Conventional pedicle screws are used as internal fixation instruments in spine surgery, and employing three-column fixation provides good segment stabilization and orthopedic support. However, screw loosening easily occurs when osteoporotic patients undergo lumbar internal fixation due to the insufficient fixation strength of osteoporotic vertebrae. The literature states that the loosening rate of pedicle screws in osteoporotic vertebrae is approximately 60%, which is approximately 3-5 times higher than that observed in nonosteoporotic patients[5,6]. It is difficult for osteoporotic vertebrae to provide good pullout strength for pedicle screws due to the loss of bone mass and sparse bone trabeculae. Due to vertebral body forward and rotation, lumbar-pelvic sagittal imbalance and intervertebral instability[16,17], the pullout strength of pedicle screws must be higher than that of pedicle screws during surgery, especially in isthmic spondylolisthesis. Biomechanical studies confirmed that the pullout strength of the pedicle screw was significantly reduced when bone density was less than 0.777 ± 0.330 g/cm2. Okuyama et al. reported that the maximum pullout strength of the pedicle screw decreased by 60 N for every 10 mg/cm2 decrease in bone density, and the pedicle screw did not have sufficient stability in the vertebral body when the bone density was less than 80 - 90 mg/cm2. How to improve the pullout strength of pedicle screws in osteoporotic vertebral bodies is a problem that spine surgeons must face.
To prevent the failure of internal fixation of the lumbar spine due to osteoporosis, the following surgical options are primarily available. Some scholars have suggested that the pullout strength of cortical bone screws is 30 - 60% higher than that of traditional pedicle screws, which provides a feasible alternative method for traditional pedicle screws. Wu et al. evaluated 157 patients undergoing lumbar fusion surgery, and the screw loosening rate in the expansion screw group (4.1%) was significantly lower than that in the ordinary screw group (12.9%). Biomechanical studies showed that the average pullout strength of double-threaded screws (2726.8 N) was significantly higher than that of hybrid-threaded screws (1890.2 N) and single-threaded screws (2213.3 N). However, some scholars proposed that double-threaded screws and ordinary pedicle screws showed the same axial extraction force and anti-fatigue strength. Studies have shown that larger diameter screws increase pullout strength, and the pullout strength increases 35% when the screw diameter is increased by 2 mm[11,23]. Polymethylmethacrylate-augmented pedicle screws are one of the most common ways to prevent the failure of internal fixation in osteoporotic lumbar internal fixation surgery. The use of bone cement significantly improved the pullout force and anti-fatigue resistance of screws. Biomechanical studies have reported that the pullout strength of screws is 119%-213% higher than that of conventional pedicle screws[24,25].
The application of polymethylmethacrylate-augmented pedicle screws has clear advantages in the surgical treatment of osteoporosis patients, but postoperative complications cannot be ignored. The most common complication is leakage of bone cement, which may cause nerve damage, pulmonary embolism, anaphylactic shock and death[26,27,28,29]. Previous studies reported that the incidence of bone cement leakage in the strengthening of bone cement nail channels was 5.4% -66.7%[18,30,31]. The present study found 9 cases (27.27%) of bone cement leakage in the PMMA-PS group, including 3 cases of paravertebral vein leakage, 4 cases of anterior vertebral vein leakage, 1 case of anterior vertebral nail hole leakage and 1 case of spinal canal leakage. There is no systematic study of the preventive measures of bone cement leakage in the strengthening of bone cement nail channels. Based on experience and the literature review of percutaneous vertebroplasty, the author summarizes the following measures to prevent bone cement leakage. First, the slow injection of 2-3 ml of dough-like bone cement into each anterior middle of the vertebral body under low pressure improved the pullout strength of the pedicle screw and prevented bone cement leakage[32,33]. Second, high-viscosity bone cement has a lower risk of leakage, but the pressure of injection is higher, and the operating time is shorter. Third, the tip of the screw should be avoided in the middle third of the vertebral body during screw insertion to prevent bone cement from leaking into the spinal canal along the central vein of the vertebral body. When the bone cement approaches the posterior edge of the vertebral body or bone cement leaks, bone cement injection should be stopped immediately. An average of 1.5-2 ml of bone cement was injected into each pedicle in the PMMA-PS group, and there was no symptomatic bone cement leakage.
