Assessing the biofidelity of in vitro biomechanical testing of the human cervical spine.

In vitro biomechanical studies of the osteoligamentous spine are widely used to characterize normal biomechanics, identify injury mechanisms, and assess the effects of degeneration and surgical instrumentation on spine mechanics. The objective of this study was to determine how well four standards in vitro loading paradigms replicate in vivo kinematics with regards to the instantaneous center of rotation and arthrokinematics in relation to disc deformation. In vivo data were previously collected from 20 asymptomatic participants (45.5 ± 5.8 years) who performed full range of motion neck flexion-extension (FE) within a biplane x-ray system. Intervertebral kinematics were determined with sub-millimeter precision using a validated model-based tracking process. Ten cadaveric spines (51.8 ± 7.3 years) were tested in FE within a robotic testing system. Each specimen was tested under four loading conditions: pure moment, axial loading, follower loading, and combined loading. The in vivo and in vitro bone motion data were directly compared. The average in vitro instant center of rotation was significantly more anterior in all four loading paradigms for all levels. In general, the anterior and posterior disc heights were larger in the in vitro models than in vivo. However, after adjusting for gender, the observed differences in disc height were not statistically significant. This data suggests that in vitro biomechanical testing alone may fail to replicate in vivo conditions, with significant implications for novel motion preservation devices such as cervical disc arthroplasty implants.

Locating the Instant Center of Rotation in the Subaxial Cervical Spine with Biplanar Fluoroscopy during In Vivo Dynamic Flexion-Extension.

BACKGROUND: Recently, biplanar fluoroscopy is used to evaluate the cervical kinematics, especially to locate the instant center of rotation (ICR) during in vivo motion. This study aims to ascertain the ICR at each cervical segment in the sagittal plane during dynamic motion and assess the differences from previous studies. METHODS: While three healthy subjects were performing full flexion-extension, two oblique views aligned horizontally and angled at approximately 55° were obtained by biplanar fluoroscopy. The minimum degree to detect significant movement in a helical axis model was set at 2°, and anterior-posterior and superior-inferior locations of each ICR were defined. To evaluate the possible distribution area and overlapping area of the ICR with disc space, we drew a circle by using the calculated distance between each coordination and the mean coordination of ICR as the radius. RESULTS: During flexion-extension motion, the mean superior-inferior location of the ICR became progressively more superior, except the C5-6 segment (p = 0.015), and the mean anterior-posterior location of the ICR became progressively more anterior without exception from C2-3 to C6-7 segments, but anterior-posterior ICR locations were not significantly different among segments. The overlapping area with the distribution circle of ICR was mainly located in the posterior half in the C3-4 segment, but the overlapping area was about 80% of the total disc space in C4-5 and C6-7 segments. The overlapping was more noticeable in the lower cervical segments after exclusion of the outlier data of the C5-6 segment in subject 1. CONCLUSIONS: The ICR in the cervical spine showed a trend of moving progressively more superiorly and anteriorly and the disc space overlapping the distribution circle of ICR increased along the lower motion segments except the C5-6 segment. These findings could provide a good basis for level-specific cervical arthroplasty designs.

Locating the Instant Center of Rotation in the Subaxial Cervical Spine with Biplanar Fluoroscopy during In Vivo Dynamic Flexion-Extension

BACKGROUND: Recently, biplanar fluoroscopy is used to evaluate the cervical kinematics, especially to locate the instant center of rotation (ICR) during in vivo motion. This study aims to ascertain the ICR at each cervical segment in the sagittal plane during dynamic motion and assess the differences from previous studies. METHODS: While three healthy subjects were performing full flexion-extension, two oblique views aligned horizontally and angled at approximately 55° were obtained by biplanar fluoroscopy. The minimum degree to detect significant movement in a helical axis model was set at 2°, and anterior-posterior and superior-inferior locations of each ICR were defined. To evaluate the possible distribution area and overlapping area of the ICR with disc space, we drew a circle by using the calculated distance between each coordination and the mean coordination of ICR as the radius. RESULTS: During flexion-extension motion, the mean superior-inferior location of the ICR became progressively more superior, except the C5–6 segment (p = 0.015), and the mean anterior-posterior location of the ICR became progressively more anterior without exception from C2–3 to C6–7 segments, but anterior-posterior ICR locations were not significantly different among segments. The overlapping area with the distribution circle of ICR was mainly located in the posterior half in the C3–4 segment, but the overlapping area was about 80% of the total disc space in C4–5 and C6–7 segments. The overlapping was more noticeable in the lower cervical segments after exclusion of the outlier data of the C5–6 segment in subject 1. CONCLUSIONS: The ICR in the cervical spine showed a trend of moving progressively more superiorly and anteriorly and the disc space overlapping the distribution circle of ICR increased along the lower motion segments except the C5–6 segment. These findings could provide a good basis for level-specific cervical arthroplasty designs.