The traumatic spondylolisthesis of the axis A biomechanical in vitro evaluation of an instability model and clinical relevant constructs for stabilization Markus Arand a, * , Sylvia Neller b , Lothar Kinzl a , Lutz Claes b , Hans Joachim Wilke b a Department of Trauma Surgery, Hand- and Reconstructive Surgery, University of Ulm, Steinh€ ovelstrasse 9, 89075 Ulm, Germany b Institute for Orthopaedic Research and Biomechanics, University of Ulm, Ulm, Germany Received 29 November 2001; accepted 14 May 2002 Abstract Objective. Stepwise destabilization of the traumatic spondylolisthesis C2 with an increasing anterior defect of C2–C3 was in- vestigated. The three-dimensional stabilizing capabilities of posterior transpedicle screw osteosynthesis and two anterior plate systems C2–C3, the H-plate and the titanium locking plate were tested. Design. A biomechanical in vitro study was performed using a standardized experimental protocol in a spine tester. Background. The extent of the instability of the traumatic spondylolisthesis of C2 within its different types remains unclear. Posterior and anterior approaches for stabilization exist for patients with isthmusfractures at C2, the stabilizing effect has not been demonstrated yet. Methods. The motion levels from C2–C3 in six human specimen were tested in flexion, extension, right and left lateral bending and left and right axial rotation. The specimens were tested intact, after destabilization and after stabilization. Results. In extension and axial rotation, each step of destabilization decreased the moment significantly, to achieve the range of motion of the intact specimen. In flexion a statistical significant destabilization after separation of the posterior longitudinal lig- ament was present. The flexibility tests showed an increasing range of motion of the posterior transpedicle screws, with increasing anteriorinstabilitymarkedlyinflexionandextension.AfterH-plateandlockingplatefixation,asignificantdecreaseoftherangeof motion resulted in flexion. The stiffness in flexion and extension increased consecutively, while in lateral bending and axial rotation the transpedicle screw fixation showed the highest stiffness. Conclusions/Relevance. The traumatic spondylolisthesis of C2 is a significantly unstable injury in case of additional segmental damage of C2–C3. Anterior stabilization in these injuries is mandatory. Ó 2002 Elsevier Science Ltd. All rights reserved. Keywords: Upper cervical spine; Spinal fusion; Instability; Biomechanics; Spinal stability; Hanged man’s fracture 1. Introduction The incidence of traumatic spondylolisthesis C2 or so-called ‘‘hanged man’s fracture’’ is most frequent in the lesions of the upper cervical spine, following the odontoid fracture [1,2]. The traumatic spondylolisthesis C2isahyperextensioninjury,inwhichtheoccipitalbase bends the posterior structures of C2 [3,4]. This causes the fracture at the weakest structure, the isthmus of C2 bilaterally. Depending on the amount of hyperexten- sion, additional damage of the anterior structures such as ligaments and the disc between C2 and C3 [5] may extend the instability. Different classifications for these fractures have been suggested, based on the dislocation of the vertebral arch C2 in relation to the vertebral body C2 and the verte- bral body C2 in relation to the vertebral body C3 [6,7]. Bucholz [8] was the first author, who divided the trau- matic lesions of the isthmus C2 into stable and unstable situations, concerning the integrity of the disc C2–C3. Effendi [6] referred his classification on the pretreat- ment radiologic configurations, which was finally mod- ified by Levine [9] in terms of consideration of trauma mechanisms, resulting in four different fracture types: * Corresponding author. E-mail address: markus.arand@medizin.uni-ulm.de (M. Arand). 0268-0033/02/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved. PII:S0268-0033(02)00037-2 Clinical Biomechanics 17 (2002) 432–438 www.elsevier.com/locate/clinbiomech