PEEK and titanium are the most widely used cage materials in PLIF, with satisfactory results and a high fusion rate. However, PEEK cages are also associated with complications. Thus, this trial investigated the non-inferiority of BGS-7 spacers to PEEK cages. The non-inferiority of BGS-7 spacer compared to PEEK cage was inconclusive. However, the fusion rates, clinicoradiographic outcomes, and adverse events of the BGS7 spacer were similar to those of the PEEK cage.
The intervertebral cage in PLIF surgery is intended to maintain intervertebral disc space stability and facilitate intervertebral fusion.[1–3] An ideal interbody spacer for PLIF is one with a similar elastic modulus to the bone while providing stability and achieving a high fusion rate. PEEK cages have several advantages over other synthetic cages. First, because it is radiolucent, it is easy to perform x-ray and CT scans when evaluating the fusion status after surgery, and magnetic resonance imaging can also be easily performed because there are limited implant artifacts. Second, the PEEK cage has a similar elastic modulus to that of the bone, and thus, the cage subsidence is lower and the load is evenly distributed between the bone and the cage.[23, 24] However, the PEEK cage does not have bone conduction and bone induction properties and therefore cannot directly fuse with the bone. To overcome these limitations, autobone and bone substitute materials such as allobone, demineralized bone matrix, and hydroxyapatite (HA), are used inside the PEEK cage. However, although materials incorporating various bone substitute materials inside the PEEK cage are used, data on these materials are still limited.[4, 24]
Compressive strength is an important characteristic of intervertebral disc spacers because it is related to the resistance to intraoperative damage and repetitive loads after surgery. HA spacers have been reported to have a weak compressive strength and are thus greatly damaged during surgery. BGS-7 spacers have stronger mechanical strength than do HA spacers and are also slightly stronger than PEEK and titanium cages. In this study, only one case of BGS-7 spacer fracture occurred, which was not statistically significant. Subsidence occurs when the mechanical strength is high, but the results of this study show lesser subsidence in BGS-7 spacers than that in PEEK cages. This result is thought to be due to the specific characteristics of the PEEK cage (CAPSTONE CONTROL™, Medtronic) used in this study (rotation in the intervertebral space) rather than the general characteristics of the PEEK cage. The degree of subsidence in this study is excellent at less than 1 mm and is similar to that in previous studies. Further, the mechanical properties is judged to only have a slight effect on subsidence. Two BGS-7 spacer migrations occurred in our study, but this was not caused by the mechanical properties of the spacer and is rather considered to be a spacer design problem. Specifically, it is caused by the smooth upper and lower surfaces of the spacer (Fig. 1), and this design problem needs to be corrected.
Spacers made of bone graft substitutes such as bioactive glass ceramics are currently being studied as alternatives to cages in which bone substitutes are fused. The BGS-7 spacer has a high bioactive capacity, showing chemical bonding capability.[6, 25] In a previous clinical study, the 12- and 48-month union rates on CT were 89.7% and 90.6%, respectively, in the BGS-7 spacer group and were similar to those in the titanium group.[12, 26] The 12-month fusion rate of the BGS-7 spacer group in this study is slightly lower than that in the previous study at 75.0%. There was no significant difference in the fusion rate compared to that of the PEEK cage, but the 95% CI range included the noninferiority limit and zero, and thus, the result was inconclusive. However, not only the fusion rate, but also the radiographic outcomes, clinical outcomes, and adverse events were similar to those of the PEEK cage. Collectively, these results support that BGS-7 spacers can be an alternative to PEEK cages. Given that the BGS-7 spacer has a wide contact surface and can bond directly to the bone, the fusion area of the end plates is much wider in the BGS-7 spacer than that in the PEEK cage. The large fusion area plays an important role in the stress distribution of the spacer in interbody fusion and has biomechanical advantages because it effectively distributes the load on the vertebral body to lower the subsidence rate.[27, 28] In this study, the bone fusion rate of the BGS-7 spacer was not higher than that of the PEEK cage, but in theory it is considered to be more advantageous than the PEEK cage.
The CaO-SiO2-P2O5-B2O3 glass ceramic is a new material made for spacer. A toxicity study in previous animal experiments showed no specific side effects even in high-dose intravenous injections of an aqueous extract of CaO-SiO2-P2O5-B2O3 glass ceramic for 90 days. The rapid absorption of glass ceramics in the body has a negative effect on its role as a bone conduction support, but CaO-SiO2-P2O5-B2O3 glass ceramics have very low bio-absorption. In addition, CaO-SiO2-P2O5-B2O3 glass ceramics are highly unlikely to cause adverse tissue reactions or systemic reactions even if it remains in the body after surgery. Thus, they are safe to use as spacers. No device-related severe adverse events were found in this study during the 1-year follow-up period, but a longer follow-up is necessary because the substance is present in the body for a long time.
To our best knowledge, this is the first trial to compare the clinicoradiographic outcomes between BGS-7 spacers and PEEK cages at 12 months after surgery. However, our trial has some limitations. First, this trial had a small sample size, which prevents more generalized conclusions on the potential differences between the two interventions. The initial sample size calculation determined 27 participants would be necessary in each group, with a 20% drop out rate. The findings may be inconclusive because of the small number of participants included in the follow-up. However, the final fusion rate and other outcomes were measured similarly. We cannot conclude that the outcomes of BGS-7 spacers and PEEK cages were similar, but they may be replaceable. Second, the 12-month end-point of our trial was inadequate to assess the advantages and disadvantages of BGS-7 spacers for PLIF surgery. Thus, we are planning to continue the follow-up. Third, this study is a single-blind study in which only the participants were blinded to the intervention. Although the assessor who collected the data on clinical outcomes was also blinded to the group allocation, it was impossible to maintain blinding of the assessor who evaluated the radiographic outcomes. To lower the impact of this limitation, the fusion rate was evaluated by a third-party investigator not involved in the study. Thus, it is unlikely that the assessor judgment will has affected the results of this study. Finally, multiple comparisons were used in the analysis of clinical outcomes. Caution should be taken when interpreting the statistical effects of interventions as these multiple comparisons increase the risk of type 1 errors.