PREVALÊNCIA DE OSSIFICAÇÃO HETEROTÓPICA ENTRE PACIENTES CINCO ANOS DEPOIS DE ARTROPLASTIA DE DISCO CERVICAL

coluna Coluna/Columna Coluna/Columna 1808-1851 2177-014X Sociedade Brasileira de Coluna RESUMO Objetivo: Este estudo foi desenhado para avaliar a prevalência e o grau de ossificação heterotópica (HO) entre pacientes depois de cinco anos de artroplastia de disco cervical (CDA). Métodos: O procedimento de CDA com próteses Activ C e M6-C foi realizado em 127 pacientes. A média de idade da coorte de pacientes foi de 38,4 anos (variação de 18 a 49). O tempo médio de acompanhamento foi de 58,4 meses, variando de 51 a 66 meses. Resultados: Ossificações de grau 1 foram encontradas em 11 (8,6%) níveis. Um total de 45 (35,4%) segmentos apresentou HO de grau 2. As HO que levaram à restrição da amplitude de movimento foram verificadas em 13 (10,2%) casos. Cinco anos depois da cirurgia, 9 (7,0%) pacientes com ossificações de grau 4 foram vistas apenas no grupo de prótese do disco artificial M6-C. Na análise de sobrevida depois da ocorrência de HO, todos os pacientes tinham sobrevida mediana de 28,3 ± 5,6 meses. O grupo que recebeu a prótese de disco artificial Activ C tinha sobrevida estatisticamente maior (49,5 ± 7,8 meses) do que a do grupo com disco M6-C. Conclusões: Em nosso estudo, 61,4% dos pacientes apresentaram HO no acompanhamento médio de 58,4 meses. Na análise de sobrevida depois de ocorrência de HO, todos os pacientes tinham sobrevida mediana de 28,3 ± 5,6 meses. O grupo que recebeu a prótese de disco artificial Activ C teve sobrevida estatisticamente maior (49,5 ± 7,8 meses) do que o grupo de disco M6-C. Nível de evidência III; Estudo Observacional Transversal. INTRODUCTION Anterior cervical diskectomy and fusion (ACDF) is the gold standard for surgical treatment of cervical degenerative disk disease, and has shown long-term clinical success.1,2 In recent years, cervical disk arthroplasty (CDA) has become widely used in patients as a substitute for ACDF.3 The introduction of CDA to treat cervical disk degenerative disease is based on the concept of preserving the patient's range of motion. Motion preservation most closely mimics the natural motion of the cervical spine, and CDA is believed to preserve the adjacent segments from degeneration over the long term, compared with ACDF.4 Randomized controlled trials have reported good outcomes and high patient satisfaction after CDA.5–7 However, heterotopic ossification (HO) and spontaneous fusion after CDA have been reported, and maintenance of motion following arthroplasty can be hindered by the development of HO.8 HO is defined as the formation of bone outside the skeletal system. It is a well-known phenomenon in the field of total hip or knee arthroplasty and immobilized the activity of patients after surgery.9 McAfee et al.,10 have already described and classified the phenomenon of HO in lumbar total disk arthroplasty. The rate of HO following CDA is unclear, but reported rates vary drastically, creating further debate and concern around the true rate and impact of HO. The long-term effects of HO resulting in unintended fusion have not been sufficiently studied. The purpose of this study was to evaluate the prevalence and graduation of HO among patients after cervical disk arthroplasty at 5 years. METHODS Patient population The CDA procedure with Activ C (B. Braun, Sheffield, UK) and M6-C (Spinal Kinetics, Sunnyvale, CA, USA) prostheses was performed from January 2009 to June 2011, on 127 patients, including 65 (51.1 %) women and 62 (48.8 %) men with symptomatic cervical disk degenerative disease but who had not responded to conservative treatment. All patients included in our study were below 50 years old. The mean age of the cohort of patients was 38.4 years (range 18-49). There were no statistically significant differences in baseline characteristics between patients with low- and high-grade HO. (Table 1) Table 1 Baseline characteristics of the patients with low- and high-grade HO 5 years after CDA. Low-grade HO