Studies about the differences in radiological and functional outcomes between patients with OPLL and CSM after laminoplasty are scarce. Herein, we focused firstly on the changes in cervical lordosis, ROM, and SVA in radiological outcomes. Sakai et al.  and Lee et al.  reported that both cervical lordosis and SVA worsened following laminoplasty in both CSM and OPLL groups, while Sakaura et al.  and Kato et al.  reported no significant differences of cervical lordosis and SVA after laminoplasty in both CSM and OPLL groups. Our results were inconsistent with the results of these previous studies. The CSM group showed a significant increase of C27SVA at the 12-month follow-up, but there was no significant difference in the OPLL group. Additionally, there was a slight loss of cervical lordosis, with no statistical significance, in both groups. Considering the changes in radiological parameters, there was a deterioration of sagittal balance with increase of C27SVA in the CSM group compared to the OPLL group after multi-level laminoplasty.
There was a significant reduction in ROM at 12 months after laminoplasty in both groups, and the CSM group showed a greater reduction of ROM than the OPLL group. Some previous reports have described a reduction in ROM as well as a progression of sagittal imbalance after laminoplasty, but the etiology of these problems remain debatable [10, 11]. The damage and atrophy of the posterior deep muscles including the semispinalis cervicis muscle, injury of the nuchal ligament, and bony fusion of facet joints after laminoplasty are considered the main reasons for these postoperative changes [12–14]. The reduction in ROM following laminoplasty is reported to range from 30–70% of the preoperative ROM [15–17]. Seichi et al.  reported a reduction in ROM from 13° to 47° in CSM patients. Our study showed a postoperative reduction of approximately 30% in the CSM group and 20% in the OPLL group. The smaller preoperative ROM of the OPLL group compared to the CSM group may have originated from the stiffed segments with calcification of the posterior longitudinal ligaments. As a result, we expect that postoperative deterioration of sagittal balance in the OPLL group would be less than that of the CSM group after laminoplasty. In addition, Hyun et al.  reported a small ROM in OPLL after laminoplasty than in CSM (47.2% vs. 72.7% of preoperative ROM) because of the postoperative higher frequency of laminar auto-fusion in OPLL than in CSM.
It is necessary to consider the effect of age on the development of sagittal imbalance with kyphotic change. Sakai et al.  reported that advanced age was a risk factor for kyphotic deformity after laminoplasty in CSM patients with normal preoperative sagittal alignment. In the thoracolumbar spine, a decrease of lumbar lordosis and an increase of thoracic kyphosis are associated with increasing age, which results in increased SVA as an indicator of the sagittal global balance [20–22]. However, unlike the thoracolumbar spine, the sagittal balance of the cervical spine is relatively well maintained in elderly patients because of increased cervical lordosis as a compensatory mechanism . To maintain the lordotic curvature of the cervical spine in the condition with sagittal imbalance, it is necessary to preserve and enhance the strength of the cervical extensor muscles. There are several less-invasive methods for saving cervical posterior structures to maintain cervical lordosis after laminoplasty . However, it is generally assumed that extensor muscles already show age-related degradation in elderly patients, which will worsen the deterioration of the cervical extension mechanism postoperatively.
The age-related cervical ROM and curvature should also be evaluated in asymptomatic individuals. Yukawa et al.  reported that the cervical motions (flexion, extension, and total range) and the curvature of neutral neck posture were 25.5º ± 9.5º, 16.2º ± 9.4º, 41.8º ± 12.7º, and 18.4º ± 11.6º in male, and 26.3º ± 98.4º, 26.7º ± 8.8º, 53.0º ± 10.9º, and 16.9º ± 10.8º in female patients in their sixties, respectively, which was the average age of our patients. These results show a greater range of flexion and similar cervical lordosis than the preoperative parameters of our patients. With increasing age, there was decreased ROM and increased cervical lordosis, and the range of extension decreased more than the flexion. These results show that cervical lordosis naturally increases and the range of extension decreases with aging caused by compensatory mechanism in asymptomatic individuals. This phenomenon was different from our postoperative radiological findings including the reduction of both ROM and cervical lordosis. Especially, the effects of age for ΔC27AF (positive correlation) and ΔC27SVA (negative correlation) were limited, and the final result showed a significant decrease of cervical flexion and increase of SVA compared to no significant difference in cervical extension 12-month postoperatively.
