Cervical kinematics and radiological changes after Discover artificial disc replacement versus fusion

The Spine Journal

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Clinical Study

Cervical kinematics and radiological changes after Discover artificial disc replacement versus fusion Yang Hou, MD, Yang Liu, MD, PhD, Wen Yuan, MD, PhD*, Xinwei Wang, MD, PhD, Huajiang Chen, MD, PhD, Lili Yang, MD, PhD, Ying Zhang, MD, PhD Department of Orthopaedic Surgery, Changzheng Hospital, No. 415 Feng Yang Rd, Shanghai 200003, China Received 21 August 2011; revised 12 April 2013; accepted 10 July 2013

Abstract

BACKGROUND CONTEXT: The cervical disc arthroplasty has emerged as a promising alternative to the anterior cervical discectomy and fusion (ACDF) in patients with radiculopathy or myelopathy with disc degeneration disease. The advantages of this technique have been reported to preserve the cervical mobility and possibly reduce the adjacent segment degeneration. However, no studies have compared the clinical outcomes and radiological results in patients treated with Discover artificial disc replacement to those observed in matched group of patients that have undergone ACDF. PURPOSE: We conducted this clinical study to compare the cervical kinematics and radiographic adjacent-level changes after Discover artificial disc replacement with ACDF. STUDY DESIGN: Analysis and evaluation of data acquired in a comparative clinical study. PATIENT SAMPLE: The number of patients in the Discover and ACDF group were 149 and 196, respectively. OUTCOME MEASURES: The Neck Disability Index (NDI) and visual analog scale (VAS) pain score were evaluated. The range of movement (ROM) by the shell angle, the functional segment unit and global angles were measured, and the postoperative radiological changes at adjacents levels were observed. METHODS: A total of 149 patients with symptomatic single or two-level cervical degenerative diseases received the Discover cervical artificial disc replacement from November 2008 to February 2010. During the same period, there were a total of 196 patients undergoing one or two-level ACDF. The average follow-up periods of the Discover disc group and ACDF group were 22.1 months and 22.5 months, respectively. Before surgery, patients were evaluated using static and dynamic cervical spine radiographs in addition to computerized tomography and magnetic resonance imaging. Static and dynamic cervical spine radiographs were obtained after surgery and then at 3- and 6-month follow-up. Then, the subsequent follow-up examinations were performed at every 6-month interval. The clinical results in terms of NDI and VAS scores, the parameters of cervical kinematics, postoperative radiological changes at adjacent levels, and complications in the two groups were statistically analyzed and compared. No funding was received for this study, and the authors report no potential conflict of interest–associated biases in the text. RESULTS: Although the clinical improvements in terms of NDI and VAS scores were achieved in both the Discover and ACDF group, no significant difference was found between the two groups for both single- (VAS p5.13, NDI p5.49) and double-level surgeries (VAS p5.28, NDI p5.21). Significant differences of cervcial kinematics occurred between the Discover and the ACDF group for both the single- and double-level surgeries at the operative segments (p!.001). Except the upper

FDA device/drug status: Investigational (The DISCOVER Artificial Cervical Disc). Author disclosures: YH: Nothing to disclose. YL: Nothing to disclose. WY: Nothing to disclose. XW: Nothing to disclose. HC: Nothing to disclose. LY: Nothing to disclose. YZ: Nothing to disclose. No funds were received in support of this work. No benefits in any form have been or will be received from a commercial party related directly or indirectly to the subject of this manuscript. 1529-9430/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.spinee.2013.07.432

The authors YH and YL contributed equally to this work and should be considered as co-first authors. * Corresponding author. Department of Orthopaedic Surgery, Changzheng Hospital, No. 415 Feng Yang Rd, Shanghai 200003, China. Tel.: (86) 13501726651; fax: (86) 021-63520020. E-mail address: [email protected] (W. Yuan)

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adjacent levels for the single-level Discover and ACDF groups (p5.33), significant increases in adjacent segment motion were observed in the ACDF group compared with the minimal ROM changes in adjacent segment motion noted in the Discover group, and the differences between the two groups for both single and double-level procedures were statistically significant (p!.05). There were significant differences in the postoperative radiological changes at adjacent levels between the Discover and ACDF groups for the single-level surgery (p!.001, c2518.18) and the double-level surgery (p5.007, c257.2). No significant difference of complications was found between the Discover and ACDF groups in both single (p5.25, c251.32) and double-level cases (p5.4, c250.69). CONCLUSIONS: The adjacent segment ROM and the incidence of radiographic adjacent-level changes in patients undergoing ACDF were higher than those undergoing Discover artificial disc replacement. The cervical mobility was relatively well maintained in the Discover group compared with the ACDF group, and the Discover cervical disc arthroplasty can be an effective alternative to the fusion technique. Ó 2013 Elsevier Inc. All rights reserved. Keywords:

Cervical arthroplasty; Anterior cervical discectomy and fusion; Discover artificial disc prothesis; Comparative clinical study; Cervical degenerative disease

Introduction The cervical total disc replacement is designed to preserve segmental motion, and this technique is becoming more and more popular as an alternative to the so-far gold standard in the surgical treatment of degenerative disc disease, for example, anterior cervical discectomy and fusion (ACDF) [1,2]. The primary disadvantage of ACDF is that it converts a functionally mobile, mechanically stable spinal unit into a fixed, nonfunctional unit. Analysis of the strain distribution of intervertebral discs after anterior cervical disc fusion has shown an increase in longitudinal strain, most frequently at the levels immediately adjacent to the fused segment [3]. Many reports have shown evidence of the development of junctional degeneration adjacent to fused levels due to increased biomechanical stress [3–7] and the compensatory increase in motion at the adjacent level resulting from the reduced physiologic motion of the spine after fusion [8,9]. In addition to the adjacent-segment degeneration, other complications such as nonunion graft migration and kyphotic malunion have also been reported [10–13]. Compared with ACDF, cervical disc arthroplasty offers several theoretical and obvious advantages [14] and has gained attention as an alternative to traditional arthrodesis. The cervical artificial disc prosthesis can be used to restore and maintain mobility and function of the involved cervical spinal segments [15,16]. The Discover artificial cervical disc (DePuy Spine, Raynham, MA, USA) is an advanced, unconstrained prosthesis that is designed to allow motion similar to the normal cervical spine functional unit, following surgery for symptomatic cervical disc disease (Fig. 1). It consists of two end plates constructed from titanium alloy and a polyethylene core and has a fixed core ball-andsocket design with objectives to provide sufficient implant range of motion and maintain physiologic range of motion at treated level. Results of biomechanical study have shown that the Discover disc at two levels can provide near-normal mobility at both levels without destabilizing the implanted segments or

affecting adjacent segment motions [17]. In addition, good clinical and radiological results have been reported in a series of 25 patients, who underwent cervical arthroplasty using Discover disc, with an average follow-up period of 15.3 months [18]. However, the clinical efficacy of the Discover disc cannot be definitively determined without comparing with ACDF group and the small number of patients in this study may decrease the power of test. Therefore, we conducted this novel study to compare the clinical and radiologic outcomes of ACDF and Discover cervical disc arthroplasty in single and bi-level cases. We also aim to answer the question whether cervical artificial disc replacement is more efficacious and safer than the fusion in patients with clinical symptoms due to disc degeneration.

Materials and methods This clinical study has been approved by the institutional review board of our hospital and every patient has signed the consent form before participating in this study. In our study, patients inclusion criteria include symptoms of radiculopathy and/or myelopathy, not responding to conservative treatment for greater than or equal to 6 weeks and objective evidence of cervical disc disease at one or two vertebral levels between C3–C7. Exclusion criteria include congenital or post-traumatic deformity, infection, tumor, metabolic bone disease, severe multilevel cervical disc degeneration, medical history of fusion procedure at any level (C1–C7), allergy to the metal alloy or polyethylene, and any serious general illness (eg, heart failure, HIV) and a follow-up period less than 12 months. The eligible patients were allocated into different treatment groups according to the patient’s conditions. The patients with cervical instability (translation O3 mm and/or O11 rotational difference to that or either adjacent level), facet joint degeneration, severe spondylosis (bridging osteophytes, disc height loss O50%, and absence of motion !2 ), and osteoporosis/osteopenia were considered

Y. Hou et al. / The Spine Journal

Fig. 1. The Discover artificial cervical disc is a fixed core ball-and-socket design consisting of superior and inferior end plates manufactured from titanium alloy. Each Discover disc assembly is designed to provide seven degrees of lordosis when implanted in the cervical spine and allows motion in all rotational directions.

inappropriate for the artificial disc replacement and were treated with fusion surgery. Before surgery, patients were evaluated using static and dynamic cervical spine radiographs in addition to computerized tomography and magnetic resonance imaging. Static and dynamic cervical spine radiographs were obtained after surgery and then at 3- and 6-month follow-up. Then, the subsequent follow-up examinations were performed at every 6 months interval. All the surgical procedures in this study were performed by a single surgeon. After the standard anterior cervical discectomy, implantation of the discover artificial cervical disc

