Real time noninvasive assessment of external trunk geometry during surgical correction of adolescent idiopathic scoliosis
© Duong et al; licensee BioMed Central Ltd. 2009
Received: 01 December 2008
Accepted: 24 February 2009
Published: 24 February 2009
The correction of trunk deformity is crucial in scoliosis surgery, especially for the patient's self-image. However, direct visualization of external scoliotic trunk deformity during surgical correction is difficult due to the covering draping sheets.
An optoelectronic camera system with 10 passive markers is used to track the trunk geometry of 5 scoliotic patients during corrective surgery. The position of 10 anatomical landmarks and 5 trunk indices computed from the position of the passive markers are compared during and after instrumentation of the spine.
Internal validation of the accuracy of tracking was evaluated at 0.41 +/- 0.05 mm RMS. Intra operative tracking during surgical maneuvers shows improvement of the shoulder balance during and after correction of the spine. Improvement of the overall patient balance is observed. At last, a minor increase of the spinal length can be noticed.
Tracking of the external geometry of the trunk during surgical correction is useful to monitor changes occurring under the sterile draping sheets. Moreover, this technique can used be used to reach the optimal configuration on the operating frame before proceeding to surgery. The current tracking technique was able to detect significant changes in trunk geometry caused by posterior instrumentation of the spine despite significant correction of the spinal curvature. It could therefore become relevant for computer-assisted guidance of surgical maneuvers when performing posterior instrumentation of the scoliotic spine, provide important insights during positioning of patients.
Adolescent idiopathic scoliosis (AIS) is characterized by a complex lateral shift of the spinal curve in the frontal plane, associated with a complex 3-D deformity of the trunk [1, 2]. In some severe cases of scoliosis, posterior spinal instrumentation and fusion is performed to partially correct and prevent worsening of the deformity . During this procedure, most of the surgical maneuvers are aimed at correcting the scoliotic spine and to ensure adequate spinal balance. However, scoliosis is also associated with an external trunk deformity, which has to be addressed during correction of scoliosis .
Direct intra-operative visualization of trunk deformity is difficult due to the draping sheets covering the whole trunk (including shoulders and pelvis). In particular, it is very difficult to evaluate precisely the shoulder, trunk and pelvic balance during surgery. Using intra-operative radiographs, Martin-Benlloch et al. have evaluated the lateral shift of C7 vertebra with respect to the sacrum and the shoulder asymmetry . Although intra-operative radiographs are routinely used to verify position of the vertebral implants, they only provide limited information about the trunk geometry. Furthermore, they cannot be used for dynamic evaluation of the trunk deformity. Mac-Thiong et al. have reported the feasibility of intraoperative tracking of the trunk during scoliosis surgery using an electromagnetic motion capture device [6, 7]. This technique was intended to be the first step towards computer-assisted guidance of surgical maneuvers in scoliosis surgery. Added to the fact that they only tested their technique on one patient, electromagnetic tracking involves many drawbacks. Ferromagnetic material present in the operating room (e.g. operating table, instruments, anesthetic equipment) can interfere with the tracking device and affects its accuracy [8–11]. In addition, the electromagnetic emitter can alter readings from electromyographic and anesthetic monitors. Also, the wiring used in electromagnetic tracking can be cumbersome in a surgical environment. Therefore, an optical camera system for intraoperative tracking of trunk geometry is introduced to overcome the technical limitations associated with electromagnetic tracking. The objective of this study is to evaluate the clinical relevance of an optical tracking technique in documenting the variation of the trunk geometry during scoliosis surgery.
Patient clinical characteristics, with Cobb angle measured during and after surgical correction of the spine.
Double right thoracic, left lumbar
Double right thoracic, left lumbar
Single, right Thoracic
Single, left Thoracic
Single, right Thoracic
Primary Cobb angle (degrees)
63 (L: 56)
61 (L: 55)
the spinous process of C7 vertebra,
the right acromion,
the left acromion,
the right thoracic prominence,
the left thoracic prominence,
the right lumbar prominence,
the left lumbar prominence,
the right superior posterior iliac crest,
the left superior posterior iliac crest, and
the spinous process of S1 vertebra.
The clinical indices computed for this study are adapted from a previous study done using a magnetic tracking system, and are outlined on Figure 2B. Indices include the shoulder rotation (using landmark 2–3), the overall balance (using landmark 1–10), and finally the spine length (landmark 1–10). Rotation indices from shoulder were considered in the axial plane and in the frontal plane. Balance in the axial and frontal plane was computed.
The remaining landmarks (4–7) are gathered to provide a complete schematic representation of the trunk including the thoracic and the lumbar spine, and thus can be easily modified to visualize the effect of surgical maneuvers on the thoracic and lumbar hump in future studies.
Since the camera system is acquiring real-time data, respiratory motion of the patient during the acquisition might affect the accuracy of the indices computation. Landmarks located on the thoracic hump (4–5) were therefore used for visual gating, so to keep only 3-D data at the end of the respiratory cycle for indices computation.