group (n=56) High-grade HO group (n=22) p Mean age, years 39.2±7.3 38.6±6.6 0.81 Female sex, n (%) 11 (19.6 %) 16 (72.2 %) 0.07 Height, cm 173.5±11.3 175.5±11.2 0.26 Weight, kg 75.2±10.8 79.3±17.4 0.08 BMI* 25.4±2.5 25.7±3.8 0.74 Level operated C5-C6, % 16 (28.5 %) 12 (54.5 %) 0.19 * – The body mass index is weight in kilograms divided by the square of the height in meters. Eligibility criteria and follow-up The enrollment inclusion criterion was symptomatic cervical disk herniation at levels C3-C4 to C6-C7 with preserved mobility (> 30 and < 110) within 1 affected segment. The exclusion criteria were trauma, kyphotic deformity, ossification of the posterior longitudinal ligament, and instability of the cervical spine. Other exclusion criteria were advanced osteoporosis, rheumatoid arthritis, and ankylosing spondylitis (Bekhterev's disease). The mean follow-up time was 58.4 months, ranging from 51 to 66 months. The study protocol was approved by the local ethics committee, in accordance with the Declaration of Helsinki.11 Surgical procedure After a standard microsurgical anterior Cloward's approach, the midline was marked under fluoroscopic control. Diskectomy and decompression were performed, and the segment was distracted and held in distraction by retaining screws. After testing the intervertebral disk height and width by fluoroscopy, appropriate prosthesis was implanted. Patients were asked to get off the bed 24 hours later. A neck collar was required to wear no > 1 week. Radiological assessment Cervical lateral radiographs obtained at scheduled time points before and after surgery were used to identify HO. (Figure 1) HO in some cases was confirmed using computed tomography (CT). (Figure 2) The HO was graded according to the McAfee et al. classification. Occurrence rate, occurrence-free period, location, and grade of HOs were investigated according to the different prosthesis types. The performance of the HO was observed by two independent spine surgeons. Figure 1 A 41-year-old male patient with C5-C6 intervertebral disk degenerative disease treated by CDA. Clear signs of HO grade III can be seen, with no significant reduction in motion of the prosthesis on flexion and extension at 5-year follow-up: A – frontal radiography, B – lateral radiography with flexion, C – lateral radiograph with extension. Figure 2 CT scans of HO grade III (previous clinical case): A – sagittal CT scan, B – axial CT scan, C – 3-dimensional CT scan. Statistical analysis Data analyses were conducted in Microsoft Office Excel 2016 and IBM SPSS 21. A significance level of 0.05 was adopted for all the tests. Statistical analysis was performed using the t test and Wilcoxon test. RESULTS In 49 (38.5 %) treated segments, no HO was detectable. Grade 1ossifications were present in 11 (8.6 %) levels. A total of 45 (35.4 %) segments showed grade 2 HO. HO that led to restrictions of the range of motion were present in 13 (10.2 %) cases. Five-year postoperatively, 9 (7.0 %) patients with grade 4 ossifications were found only in the M6-C artificial disk prosthesis group. Anterior ossification was more frequent than posterior, though the difficulty in detecting posterior ossification on plain radiographs should be considered. The distribution of the different grades of HO according to prosthesis type is shown in Table 2. Table 2 Distribution of the different grades of HO according to the prosthesis type. Grade 1 Grade 2 Grade 3 Grade 4 Activ C 5 (3.9 %) 27 (21.3 %) 8 (6.3 %) 7 (5.5 %) M6-C 6 (4.7 %) 18 (14.1 %) 5 (3.9 %) 2 (1.5 %) All prostheses 11 (8.6 %) 45 (35.4 %) 13 (10.2 %) 9 (7.0 %) In the analysis of survival after HO, all patients showed median survival of 28.3±5.6 months. The Activ C artificial disk prosthesis group showed statistically longer survival (49.5±7.8 months) than the M6-C disk group (p=0.003). (Figure 3) Figure 3 Analysis of survival after the occurrence of HO: A – all patients showed median survival of 28.3±5.6 months, B – the Activ C artificial disk prosthesis group showed statistically