Are insufficient corrections a major factor in distal junctional kyphosis? A simulated analysis of cervical deformity correction using in-construct measurements

OBJECTIVE The present study utilized recently developed in-construct measurements in simulations of cervical deformity surgery in order to assess undercorrection and predict distal junctional kyphosis (DJK). METHODS A retrospective review of a database of operative cervical deformity patients was analyzed for severe DJK and mild DJK. C2–lower instrumented vertebra (LIV) sagittal angle (SA) was measured postoperatively, and the correction was simulated in the preoperative radiograph in order to match the C2-LIV by using the planning software. Linear regression analysis that used C2 pelvic angle (CPA) and pelvic tilt (PT) determined the simulated PT that matched the virtual CPA. Linear regression analysis was used to determine the C2–T1 SA, C2–T4 SA, and C2–T10 SA that corresponded to DJK of 20° and cervical sagittal vertical axis (cSVA) of 40 mm. RESULTS Sixty-nine cervical deformity patients were included. Severe and mild DJK occurred in 11 (16%) and 22 (32%) patients, respectively; 3 (4%) required DJK revision. Simulated corrections demonstrated that severe and mild DJK patients had worse alignment compared to non-DJK patients in terms of cSVA (42.5 mm vs 33.0 mm vs 23.4 mm, p 0.57, p < 0.001). A cSVA of 40 mm corresponded to C2–T4 SA of 10.4° and C2–T10 SA of 28.0°. A DJK angle change of 10° corresponded to C2–T4 SA of 5.8° and C2–T10 SA of 20.1°. CONCLUSIONS Simulated cervical deformity corrections demonstrated that severe DJK patients have insufficient corrections compared to patients without DJK. In-construct measures assess sagittal alignment within the fusion separate from DJK and subjacent compensation. They can be useful as intraoperative tools to gauge the adequacy of cervical deformity correction.