Restoration of normal sagittal profile is an important goal of scoliosis correction surgery. The aim of this study was to provide a detailed analysis of sagittal profile correction following TASF, using both standing plane radiographs and supine low dose CT scans of the same patient group. CT was useful in addition to lateral radiographs due to the superior endplate clarity afforded by the reformatted CT images. The use of supine CT also potentially avoids the inherent variability in upright standing posture due to stance variations between subsequent sagittal radiographs
. However, the use of supine CT also raises questions about the applicability of the resulting sagittal profile measurements to clinically relevant standing postures, and in this study we provide a detailed comparison of standing versus supine sagittal profile measurements for the same patients. To our knowledge, no previous study has compared sagittal profile measurements before and after scoliosis surgery between standing radiographs and supine CT, nor reported on detailed changes in sagittal profile using the clearly defined vertebral endplate visualisation afforded by low dose CT. We also wished to compare the results of the current study with existing literature using plane radiographic measurements of sagittal profile following other selective thoracic fusion procedures (Table
After selective thoracic fusion, the lumbar spine needs to adapt to the altered shape of the thoracic spine to maintain coronal and sagittal balance
. This spontaneous correction of the lumbar compensatory curve in the coronal plane has been evaluated for various surgical approaches with varying reports as to the superiority of correction between anterior and posterior approaches
[6, 7, 17, 20, 56, 57]. The post-operative response of the lumbar spine in the sagittal plane is thought to be a consequence of the change in thoracic kyphosis achieved during surgery. A number of recent studies
[4, 5, 7, 14, 16] have found that anterior techniques for the correction of thoracic scoliosis are more kyphogenic than posterior approaches. Multiple discectomies and compression along the rod lead to shortening of the anterior column and immediate increases in the thoracic kyphosis at the first erect radiograph after surgery, with further increases reported two years after anterior selective thoracic fusion
[5, 7, 8, 14, 16, 18]. Prior studies by our group on the larger cohort have reported complications associated with this type of surgery
[8, 41, 58]. In the current cohort of 30 patients, there were 3 rod fractures and 3 top screw pullouts found by the most recent follow-up. Note that as previously reported, rod fracture is associated with a minimal loss of correction and tended to occur in the earliest patients in the series with only 3 rod fractures found from the most recent 150 cases in the larger series.
A number of prior studies have noted the poor quality of sagittal radiographs with regard to the visualisation of vertebral endplates, especially in the mid and upper thoracic regions of the spine. For example, Dang et al reported excellent intra-observer reproducibility for coronal plane radiograph measurements but for sagittal radiographs, examiners were found to have only fair to good reproducibility for angles measured from upper thoracic vertebrae, such as T2 or T5, and poor inter-observer agreement when measuring spinal levels below T9. Dang et al’s paper concluded that sagittal parameters measured on traditional radiographs do not provide valuable information because they cannot be measured reproducibly or reliably. The difficulties with measuring sagittal parameters on lateral radiographs
[37–39] are avoided with reformatted sagittal CT reconstructions due to the superior endplate clarity afforded by this imaging modality. In the current study, the 95% confidence intervals for inter-observer variability of sagittal Cobb angle measurements (range 5-8°, Table
5) are comparable with previously published 95% confidence intervals for coronal Cobb angle measurement from supine CT scans
. This suggests that the use of CT allows equivalent clarity for either sagittal or coronal plane Cobb angle measurements.
