#296 Do Facets Govern Flexion-extension Motion Pattern in the Cervical Spine? Implications to Cervical Prosthesis Design

General Session: Cervical Motion Preservation

Presented by: A. Patwardhan

Author(s):

A.G. Patwardhan (1)
T. Potluri (2)
B. McIntosh (1)
P. Tsitsopoulos (1)
L.I. Voronov (1)
G. Carandang (2)
M.R. Zindrick (1)
R.R. Havey (1)

(1) Loyola University Chicago, Orthopaedic Surgery and Rehabilitation, Maywood, IL, USA
(2) Edward Hines VA Hospital, Hines, IL, USA

Abstract

Introduction: Cervical vertebrae undergo a combination of angular and translational motions in flexion-extension. In the lower cervical segments (such as C6-C7) there is less translation for a given amount of angular motion and hence the Center of Rotation (COR) is located closer to the disc space than in the upper segments (e.g., C3-C4), where the COR is located much more caudal. Does the geometry of facets have anything to do with these differences in cervical motion patterns? The answer to this question will have relevance to the effect of cervical disc prosthesis designs on long term health of facet joints after total disc replacement. In this study, we investigated the correlation between the location of the COR a parameter that describes the pattern of flexion-extension motion and two parameters that define facet geometry: sagittal facet inclination, and facet joint location relative to the disc space.

Methods: We tested nine fresh human cadaveric cervical spine specimens (C3-T1, 42±10.5 years) yielding kinematics of 45 cervical motion segments. Spines were tested in flexion-extension and segmental motions were measured using optoelectronic instrumentation. 3D CT reconstruction of each specimen was used to define the facet planes and superior endplate plane of each vertebra so that facet inclination in the sagittal plane could be measured. Facet location was defined by measuring the anterior-posterior (A-P) and Cranial-Caudal (C-C) locations of the facet joint midpoint relative to the superior endplate of the inferior vertebra. COR locations for each cervical motion segment from neutral-to-flexion were calculated using the vertebral motion data. The A-P and C-C coordinates of the COR were correlated with the facet angles as well as the facet joint location using multiple linear regression.

Results: The sagittal facet angle made by the facet plane with the superior endplate decreased, therefore the facet plane became steeper at caudal levels (P=0.017). The facet joint position along the anterior-posterior direction did not vary by cervical level (P=0.414). However, the C-C facet joint position varied significantly by level (P< 0.001); the facet joint position relative to the disc moved cranially at more caudal levels. The COR location along the A-P direction did not vary by level (0.944); whereas the COR location along the C-C direction varied significantly by level (P< 0.001). The COR location moved cranially at caudal levels. Regression analyses showed the COR moves cranially towards the disc as (1) the facets become steeper at lower cervical levels (P=0.034); and (2) the facet joints move cranially relative to the disc (P< 0.001). Both variables together explained nearly 50% of the variability in COR location along the C-C direction (P< 0.001, R2=0.46).

Conclusions: Facet geometry and the COR location seem coupled in a characteristic manner in a cervical segment. In consideration of cervical arthroplasty, inconsistency in the COR location of a disc prosthesis with that of the native segment may lead to incompatibility with facet geometry, and hence to abnormal motions and facet loading. Six degree of freedom disc designs, in theory, should allow a better match to the native facet anatomy.