<p>  Purpose : To compare the efficacy of hydroxyapatite (HAP) spacers and beta-triphosphate calcium-HAP complex (HAP-TCP) spacers in spinous process-splitting open-door cervical laminoplasty.</p><p>  Methods : Among 299 patients who underwent laminoplasty between January 2006 and December 2012, 83 patients with cervical spondylotic myelopathy without ossification of the longitudinal ligament or history of trauma were included in the study. In all patients, spacers were inserted into each split spinous process (C3-6). The patients were divided into two groups : the HAP spacer group (n=32) and the HAP-TCP spacer group (n=51). Computed tomography scans were obtained at 7 days, 3 months, and 6 months post-surgery to assess the extent of bone-spacer bonding and ossifications at the laminar hinges (the gutter). Bone-spacer bonding was classified according to Ichikawa's classification A-E¹⁾ , whereby grades above C represent bone growth, and grades above D represent bone union. Bone changes at the gutter were classified into three grades : none, growth, and union. The chi-square test was used to evaluate frequencies. Statistical significance was defined as p<0.05.</p><p>  Results : Bone growth and union were observed in the spacer-bone interface in the HAP-TCP spacer group in 98.0% and 61.3% at 3 months and 97.6% and 53.4% at 6 months. The corresponding rates in the HAP spacer group were 96.3% and 7.8% at 3 months and 88.3% and 6.3% at 6 months, respectively. Bone growth and union rates at the bone-spacer were higher in the HAP-TCP spacer group than in the HAP spacer group, with a significant difference between the two groups at 3 months (χ²=93.42, df=2, p<0.01) and 6 months (χ²=80.52, df=4, p<0.01)]. The bone-spacer bonding rates decreased from 3 to 6 months post-surgery in both groups [HAP group : χ²=2.97, df=3, p>0.05, HAP-TCP group : χ²=8.40, df=4, p<0.05). The bone regrowth and union rates at the gutter at 6 months post-surgery were 93.4% and 63.1% in the HAP-TCP spacer group, and 98.8% and 80.3%. The differences were significant between the two groups (χ²=16.90, df=2, p<0.05).</p><p>  Conclusion : Compared to the HAP spacers, the HAP-TCP spacers demonstrated superior bone-spacer bonding but inferior bone union at the gutter. The effectiveness of both spacers was comparable in spinous process-splitting open-door cervical laminoplasty.</p>