Preoperative CT examination for accurate and safe anterior spinal instrumentation surgery with endoscopic approach

Change in Lung Volume Following Thoracoscopic Anterior Spinal Fusion Surgery: A 3-Dimensional Computed Tomography Investigation

STUDY DESIGN

Lung volumes and thoracic anatomy were measured from low-dose computed tomography (CT) scans preoperatively and 2 years following thoracoscopic anterior spinal fusion (TASF) for adolescent idiopathic scoliosis (AIS).

OBJECTIVE

The aim of this study was to assess changes in lung volume after TASF surgical correction.

SUMMARY OF BACKGROUND DATA

AIS patients are known to have decreased pulmonary function as a consequence of their spinal and ribcage deformity. Several studies have evaluated changes in pulmonary function clinically after scoliosis correction surgery showing varied results. To date, there have been no published studies using CT to evaluate lung volume changes following TASF.

METHODS

Twenty-three female AIS patients with both pre- and 2 years postoperative low-dose CT scans were selected from an ethically approved, historical databank. Three-dimensional lung volumes were reconstructed to determine anatomical lung volumes. Right and left lung volumes, total lung volume, and right-to-left lung volume ratio were obtained as well as hemithoracic symmetry, to indicate the extent of thorax deformity. Cobb angle, rib hump, levels fused in surgery, and patient height were used for correlation analysis with the lung volume results.

RESULTS

Left lung volume, total lung volume, and hemithoracic ratio all increased significantly 2 years after surgery. There was no significant change in right-to-left lung volume ratio (P = 0.36). Statistical regression found significant positive correlation between lung volume changes, reduction in Cobb angle, increase in height, and improvement in hemithoracic symmetry ratio.

CONCLUSION

TASF resulted in a statistically significant increase in lung volume following surgery, as well as improvement in the symmetry of the thoracic architecture; however, the postoperative lung volumes remained in the lower 50th percentile relative to females without thoracic deformity. Furthermore, change in lung volume was significantly correlated with changes in Cobb angle, hemithoracic asymmetry, and increased patient height, which are important consequences of thoracic deformity correction surgery.

LEVEL OF EVIDENCE

3.

Is There Asymmetry Between the Concave and Convex Pedicles in Adolescent Idiopathic Scoliosis? A CT Investigation

BACKGROUND

Adolescent idiopathic scoliosis is a complex three-dimensional deformity of the spine characterized by deformities in the sagittal, coronal, and axial planes. Spinal fusion using pedicle screw instrumentation is a widely used method for surgical correction in severe (coronal deformity, Cobb angle > 45°) adolescent idiopathic scoliosis curves. Understanding the anatomic difference in the pedicles of patients with adolescent idiopathic scoliosis is essential to reduce the risk of neurovascular or visceral injury through pedicle screw misplacement.

QUESTIONS/PURPOSES

To use CT scans (1) to analyze pedicle anatomy in the adolescent thoracic scoliotic spine comparing concave and convex pedicles and (2) to assess the intra- and interobserver reliability of these measurements to provide critical information to spine surgeons regarding size, length, and angle of projection.

METHODS

Between 2007 and 2009, 27 patients with adolescent idiopathic scoliosis underwent thoracoscopic anterior correction surgery by two experienced spinal surgeons. Preoperatively, each patient underwent a CT scan as was their standard of care at that time. Twenty-two patients (mean age, 15.7 years; SD, 2.4 years; range, 11.6-22 years) (mean Cobb angle, 53°; SD, 5.3°; range, 42°-63°) were selected. Inclusion criteria were a clinical diagnosis of adolescent idiopathic scoliosis, female, and Lenke type 1 adolescent idiopathic scoliosis with the major curve confined to the thoracic spine. Using three-dimensional image analysis software, the pedicle width, inner cortical pedicle width, pedicle height, inner cortical pedicle height, pedicle length, chord length, transverse pedicle angle, and sagittal pedicle angles were measured. Randomly selected scans were remeasured by two of the authors and the reproducibility of the measurement definitions was validated through limit of agreement analysis.

