Posterior open-door laminoplasty secured with titanium miniplates vs anchors: a comparative study of clinical efficacy and cervical sagittal balance

Cervical degenerative disease is a major cause of spinal diseases, and its incidence has been rising in the elderly people [1‐3]. Posterior open-door laminoplasty (PODL) is the primary treatment for multi-segmental cervical spondylotic myelopathy (MCSM), developmental cervical spinal canal stenosis, ossification of cervical posterior longitudinal ligament, and so on. Studies have shown this surgical approach has definite and enduring efficacy in releasing spinal compression and improving the function of nervous system [1‐7]. After PODL, it is usually necessary to firmly fix the opened laminae in order to maintain the enlargement and decompression of the spinal canal. To do so, the commonly used methods include silk suspension, suture anchor, and titanium miniplate (centerpiece), which can pry open the lamina on the open-door side [1, 2, 5‐10].

Silk suspension is to tie the silk into the space between the spinous process and articular capsule of the door-shaft side. But this method has lower biomechanical strength, and re-closure of the opened lamina may occur, resulting in relapse of the neurological symptoms [1, 8]. It has been reported that PODL secured with either miniplates or anchors can achieve a good clinical effect [1, 2, 8, 10], but few reports are concerned with the comparison of the two approaches. Whatever the approach, damage to muscle group and ligaments of the neck seems inevitable, which may further disrupt cervical sagittal balance, leading to straightening of normal cervical lordosis or even kyphosis [11‐13]. More studies are devoted to coronal-sagittal balance of the lumbar spine and the influence of spinal-pelvic parameters on postoperative lumbar spine [14, 15], while few are focused on the postoperative cervical sagittal parameters and changes in cervical sagittal balance.

In this study, clinical efficacy and radiologic parameters were compared between PODL secured with titanium miniplates and anchors. The influence of different surgical approaches on cervical sagittal balance was discussed, and the potential correlation between cervical sagittal parameters and clinical efficacy was analyzed.

MCSM can cause severe damage to the spinal function. Given to the grim consequences of progression, early surgery is important after confirmed diagnosis, so as to release spinal compression [1‐8]. When three or more segments are involved, posterior cervical approach is preferred to ensure the clinical outcomes and safety [1, 2, 6]. PODL is one of the common treatments for MCSM, which can increase vertebral canal volume and release compression of spinal cord and nerve roots, and the aim is to create enough room for the recovery of the nerves and spinal cord [1, 2, 6, 7].

In our study, PODL was performed using either titanium miniplates or anchors. Silk is fixed in screw, and they are made into a unity of anchor. The unity can prevent slip and rupture of the silk at the root of the screw in the lateral mass; moreover, the anchor has locking function and can prevent screw dislocation, which may otherwise lead to reclosure. This method proves to have a reliable open-door effect. Titanium miniplate [1, 2, 7, 8] is to install the centerpiece miniplate between the lamina and lateral mass on the open-door side, so as to enlarge the vertebral canal and to release spinal compression. The rigid support provided by the centerpiece miniplate can effectively prevent postoperative reclosure or loss of door-closing amplitude, so as to ensure the clinical efficacy. In our study, group A received immobilization with titanium miniplates, and group B immobilization with silk anchors. At 2 years after surgery, VAS, JOA, and NDI scores were all improved significantly than before (P < 0.05). This indicated that either approach could achieve a satisfactory clinical effect. It was found that at 2 years after surgery, NDI scores of group A were much better than those of group B (P<0.05). NDI is a subjective evaluation of patients’ cervical functional status. Several items of NDI deal with the subjective perception of comfort. Though bias seems inevitable with NDI, postoperative cervical functional status of the patient does have a considerable impact on the patients’ life quality [18]. We think that NDI has a certain clinical significance and serves as an important measure for surgical outcomes. Although PODL-secured titanium miniplates incurred greater hospitalized cost than using anchors (P < 0.05), the former provided higher cervical comfort for the patient after surgery.

