Clin Orthop Surg.  2019 Dec;11(4):482-489. 10.4055/cios.2019.11.4.482.

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

• Kim SH
• Ham DW
• Lee JI
• Park SW
• Ko MJ
• Koo SB
• Song KS

• KMID: 2462544
• DOI: http://doi.org/10.4055/cios.2019.11.4.482

Abstract

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.

Keyword

Cervical spine; Rotation; Fluoroscopy; Kinematics; Instant center of rotation; Biplanar fluoroscopy

MeSH Terms

Arthroplasty
Biomechanical Phenomena
Fluoroscopy*
Healthy Volunteers
Radius
Spine*

Figure

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  Fig. 1 Biplanar fluoroscopic images aligned horizontally and angled at approximately 55° were collected from a system composed of two focal-spot radiograph tubes (KMC–1400ST; COMED Technologies Inc., Gwangju, Korea).

• 

  Fig. 2 (A) Three-dimensional reconstruction of the cervical spine from biplanar fluoroscopic projection. The arrow to superior direction represents Y axis; anterior, X axis; horizontal, Z axis. (B) The location of instant center of rotation at the C2–3 segment was calculated by using the (X, Y) coordinate system.

• 

  Fig. 3 The overall range of motion in subaxial cervical segments. Positive values mean flexion and negative values mean extension. The greater range of motion was observed in the C4–5 and C5–6 segments.

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  Fig. 4 (A) The radius of circle was measured as 6.3 mm. There was no overlapping area between the circle and intervertebral space in C2–3 segment. (B) The radius of circle was measured as 9.7 mm. The overlapping area was located in the posterior half of intervertebral space in C3–4 segment. (C) The radius of circle was measured as 11.2 mm. The overall posterior area and half of anterior intervertebral space was overlapped by the distribution circle in C4–5 segment. (D) The radius of circle was measured as 5.7 mm. There was no overlapping area between the circle and intervertebral space in C5–6 as in C2–3 segment. (E) The radius of circle was measured as 10.8 mm. The overall intervertebral space was overlapped by the distribution circle in C6–7 as in C4–5 segment. (F) The overall locations of ICR showed a trend of moving progressively superiorly and anteriorly in the lower cervical segments except in C5–6 segment.

• 

  Fig. 5 (A) The location of instant center of rotation of C5–6 segment was changed more superiorly and anteriorly after exclusion of the values of C5–6 segment in subject 1. (B) The overall intervertebral space was overlapped by the distribution circle after exclusion of the values of subject 1.

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