BACKGROUND: The purpose of this biomechanical in vitro study was to compare the kinematics and intradiscal pressure achieved with 2 methods: L4–L5 pedicle screw-rod fixation (PSRF) with an upper L3–L4 Coflex device and L4–L5 PSRF alone. The results were used to characterize the biomechanics of the topping-off operation with a Coflex device for the lumbar motion segment adjacent to single-level rigid fixation.

MATERIAL AND METHODS: Six human cadaveric spine specimens were biomechanically tested in vitro (6 males, 0 females). The 3-dimensional specimen motion in response to applied loads during flexibility tests was determined. Loads were applied along anatomic axes to induce flexion-extension, lateral bending, and axial rotation. All specimens were first studied with intact lumbar motion segments, then with L4–L5 PSRF alone, and finally with L4–L5 PSRF with an upper L3–L4 Coflex device. A non-paired comparison of the 3 configurations under 3 different conditions was made.

RESULTS: PSRF, with or without a Coflex device, significantly increased the range of motion (ROM) in the upper adjacent motion segments in all directions of loading. The intradiscal pressure (IDP) changed slightly. A correlation analysis showed that the ROM and IDP are significantly positively correlated. The application of the upper motion segment of the Coflex device provided greater stability in all directions of motion than did PSRF alone, particularly for extension (p<0.05), while use of a Coflex device did not significantly decrease the IDP compared with PSRF alone (p>0.05).

CONCLUSIONS: These results suggest that L4–L5 PSRF with an L3–L4 Coflex device is more stable than L4–L5 PSRF alone. PSRF with an upper Coflex device is a promising alternative to PSRF alone. Based on these biomechanical tests, it might be considered a protective method to prevent adjacent segment degeneration (ASD), although some limitations with this in vitro study must be addressed in the future.

Keywords: Biomechanical Phenomena, Intervertebral Disc - physiology, Lumbar Vertebrae - physiology, Orthopedic Fixation Devices, Range of Motion, Articular, Torque