The n-HA/PA66 cage has been used as a bionic non-metallic material in reconstructive operations of spine in the past decade. HA has good osteoconductivity and has been well accepted as a bone repair substitute, while PA66 is a polymer with strong intensity, high flexibility, and good stability[22–24]. Furthermore, the n-HA/PA66 composite exhibits excellent biocompatibility and osteogenesis in vivo[25], and is an ideal microstructure material with a dynamic perfusion culture condition that improves osteogenesis[26]. In the present study, the n-HA/PA66 cage showed great bionic ability to achieve osteogenesis and osseointegration in the long-term (bone fusion rate of 96.2%). Bone grew to the outside surfaces of the n-HA/PA66 cage, with cortical bone and newly formed bone tracked along the inside surface. In contrast, the non-bionic surface of the PEEK cage often presents as the typical non-reactive fibrous tissue interface[27, 28], resulting in inferior osseointegration[28]. A previous study reported that 20% of PEEK cages exhibit impaction on the upper plateau during a minimum follow-up of 8 months[16].
As a bioactive material with the ability to promote new bone formation and provide a scaffold for osteogenesis, the n-HA/PA66 strut has advantages in the anterior reconstruction of thoracic and lumbar corpectomy. Ou et al.[29] reported that the n-HA/PA66 strut achieved a satisfactory short-term clinical outcome, with an excellent fusion rate of nearly 100%. Yang et al.[20] also reported that the n-HA/PA66 cage achieved a low subsidence rate of 19.6% and great fusion rate of 90.2% during 2 years of follow-up. The elastic modulus of the n-HA/PA66 strut is 5.6 GPa, which is similar to the elastic modulus of natural bone[18–20] and much lower than that of the TMC (110 GPa). Furthermore, the n-HA/PA66 strut avoids some of the stress shielding caused by metallic implants and promotes bony fusion. The use of the TMC in anterior column reconstruction of the thoracolumbar spine often results in severe subsidence[12, 13, 30]. Dvorak[12] reported an average TMC subsidence of 4 mm, but with acceptable correction of vertebral kyphosis at final follow-up. Jang et al.[30] found that TMC subsidence occurred in 93.3% of patients after anterior cervical corpectomy and reconstruction. In the present study, the TMC group had a subsidence rate of 58.3% during 7 years of follow-up.
The long-term subsidence rate in the n-HA/PA66 group in our study was 24.5%, with a mean subsidence of 2.3 mm; this was higher than the subsidence rate reported in a previous study[20] and higher than the subsidence rate of nearly 20% reported in the cervical spine[19]. The elastic modulus of the cartilage endplate and cancellous bone (0.1–0.5 GPa) is lower than the elastic modulus of the n-HA/PA66 cage, causing posterior subsidence at the interface with the cancellous bone. The relatively high subsidence rate in the n-HA/PA66 group may be due to the difficulty in the shaping of the n-HA/PA66 cage during surgery, which makes it harder to match both the superior and inferior endplates. Large mismatched angles are an important factor leading to increased cage subsidence[31, 32].
Owing to the greater loss of height of the fusion segments, severe subsidence was correlated with loss of BKA correction and subsidence-related complications. Deml reported 10.5° correction of kyphosis and 1.6° loss of correction using the PEEK cage in anterior reconstruction after thoracolumbar corpectomy through the anterior-posterior approach[17]. In surgery via the anterior approach, many previous studies have showed a mild loss of correction using a PEEK cage or TMC with short-term follow-up[10, 13, 33]. However, with significant subsidence of the TMC, the correction of kyphosis can no longer be kept stable[12]. Brandao et al.[16] also found no significant difference in the loss of correction between a TMC and a PEEK expandable cage, despite much higher loss of correction in both groups (8.86° vs. 3.65°). In our study, the loss of correction was 5.2° in the TMC group, which was similar to the 4.2° reported in a previous study[12]. In addition, the n-HA/PA 66 cage showed significantly better correction of the BKA and lower loss of correction than the TMC group after 7 years of follow-up than the TMC, which might be due to the lower elastic modulus and better osteoconductivity of the n-HA/PA 66 cage.
There were no significant differences in the ODI and VAS between the n-HA/PA66 and TMC groups at final follow-up [34.35]. In addition, the n-HA/PA66 cage exhibited excellent biocompatibility and osteoconductive ability. Considering the lower elastic modulus with more stable correction of kyphosis and earlier bony fusion of the n-HA/PA66 group compared with the TMC group, the n-HA/PA66 seems to be an ideal cage to replace the TMC in anterior reconstruction of thoracolumbar fractures.
The present study has some limitations. The sample size was small, the choice of the cage was not randomized, and the results may have been influenced by physician factors to a certain degree. A future multicenter study with a larger sample size is warranted to compare the long-term effects of these two cages.