Biomechanical evaluation of posterior lumbar dynamic stabilization: an in vitro comparison between Universal Clamp and Wallis systems

Abstract

Treatment of chronic low back pain due to degenerative lumbar spine conditions often involves fusion of the symptomatic level. A known risk of this procedure is accelerated adjacent level degeneration. Motion preservation devices have been designed to provide stabilization to the symptomatic motion segment while preserving some physiologic motion. The aim of this study was to compare the changes in relative range of motion caused as a result of application of two non-fusion, dynamic stabilization devices: the Universal Clamp (UC) and the Wallis device. Nine fresh, frozen human lumbar spines (L1-Sacrum) were tested in flexion-extension, lateral bending, and axial rotation with a custom spine simulator. Specimens were tested in four conditions: (1) intact, (2) the Universal Clamp implanted at L3-4 (UC), (3) the UC with a transverse rod added (UCTR), and (4) the Wallis device implanted at L3-4. Total range of motion at 7.5 N-m was determined for each device and compared to intact condition. The UC device (with or without a transverse rod) restricted motion in all planes more than the Wallis. The greatest restriction was observed in flexion. The neutral position of the L3-4 motion segment shifted toward extension with the UC and UCTR. Motion at the adjacent levels remained similar to that observed in the intact spine for all three constructs. These results suggest that the UC device may be an appropriate dynamic stabilization device for degenerative lumbar disorders.

Fig. 1

Implants tested in this study; a Universal Clamp with transverse rod; b Wallis

Fig. 2

Specimen mounted in the spine testing apparatus

Fig. 3

Shift in L3–4 neutral position after application of the Universal Clamp (UC), Universal Clamp with transverse rod (UCTR), and Wallis device for flexion–extension. The UC and UCTR shift to further extension was significantly different than intact (P < 0.05)

Fig. 4

Average range of motion at the instrumented level (L3–4) for the intact, Universal Clamp (UC), Universal Clamp with transverse connector (UCTR), and Wallis conditions. AR axial rotation, LB lateral bending. *P < 0.05 for comparisons between intact and instrumented conditions; +P < 0.05 for comparisons between UCTR and Wallis; **P < 0.05 for comparisons between UC and Wallis