longer survival (49.5±7.8 months) than the M6-C prosthesis group (p=0.003). DISCUSSION It is common knowledge that the occurrence of HO is an inevitable postoperative complication after CDA, and can decrease the range of motion of the operated segment, which goes against the fundamental goal of inserting an artificial disk12,13 Previous studies have reported various results in relation to the occurrence of HO. Lee et al.,14 reported that 78.6 % patients exhibited HO after a mean follow-up of 43.4 months, but Leung et al.,15 reported 17.8% of HO occurrence at 12 months of follow-up. In the study conducted by Yang et al.,16 the prevalence of HO was 90%, but their results were based on a 30-year follow-up. In our investigation, only 38.5% of patients did not show any signs of HO after CDA. There is a hypothesis that HO is not a static, but rather a dynamic and progressive phenomenon that is affected by environment.17 If this is the case, different follow-up times would definitely affect the final results. The factors associated with HO occurrence have not been clarified. According to Nunley et al.,18 odds ratios indicated that follow-up visit, male sex, and preoperative Visual Analogue Scale neck pain are related to the development of HO, whereas hazard ratios indicated male sex, obesity, endplate coverage, levels treated, and preoperative Visual Analogue Scale neck pain. Yi et al.,19 found that the occurrence of HO varies according to the different types of prosthesis used. The Bryan Disk, which has the most unconstrained motion, showed significantly lower incidence of HO compared to other prostheses. The authors proposed that differences in design, biomechanical properties, and prosthesis-specific endplate articulation could contribute to the formation of HO. In our study, the M6-C prosthesis also showed significantly lower occurrence of HO compared to the Activ C artificial disk. Park et al.,20 found that CDA influences the development of HO. In that study, two spine surgeons performed all the CDA procedures. However, they used different techniques for trimming the endplates; one spine surgeon used a fluted ball-type burr, while the other used a diamond-type burr. The study showed that the use of a fluted ball-type burr resulted in significantly higher levels of HO. Several other possible causal factors of HO have been discussed, such as not treating patients with nonsteroidal anti-inflammatory drugs (NSAIDs) after different surgical procedures. The use of NSAIDs to prevent HO after total hip replacement has been reported previously.21 The study protocols of clinical trials for CDA conducted by the US Food and Drug Administration included perioperative use of NSAIDs aimed at preventing the occurrence of HO. One study has reported a trend toward decreased HO formation in patients who used NSAIDs after CDA compared with those who did not, but the difference was not statistically significant.22 NSAIDs were not used routinely in the present investigation, and further studies are needed to assess the role of NSAIDs in the development of HO after CDA. Other predisposing factors that have been discussed are age and sex. Male sex has been reported to correlate with HO formation23 and could be a contributing factor to the differences in HO occurrence observed, compared with other reports. However, the male-to-female ratio reported in our study was not very different from that reported in other studies. There was no relationship between high- and low grade HO and age or sex in our study. Limitations Some limitations of this study are that only two types of artificial disk were investigated, the relatively small patient population, and the fact that HO was determined based only on the McAfee classification alone. CONCLUSIONS In our study, 61.4% patients exhibited HO after a mean follow-up of 58.4 months. In the analysis of survival after HO, all patients showed median survival of 28.3±5.6 months. The Activ C artificial disk prosthesis group showed statistically longer survival (49.5±7.8 