Few studies have focused on restriction of each cervical flexion or extension after laminoplasty, although many studies have reported a reduction in overall ROM with loss of cervical lordosis. Suk et al.  reported significant restriction in both flexion and extension, while Hyun et al.  showed restriction in flexion but not in extension. However, these results did not highlight any differences between the CSM and OPLL groups. Our results showed significantly more restriction of cervical flexion in the CSM group than the OPLL group after laminoplasty. With a reduction in overall ROM, the composition of ROM, comprising flexion and extension, changed from 1:2 to 1:3 postoperatively in the CSM group; however, there was no change in the OPLL group. We believe that the reason for restriction of cervical flexion primarily observed in the CSM group after laminoplasty is the compensatory mechanism against the progression of sagittal imbalance caused by a postoperative dysfunction of cervical posterior structures. In other words, a reduction of ROM with distinct restriction of cervical flexion has occurred to correct the sagittal imbalance in which the center of gravity of the head leans forward with loss of cervical lordosis and increase of SVA (Fig. 2).
In addition to age, the effect of preoperative T1S should be also considered to evaluate the radiological outcomes after laminoplasty between the CSM and OPLL groups. The preoperative high T1S is already known to be an important factor related to the development and deterioration of sagittal imbalance after laminoplasty [3, 4]. In our study, T1S showed no significant differences between the two groups pre- and postoperatively. There were no significant correlations between T1S and the changes in radiological parameters in the CSM group. However, higher preoperative T1S was related to restricted cervical extension and loss of cervical lordosis postoperatively in the OPLL group. In a comprehensive review of the effects of age, ROM, and T1S; we expect that C27ROM, unlike age and T1S, is affected mainly by the dysfunction of posterior cervical structures. The effect of posterior surgery with the damages of posterior extensor muscles and ligaments are bigger in the CSM group presenting greater preoperative ROM, which led to the increase of SVA. The reduction of ROM is also thought to be involved in the compensatory mechanism as mentioned above. Unfortunately, the compensatory mechanism related with reduction of ROM in the CSM group was insufficient to protect deterioration of sagittal balance with increasing SVA, even though it minimized loss of cervical lordosis. However, the stiffed and less-flexible segments with the formation of ossification of the posterior longitudinal ligaments combined with a compensatory mechanism of ROM preserved SVA and cervical lordosis in the OPLL group.
Unfortunately, the long-term changes in ROM and cervical curvature could not be determined from the 12-month follow-up study; thus, these can be predicted indirectly by referring to the results of previous studies. Kawaguchi et al.  reported a rapid decrease in ROM within the first year following surgery, and no further decreases thereafter without distinction between the CSM and OPLL groups. Hyun et al.  reported the differences in the reduction of ROM between the CSM and OPLL groups. The OPLL group showed a continuous reduction of ROM for 5 years; however, there was a recovery after initial reduction for 2 years in the CSM group. However, there were no significant differences in functional outcomes depending on the radiological changes. In our results, we found significant improvements in functional outcomes in the CSM and OPLL groups with no significant differences 12 months postoperatively. When considering these favorable functional outcomes, the radiological changes including the reduction of ROM and increase of SVA in the CSM group can be regarded as meaningless. However, we think that careful observation is required even after 12 months when considering the greater deterioration of sagittal balance in the CSM group than in the OPLL unlike the results of previous literature.
Our study has several limitations. First, it was a retrospective study with a relatively small number of participants. Second, the 12 months follow-up period was quite short to expect to observe overall changes in postoperative sagittal balance. Third, there was no consideration of whole-spine sagittal balance, which can influence cervical sagittal balance. Further studies with a larger number of participants during the long-term follow-up period are required to demonstrate the exact radiological and clinical differences between CSM and OPLL groups.