The incidence of bifid c7 spinous processes.

For posterior cervical surgery, if the operation only involves the lower cervical area, counting from C2 is impractical and the level may not be visible on X-rays. In such cases, we usually place a marker at the top of the incision and also rely on the size and monofid shape of the C7 spinous process. Relying on the C7 morphology, however, we initially instrumented the wrong levels in a case where the patient had a bifid C7 spinous process. We therefore sought to determine the frequency of bifid cervicothoracic spinous processes. Computed tomography axial images of C6, C7, and T1 from 516 patients were evaluated. The spinous processes were classified into three categories: "bifid," "partially bifid," and "monofid." C6 spinous process was monofid in 47.9% of cases, partially bifid in 4.2% of cases, and bifid in 47.9% of cases. C7 spinous process was monofid in 99.2% of cases, partially bifid in 0.5% of cases, and bifid in 0.3% of cases. T1 was monofid in all cases. A truly bifid C7 spinous process occurs 0.3% of the time and therefore is not a reliable landmark for choosing fusion levels. This knowledge hopefully helps prevent the type of wrong-level instrumentation that we performed.