The use of anterior plating for long (multilevel)

Transcription

1 J Neurosurg Spine 21: , 2014 AANS, 2014 A cadaveric analysis of cervical fixation: the effect of intermediate fixation points and dynamization in multilevel cervical fusions Laboratory investigation Daniel Lubelski, B.A., 1,2 William E. McCormick, M.D., 3 Lisa Ferrara, Ph.D., 4 Edward C. Benzel, M.D., 1,2 and Mark Kayanja, M.D., Ph.D. 5 1 Center for Spine Health and 2 Lerner College of Medicine, Cleveland Clinic, Cleveland, Ohio; 3 South Shore Brain and Spine Specialists, West Islip, New York; 4 OrthoKinetic Technologies, LLC, Southport, North Carolina; and 5 CORE Institute, Phoenix, Arizona Object. The authors conducted a study to compare biomechanical effects on the cervical spine of bridging fixation and intermediate fixation techniques, in both fixed and dynamic modes. Methods. A biaxial, servohydraulic machine biomechanically tested 23 human cervical spines for stiffness and strain in compression, extension, flexion, and lateral bending through 3 specimen states: 1) intact, 2) defect (corpectomy and discectomy), and 3) grafting with plate application in 1 of 4 constructs: C3 7 dynamized long strut (DLS), C3 7 fixed long strut (FLS), C3 5 7 dynamized multisegment (DMS), and C3 5 7 fixed multisegment (FMS). Results. Compared with FMS, FLS had significantly greater strain in extension (at C-3 and at the rostral and caudal parts of the graft) and in lateral bending (at C-3 and at the caudal part of the graft). Fixed (FLS and FMS) constructs had greater flexion stiffness than did dynamized (DLS and DMS) constructs and showed a trend toward greater lateral bending stiffness. Instrumentation revealed greater with the long fixed (FLS and DLS) constructs than with the multifixed (FMS and FMS) constructs at the rostral and caudal parts of the graft but no significant differences between the dynamized (DLS and DMS) and fixed (FLS and FMS) constructs. Conclusions. Multisegmental fixation provided greater stabilizing forces than did bridging constructs for both dynamized and fixed plates. Use of multisegmental fixation can potentially decrease strain at the screw-plate interface and reduce the rate of hardware failure. ( Key Words cervical fixation intermediate fixation fixed plates multisegmental fixation dynamic plates cadaveric biomechanical study Abbreviations used in this paper: BMD = bone mineral density; COR = center of rotation; DLS = dynamized long strut; DMS = dynamized multisegment; DOC = dynamic osteosyntheos cervical; FLS = fixed long strut; FMS = fixed multisegment. 736 The use of anterior plating for long (multilevel) cervical fusion has become widely accepted for cervical pathologies. It is believed to increase stability and fusion rate, maintain sagittal alignment, and decrease the duration of and/or requirement for immobilization. 8,14,18 Complications such as pseudarthrosis, plate/screw fracture, loss of alignment, and progressive kyphotic deformity can lead to early fusion failures. 8,16 Since the development and use of the early rigid cervical plate designs, advances in cervical plating technology have resulted in various plating techniques that may provide improved load transfer across the fusion graft. 2,9 Both rigid and dynamic plates can be used for cervical fusion fixation. Rigid plates were the first to be used, providing benefit by preventing settling across the fusion site, but they inhibited the normal compressive load transfer across a fusion graft necessary for optimal healing (i.e., forces enhancing bone healing, as described originally by Julius Wolff). 5,6,10,20 This increased stiffness attributed to rigid plate fixation has led to reports of construct failure due to stress shielding across the fusion site, thus leading to the development of dynamic plate fixation. 7,10 Many of the dynamic cervical plates are designed to provide controlled micromotion and compression across a fusion graft while minimizing the load transfer to the screws at the bone interface. Reduced stress transfer to the screws at the bone interface further minimizes the risk of screw backout by permitting sliding of the screws within slots in the plate. 3 In contrast to rigid plates, dynamic plates permit settling and facilitate load sharing, 1 thereby potentially reducing the chance of construct failure. However, the use of dynamic plates has been suggested to be less effective in stabilizing long strut grafts. 10 Multilevel ventral cervical spine decompression proce- This article contains some figures that are displayed in color on line but in black-and-white in the print edition.

