Background/Objectives: We investigated the impact of ossification of the posterior longitudinal ligament (OPLL) and cervical alignment on K-line assessment, proposed the K-line edge category for transitioning from K-line (+) to (−), and demonstrated a quantitative evaluation using the K-line edge. Methods: The 268 patients with OPLL who underwent computed-tomography-based K-line assessment were retrospectively stratified into three groups [K-line (+)/(±)/(−)]. We graphically plotted their distributions based on the OPLL-occupying ratio (OPLL-OR) and cervical angle (θ C_2–7). Results: The K-line (+), (±), and (−) groups comprised 159, 37, and 72 patients, respectively. The K-line (+) group demonstrated the lowest alignment value at −14.1°, suggesting a potential border for kyphosis at 14°. By examining the K-line (±) region, we successfully identified a clustered group and proposed the “K-line edge” for K-line (±), which was derived as y = 0.98x + 46.82 (R² = 0.67). The K-line edge calculation depended on the OPLL-OR and θ C_2–7 at each cervical level and determined the corresponding value for either OPLL-OR or θ C_2–7. The slope of the K-line edge was almost horizontal at both ends and was steeper in the middle, with the alignment playing a dominant role in the mid-cervical region. The calculated borders were approximately at 12° kyphosis at C4-4/5 and at 11° at C5-5/6. Conclusions: Focusing on K-line (±) identified three major factors during the K-line assessment: the OPLL-OR, cervical alignment (θ C_2–7), and cervical level. The K-line edge could be useful as a quantitative parameter for surgical decision-making.