Bone density of first and second segments of normal and dysmorphic sacra

Patient-specific bone material modelling can improve the predicted biomechanical outcomes of sacral fracture fixation techniques: A comparative finite element study

OBJECTIVE

To evaluate and compare the biomechanical efficacy of six iliosacral screw fixation techniques for treating unilateral AO Type B2 (Denis Type II) sacral fractures using literature-based and QCT-based bone material properties in finite element (FE) models.

METHODS

Two FE models of the intact pelvis were constructed: the literature-based model (LBM) with bone material properties taken from the literature, and the patient-specific model (PSM) with QCT-derived bone material properties. Unilateral transforaminal sacral fracture was modelled to assess different fixation techniques: iliosacral screw (ISS) at the first sacral vertebra (S1) (ISS1), ISS at the second sacral vertebra (S2) (ISS2), ISS at S1 and S2 (ISS12), transverse iliosacral screws (TISS) at S1 (TISS1), TISS at S2 (TISS2), and TISS at S1 and S2 (TISS12). A 600 N vertical load with both acetabula fixed was applied. Vertical stiffness (VS), relative interfragmentary displacement (RID), and the von Mises stress values in the screws and fracture interface were analysed.

RESULTS

The lowest and highest normalised VS was given by ISS1 and TISS12 techniques for LBM and PSM, with 137 % and 149 %, and 375 % and 472 %, respectively. In comparison with the LBM, the patient-specific bone modelling increased the maximum screw stress values by 19.3, 16.3, 27.8, 2.3, 24.4 and 7.8 % for ISS1, ISS2, ISS12, TISS1, TISS2 and TISS12, respectively. The maximum RID values were between 0.10 mm and 0.47 mm for all fixation techniques in both models. The maximum von Mises stress results on the fracture interface show a substantial difference between the two models, as PSM (mean ± SD of 15.76 ± 8.26 MPa) gave lower stress values for all fixation techniques than LBM (mean ± SD of 28.95 ± 6.91 MPa).

CONCLUSION

The differences in stress distribution underline the importance of considering locally defined bone material properties when investigating internal mechanical parameters. Based on the results, all techniques demonstrated clinically sufficient stability, with TISS12 being superior from a biomechanical standpoint. Both LBM and PSM models indicated a consistent trend in ranking the fixation techniques based on stability. However, long-term clinical trials are recommended to confirm the findings of the study.

Development of a sacral fracture model to demonstrate effects on sagittal alignment

PURPOSE

To develop a modeling framework to predict the secondary consequences on spinal alignment following correction and to demonstrate the impact of pedicle subtraction osteotomy (PSO) location on sagittal alignment.

METHODS

Six patients were included, and pelvic incidence (PI) was measured. Full-length standing radiographs were uploaded into PowerPoint and manipulated to model S1-S2 joint line sacral fractures at 15°, 20°, 25°, and 30°. PSO corrections with hinge points at the anterior superior corner and vertical midpoint of the L3-5 vertebral bodies were modeled. Anterior translation (AT) and vertical shortening (VS) were calculated for the six PSO locations in the four fracture angle (FA) models.

RESULTS

PI had a strong effect in the mixed AT and VS models (P < 0.001). Both AT and VS were significantly different from zero at all FA (p < 0.001), and pairwise comparisons revealed all FA were different from each other with respect to both AT and VS after adjusting for PSO location (p < 0.001), increasing as FA increased. Varying PSO location resulted in significant differences in AT when comparing all locations (p < 0.001). AT was greatest for all FA in all patients when the PSO correction was performed at the L3-AS (p < 0.001). There were significant differences in VS when comparing the L5-Mid PSO location to the L3-AS, L3-Mid, L4-AS, and L4-Mid PSO locations (p < 0.034).

CONCLUSION

PSO correction superior to a sacral fracture resulted in AT and VS of the spine. It is crucial that these changes in spinal measures be predicted and accounted for to optimize patient sagittal alignment and outcomes.

Clinical evaluation of S1 alar screws application in short-segment lumbosacral fixation and fusion for spine infection with severe S1 vertebral body loss

Background

The one-stage posterior approach for treating spinal infection has recently been generally accepted. However, severe vertebral body loss caused by infection remains a major challenge in posterior surgery. This study was conducted to evaluate the clinical application and outcomes of S1 alar screws used in the one-stage posterior surgery of short-segment lumbosacral fixation and fusion after debridement for infection with severe S1 vertebral body loss.

Methods

The clinical features and treatment outcomes of 7 patients with spinal infections from August 2016 to August 2021 who were treated with one-stage posterior surgery using S1 alar screws were retrospectively analyzed. The clinical data, including patient data, visual analogue scale (VAS), Oswestry Disability Index (ODI), fusion time and complications of the patients, were recorded.

Results

All 7 patients were followed up for an average duration of 14.57 months (range, 12—18 months). The VAS score decreased significantly from 7.3 preoperatively (range, 6—8) to 2.6 postoperatively (range, 2—3). The ODI score demonstrated a steady and gradual increase from 73.8 preoperatively (range, 68—75) to 33.6 postoperatively (range, 30—37). Bony fusion time was observed approximately 6.8 months after surgery. Two patients in our study experienced the postoperative local pain, which could be relieved by analgesics and disappeared 3 months after the operation. There were no complications of intraoperative fracture, posterior wound infection or neurovascular injury.

Conclusions

S1 alar screws are suitable for use in the operation and could be an alternative option to S1 pedicle screws for short-segment lumbosacral fixation and fusion with severe S1 vertebral body loss caused by spinal infection, which could provide satisfactory clinical outcomes.