The Parallax Effect in the Evaluation of Range of Motion in Lumbar Total Disc Replacement

Measurements

A flexible lumbar Sawbones (Pacific Research Laboratories, Inc., Vashon, Washington) model was implanted with a large-sized ProDisc-L device (Synthes Spine, West Chester, Pennsylvania) using an 11° inlay at the L4-5 level. Maximal lateral flexion and maximal extension radiographs were obtained using standard settings (96 kV X-ray beam, at 0.71 mA per second, with the object consistently placed 40 cm from the X-ray tube, onto 10x12-inch cassettes) with the beam 0°, 5°, 10°, and 15° incident to the device center (Figures 1 and 2). This range of tested parallax angles was selected based on the normal parallax variation seen in the clinical setting.

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Figure 1

Figure 1 is a schematic of the technique used for variable X-ray beam acquisition. With the X-ray tube aiming parallel to the floor, the angle subtended between the X-ray beam and the device center was decreased from 15°, 10°, 5°, and 0° in the sagittal plane by sequentially lowering the X-ray beam without changing the distance between the tube and the device (black arrow).

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Figure 2

Sawbones model (Pacific Research Laboratories, Inc., Vashon, Washington) with lumbar total disc replacement (ProDisc-L, Synthes Spine, West Chester, Pennsylvania) implanted at the L4-5 interbody space in flexion. With the X-ray beam in the neutral position (A), the edges of the endplates are clearly identifiable. With increasing superior movement of the X-ray beam at 5° (B), 10° (C), and 15° (D), notice the increasingly distorted projection of the footplates. The keel method, using lines drawn along the superior and inferior aspects of the keels, was used to determine Cobb measurements from which range of measurements were calculated.8 A comparison of (A: neutral beam angle) and (D: 15° beam angle) reveals a difference of only 0.3° despite an X-ray beam angle of 15°.

ILA: index level angle

In order to ensure maintenance of maximal motion implant flexion, a custom wooden spacer was placed in between the spinous processes and reinforced with elastic bands around the L4 and L5 transverse processes. Similarly, to ensure that maximal extension was achieved and maintained, multiple elastic bands were secured around the L4 and L5 transverse processes once the device was maximally extended.

The exact measurements for vertical displacement (Vd) of the X-ray tube to produce angle θ were calculated trigonometrically using the following equation: Vd = (40 cm)(tan θ). The X-ray tube vertical displacements needed in order to achieve the above desired angles were as follows: 0° - 0 cm; 5° - 3.5 cm; 10° - 7.1 cm; 15° - 10.7 cm. The technique for generating variable X-ray beam angles is detailed in Figure 1. A calibrated, digital meter on the X-ray unit was used to verify angle coordinates given the fixed distance from the device center. The beam was placed directly in line with the device center using a digitized laser localizer, which confirmed that there was no variability in the anterior-posterior direction. Equipment setup was supervised by an experienced X-ray technician to ensure proper calibration and coordinate determination.

The flexion to extension ROM was calculated as the difference between the flexion ILA and the extension ILA for each pair. Comparative measurements for angular ROM were obtained using Quantitative Motion Analysis software (QMA, Medical Metrics, Inc., Houston, Texas), which uses digital superposition methods to assess intervertebral motion. QMA is a computer-assisted technique similar to the method of Penning et al.,⁹ with patented modifications to optimize accuracy and reproducibility, and to circumvent parallax effects. The technique has been validated, with an average reported measurement error of 0.5°.^9–11 Cobb angle measurements of all 8 images were also performed by 8 orthopaedic surgeons and residents on 2 separate occasions using a #2 pencil and hinged goniometer (or Cobb ruler of the surgeon's choice) at least 1 week apart, with the image order randomized prior to each occasion.

Participants were blinded to the purpose of the study and were instructed to use only the keel method for ILA measurement (Figure 2), as this method has recently been shown to be more precise than use of the metallic prosthesis endplate as a landmark reference.⁸

Statistical Analysis

The effect of beam angle on both inter- and intraobserver precision and accuracy was evaluated. Mean ROM and standard deviation were computed at each beam angle. Error terms for each observation were calculated by subtracting participants’ ROM measurements from the corresponding gold standard values from the QMA software, allowing for the computation of mean error and standard deviation at each beam angle. Interobserver precision was assessed by comparing the distributions of ROM measurements at each of the 4 beam angles using 1-way analysis of variance (ANOVA). The distributions of the absolute values of these error terms were analyzed with 1-way ANOVA to determine the effect of beam angle on interobserver accuracy. The absolute values of the differences between the participants’ first and second measurements were calculated, and the distributions at each of the beam angles were compared with the Friedman rank sum test to assess intraobserver precision. The effects on intraobserver accuracy were determined by comparing the distributions of the differences in error between participants’ first and second observations at each of the beam angles with the Friedman rank sum test. Statistical analysis was performed using Statistical Package for the Social Sciences (SPSS, Chicago, Illinois), version 15.0, with P values less than .05 considered significant.