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The Effect of Soft-Tissue Restraints After Type II Odontoid Fractures in the Elderly: A Biomechanical Study

McCabe, C. Maxwell J. MD*; McLachlin, Stewart D. MESc†,‡; Bailey, Stewart I. MD*,§; Gurr, Kevin R. MD*,§; Bailey, Christopher S. MD*,§; Dunning, Cynthia E. PhD*,†,‡

doi: 10.1097/BRS.0b013e31823b4ed0

Study Design. A biomechanical analysis of soft-tissue restraints to passive motion in odontoid fractures.

Objective. To quantify the role of the C1–C2 facet joint capsules and anterior longitudinal ligaments (ALLs) in the setting of a type II odontoid fracture in the elderly.

Summary of Background Data. The odontoid process itself is the primary stabilizer at the C1–C2 level; however, little is known about the role of the soft-tissue structures that remain intact in the setting of an odontoid fracture after a low-energy mechanism.

Methods. Ten cadaveric C0–C2 spinal segments were studied. Specimens were tested under simulated axial rotation with an applied moment of ±1 Nm and with an application of a 10 N anteriorly directed force to the body of C2 to induce sagittal translation. Optical motion data were initially collected for the intact state and after a simulated dens fracture. The specimens were then divided into 2 groups, where 1 group underwent unilateral and then bilateral C1–C2 facet capsular injuries followed by an ALL injury. The second group underwent the ALL injury before the same capsular injuries. Changes in axial range of motion and C1–C2 translation were analyzed using 2-way repeated measures analyses of variance and post hoc Student-Newman-Keuls tests (α = 0.05).

Results. In axial rotation, there was an increase in range of motion by approximately 13%, with the fracture of the dens compared with the intact state (P < 0.05). An increase was also present for each subsequent soft-tissue injury state compared with the previous (P < 0.05); however, there was no difference found between the 2 sectioning protocols. For sagittal translation testing, it was found that the odontoid fracture alone showed an increase of 3 mm of C1–C2 translation compared with intact (P < 0.05). Further soft-tissue injuries did not show an increase until the complete injury state.

Conclusion. This study identifies that type II odontoid fractures without associated soft-tissue injury may be stable under certain loading modes.

The biomechanical stability provided by the soft tissues in a type II odontoid fracture was determined under applied axial rotation and sagittal C1–C2 translation loading. Changes in range of motion and translation with fracture were identified, but supporting soft-tissue restraints provide sufficient stability to C1–C2 despite the odontoid fracture.

*Division of Orthopaedics, Department of Surgery, The University of Western Ontario, London, Ontario, Canada;

Jack McBain Biomechanical Testing Laboratory, Thompson Engineering Building, The University of Western Ontario, London, Ontario, Canada;

Department of Mechanical and Materials Engineering, The University of Western Ontario, London, Ontario, Canada; and

§Orthopaedic Spine Program, London Health Sciences Centre, London, Ontario, Canada.

Address correspondence and reprint requests to Cynthia E. Dunning, PhD, Department of Mechanical and Materials Engineering, The University of Western Ontario, 1151 Richmond, St, London, ON, Canada N6A 5B9; E-mail:

Acknowledgment date: July 7, 2011. First revision date: September 26, 2011. Acceptance date: September 30, 2011.

The manuscript submitted does not contain information about medical device(s)/drug(s).

The Natural Sciences and Engineering Research Council of Canada, Canadian Foundation for Innovation, Ontario Innovation Trust, Lawson Health Research Institute, and The University of Western Ontario funds were received to support this work.

No benefits in any form have been or will be received from a commercial party related directly or indirectly to the subject of this manuscript.

© 2012 Lippincott Williams & Wilkins, Inc.