Safety and Validity of Anterior Cervical Disc Replacement for Single-level Cervical Disc Disease: Initial Two-year Follow-up of the Prospective Observational Post-marketing Surveillance Study for Japanese Patients

Abstract

Anterior cervical disc replacement (ACDR) using cervical artificial disc (CAD) has the advantage of maintaining the range of motion (ROM) at the surgical level, subsequently reducing the postoperative risk of adjacent disc disease. Following the approval for the clinical use in Japan, a post-marketing surveillance (PMS) study was conducted for two different types of CAD, namely, Mobi-C (metal-on-plastic design) and Prestige LP (metal-on-metal design). The objective of this prospective observational multicenter study was to analyze the first 2-year surgical results of the PMS study of 1-level ACDR in Japan. A total of 54 patients were registered (Mobi-C, n = 24, MC group; Prestige LP, n = 30, PLP group). Preoperative neurological assessment revealed radiculopathy in 31 patients (57.4%) and myelopathy in 15 patients (27.8%). Preoperative radiological assessment classified the disease category as disc herniation in 15 patients (27.8%), osteophyte in 6 patients (11.1%), and both in 33 patients (61.1%). The postoperative follow-up rates at 6 weeks, 6 months, 1 year, and 2 years after ACDR were 92.6%, 87.0%, 83.3%, and 79.6%, respectively. In both groups, patients' neurological condition improved significantly after surgery. Radiographic assessment revealed loss of mobility at the surgical level in 9.5% of patients in the MC group and in 9.1% of patients in the PLP group. No secondary surgeries at the initial surgical level and no serious adverse events were observed in either group. The present results suggest that 1-level ACDR is safe, although medium- to long-term follow-up is mandatory to further verify the validity of ACDR for Japanese patients.

Introduction

The first clinical trial of cervical artificial disc (CAD) was reported in 1966, but subsequent development of CAD was suspended until its reemergence in the 1980s.¹⁾ CAD has been in clinical use since the 1990s.^2,3) Anterior cervical surgery using CAD, also termed anterior cervical disc replacement (ACDR), was approved by the Food and Drug Administration of the USA in 2007. To date, at least nine types of CAD are available for clinical use.^4-9) The advantage of ACDR is that it maintains the range of motion (ROM) at the surgical level, thus reducing the postoperative risk of adjacent disc disease compared with conventional anterior cervical discectomy and fusion (ACDF). Although a number of studies have suggested beneficial effects of ACDR compared with ACDF in the medium to long term,^10-33) many others have questioned the superiority of ACDR.^34-40) Specific ACDR-related complications should also be carefully noted to ensure surgical safety.^41,42) A firm consensus on ACDR is yet to be established. There is another concern that different mechanical design of CAD may result in the different intervertebral mobility and stability.^43,44)

In Japan, two types of CAD were approved for clinical use in December 2017, namely, Mobi-C (metal-on-plastic, unconstrained three-piece design; ZimVie Japan G.K.) and Prestige LP (metal-on-metal, semiconstrained two-piece design; Medtronic Sofamor Danek, Co., Ltd.). Zimmer Biomet G.K. and Medtronic Sofamor Danek, Co., Ltd., subsequently conducted a post-marketing surveillance (PMS) in accordance with the principles of Good Post-Marketing Study Practice in Japan. The PMS study of ACDR in Japan was continued as a part of multicenter clinical research using the Japan Neurosurgical Database.^45,46) The objective of this prospective observational multicenter study was to analyze the first 2-year surgical results of ACDR surgeries that were performed by neurosurgeons in Japan and to verify the safety and validity of 1-level ACDR in Japanese patients.

Materials and Methods

Study design and eligibility

The protocol of this prospective observational multicenter study was approved by the academic committee of JNS. Institutional Review Board approval regarding research ethics was granted first by Juntendo University and further obtained from the institutions of participating researchers. Prior to the start of this study, standard guidelines for safe ACDR for Japanese patients were jointly formulated by the following four academic societies: the Japan Neurosurgical Society, the Neurospinal Society of Japan, the Japanese Orthopaedic Association, and the Japanese Society of Spine Surgery and Related Research. All surgeons who participated in this study were board-certified neurosurgeons of the Japan Neurosurgical Society and certified spine surgeon-instructors of Neurospinal Society of Japan and completed the prescribed training course prior to the start of ACDR. Informed consent to participate in this clinical research was obtained from all patients.

