone implants in a mouse model of craniectomy demonstrates an attractive method for rapidly and simply 3D printing biocompatible and bone forming implants . Developed by Particle3D, the fatty acid and β-tricalcium phosphate based 3D printing technology delivers several dimensions of implant customization. By utilizing CT or MRI scan data combined with 3D modeling capabilities, it is possible to reconstruct the geometric data regarding the bone structure – with clinically relevant porosity according to the recipient’s tissue type in the relevant area – as well as the defect.
With the fabrication technique, implants may be designed with balanced morphological and mechanical properties that meet the specific needs of the bone repair site. That is, to ensure that the bone implants deliver patient specific shapes tailored to the bone repair site while also mimicking the internal structure of the relevant native human bone. Especially, the 3D printing technology can deliver large interconnected macropores to offer space for the migration and proliferation of osteoclasts, osteoblasts, and mesenchymal stem cells as well as vascularization throughout the implant. In addition, as the fatty acid subsequently is removed through sintering of the manufactured scaffold, a significant microporosity is added, producing micropores below 10 micrometers in diameter. This increases the surface area onto which cells can attach, thereby enhancing cell growth sites.
At present, Particle3D has established pre-clinical proof through laboratory tests and five animal trials – three in mice and two in pigs. In the in vivo mouse study, which was recently published in the Journal of Tissue Engineering and Regenerative Medicine, the implants promoted defect healing with osseointegration to adjacent bone and formation of new bone and bone marrow tissue in the implant pores. The implants used in the study deliver homogenous square pores which since then have been further developed into a more organic bone structure of heterogeneous micro and macropores. The published results demonstrate that the 3D printing method is attractive for creating biocompatible and bone forming β-tricalcium phosphate implants for larger patient specific craniofacial defects.
In 2019 and 2020, further pre-clinical proof was established in large animal models, based on which additional data on the 3D engineered ceramic implants is underway.
 Karageorgiou V, Kaplan D. Porosity of 3D biomaterial scaffolds and osteogenesis. Biomaterials 2005;26(27):5474–91.
 Tsuruga E, Takita H, Itoh H, Wakisaka Y, Kuboki Y. Pore size of porous hydroxyapatite as the cell-substratum controls BMP-induced osteogenesis. J Biochem 1997;121(2):317–24.
 Huri PY, Ozilgen BA, Hutton DL, Grayson WL. Scaffold pore size modulates in vitro osteogenesis of human adipose-derived stem/stromal cells. Biomed Mater 2014;9(4):045003.
 Jensen MB, Slots C, Ditzel N, et al. Treating mouse skull defects with 3D printed fatty acid and tricalcium phosphate implants. Journal of Tissue Engineering and Regenerative Medicine. 2020; 1-11.
About the Author:
Casper Slots has research experience in materials science and development, tissue engineering and drug delivery – all in the fields of 3D printing. He has spent the last five years developing new 3D printable biological inks and drug delivery systems. He is a registered nurse with clinical experience and has extensive experience working with materials scientists, as well as with patients, clinicians, and surgeons. Casper is the Chief Commercial Officer of Particle3D where he oversees the development and commercial strategies of the company. Casper holds an MSc (Eng.) in Health & Welfare Technology, a BSc in Nursing, and is a TEDx speaker.