Ebook: Research into Spinal Deformities 7
In choosing Montreal for its 8th biennial meeting, the International Research Society of Spinal Deformities (IRSSD), is returning to an auspicious and important venue: their 1992 meeting in Montreal marked the turning point from a focus on the morphological aspects of spinal deformity, towards three-dimensional evaluation and interpretation of scoliotic deformities and their biomechanics. Since then, the IRSSD meetings have had an instrumental role in the advancement of scientific research on problems affecting the spine. This book contains the proceedings of the 2010 conference in the form of peer-reviewed, short papers and abstracts, summarizing the 140 papers and posters presented at the Montreal meeting. With contributions from scientific and clinical experts from around the world, it covers all aspects of spinal deformity research including: etiology, genetics, biology, metabolism, biomechanics, imaging technologies, innovations in treatment and treatment outcomes. It explores current research developments, the underlying mechanisms that cause scoliosis and the clinical effectiveness of a wide range of treatments. Of interest to all those involved in the research into and treatment of spinal deformities, the book provides an opportunity to learn more about the latest developments in this field.
In July 2010, the International Research Society of Spinal Deformities (IRSSD) will hold its 8th biennial meeting in Montreal, Canada. The meeting is returning to one of its important venues. The 1992 meeting in Montreal marked the turning point of a series of preceding meetings that had a primary interest in morphological aspects of spine deformities and trunk asymmetry, towards three-dimensional evaluation and interpretation of scoliotic deformities and their biomechanics. Since then, the IRSSD and the biennial meetings have played an instrumental role in the advancement of scientific research on problems affecting the spine. Over the years, the interest has evolved and diversified towards improved understanding of mechanisms involved in the etiology, biology, biomechanics, metabolism, genetics, and treatment of spinal deformities. It is attended by researchers and clinicians from all over the world, providing a collegial forum for presentations, discussion and collaboration on subjects relating to spinal deformities.
A Pre-Meeting on Clinical applications of 3D technologies and research in biomechanics, biology and genetics of scoliosis also is to be held at Sainte-Justine University Hospital Center to allow the participants to see directly how research activities of our group are linked to the clinical world.
This book contains the Proceedings of the IRSSD 2010 Conference in the form of either peer-reviewed short papers or abstracts summarizing the 140 papers and posters that will be presented. It covers to all aspects of spinal deformity research, including etiology, genetics, biology, metabolism, biomechanics, imaging technologies, innovations in treatment, and treatment outcomes. These scientific proceedings provide the opportunity for readers to learn more about the latest development in this field that will be presented and discussed during the meeting. This meeting represents a multidisciplinary taskforce giving scientific and clinical experts from around the world an opportunity to meet and discuss the latest developments in the fields of scoliosis research, on the search for the underlying mechanisms that cause scoliosis as well as the clinical effectiveness of a wide range of interventions.
I would like to thank all participants, and authors for submitting their work, as well as the members of the scientific committee, chaired by Ian Stokes, that provided very valuable comments and critics that helped in adding to the impact of the work that will be presented at the meeting.
I hope this meeting will be a unique opportunity to meet, exchange, and discuss of subjects of mutual interest related to spinal deformities, and to advance the study and research into spinal deformities for the patients' benefit.
Carl-Eric Aubin, Montreal, Quebec, Canada, April 2010
We used a microarray approach to evaluate gene expression profiles in human AIS osteoblasts, and to identify genes that are differentially expressed following estrogen exposure in non-AIS and AIS human osteoblasts. We found that more than one gene is likely responsible for AIS. Furthermore, some of these genes are estrogen-regulated, suggesting a possible role of estrogens in the etiology of scoliosis.