Patients and surgical indications

Patients with cervical radiculopathy or myelopathy caused by cervical disc herniation or spondylosis were recruited for this study. Patients with neck pain alone and pediatric patients were excluded. The applicable spine level was limited to a single level between C3/4 and C6/7. Patients received conservative therapy for 3 months or longer prior to surgery, unless progressive neurological impairment occurs. Table 1 summarizes the surgical indications, contraindications, and precautions for ACDR.

Table 1.

Surgical indications, contraindications, and precautions for ACDR

Indications
∙	Cervical radiculopathy or myelopathy caused by disc herniation or spondylosis
∙	No indication for neck pain alone
∙	The applicable spine level is limited to 1-level disc or 2-level contiguous discs of C3/4 to C6/7
Specific notes
∙	Before consideration for surgery, conservative therapy should be followed for 3 months or longer, unless progressive neurological impairment occurs.
∙	The safety and validity of one-stage or two-stage hybrid surgery with ACDF and ACDR are not established.
∙	The safety and validity of ACDR are not established for pediatric patients with immature skeletal structure.
Contraindications
∙	Cervical infection
∙	Cervical tumor or neoplasm
∙	Cervical trauma with accompanying injuries such as bone fractures and ligament damage
∙	Allergy to implant materials
∙	Severe bone fragility
∙	Marked instability at the surgical levels
∙	No mobility between the surgical levels due to bridging osteophytes
∙	Severe cervical spondylosis (such as severe narrowing of disc height or severe facet joint disease)
∙	Significant anatomical abnormalities
∙	Significant involuntary movements in the head and neck
∙	Significant cervical spine malalignment (e.g., local kyphosis)
∙	Multilevel cervical stenosis
Precautions
∙	Patients who are highly active or who are engaged in highly active occupations
∙	Patients with mental illness, alcoholism, drug addiction, or Alzheimer’s disease
∙	Patients with congenital or developmental fusion of the cervical spine
∙	Patients with rheumatoid arthritis, destructive spondyloarthropathy, steroid treatment, spinal metastasis, acute or chronic renal failure, diabetes mellitus that requires insulin treatment, severe obesity, ankylosing spondylitis, medical history of other cervical spine disease that may affect bone metabolism, or comorbidities/medical history that may affect implant placement or surgical outcomes

ACDR anterior cervical disc replacement, ACDF anterior cervical discectomy and fusion

Data collection

Clinical data were collected and compiled using the web-based Research Electronic Data Capture (REDCap) system at Osaka Metropolitan University.^47-49) Basic patient characteristics, including age, sex, height, weight, history of cervical spine disease, past medical history, neurological condition, and radiographic assessment, were collected before surgery. The following surgical information was also collected: surgical level, type of CAD (Mobi-C or Prestige LP), size of CAD, operation time, and estimated blood loss during surgery. Postoperative assessment was conducted regularly within 6 weeks after surgery and at 6 months, 1 year, and 2 years after surgery. Neurosurgical Cervical Spine Scale (NCSS) score, Japanese Orthopedic Association (JOA) score, and the Numerical Rating Scale (NRS) were used to evaluate neurological condition before and after surgery.^50,51) NCSS and JOA scores were assessed by a surgeon before and after surgery. The recovery rate after ACDR was calculated based on the preoperative score. NRS, before and after surgery, was scored by the patient himself or herself. Radiographic assessment included plain radiographs, computed tomography, and magnetic resonance imaging of the cervical spine. Plain radiographs of the cervical spine, including lateral flexion and extension, were examined to evaluate ROM before and after surgery. All systemic and surgery-related complications were collected. Adverse events reported as Grade 2 or higher based on the Common Terminology Criteria for Adverse Events v5.0 (CTCAE) were analyzed.⁵²⁾

Statistical analysis

All data are expressed as the average ± standard deviation. Change in neurological score between preoperative status and postoperative status at each of 6 weeks, 6 months, 1 year, and 2 years was analyzed for each implant using Wilcoxon sign ranked test. All statistical analyses were performed using JMP Pro 16.2 (SAS, USA). A p value < 0.05 was considered statistically significant.

