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Anterior cervical interbody fusion with a titanium box cage: early radiological assessment of fusion and subsidence
The Spine Journal 5 (2005) 645–649
Technical Review
Anterior cervical interbody fusion with a titanium box cage: early radiological assessment of fusion and subsidence Hans-Peter W. van Jonbergen, MDa,*, Maarten Spruit, MDa, Patricia G. Anderson, MAb, Paul W. Pavlov, MD, PhDa a
Institute of Spinal Surgery and Applied Research, Sint Maartenskliniek, Hengstdal 3, 6522 JV Nijmegen, The Netherlands b Orthopaedic Research, Sint Maartenskliniek, Hengstdal 3, 6522 JV Nijmegen, The Netherlands Received 12 October 2004; accepted 7 July 2005
Abstract
BACKGROUND CONTEXT: The use of stand-alone cervical interbody cages in anterior cervical discectomy with fusion (ACDF) has become popular, but high subsidence rates have been reported in the literature. PURPOSE: The authors present short-term radiological results of a titanium box cage with regard to fusion and subsidence. Reliable fusion and lack of subsidence may influence long-term clinical results. Early radiological data are necessary before implementation of this device on a larger scale can be accepted. STUDY DESIGN/SETTING: Retrospective radiological quality assessment study. PATIENT SAMPLE: ACDF using the titanium cage was performed in 71 consecutive patients at 106 levels. Diagnoses included cervical disc disease (57) and cervical spinal stenosis (14) after failed conservative treatment. OUTCOME MEASURES: Subsidence and kyphosis were assessed on lateral cervical radiographs made directly postoperative and at 3- and 6-month follow-up. At 6-month follow-up, lateral flexion-extension radiographs were made to assess fusion. METHODS: Subsidence of the cage was defined as a decrease in total vertical height of the two fused vertebral bodies as measured on the lateral cervical radiographs made 3 and 6 months postoperatively compared with the directly postoperative radiographs. Segmental kyphosis was measured as the angle between the posterior borders of the two vertebral bodies on the lateral radiograph. RESULTS: No patients were lost to follow-up. Fusion was achieved after 6 months in all patients. At 3 and 6 months postoperative the same 10 cages (each in a different patient) had subsided. The C6-C7 level was significantly more frequently involved compared with all other levels. A segmental kyphotic alignment was observed in five patients at the C6-C7 level and in one patient at the C4-C5 level. CONCLUSIONS: For patients with cervical disc disease, the high subsidence tendency of the cage into the end plate of predominantly C7 is a disturbing phenomenon found in this study. A modified cage design that improves and extends contact with the inferior surface could be expected to reduce subsidence into C7. Ó 2005 Elsevier Inc. All rights reserved.
Keywords:
Cervical cage; Anterior cervical interbody fusion; Subsidence
Introduction
FDA device/drug status: approved for this indication (SynCage C). Nothing of value received from a commercial entity related to this manuscript. * Corresponding author. Department of Orthopaedics, Deventer Hospital, P.O. Box 5001, 7400 GC Deventer, The Netherlands. Tel.: 131 570 646858; fax: 131 570 646085. E-mail address: [email protected] (H.-P.W. van Jonbergen) 1529-9430/05/$ – see front matter Ó 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.spinee.2005.07.007
Anterior cervical discectomy and fusion (ACDF) is an effective procedure to treat a variety of spinal disorders, such as radiculopathy, myelopathy, herniated discs, and cervical disc disease [1–5]. The objective of anterior cervical interbody fusion is to provide segmental stability and solid arthrodesis with minimal surgical risks. Although ACDF as described by Smith and Cloward has been considered to be the standard surgical procedure after conservative treatment
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has failed [6,7], Van Limbeek et al. [8] concluded in a systematic literature review that no gold standard for the treatment of single level cervical disc disease could be identified. Various materials have been used as interbody graft in anterior cervical fusion [9]. High fusion rates have been documented with autogenous graft [4,10], and high pseudarthrosis rates have been described when using allografts [11]. To supplement the bone graft, in the past 40 years a number of fusion devices have been developed, either for stand-alone use or in conjunction with anterior or posterior instrumentation [9,12]. The objective of all such spinal devices is to immobilize the unstable degenerated motion segment so that bony fusion can occur. Currently three types of spinal fusion devices are available: horizontal cylinders, vertical rings, and open box cages like the SynCage C (Synthes, Oberdorf, Switzerland) [13,14]. Only one published report describes the in vivo characteristics of the SynCage C [15]. In this retrospective study, five of nine fused levels had radiological signs of cage subsidence. The purpose of the present cage study was to assess early fusion and subsidence. Reliable fusion and lack of subsidence may influence satisfactory long-term clinical results, and therefore early radiological data are necessary before implementation of such a device on a larger scale can be accepted.
