Individual humeral head replacement by C/C composite implants coated with hydroxyapatite via rotation plasma spraying

1 Introduction

Carbon is one of the major elements that constitute organic matter and serves as a vital element in the human body [1]. Carbon-based materials have been widely applied in artificial products such as cardiac valves, bone, root, vessels, and tendons. C/C composite materials have high strength, high tenacity, corrosion resistance, and high temperature resistance and do not have the brittleness of single-carbon materials [2,3]. Newly developed C/C composite materials consist of carbon substrates enhanced with carbon fiber. These materials can be used as potential biomedical materials. C/C composites and human bones have similar moduli; thus, C/C composites can be used as joint and bone prosthetics [4].

Artificial bone replacement surgery is an effective and widely applied treatment. Although the general morphology of the human skeleton is uniform, skeletal structures differ among individuals. For instance, the femoral collodiaphyseal angle varies from 110° to 140° with an average of 125°. Uniform prostheses will not be applicable to all patients. Common prostheses do not fit all cavitas dentis, thus leaving gaps between prostheses and bones. The prosthesis also shakes slightly when stressed. By using micromotion experiments on the bone trabecula of cows, Bowman et al. [5] found that micromovements cause fatigue damage to the bone trabecula. Close integration enables the balanced transfer of stress from the prosthesis to the skeleton, thus reducing the osteomalacia and bone resorption caused by stress shielding [6]. Therefore, artificial prostheses are highly individualized, that is, standard bone products are unsuitable for most patients. The current parameters used for artificial prostheses are generally tailored to Caucasians [7]. Individual prostheses should be designed to satisfy the requirements of the skeletal structure of specific patients.

C/C composite materials are also biologically inert. Thus, the surfaces of these materials should be modified to increase their biocompatibility and prevent carbon particles from sloughing. Existing active coatings for C/C composite materials are mainly focused on hydroxyapatite (HA) [8–10]. HA is present in bones and has the same physical and chemical characteristics as bones; these properties facilitate the acceptance of HA in human tissues [11]. Coating preparations are diverse [12,13], and plasma spraying has been clinically applied for HA coatings [14,15]. Thus, the preparation of HA for applications in human bones will be the future focus of bone research [16,17]. C/C composite materials and HA can be combined to maximize their advantages.

In this study, rapid prototyping was used in the fabrication of artificial bones from individual C/C composites. First, human bones were treated with argon plasma to improve the bond strength between the coating and substrate. The best working conditions of argon plasma treatment are as follows: 600 V direct current power supply voltage, 20 min work time, and 60 Pa argon gas pressure [18]. Rotation plasma spraying was used to apply the HA coating onto areas that will come in contact with the bone after bone insertion into the cavum medullare. Finally, the humeral heads of rabbits were replaced to study the biocompatibility of the coating interface.

2 Experimental procedure

3 Results and discussion

The humeral head of the individual C/C composite materials was processed by a CNC machining center (Figure 1). Part a was treated for wear prevention by using the gradient CVD method, which contributed to the friction between joints without inducing carbon particle shedding. Part b was covered with the HA coating because this part is inserted into the cavum medullare and comes into contact with the remaining osseous tissues. The HA coating of Part b promoted the integration of osseous tissues and C/C composite materials.

Figure 1

Gross appearance of the implants.

Figure 2 shows the surface morphology of the HA coating. The surface was rough with interstices, which facilitated osteoblast growth. The examination of the cross-section of the coating indicated a thickness of 70 μm (Figure 3), which met the required thickness for HA coatings in the human body [20,21].

Figure 2

HA coating morphology (arrows represent the interstice).

Figure 3

Scanning electron micrograph of the HA coating section.

The electron spectrum of the coating indicated that the coating was mainly composed of Ca, P, C, O, and other elements. The detected C and O were from the C/C composite. The XRD examination of the HA coating spectrum showed characteristic diffraction peaks of the HA crystal surface at (002), (211), (210), (112), and (202) as well as part of the diffraction peaks of calcium phosphate and calcium oxide. The diffraction peaks of other impurities that may be related to the presence of contaminants in the plasma gun were also detected. The energy-dispersive X-ray spectroscopy (EDS) and XRD results are shown in Figure 4. The detected area is the squares symbolized in Figure 2A.

Figure 4

(A) EDS and (B) XRD results of the HA coating.