2 Intermediate fixation and dynamization in cervical fusion dures can use bridging implants, with 2 points of screw fixation, at each terminal, and without intermediate points of fixation; or long plate fixation with intermediate segmental fixation, involving one or more intermediate points of fixation along the length of the construct. Relatively high rates of complications, such as graft-plate extrusion, have been reported with the bridging fixation methods for long constructs, 17 and some authors have advocated for the use of intermediate segmental fixation. 13,19 The optimal approach, however, is yet to be established, as few studies 1,3,13,16,17,19 have investigated these different fixation techniques. The present cervical cadaveric biomechanical study was designed to compare the differences in stiffness and strain between bridging fixation and intermediate fixation techniques, in both fixed and dynamic modes. Methods Specimen Preparation and Operative Technique Twenty-three human cervical spines were obtained for this study. Radiographs of the spines were taken to exclude vertebrae with preexisting fractures and deformities. The C2 7 spines were stripped of all musculature, preserving the ligamentous structures. The spines, contained in plastic bags, were surrounded by water to simulate body tissue and were scanned in the anteroposterior direction using dual-energy x-ray absorptiometry to determine bone mineral density (BMD). Each cervical spine was prepared for biomechanical testing using customized gripping fixtures designed for mounting to a biaxial servohydraulic materials test machine. Wood screws were placed into each terminal vertebra (C-2 and C-7) in a multiplanar fashion, with the end vertebrae embedded in polyester resin blocks (Lightweight Body Filler and Hardener #310, Bondo/Mar-Hyde Corp.) that secured to the gripping fixtures. The discs and facet joints were kept free of embedding material. The long-strut constructs were prepared by performing a 3-level (C4 6) corpectomy and then curetting the subchondral bone of the endplates to provide a graft interface. A fibular strut graft (Musculoskeletal Transplant Foundation) was then placed into the corpectomy site. For the multisegment constructs, C-4 and C-6 corpectomies were performed, leaving the C-5 vertebral body intact. Fibula grafts were placed into the corpectomy sites after preparing the endplates. The plate was then oriented so that the 14-mm rostral screws were adjacent to the endplates and directed rostrally and the 14-mm caudal screws were adjacent to the endplates and directed caudally. The plate was positioned flush with the vertebral bodies. Corpectomies, graft placement, and plate and screw placements were all performed by M.K. and W.E.M. Rectangular rosette strain gauges were applied onto the ventral surfaces of the C-3 and C-7 vertebrae and onto the rostral and caudal aspects of the fibular struts after surface preparation. The strain gauges were used to monitor strain during the biomechanical testing. In specimens undergoing multisegmental fixation, an additional rectangular rosette strain gauge was placed on the C-5 vertebra. Lead wires were attached using the 3-lead-wire method to reduce measurement errors. All strain measurements and relative strain were recorded with Strain Smart 5000 software and data acquisition system (version 2.23; Vishay Precision Group, Inc.). Biomechanical Testing Scheme All testing was conducted using a biaxial, servohydraulic testing machine (Instron 8874, Instron Corp.). A testing sequence of nondestructive loading in compression, left lateral bending, extension, and flexion was applied to each mounted spine for each treatment group in each of 3 states: 1) intact, 2) defect (corpectomy and discectomy), and 3) grafting with plate application. The mounted fixtures gripped each specimen at the rostral and caudal ends; the upper gripping fixture was free to rotate in the sagittal plane during testing, while the lower gripping fixture remained fixed. Rotation of both the superior and fixed inferior gripping fixtures was constantly monitored with rotational potentiometers. In addition, an x-y translation table was positioned under the caudal gripping fixture to allow for translation of the specimen during loading, measured with linear variable differential transformers. The test procedure involved locating the center of rotation (COR) for each specimen