Biomechanical modeling of the lateral decubitus posture during corrective scoliosis surgery

Predicting radiographic outcomes of vertebral body tethering in adolescent idiopathic scoliosis patients using machine learning

Anterior Vertebral Body Tethering (AVBT) is a growing alternative treatment for adolescent idiopathic scoliosis (AIS), offering an option besides spinal fusion. While AVBT aims to correct spinal deformity through growth correction, its outcomes have been mixed. To improve surgical outcomes, this study aimed to develop a machine learning-based tool to predict short- and midterm spinal curve correction in AIS patients who underwent AVBT surgery, using the most predictive clinical, radiographic, and surgical parameters. After institutional review board approval and based on inclusion criteria, 91 AIS patients who underwent AVBT surgery were selected from the Shriners Hospitals for Children, Philadelphia. For all patients, longitudinal standing (PA or AP, and lateral) and side bending spinal Radiographs were retrospectively obtained at six visits: preop and first standing, one year, two years, five years postop, and at the most recent follow-up. Demographic, radiographic, and surgical features associated with curve correction were collected. The sequential backward feature selection method was used to eliminate correlated features and to provide a rank-ordered list of the most predictive features of the AVBT correction. A Gradient Boosting Regressor (GBR) model was trained and tested using the selected features to predict the final correction of the curve in AIS patients. Eleven most predictive features were identified. The GBR model predicted the final Cobb angle with an average error of 6.3 ± 5.6 degrees. The model also provided a prediction interval, where 84% of the actual values were within the 90% prediction interval. A list of the most predictive features for AVBT curve correction was provided. The GBR model, trained on these features, predicted the final curve magnitude with a clinically acceptable margin of error. This model can be used as a clinical tool to plan AVBT surgical parameters and improve outcomes.

Image-Guided Tethering Spine Surgery With Outcome Prediction Using Spatio-Temporal Dynamic Networks

Recent fusionless surgical techniques for corrective spine surgery such as Anterior Vertebral Body Growth Modulation (AVBGM) allow to treat mild to severe spinal deformations by tethering vertebral bodies together, helping to preserve lower back flexibility. Forecasting the outcome of AVBGM from skeletally immature patients remains elusive with several factors involved in corrective vertebral tethering, but could help orthopaedic surgeons plan and tailor AVBGM procedures prior to surgery. We introduce an intra-operative framework forecasting the outcomes during AVBGM surgery in scoliosis patients. The method is based on spatial-temporal corrective networks, which learns the similarity in segmental corrections between patients and integrates a long-term shifting mechanism designed to cope with timing differences in onset to surgery dates, between patients in the training set. The model captures dynamic geometric dependencies in scoliosis patients, ensuring long-term dependency with temporal dynamics in curve evolution and integrated features from inter-vertebral disks extracted from T2-w MRI. The loss function of the network introduces a regularization term based on learned group-average piecewise-geodesic path to ensure the generated corrective transformations are coherent with regards to the observed evolution of spine corrections at follow-up exams. The network was trained on 695 3D spine models and tested on 72 operative patients using a set of 3D spine reconstructions as inputs. The spatio-temporal network predicted outputs with errors of 1.8 ± 0.8mm in 3D anatomical landmarks, yielding geometries similar to ground-truth spine reconstructions obtained at one and two year follow-ups and with significant improvements to comparative deep learning and biomechanical models.

Lateral Position versus Prone Position for Cervical Laminoplasty: A Retrospective Comparative Study

Purpose

To examine the safety of lateral decubitus positions for cervical laminoplasty.

Patients and Methods

A retrospective comparative study was conducted on the safety between the lateral and prone positions in cervical laminoplasty. After screening, 466 patients who underwent cervical laminoplasty at a single medical center were enrolled and categorized into the lateral (n=229) and prone (n=237) groups. Data on positioning time, surgical time, blood loss, complication rates, and surgical outcomes were collected and compared between the two groups. The patients were further divided into underweight, normal weight, overweight, and obesity subgroups according to their body mass index, and the collected data were compared between the lateral and prone groups.

