Kinematic evaluation of atlantoaxial joint instability: an in vivo cineradiographic investigation

Biomechanical effect of the C2 laminar decortication on the stability of C2 intralaminar screw construct and biomechanical comparison of C2 intralaminar screw and C2 pars screw

BACKGROUND

There have been no reports of biomechanical stability of C1-2 constructs after decortication of the C2 lamina. In addition, few studies have compared the stability of C2 laminar screw and pars screw constructs.

OBJECTIVE

To compare the biomechanical stability of 3 different C1-2 construct conditions (C2 pars screw, C2 intralaminar screw, C2 intralaminar construct with C2 laminar decortication).

METHODS

Fourteen fresh-frozen cadaveric cervical specimens (C1-3) were used. In 7 specimens, pure moments of 1.5 Nm were applied in flexion/extension, lateral bending, and axial rotation. Each specimen was tested in the normal state, in the destabilized state (after odontoidectomy and resection of transverse atlantal ligament), and after application of constructs. After kinematic study, these 7 specimens underwent axial pullout strength testing of pars screw and 50% decorticated C2 intralaminar screws. In another 7 specimens, insertion torque and pullout strength were measured to compare the pars screw and intact C2 intralaminar screw.

RESULTS

: There were no statistically significant differences between the intact C2 intralaminar and 50% decorticated C2 intralaminar screw constructs in terms of range-of-motion limitations. The C2 pars screw construct was significantly superior to the C2 laminar screw construct in lateral bending (P < .01) and axial rotation (P < .01) and equivalent to the C2 laminar screw construct in flexion/extension (P = .42). There was no significant pullout strength difference between the 3 kinds of C2 screw.

CONCLUSION

The C1 lateral mass-C2 pars screws construct was stronger than the C1 lateral mass-C2 intralaminar screw construct. Decortication of C2 laminar (up to 50%) did not affect the immediate stability of the C1-2 construct.

The use of Roentgenstereofluorogrammetry to predict the 3-D spatial coordinates of points in low speed events

Imaging technologies such as cine-radiography, cine-MRI, and X-ray stereo photogrammetry have become popular diagnostic tools in biomechanical studies of musculoskeletal systems. However, their widespread use for research purposes has been restricted due to their high cost and somewhat limited availability. In an attempt to develop a reliable low cost system, a dual-fluoroscopic system capable of tracking the 3-D spatial motion of discrete landmark points in real time was developed. A simple methodology was developed to convert the analog fluoroscopic images to digital files for post-processing. A custom computer code based on the principles of X-ray stereo photogrammetry was also developed to predict 3-D coordinates from the 2-D images from the individual flouroscopes. The goal of the current study was to assess the accuracy and resolution of this system by using it to predict the motion of a test point following a known curvilinear trajectory. Our system predicted the time-varying motion and path of the test point within 0.25%. However, the current system is limited to studying low speed events only (max event frequency of 3Hz) due to the limited sampling frequency of the A/D conversion employed here.