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International Journal of Bioprinting 3D-printed bioceramic scaffolds for bone regeneration

tissue easily invades the bone defect and forms scars, segmental bone defect model, the pore orientations
resulting in an increased risk of non-union fractures. 5-7 in porcine collagen grafts affected cell migration and
Thus, the risk of non-union can be reduced by preventing alignment of collagen fibers in the defect. Porcine
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the invasion of excess fibrous tissue into the bone defects, collagen grafts with pores parallel to the bone axis formed
during bone regeneration. 5,6 bones faster than those with pores oriented perpendicular
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Remarkably, fibrous tissue grows faster than bone and to the bone axis or in random directions. Furthermore,
occupies more space than newly formed bone. Therefore, the collagen fibers in the bone matrix were parallel to the
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although several researchers have attempted to reconstruct bone axis, similar to those in the original long bone. 25
bone after segmental bone loss using artificial bone Thus, CAp grafts with unidirectional pores, whose
grafts, 9-13 the pores of the grafts were occupied by fibrous orientations are parallel to the bone axis and whose
tissue prior to new bone formation, resulting in the failure apertures open only onto the bone stumps, are expected
of bone regeneration. In implant dentistry, to avoid the to preferentially allow bone ingrowth and prevent the
rapid invasion of fibrous tissue into the graft, the bone is invasion of fibrous tissue for bone regeneration in the
augmented by reserving space for bone formation using a case of segmental bone loss in long bones. Based on this
barrier membrane to prevent the invasion of fibrous tissue. hypothesis, we clarified the effects of pore direction on
This process is known as guided bone regeneration. the amount and orientation of newly formed bone and
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However, barrier membranes are difficult to handle; when the prevention of fibrous tissue invasion in a rabbit ulnar
not handled properly, they do not prevent soft tissue segmental bone loss model using rectangular CAp grafts
penetration and can cause infection. Ideally, an artificial
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bone graft alone should realize both bone ingrowth and with different pore directions. The pores were oriented
prevention of fibrous tissue invasion into bone defects. along the long, short, or both long and short axes of the
grafts, which were oriented parallel, perpendicular, or both
Therefore, bone grafts should allow enhanced bone parallel and perpendicular to the bone axis, respectively.
ingrowth to predominate over soft tissue penetration.
Furthermore, when a bone graft is resorbed before bone 2. Methods
formation, soft tissue invades the space created by bone graft
resorption. Therefore, the bone graft should be maintained 2.1. Fabrication of L-, S-, and LS-grafts using
until bone formation, and then gradually resorbed. Bone 3D-printing techniques
ingrowth, osteoconductivity, and bioresorption are Three types of L-, S-, and LS-grafts were fabricated as
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affected by the chemical components of bone grafts. 16-20 described previously. Briefly, the graft structures were
Calcium sulfate and beta-tricalcium phosphate (β-TCP) designed using Fusion 360 (Autodesk, San Rafael, CA,
are resorbed spontaneously, leading to the excessive USA), resulting in a stereolithography (STL) file. A mixture
formation of fibrous tissue in the resorbed region. 16,17 of calcium carbonate powder (50% volume; Sakai Chemical,
In contrast, sintered hydroxyapatite (HAp) is resorbed Sakai, Japan) and a photosensitive resin (50% volume;
and remains intact in the human body, and carbonate SPR302, SK Fine, Kusatsu, Japan) was prepared using a
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apatite (CAp), an analog of bone minerals, is resorbed by planetary centrifugal mixer (SK-350TV; Shashin Kagaku,
osteoclasts and subsequently forms bone in the resorption Kyoto, Japan) to form a precursor slurry. An SLA-based
region. 19,20 Furthermore, the osteoconductivity of CAp is 3D printer (SZ-1100, SK Fine) was used to polymerize and
higher than that of β-TCP and HAp. 19,20 Therefore, CAp is a crosslink the precursor slurry based on STL files. The laser
potential candidate for bone regeneration, as it can inhibit of the printer had a diameter of 12 μm, power of 6 mW,
fibrous tissue invasion. scan speed of 1000 mm/s, and a wavelength of 355 nm. The

Furthermore, pore architecture is known to have a printed green bodies were subjected to ultrasonic cleaning
crucial impact on both bone formation and inhibition in ethanol for 60 s to remove uncured resin. Subsequently,
of fibrous tissue invasion. Based on previous findings the green bodies were debound following the heating
obtained using rabbit calvaria models, CAp grafts with process, which began at 232°C for 0.25°C/min, followed
uniaxial pores can promote bone ingrowth within the pores by an increase to 412°C at 0.21°C/min, another increase
while preventing the invasion of fibrous tissue. 21-23 The to 600°C at 2.3°C/min, and finally an increase to 650°C at
direction of graft pores also affects bone regeneration. 24,25 0.4°C/min, before being maintained at 650°C for 24 h. At
Using a rabbit radius segmental bone defect model Feng et 550°C, the atmosphere in the heating box changed from
al. revealed that tubular β-TCP grafts were more favorable air to carbon dioxide. Pure CaCO grafts were obtained via
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for bone ingrowth and vascularization than those with heating. After heating, the CaCO grafts were immersed in
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multidirectional pores, in a rabbit radius segmental bone a 1 mol/L Na HPO solution at 80°C for 7 days to convert
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4
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defect model. Petersen et al. reported that in a rat femoral the chemical composition from CaCO to CAp.
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Volume 10 Issue 2 (2024) 446 doi: 10.36922/ijb.2323