SPINE Technique Assessment NEUROSURGERY VOLUME 64 | OPERATIVE NEUROSURGERY 1 | MARCH 2009 | ons137 Rogério Rocha, M.D. Spinal Biomechanics Laboratory, Barrow Neurological Institute, St. Joseph’s Hospital and Medical Center, Phoenix, Arizona Anna G.U. Sawa, M.S. Spinal Biomechanics Laboratory, Barrow Neurological Institute, St. Joseph’s Hospital and Medical Center, Phoenix, Arizona Seungwon Baek, M.S. Spinal Biomechanics Laboratory, Barrow Neurological Institute, St. Joseph’s Hospital and Medical Center, Phoenix, Arizona Sam Safavi-Abbasi, M.D., Ph.D. Spinal Biomechanics Laboratory, Barrow Neurological Institute, St. Joseph’s Hospital and Medical Center, Phoenix, Arizona Fadimatou Hattendorf, M.S. Spinal Biomechanics Laboratory, Barrow Neurological Institute, St. Joseph’s Hospital and Medical Center, Phoenix, Arizona Volker K.H. Sonntag, M.D. Division of Neurological Surgery, Barrow Neurological Institute, St. Joseph’s Hospital and Medical Center, Phoenix, Arizona Neil R. Crawford, Ph.D. Spinal Biomechanics Laboratory, Barrow Neurological Institute, St. Joseph’s Hospital and Medical Center, Phoenix, Arizona Reprint requests: Neil R. Crawford, Ph.D., c/o Neuroscience Publications, Barrow Neurological Institute, 350 W. Thomas Road, Phoenix, AZ 85013. Email: neuropub@chw.edu Received, March 24, 2008. Accepted, August 15, 2008. A tlantoaxial rotatory subluxation, also called atlantoaxial fixation in the pedi- atric population (10, 11, 27), can occur when forces applied to the head cause C1 and C2 to rotate excessively and displace relative to each other. The vertebrae then become locked in this displaced configuration. Depending on the axis of rotation, the subluxation can be uni- lateral anterior, unilateral posterior, or bilateral (1 anterior and 1 posterior). That is, one or both of the articular masses on C1 can rotate and become displaced relative to the corresponding articular mass(es) on C2 (10). Fixed atlantoax- ial subluxation is often misdiagnosed and managed incorrectly (33). Most cases can be managed with conservative measures such as halo fixation; however, persistent instability can require atlantoaxial arthrodesis (18, 32, 33). In the past 2 decades, the introduction of posterior atlantoaxial screw fusion techniques has significantly improved the outcome of pos- terior C1–C2 arthrodesis (6, 9, 13, 17). Previous laboratory studies have evaluated the biome- chanical effectiveness of various posterior A TLANTOAXIAL ROTATORY SUBLUXATION WITH LIGAMENTOUS DISRUPTION: A BIOMECHANICAL COMPARISON OF CURRENT FUSION METHODS OBJECTIVE: We evaluated the biomechanical effects of 4 instrumented configurations after induced atlantoaxial rotatory subluxation: transarticular screw fixation (T/A) and polyaxial C1 lateral mass and C2 pedicle screw and rod fixation (LC1-PC2) for atlantoax- ial arthrodesis with unilateral and bilateral instrumentation. METHODS: Three-dimensional intervertebral motion was tracked stereophotogram- metrically while 14 human cadaveric spine specimens underwent nonconstraining pure moment loading. Nondestructive loads were applied quasi-statistically in 0.25-Nm increments to a maximum load of 1.5 Nm during flexion-extension, right and left axial rotation, and right and left lateral bending. Hyperrotation injuries were created using tor- sional loads applied during left axial rotation until visible failure occurred. RESULTS: In the normal condition, the values for angular range of motion, lax zone (zone of ligamentous laxity), and stiff zone (zone of ligamentous stretching) were sim- ilar in both groups in all directions of loading, with no significant differences (P  0.05) between groups at C0–C1 or C1–C2. Both instrumentation systems (bilateral configu- rations) substantially stabilized angular motion at C1–C2 (P  0.05) during all loading modes for the T/A group, and during all but right lateral bending (P = 0.072) for the LC1-PC2 group. The mean failure load for both intact and instrumented specimens was slightly greater, but not significant for the LC1-PC2 group compared with the T/A group (P  0.14). CONCLUSION: Both methods fixated atlantoaxial subluxation equally well. Compared with unilateral instrumentation, a bilateral configuration with the LC1-PC2 technique significantly increased stability during extension (P  0.05). During axial rotation, bilat- eral T/A screws significantly increased stability compared with unilateral fixation (P  0.02). KEY WORDS: C1 lateral mass screw, Posterior atlantoaxial stabilization, Transarticular screws Neurosurgery 64[ONS Suppl 1]:ons137–ons144, 2009 DOI: 10.1227/01.NEU.0000335778.31497.5B ABBREVIATIONS: BMD, bone mineral density; LC1-PC2, polyaxial C1 lateral mass and C2 pedi- cle screw and rod fixation; LZ, lax zone; ROM, range of motion; SZ, stiff zone; T/A, transarticular screw fixation