Results

Patient characteristics

A total of 54 patients were registered in this study (40 males and 14 females). Twenty-four patients underwent ACDR using Mobi-C (MC group), and 30 patients underwent ACDR using Prestige-LP (PLP group). Average age of patient was 51.1 years in the MC group (range, 32-73 years) and 47.7 years in the PLP group (range, 23-69 years). There were 19 males and 5 females in the MC group and 21 males and 9 females in the PLP group. Neurological assessment before surgery revealed radiculopathy in 31 patients (57.4%), myelopathy in 15 patients (27.8%), and unclassified type in 8 patients. Radiographic assessment before surgery classified the disease category as disc herniation in 15 patients (27.8%), osteophyte in 6 patients (11.1%), and both in 33 patients (61.1%). The surgical level of ACDR was most commonly C5/6 (32 patients, 59.3%). Table 2 summarizes the patient characteristics.

Table 2.

Patient characteristics and surgical data

Surgical data and postoperative follow-up

Average surgical time was 184 min in the MC group and 168.7 min in the PLP group. Average estimated blood loss during surgery was 7.5 mL in the MC group and 26.1 mL in the PLP group. Overall, average surgical time was 176.4 min, and the average estimated blood loss during surgery was 16.6 mL. In the MC group, the most commonly used CAD size was depth 15 mm, width 17 mm, and height 5 mm. In the PLP group, the most commonly used CAD sizes were depth 16 mm, width 17.8 mm, and height 6 mm; and depth 16 mm, width 15 mm, and height 5 mm. Table 2 summarizes the surgical data. During the 2-year postsurgical follow-up period, the cumulative number of patients excluded due to insufficient follow-up was 4 patients at 6 weeks after surgery, 7 patients at 6 months after surgery, 9 patients at 1 year after surgery, and 11 patients at 2 years after surgery. Finally, the clinical data of 21 patients in the MC group and 22 patients in the PLP group were registered. The postoperative follow-up rates at 6 weeks, 6 months, 1 year, and 2 years after ACDR were 92.6%, 87.0%, 83.3%, and 79.6%, respectively, in Supplementary Table 1.

Neurological outcome

Compared with the preoperative scores, JOA scores improved significantly early after surgery, and the improvement was maintained at 2 years after surgery in both groups (Fig. 1A, B). The average recovery rate of JOA score at 2 years after surgery was 88.9% in the MC group and 71.4% in the PLP group. Pain intensity and frequency of neck and arm pain improved significantly early after surgery, and the improvement was maintained at 2 years after surgery in both groups (Fig. 1C, D) (Table 3).

Fig. 1.

Graphs that show chronological change in neurological condition according to Japanese Orthopedic Association (JOA) score (A, B) and Numerical Rating Scale (C, D). A, C: Mobi-C, B, D: Prestige LP. ***p<0.001.

Table 3.