This study confirms that TASF is a kyphosing technique which has a similar corrective effect on patients who are hypokyphotic or normokyphotic before surgery (Table
3). Those receiving a 4.5 mm rod had a slightly greater increase of their kyphosis across the instrumented segment than the group receiving the 5.5 mm rod which is in contrast to an earlier study using posterior approaches where the use of larger diameter titanium rods (6.35 vs 5.5 mm) resulted in larger thoracic kyphosis after surgery
. However, a recently published paper on 49 TASF cases
 found similar results to the current study reporting a greater increase in kyphosis when using a smaller diameter rod (4.0 mm stainless steel in earlier patients vs. 4.75 mm titanium alloy) but rather than interpret the difference as being the result of the different implant types, suggested evolving surgeon experience in patient selection was the most likely factor influencing the different sagittal changes. The 4.5 mm rod group in the current study were also the earlier cases in our larger series undergoing TASF so may also have been affected by a similar patient selection issue, although our differences were not statistically significant. All 30 patients in the study had some increase in thoracic kyphosis following TASF surgery according to CT (Figure
4) and X-Ray measures, with 26 patients found to be in the normokyphotic range on the minimum 24 months after surgery radiographs. One patient was classified as being hyperkyphotic two years after surgery (T5-T12 kyphosis 45° on CT and X-Ray) and continues to be monitored six years later and to date has not required additional surgery. AIS is a triplanar deformity and in Lenke type 1 scoliosis, the results presented here suggest that anterior correction is capable of addressing the sagittal component of both the instrumented and adjacent non instrumented segments. The corrective forces exerted by single rod anterior constructs results in a flexion moment which increases the kyphosis across the instrumented levels. The un-instrumented lumbar spine must in turn balance the kyphotic curve above so any increase in thoracic kyphosis will see a corresponding increase in the lumbar lordosis of the patient. This is evidenced in the current study (Table
2) where T2-T12 kyphosis increased by a mean 8.4° and the T12-S1 lordosis increased by mean 6.2°.
6 compares the results of the current study with previous studies reporting changes in sagittal contour after scoliosis correction surgery. This table shows that posterior approaches either exacerbate the existing thoracic hypokyphosis (at worst 12 degrees
), or only achieve small increases in kyphosis in the order of 1-2°. By contrast, anterior thoracic fusion procedures consistently increase T5-T12 kyphosis by between 4 - 12° at two years after surgery. With respect to lumbar lordosis, Table
6 reports lumbar lordosis flattening as much as 7.4° following posterior selective thoracic procedures, whereas again by contrast anterior approaches report a deepening of the lumbar lordosis by as much as 8.6° in response to the kyphosing surgical effect in the thoracic spine. The results of the meta-survey of prior studies in Table
6 suggest that anterior correction is more capable of addressing the sagittal component of both the instrumented and adjacent non instrumented segments for AIS patients.
Use of the supine position for CT-based sagittal profile measurement clearly changes the geometry of the spine relative to the standing posture, but the comparative results in this study (Table
6) show that there is a predictable (linear) relationship between supine and standing sagittal profile measurements. Of note is that mean kyphosis across the instrumented levels after surgery changed minimally between supine and standing, whereas the uninstrumented lumbar lordosis and T5-T12 kyphosis each demonstrated significant differences (mean 4.8°) due to the change of posture. It is not being suggested that CT scans should replace standing radiographs for scoliosis assessment, but for the group of patients examined here for research purposes, the paired CT data before and after surgery uniquely provided a superior imaging modality (in terms of image contrast and endplate clarity) for analysing the effects of TASF on sagittal plane deformity. There are both advantages and disadvantages to supine measurement of sagittal profile. Use of the supine position provides an ‘unloaded’ configuration of the spine which is not subject to variations in standing posture due to arm positioning
[46, 55, 60], time of day
, or muscle activation strategy
[62, 63], all of which can affect sagittal Cobb measurements. Further, relative rotation between the pelvis and ribcage can vary between subsequent standing radiographs whereas the supine position standardises many of these variables. Supine measurements are also valuable in biomechanical modelling of scoliosis, since the supine position provides an approximate zero load configuration for the spine which can be used as a starting point for biomechanical simulations. A disadvantage of supine imaging and a limitation of this study is that sagittal balance and the role of pelvic incidence in the standing position cannot be assessed. Further, the standing position is relevant to a condition such as scoliosis where gravity is known to affect the magnitude of the deformity. Recent advances in multi-slice CT are allowing lower radiation doses and faster acquisition times which will make CT an increasingly useful research tool for three-dimensional biomechanical studies of scoliosis correction. Also, low dose standing biplanar systems (such as EOS) are expected to play an important future role in scoliosis imaging and surgical planning.
Thoracoscopic anterior instrumented fusion significantly improves global thoracic kyphosis (T2-T12), thoracic kyphosis (T5-T12), lumbar lordosis (T12-S1) and instrumented segment kyphosis while simultaneously correcting and stabilising the coronal and rotational plane deformities. The results of this study show that the technique reliably increases thoracic kyphosis and lumbar lordosis while preserving proximal and distal junctional alignment in thoracic AIS patients.