Fusion has an important place in procedures to reconstruct the stability of the lumbar spine. It is of clinical significance to understand the indications for lumbar fusion, surgical methods, application of internal fixation, biomechanics and related research. The development of lumbar fusion and its indications The development of lumbar fusion has a history of nearly 100 years, and many improved procedures have emerged to improve clinical efficacy; especially the spinal biomechanics developed since the 1980s, through biomechanical testing of the normal anatomy of the spine, stress analysis of the intervertebral disc structure, the influence of the surgical approach on the structure and stability of the spine, and the biomechanical study of the internal fixation devices of the spine. The biomechanical study further elucidates the importance of posterior lumbar structures and lumbar intervertebral joints on the stability of the lumbar spine, and provides a theoretical basis for spinal fusion. The indications include discogenic low back pain, lumbar spondylolisthesis, segmental instability, tuberculosis, tumor, trauma and secondary surgery of the lumbar spine. (1) Lumbar spine fusion surgery and biomechanical characteristics Lumbar spine fusion surgery mainly includes: posterior implant fusion (including median spinous process splitting implant fusion, interspinous process H-shaped implant fusion, spinous process and lamina implant fusion), posterior lateral implant fusion (including small joint lateral and transverse process implant fusion), and anterior, that is, intervertebral body implant fusion (anterior and posterior). Since Briggs, Milligan and Cloward first proposed the posterior lunbar interbody fusion (PLIF) procedure in 1944-1945, the PLIF technique has been perfected through the continuous efforts of many surgeons. At present, there are some new understandings of PLIF technique [3], and it is believed that this procedure can achieve complete decompression and interbody fusion at the same time through the posterior approach, which has better efficacy for cases of spinal instability and spinal stenosis; some scholars even propose a combined interbody and posterior lateral anterior-posterior fusion procedure to achieve a 360-degree circumferential fusion of the anterior and posterior lumbar columns to further improve the fusion success rate. Some scholars even proposed a combined anterior-posterior fusion of the intervertebral and postero-lateral columns to achieve a 360-degree circumferential fusion to further improve the success rate of fusion. Biomechanical tests show that postero-lateral and anterior fusion is more stable (little intervertebral activity in the fused segment), while posterior fusion still has greater intervertebral motion in the fused segment; at the same time, all fusions will increase the biological stress in the adjacent vertebral segments, with posterior fusion being the most obvious and postero-lateral fusion having the least effect, and abnormal stress is often one of the causes of fusion failure. Generally, the two adjacent vertebrae and the intervertebral discs and small joints between them are considered as one motion segment, and the center of motion of each motion segment of the spine is mostly located in the intervertebral disc. When motion occurs in the spine, the displacement of the masses close to the center of motion is small; the masses far from the center of motion need to be displaced to a larger extent. Theoretically, intervertebral bone grafting has the best effect. (2) Mastery of indications Lumbar fusion is beneficial to the reconstruction of spinal stability, but from the biomechanical point of view, extensive fusion can produce complications such as stress concentration, destruction of normal physiological curvature of the spine and small joint degeneration. In cases of isthmuscle-type lumbar spondylolisthesis, lumbar instability leading to spinal stenosis (degenerative slippage, degenerative scoliosis) and the presence of objective segmental instability, lumbar fusion can improve the outcome, while the fusion rate has been reported inconsistently in cases of low back pain due to disc degeneration and secondary surgery. When complex deformities or significant segmental instability is present, lumbar fusion is more favorable, while single-segment fusion in cases such as disc herniation does not significantly improve outcomes compared with conventional surgical treatment. Therefore, the efficacy of lumbar fusion depends on careful consideration of the cause of the patient’s pain, the patient’s functional status, and his or her expectations. Active inflammation, severe osteoporosis, metal allergy and severe mental illness are absolute contraindications to lumbar fusion. 3. Application of internal fixation system in lumbar fusion (1) Development of internal fixation instruments Since the 1980s, the internal fixation instruments of the spine have been updated rapidly, and the Cotrel-Dubousset system has emerged, showing its diversity from Harrington instruments, which is of epoch-making significance. In recent years, the CCD, TSRH, Isola nail-rod systems and anterior internal fixation devices have been introduced and are widely used. In recent years, PLIF has been increasingly used in the treatment of lower lumbar instability, but postoperative complications such as sinking and displacement of the implanted internal block, prolapse to the posterior, and pseudarthrosis formation are prone to occur. To solve the problems of conventional intervertebral fusion, various implant fusion devices that can carry bone grafting materials (stainless steel, bioceramics, titanium alloy, carbon, polymer materials, etc.) have