Background: Scoliosis with vertebral wedging is thought to be caused by asymmetric growth (Hueter-Volkmann law), but vertebral diaphyseal remodeling (Wolff's law) may also contribute to the deformity. We investigated whether vertebral wedging in scoliosis might involve both mechanisms. Methods: An external fixator was used to impose a 30° scoliosis and compression of 0.1 or 0.2 MPa to the tails of 10 5-week-old and 20 14-week-old Sprague-Dawley rats for 6 weeks. The rats were divided into three groups of 10 animals each: Group 1: 5-week-old animals with 0.1 MPa compression; Group 2: 14-week-old animals with 0.1 MPa compression; Group 3: 14-week-old animals with 0.2 MPa compression. Vertebral wedging and diaphyseal curvature were measured from micro CT scans performed at weeks 1, 3, and 6. Wedging due to asymmetrical growth and remodeling was calculated from a Calcein label administered at week 3 and a Xylenol label at week 6. Results: The growth rate of the loaded vertebrae as a per cent of control vertebrae was 60% in Group 1, 40% in Group 2, and 30% in Group 3. The growth rate of control vertebrae in 14-week-old animals was 16% that of 5-week-old animals. The animals in all 3 groups developed a scoliosis with vertebral wedging that averaged 18.7° in Group 1, 8.2° in Group 2, and 10.1° in Group 3. Asymmetric growth was much greater in Group 1 (5-week-old) animals. The ossified epiphyses became wedged and diaphyseal remodeling occurred in all groups. Conclusions: The major contribution to the vertebral wedging was asymmetric growth in the 5-week-old animals and diaphyseal remodeling in the 14-week-old animals. The results support the concept that if appropriate loads can be applied to human vertebrae through minimally invasive techniques, scoliosis and vertebral wedging can be corrected without a spinal fusion in both adolescents and adults.
Quantec raster stereography has been recognized as an accurate and reliable tool for evaluating back contour and posterior truncal rotation in patients with scoliosis. In this prospective study, 35 patients with adolescent idiopathic scoliosis underwent both spinal radiography and Quantec three-dimensional evaluation of their idiopathic scoliosis before and after surgical intervention, consisting of anterior and/or posterior spinal fusion, with or without thoracoplasty. Comparison of established key parameters showed significant improvement in thoracic and thoracolumbar Cobb angle, as well as Quantec Q-angle. Thoracic rotation, Suzuki rib hump sum, and posterior trunk asymmetry also improved due to spinal fusion.
In subjects with scoliosis the thoracic cage deformity is a complex 3D phenomenon. There is a deficiency of simple clinical methods of thorax shape evaluation. The study aimed to introduce and assess an anthropometric technique measuring transverse plane deformity of the thorax in patients with idiopathic scoliosis. Thirty scoliotic girls, aged 14.4±1.5 years, thoracic scoliosis type Lenke 1, mean Cobb 54.1±24.7°, and 30 healthy volunteers matched for sex and age were examined. Using a Martin anthropometric caliper the length of the long and the short horizontal axes of the thorax were measured at the level of the xiphoid process (upper index) and of the costal arch (lower index), both on maximum inspiration and expiration. Asymmetry index, defined as difference of the length of the long and the short axes expressed as the percentage of the short one, was calculated. The upper asymmetry index in the study group was 35.2±18.6 (inspiration) while in the control group it was 13.6±13.6, difference significant, p<0.001. The lower asymmetry index in the study group was 26.2±12.9 (inspiration) while in the control group it was 12.5±11.7, difference significant, p<0.001. In conclusion, thorax asymmetry index revealed significantly higher values in scoliotic patients. Asymmetry of respiratory movements could be measured. This simple technique may be used as a helpful tool for clinicians.
Measured back surface topographies can be useful to monitor the external manifestation of scoliosis to avoid exposure to large doses of radiation. Manual shape fitting of back surface topographies from successive clinical visits can then be used to detect differences. Automated matching of the measured topographies has been seen as a possible improvement on manual comparisons. Recognizing that two changed surface cannot be expected to be simple rigid replicas of each other, the goal of this research has been to develop a new algorithm based on a non-rigid surface matching algorithm, suited to matching the surfaces into same reference frame while also estimating the parameters of the scoliosis deformities, and eliminating noise due to normal body change caused by growth. Back surface topography data from laser optical scanning have been automatically matched by a least squares non-rigid matching algorithm. The algorithm includes new parameters able to model shape changes caused by normal growth and scoliosis deformation. This non-rigid matching algorithm returned r.m.s. values for surface closeness which were improved by at least 10% over rigid matching. Experiments on various scoliosis data demonstrate that the non-rigid matching algorithm is able to accurately match the surfaces while simultaneously extracting parameters representing patient shape change. The non-rigid algorithm has proven to be an improvement on the classical rigid surface matching approach which allows positional fit rather than shape fit. Measured back surface topographies can be closely matched to monitor the external manifestation of scoliosis.
