![]() The technique has been validated, with an average reported measurement error of 0.5°. 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. 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. The flexion to extension ROM was calculated as the difference between the flexion ILA and the extension ILA for each pair. Equipment setup was supervised by an experienced X-ray technician to ensure proper calibration and coordinate determination. 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. A calibrated, digital meter on the X-ray unit was used to verify angle coordinates given the fixed distance from the device center. ![]() The technique for generating variable X-ray beam angles is detailed in Figure 1. 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 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). 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. 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. 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°. 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. With the X-ray beam in the neutral position (A), the edges of the endplates are clearly identifiable. 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. We hypothesized that parallax would not affect measurements of ROM when utilizing the keel method for ILA determination. The purpose of this study was to evaluate the influence of X-ray beam angle on measurement error in TDR. This effect can be exaggerated in large patients where the distance from the implant to the radiographic cassette is increased. Since the implant is positioned caudal to the beam center, relative beam divergence may distort the radiographic landmarks used for flexion-extension ILA measurement. While flexion-extension radiographs are typically performed with the beam centered on the mid lumbar spine, most TDR implants are at the L4-5 or L5-S1 segments. Specifically, parallax is created by aligning the X-ray beam at a fixed sagittal distance from the implant and then displacing the X-ray source cranially to create an angle of the X-ray beam. Second, parallax effect is a product of X-ray beam positioning during image acquisition. As the superior and inferior footplates of the implant replace the native endplates, an endplate no longer exists as a radiographic landmark for precise ILA measurement. Though this same method is commonly used for patients with TDR, 2 potential confounders of accurate measurement exist-endplate removal in TDR and parallax effect during image acquisition. Generally, the ILA is measured as the angle subtended by the lines drawn along the cranial vertebral endplates. ![]() In patients with preserved vertebral bony anatomy, flexionextension radiographs are taken, and the differences between index level angle (ILA) measurements taken in flexion and extension are utilized to calculate sagittal plane ROM. As a result, there is interest in improving methods of ROM measurement and in identifying potential sources of ROM measurement error. 4– 6 This correlation is partially responsible for the emergence of ROM measurements as pivotal radiographic outcome measures following lumbar TDR. 1– 3 Increased range of motion (ROM) of implanted lumbar TDR has recently been correlated with improved clinical outcomes. One goal of lumbar total disc replacement (TDR) is to maintain or restore motion, in order to reduce the transfer of stresses to adjacent levels.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |