The AOSpine Sacral Fracture Classification
Authors: Bellabarba Carlo , Schroeder Gregory D. , Kepler Christopher K. , Kurd Mark F. , Kleweno Conor P. , Firoozabadi Reza , Chapman Jens R. , Kandziora Frank , Schnake Klaus J. , Rajasekaran S. , Holstein Jorg H. , Sagi Henry C. , Dvorak Marcel F. , Vialle Luiz R. , Oner F. C. , Vaccaro Alexander R. , Krieg James C. 
Published: April 2016

Introduction Sacral fractures are complex and heterogeneous injuries that often include involvement of the lumbar spine and/or pelvis. Due to their complex nature, no comprehensive classification system has been accepted. Material and Methods The AOSpine Trauma Knowledge Forum partnered with orthopaedic traumatologists from AOTrauma to develop a straightforward, hierarchical classification system for sacral fractures. The classification was developed via a consensus process of clinical experts, and, prior to finalizing the classification system, a survey was sent to all members of AOSpine and AOTrauma asking for their input on key parts of the classification. Results The new AOSpine Sacral Classification is a hierarchical classification that follows the same structure as the subaxial and thoracolumbar classifications. First injuries are broadly divided into three types: Type A—Lower Sacro-coccygeal Injuries; Type B—Posterior Pelvic Injuries and Type C—Spino-Pelvic Injuries. Type A injuries have no impact on posterior pelvic or spinopelvic instability, however higher grade injuries may be associated with neurologic injuries. Type A injuries are divided into three subtypes; A1—Coccygeal or compression injuries as well as ligamentous avulsion fractures; A2—Non-displaced transverse fractures below the Sacroiliac (SI) joint, and A3—Displaced transverse fractures below the SI joint. Type B injuries are unilateral longitudinal sacral fractures in which the ipsilateral superior S1 facet is not discontinuous with medial portion of the sacrum. These injuries primarily impact posterior pelvic stability and have minimal impact on spino-pelvic stability. Type B injuries are divided into three subtypes based on the likelihood of neurologic injury, and while this is similar to the Denis classification, because B-type injuries exclude fractures with a transverse component, there is little risk of a neurologic injury with an injury medial to the foramen. The three sub-types of B injuries are: B1—Longitudinal fracture medial to the foramen; B2—Longitudinal fracture lateral to the foramen and B3—Longitudinal injury thought the foramen. Type C injuries are Injuries that result in spino-pelvic instability. They are divided into four subtypes: C0—Non displaced sacral U fracture (commonly seen in low energy insufficiency fractures); C1—Any unilateral B-subtype where the ipsilateral superior S1 facet is discontinuous with the medial portion of the sacrum; C2—Bilateral complete B type fracture without a transverse component, and C3—Displaced sacral U type fracture. In addition to the fracture morphology, the new classification also formally considers the neurologic status of the patient: Nx—The patient cannot be examined; N0—No neurological deficits; N1—Transient neurological injury; N2—Nerve root injury and N3—Cauda Equina Syndrome. Lastly there are four patient specific modifiers that may alter the treatment of these fractures: M1—Significant soft tissue injury; M2—Metabolic bone disease; M3—High-energy injury that may be associated with an anterior pelvic ring injury, acetabular fracture or vascular injury, and M4—Altered anatomy of the lumbosacral junction (may be due to a prior fusion or transitional anatomy). Conclusion The AOSpine sacral fracture classification is the first comprehensive sacral classification to consider posterior pelvic and spino-pelvic instability patterns, and validation studies are ongoing.