Lumbar internal fixation and fusion is a standard approach for the treatment of LDDs with good efficacy and reproducibility 
In the early 1980s, Blume and Rojas, and Harms and Rolinger, described a modified posterior interbody fusion (PLIF) technique called transforaminal interbody fusion (TLIF), which was later described by Harms et al. [8–10]. As an alternative to PLIF, TLIF is able to achieve similar clinical outcomes and fusion rates to PLIF [11,12]. According to previous scholars reporting the comparison of the clinical results and fusion rate of TLIF with a single cage and a double cage, it has been proven that the surgical method of single cage implantation can also provide good stability and a good fusion rate . Therefore, the TLIF surgical method using a single cage has become increasingly popular among clinicians because of its many advantages.
Closkey concluded that grafts with more than 30% of the intervertebral space area provided good stability . However, it is worth noting that the biomaterial cage involved in intervertebral fusion does not possess the basic elements of bone formation, and it can only provide a platform for bone transplantation and stabilize the height of the intervertebral space. With the passage of postoperative intervertebral fusion time, the cage inserted into the intervertebral body may have the possibility of subsidence, displacement, and prolapse. Therefore, the available bone surface area of the intervertebral space is critical for fusion success.
In the past, domestic and foreign scholars have reported on the area of intervertebral bone and the amount of bone graft in lumbar interbody fusion. Steffen and other scholars have found that the graft accounts for 30–40% of the total endplate surface, which can effectively prevent the settling of the graft . Xiao reported that successful intervertebral fusion can be achieved when the total amount of intervertebral graft bone is 2.5 times the amount of bone in the cage . The study by Takeuchi scholars showed that with a certain bone graft volume, the distribution of the internal and external bone graft volume of the cage has different effects on the intervertebral fusion . According to Wolff's law, the cage's larger surface area in contact with the bony endplate will not only decrease subsidence into the vertebral body but also benefit fusion.
Although there have been many reports on the area and amount of bone graft in posterior lumbar fusion surgery, little attention has been given to the area ratio of autologous bone graft. Therefore, the purpose of this study was to evaluate the area ratio of the autologous bone graft, which influenced the satisfaction of one-year interbody fusion and clinical outcomes.
Implantation of the intraoperative cage allowed the originally narrow intervertebral space to obtain an immediately acceptable distraction height, and the follow-up imaging data in each group showed a significant increase in the height of the intervertebral space one week after the operation. The reason why the final intervertebral space height in group A was significantly lower than that in groups B and C may be due to the low autogenous bone area ratio of the intervertebral body and insufficient effective contact with the bone graft area. force, affecting early bone growth and bone healing. When the autologous bone grafted into the intervertebral body cannot form a bone bridge with the upper and lower endplates, the free bone will be gradually absorbed, which will reduce the total area of the graft, cause the settlement of the intervertebral cage, and lead to a change in the intervertebral height. Even though the height of the intervertebral space in group A was significantly reduced during the follow-up, the height of the intervertebral space at the last follow-up was still improved compared with that before the operation. A possible explanation is that the cage plays a role in the maintenance of intervertebral height throughout the process of fusion.
Many studies have reported that the fusion rate of TLIF surgery is more than 90%. In our study, the fusion rate of both groups B and C reached approximately 90%. This finding indicates that an intervertebral cage combined with a local strengthening bone graft can achieve a satisfactory intervertebral healing rate after surgery.
In this study, the fusion rate of each group at 6 months after surgery was not high, which may be because the bone grafts during the operation were all obtained by decompression. Cancellous bone and bone block transplantation have a relatively long osteogenic cycle . Second, in the early stage of bone fusion, autologous bone grafted into the intervertebral body undergoes a certain degree of bone resorption due to the osteolysis of osteoclasts . The subjects in group A were more prone to bone nonunion after early bone resorption due to the low ratio of the average intervertebral bone area .Bone fusion can be observed from the image data of the following subjects(Figs. 4 and 5). Cases without intervertebral fusion within one year were not considered nonfusion, and there was still a possibility of intervertebral bone fusion. Studies have shown that the volume of the intervertebral bone graft increases significantly within 2–5 years after surgery, and in patients with nonunion one year after surgery, bone fusion is generally achieved during the subsequent bone fusion process .
Judging from the one-year postoperative fusion rate we calculated, when the average intervertebral autologous bone area was less than 19%, the fusion rate also increased significantly with increasing bone graft area, but the average intervertebral autologous bone area also increased. When it is between 19% and 25%, although the fusion rate has increased, the magnitude is significantly lower than before. When the average autologous bone area of the intervertebral body exceeds 25%, the rate of increase of the fusion rate slows down and plateaus.
Among the 57 subjects in the study, there was 1 incident of internal fixator rupture in group A. In group C, there was 1 case of displacement of local grafted bone particles.It was found that a large amount of autologous bone was implanted in the intervertebral space during the operation, and some of the grafted bone overflowed after the implantation of the cage. However, pursuing more bone grafts during surgery means that more operative time is needed to trim the autologous bone graft and deal with the intervertebral space endplate bone graft bed, which greatly increases the operative time and intraoperative blood loss, thereby increasing postoperative complications and the risk of occurrence. This is contrary to the original intention of clinical treatment.
Notably, the distribution of autologous bone at the intervertebral level was different among the groups. There are few studies describing or classifying the morphology of intervertebral bone grafts. Yao et al. divided the bone-grafted intervertebral plane into four parts: ipsilateral dorsal, ipsilateral ventral, contralateral dorsal, and contralateral ventral. the lowest side . In our study, a certain degree of similarity in bone graft morphology within the same group could be observed. The distribution of bone grafts in group A was mostly irregular (Fig. 6); most of the bone grafts in groups B and C showed a certain regularity, with a “C” shape surrounding the cage (Fig. 7). Bone fusion is most likely to be observed in the lateral region of the intervertebral space during lumbar interbody fusion . This means that the autologous bone transplanted around the cage can take the lead in bone fusion in the early stage, and the bone graft shape with a "C" shape distribution may be beneficial to bone fusion.
There are limiting factors in our study in terms of the small sample size, manual measurements, and short follow-up period. In addition, the surgeries of all patients were performed by three senior doctors. Further study is needed to increase the sample size and prolong the follow-up period. The relationship between the ratio of autologous bone graft and the fusion rate needs to be further studied because the specific position of cage implantation and endplate processing have a certain influence on the degree of distraction. Consequently, in the future, we plan to collect large samples of patients who were operated on by doctors of the same seniority and analyse the specific relationship by utilizing big data.