Optical tracking technique
The passive markers, their respective mounting posts and the triangular frame were sent to gas sterilization prior to surgery. After skin preparation and before installing the draping sheets, sterile magnetic bases are placed on each anatomical landmark defined on Figure 2A. Then, after installation of the draping sheets, individual mounting posts were clipped on corresponding magnetic bases, over the draping sheets. These markers correspond to landmarks 1 to 9. The triangular frame was fixed to landmark 10 and served as a reference to track the position and orientation of the spinous process of S1.
External Trunk Geometry Tracking
Landmark (1–10) displacement recorded in mm during and after surgical correction of scoliosis
Patient 1 Displacement (mm)
Patient 2 Displacement (mm)
Patient 3 Displacement (mm)
Patient 4 Displacement (mm)
Patient 5 Displacement (mm)
Trunk external indices computed during and after surgical correction of scoliosis. The indices showing improvement are outlined in bold.
Shoulders Orientation – X
Shoulders Orientation – Z
Balance – X
Balance – Y
This study presents the next step from the study of  into developing an intra operative tool to monitor changes in external trunk geometry in real time during scoliosis surgery. This technique is appealing in several ways: 1) it provides 3-D data that can be correlated with the external shape of the patient during initial positioning on the operative table 2) it paves the way to provide continuous spatial tracking during the surgery without any interference or added radiation exposure to the patient, 3) it provides a simple and feasible workflow to be used for intraoperative setup, 4) It can provide direct real time feedback to the surgeon in order to evaluate the efficacy of his correction maneuvers, and 5) It could be used to compare the efficiency of different instrumentation systems or surgical maneuvers to correct scoliosis. In particular, in patients with significant global imbalance, pelvic or shoulder obliquity, the current technique could allow the surgeon to better assess these parameters during his correction maneuvers, so to decrease the use of intra-operative radiographs.
The optical sensors recorded only slight variations of rotations angles (5–10 degrees) on the Relton-Hall frame, where any displacement of the supports during the procedure is prohibited. It is known that about 30% of the correction is attributed by the positioning of the patient on the frame [14, 15]. Accordingly, continuous tracking of the trunk geometry during the positioning step is advantageous since it would allow optimal positioning and curve correction before the surgery. Moreover, it can provide insight about the patient's external trunk geometry under draping sheets, which cannot be assessed from radiographs. This becomes extremely useful to provide quantitative information about the positioning of the patient on the Relton-Hall frame before, during and after surgery. Therefore, the technique described above could assist the surgeon in obtaining a global view of the patient's deformation over the surgical field during positioning stage and all along the surgery. Although only AIS patients were evaluated in this study, the technique could be applied to any patient with spinal deformity, such as patients with neuromuscular scoliosis who often present with severe imbalance and pelvic obliquity.
This study presents an accurate intra-operative system that could be used to assist spine surgeons to evaluate patient positioning and external trunk correction. The workflow described in this study involves only minor modifications to the current surgical setup. Moreover, this technique is not harmful to the patient and no radiations are involved for providing intraoperative guidance. This is appealing to obtain real-time continuous data in an intra operative setting. Improvements from the previous study involving magnetic tracking  are: 1) the optoelectronic camera system does not interfere in any way with the current surgical setup. 2) the tracking system does not require any wiring to connect the markers, since passive spheres do not require any electrical input to be tracked in 3-D. 3) the optoelectronic camera are not subject to inaccuracy due to ferromagnetic material, hence continuous data acquisition is feasible during the surgery.
Trunk geometry can sometimes be difficult to assess due to displacement and elasticity of the skin. Many authors, mainly targeted for gait analysis  and for spinal motion measurement , proposed external landmark tracking for real-time assessment of the trunk. In these cases, skin displacement might occur when the motion is relatively large, which was not the case in our study. Displacement on the skin could potentially alter the true location of a landmark used to compute an angle. However, in the current study, the landmarks that are used to compute angles are all distant to each other and an error of even 1 cm will affect the resulting angle by less than 2 degrees if the landmarks are 30 cm apart. Also, by considering only indices after exposure of the spine, prior to instrumentation, it is possible to reduce the errors due to skin displacement and to obtain a more robust trunk tracking. Moreover, the patient is sedated and supported by the surgical frame. Only the corrective motion by the clinician and the respiratory function might alter the positioning of the landmarks on the patient. To verify this hypothesis, a larger number of landmarks, could be disposed in a uniform distribution on the patient, to quantify skin displacement local to each landmarks.
This study presented preliminary results of intraoperative trunk tracking using an opto- electronic camera system used commonly in computer assisted orthopedic surgery system. The workflow presented in this study can be mapped easily into existing computer assisted systems to provide online guidance while correcting spinal deformity, to achieve an optimal cosmetic correction of the trunk. However, the work presented in this preliminary study requires further investigations and clinical validation to indicate how measurements during surgery can accurately predict postoperative residual deformity or imbalance such as rib hump, shoulder imbalance, coronal and sagittal trunk imbalance in the standing (or sitting) position. For this purpose, comparisons between pre-, per- and and postoperative measurements will be the focus of future studies.
Written patient consent was obtained for publication of the report.
The authors would like to acknowledge the involvement of Benoit Poitras, MD in this project. This research was conducted with the financial support of MENTOR, a strategic training program of the Canadian Institutes of Health Research.
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