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following the recommendations of manufacturer or fusion with anterior cervical plating/stand-alone cage was performed. Clinical evaluation was completed by the patient before and after the surgery at each follow-up visit using the visual analog scale (VAS) and Neck Disability Index (NDI) functional score. The range of movements (ROMs) at the implantation and adjacent levels were evaluated using the shell angle, defined as the angle of the disc space formed preoperatively by the natural end plates and postoperatively by the shells of the prosthesis (Fig. 2, Left). The sagittal alignment of the cervical spine was calculated using the global angles formed by lines along the inferior end plate of C2 to inferior end plate of C7 in a neutral position. Range of movement of the cervical spine was defined as the difference in the global angle between the full flexion and extension as shown in lateral radiographs. The functional segment units (FSU) ROM was defined as the sum of the Cobb angle formed by lines along the superior end plate of the cephalad vertebral body and along the inferior end plate of the caudal vertebral body with the artificial disc, which was measured in both full flexion and full extension at the implantation level (Fig. 2, Right). The angles were measured using Quantitative Motion Analysis software (QMA; Medical Metrics Inc, Houston, TX, USA), which calculates the intersecting angle between two lines drawn by the investigator. The radiological evidence of adjacent-level changes after surgeries included the following radiological manifestations above or below the operated levels: new osteophyte formation or enlargement of existing osteophytes, increased or new narrowing of a disc space by more than 30% of

Fig. 2. (Left) The range of movement (ROM) of the operated and adjacent levels by shell angle. (Right) ROM of the functional segment units (b, FSU) and the C2–C7 (a, global angle).

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the intervertebral disc space, new or increasing anterior longitudinal ligament (ALL) calcification. Three independent spine surgeons were assigned to follow-up the patients after surgeries and evaluate the follow-up results of the patients. Before discharge from the hospital, the patients were given the specific dates they need to receive the follow-up examinations. At each followup visit, the patients were asked to complete the questionnaires of VAS and NDI functional score, and they were also examined by using the static and dynamic radiographs of cervical spine. We have set up the tracking record for each patient and we reminded the patient to receive the followup examinations 3 days in advance by phone call. If the patient could not come to the doctor on the scheduled date, we rescheduled the follow-up date available for the patient. To correct the intraobserver and interobserver reliability of the radiologic measurement, three experienced observers were assigned to independently evaluate the radiographs of the patients. Each of them measured three times and the mean values were used for statistical analysis. The measurement results were analyzed by SPSS software for Windows (ver. 17.0; SPSS Inc, Chicago, IL, USA) and results were considered statistically significant at the p!.05 level. Results Patient flow and characteristics An overview of the participant flow is provided in Fig. 3. A total of 438 patients were screened for eligibility, and 345 patients who meet the inclusion criteria were selected for this study. This comparative clinical study consisted of 149 eligible patients with symptomatic single or two-level cervical disc disease (Table 1), who received the Discover cervical artificial disc prosthesis (DePuy Spine, Raynham, MA, USA) from November 2008 to February 2010. The patients in the Discover group had a mean age of 45.8 years (range from 30 to 70 years). A single-level procedure was performed in 117 patients and a two-level procedure in the other 32. During the same period, there were in all 196 eligible patients with a mean age of 46.9 years (range from 29 to 77 years), who received ACDF with autogenous bone and different types of anterior cervical plates or stand-alone cages. In the ACDF group, 108 patients received one-level procedures and 88 patients underwent double-level ACDF. The average follow-up periods of the Discover disc and ACDF group were 22.1 months and 22.5 months, respectively. The explicit data regarding the percent follow-up at each time points were shown in Fig. 3. The reasons for loss to follow-up in this study included lack of money for transportation and accommodation (62%), work (24%), and child care commitments (14%). Surgical outcome According to the preoperative clinical manifestations, there were in all 38 patients presenting with symptoms of

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myelopathy, 66 patients showing symptoms of radiculopathy, and 45 patients having symptoms of combined myelopathy and radiculopathy in the Discover group. The preoperative symptoms and signs were myelopathy in 55 patients, radiculopathy in 77 patients, combined radiculopathy and myelopathy in 64 patients for the ACDF group. The average operation time for the Discover group was 137646 minutes and the ACDF group was 153649 minutes. Mean blood loss during operation was 168651 mL in the Discover group and 210661 mL in the ACDF group. According to the measurement of plate-to-disc distance (from the tip of the plate to the adjacent disc space) by the postoperative lateral radiograph of cervical spine, the mean cephalad plate-to-disc distance was 3.9761.86 mm, and the mean caudal plate-to-disc distance was 4.2261.74 mm in the ACDF group. Clinical outcome For both the Discover and ACDF groups, the patients had significant pain relief and functional activity improvement during the follow-up period. Compared with the preoperative scores, VAS and NDI scores for patients, who received the single- or double-level operational procedure, at the last follow-up significantly decreased (p!.001; Table 2) in both the Discover and ACDF groups. However, there were no significant differences of VAS and NDI improvements between the Discover and ACDF groups in both single- (VAS p5.13, NDI p5.49; Table 3) and double-level cases (VAS p5.28, NDI p5.21).

