Background: The neurocentral junction often has been identified as a potential cause of adolescent idiopathic scoliosis (AIS). Disparate growth at this site has been thought to lead to pedicle asymmetry, which then causes vertebral rotation in the transverse plane and ultimately, the development of scoliotic curves.

Objectives:

To develop a model that incorporates pedicle growth and growth modulation into an existing finite element model of the thoracic and lumbar spine already integrating vertebral growth and growth modulation

Methods: The model was personalised to the geometry of a non-pathological subject and used as the reference spinal configuration. Left/right asymmetry of pedicle geometry (i.e. initial length) and left/right asymmetry of the pedicle growth rate alone or in combination with other AIS potential pathogenesis (anterior, lateral, or rotational displacement of apical vertebra) were simulated over a period of 24 months. The Cobb angle and local scoliotic descriptors (wedging angle, axial rotation) were assessed at each monthly growth cycle.

Results: Simulations with asymmetrical pedicle geometry did not produce significant scoliosis, vertebral rotation or wedging. Simulations with asymmetry of pedicle growth rate did not cause scoliosis independently and did not amplify the scoliotic deformity caused by other initial deformations tested by Villemure (2004).

Discussion and Conclusion: The results of this biomechanical model do not support the hypothesis that asymmetrical neurocentral junction growth is a cause of AIS. This concurs with recent animal experiments in which neurocentral junction growth was unilaterally restricted and no scoliosis, vertebral wedging or rotation was noted. With regards to addressing the aetiology of scoliotic curve development, biomechanical modelling represents a powerful tool to investigate cause and affect relationships since AIS patients typically present to the scoliosis clinic well after curves have manifested.

Contact person and Presenter: Carl-Éric Aubin, Ph.D., Canada Research Chair “CAD Innovations in Orthopedic Engineering”, Department of Mechanical Engineering, Ecole Polytechnique, Montreal, Canada, Tel: (514) 340-4711, ext. 4437; Fax: (514) 340-5867; E-mail: carl-eric.aubin@polymtl.ca

Vertebral growth remains a mystery, especially with regards to the contribution of different growth plates and the mechanisms of growth after closure of these plates. As an example of vertebral growth in general, the growth of the vertebral canal was assessed in a rat model using fluorochromes. Although 80–90% of vertebral canal growth was due to growth plates, the remaining canal growth occurred via periosteal absorption and deposition. This is contrary to the traditional idea that periosteal mechanisms do not change the shape or dimensions of bone and suggests that the vertebrae exhibit a different model of growth than typical bones.

Vertebral growth remains largely a mystery. The contributions of different growth plates and the mechanisms of growth after closure of these plates requires further exploration. As an example of vertebral growth, vertebral canal growth was assessed in a living rat model using fluorochromes.

Vertebral canal growth and presumably vertebral growth in general occurred by different mechanisms at different phases of development. Growth plates accounted for the majority of growth although periosteal mechanisms also resulted in changes in the size and shape of the vertebrae. This is contrary to the traditional concept of periosteal growth and suggests that vertebrae may exhibit a different model of growth than typical bones.

The growth of the vertebrae in a particular dimension and during a particular phase of development is dependent on different mechanisms of growth, which may play a role in interpreting vertebral growth anomalies.

The interspinous junction closed by the end of the first week, whereas the neurocentral junction closed between weeks three and four. By four weeks, the vertebral canal had achieved 80–90% of its growth in area and diameter. After growth plate closure, the canal continued to grow by periosteal mechanisms and was displaced posteriorly.

Thirty-six Sprague-Dawley rats (age one week-seven weeks) were injected with tetracycline and alizarin using a dosing interval of four days. Thoracic vertebrae were sectioned using a cryostat and examined under a fluorescence microscope. In addition to noting fluoro-chrome deposition, the dimensions of the growth plates and canal were noted.

