The effect of scoliotic deformity on spine kinematics in adolescents
The goal of this research was to characterize the spinal mobility differences caused by AIS, as this has not been done before with segmental detail. It was hypothesized that the AIS group would exhibit reduced mobility in all modes of bending compared to the TA group, but this was not supported. In almost all cases, the mobility of the AIS group was statistically equivalent or significantly greater than the mobility of the TA group.
Mobility values of the current study are compared to literature values for scoliotic adolescents and typical adolescents in Table 1. The mobility values for extension were similar across studies but those presented for flexion and lateral bending were smaller than those presented in the other studies for both TA and AIS populations. Mellin and Poussa [9] use an inclinometer method that has been shown to have a 15° range of motion difference and no correlation with values calculated by the electrogoniometer method, as was used in this study. While these studies shown in Table 1 represent the best comparisons available within the current literature, these studies did not collect data, constrain motion, or select participants with the same methodologies as the current study. Because of the differences in methodologies, differences in mobility outcomes were to be expected.
Thoracic mobility in control and scoliosis subjects
Values presented are mean range of motion values with standard deviation values presented in parentheses where available. The number of subjects varied by bending mode in Galvis et al.: Flexion (n?=?5), extension (n?=?10), and left and right lateral bending (n?=?11). The groups presented for Poussa et al. represented divisions by Cobb angle, with Group 1 having 25° Cobb angles, Group 2 having 25–35° Cobb angle, and Group 3 having Cobb angles 35°
Near the primary curve apex in the AIS group, it was expected the spine would be more rigid than a typical spine. While there was no significant difference between the mobility of the groups, the average range of motion was lower in the scoliosis group compared to the control. One study found the four periapical spinal units experience “structural tethering” where the spinal units demonstrate decreased range of motion [10]. This same effect could be causing the non-significantly lower mobility in the AIS group since the apical effects could have been dampened by the inclusion of “non-tethered” individual motion levels in the lower thoracic region. While structural tethering at the apex was not definitively demonstrated in this study, it has been shown in similar studies and could be the underlying cause of the mobility assessed here but further research is needed.
Results indicate the AIS group had increased nROM in the periapical regions of the spine, particularly in flexion and flexion-type tasks. Many possibilities exist to explain this phenomenon. The segments could be more mobile due to a compensation for reduced mobility near the apex, hypokyphosis in the thoracic spine (though degree of kyphosis was unknown) which would allow for improved rotation about the spinal column, or hyperlaxity of the spine in scoliosis patients which could contribute to deformity progression. The AIS group had greater mobility in the MT region during flexion, in MT, TL, and thoracic during left anterior-lateral flexion, and in UL during right anterior-lateral flexion. The AIS group had significantly higher UL in left lateral bending. In some bending modes, significant mobility differences were seen between the lower thoracic (apical) region and the periapical regions (p??0.05). Without controlling for thoracic kyphosis and with some studies indicating those with scoliosis are no more flexible than their non-pathologic counterparts, compensatory motion may be the mechanism causing the increased mobility. While these results indicate greater mobility above and below the theoretically tethered apical region, this contradicts related findings which indicate mobility above and below long fusions is significantly reduced [11–13]. Since previous research does not agree with the current findings, further study is needed to confirm the increased mobility and its cause.
Other research has shown that thoracic mobility in an AIS population is rarely different compared to controls [1–4]. In this pilot study, there was no statistical difference between the two groups for overall thoracic flexion but there were significant differences in smaller segments of the spine. This showed evaluating thoracic and thoracolumbar mobility to the segmental level is necessary to fully characterize thoracic mobility in an AIS population, as full segmental analyses alone can miss significant mobility differences.
Although there were innovative aspects of this work, there were several limitations to consider as well. Existing three dimensional images were not available; therefore, a three dimensional description of the deformity, including degree of kyphosis and axial rotation, was unknown. Though the current study was designed to be the first to investigate axial rotation in the thoracic spine in an AIS population, the Euler method produced significant data variations when applied to axial rotation. Therefore, spinal motion was not compared in the three primary modes of bending, which would have yielded a three dimensional characterization of adolescent spinal motion. Despite research indicating brace wear can affect curve flexibility, the group was not sub-divided to accommodate for this effect, which is a limitation of the study [14]. This study design did not control for age, Risser grade, or curve severity, which have the potential to effect mobility outcomes in this population and therefore is a limitation. Motion effect from effort, diurnal, sensor placement, soft tissue, and selection variability can obfuscate the true motion and true differences between groups. Trials were the sensors exceeded the collection volume were excluded, which may have eliminated some trials from taller and more flexible participants. With such a small sample size and low power, it was difficult to discern significant differences between groups. Future research should be designed to mitigate against these limitations.
This pilot study was designed to characterize the spinal mobility differences caused by AIS. As gender was not found to have a significant effect on curve flexibility, a mixed gender population was used [15]. Although no gender differences were noted in curve flexibility, subjects were age and gender matched across groups to eliminate any possible cofounding effects. Because curve location and direction affects flexibility, only right thoracic curves with apices between T6-T10 were chosen [2, 16–19]. The age range was limited to isolate the pubertal phase in adolescent subjects to investigate motion differences prior to skeletal maturity. Of the five trials collected for each task, the last trial was used to allow for the viscoelastic effect to stabilize during testing and fatigue effect was not expected as the trials had a high level of repeatability (r??0.9) for all measures in all tasks.
Very little research has focused on comparing spinal function, as measured by spinal mobility, in adolescents both with and without AIS. Of these studies, only one presented range of thoracic mobility and none provided information about near apex mobility [2]. While scoliosis affects large portions of the spine, the deformity varies throughout its length and greatly affects thoracic and lumbar biomechanics. This pilot study was the first to examine spinal function in segmental regions of the thoracic and thoracolumbar spine in adolescents with and without AIS. This investigation shed light on mobility differences caused by the deformity and opened the door for further exploration in this area.
Future work could expand on the research in this study. Three dimensional characterization of the posture and motion would be beneficial. Future studies should control for skeletal maturity and curve severity. As shown by the low power in this study, a larger number of participants would be needed to discern significant mobility differences. This would allow for investigation into segmental mobility differences between scoliosis and control groups with the ability to discern significant differences and isolate the causes of these mobility differences.