Chronological change of pain by Numerical Rating Scale

		Mobi-C		Prestige LP
		Average ± SD	p value	Average ± SD	p value
Neck pain: Intensity
	Preoperative	3.0 ± 2.8		4.4 ± 2.8
	Postoperative 6 weeks	1.6 ± 1.5	*	1.7 ± 1.8	***
	Postoperative 6 months	1.0 ± 1.1	***	1.7 ± 2.2	***
	Postoperative 1 year	1.3 ± 2.3	***	1.4 ± 2.0	***
	Postoperative 2 years	0.8 ± 1.3	***	1.4 ± 2.1	***
Neck pain: Frequency
	Preoperative	3.8 ± 3.5		5.0 ± 2.9
	Postoperative 6 weeks	1.7 ± 1.7	**	1.8 ± 2.2	***
	Postoperative 6 months	1.2 ± 1.5	***	2.0 ± 2.4	***
	Postoperative 1 year	1.4 ± 2.3	**	1.6 ± 2.1	***
	Postoperative 2 years	0.9 ± 1.4	***	1.3 ± 1.9	***
Arm pain: Intensity
	Preoperative	4.0 ± 3.1		4.6 ± 2.5
	Postoperative 6 weeks	1.8 ± 1.4	**	1.5 ± 2.0	***
	Postoperative 6 months	1.2 ± 1.4	***	1.7 ± 2.1	***
	Postoperative 1 year	1.7 ± 2.3	***	1.5 ± 2.0	***
	Postoperative 2 years	1.0 ± 1.3	**	1.0 ± 2.0	***
Arm pain: Frequency
	Preoperative	4.6 ± 3.7		5.5 ± 3.1
	Postoperative 6 weeks	2.0 ±2.2	**	2.3 ± 3.2	***
	Postoperative 6 months	1.6 ± 2.3	***	2.4 ± 2.9	***
	Postoperative 1 year	1.8 ± 2.6	***	1.7 ± 2.3	***
	Postoperative 2 years	1.1 ± 1.3	**	1.5 ± 2.5	***

SD standard deviation; *p < 0.05; **p < 0.01; ***p < 0.001.

Radiographic outcome

Average ROM at the surgical level before surgery in the MC group was 8.8 degrees and was maintained at 13.1 degrees at 2 years after surgery (Fig. 2A-C) (Supplementary Fig. 1). Average ROM at the surgical level before surgery in the PLP group was 8.4 degrees and was maintained at 6.2 degrees at 2 years after surgery (Fig. 2D-F) (Supplementary Fig. 1). There was loss of mobility at 2 years after surgery in 2 patients in the MC group and 2 patients in the PLP group. Heterotopic ossification (HO) was found in 5 patients in the MC group and 5 patients in the PLP group. The rates of occurrence of loss of mobility and of HO at 2 years after surgery were similar between the groups. Regarding postoperative MRI evaluation, there was a slight difference in metal artifacts between the two implants (Fig. 2G, H). Table 4 summarizes the radiographic outcomes.

Fig. 2.

Example postoperative plain lateral radiographs of the cervical spine and sagittal T2-weighted magnetic resonance images of Mobi-C (A, B, C, G) and Prestige LP (D, E, F, H).

Table 4.

Radiographic outcome, adverse events, and secondary surgery

		Mobi-C	Prestige LP
ROM (degrees) ± SD
	Preoperative	8.8 ± 9.8	8.4 ± 4.2
	Postoperative 6 weeks	12.1 ± 8.8	7.9 ± 3.5
	Postoperative 6 months	13.8 ± 9.1	8.5 ± 3.9
	Postoperative 1 year	10.8 ± 7.5	7.6 ± 4.4
	Postoperative 2 years	13.1± 8.8	6.2 ± 3.6
Loss of mobility, n (%)		2 (9.5%)	2 (9.1%)
Heterotopic ossification, n (%)		5 (23.8%)	5 (22.7%)
Surgery-related adverse events (Grade 2 or higher)
	Systemic	0	0
	Wound infection	0	0
	Implant related	0	0
Secondary surgery for surgical level of cervical spine
	Revision	0	0
	Removal	0	0
	Supplemental fixation	0	0
	Miscellaneous	0	0
Secondary surgery for adjacent levels of cervical spine
	ACDR	1	0
	ACDF	0	0
	Posterior cervical surgery	0	0
	Miscellaneous	0	0

ROM range of motion, SD standard deviation, ACDR anterior cervical disc replacement, ACDF anterior cervical discectomy and fusion

Postoperative adverse events and secondary surgery

There were no surgery-related adverse events defined as Grade 2 or higher based on the Common Terminology Criteria for Adverse Events v5.0 (CTCAE) in either group and no secondary surgery for the surgical level of cervical spine in either group. One patient in the MC group underwent secondary surgery with a different ACDR due to aggravation of radiculopathy in the inferior adjacent level of the primary ACDR. No patient in the PLP group underwent secondary surgery for adjacent levels of the cervical spine. Table 4 provides a summary of postoperative adverse events and secondary surgery.

