OPLL may progress more aggressively and cervical spondylosis occurs after osteophytes compress the spinal cord. OPLL can be removed by anterior approaches. Although doing so is more technically demanding, it allows direct decompression, which may be related to better prognosis and less progression[14]. Localized OPLL is usually asymptomatic at the very beginning. Patients’ symptoms are aggravated by disc herniation at the same intervertebral space level or due to neurological symptoms caused by disc injury because of recent trauma. For this reason, ACDF can achieve the same efficacy as ACCF by resecting the herniated disc and OPLL at the intervertebral space level, with much less surgical injury than ACCF.
In this study, we found that the volume of blood loss and duration of surgical procedure were significantly less in patients undergoing ACDF than in patients undergoing ACCF. The majority of previous studies were in accordance with our results[15]. This difference is because ACCF requires resecting the corresponding vertebral body and the adjacent disc, which is more traumatic. However, Lin et al.[16] found that the duration of the ACDF surgical procedure was significantly longer than that of the ACCF procedure, which was contrary to our study. He believed that performing ACDF to remove osteophytes from the intervertebral space is time-consuming and more technically challenging. The reason why many scholars perform ACCF is that the difficulty of ACDF is much greater than that of ACCF. The limited surgical exposure and the adhesion of the OPLL to the dura make it difficult to decompress sufficiently. However, when the technical curve of ACDF is mature, it is a more advantageous surgical method.
ACCF and ACDF not only relieves compression in the spinal cord but also reconstructs the physiological parameters of the cervical spine through careful distraction of the intervertebral space[17]. Burkardt et al.[18] showed that ACDF had better maintenance of disc space height and greater improvements in cervical lordosis. Our study corresponded with it. Compared with patients who underwent ACCF, the postoperative disc space height was significantly improved in those who underwent ACDF. However, disc space height dropped slightly in both groups when they came to review and the ACCF group was more than the ACDF group. The reduced height of the treated disc space may be due to postoperative subsidence of implant settling and then migrating to the endplates. Park et al.[19] also expounded subsidence of the treated intervertebral space height at six weeks after surgery. In addition, the thickness of the removed endplate also had a significant effect on the subsidence of the cage after fusion surgery[20]. The main causes of abnormalities in patients who undergo ACCF may be related to excessive damage to the vertebral endplate and the rigid effect of the titanium cage. The contact surface of the titanium cage is sharper. Under the action of stress, it is more likely to protrude to the cancellous bone to form subsidence. However, the cage used in ACDF has a relatively large effective contact surface, which disperses the stress of adjacent vertebrae and is more advantageous in the maintenance of intervertebral height.
Biomechanically, we believe that multiple points of distraction during ACDF can more effectively correct the cervical curvature. Clinical studies have directly linked postoperative cervical kyphosis to greater neurological deficits[21]. In our study, the C2-7 Cobb angle and the segmental angle in ACDF significantly showed greater improvement than that in ACCF at each follow-up. Therefore, we speculate that the maintenance of lordosis after ACDF may be better than that after ACCF from a long-term perspective[22]. Other main radiologic factors associated with it were T1 slope and C2–C7 SVA. Zhang et al.[23] suggested that the sagittal balance of ACDF was better than that of ACCF, which was correlated with fewer axial symptoms. From our study, we found that the T1 slope had a greater improvement in the ACDF group than in the ACCF group. Thus, two approaches corrected the balance by ameliorating the T1 slope. We found that no significant difference was observed in C2–C7 SVA in our research, and the values were at a normal level. MCAVINEY et al.[24] found that the gravity center of the head will shift forward when the C2-7 SVA is greater than 40 mm, which will straighten cervical curvature and affect horizontal vision. This finding indicated that two surgical procedures caused little damage to the neck muscle, could prevent the cervical gravity center from moving forward, better maintained cervical sagittal balance and enhanced the postoperative quality of life of patients.
The patients had similar chief complaint symptoms before surgery. Both clinical outcomes showed no significant difference, which was similar to previous studies[15]. Moreover, postoperative clinical outcomes improved significantly in both groups. This finding indicated that patients improved after surgery, and their condition gradually improved at each follow-up. Thus, the effect of the anterior approach is acceptable. Statistics show that reports of improvement in neurological function is approximately between 60% and 70%[25]. The JOA recovery rate in our study improved greatly. During the follow-up, almost all patients were satisfied with the curative effect. They obviously felt that the neck pain disappeared, and the upper limb numbness was relieved. Wang et al.[26] reported that either ACCF or ACDF had a highly successful improvement of clinical outcomes and that they are good solutions.