RESULTS

The concave pedicle widths were smaller compared with the convex pedicle widths at T7, T8, and T9 by 37% (3.44 mm ± 1.16 mm vs 4.72 mm ± 1.02 mm; p < 0.001; mean difference, 1.27 mm; 95% CI, 0.92 mm-1.62 mm), 32% (3.66 mm ± 1.00 mm vs 4.82 mm ± 1.10 mm; p < 0.001; mean difference, 1.16 mm; 95% CI, 0.84 mm-1.49 mm), and 25% (4.10 mm ± 1.57 mm vs 5.12 mm ± 1.17 mm; p < 0.001; mean difference, 1.02 mm; 95% CI, 0.66 mm-1.39 mm), respectively. The concave pedicle heights were smaller than the convex at T5 (9.43 mm ± 0.98 vs 10.63 mm ± 1.10 mm; p = 0.002; mean difference, 1.02 mm; 95% CI, 0.59 mm-1.45 mm), T6 (8.87 mm ± 1.37 mm vs 10.88 mm ± 0.81 mm; p < 0.001; mean difference, 2.02 mm; 95% CI, 1.40 mm-2.63 mm), T7 (9.09 mm ± 1.24 mm vs 11.35 mm ± 0.84 mm; p < 0.001; mean difference, 2.26 mm; 95% CI, 1.81 mm-2.72 mm), and T8 (10.11 mm ± 1.05 mm vs 11.86 mm ± 0.88 mm; p < 0.001; mean difference, 1.75 mm; 95% CI, 1.30 mm-2.19 mm). Conversely, the concave transverse pedicle angle was larger than the convex at levels T6 (11.37° ± 4.48° vs 8.82° ± 4.31°; p = 0.004; mean difference, 2.54°; 95% CI, 1.10°-3.99°), T7 (12.69° ± 5.93° vs 8.65° ± 3.79°; p = 0.002; mean difference, 4.04°; 95% CI, 1.90°-6.17°), T8 (13.24° ± 5.28° vs 7.66° ± 4.87°; p < 0.001; mean difference, 5.58°; 95% CI, 2.99°-8.17°), and T9 (19.95° ± 5.69° vs 8.21° ± 4.02°; p < 0.001; mean difference, 4.74°; 95% CI, 2.68°-6.80°), indicating a more posterolateral to anteromedial pedicle orientation.

CONCLUSIONS

There is clinically important asymmetry in the morphologic features of pedicles in individuals with adolescent idiopathic scoliosis. The concave side of the curve compared with the convex side is smaller in height and width periapically. Furthermore, the trajectory of the pedicle is more acute on the convex side of the curve compared with the concave side around the apex of the curve. Knowledge of these anatomic variations is essential when performing scoliosis correction surgery to assist with selecting the correct pedicle screw size and trajectory of insertion to reduce the risk of pedicle wall perforation and neurovascular injury.

Comparison of two- and three-dimensional measurement of the Cobb angle in scoliosis

PURPOSE

The Cobb angle as an objective measure is used to determine the progression of deformity, and is the basis in the planning of conservative and surgical treatment. However, studies have shown that the Cobb angle has two limitations: an inter- and intraobserver variability of the measurement is approximately 3-5 degrees, and high variability regarding the definition of the end vertebra. Scoliosis is a three-dimensional (3D) pathology, and 3D pathologies cannot be completely assessed by two-dimensional (2D) methods, like 2D radiography. The objective of this study was to determine the intraobserver and interobserver reliability of end vertebra definition and Cobb angle measurement using X-rays and 3D computer tomography (CT) reconstructions in scoliotic spines.

METHODS

To assess interoberver variation the Cobb angle and the end vertebra were assessed by five observers in 55 patients using X-rays and 3D CT reconstructions. Definition of end vertebra and measurement of the Cobb angle was repeated two times with a three-week interval. Intraclass correlation coefficients (ICC) were used to determine the interobserver and intraobserver reliabilities. 95% prediction limits were provided for measurement errors.

RESULTS

Intraclass correlation coefficient (ICC) showed excellent reliability for both methods. The measured Cobb angle was on average 9.2 degrees larger in the 3D CT group (72.8°, range 30-144) than on 2D radiography (63.6°, range 24-152).

CONCLUSIONS

In scoliosis treatment it is very essential to determine the curve magnitude, which is larger in a 3D measurement compared to 2D radiography.

Gravity-induced coronal plane joint moments in adolescent idiopathic scoliosis

Background

Adolescent Idiopathic Scoliosis is the most common type of spinal deformity, and whilst the isk of progression appears to be biomechanically mediated (larger deformities are more likely to progress), the detailed biomechanical mechanisms driving progression are not well understood. Gravitational forces in the upright position are the primary sustained loads experienced by the spine. In scoliosis they are asymmetrical, generating moments about the spinal joints which may promote asymmetrical growth and deformity progression. Using 3D imaging modalities to estimate segmental torso masses allows the gravitational loading on the scoliotic spine to be determined. The resulting distribution of joint moments aids understanding of the mechanics of scoliosis progression.

Methods

Existing low-dose CT scans were used to estimate torso segment masses and joint moments for 20 female scoliosis patients. Intervertebral joint moments at each vertebral level were found by summing the moments of each of the torso segment masses above the required joint.

Results

The patients’ mean age was 15.3 years (SD 2.3; range 11.9–22.3 years); mean thoracic major Cobb angle 52^° (SD 5.9^°; range 42–63^°) and mean weight 57.5 kg (SD 11.5 kg; range 41–84.7 kg). Joint moments of up to 7 Nm were estimated at the apical level. No significant correlation was found between the patients’ major Cobb angles and apical joint moments.