Spinal orthosis is generally applied to the patients with adolescent idiopathic scoliosis (AIS) during puberty to mechanically support the spine and prevent further deterioration. However, the optimum location of pressure is not easy to be determined and the X-ray taken is not a real time presentation of the spinal curvature. With the advancement of clinical ultrasound, tracing spinal processes along a scoliotic spine becomes possible, which means spinous process angle (SPA) can be obtained from ultrasound images. Moreover, SPA is found to be highly correlated with Cobb's angle. Since the outcome of orthotic intervention for AIS is considered to be associated with accurate orthosis fitting, this study seeks to apply three-dimensional (3-D) ultrasound in the fitting procedure of spinal orthosis for patients with AIS. The accuracy of pressure pad location in brace can help to improve the effectiveness of spinal orthosis treatment. By means of the ultrasound assessments, spinous process angle is examined and used as the parameter to evaluate the optimal location for pressure pad. The intra-rater reliability [ICC (1, 3)] for using ultrasound to measure SPA is >0.9 (p<0.05). Furthermore, the correlation between Cobb's angle estimated from the measurement of SPA in 3-D ultrasound images and Cobb's angle measured from X-ray is highly significant (R=0.98, p<0.01). According to these findings, ultrasound can be further developed as a non-invasive real-time assessment tool for spinal curvature especially in fitting stage to improve the treatment effect of the spinal orthosis.
Magnetic Resonance Imaging (MRI) offers a valuable research tool for the assessment of 3D spinal deformity in AIS, however the horizontal patient position imposed by conventional scanners removes the axial compressive loading on the spine. The objective of this study was to design, construct and test an MRI compatible compression device for research into the effect of axial loading on spinal deformity using supine MRI scans. The device was evaluated by performing unloaded and loaded supine MRI scans on a series of 10 AIS patients. The patient group had a mean initial (unloaded) major Cobb angle of 43±7°, which increased to 50±9° on application of the compressive load. The 7° increase in mean Cobb angle is consistent with that reported by a previous study comparing standing versus supine posture in scoliosis patients (Torell et al, 1985. Spine 10:425-7).
Scoliosis surgery involves the insertion of screws and/or hooks into selected vertebrae to secure a pre-bent rod placed along the concave side of the spine. Usually conventional x-rays will be taken before the surgery to plan the alignment and positioning of the pedicle screws. However, reports state that perforation rate range from 6% to 54%. A misalignment of a pedicle screw can potentially cause permanent neurological spinal cord injury or even a lifethreatening vascular injury. Because of the importance of positioning and aligning of pedicle screws, we are working on an ultrasound method to guide the insertion of pedicle screws in real time. A pulse-echo immersion experiment was set up to study how well the edges of cortical bone could be detected using a bovine spinous process in-vitro. Two ultrasound frequencies (3.5 MHz and 5.0 MHz) were considered in this study. This preliminary study shows that ultrasound is able to penetrate cortical bone and reflect back from the outer boundary. All interfaces are clearly identified for both frequencies. Strong reflection signals are obtained when the beam is normal to the interface. Derived thickness values from the reflections are comparable with those from micro-CT image. The 5.0MHz ultrasound frequency provided better resolution than the 3.0 MHz frequency.
Minimally invasive surgical (MIS) techniques for the correction of scoliosis are under development. The installation of fusionless implants targeting the vertebral growth plate requires precise identification of spinal micro-structures. During ex vivo studies, we demonstrate that optical coherence tomography (OCT) allows visualization of spinal tissues including the growth plate, the intervertebral disc and the vertebral body. This study aims at designing a handheld probe using OCT and assessing its potential for use in MIS. An OCT handheld probe was built which satisfies criteria for resolution, penetration and field of view required for spinal MIS techniques. Ex vivo images of rat tail and porcine vertebrae enabled differentiating musculoskeletal tissues of the spine (growth plate, intervertebral disc and vertebral body). Pending in vivo studies on porcine models, we evaluated the probe on a human finger and demonstrated its ability to image human tissues at video rate (25 fps) with proper imaging depth and resolution. These preliminary results showed the potential of the OCT probe for dynamic and precise imaging of spinal tissues.