Radiographic analysis Range of motion Operated level Range of movement of the operated segments were assessed using both FSU and the shell angle. In the singlelevel Discover group, FSU ROMs increased from 11.4 62.1 to 12.8 61.7 and ROM by shell angle correspondingly increased from 10.2 61.8 to 11.6 61.1 but without statistical significance (FSU p5.14, ROM by shell angle p5.11; Tables 4 and 5). Range of movement by FSU and shell angle significantly decreased from 10.7 62.2 to 3.2 61.1 and from 10.8 62.2 to 2.0 60.8 , respectively, in the single-level ACDF group (FSU p!.001, ROM by shell angle p!.001). Compared with the single-level ACDF group, ROMs of the operated segments from pre-op to final follow-up in the single-level Discover group were relatively well maintained and the difference was statistically significant (p!.001; Table 3). In the double-level Discover group, ROMs by FSU and shell angle increased from 12.3 61.8 to 13.4 61.9 and from 9.9 62.5 to 11.0 61.9 , respectively. No statistical differences of ROM increases were found in this group (FSU p5.053, ROM by shell angle p5.052; Tables 4 and 5).

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Fig. 3. The participant flow diagram showing the number of patients through the study. ACDF, anterior cervical discectomy and fusion.

Compared with the preoperative values, ROMs by FSU and shell angle decreased approximately 71.0% and 85.3%, respectively, in the double-level ACDF group, and ROM decreases were found to have obvious statistical significance (FSU p!.001, ROM by shell angle p!.001). Range of movements of the double-level Discover group were relatively well preserved compared with the double-level ACDF group, and the difference between the two groups was statistically significant (p!.001; Table 3). Upper adjacent level According to the radiographic measurements, ROMs at the upper adjacent levels increased from 8.9 61.7 to 9.9 61.9 for the single-level Discover group and from

9.1 61.6 to 10.9 61.9 for the single-level ACDF group. Both groups showed an increase of ROM at the final follow-up compared with the preoperative ROM. However, only the change in the single-level ACDF group had statistical significance (p5.016; Table 4), and there was no difference in ROM changes at the upper adjacent levels between the single-level Discover and single-level ACDF groups (p5.33; Table 3). For the double-level Discover group, ROMs at the upper adjacent levels increased from 9.4 61.3 to 10.3 60.9 , but without statistical significance (p5.624; Table 4). The corresponding ROMs of the double-level ACDF significantly increased from 8.7 62.1 to 10.4 61.7 at the final follow-up (p5.027). Significant difference of ROM

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Table 1 Demographic data of patients Parameters

Single-level Discover

Single-level ACDF

Double-level Discover

Double-level ACDF

Patient number Age (y) Sex Clinical symptoms Myelopathy Radiculopathy Combined symptoms Operated level

117 45.6 (31–70) 66 Males, 51 females

108 44.1 (30–74) 60 Males, 48 females

32 46.3 (30–69) 20 Males, 12 females

88 51.2 (29–77) 38 Males, 50 females

32 51 34 C3/C4: 10; C4/C5: 15; C5/C6: 68; C6/C7: 24 21.8 (12–26)

31 39 38 C3/C4: 20; C4/C5: 15; C5/C6: 54; C6/C7: 19 22.1 (14–25)

6 15 11 C3/C4–C5/C6: 1; C4/C5–C5/C6: 12; C5/C6–C6/C7: 19 24.2 (13–27)

24 38 26 C4/C5–C5/C6: 40; C5/C6–C6/C7: 48 23.3 (12–27)

Average follow-up period

ACDF, anterior cervical discectomy and fusion.

changes in the superior adjacent levels were found between the two groups (p5.043; Table 3). Lower adjacent level Range of movements at the lower adjacent levels increased from 8.6 62.0 to 9.4 61.9 for the single-level Discover group and from 11.0 61.4 to 13.6 62.1 for the single-level ACDF group. Only ROM increases at the lower adjacent levels for the single-level ACDF group were found to have statistical significance (p!.001; Table 4) and the difference in ROM changes between the two groups was statistically significant (p!.001; Table 3). The corresponding ROMs of the double-level Discover group increased from 8.7 62.0 to 9.4 62.1 , but without significant difference (p5.202; Table 4). Range of movements significantly increased from 11.0 62.1 to 14.3 61.7 in the lower adjacent segments of the doublelevel ACDF group (p!.001), and the difference of ROM changes between the two groups was also statistically significant (p!.001; Table 3). Range of movement of the C2–C7 The C2–C7 ROMs of the single-level Discover group were 48.9 69.3 on admission and 52.8 66.8 at the final