Conclusions

Patients with larger Cobb angles do not necessarily have higher joint moments, and curve shape is an important determinant of joint moment distribution. These findings may help to explain the variations in progression between individual patients. This study suggests that substantial corrective forces are required of either internal instrumentation or orthoses to effectively counter the gravity-induced moments acting to deform the spinal joints of idiopathic scoliosis patients.

Electronic supplementary material

The online version of this article (doi:10.1186/s13013-015-0060-9) contains supplementary material, which is available to authorized users.

A semiautomatic method to identify vertebral end plate lesions (Schmorl's nodes)

BACKGROUND CONTEXT

There are differences in the definitions of end plate lesions (EPLs), often referred to as Schmorl's nodes, that may, to some extent, account for the large range of reported prevalence (3.8%-76%).

PURPOSE

The purpose of this study was to develop a technique to measure the size, prevalence, and location of EPLs in a consistent manner.

STUDY DESIGN/SETTING

This study proposed a method using a detection algorithm that was applied to five adolescent females (average age, 15.1 [range, 13.0-19.2] years) with idiopathic scoliosis (average major Cobb angle, 60° [range, 55°-67°]).

METHODS

Existing low-dose, computed tomography scans were segmented semiautomatically to extract three-dimensional morphology of each vertebral end plate. Any remaining attachments to the posterior elements of adjacent vertebrae or end plates were then manually sectioned. An automatic algorithm was used to determine the presence and position of EPLs.

RESULTS

End plate lesions were identified in 15 of the 170 (8.8%) end plates analyzed with an average depth of 3.1 mm. Eleven of the 15 EPLs were seen in the lumbar spine. The algorithm was found to be most sensitive to changes in the minimum EPL gradient at the edges of the EPL.

CONCLUSIONS

This study describes an imaging analysis technique for consistent measurement of the prevalence, location, and size of EPLs. The technique can be used to analyze large populations without observer errors in EPL definitions.

Supine to standing Cobb angle change in idiopathic scoliosis: the effect of endplate pre-selection

Background

Supine imaging modalities provide valuable 3D information on scoliotic anatomy, but the altered spine geometry between the supine and standing positions affects the Cobb angle measurement. Previous studies report a mean 7°-10° Cobb angle increase from supine to standing, but none have reported the effect of endplate pre-selection or whether other parameters affect this Cobb angle difference.

Methods

Cobb angles from existing coronal radiographs were compared to those on existing low-dose CT scans taken within three months of the reference radiograph for a group of females with adolescent idiopathic scoliosis. Reformatted coronal CT images were used to measure supine Cobb angles with and without endplate pre-selection (end-plates selected from the radiographs) by two observers on three separate occasions. Inter and intra-observer measurement variability were assessed. Multi-linear regression was used to investigate whether there was a relationship between supine to standing Cobb angle change and eight variables: patient age, mass, standing Cobb angle, Risser sign, ligament laxity, Lenke type, fulcrum flexibility and time delay between radiograph and CT scan.

Results

Fifty-two patients with right thoracic Lenke Type 1 curves and mean age 14.6 years (SD 1.8) were included. The mean Cobb angle on standing radiographs was 51.9° (SD 6.7). The mean Cobb angle on supine CT images without pre-selection of endplates was 41.1° (SD 6.4). The mean Cobb angle on supine CT images with endplate pre-selection was 40.5° (SD 6.6). Pre-selecting vertebral endplates increased the mean Cobb change by 0.6° (SD 2.3, range -9° to 6°). When free to do so, observers chose different levels for the end vertebrae in 39% of cases. Multi-linear regression revealed a statistically significant relationship between supine to standing Cobb change and fulcrum flexibility (p = 0.001), age (p = 0.027) and standing Cobb angle (p < 0.001). The 95% confidence intervals for intra-observer and inter-observer measurement variability were 3.1° and 3.6°, respectively.

Conclusions

Pre-selecting vertebral endplates causes minor changes to the mean supine to standing Cobb change. There is a statistically significant relationship between supine to standing Cobb change and fulcrum flexibility such that this difference can be considered a potential alternative measure of spinal flexibility.

Multidetector CT with 3-dimensional volume rendering in the evaluation of the spine in patients with Neurofibromatosis type 1: a retrospective review in 73 patients

INTRODUCTION

Neurofibromatosis type 1 (NF-1) may involve the spine as various abnormalities including bony dysplasia, scoliosis, and nerve sheath tumors. Surgery may be performed for stabilization of the spine. We have seen an increase in requests for multidetector CT (MDCT) imaging with the (three-dimensional) 3D-volume rendered (VR) images in patients evaluated at our institution. We, therefore, investigated how MDCT could be best utilized in this patient population.

METHODS

Seventy-three patients with NF-1 were identified in whom MDCT imaging was performed for diagnostic, pre-operative, or post-operative evaluation of spinal abnormalities. True axial source images and two dimensional (2D) orthogonal reconstructed MDCT images, as well as the VR images, were compared with plain radiographs and MRI. In addition, the MDCT study was compared to the VR images. These studies were reviewed to compare assessment of A) bony abnormalities such as remodeling from dural ectasia, dysplasia, and fusion, B) abnormal spinal curvature, C) nerve sheath tumors, and D) surgical instrumentation.