months) than the M6-C disk group. This information should be useful to enable surgeons and patients to gain a better understanding of HO during follow-up. Randomized controlled trials with even longer follow-up times are needed to obtain more definite answers concerning HO occurrence, the factors associated with its occurrence and its impact on mobility, as well as clinical outcomes. Study conducted at the Irkutsk State Medical University, Irkutsk, Russia. Reviewed by: Dr. Helton Delfino REFERENCES 1 Ofluoglu AE Erdogan U Aydogan M Cevik OM Ofluoglu O Anterior cervical fusion with interbody cage containing beta-tricalcium phosphate: Clinical and radiological results Acta Orthop Traumatol Turc 2017 51 3 197 200 https://doi.org/10.1016/j.aott.2017.03.001 1 Ofluoglu AE, Erdogan U, Aydogan M, Cevik OM, Ofluoglu O. Anterior cervical fusion with interbody cage containing beta-tricalcium phosphate: Clinical and radiological results. Acta Orthop Traumatol Turc. 2017;51(3):197–200. https://doi.org/10.1016/j.aott.2017.03.001 2 Wang T Wang H Liu S An HD Liu H Ding WY Anterior cervical discectomy and fusion versus anterior cervical corpectomy and fusion in multilevel cervical spondylotic myelopathy: A meta-analysis Medicine (Baltimore) 2016 95 49 e5437 https://doi.org/10.1097/md.0000000000012618 2 Wang T, Wang H, Liu S, An HD, Liu H, Ding WY. Anterior cervical discectomy and fusion versus anterior cervical corpectomy and fusion in multilevel cervical spondylotic myelopathy: A meta-analysis. Medicine (Baltimore). 2016;95(49):e5437. https://doi.org/10.1097/md.0000000000012618 3 Byval'tsev VA Kalinin AA Stepanov IA Pestryakov YY Shepelev VV Analysis of the results of total cervical disc arthroplasty using a M6-C prosthesis: a multicenter study Zh Vopr Neirokhir Im N Burdenko 2017 81 5 46 55 https://doi.org/10.17116/neiro201781546-55 3 Byval'tsev VA, Kalinin AA, Stepanov IA, Pestryakov YY, Shepelev VV. Analysis of the results of total cervical disc arthroplasty using a M6-C prosthesis: a multicenter study. Zh Vopr Neirokhir Im N Burdenko. 2017;81(5):46–55. https://doi.org/10.17116/neiro201781546-55 4 Skeppholm M Svedmark P Noz ME Maguire GQ Jr. Olivecrona H Olerud C Evaluation of mobility and stability in the Discover artificial disc: an in vivo motion study using high accuracy 3D CT data J Neurosurg Spine 2015 23 383 389 https://doi.org/10.3171/2014.12.spine14813 4 Skeppholm M, Svedmark P, Noz ME, Maguire GQ, Jr., Olivecrona H, Olerud C. Evaluation of mobility and stability in the Discover artificial disc: an in vivo motion study using high accuracy 3D CT data. J Neurosurg Spine. 2015;23:383–389. https://doi.org/10.3171/2014.12.spine14813. 5 Gornet MF Lanman TH Burkus JK Hodges SD McConnell JR Dryer RF Cervical disc arthroplasty with the Prestige LP disc versus anterior cervical discectomy and fusion, at 2 levels: results of a prospective, multicenter randomized controlled clinical trial at 24 months J Neurosurg Spine 2017 26 6 653 667 https://doi.org/10.3171/2016.10.spine16264 5 Gornet MF, Lanman TH, Burkus JK, Hodges SD, McConnell JR, Dryer RF, et al. Cervical disc arthroplasty with the Prestige LP disc versus anterior cervical discectomy and fusion, at 2 levels: results of a prospective, multicenter randomized controlled clinical trial at 24 months. J Neurosurg Spine. 2017;26(6):653–667. https://doi.org/10.3171/2016.10.spine16264 6 Rožanković M Marasanov SM Vukić M Cervical Disk Replacement With Discover Versus Fusion in a Single-Level Cervical Disk Disease: A Prospective Single-Center Randomized Trial With a Minimum 2-Year Follow-up Clin Spine Surg 2017 30 5 E515 E522 https://doi.org/10.1097/bsd.0000000000000170 6 Rožanković M, Marasanov SM, Vukić M. Cervical Disk Replacement With Discover Versus Fusion in a Single-Level Cervical Disk Disease: A Prospective Single-Center Randomized Trial With a Minimum 2-Year Follow-up. Clin Spine Surg. 2017;30(5):E515–E522. https://doi.org/10.1097/bsd.0000000000000170 7 Sundseth J Fredriksli OA Kolstad F Johnsen LG Pripp AH Andresen H The Norwegian