Biomechanical stability with a new implant for vertebral body replacement.3-dimensional motion analysis on human cadaveric spines

The authors present a new implant for vertebral body replacement in the tho racic and lumbar spine. The titanium implant is designated for reconstructi on of the anterior column in injury, posttraumatic kyphosis or tumor of the thoracolumbar spine. The instrumentation has to be supplemented by a stabi lizing implant. After positioning, the implant is distracted in situ, throu gh which best contact to adjacent end-plates and 3-dimensional stability sh ould be provided. The possibility of secondary dislocation or loss of corre ction should thereby be minimized. Objectives. We investigated the biomechanical 3-dimensional stability in vi tro, using Synex(TM) in combination with an anteriorly (Ventrofix(TM)) or a posteriorly (USS(TM)) stabilizing implant. The differences between both st abilizing implants were to be determined. Synex(TM) was compared with the " Harms titanium mesh cage" (MOSS(TM)) as vertebral body replacement. Methods. In a 3-dimensional spinal loading simulator, we determined the bis egmental (T12-L2) neutral zone (NZ), elastic zone (EZ), and range of motion (ROM) of 12 human cadaveric spines. After corpectomy of L1 we tested 4 gro ups of implant combinations: USS(TM)/Synex(TM), USS(TM)/MOSS(TM), VentrofiX (TM)/Synex(TM), Ventrofix(TM)/MOSS(TM). We analyzed the diferences between each of the instrumentations as well as differences compared to the intact spine. Results. In most directions, significantly higher stability was achieved wi th USS(TM), compared with Ventrofix(TM) and the intact specimen. For axial rotation, with no instrumentation the stability of the intact spine was res tored. With Synex(TM) a significantly higher stability was noted for extens ion, lateral bending, and axial rotation in comparison with the Harms cage. A tendency towards more stability for flexion was additionally observed wi th Synex(TM). When using MOSS(TM) in combination with USS(TM), it was neces sary to perform a third operative step for induction of intervertebral comp ression via the posterior fixator. Conclusions. The posterior fixation was found to offer superior stability c ompared to the anterior one. Synex(TM) was at least comparable to MOSS(TM) for suspensory replacement of the vertebral body in the thoracolumbar spine . The evidence of higher biomechanical stability with Synex(TM) leads to th e probability of a higher rigidity in vivo. Due to the distractability of S ynex(TM), a better intervertebral compression was achieved. Therefore, an a dditional tightening of the posterior fixator after insertion of Synex(TM) was not necessary, in contrast to the Harms cage.