Implant description The titanium SynCage C has been adapted to the cervical disc space dimensions and has been designed to restore and maintain disc height and lordosis. It features a large cranial contact surface with the vertebral end plate and a large bony distal contact area to promote fusion between the vertebral bodies. The cranial surface of the cage is convex to address the concavity of the cranial vertebral end plate. Trial implants in the distracted disc space during surgery determine the appropriate implant size. Surgical technique The cervical spine is approached anteriorly with the neck in a slight lordotic position. The intervertebral space is opened with a Caspar distractor, after which complete discectomy, removal of osteophytes, and careful end plate preparation can be performed. The SynCage C is filled with autogenous bone graft harvested from the iliac crest through a standard, minimally invasive approach with a wire-guided trocar. Postoperatively the cervical spine is immobilized for 3 months with a cervical orthosis, which is not used at night. Follow-up examination Follow-up was scheduled after 3 months and 6 months with clinical and radiological examination. Assessment of fusion and subsidence
Materials and methods Patient selection Between March 1999 and August 2001, anterior cervical discectomy and fusion using a SynCage C was performed in 71 consecutive patients (41 males) at 106 levels. Median age at operation was 49 (23–76) years. Diagnoses included cervical disc disease (57) and cervical spinal stenosis (14), both after failed conservative treatment. Forty-five patients had a single level fusion, 19 patients a 2-level fusion, five patients a 3-level fusion, and two patients had a 4-level fusion. The C5-C6 level was most frequently fused (Table 1). A retrospective review was conducted in 2004.
Table 1 Level of fusion related to subsidence Level
Number of cages
No subsidence (%)
Subsidence (%)
C3-4 C4-5 C5-6 C6-7 C7-T1 Total
10 14 47 34 1 106
10 13 46 26 1 96
0 1 1 8 0 10
(100%) (93%) (78%) (76%) (100%) (91%)
Subsidence and kyphosis were assessed on lateral cervical radiographs made directly postoperative and at 3- and 6-month follow-up. Furthermore, at 6-month follow-up lateral flexion-extension radiographs were made in order to assess fusion. Subsidence of the SynCage C was defined as a decrease in total vertical height of the two fused vertebral bodies as measured on the lateral cervical radiographs made 3 and 6 months postoperatively compared with the directly postoperative radiographs. Subsidence greater than 3 mm was considered relevant [15]. Because of intra-individual variations of the magnification factor in the radiographs, the total vertical height of the two fused vertebral bodies was corrected for magnification differences by using the anteroposterior diameter of the upper vertebral body on the lateral cervical radiograph [16]. Segmental kyphosis was measured as the angle between the posterior borders of the two vertebral bodies on the lateral radiograph. If the difference of the interbody angle on the flexion and extension radiographs was not greater than 2 degrees, stability was assumed [15,17].
(7%) (2%) (24%)
Statistical analysis
(9%)
A Student t test was performed to see whether age was related to possible subsidence. A chi-square analysis was
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647
performed to determine whether possible subsidence was related to the following factors: diagnosis, gender, cage size, number of segments fused, and level of fusion. The level of significance was set at p!.05. Results Surgical technique and complications No technical complications occurred during surgery. All immediate postoperative radiographs showed adequate cage position without signs of subsidence (Fig. 1). Follow-up No patients were lost to follow-up. Assessment of fusion and subsidence Fusion was achieved after 6 months in all patients. At 3 and 6 months postoperative, the same 10 cages (all in different patients) showed subsidence greater than 3 mm (Table 1). In 9 of 10 patients, the cage subsided into the caudal vertebral body, producing only loss of anterior intervertebral height (Fig. 2), whereas in one patient in addition to the anterior height, loss of posterior intervertebral height was also observed (C5-C6 level). A kyphotic alignment was observed in five patients at the C6-C7 level and in one patient at the C4-C5 level, all six with anterior subsidence of the cage. Mean subsidence after 3 months was 0.861.5 mm, and after 6 months 1.061.8 mm. Overall, subsidence O3 mm was observed in 9% of the cages. The C6-C7 level was significantly more involved compared with all other levels (p5.002). All patients with subsidence had been operated for cervical disc disease (Table 2). The age (p5.465) and the gender (p5.877) of the patients with subsidence did
Fig. 2. Radiograph demonstrating the subsidence into C7 of a SynCage C after 6 months follow-up.
not differ from the patients without subsidence, and subsidence was not related to cage size (p5.523). Although subsidence appeared to have occurred more frequently in patients with a single level treatment than in patients with a multilevel treatment, this difference was not statistically significant (p5.093).
Discussion In this study, the radiological outcome of 106 titanium cervical interbody fusion cages in 71 patients was evaluated with 6-month follow-up for all patients. Although fusion was achieved in all patients, cage subsidence remains a disturbing phenomenon in this study. We found that subsidence was predominantly related to the C6-C7 level. The clinical relevance of subsidence is not clear, but subsidence may result in kyphotic deformity, pseudarthrosis, and recurrence of symptoms, possibly with a need for reoperation [15,18]. Moreland et al. recently reported the short-term results of the Rabea titanium cage for ACDF [17]. Fusion was achieved in 95% at 6 months, and subsidence greater than Table 2 Details of 10 patients with subsidence
Fig. 1. Directly postoperative radiograph showing cage design complementary to the anatomy of the intervertebral space.
