Our group previously found that calcium polyphosphate fibers can improve the mechanical properties of tissue-engineered bone and provide good bore diameter and porosity after degradation to facilitate bone ingrowth and repair based on studies of calcium polyphosphate fibers, calcium phosphate bone cement, and small-particle bone repair bone defect. Based on these relevant studies, we used SF as reinforcing materials to repair the bone defect due to its good biological properties [
4,
13,
14]. SF is a fibrous protein that is produced mainly by silkworms and spiders. Its unique mechanical properties, including a tunable biodegradation rate and the ability to support the differentiation of mesenchymal stem cells along the osteogenic lineage, have made SF a favorable scaffolding material for bone tissue engineering. SF can be processed into various scaffold forms, combined synergistically with other biomaterials to form composites and chemically modified components, which provides an impressive toolbox and allows SF scaffolds to be tailored to specific applications.
This study was based on the fiber-reinforcing principle in which we prepared SF into short fibers and then combined it with CPC, so the composite had both enhanced rigid strength and increased porosity; furthermore, with the constant degradation of SF in vivo, the porosity of the biocomposite was further increased, allowing it to promote bone cell ingrowth, promote Ca deposition, and accelerate bone replacement, thus facilitating the gradual absorbance of bone cement and achieving the purpose of ideal bone defect repair [
15]. At the same time, the addition of SF can reduce the occurrence of CPC loosening after implantation, reduce the risk of pulmonary embolism [
16,
17], and increase the safety of CPC.
The use of SF/CPC biocomposite in bone tissue engineering has theoretical and practical feasibility. The tissue slices in this study showed that, due to the addition of the small bone particles, the SF/CPC/PB and PB groups exhibited bone cell growth and trabecular bone formation in the early experimental stage, and over time, the trabecular structure tended to be complete and robust, indicating that bone particles play an important role in the bone creeping and substitution process [
18]. The SF/CPC group exhibited only slight trabecular bone formation in week 4, which may have been due to the following: (1) bone replacement and bone formation start from the interface and gradually developing toward the internal portion of the composite and (2) since the SF/CPC biocomposite lacks bone mesenchymal stem cells (BMSCs) and cell growth factors, BMSCs gradually migrate into the biocomposite and then release cell growth factors [
19,
20]. In week 8, histological observation revealed the formation of a large amount of trabecular bone, indicating that the SF/CPC biocomposite may meet the bone formation conditions: (1) when SF degrades in vivo, the porosity of the composite increases; (2) with gradual BMSC growth and proliferation, they can continue to release cell growth factors; and (3) SF itself can induce vascularization, thus increasing the local nutrient supply. The biomechanical testing results revealed that, in week 4, the mechanical strength in the SF/CPC group was lower than those of the SF/CPC/PB and PB groups, which may be caused by the structure of trabecular bone [
21,
22]. Since SF will aggregate when it meets water, it will result in non-uniform distribution of the implant material; during the bone induction process, the growth speed of bone cells and the calcification extents of minerals differ; the SF/CPC/PB and PB groups exhibited a large range of bone formation in the early implantation stage, but the trabecular bone in the SF/CPC group formed earlier and was thinner, thus making it impossible to achieve the desired mechanical strength. In week 8, the maximum bending loads among the three groups showed no significant difference (
P > 0.05), indicating that the SF/CPC group also formed a strong trabecular structure at this time. During the trabecular bone formation process, SF constantly biodegrades, thus producing a variety of amino acids; together with the vasoactive roles of SF, it is conducive to bone cell growth. Meanwhile, CPC is also gradually degraded, resulting in an extracellular environment rich in Ca
2+ and P
3+, thus helping increase the Ca deposition among the pores. This study prepared a new SF/CPC biocomposite and verified the biocompatibility and osteoinductivity of SF in animal experiments. The definition of bone defect repair is not simply fusion, it also includes whether the post-fusion mechanical strength is similar to that of normal tissues (including flexural strength). The biomechanical testing results in week 8 showed that the new SF/CPC biocomposite exhibited strength similar to that of the fine bone particles; therefore, applying SF/CPC in bone tissue engineering can reduce the requirement of autologous bone particles to a certain extent. SF/CPC biocomposites are currently at the basic experimental stage; although there have been some achievements, it is not yet widely used in the human body. Many problems remain to be solved: SF is biodegradable, but how is its degradation time connected with the speed of osteogenesis and when SF is biodegraded more quickly than the bone is formed, how can we ensure the mechanical strength of the new material? This study mixed SF and CPC at a ratio of 1:20 and achieved the desired results, but the best mixing ratio of SF and CPC remains unclear; thus, we have yet to determine how we can improve the porosity of the composite material to improve the early-stage osteogenesis speed and strength. SF aggregates easily and affects osteogenesis, so we must still determine how to improve the experimental methods to reduce the above phenomenon and achieve more uniform overall bone formation.