Study design: Case-Control Study Objectives: To analyse global sagittal alignment including the cranial center of mass (CCOM) and proximal junctional kyphosis (PJK) in Adolescent Idiopathic Scoliosis (AIS) patients treated with posterior instrumentation. Summary of background data: PJK plays an important role in the global sagittal alignment in AIS patients. Maintaining the head above the pelvis allows for a minimization of energy expense in ambulation and upright posture. Numerous studies have been performed to understand PJK phenomena in AIS patients. However, to our knowledge, no study performed on AIS patients included the head in the analysis of global sagittal alignment and PJK. Methods: This study included 85 AIS patients and 51 asymptomatic adolescents. Low-dose bi-planar X-rays were acquired for each subject preoperatively and at two years follow up. Two global sagittal alignment parameters were calculated, i.e. the angle between the vertical and the line joining the center of the bi-coxofemoral axis (HA) and either the most superior point of the dentiform apophysis of C2 (OD) or the cranial center of mass (CCOM). Results: Among normal adolescents, the average OD-HA and CCOM-HA angles were-2.3°±2° and-1.5°±1.8°, respectively. Among AIS patients the average OD-HA and CCOM-HA angles were respectively-2.3°±1.9° and-1.3°±1.8° preoperatively and-2.8°±1.7° and-1.9°±1.7° at the last follow up. 13% of the patients developed PJK postoperatively. Case-bycase analysis showed that adjusting the thoracic kyphosis and the compensations required to maintain this constant could provide explanatory elements. Conclusions: OD-HA and CCOM-HA angles remain almost constant among normal group and patients, pre-and post-operatively, whether PJK or non-PJK. Five patients without PJK and only one patient with PJK produced abnormal values relative to the asymptomatic subjects. Therefore, PJK could be a compensation mechanism, which allows for CCOM-HA and, to a lesser extent, OD-HA to remain invariant.
Although the use of pedicle screws is considered safe, mechanical issues still often occur. Commonly reported issues are screw loosening, screw bending and screw fracture. The aim of this study was to develop a Finite Element (FE) model for the study of pedicle screw biomechanics and for the prediction of the intraoperative pullout strength. The model includes both a parameterized screw model and a patient-specific vertebra model. Pullout experiments were performed on 30 human cadaveric vertebrae from ten donors. The experimental force-displacement data served to evaluate the FE model performance. μCT images were taken before and after screw insertion, allowing the creation of an accurate 3D-model and a precise representation of the mechanical properties of the bone. The experimental results revealed a significant positive correlation between bone mineral density (BMD) and pullout strength (Spearman ρ = 0.59, p < 0.001) as well as between BMD and pullout stiffness (Spearman ρ = 0.59, p < 0.001). A high positive correlation was also found between the pullout strength and stiffness (Spearman ρ = 0.84, p < 0.0001). The FE model was able to reproduce the linear part of the experimental force-displacement curve. Moreover, a high positive correlation was found between numerical and experimental pullout stiffness (Pearson ρ = 0.96, p < 0.005) and strength (Pearson ρ = 0.90, p < 0.05). Once fully validated, this model opens the way for a detailed study of pedicle screw biomechanics and for future adjustments of the screw design.
After full validation, this model should offer a relevant and efficient tool for the biomechanical and clinical study of the cervical spine, which might improve the understanding of cervical spine disorders.
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