Cervical Arthroplasty Trial (NORCAT): 2-year clinical outcome after single-level cervical arthroplasty versus fusion-a prospective, single-blinded, randomized, controlled multicenter study Eur Spine J 2017 26 4 1225 1235 https://doi.org/10.1007/s00586-016-4922-5 7 Sundseth J, Fredriksli OA, Kolstad F, Johnsen LG, Pripp AH, Andresen H,et al. The Norwegian Cervical Arthroplasty Trial (NORCAT): 2-year clinical outcome after single-level cervical arthroplasty versus fusion-a prospective, single-blinded, randomized, controlled multicenter study. Eur Spine J. 2017;26(4):1225–1235. https://doi.org/10.1007/s00586-016-4922-5 8 Zeng J Liu H Chen H Ding C Rong X Meng Y Comparison of Heterotopic Ossification After Fixed- and Mobile-Core Cervical Disc Arthroplasty World Neurosurg 2018 120 e1319 e1324 https://doi.org/10.1016/j.wneu.2018.09.075 8 Zeng J, Liu H, Chen H, Ding C, Rong X, Meng Y, et al. Comparison of Heterotopic Ossification After Fixed- and Mobile-Core Cervical Disc Arthroplasty. World Neurosurg. 2018;120:e1319–e1324. https://doi.org/10.1016/j.wneu.2018.09.075 9 Arnold PM Anderson KK Selim A Dryer RF Burkus KJ Heterotopic ossification following single-level anterior cervical discectomy and fusion: results from the prospective, multicenter, historically controlled trial comparing allograft to an optimized dose of rhBMP-2 J Neurosurg Spine 2016 25 3 292 302 https://doi.org/10.3171/2016.1.spine15798 9 Arnold PM, Anderson KK, Selim A, Dryer RF, Burkus KJ. Heterotopic ossification following single-level anterior cervical discectomy and fusion: results from the prospective, multicenter, historically controlled trial comparing allograft to an optimized dose of rhBMP-2. J Neurosurg Spine. 2016;25(3):292–302. https://doi.org/10.3171/2016.1.spine15798 10 McAfee PC Cunningham BW Devine J Williams E Yu-Yahiro J Classification of heterotopic ossification (HO) in artificial disk replacement J Spinal Disord Tech 2003 16 4 384 389 10 McAfee PC, Cunningham BW, Devine J, Williams E, Yu-Yahiro J. Classification of heterotopic ossification (HO) in artificial disk replacement. J Spinal Disord Tech. 2003;16(4):384–389. 11 Williams JR The Declaration of Helsinki and public health Bull World Health Organ 2008 86 8 650 652 11 Williams JR. The Declaration of Helsinki and public health. Bull World Health Organ. 2008;86(8):650-652. 12 Mehren C Suchomel P Grochulla F Barsa P Sourkova P Hradil J Heterotopic ossification in total cervical artificial disc replacement Spine 2003 31 2802 2806 https://doi.org/10.1097/01.brs.0000245852.70594.d5 12 Mehren C, Suchomel P, Grochulla F, Barsa P, Sourkova P, Hradil J, et al. Heterotopic ossification in total cervical artificial disc replacement. Spine. 2003;31:2802–2806. https://doi.org/10.1097/01.brs.0000245852.70594.d5 13 Tu TH Wu JC Huang WC Heterotopic ossification after cervical total disc replacement determination by CT and effects on clinical outcomes J Neurosurg Spine 2011 14 457 465 https://doi.org/10.3171/2010.11.spine10444 13 Tu TH, Wu JC, Huang WC, et al. Heterotopic ossification after cervical total disc replacement determination by CT and effects on clinical outcomes. J Neurosurg Spine. 2011;14:457–465. https://doi.org/10.3171/2010.11.spine10444 14 Lee SE Jahng TA Kim HJ Correlation between cervical lordosis and adjacent segment pathology after anterior cervical spinal surgery Eur Spine J 2015 24 2899 2909 https://doi.org/10.1007/s00586-015-4132-6 14 Lee SE, Jahng TA, Kim HJ. Correlation between cervical lordosis and adjacent segment pathology after anterior cervical spinal surgery. Eur Spine J. 2015;24:2899–909. https://doi.org/10.1007/s00586-015-4132-6 15 Leung C Casey AT Goffin J Kehr P Liebig K Lind B Clinical significance of heterotopic ossification in cervical disc replacement: a prospective multicenter clinical trial Neurosurg 2005 57 4 759 763 https://doi.org/10.1093/neurosurgery/57.4.759 15 Leung C, Casey AT, Goffin J, Kehr P, Liebig K, Lind B, et al. Clinical significance of heterotopic ossification in cervical disc replacement: a prospective multicenter clinical trial. Neurosurg. 