Assessment of Magnetic Resonance Imaging Artifact Following Cervical Total Disc Arthroplasty.

BACKGROUND: Cervical disc arthroplasty has become a technique for the treatment of cervical degenerative disc disease. Clinically, the need to accurately assess the neural elements at the operative and adjacent levels is critical postoperatively. The purpose of this study was to quantitatively and qualitatively measure the amount of MRI artifact produced by various cervical total disc replacements. METHODS: T1 and T2-weighted turbo spin-echo MRI sequences were collected on the cervical spine (C2-T1) of a 68 year-old unembalmed male cadaver. A discectomy was performed at C5-6, followed by successive implantation of six different total disc replacements. The scans were quantitatively evaluated by three of the authors. The volume of artifact was measured using image analysis software. Qualitative analysis of the adjacent and index neural elements was performed. RESULTS: The artifact in the T2 weighted images was noted to be 58.6±7.3 cm3 for Prestige ST, 14.2±1.3 cm3 for ProDisc-C, 7.5±0.8 cm3 for Discover, 8.0±0.3 cm3 for Prestige LP, 6.6±0.7 cm3 for Bryan, and 7.3±0.6 cm3 for ProDisc-C titanium prototype. Acceptable intraobserver and excellent interobsever correlation was demonstrated using Pearson Correlation and Concordance Correlation Coefficient analysis. The adjacent and implanted level neural elements (spinal cord and neuroforamina) were easily visualized on the T2 weighted images after the implantation of titanium devices (ProDisc-C titanium prototype, Discover, Prestige LP and Bryan). After implantation of a cobalt chrome implant (ProDisc-C), the adjacent level neural elements were easily visualized but the implanted level could not be fully visualized due to distortion of the images. The quality of the distortion was least favorable after the implantation of the stainless steel implant (Prestige ST), where neither the adjacent nor the index level could be fully visualized. CONCLUSION: The volume of the artifact seen following cervical total disc arthroplasty is highly dependent upon the material property of the implant. Quantitative analysis described in this study demonstrated sufficiently low intraobserver and interobserver variability to be considered a reliable technique.

Positional effects of transforaminal interbody spacer placement at the L5-S1 intervertebral disc space: a biomechanical study.

BACKGROUND CONTEXT: Transforaminal lumbar interbody fusion (TLIF) is an increasingly used alternative fusion method over anterior and posterior lumbar interbody fusions. There are conflicting results on the optimal positioning of interbody devices. No study has addressed the lumbosacral segment, L5-S1, where the lordotic configuration presents unique challenges. PURPOSE: To determine if there are biomechanical and/or anatomical advantages related to the positioning of an interbody device at L5-S1, either anterior or posterior to the neutral axis. STUDY DESIGN: An in vitro biomechanical study using human cadaveric lumbar specimens. METHODS: Lumbar specimens were biomechanically tested using pure moments with and without compressive axial loading. Testing was performed in intact and after TLIF with the implant posterior (TLIF-post) and anterior (TLIF-ant) to neutral axis. Segmental range of motion (ROM) and stiffness were analyzed at the L5-S1 surgical level and the adjacent L4-L5 level. Neuroforaminal height measurements of L5-S1 were analyzed in neutral and end range positions. RESULTS: Compared with the intact condition, ROM decreased more than 75% at L5-S1 and stiffness increased up to 270% with TLIF. There was no significant difference between anterior or posterior placement for ROM and stiffness. There was a change in L5-S1 neuroforaminal height based on the placement, with posterior placement showing a significant increase compared with anterior placement. There were no relative changes in neuroforaminal height under loading after TLIF. Compressive load did not affect the magnitudes or resulting significance of outcome measures at L5-S1 after either TLIFs. CONCLUSIONS: An interbody spacer with the addition of posterior instrumentation significantly enhances the mechanical stability of L5-S1 regardless of interbody position. There were noticeable increases in terms of construct stability and stiffness after both TLIF-ant and TLIF-post in comparison with the intact condition. A posteriorly placed interbody implant did result in the distraction of the neuroforamin. Positioning an interbody implant at L5-S1 for TLIF with posterior instrumentation should be at the discretion of the surgeon without consequence to biomechanical stability.

Structural properties of 6 forearm ligaments.

PURPOSE: To first determine the structural properties of 6 forearm ligaments and then to create linear and nonlinear analytical models of each ligament from these properties. METHODS: We nondestructively tested the annular ligament, dorsal and palmar radioulnar ligaments, and the distal, central, and proximal bands of the interosseous ligament from 7 fresh cadaver forearms in a servohydraulic testing apparatus. We performed testing with the bone-ligament-bone constructs positioned corresponding to neutral forearm rotation as well as in 45° of supination and 45° of pronation. Based on a mechanical creep test of each ligament, we computed a linear and nonlinear ligament stiffness value for each ligament. We then compared these computed analytical responses to loading with loading data when each ligament was tested at 1.0 and 0.05 mm/s. We analyzed differences among ligaments and forearm positions using 1-way and 2-way analyses of variance. RESULTS: The stiffnesses for the distal band and the dorsal radioulnar ligament were statistically less when the constructs were positioned in supination compared with neutral forearm rotation. At all forearm positions, the linear stiffness of the central band was greater than that for the distal band of the interosseous ligament, the proximal band of the interosseous ligament, and the dorsal radioulnar and palmar radioulnar ligaments. In neutral forearm rotation, the linear stiffness of the central band was statistically greater than the annular ligament. The experimental loading behavior of each ligament was better modeled by a nonlinear stiffness than a linear one. CONCLUSIONS: The central band of the interosseous membrane is the stiffest stabilizing structure of the forearm. Any structure used to replace the central band or other forearm ligaments should demonstrate a nonlinear response to loading. CLINICAL RELEVANCE: In considering a reconstruction for the forearm, the graft used should have a nonlinear response to loading and be one that is similar to the normal, original ligament.