follow-up. However, there was no statistical significance in ROM changes for the single-level Discover group (p5.06; Table 5). In the single-level ACDF group, the C2–C7 ROMs showed a significant decrease by 28.8% at the final follow-up compared with preoperative measurements (p!.001). The difference between the single-level Discover and single-level ACDF group was found to be statistically significant (p!.001; Table 3). For the double-level Discover group, the C2–C7 ROMs showed an increase of 6.0% at the final follow-up but without significant difference (p5.184; Table 5). In the doublelevel ACDF group, the C2–C7 ROMs decreased by 22.1% at the final follow-up with significant difference (p!.001). The difference between the double-level Discover and double-level ACDF group was statistically significant (p!.001; Table 3).

Radiological changes at adjacent levels The radiological review demonstrated new osteophyte formation in five patients (4.3%; Fig. 4) in the singlelevel Discover group and in 19 patients (17.6%) in the single-level ACDF group (Table 6). The enlargement of the existing osteophyte was observed in two patients (1.7%) in the single-level Discover group and in eight

Table 2 Comparison of clinical outcomes between Discover disc and ACDF with VAS and NDI Parameters VAS Pre-op Post-op 3 mo Post-op 6 mo Post-op 1 y Final follow-up p Values within group NDI Pre-op Post-op 3 mo Post-op 6 mo Post-op 1 y Final follow-up p Values within group

Single-level Discover

Single-level ACDF

Double-level Discover

Double-level ACDF

8.161.1 3.660.9 2.961.0 2.860.8 2.661.0 !.001

8.261.4 4.160.9 3.461.2 3.061.0 3.160.8 !.001

8.361.0 4.260.7 3.361.1 2.760.7 2.860.9 !.001

8.060.9 4.361.0 3.960.8 3.161.3 3.061.0 !.001

49.8619.7 28.3617.4 25.1615.4 21.7616.5 17.2613.4 !.001

51.2617.3 26.7618.1 24.8616.8 20.2614.7 18.3611.4 !.001

48.6618.4 27.3616.5 24.6617.1 21.5613.8 19.5612.7 !.001

50.2617.8 30.1616.9 26.3615.8 22.3614.4 18.7612.3 !.001

ACDF, anterior cervical discectomy and fusion; VAS, visual analog scale; NDI, Neck Disability Index; Pre-op, preoperative; Post-op, post operative.

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Table 3 The clinical and radiologic results of Discover group compared with ACDF group Parameters

Single-level Discover and ACDF (p)

Double-level Discover and ACDF (p)

NDI score VAS score FSU ROM ROM of operated level ROM of upper adjacent level ROM of lower adjacent level ROM of C2–C7 Radiological changes at adjacent levels Complication

.49 .13 !.001 !.001 .33 !.001 !.001 !.001 .25

.21 .28 !.001 !.001 .043 !.001 !.001 .007 .4

ACDF, anterior cervical discectomy and fusion; NDI, Neck Disability Index; VAS, visual analog scale; FSU, functional segment units; ROM, range of movement.

patients (7.4%) in the single-level ACDF group. Increased disc space narrowing occurred in four patients (3.4%) in the single-level Discover group and in seven patients (6.5%) in the single-level ACDF group. New disc space narrowing was present in two patients (1.7%) in the former group and three patients (2.8%) in the latter group. For the single-level Discover group, ALL calcification increase was noted in one patient (0.85%), and no case with new ALL calcification was found according to the postoperative radiographs during the follow-up period. For the singlelevel ACDF group, ALL calcification increase was noted in two patients (1.9%), and no new ALL calcification was observed. A total of 14 (12.0%) patients in the singlelevel Discover group were found to develop radiological changes compared with 39 patients (36.1%) in the singlelevel ACDF group, and the difference between the two groups was statistically significant according to the Chisquare tests (p!.001, c2518.18).

For the double-level Discover group, the new osteophyte formation, increased disc space narrowing, and new disc space narrowing were observed in three patients (9.4%; Table 6), one patient (3.1%), and one patient (3.1%), respectively, and no case with osteophyte enlargement was found. For the double-level ACDF group, the corresponding numbers of patients were 17 (19.3%), 6 (6.8%), and 2 (2.3%), respectively, and the existing osteophyte enlargement was noted in 11 patients (12.5%; Fig. 5). No ALL calcification increase was observed in the double-level Discover group compared with one case (1.1%) in the double-level ACDF group, and new ALL calcification was not present in both groups. Overall, five patients (15.6%) in the double-level Discover group had postoperative radiological changes in contrast to 37 patients (42%) in the double-level ACDF group, and there was a significant difference in the postoperative radiological changes between the two groups (p5.007, c257.2).

