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A Method for Assessing Axial Vertebral Rotation Based on Differential Rod Curvature on the Lateral Radiograph

Liu, Raymond W., MD*; Yaszay, Burt, MD; Glaser, Diana, PhD†,‡; Bastrom, Tracey P., MA; Newton, Peter O., MD

doi: 10.1097/BRS.0b013e318258aa17
Biomechanics
Free
SDC

Study Design. Bench-top and retrospective radiographical analyses to determine apical vertebral rotation based on differential rod curvature on the postoperative lateral radiograph.

Objective. To develop a clinically relevant methodology for measuring apical vertebral rotation on postoperative lateral radiographs in patients with adolescent idiopathic scoliosis, based on the distance between the spinal rods.

Summary of Background Data. Traditional methods of analyzing vertebral rotation on plain radiographs are limited in the postoperative spine with segmental instrumentation. A previous methodology based on pedicle screw tip to rod distances on the posteroanterior radiograph is effective but limited by surgical technique and patient positioning relative to the x-ray beam.

Methods. The trigonometric relationship between the inter-rod distances on lateral radiographs was defined and validated on a biomechanical model, with apical rotation varying from 0° to 20°. The ability to correct for malposition on the lateral radiograph was tested on 11 postoperative radiographs and correlated against corresponding postoperative computed tomographic scans.

Results. The bench-top model had a strong correlation between actual apical rotation and calculated rotation for the full range of image rotations (intraclass correlation coefficient, 0.99). For the 11 clinical cases, comparisons of apical rotation measured on computed tomographic scans were highly correlated to the proposed lateral radiograph calculations (r = 0.84).

Conclusion. A technique for measuring apical vertebral rotation based on the inter-rod distance on the lateral and posteroanterior radiographs was developed and validated. This technique is resilient to rotation of the patient within the x-ray machine and can complement measurement of rotation on postoperative posteroanterior radiographs.

A new technique for assessing apical vertebral rotation utilizing the inter-rod distance on the lateral radiograph was validated using a bench-top model and on clinical cases. This methodology may be used to supplement a previous technique, based on the postoperative posteroanterior view, to enhance measurement of axial plane deformity correction without a computed tomographic scan.

*Department of Orthopaedic Surgery, Case Western Reserve University, Cleveland, OH

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

Orthopedic Biomechanics Research Center, San Diego, CA.

Address correspondence and reprint requests to Burt Yaszay, MD, Department of Orthopaedic Surgery, Rady Children's Hospital and Health Center, 3030 Children's Way, Ste. 410, San Diego, CA 92123; E-mail: byaszay@rchsd.org

Acknowledgment date: August 17, 2011. First revision date: March 26, 2012. Acceptance date: March 29, 2012.

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

The device(s)/drug(s) is/are FDA approved or approved by corresponding national agency for this indication.

No funds were received in support of 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.

The spinal deformity in scoliosis is commonly described in the coronal, sagittal, and transverse planes. Although deformity measurement in the first 2 planes is relatively straightforward, it can be challenging to accurately measure vertebral rotation in the transverse plane.

Multiple studies have investigated the use of plain radiographs to measure vertebral rotation based on the pedicles, spinous processes, and vertebral width.114 Although many of those studies have found good validity and reliability on the native spine, other studies have noted measurement errors when applying those techniques to the segmentally instrumented spine.15,16

Computed tomographic (CT) scanning has demonstrated good validity and reliability in assessing vertebral rotation11,1723 but carries the drawback of radiation in adolescent patients.24 Magnetic resonance imaging and ultrasound have also been investigated2527 but tend to be too costly, cumbersome, or technician-dependent for widespread use.

Recently, a methodology for grading apical vertebral rotation on the postoperative posteroanterior (PA) radiograph has been described.28 Vertebral rotation can be calculated in an instrumented vertebra by measuring the offset of each pedicle screw tip from the rod and applying trigonometric principles. This method offers the advantage of using an image that is routinely obtained postoperatively, but limited by any rotation of the patient's body when taking the image, and is dependent on screw insertion technique.

