Interbody spinal fusion using interbody fusion cages has many advantages for the treatment of spinal disorders. Fusion cages have been clinically used routinely for more than 20 years with a variety of implant materials and different designs (Albrektsson and Jacobsson, 1987; Bagby, 1988; Cloward, 2007). Most currently used fusion cages are constructed from polyether-ether-ketone (PEEK) materials. Owing to its good biocompatibility, a cortical bone with a similar elastic modulus and mechanical characteristics has now been widely used in spinal fusion or non-fusion, trauma, and neurosurgical and cranio-maxillofacial surgeries for more than 15 years (Toth et al., 2006; Kurtz and Devine, 2007). However, bone does not bond to PEEK, a potential complication of PEEK is that its limited osseointegration with the surrounding bone results in a “PEEK-halo” effect (Phan et al., 2016). This “PEEK-halo” denotes a “radiolucent zone” that can be clearly seen on X-ray radiographs at the bone-implant interface, which may inhibit a successful fusion in the spinal interbody space. Radiolucent zones around the implant may be an indication of weak osseointegration, and fibrous encapsulation may be formed at the PEEK-bone interface, potentially leading to clinical failure with long-term implantation in the body (Noiset et al., 1999; Wu et al., 2013; Walsh et al., 2015).

Weak osteointegration affects the long-term stability of the interbody cages (Yuan et al., 2018). Much work has been conducted to improve the osteointegration property of interbody cages. Several interbody cages now have additional surface coating to enhance the bone-cage interface. Achieving long-term clinical success of interbody cage implants remains a significant and multifaceted challenge (Torstrick et al., 2018). Understanding the osseointegration of the cage materials will help the selecting and designing of new interbody cages materials in the future. The nanohydroxyapatite/polyamide 66 (n-HA/PA66) and PEEK are two materials currently utilized in interbody cages. The n-HA/PA66 composite is a bioactive material with the ability to promote new bone formation and osteogenesis. However, in our previous studies, we discovered the presence of a “radiolucent gap” between the implant and the adjacent endplate bone in the radiographic images with this material also (Zhang et al., 2016). We evaluated: 1) whether the “radiolucent gap” is the same as the “Peek-halo” and 2) if n-HA/P66 exhibits an improved bone-implant interface compared to that of PEEK.

In the present study, to better identify the bone-implant interface and assess the long-term osseointegration effects between the n-HA/P66 and PEEK, we used bone defects in rabbits to create a line-to-line fit bone-implant interface model previously reported by the literature (Bertollo et al., 2011). X-ray radiography and histology were used to observe the bone-implant interface. Micro-computed tomography (micro-CT) was used to evaluate new bone formation around the implant. Additionally, to evaluate the microstructure and dynamic change in chemical elements at the bone-implant interface, scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) were performed at 4, 8, 12, 24, and 52 weeks. Finally, to evaluate the integrated strength of implants and bone, the push-out test was applied at 24 and 52 weeks.

Data are presented as mean ± standard deviation (SD). Statistical analyses were performed using SPSS16.0 software, and analyzed using analysis of variance (ANOVA) followed by Fisher’s multiple comparison tests. p < 0.05 was considered statistically significant.

Although radiolucent zones around the implant may be an indication that the implant is encapsulated by fibrous tissue, the lack of such zones does not indicate osseointegration (Albrektsson and Jacobsson, 1987). The poor osseointegration of PEEK is often attributed to its relatively bio-inert and hydrophobic properties. In contrast, n-HA/PA66 is a bioactive composite made by infiltrating nano-HA into PA66; it mimics natural bone in that the apatite is distributed within a collagen matrix (Xu et al., 2018). Thus, the composite possesses both the bioactive effects of HA and the mechanical strength of PA66. We hypothesized that the bioactive n-HA/PA66 would enhance the in vivo osseointegration effects at the bone-implant interface. We explored whether the “radiolucent gap” and “Peek-halo” were similar and if an improved bone-implant interface was exhibited with n-HA/PA66 in comparison to that with PEEK. To determine the difference between the radiolucent gaps of the two implants between the bone and the implant we assessed the differences in osseointegration effects of the two implants after implantation for 4, 8, 12, 24, and 52 weeks. These results demonstrate that the n-HA/PA66 implant exhibited a better osseointegrated bone-implant interface than PEEK. We found a radiolucent line around n-HA/PA66 in the early weeks after implantation, and the zone decreased and disappeared gradually. The PEEK implant showed a fibrous inert interface and less bone formation. The PEEK-halo line could be seen clearly during long-term observation. In addition, the push-out tests also demonstrated that the interface of n-HA/PA66 had a higher integrated strength than the PEEK implant.

