Practical Application of the AO Spine Primary Infection Classification System: Clarifying Definitions, Borderline Cases and Common Misinterpretations

Study Design Narrative review. Objective To clarify nuances when applying the AO Spine Primary Infection Classification. Methods The international validation of the AO Spine Primary Infection Classification System was successful but recurring themes caused confusion. This included the exact definition and reasons for infections to transition between categories, definition of canal involvement or neural compression and whether an epidural infection superimposed on degenerative disease escalates infection severity. There was uncertainty regarding handling of multi-level or junctional infection and clinicians encountered difficulty applying the modifiers. Results The difference between a Type 0 and Type 1 infection is reasonably distinct given the former is purely extraspinal whilst the latter involves spondylodiscitis or osteomyelitis without canal involvement. Type 2 infections include those with any canal involvement, regardless of whether CSF effacement occurs, in the absence of neural compression. The spinal canal is defined as the compartment dorsal to the posterior longitudinal ligament and excludes infections that are ventral to this ligament but bow into the canal. Any neural compression of the thecal sac centrally or within the foramen escalates categorization to a Type 3 infection. Conclusions Primary spinal infections have the potential to rapidly progress and cause devastating neurological deficits. The introduction of the AO Spine Primary Infection Classification establishes a universal language to communicate disease severity. We clarify common nuances encountered during international validation of this system including definition of canal involvement and relevance of CSF effacement, whether foraminal involvement constitutes neural compression and the nomenclature concerning multi-level or junctional disease.