Discussion

Summary of this study and current consensus of ACDR

This prospective observational multicenter study demonstrated the initial 2-year follow-up data of 1-level ACDR using Mobi-C or Prestige LP in Japanese patients. Both implants were clinically safe, and there were no serious adverse events, although it was still difficult for us to determine which CAD design or size may better fit the Japanese patients. As PMS study of 2-level ACDR in Japan is still underway, the safety of 1- and 2-level ACDR has not been fully determined. Adverse events, revision or secondary surgery, and the incidence rate of adjacent segmental disease after ACDR in this cohort need to be carefully followed with medium- to long-term perspectives.

The first clinical trial of CAD was reported in 1966, but due to poor results, its development was discontinued.¹⁾ Development resumed in the 1980s, and CAD has been in clinical use since the 1990s.^2,3) In the USA, CAD was first approved by the Food and Drug Administration in 2007. At least nine types of CAD are currently in use for 1- or 2-level ACDR, including Mobi-C and Prestige LP.^4-9) Clinical trials have reported the safety and efficacy of ACDR, including prospective, multicenter, and randomized trials comparing ACDR and ACDF, such as the Investment Device Exemption (IDE) clinical trials in the USA (Table 5). Positive results have demonstrated the superiority of ACDR over ACDF in terms of improvement in neurological condition, patient satisfaction, maintenance of cervical spine ROM, and surgery-related complications.^10-33) ACDR is safe and can be expected to reduce the risk of adjacent disc disease for longer after ACDR compared with ACDF. However, dissenting opinion has also been expressed, including negative results, which brings into question the superiority of ACDR over ACDF.^34-40)

Table 5.

Summary of data from recently published studies of ACDR vs. ACDF

Author	Year	Device	Study design	No. of treated discs	Follow-up duration	Effect of ACDR
Davis RJ, et al.11)	2015	Mobi-C	Prospective	2	4 years	Positive
			Multicenter
			Randomized
Skeppholm M, et al.35)	2015	Discover	Prospective	1 or 2	2 years	Negative
			Multicenter
			Randomized
Janssen ME, et al.10)	2015	ProDisc-C	Prospective	1	7 years	Positive
			Multicenter
			Randomized
Phillips FM, et al.13)	2015	PCM	Prospective	1	5 years	Positive
			Multicenter
			Randomized
Radcliff K, et al.16)	2016	Mobi-C	Prospective	2	5 years	Positive
			Multicenter
			Randomized
Hisey MS, et al.14)	2016	Mobi-C	Prospective	1	5 years	Positive
			Multicenter
			Randomized
Gornet MF, et al.15)	2016	Prestige LP	Prospective	1	7 years	Positive
			Multicenter
			Non-randomized
Gornet MF, et al.21)	2017	Prestige LP	Prospective	2	2 years	Positive
			Multicenter
			Randomized
Lanman TH, et al.22)	2017	Prestige LP	Prospective	2	7 years	Positive
			Multicenter
			Randomized
Rožankovic´ M, et al.18)	2017	Discover	Prospective	1	2 years	Positive
			Single center
			Randomized
Vaccaro A, et al.24)	2018	SECURE-C	Prospective	1	7 years	Positive
			Multicenter
			Randomized
Coric D, et al.26)	2018	Kineflex|C	Prospective	1	5 years	Positive
			Multicenter
			Randomized
Yang W, et al.27)	2018	Mobi-C	Prospective	2	> 6 years	Positive
			Single center
			Randomized
Macdowall A, et al.39)	2019	Discover	Prospective	1 or 2	5 years	Negative
			Multicenter
			Randomized
Macdowall A, et al.38)	2019	miscellaneous	Retrospective	1, 2, or 3	5 years	Negative
			Multicenter
			Non-randomized
Vleggeert-Lankamp CLA, et al.40)	2019	activC	Prospective	1	2 years	Negative
			Multicenter
			Randomized
Gornet MF, et al.29)	2019	Prestige LP	Prospective	1	10 years	Positive
			Multicenter
			Non-randomized
Gornet MF, et al.30)	2019	Prestige LP	Prospective	2	10 years	Positive
			Multicenter
			Randomized
Kim K, et al.31)	2021	Mobi-C	Prospective	1 or 2	10 years	Positive
			Multicenter
			Randomized
Loidolt T, et al.32)	2021	BRYAN	Prospective	1	10 years	Positive
			Multicenter
			Randomized
Spivak JM, et al.33)	2022	ProDisc-C	Prospective	1	7 years	Positive
			Multicenter
			Randomized