Patient no.
Gender
Age
Diagnosis
2 8 13 14 15 24 36 49 50 64
M M F F M M M M F F
50 66 35 49 45 47 50 41 59 56
CDD CDD CDD CDD CDD CDD CDD CDD CDD CDD
CDD5cervical disc disease.
Level
Number of levels fused
Cage size
C6-C7 C6-C7 C6-C7 C5-C6 C6-C7 C6-C7 C6-C7 C6-C7 C4-C5 C6-C7
1 2 1 1 1 1 2 1 1 2
Medium Small Medium Medium Medium Large Medium Medium Medium Medium
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2 mm occurred at the inferior anterior edge of the cage in 22% of patients. The observation of subsidence did not compromise fusion rate or clinical outcome. Gercek et al. noted subsidence in five of nine fused levels using the SynCage C after 15 months follow-up [15]. In four of five patients, subsidence did not produce clinical symptoms [19,20]. Subsidence behavior of interbody fusion cages may be influenced by various factors [21]. Three-dimensional segmental stability and the mechanics at the cage–end plate interface are important biomechanical aspects related to subsidence [21,22]. They are compromised by cage design [13,14], cage size [23], the contact area at the implant–bone interface [21], end plate geometry [24], and the bone quality of vertebral end plates [24]. Finally, postoperative neck flexion movements probably influence cage subsidence behavior [21,25]. The current cage design is an open box cage with a large cranial contact surface complementary with the vertebral end plate geometry and with a large bony contact area to promote fusion between the vertebral bodies. Kandziora et al. concluded that the SynCage C provides a high, volume-related stiffness and allows sufficient space for bone graft [14]. This might result in a favorable biologic environment for bony fusion to occur. Appropriate implant size is determined during surgery with trial implants. Although cage size may influence the biomechanical properties of lumbar interbody fusion devices [23], and overdistraction of the cervical disc space may contribute to subsidence [19], we did not find any relation between cage size and subsidence. The stress distribution at the cage–end plate interface is unfavorable because of the limited inferior cage–end plate contact area (Fig. 3). While this might be partly responsible for the high rate of subsidence into the lower vertebral end plate, it does not explain why the cage subsides predominantly into the end plate of C7. The C7 end plate itself may be responsible for this typical subsidence tendency. In an experimental study, Truumees et al. found that several
Fig. 3. Photograph of inferior contact surface of SynCage C.
factors such as increasing patient age and female gender predicted decreasing failure load of intact cervical end plates. End plate dimensions did not significantly predict a decrease in failure load [24]. The quality of the end plates has been further evaluated by measurement of the bone mineral density and end plate thickness [18]. Several studies show that the structural integrity of the underlying trabecular bone significantly contributes to the strength of the end plate [18,22]. No differences in bone mineral density were found between different cervical levels and the end plate thickness determined by computed tomographic images: all levels have similar thickness of cranial and caudal end plates [18]. In a clinical study, Kanayama et al. observed titanium cage subsidence evenly distributed between upper (42%) and lower (50%) cervical spine [26], which is in conflict with our results. Finally, minimal end plate destruction in combination with a large contact area at the cage–end plate interface seems to reduce subsidence risk [21,24]. The use of a finite element analysis will probably help define the optimum cage–end plate interface. Conclusion The subsidence behavior of this titanium cage design noted in this study is a disturbing phenomenon. Although the cage is complementary to the anatomy of the intervertebral space, and appears to allow fusion despite subsidence, a modified cage design with improved and extended lower contact surface could be expected to reduce subsidence. References [1] 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. [2] Clements DH, O’Leary PF. Anterior cervical discectomy and fusion. Spine 1990;15:1023–5. [3] Bohlman HH, Emery SE, Goodfellow DB, Jones PK. Robinson anterior cervical discectomy and arthrodesis for cervical radiculopathy: long-term follow-up of one hundred and twenty-two patients. J Bone Joint Surg Am 1993;75:1298–307. [4] Bishop RC, Moore KA, Hadley MN. Anterior cervical interbody fusion using autogeneic and allogeneic bone graft substrate: a prospective comparative analysis. J Neurosurg 1996;85:206–10. [5] Pavlov PW. Anterior decompression for cervical spondylotic myelopathy. Eur Spine J 2003;12(Suppl. 2):S188–94. [6] Cloward RB. The anterior approach for removal of ruptured cervical disks. J Neurosurg 1958;15:602–17. [7] Smith GW, Robinson RA. The treatment of certain cervical-spine disorders by anterior removal of the intervertebral disc and interbody fusion. J Bone Joint Surg Am 1958;40-A:607–24. [8] van Limbeek J, Jacobs WC, Anderson PG, Pavlov PW. A systematic literature review to identify the best method for a single level anterior cervical interbody fusion. Eur Spine J 2000;9:129–36. [9] Wigfield CC, Nelson RJ. Nonautologous interbody fusion materials in cervical spine surgery: how strong is the evidence to justify their use? Spine 2001;26:687–94. [10] Malloy KM, Hilibrand AS. Autograft versus allograft in degenerative cervical disease. Clin Orthop 2002;394:27–38.