Results

The lateral group had a lower incidence of facial pressure ulcers (2.18%) than the prone group (11.39%). However, positioning time, surgical time, blood loss, and surgical outcomes were not significantly different between the two groups. In the subgroup analysis, no significant difference in positioning time, operative time, and blood loss was observed in the underweight, normal weight, and overweight patients between the two groups, but in the obesity subgroup, the lateral group had a significantly shorter positioning time (15.23±6.44 vs 21.63±9.43 min, P=0.045) and operative time (140.16±40.48 vs 178.62±51.82 min, P=0.037) and lesser blood loss (285.31±171.75 vs 430.46±189.84 mL, P=0.044) than the prone group.

Conclusion

The lateral position is as safe as the prone position for cervical laminoplasty, but it has advantages over the prone position for patients with obesity.

Contribution of Lateral Decubitus Positioning and Cable Tensioning on Immediate Correction in Anterior Vertebral Body Growth Modulation

STUDY DESIGN

Computational simulation of lateral decubitus and anterior vertebral body growth modulation (AVBGM).

OBJECTIVES

To biomechanically evaluate lateral decubitus and cable tensioning contributions on intra- and postoperative correction.

SUMMARY OF BACKGROUND DATA

AVBGM is a compression-based fusionless procedure to treat progressive pediatric scoliosis. During surgery, the patient is positioned in lateral decubitus, which reduces spinal curves. The deformity is further corrected with the application of compression by cable tensioning. Predicting postoperative correction following AVBGM installation remains difficult.

METHODS

Twenty pediatric scoliotic patients instrumented with AVBGM were recruited. Three-dimensional (3D) reconstructions obtained from calibrated biplanar radiographs were used to generate a personalized finite element model. Intraoperative lateral decubitus position and installation of AVBGM were simulated to evaluate the intraoperative positioning and cable tensioning (100 / 150 / 200 N) relative contribution on intra- and postoperative correction.

RESULTS

Average Cobb angles prior to surgery were 56° ± 10° (thoracic) and 38° ± 8° (lumbar). Simulated presenting growth plate's stresses were of 0.86 MPa (concave side) and 0.02 MPa (convex side). The simulated lateral decubitus reduced Cobb angles on average by 30% (thoracic) and 18% (lumbar). Cable tensioning supplementary contribution on intraoperative spinal correction was of 15%, 18%, and 24% (thoracic) for 100, 150, and 200 N, respectively. Simulated Cobb angles for the postoperative standing position were 39°, 37°, and 33° (thoracic) and 30°, 29°, and 28° (lumbar), respectively, whereas growth plate's stresses were of 0.54, 0.53, and 0.51 MPa (concave side) and 0.36, 0.53, and 0.68 MPa (convex side) for the three tensions.

CONCLUSION

The majority of curve correction was achieved by lateral decubitus positioning. The main role of the cable was to apply supplemental periapical correction and secure the intraoperative positioning correction. Increases in cable tensioning furthermore rebalanced initially asymmetric compressive stresses. This study could help improve the design of AVBGM by understanding the contributions of the surgical procedure components to the overall correction achieved.

LEVEL OF EVIDENCE

Level III.

3D correction over 2years with anterior vertebral body growth modulation: A finite element analysis of screw positioning, cable tensioning and postoperative functional activities

BACKGROUND

Anterior vertebral body growth modulation is a fusionless instrumentation to correct scoliosis using growth modulation. The objective was to biomechanically assess effects of cable tensioning, screw positioning and post-operative position on tridimensional correction.

METHODS

The design of experiments included two variables: cable tensioning (150/200N) and screw positioning (lateral/anterior/triangulated), computationally tested on 10 scoliotic cases using a personalized finite element model to simulate spinal instrumentation, and 2years growth modulation with the device. Dependent variables were: computed Cobb angles, kyphosis, lordosis, axial rotation and stresses exerted on growth plates. Supine functional post-operative position was simulated in addition to the reference standing position to evaluate corresponding growth plate's stresses.

FINDINGS

Simulated cable tensioning and screw positioning had a significant impact on immediate and after 2years Cobb angle (between 5°-11°, p<0.01). Anterior screw positioning significantly increased kyphosis after 2years (6°-8°, p=0.02). Triangulated screw positioning did not significantly impact axial rotation but significantly reduced kyphosis (8°-10°, p=0.001). Growth plates' stresses were increased by 23% on the curve's convex side with cable tensioning, while screw positioning rather affected anterior/posterior distributions. Supine position significantly affected stress distributions on the apical vertebra compared to standing position (respectively 72% of compressive stresses on convex side vs 55%).

