In Situ Demonstration of Atlantoaxial Instability

There are separate poster presentation times for odd and even posters.
Odd poster #s – first hour
Even poster #s – second hour

Abstract Body : INTRODUCTION AND OBJECTIVE: The functional roles of musculoskeletal structures in movement and movement-related disorders can be challenging to illustrate and understand. Because of its complexity, the upper cervical spine represents an anatomical region which can be as difficult for educators to teach as it is for learners to comprehend. Considering the potentially serious clinical consequences of upper cervical instability, teaching methods for demonstrating elements of this condition may benefit both instruction and learning. The intent of this descriptive study was to outline a novel technique used to depict in situ atlanto-axial (AA) instability.
MATERIALS AND METHODS: A median plane section through the head and neck of a 74-year-old male cadaver was performed. Inherent to this technique is transection of the transverse ligament of the atlas. With the axis held stable, changes in the atlantodental interval (ADI) – a proxy for AA instability – and patency of the vertebral canal were observed during the application of a posterior-to-anterior (PA) force to the posterior arch of the atlas. These observations were repeated while an anterior-to-posterior (AP) force was applied to the anterior arch of the atlas. To simulate a fracture, a horizontal cut was next made through the base of the odontoid process (OP) of the axis. The same loading techniques and observations were repeated. Images were captured during the application of these procedures under each condition.
RESULTS: During application of the PA force upon the atlas prior to the horizontal cut through the OP, an increase in the ADI occurred while a corresponding decrease in the patency of the vertebral canal was observed. In applying an AP force to the atlas under these same conditions, the ADI was reduced, and the patency of the vertebral canal normalized. In applying these same procedures after detaching the OP from the body of the axis, patency of the vertebral canal was profoundly obscured when either an PA or AP force was applied to the atlas although change in the ADI was not readily observed.
CONCLUSION: This study described a method for demonstrating AA instability through cadaver dissection. Excessive anterior translation of the atlas upon the axis was shown which was likely due, in part, to insufficiency of the transverse ligament of the atlas occurring because of the dissection technique. Posterior translation of the atlas upon the axis continued to be restrained, though, when the OP remained intact. Once the OP was separated from the axis, excessive anterior and posterior translation of the atlas upon the axis was exhibited.
SIGNIFICANCE/IMPLICATION: The described procedures allow for an in situ depiction of AA instability which may be useful for both instructors and learners exploring clinical disorders of the upper cervical spine. Future work could attempt to determine the effects, if any, of using of such a technique upon associated learning outcomes.
FUNDING SOURCES: This study received no specific funding from any agency in the public, commercial, or not-for-profit sectors.