Biomechanical analysis of direct posterior C1 screws compression reduction in the selective treatment of unstable atlas fractures

English Keywords:Atlantoaxial joint  Lateral mass screws  Biomechanics  Ligament

English Abstract:

  【Abstract】 Objectives: A biomechanical analysis was performed to investigate the stability of the direct posterior C1 lateral mass screws compression reduction for treatment of instable Jefferson fracture and provide the theoretical basis for clinical applications. Methods: Six fresh cadaveric whole upper cervical spine(UCS) specimens were harvested and embedded in polymethylmethacrylate(PMMA). Three dimensional movements including flexion, extension, right and left lateral bending as well as the C1-2 displacement in flexion(atlantodental interval, ADI) were measured on an electromechanical testing machine by stereophotogrammetry. Specimens were tested in the following states in sequence: the intact specimens were tested(group A), after destabilization of C1-C2, the specimens underwent C1 lateral mass screw fixation(group B), then specimens with transverse ligament and longitudinal ligaments transected(group C) were tested. Results: The motion of ADI, flexion and extension had no signicant difference(P>0.05) between group A and group B under the physiological loading(50-90N). At the loading of 100N and 150N, significant difference was detected between the two groups(P<0.05), at the loading of 100N and 150N, the average of ADI, flexion and extension was 2.76mm, 11.66°, 14.05° and 3.15mm, 15.27°, 16.25°. The motion of left and right lateral bending between group B and group A had no statistical difference indicated under physiological loading range. At the loading of 100N, the average of left lateral bending was 7.51°(P>0.05), whereas significant difference was detected when the loading was more than 150N(P<0.05). The motion of right lateral bending had significant difference when the loading was more than 100N, and at the loading of 100N, 150N, the average of left lateral bending was 8.43°, 10.67°(P<0.05). The three-dimensional motion of group C indicated obvious instability, significant difference was found in the comparison with group A and group B. Conclusions: The stability of atlantoaxial joint can be maintained by collaboration of series of ligaments. The longitudinal ligaments can maintain the stable of atlantoaxial joint under physiological loading range.