![]() 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. ![]()
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