To complement this method, we developed a new methodology. Based on a simple trigonometric relationship, apical rotation was calculated using the ratio of the distances between the rods on the lateral view and the PA view. The objective of this study was to test this methodology for postoperative assessment of apical rotation on a biomechanical bench-top model, validate the method using postoperative clinical images, and create a simple but robust technique for clinical and research application.

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MATERIALS AND METHODS

Mathematical Calculations

The trigonometric relationship between vertebral rotation and the distances between the spinal rods on the PA and lateral views is illustrated in Figure 1. After measuring the distance between the rods on the lateral radiograph (A), and the distance between the rods on the PA radiograph (B), apical rotation (A) can be calculated using the formula:

Figure 1

Figure 1

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Biomechanical Model

A sample postoperative radiograph instrumented from T5 to T12 was used to develop our biomechanical model (Figure 2). The vertical position of each set of screws was measured from the PA radiograph. The spacing between rods was then measured and noted to gradually increase from superior to inferior, with an average distance of 40 mm between the rods. To simplify the model, a constant distance of 40 mm was used. To recreate the sagittal profile, the distance from the center of the rod to a vertical plumb line was measured at each screw on the lateral radiograph and carefully reproduced in the model on the basis of quarter turns of each screw (pitch = 0.25 mm). Initially, each pair of screws had identical positions in the sagittal plane, representing 0° of rotation. The screws were inserted into a 1 × 4-in. board. The board and screw construct was then mounted onto a rotation platform with 2° incremental markings (Product RP01, Thorlabs, NJ).

Figure 2

Figure 2

Two 4.75-mm rods were contoured and capped within the screws. We calculated the change in depth required at each screw to represent apical rotations of 5°, 10°, 15°, and 20°, with a simultaneous decrease in depth of the right-sided screws and increase in depth of the left-sided screws to represent a right thoracic curve. At each curvature, a scoliometer was used to grossly confirm apical vertebral rotation. The screws at T5 and T12 were kept at equal depths throughout the study to create 2 sets of neutral vertebrae, while T8 represented the apical vertebra.

Radiographs were obtained using a standard x-ray machine (range, 40–125 kVp, SEDECAL IDEAL, SHF 520, SEDECAL, Buffalo Grove, IL) in our scoliosis clinic. The model was placed 6 ft from the image source and oriented with the “left shoulder” appropriately spaced against the image collector. The beam was oriented slightly anterior to the center of the construct to represent a standard clinical radiograph.

Radiographs were obtained with the model set at 0° apical rotation, and the entire construct rotated from −20° to 20° in 5° increments to represent a range of imperfect lateral radiographs. A perfect lateral radiograph should have perfect overlap of the rods at the neutral vertebra. Therefore, an imperfect lateral radiograph will demonstrate 2 rods being visible at the level of the neutral vertebra. The entire series was then repeated with the model set to 5°, 10°, 15°, and 20° of apical rotation. The validity of this approach has further been validated using a 3-dimensional model in Solidworks (Waltham, Massachusetts) (see Appendix, Supplemental Digital Content 1, available at: http://links.lww.com/BRS/A659).

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Radiographical Analysis

Radiographs were analyzed on a picture archiving and communication system. The distances between the rods were measured on the lateral radiograph at the neutral and apical vertebrae. Distances were calibrated on the basis of the known rod width of 4.75 mm. Because of difficulties in visualizing the rods at the screw level, measurements were made above and below the rod-screw interface and then averaged (Figure 3). It is important to note that negative values are possible if the rods cross paths (Figure 4).

Figure 3

Figure 3

Figure 4

Figure 4

Correcting for an imperfect radiograph can be determined by visualizing the rods at the neutral vertebral. If the rods do not perfectly overlap, then the distances measured at the 2 neutral vertebrae are averaged and subtracted from the apical vertebrae rod distance. The distance between the rods on the PA view was a known value of 40 mm. Apical rotation was calculated using the aforementioned formula.

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Retrospective Analysis

The retrospective analysis was approved by our institutional review board. Eleven patients had been treated with posterior segmental instrumentation and fusion, with postoperative PA and lateral radiographs and postoperative CT scans available.

Postoperative PA radiographs were reviewed to determine the neutral and apical vertebrae. Distances were measured between the rods at the apical vertebrae on the PA view and between the rods at the neutral and apical vertebrae on the lateral view to calculate apical rotation. Both neutral vertebra measurements were averaged and used as a baseline for the comparison of the apical rotation. When only 1 neutral vertebra was included in the instrumentation, the other neutral vertebra was disregarded. The average distance of the neutral vertebrae was then subtracted from the distance at the apical vertebrae to correct for rotational positioning of the patient for the radiograph.

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Computed Tomography Validation

Postoperative axial CT images were used to measure neutral and apical vertebral rotation on the axial sequences using a method similar to that described by Ho et al.21 A line was drawn joining the junction points of the lamina and pedicles on each side. A second line was drawn perpendicular to this line, and its angle versus the vertical was measured. When 2 neutral vertebrae were included in the fusion, their values were averaged. The average rotation of the neutral vertebrae was then subtracted from the rotation at the apical vertebrae to correct for any rotational obliquity of the patient who is not lying perfectly supine on the CT table, which would potentially occur in patients with large posterior rib prominences.

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Statistics

The relationship between actual rotation measures on the bench-top model and radiographical measures of the model was correlated using intraclass correlation coefficient. To assess the relationship between apical rotation measured using the differential rod contour technique compared with the traditional CT scan, Pearson correlation coefficient (r) was calculated.

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RESULTS

Biomechanical Model

The measured values for apical vertebral rotation with the model's apical rotation varied from 0° to 20°, and the rotation of the entire model within the x-ray machine varied from −20° to 20° was plotted (Figure 5). The interclass correlation was 0.994. The 3 largest discrepancies between true apical rotation and measured apical rotation (2.0°, 2.3°, and 2.7°) all occurred with the overall construct in neutral rotation.

Figure 5

Figure 5

To assist in the clinical use of this method and avoid strict calculations for each patient, a quick rotational estimate can be determined on the basis of the distance between the rods as related to the width of the rod. For example, for a patient with a 40-mm interpedicular distance on the PA radiograph, each rod distance (after subtracting out the neutral vertebrae) is equivalent to approximately 8° of rotation. In other words, if the rods are perfectly overlapped, then rotation is zero; if the rods are side by side, then rotation is 8°; if there is 1 full rod width between the 2 rods, then rotation is 15°; if there are 2 full rod widths between the 2 rods, then rotation is 22°. The distance between the rods on the PA radiograph does influence this estimated rotation, especially at higher degrees of apical vertebra rotation (Table 1).

TABLE 1

TABLE 1

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Retrospective Analysis

Images were measured from 11 postoperative patients. Only the neutral vertebrae included within the fusion were used, with both neutral vertebrae used in 4 patients, the upper neutral vertebrae only in 2 patients, and the lower neutral vertebrae only in 5 patients. All patients were instrumented with 5.5-mm rods. Figure 6 plots the apical vertebral rotation measured on radiographs, using our differential rod curvature technique versus apical vertebral rotation measured on CT scans. Overall correlation was good with r = 0.84.

Figure 6

Figure 6

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DISCUSSION

When planning a scoliosis treatment or evaluating treatment success, it is important to consider that scoliosis is a 3-dimensional deformity. Apical vertebral rotation is especially important in regard to its relationship to the thoracic and lumbar prominences,13,29 and its relationship with curve progression, correction, and decompensation.3032

Although multiple techniques exist for measuring vertebral rotation on plain radiographs, methods using the position of pedicles are limited by postoperative instrumentation. Kuklo et al15 found intraobserver reliability of 0.74–0.85 on preoperative radiographs versus 0.50–0.73 on postoperative radiographs when using the Nash-Moe technique. Richards reported similar difficulties after Cotrel-Debousset instrumentation.16

A recently described methodology allows for grading apical vertebral rotation on the postoperative PA radiograph on the basis of pedicle screw tip location.28 This technique offers the advantage of measuring postoperative apical rotation on a readily available and commonly used radiograph. However, the method is limited because it assumes symmetric intrapedicular screw placement and does not correct for rotation of the patient within the x-ray machine.

