Authors' Response to Letter to the Editor. Fake Union.

First, we would like to thank Drs. Tsukamoto, Morimoto, Yoshihara, and Mawatari for their pertinent questions regarding our publication. We appreciate that they found value in our work and took the time to read it in detail. This study was an additional investigation using data from a multicenter, prospective, randomized study reported by Ebata et al. in 2017. The authors performed CT imaging four times: immediately postoperative and at 2, 4, and 6 months after surgery. They took the negative impact of radiation very seriously and used a low-dose protocol. We devised CT photography for decreasing radiation exposure by 50% using dose-reduction technique and iterative reconstruction method for image reconstruction. Our investigation revealed that bone fusion decisions at 2 or 4 months postoperatively had little clinical significance due to the possibility of fake union. Based on the study results, we recommended against future investigations of bone fusion being performed at those potentially misleading time points. In contrast, CT imaging immediately after surgery may be useful since bone contact immediately after surgery greatly affects subsequent bone fusion. We agree that the relationship between intravertebral bone cysts and pseudarthrosis is important, and we believe that future studies should include vertebral cysts in their evaluation. The assessment of osteoporosis in fused vertebrae using the Hounsfield unit is another interesting method. As you pointed out, the effect of teriparatide use in patients with a history of bisphosphonates cannot be ignored. In our cohort, there was one patient in the teriparatide group who had been previously treated for osteoporosis. The patient was considered to have bony fusion at 2, 4, and 6 months postoperatively, and so no fake union occurred. We also agree that a history of bisphosphonate use should be investigated to evaluate the efficacy of teriparatide. Perhaps the strongest limitation of our study was the short final evaluation period of 6 months. We have defined fake union as any event in which a vertebral body judged to have fused is later determined as not fused at the final evaluation. Moving forward, we aim to extend our observation period on the rate of fake union from 6 months to 1 or 2 years postoperatively in order to confirm our results.