Biomechanical effect of 4-rod technique on lumbosacral fixation: an in vitro human cadaveric investigation.

STUDY DESIGN: An in vitro biomechanical study of 3 lumbosacral fixation techniques in human cadaveric lumbar-pelvic spine models.

OBJECTIVE: To compare the in vitro biomechanical effect of a novel 4-rod lumbosacral reconstruction technique with conventional techniques in a human cadaveric lumbopelvic model, and to evaluate the benefit of adding supplementary rod fixation.

SUMMARY OF BACKGROUND DATA: Spinopelvic fixation involving the sacrum remains a difficult clinical challenge. Numerous lumbopelvic reconstruction methods based on the Galveston 2-rod technique have been proposed. Recently, a novel technique using supporting longitudinal rods across the lumbopelvic junction was reported. However, no comparative in vitro biomechanical testing was performed to evaluate the benefit of adding supplementary fixation at the L5-S1 levels.

METHODS: Seven fresh-frozen cadaveric lumbar-pelvic spines were prepared and tested for bone mineral density. The intact cadavers underwent a flexibility test, followed by insertion of the instrumented construct. Three constructs were tested: S1 screws alone (group 1), S1 screws plus iliac screws (group 2), and the 4-rod technique (group 3). Rotational angles of the L1-S1 and L5-S1 segments were measured to study the stability of the 3 lumbosacral fixation constructs compared with the intact spine. Nondestructive, multidirectional flexibility tests that included 4 loading methods followed by a destructive flexural load to failure were performed using an material testing machine. The lumbosacral peak range of motion (ROM) (millimeters or degrees) and ultimate failure load (Nm) of the 3 reconstruction techniques were statistically compared using a 1-way analysis of variance combined with a Student-Newman-Keuls post hoc test.

RESULTS: The average bone mineral density of the 7 specimens was 0.81 ± 0.09 g/cm. The ROM of the 3 fixation constructs was significantly smaller than that of the intact group in all 6 directions (P < 0.05). In lateral bending, the ROM of groups 2 and 3 was significantly smaller than that of group 1 (P < 0.05), but groups 2 and 3 were not significantly different from each other (P > 0.05). In flexion-extension, the ROM of groups 1 and 3 was significantly smaller than group 2 (P < 0.05), but groups 1 and 3 were not significantly different from each other (P > 0.05). In axial rotation, the ROM of group 3 was significantly smaller than those of groups 1 and 2 (P < 0.05), but groups 1 and 2 were not significantly different from each other (P > 0.05).

CONCLUSION: The 4-rod technique achieved stable biomechanical effects in lumbosacral fixation. At the L5-S1 junction, the 4-rod technique demonstrated better stability than the constructs using S1 screws or S1 screws plus iliac screws..