The success of cervical surgery often depends directly on the progress of fusion[27]. From a biomechanical point of view, additional external or internal support should be employed to prevent excessive movement of cervical cages[28]. Qiu et al.[29] reported that corpectomy and discectomy with plate fixation and autograft fusion had similar fusion rates. Several scholars have shown that ACCF has relatively good fusion rates[30]. Pseudarthrosis may occur in multi-segmented ACDF if fusion surfaces are increased[31]. However, in our study, the six-month fusion rates of patients who underwent ACDF and those who underwent ACCF were 66.2% and 40.5%, respectively. This is statistically significant. This result indicated that ACDF might promote earlier solid fusion in the six months. At one year, the difference disappeared. In the final follow-up, patients achieved successful solid fusion in each group.
In anterior cervical spine surgery, chronic dysphagia is one of the most common postoperative complications, but the mechanism remains unknown. Esophageal injury, anterior cervical soft tissue edema and postoperative hematoma might contribute to dysphagia[32]. In ACDF, many methods can be used for final internal fixation. We preferred to employ ROI-C, which is a novel zero-profile anchored spacer. Liu et al.[33] conducted a comparative study in which ROI-C was linked with a simpler surgery and a lower incidence of complications when compared to PEEK cages with anterior plates. In the ACCF group, we still employed the plate traditionally. Sixteen (20.8%) patients complained of mild dysphagia postoperatively in the ACDF group and thirty (45.9%) in the ACCF group after surgery. Some of them might be due to recurrent laryngeal nerve injury, and they recovered after we used methylprednisolone. At one month postoperatively, we still found a lower incidence of dysphagia in patients who underwent ACDF (10.4%) than in those who underwent ACCF (24.3%). We thought that the plate used in ACCF made the difference and that the presence of a plate caused anterior cervical soft tissue edema. In ACDF, We implanted a special cage into the intervertebral space and insert two anchoring clips to fix the cage into the adjacent vertebra[34]. This method could prevent the implant from contacting the anterior cervical soft tissue. Through our follow-up of patients, only one patient in the ACDF group and five patients in the ACDF group had no apparent relief finally. Thus, we conclude that ACDF that uses ROI-C is great at ensuring milder anterior soft tissue injury, which results in a lower incidence of dysphagia. Another long-term postoperative complication is ASD, because cervical spine fusion surgery immobilizes the motion segment at the expense of its range of motion. Fewer remaining segments are considered to provide more motion, which accelerates disc degeneration, chronic osteophyte formation and new adjacent segment disease[34]. Another factor is that the edge of the plate approaches the disc[35]. However, the results showed in our current study that no case of ASD was observed in the ACDF group, and the incidence was 4.1% in the ACCF group. Fortunately, the two patients showed only changes in imaging data and no definite clinical symptoms or mild symptoms, so no repeated operations were needed. The low incidence of ASD in this study may be due to the short observation period. Nevertheless, we speculate that the incidence of ASD will increase in those who undergo ACCF in the future because of the inevitable misposition of the plate.
Cerebrospinal fluid leak (CSFL) after cervical spine surgery can be troublesome, as it can cause meningitis, spinocutaneous fistula, or pseudomeningocele. Lee et al.[36] reports that dural tears are relatively common, as they occur in approximately 10.5% of ACCF procedures. The crucial point of our operation is that the herniated disc is completely removed first. Then, we can incise the normal posterior longitudinal ligament so that we can, in turn, expose the dura and remove the OPLL with a rongeur. When a rongeur was used to resect the localized OPLL, it was much more likely to break the dura if the OPLL was adhered to the dura. In our study, CSFL occurred in six patients in the ACDF group and in ten patients in the ACCF group. Although the incidence was not statistically significant, we considered that resection of more structures during ACCF is more likely to cause dural tears. Various techniques have been used to manage dural tears and the consequent CSFL after surgery. Gelatin sponges were successfully used to repair dural tears intraoperatively. These patients completely recovered after 3–5 days of bed rest and management with ceftriaxone, and their wounds healed normally. There was one patient in the ACCF group in which lumbar cistern drainage postoperatively was employed to solve the problem because the size of the dural defect was large. No patient developed further spinal or intracranial infection, cutaneous fistula or secondary surgery.
This study has some limitations. First, larger sample and longer follow-up time are necessary. Second, a randomized controlled trial study should be carried out to control the possibility of selection bias. The current findings cannot be interpreted as long-term results. Therefore, we require further observation to investigate the issue in prospective randomized trials.