ACDR anterior cervical disc replacement, ACDF anterior cervical discectomy and fusion

CAD design

CAD design or size may be a key issue influencing clinical outcome after ACDR.^43,44,53-55). It is important for surgeons to choose the appropriate CAD design or size that matches the cervical spine of each patient. Chen et al. conducted a finite element analysis that focused on the strain behavior of malalignment in cervical spines implanted with either metal-on plastic or metal-on-metal CAD. The results suggested that in situ performance was largely dependent on the design of the CAD and to the material properties in particular.⁵⁶⁾ Purushothaman et al. examined and compared the biomechanical properties of several CAD types using finite element models.⁵⁷⁾ The greatest extent of flexion was demonstrated by Mobi-C, followed by Secure-C, Prestige LP, and Prodisc C. The greatest extent of extension was demonstrated by Secure-C, followed by Mobi-C, Prodisc C, and Prestige LP. Intradiscal pressure at the adjacent levels decreased with Mobi-C, Secure-C, and Prodisc C. Conversely, intradiscal pressure was increased at both adjacent levels with Prestige LP. Coban et al. performed a meta-analysis and systemic review of previously published reports that evaluated patient-reported clinical outcomes, overall reoperation rates, complications, and rates of adjacent disc disease between CADs with metal-on-plastic and metal-on-metal designs.⁵⁸⁾ They found superior clinical outcomes for metal-on-metal CAD in 2-level ACDR but higher rates of adjacent disc disease that require secondary surgery compared with metal-on-plastic CAD during a follow-up period of 5 years or longer. The path of motion of the instant center of rotation (COR) in the cervical spine during in vivo dynamic flexion-extension is the key issue in the design of CAD.⁵⁹⁾ Sang et al. suggested that CAD design affects the change in COR location after ACDR. When a constrained or semiconstrained CAD such as Prestige LP is selected, the COR tends to shift anteriorly and/or superiorly.⁶⁰⁾ In contrast, when a non-constrained CAD such as Mobi-C is selected, the COR tends to stay close to the original location. They also concluded that the position of CAD in the intervertebral space can affect the change in COR after ACDR. It should be noted that positioning of the CAD in the intervertebral space as well as mechanical differences between CAD designs can affect clinical outcomes.