When fusion internal fixation is performed for patients with osteoporotic lumbar spine disease, most of the literature recommends strengthening of the bone cement nail channel to prevent the risk of screw loosening and fracture after surgery. However, the research subjects[35,36,37] were patients with multiple segments, mostly mixed with a single segment and double segment. Whether single segments must be strengthened is not clear. Nagahama et al. followed up 40 osteoporotic patients with lumbar spondylolisthesis who underwent single-segment PLIF and found that the fusion rate in the bisphosphonate group was as high as 95% 1 year after surgery, the vitamin D group was only 65%, and 24% of the patients had fracture of adjacent vertebral body in the vitamin D group. Chen et al. followed 79 osteoporotic patients with lumbar spondylolisthesis who underwent single-segment fusion and found that the fusion rates of the zoledronic acid group and the nonzoledronic acid group were greater than 82% on the basis of taking calcium and vitamin D regularly, but the latter group had a high incidence (17%) of fracture in adjacent segments. Fischer et al. performed a literature review and recommended that teriparatide increased bone mass and promoted intervertebral fusion. Therefore, single-segment fusion internal fixation also achieved a higher fusion rate for lumbar spondylolisthesis with osteoporosis with the cooperation of regular anti-osteoporosis, and no significant internal fixation failure was observed. The results of the present study showed that the PMMA-PS group (100%) and the CPS group (96.15%) had satisfying fusion rates, and there was no significant difference between the two groups. There was 1 case of postoperative adjacent segment fractures, 1 case of postoperative revision and no screw loosening or postoperative infection in the PMMA-PS group. There was 1 case of postoperative revision, 2 cases of screw loosening, 1 case of postoperative infection and no postoperative adjacent segment fracture in the CPS group. There were no significant differences in postoperative fusion, screw loosening or postoperative adjacent segment fractures between the 2 groups. The situation may be related to the combined use of zoledronic acid on the basis of regular anti-osteoporosis treatment in postoperative patients.
In lumbar internal fixation surgery, postoperative intervertebral fusion is highly important. Intervertebral fusion is prone to screw loosening and breaking. especially in patients with osteoporosis[4,5,6]. Therefore, when osteoporotic patients with lumbar spondylolisthesis undergo lumbar internal fixation and fusion surgery, it is necessary to pay attention to the relevant factors of intervertebral fusion. Okuda et al. retrospectively analyzed 101 patients with lumbar spondylolisthesis through at least 3 years of follow-up and found that the incidence of delayed fusion was significantly higher in patients over 70 years of age than in patients under 70 years of age, but age did not affect clinical efficacy. Park et al. studied 881 intervertebral spaces in 784 patients who were treated with TLIF and found that the pear-shaped intervertebral space easily caused cage backward movement and affected intervertebral fusion. Abbushi et al. analyzed 40 patients with lumbar spondylolisthesis who underwent lumbar fusion surgery and found that bullet-shaped cages, cages in the central vertebral body, insufficient cage height, stress in the posterior column, and endplate damage were risk factors that led to postoperative fusion failure. Kimura et al. followed up 1,070 patients who underwent PLIF, including 76 patients with isthmic spondylolisthesis, and suggested that because of the angle and pear shape of the intervertebral space, L5/S1 had a large degree of spatial mobility, which easily caused the cage to move backward. For the effects of the abovementioned factors on intervertebral fusion, based on our experience and literature review, we performed the following measures to avoid risks. First, cages were not placed in the middle of the weak endplate, especially in patients with osteoporosis and pear-shaped intervertebral spaces. Second, damage to the cartilage endplate was avoided during the surgery. Third, the size of the cage was slightly larger than the size measured during the surgery. When the fusion segment was L5/S1, an angled cage was selected, which fit the upper and lower endplates such that the cage obtained a larger weight-supporting area, which reduced the loosening rate of the screws and promoted fusion.
The data of the present study showed that the operation time and intraoperative blood loss in the CPS group were lower than those in the PMMA-PS group, but there was no significant difference in the length of hospital stay between the two groups. The higher blood loss in the PMMA group may be related to the longer operation time. The last postoperative LL, PT, and SS of all patients were significantly improved compared with those before surgery, and the sagittal balance was corrected. The clinical symptoms of the two groups of patients improved significantly compared to before surgery, which was related to nerve decompression, reduction of spondylolisthesis and improvement of spine-pelvic parameters. At the final follow-up, there were no significant differences in VAS or ODI scores between the two groups.
There are several deficiencies in this study. First, this study was a single-center retrospective study, and more prospective studies are needed. Second, the follow-up time was short, and the sample size was small. Only patients with I° and II° lumbar spondylolisthesis were included, and cases with III° lumbar spondylolisthesis were not included, which may bias the conclusions. Third, the sequence of bone cement injection and screw placement was not clear.