. 2021 Jan 11;11:552719.
doi: 10.3389/fendo.2020.552719.
eCollection 2020.
Affiliations
Item in Clipboard
et al.
Front Endocrinol (Lausanne).
.
Abstract
Objective:
Decreased bone mineral density (BMD) impairs screw purchase in trabecular bone and can cause screw loosening following spinal instrumentation. Existing computed tomography (CT) scans could be used for opportunistic osteoporosis screening for decreased BMD. Purpose of this case-control study was to investigate the association of opportunistically assessed BMD with the outcome after spinal surgery with semi-rigid instrumentation for lumbar degenerative instability.
Methods:
We reviewed consecutive patients that had primary surgery with semi-rigid instrumentation in our hospital. Patients that showed screw loosening in follow-up imaging qualified as cases. Patients that did not show screw loosening or-if no follow-up imaging was available (n = 8)-reported benefit from surgery ≥ 6 months after primary surgery qualified as controls. Matching criteria were sex, age, and surgical construct. Opportunistic BMD screening was performed at L1 to L4 in perioperative CT scans by automatic spine segmentation and using asynchronous calibration. Processing steps of this deep learning-driven approach can be reproduced using the freely available online-tool Anduin (https://anduin.bonescreen.de). Area under the curve (AUC) was calculated for BMD as a predictor of screw loosening.
Results:
Forty-six elderly patients (69.9 ± 9.1 years)-23 cases and 23 controls-were included. The majority of surgeries involved three spinal motion segments (n = 34). Twenty patients had low bone mass and 13 had osteoporotic BMD. Cases had significantly lower mean BMD (86.5 ± 29.5 mg/cm³) compared to controls (118.2 ± 32.9 mg/cm³, p = 0.001), i.e. patients with screw loosening showed reduced BMD. Screw loosening was best predicted by a BMD < 81.8 mg/cm³ (sensitivity = 91.3%, specificity = 56.5%, AUC = 0.769, p = 0.002).
Conclusion:
Prevalence of osteoporosis or low bone mass (BMD ≤ 120 mg/cm³) was relatively high in this group of elderly patients undergoing spinal surgery. Screw loosening was associated with BMD close to the threshold for osteoporosis (< 80 mg/cm³). Opportunistic BMD screening is feasible using the presented approach and can guide the surgeon to take measures to prevent screw loosening and to increase favorable outcomes.
Keywords:
bone mineral density; computer neural networks; computer-assisted image analysis; degenerative spine surgery; multidetector computed tomography; osteoporosis; spinal fusion and instrumentation.
Copyright © 2021 Löffler, Sollmann, Burian, Bayat, Aftahy, Baum, Meyer, Ryang and Kirschke.
Conflict of interest statement
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
Figures
Figure 1
Top row: Segmentation masks in 3D-rendering. Bottom row: Segmentation masks in sagittal view overlaid on preoperative CT scan. Left to right columns: Segmentation masks of L1 to L4 were created after manual labeling of vertebral centroids and automatic segmentation by a fully convolutional network (A). Posterior elements were removed from segmentation masks (B). The outer 10 mm were eroded from segmentation mask and L4 mask was removed due to osteochondrosis (C). These final segmentation masks served as ROIs to extract HU from the CT scan for opportunistic BMD screening. HU, Hounsfield units; BMD, bone mineral density; ROI, region of interest.
Figure 2
Top to bottom row: Segmentation masks in 3D-rendering, in sagittal view, and in axial view overlaid on postoperative CT scan. Left to right columns: Automatically created segmentation masks of L1 to L4 excluded screw contours by HU thresholding (A). Posterior elements were removed from segmentation masks (B). The outer 10 mm were eroded from segmentation mask and L3 mask was removed due to degenerative changes (C). These final segmentation masks served as ROIs to extract HU from the CT scan for opportunistic BMD screening. HU, Hounsfield units; BMD, bone mineral density; ROI, region of interest.
Figure 3
Case of a 48-year-old woman who underwent spinal surgery with semi-rigid instrumentation of levels L4 to S1 for lumbar instability. Of note, this patient has a partially lumbarized S1 vertebra that shows bony fusion at the lateral mass to the os sacrum. Opportunistic BMD evaluation yielded low bone mass (BMD = 95.8 mg/cm³). An immediate postoperative CT scans shows all six pedicle screws at L4, L5, and S1 and an intervertebral cage at L5/S1 in place (A). In 3-months survey the patient reported severe disability according to Oswestry Disability Index (score 42). A control CT scan 178 days after index surgery showed loosening of right L4 screw and both S1 screws (screws not fully depicted; B). BMD, bone mineral density.
Figure 4
Matched control case of a 48-year-old woman who underwent spinal surgery with semi-rigid instrumentation of levels L4 to S1 for lumbar instability. Opportunistic BMD evaluation yielded normal bone density (BMD = 133.2 mg/cm³). An immediate postoperative CT scans shows all six pedicle screws at L4, L5, and S1 and an intervertebral cage at L5/S1 in place (A). In 12-months survey the patient reported overall benefit from surgery with moderate disability according to Oswestry Disability Index (score 24). A follow-up CT scan more than 7 years after index surgery showed no signs of screw loosening or breakage (screws not fully depicted; B). BMD, bone mineral density.
Figure 5
Receiver operating characteristics (ROC) curve for predicting screw loosening after semi-rigid instrumentation by opportunistically assessed BMD at the lumbar spine. BMD was a significant predictor of screw loosening with an AUC = 0.769 (CI: 0.634–0.905, p = 0.002). For BMD 3 screw loosening can be predicted with a sensitivity of 91.3% and specificity of 56.5% (maximum Youden’s index = 0.478). AUC, area under the ROC curve; CI, 95% confidence interval; BMD, bone mineral density.