The vertebral body’s Hounsfield unit (HU) value can credibly reflect patients’ bone mineral density (BMD). Given that poor bone-screw integration initially triggers screw loosening and regional differences in BMD and strength in the vertebral body exist, HU in screw holding planes should better predict screw loosening. According to the stress shielding effect, the stress distribution changes in the fixation segment with BMD reduction should be related to screw loosening, but this has not been identified. We retrospectively collected the radiographic and demographic data of 56 patients treated by single-level oblique lumbar interbody fusion (OLIF) with anterior lateral single rod (ALSR) screw fixation. BMD was identified by measuring HU values in vertebral bodies and screw holding planes. Regression analyses identified independent risk factors for cranial and caudal screw loosening separately. Meanwhile, OLIF with ALSR fixation was numerically simulated; the elastic modulus of bony structures was adjusted to simulate different grades of BMD reduction. Stress distribution changes were judged by computing stress distribution in screws, bone-screw interfaces, and cancellous bones in the fixation segment. The results showed that HU reduction in vertebral bodies and screw holding planes were independent risk factors for screw loosening. The predictive performance of screw holding plane HU is better than the mean HU of vertebral bodies. Cranial screws suffer a higher risk of screw loosening, but HU was not significantly different between cranial and caudal sides. The poor BMD led to stress concentrations on both the screw and bone-screw interfaces. Biomechanical deterioration was more severe in the cranial screws than in the caudal screws. Additionally, lower stress can also be observed in fixation segments’ cancellous bone. Therefore, a higher proportion of ALSR load transmission triggers stress concentration on the screw and bone-screw interfaces in patients with poor BMD. This, together with decreased bony strength in the screw holding position, contributes to screw loosening in osteoporotic patients biomechanically. The trajectory optimization of ALSR screws based on preoperative HU measurement and regular anti-osteoporosis therapy may effectively reduce the risk of screw loosening.

Background Cage subsidence causes poor prognoses in patients treated by oblique lumbar interbody fusion (OLIF). Deterioration of the biomechanical environment initially triggers cage subsidence, and patients with low bone mineral density (BMD) suffer a higher risk of cage subsidence. However, whether low BMD increases the risk of cage subsidence by deteriorating the local biomechanical environment has not been clearly identified. Methods OLIF without additional fixation (stand-alone, S-A) and with different additional fixation devices (AFDs), including anterolateral single rod screws (ALSRs) and bilateral pedicle screws (BPSs) fixation, was simulated in the L4-L5 segment of a well-validated finite element model. The biomechanical effects of different BMDs were investigated by adjusting the material properties of bony structures. Biomechanical indicators related to cage subsidence were computed and recorded under different directional moments. Results Overall, low BMD triggers stress concentration in surgical segment, the highest equivalent stress can be observed in osteoporosis models under most loading conditions. Compared with the flexion-extension loading condition, this variation tendency was more pronounced under bending and rotation loading conditions. In addition, AFDs obviously reduced the stress concentration on both bony endplates and the OLIF cage, and the maximum stress on ALSRs was evidently higher than that on BPSs under almost all loading conditions. Conclusions Stepwise reduction of BMD increases the risk of a poor local biomechanical environment in OLIF patients, and regular anti-osteoporosis therapy should be considered an effective method to biomechanically optimize the prognosis of OLIF patients.