Table 4 The sagittal ROM at the operated and adjacent levels Parameters

Single-level Discover

The sagittal ROM at the operated levels Pre-op (  ) 10.261.8 10.962.1 Post-op 3 mo (  ) Post-op 6 mo (  ) 11.161.7 Post-op 1 y (  ) 11.361.9 Final follow-up (  ) 11.661.1 p Values within group .11 The sagittal ROM at the upper adjacent levels Pre-op (  ) 8.961.7 Post-op 3 mo (  ) 9.262.1 9.061.9 Post-op 6 mo (  ) Post-op 1 y (  ) 9.461.6 Final follow-up (  ) 9.961.9 p Values within group .206 The sagittal ROM at the lower adjacent levels Pre-op (  ) 8.662.0 Post-op 3 mo (  ) 8.862.1 9.161.8 Post-op 6 mo (  ) Post-op 1 y (  ) 9.362.2 Final follow-up (  ) 9.461.9 p Values within group .096

Single-level ACDF

Double-level Discover

Double-level ACDF

10.862.2 2.560.9 2.361.1 2.261.2 2.060.8 !.001

9.962.5 10.162.0 10.862.1 10.462.0 11.061.9 .052

11.662.1 2.260.4 2.060.3 1.860.3 1.760.2 !.001

9.161.6 8.961.8 9.861.6 10.462.1 10.961.9 .016

9.461.3 9.562.0 9.661.9 9.961.4 10.360.9 .624

8.762.1 8.560.8 9.161.1 9.560.9 10.461.7 .027

11.061.4 10.761.8 11.962.0 12.861.6 13.662.1 !.001

8.762.0 8.961.8 9.062.1 9.361.9 9.462.1 .202

11.062.1 11.261.7 12.162.1 13.261.8 14.361.7 !.001

ROM, range of movement; ACDF, anterior cervical discectomy and fusion; Pre-op, preoperative; Post-op, post operative.

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Table 5 Range of movement of FSU and C2–C7 Parameters FSU ROM Pre-op (  ) Post-op 3 mo (  ) Post-op 6 mo (  ) Post-op 1 y (  ) Final follow-up (  ) p Values within group ROM of the C2–C7 Pre-op (  ) Post-op 3 mo (  ) Post-op 6 mo (  ) Post-op 1 y (  ) Final follow-up (  ) p Values within group

Single-level Discover

Single-level ACDF

Double-level Discover

Double-level ACDF

11.462.1 10.961.9 11.662.3 12.161.8 12.861.7 .14

10.762.2 2.860.9 2.760.6 2.961.0 3.261.1 !.001

12.361.8 11.762.2 12.561.6 13.162.1 13.461.9 .053

12.462.3 3.260.8 2.960.7 3.361.1 3.660.9 !.001

48.969.3 50.868.6 51.367.9 52.268.7 52.866.8 .06

49.7610.2 27.464.1 30.163.9 32.362.9 35.464.7 !.001

52.168.7 51.869.5 53.967.2 54.766.8 55.266.9 .184

50.767.6 30.166.9 32.368.1 34.266.6 39.567.1 !.001

FSU, functional segment units; ACDF, anterior cervical discectomy and fusion; ROM, range of movement; Pre-op, preoperative; Post-op, post operative.

was found between the Discover and ACDF groups in both single (p5.25, c251.32) and double-level cases (p5.4, c250.69; Table 7).

Complications In the Discover group, prevertebral hematoma and wound infection occurred in one patient. Cerebrospinal fluid leak, epidural hematoma, and migration of the prosthesis were noted in two patients. The anterior displacement of the Discover disc was less than 2 mm in the two patients with no symptoms and no treatment was undertaken. Three cases were found to have postoperative C5 palsy and hoarseness. Dysphagia was present in 11 patients postoperatively. No vertebral artery injury and implant subsidence were found in this group. A total of 25 patients (16.8%) including 18 patients (15.4%) with single-level Discover and 7 patients (21.9%) with double-level Discover developed complications postoperatively (Table 7). In the ACDF group, prevertebral hematoma, cerebrospinal fluid leak, wound infection, and subsidence were found in three patients. Two patients were observed to have cage migration, and epidural hematoma was present in four patients. There were six patients showing C5 radiculopathy and hoarseness. Dysphagia occurred in 19 patients and no vertebral artery injury was found. There were in all 49 patients (25%) including 23 patients (21.3%) with single-level ACDF and 26 patients (29.5%) with double-level ACDF having postoperative complications. According to the Chi-square tests, no significant difference of complications

