Thesis:Studies On Porcine Model For Early Onset Scoliosis And Its Related Radiographic Investigations

Chapter1CT-based quantification of the thoracic and lumbar pedicle morphology of immature porcine and the significance for pedicle screw implantationObjective:To measure the morphologic parameters of Yorkshire pig's thoracic and lumbar vertebrae with CT scan and provide anatomic reference data for pedicle screw placement for animal experiment.Methods:Ten Yorkshire pigs (6-week aged, female) were included. All the pigs underwent CT scan from T1to L5. The following parameters were measured:the height and transverse diameter of vertebral body (HVB, TDVB). anteroposterior distance of vertebral body (APDB). transverse diameter of vertebral canal (TDVC). the total length and width of the pedicle (TL, PedW). Pedicle screws were inserted according to the parameters and postoperative CT scan was performed to evaluate the accuracy of the pedicle screws.Results:The CT scans revealed that the width of the pedicle and the total length of the pedicle path were5.3-7.4mm and18.8-22.6mm, respectively. The transverse screw angle was32.1-44.4°. Based on the above parameters, pedicle screws with4.5mm in diameter and25mm in length were inserted successfully.Conclusions:Despite the small pedicle of the immature porcine, it was possible to insert pedicle screws effectively with the guidance of preoperative CT scan.Chapter2Establishment of rapidly progressive scoliosis in immature pigs with posterior unilateral tetheringObjective To evaluate the feasibility of developing a rapidly progressive scoliosis model in immature pigs with posterior asymmetric tethering.Methods Six female pigs (age:6weeks old, weight:6-8kg) were instrumented and tethered using a three separate incisions fashion. Minimal dissection at the left T5-T6and L2-L3allowed for pedicle screw placement. Ipsilateral ribcage tethering of left T10-T13ribs was then performed after a left-sided paramedian incision was made. Through mild intraoperative tensioning and repositioning of the animal with a right coronal curvature, a mild deformity was achieved. The tethering period of our scoliosis model was8weeks. Progressive spinal deformity was documented with serial X-ray and computed tomography (CT). After8weeks, the posterior instrument were removed and the pigs were followed over a4-week period with serial radiographs to document changes of the deformity.Results One pig encountered substantial complications (infection and failure of the tether). Of the5available for analysis, all pigs developed progressive, structural scoliosis. Initial Cobb angle of the scoliosis after posterior asymmetric tethering was19.6°on average (range14-27°) and progressed to55.2°on average (range44-73°) after8-week tethering period. The curvature remained stable (51.2°) during the subsequent4weeks after removal of the tethering. All curves demonstrated characteristic radiographic features including significant translation of the apical vertebrae from the midline, apical vertebrae rotation, etc.. CT three-dimensional reconstruction showed that the anterior and posterior elements of the curvature were intact and no signs of fusion were detected.Conclusions This study establishes a rapidly progressive scoliosis model demonstrating progressive, structural curves in immature pigs. In addition, this method of creating experimental scoliosis avoids violation of the spinal elements throughout the maximal portion of the curve and provide an ideal model for further study regarding corrective techniques.Chapter3Evolution of curve pattern in porcine scoliosis model and its significanceObjective To investigate the evolution of curve pattern in progressive scoliosis model in immature pigs and its significance. Methods Eight female pigs (age:6weeks old, weight:6-8kg) were instrumented and tethered through three separate incisions. Pedicle screws were inserted at the left sides of T5-T6and L2-L3. Ipsilateral ribcage tethering of left T10-T13ribs was also performed through a left-sided paramedian incision. Through mild intraoperative tensioning and repositioning of the animal with a right coronal curvature, a mild deformity was achieved. Progressive spinal deformity was documented with serial X-ray examinations at an interval of4weeks. After8weeks, the posterior instrumentations were removed.Results One pig was eliminated from this study for deep infection. Of the7available for analyses, all pigs developed a progressive, structural scoliosis with convex to the right side. Initial Cobb angle of the scoliosis after posterior asymmetric tethering was34.5°on average and progressed to59.3°after8-week tethering period. All curves demonstrated characteristic radiographic features, such as remarkable apical vertebrae translation and apical vertebrae rotation. On the corronal plane, the apex location changed from the mid-thoracic to the lower thoracic region. On the saggittal plane, the curve pattern changed from a totally smooth kyphosis postoperativly to thoracic lordosis and thoracolumbar kyphosis.Conclusions In the porcine scoliosis model, the apex may change and the radiographic features of the sagittal plane may change during the tethering period. All of these should be taken into account for testing new spinal implants pre-clinically.Chapter4Evolution of vertebral and disc wedging in immature porcine scoliosis model and its significanceObjective:To investigate the evolution of the disc and vertebral wedging under unilateral tethering in porcine scoliosis model and its significance.Methods:Seven female pigs underwent posterior asymetric tethering surgery. All pigs were observed with serial postero-anterior X-ray films at4-week intervals to document progression of the deformity. The disc and vertebral wedging in the curve was measured by Cobb's method. The wedging of5vertebrae and four discs which included apex and two superior and two inferior vertebrae as well as discs between these vertebrae was analyzed. The wedging angle of every disc and vertebra in whole major curve was measured, and the proportion to the Cobb angle was calculated (wedging percentage) respectively.Results:The wedging angle of the vertebra in the apical area increased over time. The wedging of the apical vertebra and disc was found to larger than the adjacent vertebrae. Immediate postoperativly, the wedging of intervertebral discs made almost whole contribution to the scoliosis. However, the contribution of the vertebral wedging to the scoliosis increased over time. The wedging of the vertebra made the major contribution (71.5%) to the scoliosis8-week postoperativly.Conclusion:The relative contributions of vertebral and disc wedging to the Cobb angle varied over time under asymmetric tethering. To obtain a reliable scoliosis animal model, adequate tethering period is required to get prominent vertebral wedging.Chapter5Discrepancy of vertebral and disc wedging between porcine scoliosis model and early onset scoliosis in humansObjective:To compare the disc and vertebral wedging in patients with early onset scoliosis (EOS) and porcine scoliosis model.Method:Eight scoliosis pigs and13EOS cases (all without congenital spinal deformity) were reviewed radiographically. Each disc and vertebral wedging in the major curve was measured by Cobb's method. The wedging angle as a proportion of the curve's Cobb angle was then calculated. The average wedging angle of5vertebrae and4discs were calculated which included apex and two superior and two inferior vertebrae as well as4discs between these vertebrae. Additionally, the wedging of the5vertebrae and4discs in the apical region was also calculated as a proportion of the sum of those. The statistical analysis was conducted using SPSS13.0software.Result:No difference with respect to the Cobb angle was observed between the porcine scoliosis model and EOS patients (P=0.499). The wedging of both vertebrae and discs was greatest at the apex of the scoliosis deformity. Vertebral wedging played a major role in the contribution of Cobb angle in both groups. However, the wedging of the vertebra was found to be greater in porcine scoliosis than EOS patients (P<0.001). No significant difference with respect to the average wedging angle of the five vertebrae was found between porcine scoliosis and EOS patients (P>0.05). However, the vertebral wedging as a proportion of the apical Cobb angle of the porcine scoliosis model was larger than that of EOS patients (75.4%vs.60.2%, P<0.001). Conversely, the wedging angle of the four discs and their proportion to the apical Cobb angle of the porcine scoliosis model were smaller than that of EOS patients (P<0.05).Conclusion:The pattern of vertebrae and discs wedging is different between porcine scoliosis and EOS patients. It should be taken into account when appling the results extracted from the porcine model to humans and exploration of new correction techniques.