2005;57(4):759–763. https://doi.org/10.1093/neurosurgery/57.4.759 16 Yang H Lu X Yuan W Wang X Chen D Zhao D Artificial Disk Replacement in the Treatment of Degenerative Cervical Disk Disorder Spine 2014 39 19 1564 1571 http://dx.doi.org/10.1097/brs.0000000000000484 16 Yang H, Lu X, Yuan W, Wang X, Chen D, Zhao D. Artificial Disk Replacement in the Treatment of Degenerative Cervical Disk Disorder. Spine. 2014;39(19):1564–1571. http://dx.doi.org/10.1097/brs.0000000000000484 17 Jin YJ Park SB Kim MJ Kim K-J Kim H-J An analysis of heterotopic ossification in cervical disc arthroplasty: a novel morphologic classification of an ossified mass Spine J 2013 13 4 408 420 http://dx.doi.org/10.1016/j.spinee.2012.11.048 17 Jin YJ, Park SB, Kim MJ, Kim K-J, Kim H-J. An analysis of heterotopic ossification in cervical disc arthroplasty: a novel morphologic classification of an ossified mass. Spine J. 2013;13(4):408–420. http://dx.doi.org/10.1016/j.spinee.2012.11.048 18 Nunley PD Cavanaugh DA Kerr EH Utter Pa Campbell PG Frank KA Heterotopic Ossification After Cervical Total Disc Replacement at 7 Years – Prevalence, Progression, Clinical Implications, and Risk Factors Int. J. Spine Surg 2018 12 3 352 361 http://dx.doi.org/10.14444/5041 18 Nunley PD, Cavanaugh DA, Kerr EH, Utter Pa, Campbell PG, Frank KA, et al. Heterotopic Ossification After Cervical Total Disc Replacement at 7 Years – Prevalence, Progression, Clinical Implications, and Risk Factors. Int. J. Spine Surg. 2018;12(3):352–361. http://dx.doi.org/10.14444/5041 19 Yi S Kim KN Yang MS Yang JW Kim H Ha Y Difference in Occurrence of Heterotopic Ossification According to Prosthesis Type in the Cervical Artificial Disc Replacement Spine 2010 35 16 1556 1561 http://dx.doi.org/10.1097/brs.0b013e3181c6526b 19 Yi S, Kim KN, Yang MS, Yang JW, Kim H, Ha Y, et al. Difference in Occurrence of Heterotopic Ossification According to Prosthesis Type in the Cervical Artificial Disc Replacement. Spine. 2010;35(16):1556–1561. http://dx.doi.org/10.1097/brs.0b013e3181c6526b 20 Park JH Rhim SC Roh SW Mid-term follow-up of clinical and radiologic outcomes in cervical total disk replacement (Mobi-C): incidence of heterotopic ossification and risk factors J Spinal Disord Tech 2013 26 141 145 http://dx.doi.org/10.1097/BSD.0b013e31823ba071 20 Park JH, Rhim SC, Roh SW. Mid-term follow-up of clinical and radiologic outcomes in cervical total disk replacement (Mobi-C): incidence of heterotopic ossification and risk factors. J Spinal Disord Tech. 2013;26:141–145. http://dx.doi.org/10.1097/BSD.0b013e31823ba071 21 Sodemann B Persson PE Nilsson OS Prevention of heterotopic ossification by nonsteroid antiinflammatory drugs after total hip arthroplasty Clin Orthop Relat Res 1988 237 158 163 21 Sodemann B, Persson PE, Nilsson OS. Prevention of heterotopic ossification by nonsteroid antiinflammatory drugs after total hip arthroplasty. Clin Orthop Relat Res. 1988;237:158–163. 22 Tu TH Wu JC Huang WC Postoperative nonsteroidal anti-inflammatory drugs and the prevention of heterotopic ossification after cervical arthroplasty: analysis using CT and a minimum 2-year follow-up J Neurosurg Spine 2015 22 447 453 http://dx.doi.org/10.3171/2014.10.spine14333 22 Tu TH, Wu JC, Huang WC. Postoperative nonsteroidal anti-inflammatory drugs and the prevention of heterotopic ossification after cervical arthroplasty: analysis using CT and a minimum 2-year follow-up. J Neurosurg Spine. 2015;22:447–453. http://dx.doi.org/10.3171/2014.10.spine14333 23 Yi S Shin DA Kim KN Choi G Shin HC Kim KS The predisposing factors for the heterotopic ossification after cervical artificial disc replacement Spine J Off J North Am Spine Soc 2013 13 1048 1054 http://dx.doi.org/10.1016/j.spinee.2013.02.036 23 Yi S, Shin DA, Kim KN, Choi G, Shin HC, Kim KS, et al. The predisposing factors for the heterotopic ossification after cervical artificial disc replacement. Spine J Off J North Am Spine Soc. 2013;13:1048–1054. http://dx.doi.org/10.1016/j.spinee.2013.02.036