Biomechanical comparison of adjacent segmental motion after ventral cervical fixation with varying angles of lordosis.

BACKGROUND CONTEXT: Complications, such as graft subsidence and adjacent segment degeneration, are not uncommon after ventral cervical fusion. It has been theorized, but not proven, that sagittal alignment may affect this process. It is therefore hypothesized that increasing lordosis during anterior cervical fusion decreases adjacent segment motion (ASM) and thus decreases the rate of adjacent disc degeneration. A study was designed to test the first portion of this hypothesis; ie, that increasing lordosis during anterior cervical fusion decreases ASM.

PURPOSE: To determine the effect on the adjacent segment motion (ASM) after ventral cervical spine fusion obtained by varying the angle of lordosis using interbody spacers with different heights (small: 6-mm interbody spacer; large: 9-mm interbody spacer).

STUDY DESIGN: A biomechanical study comparing the segmental motion at adjacent disc levels after cervical fusion with varying angles of lordosis. Sample and outcome measures: six human cadaveric spines C2-C7, range of motion (ROM).

METHODS: Six fresh human cadaveric cervical spines (C2-C7) were embedded at C2 and C7 and biomechanically tested to 0.7 Nm flexion and 0.5 Nm extension. Lordosis was measured at C4-C5 from radiographs; range of motion (ROM) at C3-C4, C4-C5, and C5-C6 was measured using markers during flexion and extension in the intact state, after ventral cervical fixation at C4-C5 with a small (6-mm) and with a large (9-mm) interbody spacer. A repeated measures analysis of variance was used to compare lordosis and the ROM for the different states.

RESULTS: Six cervical spines with a mean age of 55.3+/-1.6 years were studied. The mean sagittal angles of the specimens measured at C4-C5 using the Cobb angle method were -6.4+/-1.3 degrees intact, -8.8+/-1.4 degrees with small interbody spacer (intact vs. small spacer p=.02), and -12.4+/-0.9 degrees with large interbody spacer fixation (intact vs. large spacer p=.005). The lordotic angle of the specimens was lowest in the intact state, higher with the small spacer, and highest with the large spacer. The greatest ROM in the intact state testing was at C4-C5 (10.6+/-1.3 degrees), followed by at C5-C6 (7.2+/-1.5 degrees), and then at C3-C4 (7.1+/-0.9 degrees). After C4-C5 fusion, the ROM at C3-C4 and C5-C6 was significantly increased with the small spacer only. No significant change in ROM was observed with the large spacer. The greatest overall ROM (all three motion segments) was observed in the intact state (24.9+/-1.8 degrees), followed by the small spacer (21.4+/-2.0 degrees) and the large spacer (15.1+/-1.7 degrees).

CONCLUSIONS: Under the conditions of this study, there is a significant increase in ASM with the achievement of a modest increase in lordosis (small spacer) that is not observed with a greater increase in lordosis (large spacer).