H.-P.W. van Jonbergen et al. / The Spine Journal 5 (2005) 645–649 [11] Zdeblick TA, Ducker TB. The use of freeze-dried allograft bone for anterior cervical fusions. Spine 1991;16:726–9. [12] Zdeblick TA, Phillips FM. Interbody cage devices. Spine 2003;28:S2–7. [13] Weiner BK, Fraser RD. Spine update lumbar interbody cages. Spine 1998;23:634–40. [14] Kandziora F, Pflugmacher R, Schafer J, et al. Biomechanical comparison of cervical spine interbody fusion cages. Spine 2001;26:1850–7. [15] Gercek E, Arlet V, Delisle J, Marchesi D. Subsidence of stand-alone cervical cages in anterior interbody fusion: warning. Eur Spine J 2003;12:513–6. [16] Siddiqui AA, Jackowski A. Cage versus tricortical graft for cervical interbody fusion: a prospective randomised study. J Bone Joint Surg Br 2003;85:1019–25. [17] Moreland DB, Asch HL, Clabeaux DE, et al. Anterior cervical discectomy and fusion with implantable titanium cage: initial impressions, patient outcomes and comparison to fusion with allograft. Spine J 2004;4:184–91. [18] Lim TH, Kwon H, Jeon CH, et al. Effect of endplate conditions and bone mineral density on the compressive strength of the graft– endplate interface in anterior cervical spine fusion. Spine 2001;26: 951–6.
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[19] Borm W, Seitz K. Use of cervical stand-alone cages. Eur Spine J 2004;13:474–5. [20] Arlet V. Answer to the ‘‘Letter to the Editor’’ of Dr. Wolfgang Bo¨rm. Eur Spine J 2004;13:476–7. [21] Wilke HJ, Kettler A, Goetz C, Claes L. Subsidence resulting from simulated postoperative neck movements: an in vitro investigation with a new cervical fusion cage. Spine 2000;25:2762–70. [22] Oxland TR, Lund T. Biomechanics of stand-alone cages and cages in combination with posterior fixation: a literature review. Eur Spine J 2000;9(Suppl. 1):S95–S101. [23] Goh JC, Wong HK, Thambyah A, Yu CS. Influence of PLIF cage size on lumbar spine stability. Spine 2000;25:35–9. [24] Truumees E, Demetropoulos CK, Yang KH, Herkowitz HN. Failure of human cervical endplates: a cadaveric experimental model. Spine 2003;28:2204–8. [25] Kettler A, Wilke HJ, Claes L. Effects of neck movements on stability and subsidence in cervical interbody fusion: an in vitro study. J Neurosurg 2001;94:97–107. [26] Kanayama M, Hashimoto T, Shigenobu K, Oha F, Ishida T, Yamane S. Pitfalls of anterior cervical fusion using titanium mesh and local autograft. J Spinal Disord Tech 2003;16:513–8.
COMMENTARY Harvinder S. Sandhu, MD, New York, NY
Subsidence following anterior cervical decompressive discectomy and fusion has been a foremost concern ever since Ralph Cloward’s popularization of the surgical procedure. A variety of risk factors have been touted, including end plate preparation, graft type, metabolic condition of bone, and postoperative immobilization. Modern anterior fixation technologies have been credited with reducing the frequency and degree of subsidence. The authors of this study should be praised for highlighting another potential risk factor: location of the discectomy and fusion. Ten patients (9%) among 106 included in the study cohort satisfied their inclusion criteria for postoperative subsidence. However, 8 of these 10 cases were identified at the C6-7 level (p5.002). This fascinating observation could not be attributed to age, gender, or cage size. In fact, insufficient data exists to indicate a reliable cause but there is enough to raise further questions. One possibility relates to the implant design. The SynCage S is an open box–type cage. However, the cervicothoracic discs (C6-T2) tend to be more wedge-shaped in the sagittal plane. This may have led to greater subchondral penetration of the device into the C7 end plate, resulting in
less resistance to settling. Another possibility relates to surgical technique. A suboptimal surgical approach to the lower cervical discs can make parallel decortication of the inferior end plate awkward and difficult. Eccentric end plate preparation can again weaken resistance to settling. Finally, as the authors point out, another possibility is that the C7 end plate itself may be responsible for the subsidence tendency. Although studies using finite element analysis may provide further insight, I would suggest that the authors follow this report with further clinical observations. Reproduction of this study with different fusion devices may determine whether the findings are implant-related. Alternatively, useful information can be derived from other investigators who use the SynCage S device for similar indications. Failure to observe similar subsidence would suggest that the findings of this study may be technique related. Finally, if the anatomic characteristics of the C7 end plate are ultimately found to be at fault, then surgeons would be wise to consider fusion location when choosing implant and fixation strategies for their anterior cervical intervertebral fusions. doi:10.1016/j.spinee.2005.09.003
Technical Review