Osseointegration has been used to describe the successful healing of an implant within the host bone for a long time (Kuzyk and Schemitsch, 2011; Shah et al., 2019). Knowledge of this bone-implant interface is useful for interpreting the reasoning behind orthopedic implant design strategies and their long-term success, and it is imperative for surgeons to have a basic understanding of the bone-implant interface healing process (Shah et al., 2019). In the present study, we discovered a radiolucent gap at the margin of the n-HA/PA66 implant by X-ray radiography and histological sections in the early weeks after implantation (4–8 weeks). Subsequently, the gap decreased and disappeared gradually at 24 weeks and becoming more narrow at 52 weeks, which is consistent with the healing process. Through histological analysis, we could clearly observe the space between the bone and implant in the early weeks after implantation (4–12 weeks). The space was gradually filled with new bone, which suggested that n-HA/PA66 had good osseointegration properties with the surrounding bones. At 24 and 52 weeks, we found that n-HA/PA66 could integrate and be connected with host bone in some regions where new bone grew into the n-HA/PA66 implant after implantation. In contrast, in the PEEK group, a radiolucent halo line could be identified by X-ray film at 4 and 8 weeks, and the fibrous gap could be clearly observed by histological assessment. Furthermore, there was no change in the fibrous interface during the healing process after 12, 24, and 52 weeks. This indicates that the PEEK-bone interface becomes inert, and the surrounding tissue has formed a stable bioinert structure. The same results were reported by Walsh and etc. who tried to incorporate HA into PEEK and plasma spraying of titanium to PEEK in order to improve osseointegration (Pelletier et al., 2016; Walsh et al., 2018).

Clinically, the success or failure of an implanted device is often measured by X-ray radiography and CT examination. The “PEEK-halo” and “radiolucent gap” effects in many clinical studies reflect the inadequacy of clinical methods for evaluating osseointegration. These tests are indirect methods, that indicate, but do not verify osseointegration. The classical description of osseointegration is a direct contact between the implant surface and bone at the light microscopic level (Shah et al., 2019), which is mainly distinguished from fibrous integration. However, this definition ignores the presence of other interposed components of the tissue along the interface. The interface should include bone-implant areas and the peri-implant bone. Micro-CT is frequently used to evaluate bone formation around the implant, but the bone-implant interface is often obscured and cannot be clearly determined (Palmquist et al., 2017). In the present study, the results of micro-CT and histological analysis showed that the n-HA/PA66 implant had higher BV/TV values and bone volumes than PEEK at 4, 8, and 12 weeks, which remained stable from 24 to 52 weeks. This demonstrated that the n-HA/PA66 surface may favor new bone formation. To assert that an implant achieved good osseointegration and solid fusion must imply that most of it is anchored with bone tissue. We therefore investigated the ultrastructure of the bone-implant interface using SEM and EDX mapping. Electron microscopy of the tissue interface is capable of producing interesting information, but it is difficult to identify the precise components of tissue around the entire implant. The results demonstrated that the n-HA/PA66 implant has a thin interface gap and a continuous mineralized interface zone, and the EDX analysis showed that a fiber medial layer was found between the PEEK implant and bone, whereas the n-HA/PA66 implant exhibited an improved mineralized implant-bone interface.

The aim of determining an optimized spinal fusion device is to achieve good osseointegration and promote long-term success (Shah et al., 2019). Through push-out tests, we could see that n-HA/PA66 implants can possess stronger bonding strength with the surrounding bone tissue and could obtain sufficient stability at 24 and 52 weeks, whereas the PEEK implant could be pushed out easily, which indicates that the surrounding bone tissue cannot adhere to PEEK. Therefore, our hypothesis that n-HA/PA66 would have better osseointegration properties and higher bonding strength than PEEK was confirmed. Several efforts have been made to improve the bioactivity and osteointegration properties of PEEK to prolong its long-term stability (Johansson et al., 2015; Torstrick et al., 2017; Liu et al., 2018); for example, plasma-sprayed titanium or HA coatings on PEEK and the incorporation of HA into PEEK (Walsh et al., 2016; Torstrick et al., 2017). However, these studies only showed good improvements in bone formation by histological analysis and micro-CT evaluation. Bone formation begins at the implant surface in response to the surface physicochemical properties of the implant surface (Liu et al., 2018). Remodeling of the bone in contact with the implant surface continues throughout the lifetime of the implant. This remodeling may allow for increased contact between the implant and the bone over time and likely continues for longer than six months. Our previous studies have shown that a bone-like apatite layer can be formed on the surface of n-HA/PA66 in simulated body fluid (Wang et al., 2002), and the n-HA/PA66 composite could promote the growth and osteogenic differentiation of mesenchymal stem cells (Xu et al., 2018). The EDX mapping results revealed that Ca and phosphonium were successfully incorporated on the surface of n-HA/PA66 and were homogeneously distributed. Thus, the n-HA particles on the surface provide a good osteoconductive surface, which has a positive effect on bone apposition and mineralization.

In summary, we found that the n-HA/PA66 implant has good osseointegration properties and exhibits an improved bone-implant interface. Thus, this composite is appropriate for bone formation and bonding with the surrounding bone and provides better biomechanical stability than PEEK. Consequently, the n-HA/PA66 composite exhibited good osseointegration properties for clinical applications. Additional clinical and experimental studies are required to elucidate the precise bonding mechanisms underlying the bone-implant interface.