Multimodal fusion of 2D thoracoscopic images with a pre-operative 3D anatomical model of the spine is useful for minimally invasive surgical procedures using an angled monocular endoscope with varying focal length. An offline calibration procedure has been developed to compute initial endoscope parameters, such as lens distortion, focal length and optical center before surgery. An optical tracking system is used to update extrinsic parameters describing the position and orientation of the endoscope in real-time during the procedure. This calibration allows the registration of the thoracoscopic image sequence with a pre-operative MRI 3D model of the spine. Two visualization methods merging the 3D model and thoracoscopic image sequence have been developed using both augmented reality and augmented virtuality paradigms primarily as an aid for discectomy. Augmented views are generated by adding annotations and projecting the MRI 3D model onto real thoracoscopic images. Virtual views are generated by projecting the real thoracoscopic images on a virtual view of the 3D model. Experimental results showed that the calibration procedure accuracy obtained by computing the relative 3D reconstruction error on a known object was 1.0 mm. Two orthopedic surgeons assessed the generated views, confirming the relevance and added value of the proposed visualization tool for minimally invasive discectomy assistance.
The development of scoliosis in animal models with induced asymmetric rib growth suggested the possible role of asymmetric rib growth in the etiopathogenesis of adolescent idiopathic scoliosis (AIS). Asymmetric rib length is well recognized in idiopathic scoliosis, however, whether this rib asymmetry was primary or secondary has not been clearly documented. We measured the lengths of all ribs in 48 patients with AIS and 29 patients with scoliosis associated with syringomyelia and Chiari I malformation (SS). In both groups, ribs on the concave side were significantly longer than those on the convex side in apical area. The rib length difference was significantly associated with magnitude of the Cobb angle of thoracic curve. However, the rib length discrepancy showed no difference between AIS and SS group. The finding of similar asymmetry of rib length in both AIS and SS patients with thoracic curve indicated the discrepancy found in AIS might be secondary to the spinal deformity rather than its etiological factor.
Human bipedalism appears to be a prerequisite for the development of idiopathic scoliosis. The objective of this study was to examine the effect of different positions of the human spine on vertebral rotation in vivo. Thirty asymptomatic volunteers underwent MRI scanning of the spine in three different body positions; upright, quadrupedal-like and supine. Vertebral rotation in the local transverse plane was measured and compared at different spinal levels between the three body positions. It was shown that in all three positions the mid and lower thoracic vertebrae were predominantly rotated to the right. However, rotation was significantly less in certain areas of the spine in the quadrupedal-like position than in both the standing upright and supine positions. We hypothesize that the erect position of the human spine, but also the supine position, increases dorsally directed shear loads that may increase the tendency of certain spinal segments to rotate.
The value of the lateral bending test is important in the assessment of spinal curve mobility and prediction of surgical outcome in the treatment of adolescent idiopathic scoliosis (AIS). However, radiographic bending tests are unable to assess the reducibility of trunk asymmetry. This study aims to exploit surface topography measurement in order to evaluate the changes in shape of the trunk (a) between bending and neutral standing positions, and (b) between standing pre- and post-operative visits, in a cohort of adolescents with AIS having undergone surgical correction; and to correlate the differences measured in cases (a) and (b). Our cohort includes 13 patients with right thoracic AIS. Each patient had their 3D trunk surface digitized with a multi-head InSpeck system in standing posture (at the pre-op and post-op visits) and in maximum voluntary right and left bending (at the pre-op visit). We developed a novel trunk shape analysis method which produces a set of inclined trunk cross-sections allowing comparison between different postures. Two asymmetry indices, trunk rotation (TR) and back surface rotation (BSR), were computed in all cases and a statistical analysis was performed. Our correlation study (Pearson test) showed fair correlations in most cases between the changes in side-bending and those post-surgery, with the strongest relationship (p-value < 0.01) when combining the TR measurements from both bendings. These results provide evidence that the bending test can be used to assess trunk asymmetry reducibility. The proposed approach could provide a non-invasive trunk asymmetry reducibility test for routine clinical use in AIS surgery planning.
