Three-dimensional motion analysis with Synex - Comparative biomechanical test series with a new vertebral body replacement for the thoracolumbar spine

The authors present a new implant for vertebral body replacement in the tho racic and lumbar spine. Synex is a titanium implant designated for reconstr uction of the anterior column in injury, post-traumatic kyphosis or tumour of the thoracolumbar spine and must be supplemented by a stabilizing implan t. After positioning, the implant is distracted in situ, thus ensuring best contact with adjacent end-plates and three-dimensional (3D) stability, and minimizing the possibility of secondary dislocation or loss of correction. We compared the effectiveness of the Synex implant with that of the "Harms cage" (MOSS) in combination with two alternative stabilizing instrumentati ons: the USS and Ventrofix. In a 3D spinal loading simulator, we determined the bisegmental (T12-L2) neutral zone (NZ), elastic zone (EZ), and range o f motion (ROM) of 12 human cadaveric spines. After corpectomy of L1, we tes ted the four possible combinations of stabilizing instrumentation and verte bral replacement implant: USS/Synex, USS/MOSS, Ventrofix/Synex, Ventrofix/M OSS. We analysed the differences between each of the instrumentations as we ll as differences compared to the intact spine. Comparing the two stabilizi ng implants, a significantly higher stability was achieved with the USS for flexion, extension, and lateral bending, regardless of whether Synex or MO SS was used as vertebral body replacement. No differences were observed for axial rotation. In addition, no implant combination was able to restore th e rotational stability of the intact spine. Comparing the vertebral body re placing implants, significantly higher stability was noted with Synex in co mbination with USS for extension, lateral bending, and axial rotation. No d ifferences between Synex and MOSS were observed in combination with Ventrof ix. Posterior fixation was found to offer superior stability compared to an terior fixation. Synex was at least comparable to MOSS for suspensory repla cement of the vertebral body in the thoracolumbar spine. The increased biom echanical stability demonstrated for Synex suggests that a more rigid const ruction would also be achieved in vivo. When using MOSS in combination with posterior stabilization, the induction of intervertebral compression via t he posterior fixator is recommended. This surgical step was not necessary w ith Synex.