ACDR-specific complications

Some ACDR-specific complications may require revision or secondary surgery.^41,42) In ACDR surgery, the surgeon needs to achieve sufficient spinal cord and nerve roots decompression because the ROM of the surgical level is maintained, unlike in ACDF. Particular attention should be paid to ensuring that the neural foramen is sufficiently open in cases of stenosis due to mild formation of osteophyte (Fig. 3). Insufficient decompression of the spinal cord and nerve roots may lead directly to continuous impingement of the nerve roots and an outcome of insufficient recovery of neurological condition after ACDR. Implant-related complications such as hypermobility, HO, implant subsidence or migration, vertebral osteolysis or anterior bone loss, vertebral fracture, or local kyphosis are serious complications that must be avoided.^41,42,61-66) HO is one of the most common complications following ACDR. Although its risk factors are unclear, HO after ACDR causes a reduction in ROM at the indexed level after ACDR. Kim et al. reported 10-year outcomes of 1- and 2-level ACDR using Mobi-C. Grade 3 or 4 HO was found in 11.3% of patients at 2 years after surgery, in 28.7% at 7 years after surgery, and in 30.6% at 10 years after surgery.³¹⁾ Gornet et al. reported 10-year outcomes of 1-level ACDR using Prestige LP.⁶⁷⁾ Grade 3 or 4 HO was found in 10% of patients 2 years after surgery, in 20.5% at 7 years after surgery, and in 28.5% at 10 years after surgery. Considering maintenance of ROM at the surgical level, the rate of occurrence of HO after ACDR may be higher than expected. Although there is still no effective way of reducing the risk of HO long after ACDR, Xu et al. suggested that the occurrence of HO after ACDR may be related to insufficient sagittal coverage of the endplate by CAD.⁶⁸⁾ Migration or dislocation of CAD after ACDR is a rare complication but may result in serious problems once it occurs. Previous reports of such complications are limited to case reports. Viezens et al. reported the case of a healthy 53-year-old man who suffered incomplete paraplegia due to posterior dislocation of CAD into the spinal canal.⁶⁹⁾ Brenke et al. reported a case of core herniation of CAD that presented at 8 years after the initial ACDR.⁷⁰⁾ Niu et al. reported a rare complication of CAD extrusion that occurred due to minor trauma at 14 months after placement.⁷¹⁾ Brophy et al. reported inflammatory granulomatosis as an extremely rare complication secondary to 2-level ACDR, which was recovered by a second surgery of additional posterior fixation of the cervical spine.⁷²⁾ Anterior bone loss at the level of the ACDR is a more common concern but does not affect the postoperative outcome or reoperation rate. The mechanism is presumed to be excessive load of the CAD on the inferior vertebral body.^73,74) Kieser et al. retrospectively analyzed 146 cases of ACDR and reported that anterior bone loss at the anterior endplate of the inferior vertebral body of ACDR occurred in 57.1% of their cases.⁷⁴⁾ The anterior bone loss was generally mild; however, they considered that 3% of cases were severe. Selection of CAD size in ACDR may be another key issue in securing safe ACDR surgery.

Fig. 3.

A. Example preoperative plain oblique radiograph of the cervical spine that shows neural foraminal stenosis due to mild osteophyte formation. B. Intraoperative fluoroscopic image that shows removal of the osteophytes.

Study limitations

There were several study limitations in this study. First, each surgeon was assigned to either the MC group or PLP group before the start of this study. Therefore, it was impossible for each surgeon to make a strict comparison between Mobi-C and Prestige LP. Second, it was not a comparative study of ACDF and ACDR but a single-arm study of ACDR only. Therefore, it was also difficult to determine the effect of ACDR on decreasing the risk of adjacent segmental disease after surgery. Third, a limited number of cases were analyzed, and the minimum follow-up period was 2 years after surgery. It is absolutely necessary to analyze a large number of cases in the medium to long period after surgery, focusing on the risk of adjacent disc disease, local angle, ROM of the surgical level of cervical spine, and any problems encountered with the implant itself. We plan to continue the analysis of this cohort in a second report, including the complications or adverse events associated with ACDR. Fourth, this study was conducted as an extension of PMS. Although the analysis of surgical outcomes was conducted independently without any influence of the involved companies, it is impossible to completely avoid any bias in the analysis.

Conclusions

There was delay in the adoption of ACDR in Japan compared with other countries. However, the delay has granted Japanese spine surgeons the opportunity to carefully judge the published results of ACDR. It may be necessary for us to understand that CAD with different design may have different characteristics of cervical segmental motion. The surgical results obtained in the first 2 years of this ACDR study in Japan suggest that single-level ACDR is safe and is an appropriate surgical selection for patients with cervical disc disease, although medium- to long-term follow-up is absolutely required to further verify the safety and validity of ACDR in Japanese patients.

Funding

This study received financial support from the Japan Neurosurgical Society.

Conflicts of Interest Disclosure

Each author signed a contact regarding post-marketing surveillance with either ZimVie Japan G.K. or Medtronic Sofamor Danek, Co., Ltd. before the start of this study. The analysis of surgical outcomes was conducted independently without any influence of the companies. All authors who are members of JNS have registered self-reported conflict of interest disclosure statement forms online through the website for JNS members.

Supplementary Material

Supplementary Figure 1

Graphs that show chronological change of ROM at the surgical level of ACDR.