Anterior transdiscal axial screw fixation for subaxial cervical spine: A biomechanical study.

OBJECTIVE: To evaluate stability of anterior transdiscal axial screw (ATAS) fixation for anterior instrumentation and to compare with standard anterior cervical decompression and fusion and plate (ACDFP) fixation in human subaxial cervical spine. METHODS: Flexibility tests were conducted on 7 cadaveric specimens (C5-T1) in an intact and injured state and instrumented with ACDFP fixation, ATAS fixation, and ACDFP plus ATAS fixation at the C6-7 segment after section of the anterior and posterior longitudinal ligaments and discectomy. A pure moment of +/- 2.0 N-m was applied to the specimen in flexion-extension, lateral bending, and axial rotation. Range of motion (ROM) and neutral zone were calculated for the C6-7 segment. RESULTS: ROM was reduced significantly compared with the intact or injured condition for 3 configurations under all motions. ATAS fixation resulted in similar ROM in C6-7 compared with ACDFP fixation in flexion (2.7 degrees vs. 2.6 degrees, P = 0.419), extension (2.7 degrees vs. 2.1 degrees, P = 0.152), and lateral bending (4.6 degrees vs. 4.2 degrees, P= 0.295) but largerROMin axial rotation (6.1 degrees vs. 5.3 degrees, P= 0.014). When combined with an anterior plate, ATAS fixation reduced ROMto 1.2 degrees in flexion, 1.1 degrees in extension, 3.3 degrees in lateral bending, and 3.8 degrees in axial rotation, which were significantly smaller than ACDFP or ATAS fixation alone. CONCLUSIONS: ATAS fixation is a biomechanically effective alternative or supplemental method of anterior fixation in subaxial cervical spine.