Anterior cervical interbody fusion with a titanium box cage: early radiological assessment of fusion and subsidence Hans-Peter W. van Jonbergen, MDa,*, Maarten Spruit, MDa, Patricia G. Anderson, MAb, Paul W. Pavlov, MD, PhDa a
Institute of Spinal Surgery and Applied Research, Sint Maartenskliniek, Hengstdal 3, 6522 JV Nijmegen, The Netherlands b Orthopaedic Research, Sint Maartenskliniek, Hengstdal 3, 6522 JV Nijmegen, The Netherlands Received 12 October 2004; accepted 7 July 2005
Abstract
BACKGROUND CONTEXT: The use of stand-alone cervical interbody cages in anterior cervical discectomy with fusion (ACDF) has become popular, but high subsidence rates have been reported in the literature. PURPOSE: The authors present short-term radiological results of a titanium box cage with regard to fusion and subsidence. Reliable fusion and lack of subsidence may influence long-term clinical results. Early radiological data are necessary before implementation of this device on a larger scale can be accepted. STUDY DESIGN/SETTING: Retrospective radiological quality assessment study. PATIENT SAMPLE: ACDF using the titanium cage was performed in 71 consecutive patients at 106 levels. Diagnoses included cervical disc disease (57) and cervical spinal stenosis (14) after failed conservative treatment. OUTCOME MEASURES: Subsidence and kyphosis were assessed on lateral cervical radiographs made directly postoperative and at 3- and 6-month follow-up. At 6-month follow-up, lateral flexion-extension radiographs were made to assess fusion. METHODS: Subsidence of the cage was defined as a decrease in total vertical height of the two fused vertebral bodies as measured on the lateral cervical radiographs made 3 and 6 months postoperatively compared with the directly postoperative radiographs. Segmental kyphosis was measured as the angle between the posterior borders of the two vertebral bodies on the lateral radiograph. RESULTS: No patients were lost to follow-up. Fusion was achieved after 6 months in all patients. At 3 and 6 months postoperative the same 10 cages (each in a different patient) had subsided. The C6-C7 level was significantly more frequently involved compared with all other levels. A segmental kyphotic alignment was observed in five patients at the C6-C7 level and in one patient at the C4-C5 level. CONCLUSIONS: For patients with cervical disc disease, the high subsidence tendency of the cage into the end plate of predominantly C7 is a disturbing phenomenon found in this study. A modified cage design that improves and extends contact with the inferior surface could be expected to reduce subsidence into C7. Ó 2005 Elsevier Inc. All rights reserved.
Keywords:
Cervical cage; Anterior cervical interbody fusion; Subsidence
Introduction
FDA device/drug status: approved for this indication (SynCage C). Nothing of value received from a commercial entity related to this manuscript. * Corresponding author. Department of Orthopaedics, Deventer Hospital, P.O. Box 5001, 7400 GC Deventer, The Netherlands. Tel.: 131 570 646858; fax: 131 570 646085. E-mail address: [email protected] (H.-P.W. van Jonbergen) 1529-9430/05/$ – see front matter Ó 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.spinee.2005.07.007
Anterior cervical discectomy and fusion (ACDF) is an effective procedure to treat a variety of spinal disorders, such as radiculopathy, myelopathy, herniated discs, and cervical disc disease [1–5]. The objective of anterior cervical interbody fusion is to provide segmental stability and solid arthrodesis with minimal surgical risks. Although ACDF as described by Smith and Cloward has been considered to be the standard surgical procedure after conservative treatment
646
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has failed [6,7], Van Limbeek et al. [8] concluded in a systematic literature review that no gold standard for the treatment of single level cervical disc disease could be identified. Various materials have been used as interbody graft in anterior cervical fusion [9]. High fusion rates have been documented with autogenous graft [4,10], and high pseudarthrosis rates have been described when using allografts [11]. To supplement the bone graft, in the past 40 years a number of fusion devices have been developed, either for stand-alone use or in conjunction with anterior or posterior instrumentation [9,12]. The objective of all such spinal devices is to immobilize the unstable degenerated motion segment so that bony fusion can occur. Currently three types of spinal fusion devices are available: horizontal cylinders, vertical rings, and open box cages like the SynCage C (Synthes, Oberdorf, Switzerland) [13,14]. Only one published report describes the in vivo characteristics of the SynCage C [15]. In this retrospective study, five of nine fused levels had radiological signs of cage subsidence. The purpose of the present cage study was to assess early fusion and subsidence. Reliable fusion and lack of subsidence may influence satisfactory long-term clinical results, and therefore early radiological data are necessary before implementation of such a device on a larger scale can be accepted.
