Does Resection of the Posterior Longitudinal Ligament Affect the Stability of Cervical Disc Arthroplasty?

BACKGROUND: The need for posterior longitudinal ligament (PLL) resection during cervical total disc arthroplasty (TDA) has been debated. The purpose of this laboratory study was to investigate the effect of PLL resection on cervical kinematics after TDA. METHODS: Eight cadaveric cervical spine specimens were tested in flexion-extension (FE), lateral bending (LB), and axial rotation (AR) to moments of ±1.5 Nm. After testing the intact condition, anterior C5-C6 cervical discectomy was performed followed by PLL resection and implantation of a compressible, 6-degrees-of-freedom disc prosthesis (M6-C, Spinal Kinetics Inc, Sunnyvale, California). Next, a second prosthesis was implanted at C6-C7 with PLL intact. Finally, the C6-C7 PLL was resected while the disc prosthesis remained in place. Segmental range of motion (ROM) and stiffness in the high flexibility zone around the neutral posture were analyzed using repeated measures ANOVA. RESULTS: At C5-C6, following TDA and PLL resection, FE, LB, and AR ROMs decreased significantly. Anterior and posterior disc height, segmental lordosis, and flexion stiffness increased significantly. At C6-C7, TDA with the PLL intact resulted in a significant increase in anterior disc height and segmental lordosis with no change in posterior disc height. FE, LB, and AR ROMs all decreased significantly, while flexion stiffness increased significantly compared to intact. PLL resection at C6-C7 did not result in a notable change compared to TDA with PLL intact. At the same level, flexion stiffness decreased following PLL resection compared to TDA with a value closer to intact. Two-level TDA (C5-C7) with PLL resection did not result in a loss of segmental stability. CONCLUSION: PLL resection did not significantly affect motion segment kinematics following cervical TDA using a prosthesis with inherent stiffness. Motion segment stiffness loss after PLL resection can be compensated for by a TDA design that can provide resistance to angular motion.

Interpolation of three dimensional kinematics with dual-quaternions.

Devising patient-specific kinematic assessment techniques are critical for both patient diagnosis and treatment evaluation of complex biomechanical joints within the body. New non-invasive kinematic assessment techniques, such as bi-planar fluoroscopic registration, provide improved insight on joint biomechanics compared to traditional techniques, but at the expense of higher radiation exposure to the patient. The purpose of this study was to minimize the x-ray sample size required for evaluating spine kinematics, ultimately reducing radiation exposure, while maintaining a high degree of accuracy by improving upon existing 3D kinematic interpolation techniques. Existing interpolation methods were improved to account for non-uniformly sampled control points and applied to new motion descriptors, thus creating a new approach to 3D kinematic interpolation utilizing dual-quaternions. Interpolation reconstruction methods were applied to decimated gold standard ex vivo spinal kinematic data originally acquired at 30Hz. The effects of interpolation method and variables (motion descriptor, sample spacing, sampling correction factors) on accuracy were compared. Dual-quaternion interpolation methods and equal interval angular sampling showed superior reconstruction results. Accuracy also improved when using temporal correction factors. Less than 1% normalized root-mean-squared error and less than 2% normalized maximum error were achieved from 0.36% of the original data set. The new approach also demonstrated its scalability for larger movements. However, accuracy may vary when interpolating more complex motion patterns. Overall, multiple interpolation methods and factors were evaluated in reconstructing 3D spine kinematics. High accuracy at low sample sizes and advantageous scalability to motions with larger total displacement illustrate its viability for bi-planar fluoroscopy.

Three-Dimensional Computed Tomography-Based Specimen-Specific Kinematic Model for Ex Vivo Assessment of Lumbar Neuroforaminal Space.

STUDY DESIGN: Cadaveric study to accurately measure lumbar neuroforaminal area and height throughout the flexion-extension range of motion (ROM). OBJECTIVE: Create a new computed tomography (CT)-based specimen-specific model technique to provide insight on the effects of kinematics on lumbar neuroforamen morphology during flexion-extension ROM. SUMMARY OF BACKGROUND DATA: Nerve root compression is a key factor in symptomatic progression of degenerative disc disease because these changes directly affect neuroforaminal area. Traditional techniques to evaluate the neuroforamen suffer from poor accuracy, have inherent limitations, and fail to provide data throughout the ROM. METHODS: Six cadaveric specimens (L1-sacrum) were instrumented with radiopaque spheres and CT scanned. 3-Dimensional reconstructions were made of each vertebra and the sphere locations determined. During kinematic testing, the spheres were located in relation to optoelectronic targets attached to each vertebra. The result was a 3-dimensional representation of the specimen's CT reconstruction moving in response to experimental data. Bony contours of the L2-L3 and L4-L5 neuroforamen were digitized producing continuous neuroforaminal area and height data throughout the ROM. RESULTS: Neuroforaminal area and height linearly increased in flexion and decreased in extension. There was significant correlation between flexion-extension motion and percent change in area (L2-L3: 3.1%/deg, R = 0.94, L4-L5: 2.5%/deg, R = 0.90) and neuroforaminal height (L2-L3: 2.1%/deg, R = 0.95, L4-L5: 1.6%/deg, R = 0.93). Regression analysis showed that the ratio between neuroforaminal height and area is at least 1:1.5 such that a 100% increase in height is associated with an area increase of more than 150%. CONCLUSION: This is the first study to measure lumbar neuroforaminal area and height throughout flexion-extension ROM. The CT-based specimen-specific model technique can accurately evaluate the effect of kinematics on morphological features of the spine. The demonstrated increase in neuroforaminal dimension in flexion is consistent with treatment modalities used in clinical therapies to relieve radicular symptoms. LEVEL OF EVIDENCE: N/A.