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Postoperative Changes in Spinal Rod Contour in Adolescent Idiopathic Scoliosis: An In Vivo Deformation Study

Cidambi, Krishna R., MD*; Glaser, Diana A., PhD†,‡; Bastrom, Tracey P., MA; Nunn, Thomas N., BS†,‡; Ono, Takashi, MD; Newton, Peter O., MD*,†,‡

doi: 10.1097/BRS.0b013e318252ccbe
Deformity
Free

Study Design. Prospective case series.

Objective. To evaluate the change in spinal rod contour from before implantation to after surgical correction of thoracic curves in patients with adolescent idiopathic scoliosis.

Summary of Background Data. With segmental pedicle screw spinal instrumentation and vertebral derotation, many authors have reported a loss of thoracic kyphosis postoperatively. Although surgeons anticipate some flattening of the preimplantation rod contour in the sagittal plane, the magnitude of this change in shape has not been documented.

Methods. The concave and convex rod shapes of 5.5-mm ultrahigh-strength steel spinal rods (200 KSI) from patients with thoracic adolescent idiopathic scoliosis (n = 27), which were contoured with benders by the surgeon, were traced prior to insertion. Postoperative (average, 5 weeks) sagittal rod shape was determined from lateral 2-dimensional radiographs. Maximal rod deflection and angle of the tangents to rod end points (Cobb) were measured. Repeated measures analysis of variance assessed differences between pre- and postoperation.

Results. The scoliosis of 55° ±14° was corrected 72% to 15° ± 5°. The preinsertion rod shapes were more kyphotic for the concave (45.6°) than for the convex (31.4°) rods. Following correction, the concave rods flattened, with decrease in deflection of 13 mm and reduction in angle of 21° (both P < 0.001). The convex rods increased 1.5 mm in deflection and 2° in angle (P < 0.01, P = 0.18). The sagittal profile was maintained postoperatively as measured from T5–T12: 19° ±14° versus 22° ± 6° (pre vs. post, P > 0.1).

Conclusion. We found a significant difference between pre- and postoperative rod contour, particularly for concave rods. Rod overcontouring (by ∼20° for concave rod) resulted in high degrees of correction without loss of sagittal alignment. The resulting deformations are likely associated with substantial in vivo deforming forces.

Overcontoured concave rods flattened significantly after surgical correction of thoracic curves in patients with adolescent idiopathic scoliosis. Rod overcontouring permits substantial force application that allows a high degree of scoliosis correction in 3 dimensions and maintains sagittal alignment.

*Department of Orthopedic Surgery, University of California San Diego, San Diego, CA

Department of Orthopedic Surgery, Rady Children's Hospital and Health Center, and Children's Way, San Diego, CA

Orthopedic Biomechanics Research Center, and Children's Way, San Diego, CA.

Address correspondence and reprint requests to Peter O. Newton, MD, 3030 Children's Way, Ste., 410, San Diego, CA 92123; E-mail: pnewton@rchsd.org

Acknowledgment date: August 10, 2011. First revision date: December 7, 2011. Second revision date: February 17, 2012. Acceptance date: February 21, 2012.

This study was conducted at Rady Children's Hospital and Health Center San Diego, CA, and the Orthopedic Biomechanics Research Center, San Diego, CA.

The device(s)/drug(s) is/are FDA approved or approved by corresponding national agency for this indication. Corporate/Industry and Professional funds were received to support this work.

One or more of the author(s) has/have received or will receive benefits for personal or professional use from a commercial party related directly or indirectly to the subject of this manuscript: e.g., honoraria, gifts, consultancies, royalties, stocks, stock options, decision-making position.

Idiopathic scoliosis is a complex 3-dimensional deformity1 that affects up to 2% to 3% of the population. Accumulating evidence supports a genetic etiology.25 Although the exact pathway remains unclear, Deacon et al6 put forth the idea that the development and progression of adolescent scoliosis is due to an imbalance in sagittal alignment. Progression of idiopathic scoliosis is related to the accelerated growth phase of puberty.710

