Postoperative Increase in Occiput–C2 Angle Negatively Impacts Subaxial Lordosis after Occipito–Upper Cervical Posterior Fusion Surgery

STUDY DESIGN: Retrospective case series. PURPOSE: To elucidate the impact of postoperative occiput–C2 (O–C2) angle change on subaxial cervical alignment. OVERVIEW OF LITERATURE: In the case of occipito–upper cervical fixation surgery, it is recommended that the O–C2 angle should be set larger than the preoperative value postoperatively. METHODS: The present study included 17 patients who underwent occipito–upper cervical spine (above C4) posterior fixation surgery for atlantoaxial subluxation of various etiologies. Plain lateral cervical radiographs in a neutral position at standing were obtained and the O–C2 angle and subaxial lordosis angle (the angle between the endplates of the lowest instrumented vertebra (LIV) and C7 vertebrae) were measured preoperatively and postoperatively soon after surgery and ambulation and at the final follow-up visit. RESULTS: There was a significant negative correlation between the average postoperative alteration of O–C2 angle (DO–C2) and the average postoperative alteration of subaxial lordosis angle (Dsubaxial lordosis angle) (r=–0.47, p=0.03). CONCLUSIONS: There was a negative correlation between DO–C2 and Dsubaxial lordosis angles. This suggests that decrease of mid-to lower-cervical lordosis acts as a compensatory mechanism for lordotic correction between the occiput and C2. In occipito-cervical fusion surgery, care must be taken to avoid excessive O–C2 angle correction because it might induce mid-to-lower cervical compensatory decrease of lordosis.