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Research Papers

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

[+] Author and Article Information
William J. Anderst

e-mail: anderst@pitt.edu

James D. Kang

Department of Orthopaedic Surgery,
University of Pittsburgh,
Pittsburgh, PA 15203

1Corresponding author.

Contributed by the Bioengineering Division of ASME for publication in the JOURNAL OF BIOMECHANICAL ENGINEERING. Manuscript received October 5, 2012; final manuscript received January 7, 2013; accepted manuscript posted January 29, 2013; published online May 9, 2013. Assoc. Editor: Yener N. Yeni.

J Biomech Eng 135(6), 061007 (May 09, 2013) (8 pages) Paper No: BIO-12-1464; doi: 10.1115/1.4023524 History: Received October 05, 2012; Revised January 07, 2013; Accepted January 29, 2013

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 flexion-extension using dynamic stereo X-ray and conventional reflective markers. Subject-specific 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.

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Figures

Grahic Jump Location
Fig. 1

The biplane X-ray system. X-ray tubes (left) directed X-rays through the subject to image intensifiers (right). 2.5 ms X-ray pulses (70 kV, 160 mA) were generated by cardiac cine-angiography generators at a rate of 30 Hz and radiographic images were collected by high-speed cameras synchronized to the X-ray pulses.

Grahic Jump Location
Fig. 2

The skull and cervical spine model for a representative subject in an (a) extended orientation, (b) the neutral position, and (c) a flexed orientation. Reflective markers (green) determined skull and torso position, while vertebral positions were determined from model-based tracking of biplane radiographs. Muscle forces (red vectors) were generated by flexor muscles (yellow spheres identify insertions) when the head was in an extended orientation and by extensor muscles (magenta spheres identify insertions) when the head was in a flexed orientation. The orange vector represents the weight of head. The teal vector represents the total force applied by the head to C2. Anatomic coordinate systems were generated within each vertebra (only C7 is shown for clarity).

Grahic Jump Location
Fig. 3

The total force made by the skull on C2 while the head moved in flexion (blue diamonds) and extension (red squares). Data points indicate the average of all subjects (asymptomatic plus arthrodesis). Error bars indicate intersubject variability (the 95% confidence interval of the mean), however, within-subject comparisons were performed to identify differences between flexion and extension movement directions. P values comparing flexion versus extension, adjusted for multiple comparisons, are provided for each 10 deg increment of head flexion-extension. The horizontal axis indicates head flexion-extension angle relative to the neutral position.

Grahic Jump Location
Fig. 4

The perpendicular distance from the midpoint of the C2 superior facets to the line of action of the flexor muscles during the flexion (solid lines) and extension (dashed lines) motion. Data points indicate the average of all subjects (asymptomatic plus arthrodesis). Positive values indicate a flexion moment, negative values indicate an extension moment. Error bars indicate intersubject variability (the 95% confidence interval of the mean), however, within-subject comparisons were performed to identify differences between flexion and extension movement directions and to identify differences among 10 deg increments of head extension. Each flexion value was significantly greater than the corresponding extension value for longus capitus (LGC) (all p ≤ 0.003) and infrahyoid (all p ≤ 0.002) but only at 0 deg, 10 deg, and 20 deg of extension for the SCM moment arm (p ≤ 0.033). For the SCM and LGC, the average moment arm (flexion + extension movement direction) significantly decreased at each 10 deg increment of head extension from neutral to 40 deg of extension (all p ≤ 0.020 and all p ≤ 0.001, respectively). The infrahyoid moment arm significantly increased each 10 deg of head flexion-extension from neutral to 40 deg of extension (all p ≤ 0.001).

Grahic Jump Location
Fig. 5

The perpendicular distance from the midpoint of the C2 superior facets to the line of action of the extensor muscles during the flexion (solid lines) and extension (dashed lines) motion. Data points indicate the average of all subjects (asymptomatic plus arthrodesis). Negative values indicate an extension moment. Error bars indicate intersubject variability (the 95% confidence interval of the mean), however, within-subject comparisons were performed to identify differences between flexion and extension movement directions and to identify differences among 10 deg increments of head extension. Each extension value was significantly greater than the corresponding flexion value for the trapezius (Trap) (all p ≤ 0.001), semispinalis (SSP) (all p ≤ 0.001), and splenius capitis (SPL) (all p ≤ 0.002). Each extension value was significantly greater than the corresponding flexion value for the rectus capitis posterior major (RCP) from 10 deg to 40 deg of head flexion (all p ≤ 0.022). The Trap and SPL average moment arms (flexion + extension movement direction) were significantly different at each 10 deg increment of head flexion (all p ≤ 0.016 and all p ≤ 0.038, respectively), while the SSP average moment arm decreased significantly at each 10 deg increment of head flexion from neutral to 30 deg flexion (all p ≤ 0.015). The rectus capitis posterior major moment arm significantly decreased from 30 deg to 40 deg of head flexion (p = 0.019).

Grahic Jump Location
Fig. 6

Sensitivity of the model to changes in the sternocleidomastoid attachment site on the skull. The vertical axis is the percent change in force applied to C2, relative to the force determined using the original attachment site. The attachment site was repositioned 8, 4, 2, or 1 mm anterior (+) or posterior (−) to the original attachment site. Changing the SSP and infrahyoid attachment sites yielded changes in the C2 force curves that were smaller in magnitude but similar in pattern.

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