RESULTS

When compared to plain radiographs, the MDCT and VR images were rated as helpful for evaluating the abnormalities of the spine in 19 of 24 patients for a total of 30 findings. This included the following categories A) (n = 6), B) (n = 5), C) (n = 7), and D) (n = 12). Compared to MR, the MDCT and VR study was helpful in evaluating the findings of NF-1 in 24 of 36 patients for a total of 40 findings. This included the following categories A) (n = 12), B) (n = 10), C) (n = 3), and D) (n = 15). When the VR images were compared to the orthogonal MDCT, the VR images was rated as helpful in 41 of 73 patients for a total of 60 findings, including the following categories: A) (n = 11), B) (n = 24), C) (n = 0), and D) (n = 25).

CONCLUSION

MDCT has distinct advantages over plain radiographs and MR imaging, and the VR images over MDCT in the evaluation of the spine in patients with NF-1, especially for the assessment of bony abnormalities, abnormal spinal curvature, and spinal instrumentation.

Segmental torso masses in adolescent idiopathic scoliosis

BACKGROUND

Adolescent idiopathic scoliosis is the most common type of spinal deformity whose aetiology remains unclear. Studies suggest that gravitational forces in the standing position play an important role in scoliosis progression, therefore anthropometric data is required to develop biomechanical models of the deformity. Few studies have analysed the trunk by vertebral level and none have performed investigations of the scoliotic trunk. The aim of this study was to determine the centroid, thickness, volume and estimated mass, for sections of the scoliotic trunk.

METHODS

Existing low-dose CT scans were used to estimate vertebral level-by-level torso masses for 20 female adolescent idiopathic scoliosis patients. ImageJ processing software was used to analyse the CT images and enable estimation of the segmental torso mass corresponding to each vertebral level.

FINDINGS

The patients' mean age was 15.0 (SD 2.7) years with mean major Cobb angle of 52 (SD 5.9)° and mean patient weight of 58.2 (SD 11.6) kg. The magnitude of torso segment mass corresponding to each vertebral level increased by 150% from 0.6kg at T1 to 1.5kg at L5. Similarly, segmental thickness from T1-L5 increased inferiorly from a mean 18.5 (SD 2.2) mm at T1 to 32.8 (SD 3.4) mm at L5. The mean total trunk mass, as a percentage of total body mass, was 27.8 (SD 0.5) % which was close to values reported in previous literature.

INTERPRETATION

This study provides new anthropometric reference data on segmental (vertebral level-by-level) torso mass in adolescent idiopathic scoliosis patients, useful for biomechanical models of scoliosis progression and treatment.

An FE investigation simulating intra-operative corrective forces applied to correct scoliosis deformity

Background

Adolescent idiopathic scoliosis (AIS) is a deformity of the spine, which may require surgical correction by attaching a rod to the patient’s spine using screws implanted in the vertebral bodies. Surgeons achieve an intra-operative reduction in the deformity by applying compressive forces across the intervertebral disc spaces while they secure the rod to the vertebra. We were interested to understand how the deformity correction is influenced by increasing magnitudes of surgical corrective forces and what tissue level stresses are predicted at the vertebral endplates due to the surgical correction.

Methods

Patient-specific finite element models of the osseoligamentous spine and ribcage of eight AIS patients who underwent single rod anterior scoliosis surgery were created using pre-operative computed tomography (CT) scans. The surgically altered spine, including titanium rod and vertebral screws, was simulated. The models were analysed using data for intra-operatively measured compressive forces – three load profiles representing the mean and upper and lower standard deviation of this data were analysed. Data for the clinically observed deformity correction (Cobb angle) were compared with the model-predicted correction and the model results investigated to better understand the influence of increased compressive forces on the biomechanics of the instrumented joints.

Results

The predicted corrected Cobb angle for seven of the eight FE models were within the 5° clinical Cobb measurement variability for at least one of the force profiles. The largest portion of overall correction was predicted at or near the apical intervertebral disc for all load profiles. Model predictions for four of the eight patients showed endplate-to-endplate contact was occurring on adjacent endplates of one or more intervertebral disc spaces in the instrumented curve following the surgical loading steps.

Conclusion

This study demonstrated there is a direct relationship between intra-operative joint compressive forces and the degree of deformity correction achieved. The majority of the deformity correction will occur at or in adjacent spinal levels to the apex of the deformity. This study highlighted the importance of the intervertebral disc space anatomy in governing the coronal plane deformity correction and the limit of this correction will be when bone-to-bone contact of the opposing vertebral endplates occurs.

CT and radiographic analysis of sagittal profile changes following thoracoscopic anterior scoliosis surgery

Background

Previous studies report an increase in thoracic kyphosis after anterior approaches and a flattening of sagittal contours following posterior approaches. Difficulties with measuring sagittal parameters on radiographs are avoided with reformatted sagittal CT reconstructions due to the superior endplate clarity afforded by this imaging modality.

