Subject-Specific Inverse Dynamics of the Head and Cervical Spine During in Vivo Dynamic Flexion-Extension

Abstract

The effects of degeneration and surgery on cervical spine mechanics are commonly evaluated through in vitro testing and finite element models derived from these tests. The objectives of the current study were to estimate the load applied to the C2 vertebra during in vivo functional flexion-extension and to evaluate the effects of anterior cervical arthrodesis on spine kinetics. Spine and head kinematics from 16 subjects (six arthrodesis patients and ten asymptomatic controls) were determined during functional flexionextension using dynamic stereo X-ray and conventional reflective markers. Subjectspecific inverse dynamics models, including three flexor muscles and four extensor muscles attached to the skull, estimated the force applied to C2. Total force applied to C2 was not significantly different between arthrodesis and control groups at any 10 deg increment of head flexion-extension (all p values ! 0.937). Forces applied to C2 were smallest in the neutral position, increased slowly with flexion, and increased rapidly with extension. Muscle moment arms changed significantly during flexion-extension, and were dependent upon the direction of head motion. The results suggest that in vitro protocols and finite element models that apply constant loads to C2 do not accurately represent in vivo cervical spine kinetics.

Cervical spine intervertebral kinematics with respect to the head are different during flexion and extension motions

“…Future studies are necessary to assess the kinematic results (bone kinematics and tissue-level kinematics) when imaging static and dynamic activities, and to assess the effect of movement direction on kinematics in other joints. Third, considering the fact that musculoskeletal models of the head and cervical spine assume the active muscles are determined by head angle, not movement direction (Anderst et al, 2013, in press; Liu et al., 2007), the current results suggest that these models should account for the direction of head motion when determining muscle moment arms because vertebral orientations (and therefore muscle attachment sites) are dependent on the direction of head motion. The direction-dependent differences at each motion segment are cumulative, leading to substantially different spine configurations for identical head orientations when moving in flexion or extension.…”

“…In order to address these limitations associated with static imaging of the spine, imaging during dynamic, functional motion has become more common (Anderst et al, 2013, in press; McDonald et al, 2010; Reitman et al, 2004; van Mameren et al, 1992; Wu et al, 2010). Previous dynamic imaging studies of the cervical spine have focused entirely on intervertebral kinematics while neglecting to investigate the relationship between head motion and intervertebral motion.…”

“…Detailed knowledge of the relationship between head and vertebral kinematics is necessary to optimize musculoskeletal models that require accurate head and vertebral kinematics to determine moment arms for inverse dynamics calculations (Liu et al, 2007; Mathys and Ferguson, 2012; Moroney et al, 1988). A common assumption of these musculoskeletal models is that the active muscles are determined by the head angle, not the direction of movement (Anderst et al, 2013, in press; Liu et al, 2007; Moroney et al, 1988). Therefore, it is critical to determine if the relationship between head angle and intervertebral orientation (and therefore muscle moment arms) is affected by movement direction.…”

Narrative review of the in vivo mechanics of the cervical spine after anterior arthrodesis as revealed by dynamic biplane radiography

“…). This novel imaging and tracking technology has recently been combined with custom computational algorithms to characterize cervical spine mechanics in young healthy adults, in middle‐aged asymptomatic adults and arthrodesis patients, and in arthrodesis and arthroplasty patients …”

“…Using radiographic images collected continuously during the dynamic movement, dynamic biplane radiography has demonstrated that, in the sub‐axial cervical spine of asymptomatic subjects, mid‐range flexion/extension is dominated by C3/C4 and C4/C5 motion, while contributions from C5/C6, and C6/C7 motion segments increase near the start, and the end of the full ROM, when loads on the spine are greatest (Fig. B).…”