Patients with lumbar degenerative disease typically present with low back pain and leg pain, which occur especially when they are walking. This degenerative condition severely restricts function, walking ability, and quality of life [16, 17]. These lumbar symptoms have become the most common indication for spinal surgery, and studies have shown that surgical treatment in selected patients is more successful than conservative alternatives [18,19,20].
In this study, we described a modified facet joint fusion technique for the treatment of lumbar degenerative disease. In this technique, the scope of lamina excision and the method for fabricating the bone graft bed is very important. Traditional facet joint fusion uses a bone chisel to shape the joint space into a “V” shape. This bone graft bed has a small area, and the quality of the resulting graft cannot be guaranteed. In our technique, a high-speed grinding drill with a diameter of approximately 3 mm is used to remove the cartilage and bone cortex in the joint space, thus creating a “U”-shaped graft bed, which not only ensures the quality of the graft but also greatly increases the area of the bone graft bed. At the same time, it avoids the defect of creating joint process fracture to a large extent, which could be easily created by the traditional fusion technique. Different bone graft materials, such as autogenous cancellous bone and allogeneic cancellous bone granules, could be implanted into the bone graft bed. In this study, the VAS scores for lumbar pain and leg pain, the ODI, and the JOA and EQ-5D scores were significantly improved after surgery, and there was no significant difference in the treatment effect between the PLF and MFF techniques, and no complications related to MFF occurred. Therefore, we believe that MFF is a safe and convenient fusion technique, with a therapeutic effect no worse than that of PLF.
On surgical indications, MFF and PLF are feasible for patients with lumbar disc herniation and lumbar spinal stenosis combined with symptoms, such as lumbar and leg pain, paresthesia, and muscle loss caused by lumbar degenerative diseases. MFF can be considered for patients without lumbar spondylolisthesis above grade I, spondylolysis, scoliosis, and lumbar fracture.
This study selected all patients who met the inclusion and exclusion criteria during the specific period to minimize selection bias. Excluding patients with long-segment lumbar surgery and more severe lumbar degenerative disease also causes selection bias. It is good practice to obtain results from the minimally adjusted available model.
In 1999, Park et al.  described facet joint fusion techniques and reported a fusion rate of 93.8% in patients with degenerative lumbar spondylolisthesis (n = 32) followed up for more than 1 year. However, in their research, the bone graft material was autogenous iliac cancellous bone. Although autologous bone is the most ideal bone graft material, the procedure used to acquire it may lead to pain, bleeding, infection and other complications . In our technique, autologous cancellous bone was extracted from the intraoperative spinous process and lamina, supplemented by allogeneic cancellous bone particles, which not only avoids the above complications but also achieves the same therapeutic effect. However, it has not been widely used in clinical practice.
In 2015, Miyashita et al.  studied facet joint fusion in patients with degenerative lumbar scoliosis (n = 88) and achieved a fusion rate of 88%. The production criterion for the bone graft bed was a length or depth of at least 1 cm, which we thought was not very accurate. Because most of the facet joints in patients with lumbar degenerative disease show obvious proliferative sclerosis, the quality of the bone graft bed cannot be guaranteed simply by making the bone graft bed according to a standard of length or depth. In our research, we used a high-speed grinding drill to remove the cartilage and cortical bone in the facet joint space until the cancellous bone surface was exposed to ensure the quality of the bone graft bed.
In previous studies, the fusion rate of PLF ranges from 72.3 to 90% [1, 23,24,25]. In our study, the fusion rate was 76.2%, which was similar with the result in the other studies. PLF is a quite widely used surgical method to treat lumbar degenerative disease. However, because of the need to expose paraspinal muscles, patients are more likely to experience postoperative low back pain compared with the other surgical methods . In our study, the fusion rate of MFF was significantly higher than PLF and the treatment effect was similar between the two groups. Therefore, for short-segment lumbar degenerative disease, MFF should be a better choice compared with PLF.
Because the gap between the facet joints is narrow and the bone graft is firmly in contact with the surrounding bone, theoretically, the fusion rate is higher. In the future, MFF can be used in patients with the aforementioned surgical indications and has broad clinical applications. If it can be widely performed in clinical practice, it will reduce patients' surrounding tissue injury and improve the fusion rate. As for mechanical strength, solid fusion can effectively reduce the stress of the internal fixation system. Thus, it reduces the incidence of internal fixation fatigue fractures, prevents mechanical failure, and ensures long-term postoperative effects. There is still a lack of relevant studies on whether there is a difference in spinal stability after MFF and PLF. Presently, no complications, such as internal fixation fracture, were observed in the one-year follow-up. However, systematic biomechanical studies and long-term clinical efficacy and complications still need further study.
In our study, there was no significant difference between the MFF group and the PLF group in terms of both the ODI and the VAS, JOA, or EQ-5D scores and in the length of hospital stay, length of operation, intraoperative blood loss, cost of hospitalization, and perioperative complications, indicating that the treatment effect for lumbar degenerative diseases in the MFF group was no worse than that in the PLF group. In addition, the intraoperative blood loss of the MFF group was significantly lower than that of the PLF group, which may be related to the fewer injuries to the paravertebral muscle, the smaller wound surface and the relatively shorter operation time in the MFF group than in the PLF group. If the modified facet joint fusion technique can be widely used in lumbar surgery, the intraoperative blood loss of patients will be effectively reduced. According to our study, the MFF technique can be used for lumbar fusion in patients with degenerative lumbar diseases undergoing short-segment surgery without severe lumbar spondylolisthesis, scoliosis, spondylolysis, and lumbar fracture.
Compared with the currently widely used intervertebral fusion, the advantage of modified facet joint fusion is that the latter technique is simple and does not need to be performed on the anterior column or middle column, so it can effectively avoid complications associated with intervertebral fusion [6, 26]. In this study, no complications related to the MFF technique occurred. In addition, the modified facet joint fusion technique does not require special equipment, such as cages, so the cost to the patient should be reduced. However, a detailed comparison between modified facet joint fusion and intervertebral fusion requires further study and analysis.
This study has the following limitations. First, we only evaluated the fusion rate at 1 year postoperatively and did not assess changes of fusion rates dynamically. Second, the 1 year for the postoperative follow-up is relatively short, so it is necessary to further extend the follow-up duration to evaluate the long-term effectiveness of the treatments. Third, we only selected patients with 1–2 levels of lumbar degenerative disease for evaluation. In further study, patients with multiple levels of lumbar degenerative disease can be included.