Biomechanical study of posterior plate-rod system for instable atlas fractures

English Keywords:Atlas fracture  Posterior fixation system  Range of motion  Biomechanics

English Abstract:

  【Abstract】 Objectives: To test the stability of atlas fracture fixed with posterior plate-rod system, and to provide the rationale for its clinical use. Methods: Six fresh human cadaveric upper cervical spine specimens(C0-C3) were harvested and embedded in polymethyl methacrylate(PMMA). Three dimentional movements including extension, flexion, left and right lateral bending, left and right axial rotation were tested on the mechanical testing machine. The movements were measured by range of motions(ROM). The C0-C1 and C1-C2 segments of specimens were distributed and tested in these status: the intact specimens(group A), the specimens with atlas fractures(group B, including the posterior arch fracture as subgroup B1 and Jefferson fracture as subgroup B2) and the specimens of group B with the posterior plate-rod system placement(group C). Then the three dimensional motion stability index(Sf) of group A was defined as 100% in the six types of motions. The Sf of group B and group C was calculated, also in six motions to compare the stability among different specimens. Results: Level of C0-C1: the ROM of group A of six movements was 11.06°±1.00°(extension, flexion), 7.08°±0.62°(left and right lateral bending), 13.24°±1.24°(left and right axial rotation) respectively. ROM of group B(including B1 and B2) increased significantly compared with that of group A(P<0.05). ROM of group C1 decreased significantly compared with that of group B1 in all movements(P<0.05). And the meaningful decrease could be seen between group C2 and B2(P<0.05). ROM of group C1 decreased compared with that of group A, but it was not significant(P>0.05). ROM of C2 increased insignificantly compared with that of group A(P>0.05). Level of C1-C2: the ROM of group A of six movements was 10.07°±1.12°(extension, /flexion), 5.56°±0.54°(left and right lateral bending), 20.8°±32.12°(left and right axial rotation) respectively. ROM of group B1 and B2 increased significantly compared with that of group A(P<0.05). ROM of group C1 decreased compared with that of group B1(P<0.05). When compared with group B2 in all the movements, ROM of group C2 decreased significantly(P<0.05). ROM of C1 decreased insignificantly compared with that of group A(P>0.05). ROM of C2 increased compared with that of group A, but the increase was not significant(P>0.05). In level of C0-C1, the Sf of group B1 was 71%(extension, /flexion), 70%(left and right lateral bending), 64%(left and right axial rotation) respectively. The Sf of group B2 was 61%(extension, /flexion), 55%(left and right lateral bending), 58%(left and right axial rotation). The Sf of group C1 of the movements was respectively 102%, 108%, 108%. The Sf of group C2 was respective 91%, 90%, 92%. Conclusions: After placing posterior plate-rod system, the stability of fractured atlas recovers. Moreover, the flexibility of the atlas is maintained, which has better outcome for Levine Ⅰ atlas fracture than Jefferson fracture.