by applying a 100-N compressive load to the upper fixture while adjusting the position of the specimen in the fixture, identifying the position at which compression caused no angular displacement. Nondestructive loading in compression, left lateral bending, extension, and flexion followed. The specimens with their respective fixation states are depicted in Figs. 1 4; the testing apparatus and setup is depicted in Fig. 5. Fixation was performed using a dynamic osteosyntheos cervical (DOC) plate (DePuy Spine, Inc.), which allowed for either fixed or dynamized fixation states. With this system, dynamized fixation allows for subsidence along a trajectory predetermined by the surgeon. In contrast, dynamic stabilization systems, which allow for persistent motion, were not used here and should not be confused with the dynamized fixation of the DOC plate. The clinical/operative technique with the DOC has been previously described. 15 Briefly, the DOC plate consists of end platforms connected by rods. The platforms are rigidly fixed to the vertebral bodies via screws. The caudal platform is secured to the rods via screws; the rostral platform is not secured and is able to settle (slide) along the rods as the spine subsides. A cross-fixator attached to the rods at the cranial end of the plate limits the amount of subsidence. While the cross-fixator is preset to permit 3 mm of subsidence (dynamized state, Fig. 1), it can be adjusted to biomechanically act as a rigid system (Fig. 2). Additionally, an intermediate platform can be placed and affixed to an intermediate vertebral body (Figs. 3 and 4). Compression. During compression testing, the fixtures were locked into a fixed position to allow for pure compression with the specimen at the COR. Specimens were cycled nondestructively in displacement control from 0 to 1 mm displacement for 6 cycles at a rate of 3.6 mm/minute (0.03 Hz). Load-displacement data were collected at 0.5 khz. 737

3 D. Lubelski et al. Fig. 1. DLS fixation in anterior (left) and lateral (right) views. Reprinted with Fig. 3. DMS in anterior (left) and lateral (right) views. Reprinted with Left Lateral Bending. For loading in left lateral bending, the specimen was rotated through 90 in the fixture and then shifted 3 cm from the COR to the specimen s right to induce left lateral bending. The specimen was then cyclically loaded nondestructively from 0 to 2 mm displacement in displacement control for 6 cycles at a rate of 8 mm/minute (0.033 Hz). Load-displacement data were collected at 0.5 khz. cally loaded nondestructively from 0 to 2 mm displacement in displacement control (as has been previously described)12,13 for 6 cycles at a rate of 8 mm/minute (0.033 Hz). Load-displacement data were collected at 0.5 khz. Extension. During extension loading, specimens were shifted 3 cm ventrally in the sagittal plane in the fixtures from the previously established COR. Specimens were then cyclically loaded nondestructively from 0 to 2 mm displacement in displacement control for 6 cycles at a rate of 8 mm/minute (0.033 Hz). Load-displacement data were collected at 0.5 khz. Data Collection and Management The first 3 cycles for each loading paradigm were used for preconditioning, and the data analysis was conducted on data from the last 3 cycles. Stiffness was measured as the slope of the tangent within the linear elastic region of the nondestructive load versus displacement cycle for each of these cycles, calculating the mean. Strain on C-3, C-5, and C-7 and from the rostral and caudal parts of the graft were measured from the loading portions of each of the last 3 cycles, determining the means. Flexion. For flexion loading, specimens were shifted 3 cm dorsally in the sagittal plane in the fixtures from the previously established COR. Specimens were then cycli- Statistical Comparisons Fig. 2. FLS in anterior (left) and lateral (right) views. Reprinted with Fig. 4. FMS in anterior (left) and lateral (right) views. Reprinted with 738 All data were analyzed using JMP 9.0 (SAS Institute Inc.). Independent-sample t-tests then compared strain

