HA is the main inorganic phase of the bone matrix, so it is widely applied to make scaffolds for bone tissue repair . PA66 is a form of opalescent crystalline polymer with excellent mechanical properties (good elasticity, strength, and high elongation), and its unique molecular structure is beneficial for its modification . The nHA crystals and the carboxy and amide groups of PA66 are similar to bone apatite and collagen, respectively, so the n-HA/PA66 composites were used to mimic natural bone in previous studies. They found that the physical, chemical, and mechanical characteristics of n-HA/PA66 match well with those of natural bone . The biocompatibility and oseogenesis of the n-HA/PA66 composite scaffolds were also indicated to be good both in vitro and in vivo .
The n-HA/PA66 cage used in ACDF was firstly designed as a hollow cylindrical shape and some scholars has reported its clinical and radiological outcomes. Yang et al.  investigated the outcomes for the use of hollow cylindrical n-HA/PA66 cages for single-level ACDF and concluded that the hollow cylindrical n-HA/PA66 cage is a satisfactory reconstructing implant after ACDF. Hu et al.  further analyzed the long-term follow-up (> 7 years) results of hollow cylindrical n-HA/PA66 cages used in single-level ACDF in comparison to PEEK cages. Although there were similar radiological and clinical outcomes with the hollow cylindrical n-HA/PA66 cage as the PEEK cage, the subsidence rates of the hollow cylindrical n-HA/PA66 cage were relatively high (10.6%). The authors speculated that the main reason may be that the shape of the hollow cylindrical n-HA/PA66 cage was less suitable than that of the PEEK cage, which had an impact on the biomechanical properties of the cage.
Sequentially, to reduce the subsidence rate, the hollow cylindrical n-HA/PA66 cage was redesigned into a horseshoe-shaped cage with a more suitable shape and larger bone graft volume (novel n-HA/PA66 cage). In this study, 52 patients with novel n-HA/PA66 cages (Group A) and 55 patients with hollow cylindrical n-HA/PA66 cages (Group B) were included. The 6-month postoperative IHs in both groups were comparable and significantly improved after the surgery. However, the IH at the final follow-up of Group A was significantly higher than that of Group B (36.7 mm vs. 35.9 mm, p < 0.05). Meanwhile, the subsidence rate at the final follow-up of Group A was 5.8% (3/52), which was significantly lower than that of Group B (18.2%, 10/55). These results indicated that the modifications to the morphology of the hollow cylindrical n-HA/PA66 cage were effective in lowering the subsidence rate.
Subsidence is associated with multiple factors, such as bone mineral density, smoking, operative segments, and cage characteristics (material, shape, height, etc.) [9, 21]. In the present study, the patient demographic data (age, sex, operative time, blood loss, and segments) of Groups A and B were similar, while the primary difference between the two groups was cage shape. Different cage shapes correlated with different cage-endplate interfaces and bone graft shapes and volumes and finally led to variational postoperative fusion and subsidence resistance [14, 22]. Compared to the hollow cylindrical n-HA/PA66 cage, the novel n-HA/PA66 cage’s horseshoe shape has more inverse distributed jags, which can better mimic the concave contour of the vertebral endplate and improve the contact area and anchorage. Interestingly, the bone graft volume of the novel n-HA/PA66 cage was higher than that of the hollow cylindrical n-HA/PA66 cage, but the fusion rates at 6 months postoperatively and at the final follow-up in both groups were comparable.
Postoperative complications such as cervical kyphosis, recurrence of neurological symptoms, and internal fixation failure may be related to cage subsidence because it can compromise IH and neural foramen stenosis [9, 10]. Fortunately, no patients in either group complained about subsidence-related symptoms during the follow-up. Furthermore, the JOA and VAS scores in Groups A and B were similar; they improved postoperatively and were maintained at the final follow-up. These findings could explain the comparably ideal patient satisfaction at the final follow-up in both groups.
There were several limitations in the present study. First, this was a retrospective study with a small sample size. Second, this study did not analyze patients with multiple CDDD. Hence, future prospective studies with more patients with multiple CDDD are required to investigate the outcomes when using novel n-HA/PA66 cages.