Implant description The titanium SynCage C has been adapted to the cervical disc space dimensions and has been designed to restore and maintain disc height and lordosis. It features a large cranial contact surface with the vertebral end plate and a large bony distal contact area to promote fusion between the vertebral bodies. The cranial surface of the cage is convex to address the concavity of the cranial vertebral end plate. Trial implants in the distracted disc space during surgery determine the appropriate implant size. Surgical technique The cervical spine is approached anteriorly with the neck in a slight lordotic position. The intervertebral space is opened with a Caspar distractor, after which complete discectomy, removal of osteophytes, and careful end plate preparation can be performed. The SynCage C is filled with autogenous bone graft harvested from the iliac crest through a standard, minimally invasive approach with a wire-guided trocar. Postoperatively the cervical spine is immobilized for 3 months with a cervical orthosis, which is not used at night. Follow-up examination Follow-up was scheduled after 3 months and 6 months with clinical and radiological examination. Assessment of fusion and subsidence
Materials and methods Patient selection Between March 1999 and August 2001, anterior cervical discectomy and fusion using a SynCage C was performed in 71 consecutive patients (41 males) at 106 levels. Median age at operation was 49 (23–76) years. Diagnoses included cervical disc disease (57) and cervical spinal stenosis (14), both after failed conservative treatment. Forty-five patients had a single level fusion, 19 patients a 2-level fusion, five patients a 3-level fusion, and two patients had a 4-level fusion. The C5-C6 level was most frequently fused (Table 1). A retrospective review was conducted in 2004.
Table 1 Level of fusion related to subsidence Level
Number of cages
No subsidence (%)
Subsidence (%)
C3-4 C4-5 C5-6 C6-7 C7-T1 Total
10 14 47 34 1 106
10 13 46 26 1 96
0 1 1 8 0 10
(100%) (93%) (78%) (76%) (100%) (91%)
Subsidence and kyphosis were assessed on lateral cervical radiographs made directly postoperative and at 3- and 6-month follow-up. Furthermore, at 6-month follow-up lateral flexion-extension radiographs were made in order to assess fusion. Subsidence of the SynCage C was defined as a decrease in total vertical height of the two fused vertebral bodies as measured on the lateral cervical radiographs made 3 and 6 months postoperatively compared with the directly postoperative radiographs. Subsidence greater than 3 mm was considered relevant [15]. Because of intra-individual variations of the magnification factor in the radiographs, the total vertical height of the two fused vertebral bodies was corrected for magnification differences by using the anteroposterior diameter of the upper vertebral body on the lateral cervical radiograph [16]. Segmental kyphosis was measured as the angle between the posterior borders of the two vertebral bodies on the lateral radiograph. If the difference of the interbody angle on the flexion and extension radiographs was not greater than 2 degrees, stability was assumed [15,17].
(7%) (2%) (24%)
Statistical analysis
(9%)
A Student t test was performed to see whether age was related to possible subsidence. A chi-square analysis was
H.-P.W. van Jonbergen et al. / The Spine Journal 5 (2005) 645–649
647
performed to determine whether possible subsidence was related to the following factors: diagnosis, gender, cage size, number of segments fused, and level of fusion. The level of significance was set at p!.05. Results Surgical technique and complications No technical complications occurred during surgery. All immediate postoperative radiographs showed adequate cage position without signs of subsidence (Fig. 1). Follow-up No patients were lost to follow-up. Assessment of fusion and subsidence Fusion was achieved after 6 months in all patients. At 3 and 6 months postoperative, the same 10 cages (all in different patients) showed subsidence greater than 3 mm (Table 1). In 9 of 10 patients, the cage subsided into the caudal vertebral body, producing only loss of anterior intervertebral height (Fig. 2), whereas in one patient in addition to the anterior height, loss of posterior intervertebral height was also observed (C5-C6 level). A kyphotic alignment was observed in five patients at the C6-C7 level and in one patient at the C4-C5 level, all six with anterior subsidence of the cage. Mean subsidence after 3 months was 0.861.5 mm, and after 6 months 1.061.8 mm. Overall, subsidence O3 mm was observed in 9% of the cages. The C6-C7 level was significantly more involved compared with all other levels (p5.002). All patients with subsidence had been operated for cervical disc disease (Table 2). The age (p5.465) and the gender (p5.877) of the patients with subsidence did
Fig. 2. Radiograph demonstrating the subsidence into C7 of a SynCage C after 6 months follow-up.
not differ from the patients without subsidence, and subsidence was not related to cage size (p5.523). Although subsidence appeared to have occurred more frequently in patients with a single level treatment than in patients with a multilevel treatment, this difference was not statistically significant (p5.093).
Discussion In this study, the radiological outcome of 106 titanium cervical interbody fusion cages in 71 patients was evaluated with 6-month follow-up for all patients. Although fusion was achieved in all patients, cage subsidence remains a disturbing phenomenon in this study. We found that subsidence was predominantly related to the C6-C7 level. The clinical relevance of subsidence is not clear, but subsidence may result in kyphotic deformity, pseudarthrosis, and recurrence of symptoms, possibly with a need for reoperation [15,18]. Moreland et al. recently reported the short-term results of the Rabea titanium cage for ACDF [17]. Fusion was achieved in 95% at 6 months, and subsidence greater than Table 2 Details of 10 patients with subsidence
Fig. 1. Directly postoperative radiograph showing cage design complementary to the anatomy of the intervertebral space.
