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 modalities, application of internal fixation, biomechanics and related research. 1, Indications for lumbar fusion Its indications mainly include discogenic low back pain, lumbar spondylolisthesis, segmental instability, tuberculosis, tumor, trauma and secondary surgery of the lumbar spine [2]. Lumbar spine fusion techniques Lumbar spine fusion techniques and biomechanical characteristics Lumbar spine fusion techniques mainly include: 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, i.e. intervertebral body implant fusion (anterior and posterior). Mastery of indications Lumbar fusion is beneficial to reconstruct the stability of the spine, but from the biomechanical point of view, extensive fusion can produce complications such as stress concentration, disruption of the normal physiological curvature of the spine and small joint degeneration [5]. In cases of isthmuscle-type lumbar spondylolisthesis, lumbar instability leading to spinal stenosis (degenerative slippage, degenerative scoliosis) and the presence of objective segmental instability, fusion of the lumbar spine is performed to improve the outcome, while the fusion rate has been reported inconsistently in cases of low back pain due to disc degeneration and secondary surgery. Lumbar fusion is beneficial when complex deformities or significant segmental instability is present, whereas single-segment fusion in cases such as herniated discs 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 The development of internal fixation instrumentation 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 [6]. To solve the problems of conventional intervertebral fusion, various implant fusion devices that can carry bone graft 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 in the posterior but also in the anterior approach. The use of internal fixation has enabled many successful lumbar fusion procedures. Implant fusion with appropriate internal fixation increases stability after repositioning, improves the success rate of implant fusion, and shortens postoperative recovery 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 such as secondary lumbar spine surgery, medically induced or degenerative lumbar spine slippage, the application of internal fixation devices may improve the fusion rate of the spine [8]. However, this is not true for single-segment lumbar spine slippage (mild) or degenerative lumbar instability. In the principle that the advantages outweigh the disadvantages, the possibility of complications is minimized. 4, the criteria for judging the success of fusion There is no uniform standard, roughly through the following aspects to help assess: (1) in the X-ray plain film always maintain the same intervertebral space 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 photographic, such as in flexion, posterior extension position found in the intervertebral space 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. 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 blood 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. Influence of physical factors The bone block 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 in 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 for 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 to the comparison group. The influence of biological factors The development trend of spinal fusion, namely the application of biological materials and tissue substances. In the last decade, there has been significant progress in the study of the application of materials of biological bone origin as osteoconductive and osteoinductive mediators. Materials used to make internal fixation devices The results show that the former has better screw-bone interface bonding for pedicle screws made of titanium alloy material compared to stainless steel devices; screw torsion tests show that titanium alloy has a higher torsional moment than stainless steel pedicle screws. 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. Internal fixators made of tantalum metal are also being developed because of its role in promoting bone growth. Individual factors Patients who are physically strong and well-nourished have a high and rapid success rate of lumbar fusion, while osteoporotic and smokers have a relatively low success rate of lumbar fusion. 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 [22] found that stopping smoking before surgery increased fusion success through a study of a rabbit spinal fusion model.