Multidirectional stabilizing potential of BAK interbody spinal fusion system for anterior surgery

Improvement in laparoscopic surgery requires spinal-fusion devices appropriately designed for this technique. The BAK interbody fusion device (Spine Tech Inc., Minneapolis, MN, U.S.A.), which consists of two titanium screw cages, meets this requirement. Multidirectional stabilizing potential of this device was investigated by using an in vitro human cadaveric model. Four fresh-frozen human lumbosacral spine specimens (L5-S1) were used. The flexibility test consisted of applying six pure moments (flexion, extension, bilateral axial torques, and lateral bending moments) and measuring the ensuing three-dimensional motion. Moments were applied in four load steps: 0, 2.5, 5.0, 7.5, and 10.0 Nm, and for three load and unload cycles. Motion of the top vertebra was recorded during the third load cycle by using a three-dimensional optoelectronic motion-measurement system. The motion parameters studied were the ranges of motion (ROM) and the neutral zone (NZ). Comparing the ROM of the intact specimen and after the fixation, all motions except extension were reduced significantly (p < 0.005). Average percentage decrease in ROM were 45.8% in flexion, 40.4% in axial rotation, and 65.6% in lateral bending. The only significant changes in NZ were a 255.7% increase in extension, a 90.9% increase in axial rotation, and a 70.8% decrease in lateral bending. This biomechanical study revealed that the BAK system provided decreases in ROM in all directions except in extension. The increased NZ in extension and axial rotation is most likely related to the positioning of the implant. Because these implants were placed from the anterior, damage to anterior annulus and anterior longitudinal ligament is inevitable. For clinical relevance, the patients undergoing this surgical procedure should avoid extension motions.