4 Intermediate fixation and dynamization in cervical fusion = 0.007). Lateral was also greater for the FLS construct (Fig. 2) than for the FMS construct (Fig. 4), with a trend toward significance at C-3 (733 vs 318 mstrain; p = 0.06) and at the caudal part of the graft (66 vs 26 mstrain; p = 0.03). Fixed constructs (FLS and FMS) had greater flexion stiffness than did dynamized constructs (DLS and DMS) (0.40 vs 0.16 Nm; p = 0.03) and showed a trend toward greater lateral bending stiffness (0.60 vs 0.35 Nm; p = 0.07) but toward lower lateral measured at the rostral part of the graft (34 vs 90 mstrain; p = 0.08; Table 3). Bridging versus multisegment fixation was also compared in the dynamized condition (DLS [Fig. 1] vs DMS [Fig. 3]; Table 4) and in the fixed condition (FLS [Fig. 2] vs FMS [Fig. 4]; Table 5). For dynamized constructs, was greater with the DLS than with the DMS construct (rostral part of graft: 108 vs 32 mstrain, p = 0.04; caudal part of graft: 141 vs 18 mstrain, p = 0.06). The same was seen with fixed plates: was greater with the FLS than with the FMS construct (rostral part of graft: 81 vs 33 mstrain, p = 0.04; caudal part of graft: 89 vs 34 mstrain, p = 0.02). Fig. 5. Testing apparatus using a biaxial, servohydraulic testing machine, with the fixtures gripped at the rostral and caudal ends, demonstrating flexion/extension tests being performed. Reprinted with and stiffness between each of the four conditions: C3 7 dynamized long strut (DLS), C3 7 fixed long strut (FLS), C3 5 7 dynamized multisegment (DMS), and C3 5 7 fixed multisegment (FMS) (Figs. 1 4, respectively). All p values 0.05 were considered statistically significant. Multivariate analysis was then subsequently performed with BMD as a covariate. Results Twenty-three specimens were included: 16 males and 7 females with a mean age of 52.9 ± 8.1 years (Table 1). The specimens had a mean BMD of 0.65 ± 0.1 g/cm 2. Bridging fixation (DLS and FLS) was associated with greater compressive stiffness and greater strain in compression, lateral bending, extension, and flexion than was multipoint segmental fixation (DMS and FMS) (Table 2). Large variability in the values precluded many of the differences from reaching statistical significance, though the difference in between the FLS and FMS constructs was statistically significant at C-3 (558 vs 229 mstrain, respectively; p = 0.02), at the rostral part of the graft (94 vs 32 mstrain, respectively; p = 0.003), and at the caudal part of the graft (117 vs 26 mstrain, respectively; p Discussion Patients with multilevel cervical spondylosis and myelopathy are commonly surgically treated with either multilevel discectomies or corpectomies. Both postoperative stability and adequacy of decompression are of significant concern. Whereas discectomies are technically easier to perform and have lower graft dislodgement rates than do corpectomies, they may also not achieve complete decompression, particularly when multiple disc levels are involved. This is due in part to the limited exposure of the dural sac. 19 In a retrospective review of 16 patients by Emery et al., 4 only 9 patients (56%) who underwent a 3-level anterior cervical discectomy and fusion without instrumentation achieved solid arthrodesis at all 3 levels after an average follow-up of about 3 years. Multilevel corpectomies typically involve long strut graft and plate (bridging) fixation, but the large moment arm that is created at the bottom of the construct can stress the screws and lead to plate dislodgment and instrument failure. 13,17,19 Vaccaro et al. 17 found a 50% (6/12 patients) plate dislodgement rate among patients with 3-level corpectomy and fusion with plating. Wei-bing et al. 19 found a screw loosening rate of 18% (7/39) in patients who underwent 3-level cervical corpectomies and long-segment endconstruct plate fixation. Riew et al. 11 reviewed patients with 2- or 3-level corpectomies and found a 35.7% complication rate that included plate dislodgement, graft extrusion, and pseudarthrosis. Because of the high rate of failure of instrumentation, Rhee et al. 10 suggested that patients with multilevel stenosis should not be treated with the standard multilevel corpectomy with a long strut graft but, rather, should be treated with alternative surgical procedures and/ or different corpectomy constructs. The findings of the present study biomechanically support the aforementioned clinical data. The bridging fixation method (FLS and DLS) led to statistically sig- 739