been successfully developed since the 1990s. It is relatively unanimous that the choice of this design has more advantages than other procedures and is easy to handle, but its clinical long-term efficacy needs to be further observed [7]. These implants can be used not only for posterior but also for anterior surgery. (2) Mastery of indications The application of internal fixation has enabled many lumbar fusion procedures to be successful. Implant fusion with appropriate internal fixation increases the stability after repositioning, improves the success rate of implant fusion, and shortens the postoperative rehabilitation time. However, spinal fixation devices can never replace a good fusion and osteotomy. All fusions with instrumentation will ultimately fail if bony healing is not obtained. In addition, the use of internal fixation devices as an adjunct in the treatment of lumbar degeneration is controversial. In specific cases of secondary lumbar spine surgery, medical or degenerative lumbar spondylolisthesis, the use of internal fixation devices may improve the fusion rate of the spine. However, this is not true for single-segment lumbar spine slippage (mild) or degenerative lumbar instability. In a prospective study of the effect of CD instrumentation on the fusion of posterior lateral lumbar implants, Christerson conducted a study of cases of spondylolisthesis (I-II) or degenerative segmental instability and found no correlation between lumbar fusion rates and CD application. In view of the fact that the application of internal fixation brings with it many disadvantages such as increased costs, prolonged operative time, increased infection and reoperation rates, residual low back pain, flat back deformity, osteoporosis of the fused segment due to stress masking of the strong internal fixation and fatigue fractures and pseudarthrosis formation above and below the fixed structure, we should be cautious in the selection of internal fixation devices, based on the principle that the advantages outweigh the disadvantages, to minimize the possibility of complications. The possibility of complications should be reduced to a minimum. 4, the criteria for judging the success of fusion There is no unified standard, roughly through the following aspects to help assess: (1) in the X-ray plain film always maintain the same gap height, 3-6 months after surgery transplantation bone gap contour is unclear, a year later there are obvious bone trabeculae through; (2) lumbar spine dynamic photography, such as in flexion, posterior extension position found in the gap height changes, suggesting that there is abnormal activity between the vertebral body, bone (3) lumbar spine tomography to observe bone fusion at different levels of the intervertebral space; (4) CT examination to observe the process of bone fusion from the intervertebral space in cross-section. However, these determination criteria are uncertain, and as has been pointed out, the implant fusion rate depends largely on the subjective assumptions of the investigator. Therefore, comparing the results of different studies is particularly difficult. If bone fusion has occurred, and it is difficult to determine this from imaging alone. In addition, tests of spinal biomechanics do not always agree exactly with the basis for fusion on imaging. Animal tests have shown that a spine with a strong fusion as evidenced by the formation of a continuous bone scab on X-rays is not as strong as a spine with 2-3L fibrous connections between vertebrae. Two criteria for fusion were compared: one was determined by comparing the slip of the spine in hyperextension and hyperextension radiographs, and the other was the presence of continuous trabeculae passing through the fused segment on the radiograph; the former was found to be nearly 20% higher than the fusion rate determined by the latter. The criteria for determining fusion by X-rays are still controversial. It is generally accepted that successful fusion means that there is continuous bone fragmentation and no motion in the fused segment, but it is often difficult to determine fusion based on the change in motion shown on radiographs in hyperextension and hyperflexion of the spine. In recent studies of intervertebral fusion devices, clinical cases with 5% motion of the fused segment have been judged as bone healing; some believe that a 5% motion in hyperextension-hyperflexion should be judged as fusion failure, while most believe that a 2%-3% motion is acceptable. It has also been suggested that fusion failure is indicated if there is a translucent area within the fusion zone that is more than 2L wide and crosses 50% of the implant surface. Kant et al [13] compared the fusion assessment results obtained by applying radiographic plain films with the results of direct surgical exploration in order to further explore the criteria for determining lumbar fusion. They selected 75 patients who had re-experienced lumbar fusion with different internal fixation devices, including postero-lateral fusion, postero-lateral plus interbody fusion, and fusion materials including autologous bone, allograft bone, or a mixture of both; the study was double-blinded, and the fusion rate was determined by preoperative radiographs from the same examiner and compared with the results of immediate surgical exploration. The results showed that the fusion success rate determined by radiograph was only 68%, with L4-5 being the least fusion-prone segment and the most difficult to determine by radiograph in this plane. Therefore, even if the X-ray film indicates a strong fusion, if the patient still has persistent back pain after surgery, after excluding non-mechanical factors, surgery should still be actively taken to investigate. 