Discussion In the present study, the Discover cervical disc arthroplasty was performed in patients with cervical degenerative diseases, and the clinical and radiological results for Discover group were statistically analyzed in contrast to the fusion group. In our series, significant improvements in NDI and VAS scores for both the Discover and ACDF groups were achieved at the final follow-up but statistically insignificant between the arthroplasty and arthrodesis groups. There were no deteriorations in either group. In addition, no significant difference of the complications was found between the Discover and ACDF groups. One of the major disadvantages of ACDF is that it does not preserve the normal kinematics of the spine and therefore may result in degeneration at adjacent segments potentially requiring an additional fusion [3,5–7,19]. Goffin et al. [20] reported a 92% rate of adjacent-level radiologic degeneration after fusion over an average of 8.6 years. In addition, Hilibrand et al. [6] found that symptomatic adjacent segment disease occurred at an average rate of 2.9% per

Table 6 Summary of radiological changes after surgeries Parameters

Single-level Discover (N5117)

Single-level ACDF (N5108)

Double-level Discover (N532)

Double-level ACDF (N588)

New osteophyte formation (%) Osteophyte enlargement (%) Increased disc space narrowing (%) New disc space narrowing (%) ALL calcification increase (%) New ALL calcification Total of new X-ray changes (%) p Value between groups

5 (4.3) 2 (1.7) 4 (3.4) 2 (1.7) 1 (0.85) 0 14 (12.0) p!.001, c2518.18

19 8 7 3 2 0 39

3 (9.4) 0 1 (3.1) 1 (3.1) 0 0 5 (15.6) p5.007, c257.2

17 11 6 2 1 0 37

(17.6) (7.4) (6.5) (2.8) (1.9) (36.1)

ACDF, anterior cervical discectomy and fusion; ALL, anterior longitudinal ligament.

(19.3) (12.5) (6.8) (2.3) (1.1) (42.0)

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Table 7 Summary of the complications after surgeries Parameters Prevertebral hematoma (%) Cerebrospinal fluid leak (%) Epidural hematoma (%) Vertebral artery injury Dysphagia (%) Hoarseness (%) C5 radiculopathy (%) Wound infections (%) Migration of a prosthesis (%) Subsidence (%) Total (%)

Single-level Discover (N5117)

Single-level ACDF (N5108)

Double-level Discover (N532)

Double-level ACDF (N588)

1 (0.9) 2 (1.7) 1 (0.9) 0 8 (6.8) 2 (1.7) 2 (1.7) 0 2 (1.7) 0 18 (15.4) (p5.25, c251.32)

2 1 2 0 9 3 3 1 1 1 23

0 0 1 (3.1) 0 3 (9.4) 1 (3.1) 1 (3.1) 1 (3.1) 0 0 7 (21.9) (p5.4, c250.69)

1 2 2 0 10 3 3 2 1 2 26

(1.9) (0.9) (1.9) (8.3) (2.8) (2.8) (0.9) (0.9) (0.9) (21.3)

(1.1) (2.3) (2.3) (11.4) (3.4) (3.4) (2.3) (1.1) (2.3) (29.5)

ACDF, anterior cervical discectomy and fusion.

year, and 25.6% of patients were predicted to have new onset of symptoms within 10 years of fusion. Different studies have been conducted to investigate the cause of adjacent segment disease in patients undergoing ACDF, and the mechanisms of adjacent segment degeneration have been hypothesized to be the increased tissue strain and/or increased intradiscal pressures at levels adjacent to the fusion [3,20]. However, there are several studies that are not consistent with the previous results. Gore and Sepic [21] reviewed 146 patients receiving ACDF with an average follow-up period of 563.3 years and found that there was no difference in the incidence of adjacent segment degeneration between the ACDF and control groups. Ishihara et al. [22] reported that there was no relationship between the incidence of ASD and the number of levels fused and thought that adjacent segment disease is the result of a continuing degenerative process at adjacent levels and is not caused by the spinal fusion itself. Jawahar et al. [23] reported a 16% rate of adjacent segment degeneration after total disc

arthroplasty and a 18% rate after ACDF with a follow-up period of 4 years. They found that the risk of developing adjacent segment degeneration is equivalent after both procedures but is significantly higher in patients with concurrent degenerative disc disease in lumbar spine. We believe that more long-term clinical studies will be required to ascertain whether the artificial disc is able to circumvent adjacent-level disease because it simulates the motion of the native spine at the adjacent level. It has been reported that an appropriate plate length should be selected to maintain a minimum distance of 5 mm between the ends of the plate and adjacent discs, which can help to decrease adjacent-level ossification [24]. When performing ACDF, we strive to insert the plate as far as possible from the adjacent disc space. The shortest plate can be placed by drilling fixation holes as close to the fused disc and keeping the trajectory of the drill away from the fused disc both cranially and caudally. All the surgeries in this study have been completed by the director of the department of orthopaedic surgery in our hospital, who has

Fig. 4. (Left) The preoperative, (Middle) immediately postoperative, (Right) and final follow-up radiographs showing new anterior osteophyte formation in this patient after placement of a C5/C6 Discover disc prosthesis. The arrows indicate the lesion sites.