There is no consensus on which surface topography (ST) parameters may be used to detect scoliosis progression. The sensitivity to change of common ST parameters has not yet been compared. The goal of this study was to determine which ST parameters are most sensitive to scoliosis progression in patients with adolescent idiopathic scoliosis (AIS) receiving conservative treatment. Fifty-eight subjects with AIS were included whose Cobb angle had progressed by at least 5° during a 1 year interval. All had had ST scans and frontal radiographs at a 12 month interval at our clinic. Commonly used back-only ST parameters and contributing scores were derived by one evaluator. Standardized response mean (SRM) and 95% confidence intervals (CI) were calculated using the absolute value of the changes between baseline and follow-up to reflect change in deformity, independent of direction. Decompensation, cosmetic score, Deformity in the Axial Plane Index (DAPI), trunk rotation, Hump Sum, and lordosis angle were highly sensitive to scoliosis progression (SRM>0.8). Cosmetic score, Posterior Trunk Symmetry Index (POTSI), and kyphosis angle had significantly poorer SRM values than the Cobb angle. All other ST parameters had SRM estimates that did not differ significantly from the Cobb angle, suggesting that they have a similar ability to detect progression The ST measures that were most sensitive to detection of scoliosis progression in the frontal, transverse, and sagittal planes were decompensation, trunk rotation, and lordosis angle, respectively. Absolute changes in surface parameters representing either worsening or improvement externally could reflect worsening of the internal deformity. The majority of ST parameters are potentially sensitive to scoliosis progression.
A multi-functional positioning frame (MFPF) has been developed which includes a number of positioning features allowing for hip flexion and extension, thorax vertical displacement, lateral leg displacement, pelvic torsion and thorax lateral displacement. The objective of this study was to develop a method allowing for optimized combined use of the MFPF features. Finite element models (FEMs) representing the osseo-ligamentous structures of the spine, ribcage, pelvis and lower limbs, including muscles, were created for three different curve types (main thoracic, double major, and triple major) using a radiographic bi-planar reconstruction technique. Each FEM was subjected to an experimental design in which MFPF features were independently and simultaneously varied between extreme positions and the resultant changes in spinal geometry measured. Optimization of individual spinal geometrical parameters showed variability between curve types and some patterns such as minimum Cobb with lower limbs displaced laterally towards the convexity, pelvis raised on the side of concavity, and thorax laterally displaced towards the thoracic concavity. A weighted and normalized global optimization equation was developed which accounts for the relative importance and desired values of each geometrical parameter. Combined use of MFPF features and adjustments offers a wider range of possible intra-operative spinal geometries than their individual use.
Fusionless growth sparring implants seek to restore spinal alignment through the early intervention of pediatric scoliosis. Amongst a growing number of concepts, the stainless steel (SS) staple, flexible tether and shape memory alloy (SMA) staple have demonstrated their validity by retarding convex vertebral growth while modifying spinal alignment. The purpose of this study was to explore the biomechanics of these devices in a human scoliotic finite element model (FEM) constructed from patient data. A FEM of a scoliotic anterior spine (28° thoracic curve) was developed to include growth dynamics and shown to represent typical scoliotic progression. The explored implant concepts were alternatively introduced around the apical vertebra of the FEM (T5–T9). Immediate impact (asymmetrical loading of the vertebral growth plates and correction of scoliotic curve) and long term impact (correction of scoliotic curve after 2 years of growth) were simulated and compared to the behavior of the non-instrumented model and patient data. Results of the difference in asymmetrical growth plate stress between instrumented and non-instrumented models reveal: insignificant initial impact by the SS staple, a 52% reduction with the flexible tether, and a 31% reduction with the SMA staple. Initial and long term modifications of coronal spinal alignment following simulated growth was respectfully 28° to 62°in non-instrumented model and patient data, 28° to 31° with SS staple, 23° to 31° with flexible tether, and 27° to 34° with SMA staple. The interpretation of such methods suggests that the long term correction, achieved via growth modulation, would benefit from improved control of asymmetrical stresses within the growth plates. From a biomechanical perspective, fusionless growth sparring techniques for the early treatment of idiopathic scoliosis show promising preliminary results.
The intra-operative prone position used for the posterior instrumentation of scoliotic patients has been shown to reduce the spinal deformities prior to instrumentation by 37% on average. However, the effects of the lateral decubitus position used for anterior approaches and minimally invasive techniques have not been investigated. The objectives were to characterize, model and study the biomechanics of this intra-operative posture. Several clinical indices were measured on the pre- and intra-operative radiographs of six scoliotic patients. A personalized finite element mode (FEM) was developed using the pre-op 3D reconstruction, and a three-step method was developed to simulate the lateral decubitus positioning. Two additional intra-op postures, simulating different pelvic obliquities, were also tested by varying the inclination of L5. The radiographic evaluation of the lateral decubitus position showed a significant reduction of 44% of the major curve with 18 mm of apical vertebra translation. The FEM was able to reproduce the intra-op spine geometry with no significant difference with the measured values. Simulations also showed that the pelvic obliquity had different effects on the lumbar and major Cobb angles depending on the scoliotic curve type. The lateral decubitus posture reduces significantly the scoliotic curvatures prior to instrumentation, which was dependent on the pelvic obliquity.