5 D. Lubelski et al. TABLE 1: Specimen demographics FLS Specimen No. Age (yrs), Sex BMD (g/cm 2 ) Cause of Death DLS FMS DMS 1 59, M atherosclerotic cardiovascular disease 2 47, M acute myocardial infarction 3 59, M end-stage chronic obstructive pulmonary disease 4 59, M congestive cardiomyopathy 5 60, M cardiac arrest 6 66, M hypertensive, cardiovascular disease 1 59, M metastatic cancer in liver 2 54, F amyotrophic lateral sclerosis 3 38, M gunshot wound to chest 4 48, M liver cirrhosis 5 49, M cardiac tamponade 6 48, F adult respiratory distress syndrome 1 50, M atherosclerotic cardiovascular disease 2 59, M acute hemopericardium 3 67, M renal failure 4 40, F unknown 5 41, F chronic alcoholism 1 52, M small cell carcinoma of lung 2 62, M unknown 3 47, F metastatic non small cell lung carcinoma 4 45, F unknown 5 57, F anoxia encephalopathy 6 50, M congestive heart failure nificantly greater at the top and bottom of the graft, as well as greater lateral at the bottom of the graft and greater lateral bending and extension strain at the C-3 level, than did the multisegment fixation method (FMS and DMS). These findings were true regardless of whether the graft was fixed or dynamized. There were no vertebral fractures or instrument failure in any of the specimens. These results are similar to a previous biomechanical study by Singh et al. 13 that used 7 cadaveric cervical spines with various combinations of discectomies and corpectomies to compare bridging fixation versus multisegment fixation. They found that segmental plate fixation provided greater biomechanical rigidity in flexionextension and lateral bending than did bridging fixation. Similarly, we found that multisegment fixation was associated with significantly less strain (less net deformation in response to the applied force) than was bridging fixation, thereby creating a more rigid form of fixation. The greater fixation associated with the multisegment construct may allow for the cantilever forces to be spread across the multiple fixation sites. 13 In contrast to the significant differences found between the bridging fixation constructs and the multisegment constructs, there were few significant differences between the dynamized and fixed constructs. There were some trends in the data for greater flexion, extension, and lateral bending stiffness with the fixed construct, and greater strain at the rostral and caudal parts of the grafts, but substantial variability in the data precluded any statistically significant findings. Accordingly, the data indicate that perhaps the choice of dynamized versus fixed plate is less important in terms of postoperative instrument failure than the choice of bridging fixation versus multisegment fixation. This concept was previously described by Steinmetz et al., 15 who reported that dynamic plates are not necessary in all cervical pathologic conditions but are important when there is a risk of construct failure related to poor bone integrity (e.g., in osteoporosis, in revision surgery, and in smokers with poor bone quality). Nonetheless, several studies have found that dynamic constructs provide rostral stabilization. Dvorak et al., 3 in a cadaveric biomechanical study, found no significant differences in cervical range of motion of fixed versus dynamic plate constructs, except for extension, where dynamic plates provided statistically significantly greater stabilization. Stulik et al., 16 reporting 6-month outcomes of a randomized controlled study comparing fixed versus 740

6 Intermediate fixation and dynamization in cervical fusion TABLE 2: Strain with bridging (DLS and FLS) versus multisegment (DMS and FMS) constructs Measure Bridging (n = 12) Multisegment (n = 11) p Value compressive stiffness ± ± bending stiffness 0.49 ± ± extension stiffness 0.54 ± ± flexion stiffness 0.24 ± ± C ± ± C ± ± C ± ± * C ± ± C ± ± C ± ± C ± ± C ± ± rostral part of graft ± ± caudal part of graft ± ± rostral part of graft 63.0 ± ± caudal part of graft 66.1 ± ± * rostral part of graft 94.0 ± ± * caudal part of graft ± ± * rostral part of graft 42.9 ± ± caudal part of graft 58.6 ± ± Means are given with SDs. dynamic plating, also found that patients that received the dynamic plate had significantly less cervical segmental mobility (greater stability) and decreased rate of hardware complications than those receiving fixed plates. This is the first biomechanical cadaveric study to assess bridging fixation versus multisegment fixation, while also taking into account the differences between dynamic and fixed plate constructs. The DOC system described herein is no longer clinically available; however, the intent of this study was not to