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Fig. 5. (Left) Immediately postoperative and (Right) final follow-up radiographs showing osteophyte enlargement observed in one patient from the doublelevel anterior cervical discectomy and fusion group. The arrows indicate the lesion sites.

successfully performed more than 2,000 cervical spine operations. Good clinical results of these procedures have been acquired and no major side effects have been observed. We think that the fact all the surgeries were completed by the same surgeon using the standardized procedures can largely decrease the clinical bias resulting from different preference of several surgeons. According to the radiological measurements, ROM by shell angle, FSU, and overall cervical angle (C2–C7) were relatively well maintained in the Discover group compared with the ACDF group for both the single- and double-level surgeries at the operative and adjacent segments. Postoperatively, significant increases in adjacent segment motion in the ACDF group occurred compared with the minimal ROM changes in adjacent-level motion observed in the Discover group. In addition, our study showed that the incidences of postoperative radiological changes at adjacent levels in the ACDF group were significantly higher than the Discover group. These findings indicate that the Discover cervical disc arthroplasty with motion preservation of the cervical spine could reduce the stress on the nonoperated adjacent segments and eventually decrease the incidence of disc degeneration at these levels. In addition, the difference of the radiographic adjacentlevel changes between the two groups did not result in the difference in clinical outcomes between the Discover and ACDF groups. We believe a longer follow-up period is required to analyze the correlation between the radiological changes and the symptomatic adjacent disc disease in the two groups.

The previous results suggest that Discover cervical disc arthroplasty is as effective and safe as ACDF at treating cervical degenerative diseases. However, the limitations of this study include that this is a nonrandomized clinical study and the differences in patients’ conditions between the Discover and ACDF groups may influence patient outcomes. We believe a prospective randomized clinical study is required to adequately assess the clinical efficacy and safety of the Discover disc compared with ACDF. Conclusion In this study, there were significant increases in adjacent segment ROMs and the incidence of radiographic adjacentlevel changes in patients undergoing ACDF, compared with patients receiving the Discover cervical artificial disc replacement. The cervical kinematics of the operated segments were relatively well maintained in the Discover group compared with the ACDF group, and the Discover cervical disc arthroplasty can be an effective alternative to the fusion technique. References [1] Anderson P, Sasso RC, Riew KD. Update on cervical artificial disk replacement. Instr Course Lect 2007;56:237–45. [2] Phillips MF, Garfin SR. Cervical disc replacement. Spine 2005;30: S27–33. [3] Matsunaga S, Kabayama S, Yamamoto T, et al. Strain on intervertebral discs after anterior cervical decompression and fusion. Spine 1999;24:670–5.

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[15] Lin EL, Wang JC. Total disk arthroplasty. J Am Acad Orthop Surg 2006;14:705–14. [16] Yoon DH, Yi S, Shin HC, et al. Clinical and radiological results following cervical arthroplasty. Acta Neurochir (Wien) 2006;148: 943–50. [17] Phillips FM, Tzermiadianos MN, Voronov LI, et al. Effect of twolevel total disc replacement on cervical spine kinematics. Spine 2009;34:E794–9. [18] Du J, Li M, Liu H, et al. Early follow-up outcomes after treatment of degenerative disc disease with the discover cervical disc prosthesis. Spine J 2011;11:281–9. [19] Goffin J, Pointillart V, Lind B. Two-year clinical results from a multicenter study of the Bryan cervical disc system. Spine J 2004;4: 3S–199S. [20] Goffin J, Geusens E, Vantomme N, et al. Long-term follow-up after interbody fusion of the cervical spine. J Spinal Disord Tech 2004;17: 79–85. [21] Gore DR, Sepic SB. Anterior cervical fusion for degenerated or protruded discs: a review of one hundred forty-six patients. Spine 1984;9:667–71. [22] Ishihara H, Kanamori M, Kawaguchi Y, et al. Adjacent segment disease after anterior cervical interbody fusion. Spine J 2004;4: 624–8. [23] Jawahar A, Cavanaugh DA, Kerr EJ 3rd, et al. Total disc arthroplasty does not affect the incidence of adjacent segment degeneration in cervical spine: results of 93 patients in three prospective randomized clinical trials. Spine J 2010;10:1043–8. [24] Park JB, Cho YS, Riew KD. Development of adjacent-level ossification in patients with an anterior cervical plate. J Bone Joint Surg Am 2005;87:558–63.