Cervical laminoplasty was introduced in the early 1980s as a successful means of addressing multilevel cervical spinal stenosis secondary to either cervical spondylotic myelopathy (CSM) or ossification of posterior longitudinal ligament (OPLL)23). The goal of laminoplasty is to indirectly decompress the spinal canal by opening one or both sides of the lamina. In this procedure, the "open-door" technique, which was popularized by Hirabayashi et al.2) can be used. Later on, other adaptations of this surgical technique including the double-door approach where the spinous process is split open were developed. Although both techniques are effective in improving neurologic outcomes, double-door laminoplasty is better than open-door laminoplasty when it comes to clinical outcomes such as axial neck pain, perioperative complication, and anatomical alignment131518). Double-door laminoplasty has been considered as an effective and safe method for decompressing the multi-segmental cervical lesions caused by CSM, OPLL, or intervertebral disc herniation with developmental canal stenosis, as well as for achieving the immediate stability of these lesions 13891419). For spinal canal decompression and stability of the spine, various materials were used in this procedure, which include : autologous grafts, hydroxyapatite (HA) spacers, and allogeneic bone (AB) spacers. However, the pros and cons of individual implants have not been thoroughly investigated. In addition, many studies have compared heterogeneous groups or small individual case series, but there is still a lack in direct comparisons among these options. In this study, we compared double-door laminoplasty using HA spacers (Apacerum®; Asahi Optical Co., Ltd., Tokyo, Japan) and AB spacers (Allo-Spine®; CG Bio, Seoul, Korea) to analyze the clinical and radiological outcomes in patients.

The mean follow-up duration was 23.1 months (range : 4–69 months). Of the 79 included patients, 22 were diagnosed with CSM and 57 with OPLL. Surgeries were performed on 280 levels : 182 for the HA group and 98 for the AB group. Mean cervical lordosis changed from 13.54°±9.21° to 12.16°±8.68° in the HA group and from 16.93°±9.07° to 11.13°±7.08° in the AB group (p=0.131) (Table 2). Immediately after the surgery, the spinal canal dimension increased from 189.31 mm² to 275.25 mm² in the HA group and from 185.48 mm² to 307.22 mm² in the AB group, but it decreased to 243.71 mm² and 254.50 mm², respectively, at the last follow-up (p<0.001) (Table 3) (Fig. 3, 4). The dimension decreased during the last follow-up in both groups, but the degree of decrease in dimension was greater in the AB group than in the HA group. JOA scores changed from 12.35±4.67 to 15.02±3.72 in the HA group and from 13.04±4.19 to 15.29±3.13 in the AB group (p=0.674) (Fig. 5). Neck VAS scores changed from 4.65±3.10 to 1.41±1.51 in the HA group and from 4.54±1.71 to 1.46±0.88 in the AB group (p=0.598) (Fig. 6). To compare both fusion and resorption rates, we chose the group with a similar follow-up term of 18 months. In the HA group, a total of 74 implants were used from which 55 achieved union. Meanwhile, in the AB group, a total of 37 implants were used from which 24 achieved union. However, there was no statistically significant difference found between the two groups (p=0.3) (Table 4). In terms of resorption rate, a total of 74 spacers in the HA group were used from which resorption occurred in 6. On the other hand, a total of 37 spacers in the AB group were used from which resorption occurred in 19 p<0.001) (Table 5).

Numerous modifications and supplementary instruments were utilized to retain hinge patency and provide secure fixation in open-door or double-door laminoplasties417). Some examples of these are as follows : HA spacer, AB spacer, ceramic lamina, miniplate osteosynthesis, and autogenous bone grafting424). To be specific, AB spacer, HA spacer, autogenous bone graft, and ceramic block were used for the maintenance of an opened lamina. Although these promote the bony healing of the grafted side and the maintenance of the laminar arch when spaces were used, these posed the risk of root or cord compression because of a graft kick-out4522). In laminoplasty, HA and AB spacers have several benefits compared to the autologous bone graft. These include good biomechanical stability and reductions in operation time, blood loss, and donor site morbidity 710111216). It was reported that the fusion rates after the use of an HA spacer and AB spacer were 48.8% and 51%, respectively16). However, no study has actually directly compared these two groups. In this study, the fusion between the lamina and the spacer, as well as the resorption, was checked through the fine-section reconstruction of the CT. Since an HA spacer only has an osteoconduction activity compared with an AB spacer with both osteoconduction and osteoinduction activities, the fusion rate was expected to be higher in the AB group. In the current study, 55 of the 74 changes in the HA group and 24 of the 37 changes in the AB group were fused. While the result was not statistically significant, the HA group showed a similar fusion rate with AB group. Although the clinical outcomes improved in both groups, there were no statistically significant differences between them. This is because the shape and size of the spacers are similar, which allowed the canal dimension to increase in a similar proportion with the canal expansion postoperatively. In agreement with previous findings, this study observed the dimensional reduction of the spinal canal after an immediate postoperative widening716). However, the degree of dimensional reduction was greater in the AB group on the final follow-up CT imaging. This may be taken into consideration in conjunction with the fusion rate. In addition, it may be related to the fusion of the newly formed bone, which narrowed the canal dimension. Moreover, spacer resorption may be important in this change in dimension. Originally, the spacer can widen the width of the canal by occupying the space between the spinous processes. However, with the increase in spacer resorption, its shape is changed, and the space between the spinous processes is narrowed. It is believed to result in a decrease because of its canal-widening effect. Aside from having been observed in many other studies, kyphotic deformities were also observed after laminoplasty in this study. However, additional follow-up is needed 36182021).

After double-door laminoplasty was performed with an AB spacer on one group and an HA spacer on another group, the clinical outcomes of both groups improved. In addition, similar fusion rates were found in these groups. However, the AB group had a greater degree of reduction in the spinal canal dimension than the HA group at the final postoperative follow-up. Therefore, we suggest that surgeons should consider the use of larger-sized AB spacers in double-door laminoplasties.