Methods

A prospective study of 30 Lenke 1 adolescent idiopathic scoliosis (AIS) patients receiving selective thoracoscopic anterior spinal fusion (TASF) was performed. Participants had ethically approved low dose CT scans at minimum 24 months after surgery in addition to their standard care following surgery. The change in sagittal contours on supine CT was compared to standing radiographic measurements of the same patients and with previous studies. Inter-observer variability was assessed as well as whether hypokyphotic and normokyphotic patient groups responded differently to the thoracoscopic anterior approach.

Results

Mean T5-12 kyphosis Cobb angle increased by 11.8 degrees and lumbar lordosis increased by 5.9 degrees on standing radiographs two years after surgery. By comparison, CT measurements of kyphosis and lordosis increased by 12.3 degrees and 7.0 degrees respectively. 95% confidence intervals for inter-observer variability of sagittal contour measurements on supine CT ranged between 5-8 degrees. TASF had a slightly greater corrective effect on patients who were hypokyphotic before surgery compared with those who were normokyphotic.

Conclusions

Restoration of sagittal profile is an important goal of scoliosis surgery, but reliable measurement with radiographs suffers from poor endplate clarity. TASF significantly improves thoracic kyphosis and lumbar lordosis while preserving proximal and distal junctional alignment in thoracic AIS patients. Supine CT allows greater endplate clarity for sagittal Cobb measurements and linear relationships were found between supine CT and standing radiographic measurements. In this study, improvements in sagittal kyphosis and lordosis following surgery were in agreement with prior anterior surgery studies, and add to the current evidence suggesting that anterior correction is more capable than posterior approaches of addressing the sagittal component of both the instrumented and adjacent non instrumented segments following surgical correction of progressive Lenke 1 idiopathic scoliosis.

Three-Dimensional Volume-Rendered Series Complements 2D Orthogonal Multidetector Computed Tomography in the Evaluation of Abnormal Spinal Curvature in Patients at a Major Cancer Center: A Retrospective Review

Background. Abnormal spinal curvature is routinely assessed with plain radiographs, MDCT, and MRI. MDCT can provide two-dimensional (2-D) orthogonal as well as reconstructed three-dimensional volume-rendered (3-D VR) images of the spine, including the translucent display: a computer-generated image set that enables the visualization of surgical instrumentation through bony structures. We hypothesized that the 3-D VR series provides additional information beyond that of 2-D orthogonal MDCT in the evaluation of abnormal spinal curvature in patients evaluated at a major cancer center. Methods. The 3-D VR series, including the translucent display, was compared to 2-D orthogonal MDCT studies in patients with an abnormal spinal curvature greater than 25 degrees and scored as being not helpful (0) or helpful (1) in 3 categories: spinal curvature; bony definition; additional findings (mass lesions, fractures, and instrumentation). Results. In 38 of 48 (79.2%) patients assessed, the 3-D VR series were scored as helpful in 63 of 144 (43.8%) total possible categories (32 spinal curvature; 14 bony definition; 17 additional findings). Conclusion. Three-dimensional MDCT images, including the translucent display, are complementary to multiplanar 2-D orthogonal MCDT in the evaluation of abnormal spinal curvature in patients treated at a major cancer center.

Anatomical study of position of the rib head for placing anterior vertebral body screws in a chinese population

In this study, the variability of rib head position in a Chinese population in terms of the spinal canal and vertebral body was analyzed using computed tomography (CT). Images from transverse CT scan of the T4 to T12 vertebral bodies of 30 normal individuals were 3-dimensionally reconstructed, and analyzed for measurement of parameters that define the relative anatomic position of the rib head. We have found that the distance between the anterior border of the rib head and the posterior margin of the vertebral body, posterior safe angle, and the distance between the most inferior border of the rib head and inferior end plate in the sagittal plane gradually decrease. However, the distance between the anterior boarder of the rib head and the anterior margin of the vertebral body, transverse dimension, anterior safe angle, and the distance between the most inferior border of the rib head and superior end plate in the sagittal plane gradually increase from T4 to T12. This indicates that the position of the rib head is oriented from a more anterior position to a more posterior position and from a more superior position to a more inferior position as the number of the vertebra increases, which is different from what has been reported from western populations. Our study has identified useful parameters to define the position of the rib head, and provides a comprehensive reference guide for accurate and safe instrumentation of vertebral body screws in treating related spine diseases.