Introduction: Adolescent idiopathic scoliosis (AIS) can affect spine mobility and gait mechanisms. Nowadays little is known about the effects of scoliosis on gait. Objectives: To evaluate the effects of untreated scoliosis on gait. Materials and Methods: Fifty-four females (13 healthy girls and 41 AIS with thoracolumbar/lumbar curve) were assessed by gait analysis. Xrays allowed classifying AIS patients into three groups, depending on the scoliosis severity. Gait analysis included synchronous bilateral kinematic and electromyographic (EMG) measurements, assessment of external (Wext), internal (Wint), total mechanical work (Wtot), oxygen consumption (V& O2), energy cost (C) and muscular efficiency (Wtot/C). Results: Shoulder, pelvis and hip motions were significantly reduced in all AIS patients. The reduced motion correlated with decreased Wtot. V& O2 and C were increased by 30 %. Muscle efficiency was decreased by 29 %. Increased C correlated with prolonged timing activity of lumbo-pelvic muscles (LPMTA). Discussion: Reduction of Wtot can be explained by decrease of external work. Increase of V& O2 and energy cost could be due to bilateral increase of LPMTA. Conclusion: Similar gait alterations were observed in all AIS patients, whatever the severity of the scoliosis. The observed alterations may be considered as the manifestation of an underlying neuromuscular disease or as a consequence of the stiffening effect of scoliosis. The observed “careful walking” strategy could also be a compensatory mechanism to minimize energy expenditure.
The internal forces in the human body in motion could provide valuable information for the evaluation and follow-up of subjects with musculo-skeletal pathologies, such as scoliosis, but are still difficult to accurately measure. In this context, the objective of this study is to quantify the global intervertebral torques along the spine during walking, in order to compare the dynamical behavior between two subjects with different scoliosis severities. Practically, two patients, both with left lumbar adolescent idiopathic scoliosis (lumbar Cobb angles: 40°), but with different apical axial rotations (20° and 30°) and different thoracic Cobb angles (20° and 30°), walked on a treadmill at 4 km/h. The acquisition system included optokinetic sensors (reflective markers), recording the 3D-joint coordinates, and a treadmill equipped with strain gauges, measuring the external forces independently applied to both feet. The global intervertebral torques were computed using an inverse dynamic model of the human body in 3D. As results, significant differences of the subject kinematics and ground reaction forces were recorded, which lead to different internal torque behavior in magnitude, maxima and minima when normalized to the subject mass. In conclusion in this preliminary study, different dynamical behavior was found between two subjects with different scoliosis severities, suggesting that the scoliosis severity could be affected by abnormal internal torques along the spine during gait (different than in the standing posture), which could lead to a supplementary asymmetric growth modulation of the vertebrae and the further progression of the scoliotic deformities in the framework of the Hueter-Volkman principle. Finally, this is a pilot study and the results will inform future studies and biomechanical modeling.
Despite years of extensive research, the etiology of idiopathic scoliosis still has not been resolved. A hypothesis on the role of posteriorly directed shear loads was studied in several biomechanical and imaging studies. So far, it has been shown that: on the human erect spine these posteriorly directed shear loads act; these loads decrease the rotational stability of the spine vitro and in vivo; once rotation occurs, it logically follows an already built-in vertebral rotational pattern, that is pre-existent in the human spine; this pre-existent rotational pattern is related to organ anatomy, and not to handedness; certain areas in the female spine are more subject to posteriorly directed shear loads as certain areas in the female spine are more backwardly inclined. Although it is appreciated that the cause of idiopathic scoliosis is multi-factorial, we believe that the delicate upright spinal sagittal balance and the unique posteriorly directed shear loads acting on the erect human spine play a crucial role in the rotational stability of the human spine, and thus in the pathogenesis of idiopathic scoliosis.