Aplasia of the Posterior Arch of the Atlas: A Proposal for a New Clinical Classification

Partial or complete aplasia of the posterior arch of the atlas is often detected inciden‑ tally when a patient has cervical Xrays.　Patients with this anomaly are generally asymptomatic.　However some cases demonstrate neurological symptoms.　It is important to predict cases of this anomaly that may be accompanied with neurological symptoms.　Between April 2001 and March 2005 three patients were encountered who had an anomalous posterior arch of the atlas.　The anomalies encountered in the three patients were midline cleft(two pa‑ tients)and an absence of the posterior arch with persistent posterior tubercle(one patient) .　Based on our clinical experience and a review of the literature we developed a new clinical classification system for this condition.　From a clinical point of view aplasia of the pos‑ terior arch of the atlas can be classified into three types.　Type Ⅰ is complete aplasia.　Type Ⅱ is partial aplasia with the isolated posterior fragment.　Type Ⅱ is divided into subtypes depend‑ ing on the inward mobility of the isolated posterior fragment(Type Ⅱa without and Type Ⅱ b with mobility) .　Type Ⅲ is partial aplasia without the isolated posterior fragment.　Our new classification is simple and unambiguous and it also provides a clear direction for developing ap‑ propriate diagnostic and treatment strategies.