Lateral bone density variations in the scoliotic spine

Adolescent Idiopathic Scoliosis (AIS) is the most common deformity of the spine, affecting 2-4% of the population. Previous studies have shown that the vertebrae in scoliotic spines undergo abnormal shape changes, however there has been little exploration of how scoliosis affects bone density distribution within the vertebrae. In this study, existing CT scans of 53 female idiopathic scoliosis patients with right-sided main thoracic curves were used to measure the lateral (right to left) bone density profile at mid-height through each vertebral body. Five key bone density profile measures were identified from each normalized bone density distribution, and multiple regression analysis was performed to explore the relationship between bone density distribution and patient demographics (age, height, weight, body mass index (BMI), skeletal maturity, time since Menarche, vertebral level, and scoliosis curve severity). Results showed a marked convex/concave asymmetry in bone density for vertebral levels at or near the apex of the scoliotic curve. At the apical vertebra, mean bone density at the left side (concave) cortical shell was 23.5% higher than for the right (convex) cortical shell, and cancellous bone density along the central 60% of the lateral path from convex to concave increased by 13.8%. The centre of mass of the bone density profile at the thoracic curve apex was located 53.8% of the distance along the lateral path, indicating a shift of nearly 4% toward the concavity of the deformity. These lateral bone density gradients tapered off when moving away from the apical vertebra. Multi-linear regressions showed that the right cortical shell peak bone density is significantly correlated with skeletal maturity, with each Risser increment corresponding to an increase in mineral equivalent bone density of 4-5%. There were also statistically significant relationships between patient height, weight and BMI, and the gradient of cancellous bone density along the central 60% of the lateral path. Bone density gradient is positively correlated with weight, and negatively correlated with height and BMI, such that at the apical vertebra, a unit decrease in BMI corresponds to an almost 100% increase in bone density gradient.

Computed tomographic-based volumetric reconstruction of the pulmonary system in scoliosis: trends in lung volume and lung volume asymmetry with spinal curve severity

BACKGROUND

Scoliosis has been associated with reduced pulmonary capacity; however, the source of the reduction in capacity (left, right, or both lungs) is not clear. The objective of this study was to investigate trends in left, right, and total lung volume and left/right lung volume asymmetry with spinal curve severity in scoliosis.

METHODS

Three-dimensional volumetric reconstruction of the pulmonary system was performed on existing preoperative computed tomographic (CT) scans for 28 idiopathic scoliosis patients. Left, right, and total lung volumes, and left/right lung volume ratios were calculated and correlated with the following spinal curve parameters: major Cobb angle, rib hump, number of vertebrae in the major curve, most cephalad vertebra in the major curve, and thoracic kyphosis.

RESULTS

Left/right lung volume ratio increases significantly with increasing rib hump. Left, right, and total lung volumes were significantly correlated with rib hump and number of vertebrae in the major curve (P < 0.05), and near-significantly correlated with most cephalad vertebra in the major curve (P < 0.10). Shorter, higher, more rotated thoracic curves therefore restrict lung volume more than longer, lower, less rotated curves. The mean lung volume ratio for scoliosis patients was lower than for age-matched controls (P < 0.10).

CONCLUSION

CT-based volumetric reconstruction of the pulmonary system in scoliosis patients shows differences in both lung volumes and lung volume ratios compared with normal controls.

Automatic measurement of vertebral rotation in idiopathic scoliosis

STUDY DESIGN

Development of an automatic measurement algorithm and comparison with manual measurement methods.

OBJECTIVES

To develop a new computer-based method for automatic measurement of vertebral rotation in idiopathic scoliosis from computed tomography images and to compare the automatic method with two manual measurement techniques.

SUMMARY OF BACKGROUND DATA

Techniques have been developed for vertebral rotation measurement in idiopathic scoliosis using plain radiographs, computed tomography, or magnetic resonance images. All of these techniques require manual selection of landmark points and are therefore subject to interobserver and intraobserver error.

METHODS

We developed a new method for automatic measurement of vertebral rotation in idiopathic scoliosis using a symmetry ratio algorithm. The automatic method provided values comparable with Aaro and Ho's manual measurement methods for a set of 19 transverse computed tomography slices through apical vertebrae, and with Aaro's method for a set of 204 reformatted computed tomography images through vertebral endplates.

RESULTS

Confidence intervals (95%) for intraobserver and interobserver variability using manual methods were in the range 5.5 degrees to 7.2 degrees . The mean (+/-SD) difference between automatic and manual rotation measurements for the 19 apical images was -0.5 degrees +/- 3.3 degrees for Aaro's method and 0.7 degrees +/- 3.4 degrees for Ho's method. The mean (+/-SD) difference between automatic and manual rotation measurements for the 204 endplate images was 0.25 degrees +/- 3.8 degrees .

CONCLUSIONS

The symmetry ratio algorithm allows automatic measurement of vertebral rotation in idiopathic scoliosis without intraobserver or interobserver error due to landmark point selection.

Regional differences in anatomical landmarks for placing anterior instrumentation of the thoracic spine in both normal patients and patients with adolescent idiopathic scoliosis

STUDY DESIGN

A retrospective analysis of patients who had magnetic resonance imaging (MRI) of the thoracic spine, comparing those with a normal straight spine and those with a right thoracic adolescent idiopathic scoliosis (AIS).