5. Factors affecting lumbar fusion (1) Construction of the bone graft bed The success of bone grafting requires a slow crawling replacement process of the bone in the recipient area. The rich blood supply of the implant bed facilitates the close contact between the blood-transported bone in the recipient area and the bloodless graft bone, so that the active vascular granulation tissue grows over the graft bone. Therefore, an ideal bone graft bed should be created at the time of bone grafting, such as trimming the sclerotic bone, fully removing the bone cortex of the bone graft area or chiseling the surface bone cortex into a fish scale rough surface (depth about 2-4L). (2) Size and quality of the grafted bone The osteogenic induction ability of cancellous bone is better than that of cortical bone, and when applying a mixed graft of cortical and cancellous bone, placing the cancellous bone at the periphery so that it is in contact with the surrounding tissue results in rapid bone formation Ehrler et al. made a systematic study of the application of allogeneic bone in lumbar fusion and concluded that more allogeneic bone can be obtained than autologous bone, and that fresh frozen bone has a higher immunogenicity and more complete fusion than freeze-dried bone, and that there is minimal disease transmission using standard acquisition methods. Compared with autologous bone graft, the use of allogeneic bone alone or in combination with autologous bone resulted in decreased success rates in posterior lumbar fusion but higher fusion rates in anterior fusion; Wimmer et al. followed 94 patients who underwent anterior lumbar interbody fusion during 1993 and found no significant difference in the effect of the two on spinal fusion rates. In addition, mixing coral particles with autologous bone can also yield more desirable fusion results. (3) Application of internal spinal fixation devices After bone grafting, the fixation of the spine is important, especially in the first 3 weeks. This is because the movement of bone and cartilage at this time can easily damage the small vessels supplying the cancellous bone graft. Strong internal fixation can increase the stability of the spine and improve the rate of implant fusion; meanwhile, Kanayama et al [17] found that the application of internal fixation devices in the spine and the correlation between the process of implant fusion through a sheep model can also accelerate the rate of spinal fusion. (4) Influence of physical factors Bone blocks should be implanted in the recipient area as soon as possible after freeing; saline, operating room lighting, temperature (over 42°C), and antimicrobial immersion can affect the survival of cells in the bone graft block. After the grafted bone block is obtained, it is best to wrap it with a blood-soaked sponge. Ito et al [18] found that electromagnetic waves have a positive effect on reducing osteoporosis due to internal fixation and can effectively increase the rate of bone fusion; an implantable electrical stimulator has been applied in a population at high risk of spinal fusion, and the results showed that the success rate of spinal fusion and the degree of relief of clinical symptoms were significantly improved compared with the comparison group. (5) The influence of biological factors The development trend of spinal fusion, namely the application of biological materials and tissue substances. In the past decade, there has been significant progress in research on the application of biological bone derived materials as osteoconductive and osteoinductive mediators. The most important of these is the application of demineralized bone matrix (DBM) and bone morphogenetic protein (BMP) for the repair of segmental bone defects and spinal fusion models. Recently, due to the improvement of endoscopic and minimally invasive techniques and the application of advanced internal fixation devices such as intervertebral fusion (cage) and threaded allograft rings, the research on osteoinductive growth factors (OGFs) has been increasing [19, 20]. Clinical applications have shown that recombinant bone morphogenetic protein-2 (rhBMP-2), recombinant osteogenic protein-1 (rhOP-1), P-15, mitotic-derived interstitial cells, and low-dose adenovirus-mediated gene therapy can induce bone fusion, shorten surgery time and hospitalization days, and reduce surgical trauma. (6) The materials used to make internal fixation instruments Sun Changtai et al. compared the difference between the bone-screw interface of titanium alloy and stainless steel pedicle screws from histology and mechanics. The results show that: titanium alloy material made of pedicle screws compared with stainless steel instruments, the former has a better screw-bone interface bond; screw torsion test shows that titanium alloy than stainless steel pedicle screws have a higher torsion moment. There is no doubt that the application of titanium alloy internal fixators will increase the stability of the spine, thus increasing the fusion success rate. Currently, people are also developing internal fixators of tantalum metal, because tantalum has the effect of promoting bone growth. (7) Individual factors The success rate of lumbar fusion is high and fast in patients with strong constitution and good nutrition, and the success rate of lumbar fusion is relatively low in osteoporotic and smokers. Numerous studies have shown that nicotine increases the rate of bone discontinuity in spinal fusion surgery and that long-term smoking decreases the likelihood of spinal fusion and disease healing, while Wing found that stopping smoking before surgery increased fusion success through a study of a rabbit spinal fusion model.