Analysis of the reciprocal changes in upper cervical profile and the risk factors for increasing cervical sagittal vertical axis after laminoplasty for ossification of the posterior longitudinal ligament of the cervical spine

•ΔO-C2 angle was negatively correlated with ΔC2-C7.•T1S was negatively correlated with ΔC2-C7.•Higher T1S is a risk factor for increasing cSVA after laminoplasty for OPLL. The purpose of this study was to elucidate the reciprocal changes in the upper cervical profile and the risk factors for increasing cervical sagittal vertical axis (cSVA) after laminoplasty for ossification of the posterior longitudinal ligament (OPLL) of the cervical spine. This retrospective study included thirty-nine consecutive patients (30 men and 9 women) with cervical OPLL who underwent cervical laminoplasty. We recorded the operative time, blood loss, Japanese Orthopaedic Association (JOA) score recovery rate. Radiological measurements were performed to analyze the following parameters: pre and 1- year postoperative chin-brow vertical angle (CBVA), McGregor’s slope (McGS), occiput to C2 Cobb angle (O-C2 angle), C2–C7 Cobb angle (C2–C7 angle), T1-slope (T1S), C2–C7 sagittal vertical axis (cSVA) and calculated the change (Δ). Patients were divided into two groups according to ΔcSVA: positive (ΔcSVA ≥ 0) and negative (ΔcSVA < 0). Postoperative O-C2 angle (P = 0.028), ΔO-C2 angle (P = 0.019), ΔC2-C7 angle (P = 0.030) and T1S (P = 0.009) diff ;ered between the two groups. ΔcSVA showed a positive correlation with ΔO-C2 (R = 0.365, P = 0.022) and T1S (R = 0.472, P = 0.002). ΔO-C2 showed a positive correlation with T1S (R = 0.478, P = 0.002) and a negative correlation with ΔC2-C7 (R=-0.443, P = 0.005). ΔC2-C7 showed a negative correlation with T1S (R=-0.415, P = 0.009). Stepwise multiple linear regression analysis showed that ΔcSVA increased by 0.757 mm for each T1 slope and increased by 0.905 mm for each ΔMcGS. Increasing the lordosis in the O-C2 segment compensates for the loss of lordosis in the C2-C7 segment after cervical laminoplasty. Higher T1S is a risk factor for increasing cSVA after laminoplasty for OPLL of the cervical spine.