OBJECTIVE

To analyze the position of the rib head with respect to the spinal canal and vertebral body in normal patients and those with right thoracic AIS using MRI.

SUMMARY OF BACKGROUND DATA

When placing anterior vertebral body screws in thoracic AIS, the most cephalad screws are most at risk for loosening because of smaller vertebral body size and the position of the rib heads, which may obscure more of the vertebral bodies. To our knowledge, there are no studies defining the relationship of the rib head to the vertebral anatomy in thoracic AIS.

METHODS

Transverse MRIs of the vertebral bodies from the 4th thoracic (T4) vertebra to the 12th thoracic (T12) vertebra in normal patients (n = 21) and patients with AIS (n = 21) group were analyzed regarding the following parameters: (1) percent vertebra obscured by rib head (i.e., the percent of the sagittal plane vertebral body length obscured by the overlapping rib head); (2) posterior safe angle, defined as the most posterior angle a screw can be placed, which avoids the spinal canal; and (3) anterior safe angle, defined as the most anteriorly directed screw trajectory that safely obtained good screw purchase.

RESULTS

In both the normal and AIS groups, the percent vertebra obscured by rib head significantly decreased from T4 (30% in normal group and 34.7% in AIS group) to T12 (-0.4% in normal group and 3.5% in AIS group) (P < 0.05). The rib head was positioned more anterior to the vertebral body in the cephalad-thoracic spine when compared to a more posterior position in the caudal thoracic spine. In each group, the posterior safe angle significantly decreased from T4 (23 degrees in normal group and 20.8 degrees in AIS group) to T12 (-0.9 degrees in normal group and 2.1 degrees in AIS group) (P < 0.05), while the anterior safe angle significantly increased from T4 (27.5 degrees in normal group and 26.6 degrees in AIS group) to T12 (38.3 degrees in normal group and 38.5 degrees in AIS group) (P < 0.05).

CONCLUSIONS

It is important to understand the relationship of the rib head to the vertebral body to provide-excellent screw purchase within the vertebral body without risking penetration into the spinal canal. In both normal and AIS groups, the relationship of the rib head to the vertebral body and spinal canal changes so that the rib head is positioned more anteriorly in the cephalad-thoracic spine and more posteriorly in the caudal thoracic spine. When placing anterior thoracic screws, at the cephalad- thoracic spine (T4, T5, T6, and T7), removal of rib heads is recommended to allow for good screw purchase. However, at the caudal thoracic spine (T10-T12), staying anterior to the rib head is important to avoid penetration into the spinal canal.

Variability in Cobb angle measurements using reformatted computerized tomography scans

STUDY DESIGN

Survey of intraobserver and interobserver measurement variability.

OBJECTIVE

To assess the use of reformatted computerized tomography (CT) images for manual measurement of coronal Cobb angles in idiopathic scoliosis.

SUMMARY OF BACKGROUND DATA

Cobb angle measurements in idiopathic scoliosis are traditionally made from standing radiographs, whereas CT is often used for assessment of vertebral rotation. Correlating Cobb angles from standing radiographs with vertebral rotations from supine CT is problematic because the geometry of the spine changes significantly from standing to supine positions, and 2 different imaging methods are involved.

METHODS

We assessed the use of reformatted thoracolumbar CT images for Cobb angle measurement. Preoperative CT of 12 patients with idiopathic scoliosis were used to generate reformatted coronal images. Five observers measured coronal Cobb angles on 3 occasions from each of the images. Intraobserver and interobserver variability associated with Cobb measurement from reformatted CT scans was assessed and compared with previous studies of measurement variability using plain radiographs.

RESULTS

For major curves, 95% confidence intervals for intraobserver and interobserver variability were +/-6.6 degrees and +/-7.7 degrees, respectively. For minor curves, the intervals were +/-7.5 degrees and +/-8.2 degrees, respectively. Intraobserver and interobserver technical error of measurement was 2.4 degrees and 2.7 degrees, with reliability coefficients of 88% and 84%, respectively. There was no correlation between measurement variability and curve severity.

CONCLUSIONS

Reformatted CT images may be used for manual measurement of coronal Cobb angles in idiopathic scoliosis with similar variability to manual measurement of plain radiographs.

Structures at risk following anterior instrumented spinal fusion for thoracic adolescent idiopathic scoliosis

OBJECTIVES

With the increasing popularity of anterior instrumented spinal fusion for adolescent idiopathic scoliosis, there has also been a rising concern over the proximity of the descending aorta to the screw tips and the possibility of vessel wall erosion over time. This computed tomography (CT) study attempts to define the relative position of the thoracic aorta and other vital structures to the spine (preoperatively) and to the projected instrumentation (postoperatively) by level and curve magnitude in deformity patients.

