The improved n-HA/PA66 cage has been developed from the original hollow cylindrical shape to a trapezoidal and wedge shape, which has a better biomechanical property. The morphology of the improved n-HA/PA66 cage and PEEK cages is similar, and the difference between them is mainly in the material properties. PEEK is a bioinert material, while n-HA/PA66 is a bioactive material. To study the long-term outcomes of the improved n-HA/PA66 cage and PEEK cage in ACDF, focusing on material differences, only single-segment was included. The retrospective case-matched study showed that excellent radiographic fusion, low subsidence, and great clinical results were essentially similar in patients with improved n-HA/PA66 cages and PEEK cages. Although the improved n-HA/PA66 group had a slightly greater fusion rate and better intervertebral height and segmental angle maintenance on imaging than the PEEK group, these differences were not statistically significant.
An n-HA/PA66 cage composite with an n-HA to PA66 ratio of 6:4 was used as the biomimetic scaffold [18]. The n-HA crystals used in this cage have a composition and structure that is remarkably similar to those of natural bone materials. It can support osteocyte adhesion and proliferation and induce osteocytes to release a range of osteogenic differentiation factors. Moreover, it plays a significant role in osteoconduction and supplies crystal nuclei for the calcification of osteocytes throughout the calcification process. Therefore, it has been considered an ideal material for constructing bone tissue engineering scaffolds [19, 20]. The n-HA crystals impart biological activity to the n-HA/PA66 cage, providing a calcium-phospho-rich microenvironment for bone conduction and induction after implantation [18, 21]. This fact might explain why the n-HA/PA66 cage group had a slightly higher fusion rate at 6 months than the PEEK cage group. However, the difference was not statistically significant, and at the last follow-up, the fusion rates were similar in both groups. The findings were in line with earlier studies [9, 10]. We speculated that there were two main reasons for this unexpected phenomenon. On the one hand, the quantity of autografts is positively correlated with fusion rates [22, 23]. The bone graft volumes in both cages were similar and sufficiently large to be filled with bone fragments during surgery, and they had enough bone mass for osteogenesis. On the other hand, the pore size and porosity played crucial roles in the scaffold architecture and cell proliferation, differentiation and bone in-growth [24]. In the goat C3/4 partial discectomy and fusion model, the mean CT fusion scores of the porous n-HA/PA66 group were significantly higher than those of the dense strut group (porous group vs. dense group: 15.33 ± 2.55 and 10.67 ± 2.55 at 12 weeks, 23.60 ± 3.57 and 16.60 ± 4.67 at 24 weeks, P < 0.05). There was a significant difference in the fusion rate between the two groups. Histologic evaluation showed that the mean new bone volumes of the porous n-HA/PA66 group and the dense strut group after surgery were 13.27 ± 2.87 and 42.80 ± 8.56 at 12 weeks and 20.93 ± 3.39 and 68.13 ± 14.03 at 24 weeks, respectively (P < 0.05). As a result, faster bone in-growth occurred with the porous struts [25]. The improved n-HA/PA66 cage is still dense, which could limit its biological activity. To overcome these drawbacks, a novel porous n-HA/PA66 composite has been developed in recent years [24,25,26,27]. More effort is necessary to translate it into clinical use, which might enhance radiological and clinical results.
Cage subsidence in ACDF can be influenced by many factors, including end plate preparation, postoperative cervical motion, cage design and material properties, implantation of the anterior cervical plate, and bone mineral density or age [28,29,30,31]. This case-matched study was performed to reduce the confounding factors as much as logically and reasonably possible to examine the impact of the subsidence of material characteristics, and the two groups were well matched. Furthermore, the n-HA/PA66 cage was improved to be a more compatible shape with favourable biomechanics, consistent with the PEEK cage. Although the subsidence rate decreased to 6.9% from the previously reported 10.6% with the old cylindrical n-HA/PA66 cage, there was no significant difference compared with the PEEK group (12.1%) [9].
Subsidence is believed to be related to high pressures transferred through interbody spacers on a small surface area. The elastic modulus of the n-HA/PA66 cage was 5.6 GPa, and that of the PEEK cage was 3.5 to 4 GPa [32, 33]. Both cages are similar to natural bone, resulting in lower stress shielding and avoiding some stress shielding accompanied by metallic implants. However, the elastic modulus of the cartilage endplate and cancellous bone (0.1–0.5 GPa) is lower than that of both cages, indicating that subsidence at the interface seems inevitable. In this study, the subsidence of the n-HA/PA66 group was slightly lower than that in the PEEK group, likely because the former fused faster. The more important reason is that the n-HA/PA66 composite has better osseointegration properties and an improved integrated bone-implant interface. Animal experiments have demonstrated that the PEEK cage generates peri-implant inflammatory factors after implantation, gradually forming a fibrous tissue layer on the cage surface that bridges the graft for poor osteogenesis. There was an evident radiolucent rim at the bone graft/PEEK interface with no bone integration, which was called “PEEK-Halo” [34]. The same phenomena were reported by Li et al. who found that the PEEK implant showed a fibrous inert interface and less bone formation, and the PEEK halo line could be seen clearly during long-term observation. In contrast, in the n-HA/PA66 group, these authors 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 zone decreased and disappeared gradually by 24 weeks. Histological analysis confirmed that more newly formed bone was observed around the n-HA/PA66 implants than PEEK implants during the entire implantation period, and the new bone grew into part of the n-HA/PA66 implant, by which the strut could be integrated with the host bone [35]. Therefore, n-HA/PA66 has better solid anchoring with bone tissue than PEEK, so the intervertebral height and segment angle were better maintained in the n-HA/PA66 group.
In a long-term study, we observed a significant decline in neck and arm pain VAS scores, and the scores did not significantly differ between the two groups at any time point. The NDI and JOA scores showed satisfactory improvement in both groups. No serious complications occurred in our study. The satisfaction rates of the two groups reached those in previous reports [36]. Although one disc undergoing ACDF with n-HA/PA66 cages exhibited solid union on radiographs, the patient complained of neck discomfort, and the clinical results were assessed as poor. Further investigation revealed that the patient suffered from depression pre- and postoperatively. Depression can have a strong association with postoperative outcomes [37]. Therefore, we advocate that surgeons must not only master superb surgical techniques but also relate to the mental health of their patients. Overall, the present result is comparable with our previous results from a series of 98 patients who underwent single-level anterior cervical decompression and fusion (ACDF) using a PEEK cage or an n-HA/PA66 cage for cervical degenerative disease [9].
There were some limitations in the present study. It was a retrospective analysis, and the number of cases was relatively small. Prospective studies with a large number of patients are required to confirm the present findings. This study mainly focused on the comparison of two different cages, and thus the inclusion criteria were relatively strict. And the application in multi-segment ACDF was not discussed. In multi-segmental ACDF, the improved cage has also achieved good outcomes, which will be reported in the future.