Patient no.
Gender
Age
Diagnosis
2 8 13 14 15 24 36 49 50 64
M M F F M M M M F F
50 66 35 49 45 47 50 41 59 56
CDD CDD CDD CDD CDD CDD CDD CDD CDD CDD
CDD5cervical disc disease.
Level
Number of levels fused
Cage size
C6-C7 C6-C7 C6-C7 C5-C6 C6-C7 C6-C7 C6-C7 C6-C7 C4-C5 C6-C7
1 2 1 1 1 1 2 1 1 2
Medium Small Medium Medium Medium Large Medium Medium Medium Medium
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2 mm occurred at the inferior anterior edge of the cage in 22% of patients. The observation of subsidence did not compromise fusion rate or clinical outcome. Gercek et al. noted subsidence in five of nine fused levels using the SynCage C after 15 months follow-up [15]. In four of five patients, subsidence did not produce clinical symptoms [19,20]. Subsidence behavior of interbody fusion cages may be influenced by various factors [21]. Three-dimensional segmental stability and the mechanics at the cage–end plate interface are important biomechanical aspects related to subsidence [21,22]. They are compromised by cage design [13,14], cage size [23], the contact area at the implant–bone interface [21], end plate geometry [24], and the bone quality of vertebral end plates [24]. Finally, postoperative neck flexion movements probably influence cage subsidence behavior [21,25]. The current cage design is an open box cage with a large cranial contact surface complementary with the vertebral end plate geometry and with a large bony contact area to promote fusion between the vertebral bodies. Kandziora et al. concluded that the SynCage C provides a high, volume-related stiffness and allows sufficient space for bone graft [14]. This might result in a favorable biologic environment for bony fusion to occur. Appropriate implant size is determined during surgery with trial implants. Although cage size may influence the biomechanical properties of lumbar interbody fusion devices [23], and overdistraction of the cervical disc space may contribute to subsidence [19], we did not find any relation between cage size and subsidence. The stress distribution at the cage–end plate interface is unfavorable because of the limited inferior cage–end plate contact area (Fig. 3). While this might be partly responsible for the high rate of subsidence into the lower vertebral end plate, it does not explain why the cage subsides predominantly into the end plate of C7. The C7 end plate itself may be responsible for this typical subsidence tendency. In an experimental study, Truumees et al. found that several
Fig. 3. Photograph of inferior contact surface of SynCage C.
factors such as increasing patient age and female gender predicted decreasing failure load of intact cervical end plates. End plate dimensions did not significantly predict a decrease in failure load [24]. The quality of the end plates has been further evaluated by measurement of the bone mineral density and end plate thickness [18]. Several studies show that the structural integrity of the underlying trabecular bone significantly contributes to the strength of the end plate [18,22]. No differences in bone mineral density were found between different cervical levels and the end plate thickness determined by computed tomographic images: all levels have similar thickness of cranial and caudal end plates [18]. In a clinical study, Kanayama et al. observed titanium cage subsidence evenly distributed between upper (42%) and lower (50%) cervical spine [26], which is in conflict with our results. Finally, minimal end plate destruction in combination with a large contact area at the cage–end plate interface seems to reduce subsidence risk [21,24]. The use of a finite element analysis will probably help define the optimum cage–end plate interface. Conclusion The subsidence behavior of this titanium cage design noted in this study is a disturbing phenomenon. Although the cage is complementary to the anatomy of the intervertebral space, and appears to allow fusion despite subsidence, a modified cage design with improved and extended lower contact surface could be expected to reduce subsidence. References [1] 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. [2] Clements DH, O’Leary PF. Anterior cervical discectomy and fusion. Spine 1990;15:1023–5. [3] Bohlman HH, Emery SE, Goodfellow DB, Jones PK. Robinson anterior cervical discectomy and arthrodesis for cervical radiculopathy: long-term follow-up of one hundred and twenty-two patients. J Bone Joint Surg Am 1993;75:1298–307. [4] Bishop RC, Moore KA, Hadley MN. Anterior cervical interbody fusion using autogeneic and allogeneic bone graft substrate: a prospective comparative analysis. J Neurosurg 1996;85:206–10. [5] Pavlov PW. Anterior decompression for cervical spondylotic myelopathy. Eur Spine J 2003;12(Suppl. 2):S188–94. [6] Cloward RB. The anterior approach for removal of ruptured cervical disks. J Neurosurg 1958;15:602–17. [7] Smith GW, Robinson RA. The treatment of certain cervical-spine disorders by anterior removal of the intervertebral disc and interbody fusion. J Bone Joint Surg Am 1958;40-A:607–24. [8] van Limbeek J, Jacobs WC, Anderson PG, Pavlov PW. A systematic literature review to identify the best method for a single level anterior cervical interbody fusion. Eur Spine J 2000;9:129–36. [9] Wigfield CC, Nelson RJ. Nonautologous interbody fusion materials in cervical spine surgery: how strong is the evidence to justify their use? Spine 2001;26:687–94. [10] Malloy KM, Hilibrand AS. Autograft versus allograft in degenerative cervical disease. Clin Orthop 2002;394:27–38.