The goals of operative treatment of adolescent idiopathic scoliosis (AIS) are to prevent further curve progression and to obtain a balanced spine by way of deformity correction in 3 dimensions. Currently, posterior segmental spinal instrumentation and fusion has become one of the most common operative treatments of progressive scoliosis with a curvature greater than 50°.11,12 This technique commonly uses bilateral pedicle screws anchored to vertebral bodies which are then fixed to contoured rods. The use of pedicle screws permits fixation of all 3 columns of the spine.12 The surgeon bends or contours the rods to conform to the desired profile on either side of the spine and permits significant force application during correction of the scoliotic deformity in 3 planes.12 With these systems, the surgeon is able to use multiple intraoperative techniques to achieve correction, several of which hinge on rod contouring and rod mechanics. Correction strategies include rod rotation, rod cantilever, in situ rod bending, compression and distraction of convex and concave vertebral segments,13 and direct vertebral rotation (DVR).14 In addition, as described by Cotrel et al,15 the convex rod was prebent into a flatter shape at the apex to give posteroanterior pressure at the apex and improve the derotation produced by the concave rod. However, depending on instrumentation and correction techniques used, thoracic kyphosis which is generally reduced in AIS has been shown to either increase or decrease depending on the correction/instrumentation used.1620

It is clear that the spinal rod biomechanical properties and their shapes are integral components of posterior instrumentation outcome. Although the use of the rod-screw instrumentation is a commonly accepted surgical intervention, the deformation of spinal rods as well as their preimplantation contour characteristics have not been previously studied. Understanding the changes in sagittal rod contour after implantation, which carries with it implications for in vivo spinal deformity corrective forces, may help improve results in the surgical treatment of idiopathic scoliosis correction. The purpose of this study was therefore to assess the magnitude of spinal rod deformation associated with AIS correction and determine whether concave rod “overcontouring” can preserve sagittal alignment and aid in axial derotation of the apical vertebra.

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MATERIALS AND METHODS

Subjects

Rod tracings were incorporated as part of an ongoing institutional review board–approved prospective study on AIS operative patients. In the absence of sufficient pilot data, analysis was conducted after what was thought to be an adequate period of enrollment. Rods from the concave and convex sides of patients with thoracic idiopathic scoliosis who underwent corrective surgery between 2006 and 2009 were analyzed. Patients who underwent correction for a primary thoracic curve and were instrumented with bilateral segmental uniplanar pedicle screw constructs using 5.5-mm diameter 200 KSI steel rods were included. According to Lenke et al,21 we excluded patients with a positive sagittal modifier (>40°, T5–T12) because we were most interested in the patients with typical hypokyphotic thoracic scoliosis. No patients with reversal of sagittal rod contour at the thoracolumbar junction were included in this study. All patients were surgically treated by 1 surgeon at the same center.

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Operative Technique and Rod Contouring

In this study, all patients with AIS were treated with segmental posterior spinal instrumentation with pedicle screws. All patients were positioned on an OSI (Mizuho OSI, Union City, CA) table with pads placed at the chest and the iliac crest. Bilateral segmental pedicle screws were inserted with a freehand pedicle screw placement technique. Rods were contoured to aid in kyphosis creation without any particular attention to the coronal deformity (Figure 1). Following rod insertion, traditional rod rotational maneuvers were carried out as required to bring the kyphotic rod bend into the sagittal plane (rod was rotated and locked at a single distal vertebral level). The rotation of the rod into the sagittal plane transformed the remaining coronal scoliotic deformity into kyphosis. Overcontouring (particularly of the concave rod) was used to improve kyphosis and affect derotation at the apex of the curve (Figure 2). The right convex hypokyphotic/lordotic thoracic curves representative of this study cohort exhibited apical vertebral body rotation counterclockwise (as viewed from caudad).22 As such, the concave rod was contoured with increased kyphosis (overbending) to “pull” the left side of the apical vertebrae dorsally out of chest and correct apical hypokyphosis. The convex rod was contoured with less kyphosis to “push” on the right side of the apical vertebral body anteriorly and therefore assisting in axial plane derotation. To further maximize deformity correction, additional techniques included direct vertebral body derotational maneuvers and segmental compression and/or distraction (which were performed with both rods in place). In particular, concave distraction using the motion of the uniplanar screw heads allowed maximum scoliosis and kyphosis correction to occur together.

Figure 1

Figure 1

Figure 2

Figure 2

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Rod Characteristics

All patients in this study had 5.5-mm ultrahigh-strength steel (200 KSI) spinal rods implanted. The rod lengths varied from patient to patient in accordance with patient's height and scoliotic deformity, with a range of 147 to 303 mm (mean, 219 ± 37 mm) for the left side and 161 to 301 mm (mean, 228 ± 32 mm) for the right side. The number of levels instrumented varied from 6 to 11, with the upper and lower instrumented levels between T2–T5 and T11–L2, respectively.

