The Unstable Spine: A Surgeon's Perspective.

Instability of the spine is a complex clinical entity that exists on a wide spectrum encompassing many aspects of spinal pathology including traumatic, neoplastic, infectious, and degenerative processes. The importance of determining stability is paramount in the decision-making process regarding the need for operative or nonoperative care. Defining clinical instability can be a challenging and requires careful attention to the pathology involved, findings of necessary imaging, and a thorough clinical exam. Several classification systems have been developed to aid in surgical decision making, but certain limitations exist. Various imaging modalities play a crucial role in the evaluation of suspected instability. Computed tomography is the initial imaging modality of choice in the traumatic setting. Magnetic resonance imaging is an important adjunct in the setting of suspected ligamentous injury and the modality of choice in suspected infectious and neoplastic processes. Upright radiographs can be particularly useful in the setting of acute or subacute instability to glean information about how the spine responds to gravity and weightbearing. The clinical exam is also of critical importance in the determination of stability. The presence of a neurologic deficit is highly suggestive of a potentially unstable spine and appropriate spinal precautions should be maintained until instability and injury has been ruled out. Certain clinical entities, such as ankylosing spondylitis and diffuse idiopathic skeletal hyperostosis, are at high risk for instability particularly in the traumatic setting. In these situations, the spine should be considered unstable until proven otherwise. Ultimately, the determination of spinal stability, and subsequent need for surgical treatment, should be based on the individual case. Combining information from the clinical exam and imaging findings, including upright radiographs when appropriate, allows for the appropriated determination of spinal stability.