Cervical sagittal balance is an important component of balance of the entire spine. Changes in cervical sagittal parameters after PODL will affect sagittal balance of the cervical spine or even the entire spine [11‐13]. C2-7 SVA, C2-7 Cobb angle, and T1 slope are important measures of cervical sagittal balance [11‐13, 19]. After surgery, both C2-7 SVA and T1 slope increased in the two groups than before (P < 0.05), while C2-7 Cobb angle decreased (P < 0.05). The result indicated conspicuous postoperative cervical anteversion. One major cause might be the damage to posterior cervical muscle-ligament complex when exposing the bilateral lamina. Additional cause might be that during door-opening operations in C3 and C7, attachments of the muscle in the spinous processes of C3 and C7 might be damaged with severing of laminae from the lateral mass on the open-door side. The above factors may lead to damage of muscle group and bony structure, which further affects cervical instability. As a result, anteversion may occur due to failure to maintain normal cervical curvature. Cervical anteversion is most significant at an early stage after surgery. Along with the progression of time, many patients may still fail to recover to preoperative curvature. Disruption of cervical sagittal balance as measured by C2-7 SVA, C2-7 Cobb angle, and T1 slope at 1 month after surgery was most apparent. These parameters were improved (P < 0.05) at 2 year after surgery, but cervical sagittal balance still did not recover to the preoperative curvature (P < 0.05).

However, controversy is still going on as to which radiologic parameter is of the highest clinical significance among all parameters of cervical sagittal balance. According to Ling et al. [19], C2-7 SVA was the most important radiologic parameter among the followings: C0-2 Cobb angle, C2-7 Cobb angle, C4-7 Cobb angle, C0-7 SVA, C1-7 SVA, C2-7 SVA, C7 Slope, T1 slope, angle of cervical endplate, McGregor's slope, elbow angle, craniospinal angle, and thoracic inlet angle. In this study, both groups had a significant increase in C2-C7 SVA after surgery (P < 0.05), indicating cervical anteversion. But at 2 years after surgery, group A achieved a much better recovery than group B (P < 0.05). Immobilization with silk anchors might cause complete damage to the bony structure on the open-door side. As a result, the cervical spine loses the support offered by bony structure on this side, leading to cervical instability to varying degrees. Moreover, posterior cervical muscle group was damaged, and the normal physiological curvature of the cervical spine could not be effectively maintained, leading to persistent anteversion [20]. However, miniplates provides a stable bridging fixation between the opened lamina and ipsilateral lateral mass, so that the opened lamina and lateral mass for the same segment can form a whole unity. In the meantime, rigid fixation can be provided for the open-door side, so that the original support by the body structure can be restored as much as possible. The opened lamina will be subjected to uniform stress under flexion and extension and rotation of the cervical spine, thereby increasing the possibility of restoring the normal cervical stress [21]. Besides, immobilization with miniplates can also provide solid stabilization to the door-shaft side, which is conducive to bony union on this side [7]. Cervical anteversion after PODL secured with miniplates might be only the result of damage to the posterior muscle group.

As to whether the radiologic parameters of cervical sagittal balance would affect clinical symptoms of the patients, Tang et al. [11] reported 113 patients receiving PODL with multilevel fusion. It was found that C2-7 SVA was correlated negatively with SF-36 and cervical anteversion would affect the surgical efficacy. Under normal physiological curvature of the cervical spine, posterior muscle group has the lowest energy consumption, and therefore, the patient has the highest degree of comfort. But if cervical anteversion occurs due to damage of bony structure and posterior muscle group after PODL, the posterior muscle group no longer has the lowest energy consumption [19]. This will further lead to such clinical symptoms as fatigue, discomfort, and pain of the posterior neck. Two different approaches were adopted in this study. The results showed that the difference value in NDI scores was correlated negatively with that in C2-7 SVA (r = − 0.433, P < 0.05). This means disruption of cervical sagittal balance would definitely impair cervical functional status, which further lowered cervical comfort of patients.

However, the present study still had the following limitations: (1) The sample size was small. (2) The duration of follow-up was limited. Our conclusions need to be further verified through studies with larger sample size and longer follow-up.

Taken together, PODL secured with titanium miniplates or anchors could achieve satisfactory improvement of neurological function for MCSM. However, the use of miniplates led to better cervical functional status, while the use of anchors reduced the hospitalized cost. Significant changes occurred in cervical sagittal balance after surgery using either approach, mainly presenting as cervical anteversion, which was most serious at the early stage after surgery. But this condition would improve with time, and alteration of sagittal balance had an impact on cervical comfort. It was revealed that the recovery of cervical sagittal balance was better secured with the miniplates than anchors.