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Rod Analysis

Prior to implantation, following the intraoperative contouring of the rods, the surgeon traced the rod shapes on paper (Figure 1B). These tracings were then digitized into a JPEG format (Joint Photographic Experts Group, image extension format) for postoperative analysis. The contoured rods were then implanted as described previously. Postoperative sagittal rod shape was determined from lateral 2-dimensional radiographs obtained on average at 5 weeks postoperatively (range, 4–6 wk). The rod images from each preoperative tracing and the corresponding postoperative film were then digitized and isolated from the parent image using Photoshop (Adobe Systems Inc., San Jose, CA) (Figure 3). Rod tracings were initially performed by a trained research coordinator. Concave and convex rods were identified on the lateral radiographs on the basis of the difference in rod heights/lengths seen on posteroanterior films. There was unanimous agreement on rod identification by the research coordinator, primary author, and senior author. The resulting rod images were then processed by a custom MATLAB (The Mathworks, Inc. Natick, MA) program to measure the data of interest: (1) maximal deflection of the rod indicating greatest deformation due to bending and (2) angle of intersection of the tangents to the rod end points (rod angle) (Figure 4). The MATLAB rod analysis was automated but required user confirmation of tangent lines to ensure that pixelation artifact at the rod corners or distal rod flattening at the thoracolumbar junction was not causing inaccurate contour prediction. In these instances, the tangent location was chosen to be just central to aberrancy, and number of points used in the calculation of the best-fit tangent line was adjusted to best match the rod contour. Each rod image was calibrated within our application using a standardized calibration line drawn on the preoperative rod tracing prior to digitization allowing for a pixel-to-millimeter conversion. Postimplantation rod images extracted from radiographs were calibrated on the basis of the assumption that the rod arc length remained constant postoperatively.

Figure 3

Figure 3

Figure 4

Figure 4

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Radiographical Outcomes

Pre- and postoperative radiographs underwent standardized measurements of the coronal and sagittal deformities. The maximal Cobb angles of the thoracic curve and the T5–T12 thoracic kyphosis were digitally measured. Postoperative apical vertebral rotation was measured by the method of Upasani et al.23 Briefly, trigonometric relationships are used to calculate vertebral rotation from the patients' postoperative posteroanterior radiographs. This calculation is based on the projection of bilateral pedicle screws of known length. The change in rotation was assessed with pre- and postoperative scoliometer measurements.

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Statistics

Statistical Package for Social Sciences (SPSS v.12; IBM, Armonk, NY) was used for all analyses in this study. Statistical analysis using repeated measures analysis of variance assessed the differences in the pre- and postoperative contour of the rods. The significance level was set to a P value of 0.05.

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RESULTS

Overall, 27 patients were included in this analysis. The average age, average postoperative follow-up, sex, preoperative Cobb angle, Lenke-type distribution, and curve flexibility are recorded in Table 1.

TABLE 1

TABLE 1

In comparing the contours of the concave and convex rods prior to implantation, we found the rods on the concave side to be more kyphotic relative to the rods on the convex side. The concave and convex rod angles were 45.6° and 31.4°, respectively. The corresponding maximum deflection was 25.4 mm versus 15.0 mm (P < 0.001) (Figure 6). Normalized to rod length, the concave versus convex maximum deflection per millimeter of rod length was 0.12 ± 0.01 mm versus 0.07 ± 0.01 mm (P < 0.001). These results are consistent with the above-mentioned goals of differential overcontouring to aid in axial derotation and kyphosis creation.

Figure 5

Figure 5

Figure 6

Figure 6

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.

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DISCUSSION

In this study designed to analyze the change in spinal rod contour after implantation, we found a statistically significant difference between the pre- and postimplantation contour, particularly for the concave rods, after scoliosis correction. Rod overcontouring (by ∼20°) and the resulting deformations of these 5.5-mm steel rods are likely associated with substantial corrective forces permitting correction in the coronal and axial planes without a loss of sagittal alignment.

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.2426

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.

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CONCLUSION

In this study, we have demonstrated a change in spinal rod contour, particularly for the concave rods, following posterior implantation in patients with AIS. Rod overcontouring (by ∼20°) resulted in high degrees of correction in the coronal and axial planes without a loss of sagittal alignment.

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Key Points

  • Concave rods flattened significantly postoperatively.
  • Rod overcontouring (by ∼20° for concave rod) results in high degrees of correction in the coronal and axial planes with preservation of sagittal alignment.
  • The resulting deformations of ultra–high-strength steel are likely associated with substantial in vivo deforming forces.
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Acknowledgments

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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References

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              Keywords:

              adolescent idiopathic scoliosis; rod contour; differential rod contouring; deformation; sagittal alignment; biomechanics; force

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