Figure 5 shows the CT image of the implants after insertion into the cavum medullare. The size of the prosthesis was consistent with that of the cavum medullare, thus enabling the HA on the prosthesis to bind completely with osseous tissues and promoting the growth of osseous tissues into the artificial bones.

Figure 5

CT map after the implantation of humeral head.

The examination of the coated areas 5 days after implantation showed the development of a phosphorite kainotype deposit on the surface of the coating. The phosphorites were hemispherical and covered the original coatings. The phosphorites also displayed a lamellar crystal conformation under high SEM magnification (Figure 6A). The EDS analysis showed that the phosphorites mainly consisted of Ca and P, with C and O from the basal body and Na was from the animal. Some of the coated surface was covered with osteoblasts that closely adhered to the HA coating. The pseudopodia of the cells stretched and extended along the uneven coating surface and entered the interstices of the coating. The osteoblast growth on the coating surface indicated good biocompatibility.

Figure 6

Scanning electron micrograph of the surface of the HA coating at 5 days after implantation and the corresponding EDS: (A) new phosphorites and (B) osteoblasts on the coating surface.

Figure 7B shows the scanning electron micrograph of the artificial bone 150 days after implantation. The implants with the HA coating closely combined with the surrounding osseous tissues and crossed the juncture of the bones and phosphorite. As shown in the vertical direction, the flaky osteoblasts stacked together and grew along the coating with extended pseudopodia and stromal secretions. This result indicated that the osteoblasts were growing well on the coating. On the surface of the C/C composite materials, the osteoblasts were growing into the interstitium between the carbon fiber surface and the carbon fibers with extended pseudopodia and stromal secretions (Figure 7A). The proliferation speed is slow on the C/C composite surface osteoblast.

Figure 7

Scanning electron micrograph of the surface of the HA coating 150 days after implantation and the interface of HA coating: (A) C/C composite material and (B) HA coating.

Figure 8 shows the micrograph of the surface of the HA coating 150 days after implantation under hematoxylin and eosin staining. Numerous mature and maturing bone trabeculae were observed around the coating of the C/C composite. The fracture healed, and a subfiber bone callus formed. The formation of new bone trabeculae and osseous calluses (mature bone trabeculae) demonstrated that the HA coating had good biocompatibility. After 3D CT reconstruction (Figure 9), the artificial humeral head completely fused together with surrounding bone tissues. This finding indicated that the coated C/C composite combined well with osseous tissues.

Figure 8

Micrograph of the HA coating interface 150 days after implantation under hematoxylin and eosin staining (black and arrows represent the C/C composite and red bone trabeculae, respectively).

Figure 9

CT image of 3D reconstruction.

Long-term implantation causes orthopedic implants to become malleable. This issue is of great concern in orthopedics. The surfaces of implants are generally modified to improve their properties [22]. Considering the limitations of autologous and allogeneic bone grafts as well as the risk of infection, new methods of bone transplantation should be introduced. The long-term implantation of metal implants leads to the release of metallic ions, which causes side effects [23]. Furthermore, the mechanical properties of metallic materials differ greatly from that of human bones, thus increasing the risk of “stress shielding.” Uniform artificial prostheses do not fit all patients. By contrast, individualized artificial bones have the same size as that of replaced artificial bones and closely meet the dynamic requirements of patients. Therefore, the preparation of individualized HA-coated artificial bones from C/C composites has great potential in solving the existing problems from commonly used metal implants.

Ideal implant materials should facilitate osteogenesis, bone induction, and absorption [24]. HA is such a material. HA exchanges iron with animal bones to generate negative charges. Negative charges attract calcium and phosphate ions and facilitate their deposition onto the surface of the implant, thereby stimulating the growth of new bones [25,26]. In this research, phosphorite kainotype was deposited on the surface of the HA coating, thus contributing to osteoblast growth. The HA coating induced osteocyte differentiation to accelerate the integration of artificial bones and osseous tissues.

4 Conclusion

Rotating plasma spraying was used to coat HA on individualized artificial bones prepared from C/C composites. The coating allowed the implants to adapt to dynamic requirements. New phosphorite kainotype was deposited on the surface of the HA coating 5 days after implantation. Unlike uncoated individualized artificial C/C composite bones, the HA coating promoted osteoblast growth on the coating surface. Furthermore, the bonding strength of the individualized artificial bones with osseous tissues increased 150 days after implantation.