MULTIDIRECTIONAL STABILIZING POTENTIAL OF BAK INTERBODY SPINAL-FUSIONSYSTEM FOR ANTERIOR SURGERY

Improvement in laparoscopic surgery requires spinal-fusion devices app ropriately designed for this technique. The BAK interbody fusion devic e (Spine Tech Inc., Minneapolis, MN, U.S.A.), which consists of two ti tanium screw cages, meets this requirement. Multidirectional stabilizi ng potential of this device was investigated by using an in vitro huma n cadaveric model. Four fresh-frozen human lumbosacral spine specimens (L5-S1) were used. The flexibility test consisted of applying six pur e moments (flexion, extension, bilateral axial torques, and lateral be nding moments) and measuring the ensuing three-dimensional motion. Mom ents were applied in four load steps: 0, 2.5, 5.0, 7.5, and 10.0 Nm, a nd for three load and unload cycles. Motion of the top vertebra was re corded during the third load cycle by using a three-dimensional optoel ectronic motion-measurement system. The motion parameters studied were the ranges of motion (ROM) and the neutral zone (NZ). Comparing the R OM of the intact specimen and after the fixation, all motions except e xtension were reduced significantly (p < 0.005). Average percentage de crease 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 e xtension. The increased NZ in extension and axial rotation is most lik ely 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 pati ents undergoing this surgical procedure should avoid extension motions .