determine the biomechanical application of this cervical plate. Allowing for both dynamized and fixed states, the DOC plate is a unique tool that enabled us to test the biomechanical effects of these 2 fixation types in a controlled fashion. The present study has several limitations. As in all cadaveric analyses, the results are based on an in vitro system that may not completely recapitulate the human condition. As in most published biomechanical studies, we dissected away the musculature; the spine and ligaments, which are the major contributors to stability, were all preserved. The study sample comprised 23 specimens, and there was relatively wide heterogeneity in the data, which could have been improved with a larger sample size. This variability precludes generalizability of the data. The relatively large TABLE 3: Strain with dynamic (DLS and DMS) versus fixed (FLS and FMS) constructs Measure Dynamic (n = 12) Fixed (n = 11) p Value compressive stiffness 84.3 ± ± bending stiffness 0.35 ± ± extension stiffness 0.39 ± ± flexion stiffness 0.16 ± ± * C ± ± C ± ± C ± ± C ± ± C ± ± C ± ± C ± ± C ± ± rostral part of graft ± ± caudal part of graft ± ± rostral part of graft 90.4 ± ± caudal part of graft 55.9 ± ± rostral part of graft 73.0 ± ± caudal part of graft 85.2 ± ± rostral part of graft 97.9 ± ± caudal part of graft 53.3 ± ± variability in the data is a limitation in biomechanical cadaveric studies, where each specimen differs in BMD and disc degeneration. While it was difficult to perform a formal a priori power analysis, our sample size was the same as or larger than other similar studies looking at dynamic versus rigid fixation or multisegment versus long fixation. TABLE 4: Strain with DLS versus DMS constructs Measure DLS (n = 6) DMS (n = 6) p Value rostral part of graft ± ± caudal part of graft ± ± rostral part of graft 85.0 ± ± caudal part of graft 86.5 ± ± rostral part of graft ± ± * caudal part of graft ± ± rostral part of graft 56.3 ± ± caudal part of graft 83.3 ± ±

7 D. Lubelski et al. TABLE 5: Strain with FLS versus FMS constructs Measure FLS (n = 6) FMS (n = 5) p Value rostral part of graft 75.5 ± ± caudal part of graft ± ± * rostral part of graft 41.0 ± ± caudal part of graft 41.6 ± ± rostral part of graft 80.5 ± ± * caudal part of graft 89.0 ± ± * rostral part of graft 29.5 ± ± caudal part of graft 29.0 ± ± The lack of robust statistically significant findings when comparing the fixed versus dynamic states is more likely a function of specimen variability than a Type II error (false negative). Larger studies specifically designed to look at this effect are needed. Conclusions Multisegment fixation provided greater biomechanical stability than did bridging fixation, for both rigid and dynamic plates. Use of multisegment fixation can potentially decrease strain at the screw-plate interface and reduce the rate of hardware failure. With both dynamic and fixed plates, multisegment fixation provides greater stabilization compared with bridging constructs. Disclosure Dr. Benzel receives royalties from DePuy Spine, Inc., for being a patent holder of the dynamic osteosyntheos cervical fixation system. The authors report no other conflict of interest concerning the materials or methods used in this study or the findings specified in this paper. Author contributions to the study and manuscript preparation include the following. Conception and design: Kayanja, Ferrara, Benzel. Acquisition of data: Kayanja, Lubelski, McCormick, Ferrara. Analysis and interpretation of data: Kayanja, Lubelski, Benzel. Drafting the article: Kayanja, Lubelski. Critically revising the article: all authors. Reviewed submitted version of manuscript: all authors. Statistical analysis: Lubelski. References 1. 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Spine (Phila Pa 1976) 26: , Wei-bing X, Wun-Jer S, Gang L, Yue Z, Ming-xi J, Lian-shun J: Reconstructive techniques study after anterior decompression of multilevel cervical spondylotic myelopathy. J Spinal Disord Tech 22: , Wolff J: Das Gesetz der Transformation der Knochen. Berlin: A Hirschwald, 1982 Manuscript submitted October 26, Accepted July 24, Please include this information when citing this paper: published online September 5, 2014; DOI: / SPINE Address correspondence to: Mark Kayanja, M.D., Ph.D., Wright State Orthopaedics, Miami Valley Hospital, 30 E. Apple St., Ste. 2200, Dayton, OH markmakumbi@gmail.com. 742