Within this study, we developed a second measurement for postoperative radiographs to complement the aforementioned technique. Based on the lateral contour of the rods, the distance between the rods on the lateral view versus the PA view is trigonometrically related to apical vertebral rotation. Our biomechanical model demonstrated that this technique is valid for measuring apical vertebral rotation. In addition, it corrects for positioning of the patient within the x-ray machine, where the patient is often slightly rotated with respect to the beam. This correction is performed by subtracting the distances at the neutral vertebrae from the distance at the apical vertebrae. In fact, our least accurate measurements on the bench-top model were made at neutral rotation. We postulate that this occurred because of the difficulty in measuring rod-to-rod distance at the neutral vertebrae when they are nearly completely overlapped.

Although the calculations are fairly simple, we anticipate the major application of this mathematical technique to be for research purposes. However, the theory of lateral rod distances relating to apical rotation can be used clinically to quickly judge the rotational correction in a postoperative patient. In a patient with an interpedicular distance of 40 mm and instrumented with 5.5-mm rods, each rod distance on the lateral radiograph represents approximately 8° of rotation. Similarly, for PA inter-rod distances of 35 mm, 30 mm, and 25 mm, each rod distance is approximately equivalent to 9°, 10°, and 12° of rotation, respectively. It is important to note that these values pertain only to 5.5-mm rods, and other rod thickness would necessitate separate calculation.

This technique is limited in that it is applicable only to postoperative images. It would be ideal to use the same technique to measure apical vertebral rotation on preoperative and postoperative images to determine operative correction.

Also, the mathematics assumes that the screws are equally seated within the bone, which can vary on the basis of surgical technique. However, there is no theoretic effect of differential contouring of the 2 rods on the ability of this technique to measure apical vertebral rotation as long as the screws are equally seated. In addition, we expect the theoretical effect of multiaxial screws to be limited as long as the screws are seated directly on the bone, in which the amount of displacement from an oblique takeoff of the screw would be minimal. On the contrary, this technique cannot be used for hook or wire fixation at the neutral or apical vertebrae, as the equal seating of the rods would not be known.

Finally, this study is limited because the measurements can be more challenging when the rods are closer, especially when rods are just overlapping at a certain vertebrae. Despite these challenges, we obtained a good correlation of r = 0.84 when validating the study with clinical radiographs and CT scans.

Theoretically, this technique should be valid for any spinal deformity. Practically, we would expect more error with increasing spinal deformity due to obliquity of the radiographs to different segments of the spine. Much of this error can be improved with subtraction of the lateral distance at the neutral vertebrae. In the cases of lordotic and kyphotic vertebrae, it is important that the lateral distance is measured in the plane of the vertebral body. In other words, the measurement should be performed by drawing a line along the superior end plate of the vertebral body on the lateral view and then measuring the distances between the rods in reference to that line.

In conclusion, apical vertebral rotation is an important component of spinal deformity in scoliosis. We describe a simple methodology for measuring apical rotation based on inter-rod distance on the standard lateral radiograph. By subtracting the lateral distance at the neutral vertebrae, this technique is resilient to rotation of the patient within the x-ray machine and can complement other measurements for clinical and research application. A rule of thumb is presented for quick analysis of radiographs in the clinical setting. In the future, this technique can be useful for comparing the efficacy of different surgical techniques and fusion lengths on the correction of apical vertebral rotation.

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Key Points

  • There is a simple trigonometric relationship between apical vertebral rotation and the distances between spinal rods on the lateral and PA radiographs.
  • A bench-top model validated this relationship (intraclass correlation coefficient = 0.99) and demonstrated that malposition of the patient when obtaining the lateral radiograph is well corrected using this technique.
  • Good correlation (r = 0.84) was found between clinical postoperative radiographs measured with this technique and postoperative CT scans.
  • A rule of thumb technique can be used in the clinical setting to quickly estimate apical rotation based on the number of rod width distances between the 2 rods on the lateral view.
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Acknowledgment

The authors thank J. D. Bomar for his graphical and image-processing assistance.

Supplemental digital content is available for this article. Direct URL citations appear in the printed text and are provided in the HTML and PDF versions of this article on the journal's Web site (www.spinejournal.org).

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Keywords:

apical vertebral rotation; adolescent idiopathic scoliosis; radiographical measurements

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