Objective: There has been increasing concern about the importance of sagittal alignment in the evaluation and treatment of spinal scoliosis. However, recent studies have only focused on patients with mild to moderate scoliosis. To date, little is known about the sagittal alignment in patients with severe and rigid scoliosis (SRS). This study was performed to evaluate the sagittal alignment in patients with SRS, and to analyze how it was altered after corrective surgery. Methods:In this retrospective cohort study, we included 58 patients with SRS who underwent surgery from January 2015 to April 2020. Preoperative and postoperative radiographs were reviewed, and the sagittal parameters mainly included thoracic kyphosis (TK), lumbar lordosis (LL), pelvic incidence (PI), pelvic tilt (PT), sacrum slope (SS), and sagittal vertical axis (SVA). The sagittal balance state was evaluated according to whether the PI minus the LL (PI-LL) was less than 9 , and the patients were divided into thoracic hyperkyphosis and normal groups based on whether the TK exceeded 40 . The Student's t test, Pearson's test, and Receiver operating characteristic (ROC) curve analysis were used to compare related parameters between the different groups. Results:The mean follow-up duration was 2.8 years. Preoperatively, the mean PI was 43.6 AE 9.4 , and the mean LL was 65.2 AE 13.9 . Sixty-nine percent of patients showed sagittal imbalance, and they showed larger TK and LL values and smaller PI and SVA values than those with sagittal balance. Additionally, most patients (44/58) presented with thoracic hyperkyphosis; this group had smaller PI and SVA values than the normal patients. Patients with syringomyelia-associated scoliosis were more likely to present with thoracic hyperkyphosis. The TK and LL values were significantly decreased, and 45% of patients with preoperative sagittal imbalance recovered after surgery. These patients had a larger PI (46.4 AE 9.0 vs 38.3 AE 8.8 , P = 0.003) and a smaller TK (25.5 AE 5.2 vs 36.3 AE 8.0 , P = 0.000) at the final follow-up. Conclusion:Preoperative sagittal imbalance appears in the majority of SRS patients, accounting for approximately 69% of our cohort. Patients with small PI values or syringomyelia-associated scoliosis were more likely to present with thoracic hyperkyphosis. Sagittal imbalance can generally be corrected by surgery, except in patients with a PI less than 39 . To achieve good postoperative sagittal alignment, we recommend controlling the TK to within 31 .

Objective Postoperative ileus (POI) is a relatively common complication after spinal fusion surgery, which can lead to delayed recovery, prolonged length of stay and increased medical costs. However, little is known about the incidence and risk factors of POI after corrective surgery for patients with adolescent idiopathic scoliosis (AIS). This study was performed to report the incidence of POI and identify the independent risk factors for POI after postoperative corrective surgery. Methods In this retrospective cohort study, A total of 318 patients with AIS who underwent corrective surgery from April 2015 to February 2021 were enrolled and divided into two groups: those with POI and those without POI. The Student's t test, Mann–Whitney U test, and Pearson's chi‐square test were used to compare the two groups regarding patient demographics and preoperative characteristics (age, sex and the major curve type), intraoperative and postoperative parameters (lowest instrumented vertebra [LIV], number of screws, and length of stay), radiographic parameters (T5–12 thoracic kyphosis [TK], T10–L2 thoracolumbar kyphosis and height [TLK and T10–L2 height], L1–S1 lumbar lordosis [LL], and L1–5 height). Then, a multivariate logistic regression analysis was used to identify independent risk factors for POI, and a receiver operating characteristic (ROC) curve was performed to assess the predictive values of these risk factors. Results Forty‐two (13.2%) of 318 patients who developed POI following corrective surgery were identified. The group with POI had a significantly longer length of stay, more lumbar screws, higher proportions of a major lumbar curve and lumbar anterior screw breech, and a lower LIV. Among radiographic parameters, the mean lumbar Cobb angle at baseline, the changes in the lumbar Cobb angle, and T10–L2 and L1–5 height from before to after surgery were significantly larger in the group with POI than in the group without POI. Multivariate logistic regression analysis showed that large changes in T10–L2 (odds ratio [OR] =2.846, P = 0.007) and L1–5 height (OR = 31.294, p = 0.000) and lumbar anterior screw breech (OR = 5.561, P = 0.006) were independent risk factors for POI. The cutoff values for the changes in T10–L2 and L1–5 height were 1.885 cm and 1.195 cm, respectively. Conclusion In this study, we identified that large changes in T10–L2 and L1–5 height and lumbar anterior screw breech were independent risk factors for POI after corrective surgery. Improving the accuracy of pedicle screw placement might reduce the incidence of POI, and greater attention should be given to patients who are likely to have large changes in T10–L2 and L1–5 height after corrective surgery.