We retrospectively reviewed all 44 patients with AIS who underwent posterior instrumentation with pedicle screws from two centers from August 2000 through July 2007. The average age at the time of surgery was 15.1 ± 2.21 years. We included all patients with the diagnosis of AIS that had radiographs taken at all three time points: (1) preoperative long-cassette standing radiographs; (2) intraoperative, prone full-length scoliosis films; and (3) postoperative, standing radiographs. Nine patients were excluded as a result of the inability to adequately visualize the pelvis on the intraoperative images, which was necessary to assess all coronal balance parameters. We measured radiographic measurements, including curve magnitudes, coronal and sagittal balance, shoulder and pelvic balance, and disc angles, on all three films. The intraoperative, prone scoliosis film was obtained after instrumentation and correction were completed.
All surgeries were performed by the two senior authors (LGL, RAL) using a free-hand technique and stainless steel pedicle screw instrumentation as espoused by Kim et al. [5
]. Pedicle screws were placed segmentally with the major corrective maneuver being direct vertebral derotation using linked vertebral column manipulators (VCMs) (Medtronic Spinal and Biologics, Minneapolis, MN). Per routine, the VCM device was attached to the screw heads that did not allow for toggle in the mediolateral plane (ie, fixed head or uniplanar screws), and direct derotation was performed followed by placement of the correcting rod. The correcting rod was precontoured in the sagittal plane only. The rod was placed and set plug screws were captured at the three cephalad levels. The set plugs were left loose, and then the rod was rotated 180° and placed into the distal screw heads. Then in a stepwise fashion, set plugs were placed with the use of “rockers” or rod-manipulating devices in a caudad to cephalad direction. During the rod capture procedure, the assistant placed a direct, thoracic force on the rib cage while maintaining the derotation force through the VCM devices. After rod placement, set plugs were locked in place in a caudal to cephalad manner using various rod reduction tools. Then, in situ coronal bending was performed simultaneously on the correcting and/or holding rods, and compression or distraction maneuvers were used at the cephalad and caudad aspects of the construct to obtain proper shoulder balance and horizontalization of the lowest instrumented level (LIV). After final correction maneuvers, an intraoperative long-cassette prone radiograph was obtained to assess our correction and balance parameters. Three patients in our study group had inadequate curve correction based on intraoperative imaging and required revision correction maneuvers and repeat intraoperative imaging. We then decorticated and placed our crosslinks in a standard fashion at the cephalad and caudad aspect of our construct.
Postoperative standing films were obtained before discharge within 4 to 7 days after surgery when the patient was able to stand for the film with minimal discomfort. All films were routinely taken with the patient approximately 72 inches from the radiographic source as is standard protocol for scoliosis imaging at both centers. This was accomplished intraoperatively by placing the cassette on a sitting stool and lowering the operating table. The portable radiographic machine was then elevated to achieve the appropriate distance from the source to the cassette. An intraoperative, long-cassette lateral film is not possible secondary to arm positioning and shoulder overlap. Intraoperative lateral images were taken on two short cassettes and were thus only used to assess implant position and not useful for assessing sagittal alignment. We then compared all radiographic parameters on intraoperative film with the postoperative, standing film.
A single, independent reviewer (MDH) not involved in any of the surgeries measured all radiographs. Initially, all preoperative films were measured and the end vertebrae were noted for all curves. The intraoperative films were then reviewed at a separate sitting without the availability of the preoperative or postoperative films to avoid any personal bias with the measurements. All curves were measured using the same end vertebra noted from the preoperative films to ensure consistency with measurement techniques. Similarly, the postoperative films were measured at a separate time, again without any of the previous images available. One center uses plain film radiography, necessitating use of a digital protractor (x-caliper; Eisenlohr Technologies, Wilton, CA) to obtain curve measurements, whereas the other center uses all digital imaging (AGFA Impax 4.5). The same digital protractor was used for all plain film measurements.
Coronal measurements included Cobb angles, lowest instrumented vertebra tilt to the horizontal (LIV-H), angulation of disc below lowest instrumented level (LIV-α), the alignment of the C7 plumb line in relation to the center sacral vertical line (C7-CSVL), apical vertebral translation (apex-CSVL), and the coronal position of the lowest instrumented level (LIV-CSVL) (Fig. A–C). The most widely used measurement parameter in scoliosis is the Cobb angle, which is determined by the intersection of a line drawn from the superior end plate of the cephalad end vertebra and the inferior end plate of the caudal end vertebra for each curve, which measures curvature in the coronal plane. The LIV-H measures the angle of a line along the caudal end plate of the LIV and a horizontal intersecting line assessing the tilt of the LIV. Similarly, LIV-α measures the coronal angulation of the disc below the LIV by computing the angle between tangential lines drawn from the inferior end plate of the LIV and the superior end plate of the level below. Overall coronal balance is measured by the distance between the C7 plumb line and the CSVL. The C7 plumb line is a line from the middle of the C7 body parallel to the edge of the radiograph, and the CSVL is drawn from the midbody of S1 parallel to the radiograph edge. Finally, the coronal translation of the LIV and curve apex were also measured [18
Preoperative (A), intraoperative (B), and postoperative (C) radiographs show correlation between intraoperative and postoperative films
We classified the curves using the system of Lenke et al. [15
]. The system has three components: (1) a curve type (1 through 6), (2) a modifier for the lumbar spine (A, B, or C), and (3) a modifier for the thoracic spine on the sagittal radiograph (−, N, or +). The system reportedly has high reliability [15
We determined the difference in the postoperative and intraoperative parameters for all patients (Table ). Correlation between intraoperative and postoperative radiographic parameters was tested through computation of a Pearson’s coefficient. Frequency data were evaluated to determine percent of patients with less than 5° difference. Statistical analysis was performed using SPSS 14.0 for Windows (SPSS Inc, Chicago, IL) statistical software program.
Difference between intraoperative and postoperative measurements