H.-P.W. van Jonbergen et al. / The Spine Journal 5 (2005) 645–649 [11] Zdeblick TA, Ducker TB. The use of freeze-dried allograft bone for anterior cervical fusions. Spine 1991;16:726–9. [12] Zdeblick TA, Phillips FM. Interbody cage devices. Spine 2003;28:S2–7. [13] Weiner BK, Fraser RD. Spine update lumbar interbody cages. Spine 1998;23:634–40. [14] Kandziora F, Pflugmacher R, Schafer J, et al. Biomechanical comparison of cervical spine interbody fusion cages. Spine 2001;26:1850–7. [15] Gercek E, Arlet V, Delisle J, Marchesi D. Subsidence of stand-alone cervical cages in anterior interbody fusion: warning. Eur Spine J 2003;12:513–6. [16] Siddiqui AA, Jackowski A. Cage versus tricortical graft for cervical interbody fusion: a prospective randomised study. J Bone Joint Surg Br 2003;85:1019–25. [17] Moreland DB, Asch HL, Clabeaux DE, et al. Anterior cervical discectomy and fusion with implantable titanium cage: initial impressions, patient outcomes and comparison to fusion with allograft. Spine J 2004;4:184–91. [18] Lim TH, Kwon H, Jeon CH, et al. Effect of endplate conditions and bone mineral density on the compressive strength of the graft– endplate interface in anterior cervical spine fusion. Spine 2001;26: 951–6.
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[19] Borm W, Seitz K. Use of cervical stand-alone cages. Eur Spine J 2004;13:474–5. [20] Arlet V. Answer to the ‘‘Letter to the Editor’’ of Dr. Wolfgang Bo¨rm. Eur Spine J 2004;13:476–7. [21] Wilke HJ, Kettler A, Goetz C, Claes L. Subsidence resulting from simulated postoperative neck movements: an in vitro investigation with a new cervical fusion cage. Spine 2000;25:2762–70. [22] Oxland TR, Lund T. Biomechanics of stand-alone cages and cages in combination with posterior fixation: a literature review. Eur Spine J 2000;9(Suppl. 1):S95–S101. [23] Goh JC, Wong HK, Thambyah A, Yu CS. Influence of PLIF cage size on lumbar spine stability. Spine 2000;25:35–9. [24] Truumees E, Demetropoulos CK, Yang KH, Herkowitz HN. Failure of human cervical endplates: a cadaveric experimental model. Spine 2003;28:2204–8. [25] Kettler A, Wilke HJ, Claes L. Effects of neck movements on stability and subsidence in cervical interbody fusion: an in vitro study. J Neurosurg 2001;94:97–107. [26] Kanayama M, Hashimoto T, Shigenobu K, Oha F, Ishida T, Yamane S. Pitfalls of anterior cervical fusion using titanium mesh and local autograft. J Spinal Disord Tech 2003;16:513–8.
COMMENTARY Harvinder S. Sandhu, MD, New York, NY
Subsidence following anterior cervical decompressive discectomy and fusion has been a foremost concern ever since Ralph Cloward’s popularization of the surgical procedure. A variety of risk factors have been touted, including end plate preparation, graft type, metabolic condition of bone, and postoperative immobilization. Modern anterior fixation technologies have been credited with reducing the frequency and degree of subsidence. The authors of this study should be praised for highlighting another potential risk factor: location of the discectomy and fusion. Ten patients (9%) among 106 included in the study cohort satisfied their inclusion criteria for postoperative subsidence. However, 8 of these 10 cases were identified at the C6-7 level (p5.002). This fascinating observation could not be attributed to age, gender, or cage size. In fact, insufficient data exists to indicate a reliable cause but there is enough to raise further questions. One possibility relates to the implant design. The SynCage S is an open box–type cage. However, the cervicothoracic discs (C6-T2) tend to be more wedge-shaped in the sagittal plane. This may have led to greater subchondral penetration of the device into the C7 end plate, resulting in
less resistance to settling. Another possibility relates to surgical technique. A suboptimal surgical approach to the lower cervical discs can make parallel decortication of the inferior end plate awkward and difficult. Eccentric end plate preparation can again weaken resistance to settling. Finally, as the authors point out, another possibility is that the C7 end plate itself may be responsible for the subsidence tendency. Although studies using finite element analysis may provide further insight, I would suggest that the authors follow this report with further clinical observations. Reproduction of this study with different fusion devices may determine whether the findings are implant-related. Alternatively, useful information can be derived from other investigators who use the SynCage S device for similar indications. Failure to observe similar subsidence would suggest that the findings of this study may be technique related. Finally, if the anatomic characteristics of the C7 end plate are ultimately found to be at fault, then surgeons would be wise to consider fusion location when choosing implant and fixation strategies for their anterior cervical intervertebral fusions. doi:10.1016/j.spinee.2005.09.003