Cervicothoracic Segment Fusion and Its Effects on Adjacent-Level Range of Motion: A Biomechanical Study Comparing Interlaminar Hooks, Lateral Mass Screws, and Side Connectors.

BACKGROUND AND

OBJECTIVES

To investigate the effects of adjacent-segment biomechanics at the cervicothoracic junction using different fixation constructs.

METHODS

Six human cadaveric spine specimens (C5-T4) were analyzed for range of motion (ROM) in the intact condition. All specimens were destabilized and reconstructed using 4.0-mm pedicle screws at T1-T3. Two constructs included interlaminar hooks at C6 and C7 with 3.5- or 4.0-mm rods (C6-T3). Two constructs included lateral mass screws at C6 and pedicle screws at C7, with 3.5- or 4.0-mm rods (C6-T3). Two screw-connector-rod constructs included lateral mass screws at C6 and pedicle screws at C7, with 3.5- or 4.0-mm rods; 1 rod spanned C6-C7 with a connector to a second rod of the same size spanning T1-T3. ROMs at upper and lower adjacent levels were compared with the intact condition.

RESULTS

Upper adjacent ROMs for C6-T3 screw-rod fixation with either rod size did not differ from the intact condition in any direction ( P > .78). Screw-connector-rod fixation ROM with either rod size did not differ from the intact condition in any direction ( P > .07). ROMs for upper adjacent hook-rod constructs were significantly less for both rod sizes than that of the intact condition in all directions ( P < .006). The lower adjacent constructs did not differ with either rod size from the intact condition ( P > .3).

CONCLUSION

The adjacent-segment motion magnitude was dictated by construct composition at the cervicothoracic junction. Although screw constructs provided an adjacent-level ROM most like that of the intact condition, hook constructs provided a smooth transition by limiting adjacent-segment ROM.