INTERPRETATION

This comparative numerical study showed the biomechanical possibility to adjust the fusionless instrumentation parameters to improve correction in frontal and sagittal planes, but not in the transverse plane. The convex side stresses increase in the supine position may suggest that growth modulation could be accentuated during nighttime.

Biomechanical simulations of costo-vertebral and anterior vertebral body tethers for the fusionless treatment of pediatric scoliosis

Compression-based fusionless tethers are an alternative to conventional surgical treatments of pediatric scoliosis. Anterior approaches place an anterior (ANT) tether on the anterolateral convexity of the deformed spine to modify growth. Posterior, or costo-vertebral (CV), approaches have not been assessed for biomechanical and corrective effectiveness. The objective was to biomechanically assess CV and ANT tethers using six patient-specific, finite element models of adolescent scoliotic patients (11.9 ± 0.7 years, Cobb 34° ± 10°). A validated algorithm simulated the growth and Hueter-Volkmann growth modulation over a period of 2 years with the CV and ANT tethers at two initial tensions (100, 200 N). The models without tethering also simulated deformity progression with Cobb angle increasing from 34° to 56°, axial rotation 11° to 13°, and kyphosis 28° to 32° (mean values). With the CV tether, the Cobb angle was reduced to 27° and 20° for tensions of 100 and 200 N, respectively, kyphosis to 21° and 19°, and no change in axial rotation. With the ANT tether, Cobb was reduced to 32° and 9° for 100 and 200 N, respectively, kyphosis unchanged, and axial rotation to 3° and 0°. While the CV tether mildly corrected the coronal curve over a 2-year growth period, it had sagittal lordosing effect, particularly with increasing initial axial rotation (>15°). The ANT tether achieved coronal correction, maintained kyphosis, and reduced the axial rotation, but over-correction was simulated at higher initial tensions. This biomechanical study captured the differences between a CV and ANT tether and indicated the variability arising from the patient-specific characteristics. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:254-264, 2018.

The Use of Finite Element Models to Assist Understanding and Treatment For Scoliosis: A Review Paper

INTRODUCTION

Scoliosis is a complex spinal deformity whose etiology is still unknown, and its treatment presents many challenges. Finite element modeling (FEM) is one of the analytical techniques that has been used to elucidate the mechanism of scoliosis and the effects of various treatments.

METHODS

A literature review on the application of FEM in scoliosis evaluation and treatment has been undertaken. A literature search was performed in each of three major electronic databases (Google Scholar, Web of Science, and Ovid) using the key words "scoliosis" and "finite element methods/model". Articles using FEM and having a potential impact on clinical practice were included.

RESULTS

A total of 132 abstracts were retrieved. The query returned 105 articles in which the abstracts appeared to correspond to this review's focus, and 85 papers were retained. The current state of the art of FEM related to the biomechanical analysis of scoliosis is discussed in 4 sections: the etiology of adolescent idiopathic scoliosis, brace treatment, instrumentation treatment, and sensitivity studies of FEM. The limitations of FEM and suggested future work are also discussed.

Computational Biomechanical Modeling of Scoliotic Spine: Challenges and Opportunities

BACKGROUND

Biomechanical computer models of the spine have important roles in the treatment and correction of scoliosis by providing predictive information for surgeons and other clinicians.

OBJECTIVES

This article reviews computational models of intact and scoliotic spine and its components; vertebra, intervertebral disc, ligament, facet joints, and muscle. Several spine models, developed using multi-body modelling and finite element modelling schemes, and their pros and cons are discussed.

CONCLUSIONS

The review reveals that scoliosis modelling is performed for 3 main applications: 1) brace simulation; 2) analysis of surgical correction technique; and 3) patient positioning before surgical instrumentation. The models provide predictive information for a priori choice of brace configurations and mechanically effective surgical correction techniques and the expected degree of correction. However, they have many shortcomings: for instance, they do not fully reproduce the active behaviour of the spine and the models' properties are not personalized.