METHODS

Twenty consecutive patients (17 female, 3 male) with an average age of 14.5 years (range 12.4-18.5 years) and a right main thoracic/Lenke 1 curve (average 55.2 degrees , range 50-66 degrees ) underwent preoperative and postoperative CT scanning as part of their planned open anterior spinal fusion with instrumentation. All images were analyzed for proximity (distance from the midvertebral body) and position to (as defined relative to the center of the vertebral body in the axial plane) the spine preoperatively and the projecting screw tip postoperatively. As a control, 10 age-matched nondeformity thoracic CT scans were analyzed to assess the relative position of the thoracic aorta to the vertebral bodies by level. Preoperative and postoperative plain radiographs were also analyzed for curve magnitude, correction, and fusion levels to assess the possible effect of these variables on the various thoracic structures.

RESULTS

The postoperative curve magnitude averaged 26.9 degrees (range 17-40 degrees ; 51% correction) using 151 screws (7.5 screws/patient) and an average follow-up of 4.1 years (range 3.2-7.0 years). Screw-to-spinal-canal distance averaged 5.3 mm (range 3.5-8.2 mm) at an average of +4.5 degrees (range -11 degrees to +15 degrees ) from the coronal axis. Screw tip extrusion (distance beyond the far cortex) averaged 2.8 mm (0-5 mm). The trachea, azygous vein, esophagus, and lungs/pleura were not found to be at risk from screw penetration. The postoperative screw-tip-to-descending-aorta distance varied by vertebra level, with the periapical and distal screws being positioned closer to the aorta (1.6-2.4 mm) (P < 0.05). Additionally, 23 of 151(15%) screws were thought to be adjacent (</=2 mm) to the aorta. This includes 4 of 60 (7%) of the proximal screws but 6 of 40 (15%) of the periapical screws and 13 of 51 (26%) of the distal screws (P < 0.05). There were no screws compressing the aorta and no perioperative or postoperative complications.

CONCLUSIONS

The course of the thoracic aorta may vary in individuals; however, in deformity patients, it generally moves from a relatively anterolateral position proximally to a posteromedial position at the apex. Distally, it moves more anteriorly. Consequently, the aorta moves closer to the screw tips both at the apex and distally, whereas the distal screws are more frequently juxtaposed to the descending aorta (P < 0.05).

Anterior vertebral body screw position placed thoracoscopically: a function of anatomy and surgeon experience in a porcine model

STUDY DESIGN

Manual digital measurements, in a porcine model of thoracoscopic anterior instrumentation, divided into two studies: 1) comparing the actual screw position within each vertebra with the perceived ideal screw position and correlation with the learning curve of the surgeon; and 2) a quantitative analysis of the vertebral bodies from T3 to T12 to determine the ideal screw insertion point and trajectory.

OBJECTIVE

To investigate whether the position of thoracoscopically placed screws is related to the portion of the thoracic spine instrumented and to the experience of the surgeon and to define the ideal starting position and direction of anterior thoracic screws in a porcine model.

SUMMARY OF BACKGROUND DATA

Anterior screw insertion using a thoracoscopic approach is generally considered to be technically demanding and potentially dangerous. To our knowledge, there is no study analyzing the proper position of anterior vertebral screws using thoracoscopic technique and no study analyzing the ideal starting position of anterior vertebral body screws.

METHODS

In Study 1, 26 pigs were assigned to two groups (early experience, n = 16; late experience, n = 10) and underwent thoracoscopic anterior instrumentation and fusion from T5 to T10. The screw position was determined in the sagittal plane and in the frontal plane, respectively. In Study 2, 10 thoracic pig spines (T3-T12) were obtained. Each vertebra was digitally measured in the midtransverse plane. Using a point directly adjacent to the rib head as the assumed ideal screw starting point, the percent of vertebral body "obscured" by the rib head and the maximum safe anterior and posterior screw insertion angles were determined.

RESULTS

In Study 1, screw positions of Group 1 were significantly more anterior from T5 to T7 compared with the ideal screw position (P < 0.05). Most screws were in the ideal position in Group 2. In both groups, the screw position within T5 was too inferior (P < 0.05). The majority of posterior cortical disruptions occurred in the distal vertebrae. Most anterior disruptions occurred in the proximal vertebrae. In Study 2, the percent of vertebral body behind the rib head significantly decreased from T3 (61.5%) to T12 (25.7%) (P < 0.0001). The maximum anterior insertion angle significantly increased from T3 (11 degrees) to T12 (24 degrees) (P < 0.0001) while the maximum posterior insertion angle significantly decreased from T3 (28 degrees) to T12 (11 degrees) (P < 0.0001).

CONCLUSIONS

The position of thoracoscopically placed vertebral screws is dependent on the level of the spine instrumented and surgeon experience. Screws placed in the proximal thoracic spine tend to be too anterior and too inferior while posterior placement of screws occurs in the distal thoracic spine. When using the rib head as the starting point for anterior screw insertion, a slight anterior angle is required in the distal thoracic spine while a slight posterior insertion angle is required in the proximal thoracic spine. A learning curve does exist, however; the proximal screw positioning continues to be a challenge.