Funding: Edmonton Orthopaedic Research Association and University of Alberta Department of Radiology and Diagnostic Imaging

This study was designed to examine the components of the MR image of the neurocentral junction (NCJ) and to explore the discrepancy between the age of closure of the NCJ as determined by anatomic and imaging studies. MR images of one hundred and fourteen porcine NCJs were correlated with anatomic and histologic sections. Whereas gross anatomic visualization did not reveal the NCJ site, MRI was sensitive for cartilage detection and accurately determined the age of NCJ closure although it overestimated the extent of closure. Based on this study, MRI characterization of the NCJ appears reliable and the NCJ cartilage does not close until adolescence.

This study examined the composition of the MR image of the neurocentral junction (NCJ) and the discrepancy between the age of closure of the NCJ as determined by anatomic and imaging studies.

MRI was sensitive for cartilage detection and accurately determined the age of NCJ closure (i.e. absence of cartilage on histologic examination).

Disparate NCJ development has been implicated as a potential cause of adolescent idiopathic scoliosis. Whereas autopsy studies have refuted this theory by suggesting that the NCJ closes before adolescence, MRI studies have resurrected this idea by suggesting later closure. MRI-histologic correlation suggests that the NCJ cartilage remains present until adolescence and therefore further exploration of the disparate growth hypothesis is required.

Gross anatomic visualization did not reveal the NCJ site, even after removal of the periosteum. In contrast, the presence or absence of an NCJ image correlated with the presence or absence of cartilage although MRI overestimated the extent of this cartilage.

Vertebrae were grossly examined for any evidence of the NCJ site. Sagittal and transverse MR images of one hundred and fourteen porcine NCJs in various stages of development (thirty-eight open, sixty-four closing, twelve closed) were correlated with anatomic and histologic sections acquired at the same position.

Funding: Edmonton Orthopaedic Research Association and University of Alberta Department of Radiology and Diagnostic Imaging

Introduction: Although there are several known causes of scoliosis, most are of unknown cause and develop during adolescence, making adolescent idiopathic scoliosis (AIS) the most common form. It has long been hypothesised that unilateral closure of the neurocentral junction accompanied by continued growth on the opposite side could lead to vertebral rotation and subsequent lateral curvature. However, autopsy studies of neurocentral junction closure in children has revealed that these joints close at approximately six years of age consequently excluding this hypothesis as a cause of AIS. In contrast, a recent MRI study has suggested that in some children at least, the NCJ does not close until much later in development around the time of puberty thereby resurrecting this hypothesis as a potential cause of AIS. This study was designed to investigate closure time and pattern of closure of the NCJ in normal patients to determine whether further examination of this hypothesis might be warranted.

Methods and results: The morphology of the NCJs in 20 patients between the ages of 3 and 15 were observed in MR images taken for purposes other than spinal anomaly. The structure of individual NCJs were observed and reconstructed in 3-dimensions. The age at which NCJs became closed was determined and pattern of closure of a typical NCJ was created using the reconstructed images. The pattern of closure of the NCJs along the vertebral column was also determined and any differences between right and left sides at the same level was also noted.

The results showed that there was a sequence of closure along the vertebral column for the NCJs with those in the cervical and lumbar regions being the first to close and those at the approximate level of T8 being the last to close. While the NCJs in the cervical and lumbar regions close at 5–6 years of age, those in the thoracic region, that are the last to close, do so at approximately 12 years of age. No significant difference between the stage of closure of the left and right sides was seen at any level.

Conclusion: The results of this study have shown that the closure of the NCJs in those vertebrae that form at approximately the most common level for the apical vertebra associated with AIS (midthoracic) does not occur until the time of puberty. This contrasts sharply with previously held views on the age of closure. Although no significant difference in closure between left and right sides was seen among these particular patients it does not exclude unilateral closure as a cause of AIS at least in some patients. These results suggest that examination of this hypothesis should be resurrected and that further study is well warranted. MR examination of young patients with small, initial curves could be well worthwhile.