Following scoliosis correction, the concave rods had a reduction in kyphotic shape, with a decrease in deflection of 13 mm and a decrease in rod contour angle of 21° (both P < 0.001) (Figures 5A, B). The concave rods showed a 0.06 ± 0.02 mm decrease in deflection per millimeter of rod length. The convex rods trended toward increased kyphosis, with an increase in deflection of 1.5 mm and an increase in Cobb angle of 2° (P < 0.01, P = 0.18, respectively) (Figures 5A, B). The convex rods showed a 0.01 ± 0.01 mm increase in deflection per millimeter of rod length.
In the coronal plane, the preoperative scoliosis curves of 55° ±14° corrected 72% to 15° ± 5°. Concomitant axial plane correction was observed as indicated by a decrease in the thoracic rib hump from 17° ± 4° to 8° ± 5° (P < 0.001) and a postoperative apical vertebral rotation of 14° ± 8°. These high degrees of correction in the coronal and axial planes occurred without a loss of sagittal alignment, which was maintained postoperatively as measured from T2–T12: 27° ± 12° versus 30° ± 7° and T5–T12: 19° ± 14° versus 22° ± 6° (pre vs. post, P > 0.1) and reduced to a more normal range (Figure 6).
In this study group, 6 patients were instrumented to T11, 11 to T12, 9 to L1, and 1 to L2. The average increase in thoracolumbar junctional kyphosis (as measured from T10–L2) was 4° ± 9°—from a preoperative lordosis of 3° ± 10° (range: 16° kyphosis to 21° lordosis) to postoperative kyphosis of 1° ± 6° (range: 12° kyphosis to 13° lordosis) (P = 0.04). Although the total number of patients with T10–L2 kyphosis increased postoperatively from 10 to 16, the average kyphosis for these patients decreased from 8° ± 5° to 5° ± 4°, P = 0.085). Two patients had postoperative T10–L2 kyphosis of 10° or greater, compared with 5 patients with such values preoperatively. Of these 2 patients, there was reduction in kyphosis in 1 patient and kyphosis was unchanged in the other patient.
Although the emphasis in the past was on coronal plane correction, with the evolution of correction techniques and understanding of 3-dimensional deformity in scoliosis, efforts at correction have moved to include the sagittal plane (initially with Cotrel-Debousset instrumentation15) and increasingly the axial planes (with segmental thoracic pedicle screws and DVR14). In addition, it has been noted that the ideal correction requires a balance of fused and unfused segments with the goal of achieving a lasting, balanced correction that minimizes fusion levels. Initial attempts at derotation often resulted in a loss of kyphosis, which has been associated with an increased incidence of proximal junctional kyphosis, reduced lumbar lordosis, and detrimental effects on pulmonary function testing.24–26
Both steel and titanium rods have been noted to “flatten” during insertion and/or rod derotation maneuvers. Many methods of DVR also seem to accentuate rod flattening. Some of this may be due to technique, but it is also related to the relative thoracic apical lordosis present in the scoliosis deformity. Derotating the axial deformity repositions this relative lordosis into the “global” sagittal plane in which we routinely obtain our radiographs. When measured by our 2-dimensional methods, postoperatively there appears to be a reduction in the measured thoracic kyphosis.27 In truth, our initial measure of thoracic kyphosis is erroneously high. This is due to projectional “distortion” related to the axial plane deformity, which rotates the “local” sagittal plane through the apical vertebra out of the “global” sagittal plane in which we routinely obtain our radiographs. Irrespective of the cause, recent use of thoracic pedicle screws and DVR often give postoperative thoracic kyphosis below the normal and presumably ideal range.
Realizing this, undercorrection of kyphosis encourages the consideration of different surgical strategies for further correction. These include the use of stiffer, stronger, and larger diameter and/or hypercontoured rods, in addition to the methods that increase spinal flexibility (anterior or posterior release). A work-hardened steel rod was used in this series. Steel is stiffer than titanium alloy, and the high-strength (200 KSI) steel has a yield point that is substantially higher than standard steel (120 KSI). Although not measured, we anticipate that forces on the order of several hundred Newtons are associated with the deformation observed in the concave rods in the cases of this series on the basis of the 21° change in contour and the 13-mm change in deflection. Although the amount of overcorrection must be estimated by anticipating the corrective forces required, one can use overcontouring, particularly of the concave rod, to achieve a final sagittal alignment near anatomic levels. In this series, significant screw pullout due to the forces associated with rod deformation did not occur, however, the number of screws needed was relatively high.
Screw type (direction of and amount of head excursion) is an important variable in the amount of force and subsequent deformation the rod sees during seating and deformity correction. Uniplanar screws (screws that allow motion only in the sagittal plane) were used in this study. This screw design not only allows axial derotation forces to be delivered during DVR maneuvers, but also allows fine tuning of the sagittal alignment with additional compression and distraction.28
Although less clearly demonstrated because of lack of precise pre- and postoperative axial measures, we think that the concept of differentially contouring the 2 rods is a useful method for improving axial derotation. The similar postoperative sagittal rod contour of the concave and convex rods is an indirect means of inferring axial correction as is the more direct measure using the screw positions on the posteroanterior radiographs. It is impossible to know how much of the axial correction is due to the differing rod shapes versus that due to direct manipulation.
The authors recognize and thank J. D. Bomar for helping out with the image processing. This study was supported in part by a grant to the Harms Study Group Foundation from DePuy Spine Inc. and in part by a grant from the Pediatric Orthopaedic Society of North America.
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