Anterior Cervical Fixation: Analysis of Load-Sharing and Stability with Use of Static and Dynamic Plates

Brodke, Darrel S. MD; Klimo, Paul Jr. MD, MPH; Bachus, Kent N. PhD; Braun, John T. MD; Dailey, Andrew T. MD

Journal of Bone & Joint Surgery - American Volume:
doi: 10.2106/JBJS.E.00305
Scientific Articles
Abstract

Background: Anterior plates provide stability following decompression and fusion of the cervical spine. Various plate designs have emerged, and they include static plates with fixed-angle screws, rotationally dynamic plates that allow the screws to toggle in the plate, and translationally dynamic plates that allow the screws to both toggle and translate vertically. The goal of this study was to document the effects of plate design following a single-level corpectomy and placement of a full-length strut graft and the effects following 10% subsidence of the graft.

Methods: A total of twenty-one cadaveric cervical spines (C2-T1) were randomized into three treatment groups and were tested for initial range of motion. A C5 corpectomy was performed, reconstruction was done with a full-length interbody spacer containing a load-cell, and an anterior cervical plate was applied. Load-sharing data were recorded with incremental axial loads. The range of motion was measured with ±2.5 Nm of torque in flexion-extension, lateral bending, and axial rotation. Then, the total length of the interbody spacer was reduced by 10% to simulate subsidence, and load-sharing and the range of motion were retested.

Results: With the full-length interbody spacer, there were no significant differences in the abilities of the constructs to share load or limit motion. Following shortening of the interbody spacer, the static plate construct lost nearly 70% of its load-sharing capability, while neither of the dynamic plate constructs lost load-sharing capabilities. Also, the static plate construct allowed significantly more motion in flexion-extension following simulated subsidence than did either of the dynamic plate constructs (p < 0.05).

Conclusions: Although all of the tested anterior cervical plating systems provide similar load-sharing and stiffness following initial placement of the interbody spacer, the static plate system lost its ability to share load and limit motion following simulated subsidence of the interbody spacer. Both dynamic plate systems maintained load-sharing and stiffness despite simulated subsidence.

Clinical Relevance: This study provides an improved understanding of the immediate performance of anterior cervical fusion surgery with plate fixation.

Author Information

1 Department of Orthopaedics (D.S.B., K.N.B., and J.T.B.) and Orthopaedic Research Laboratory (K.N.B.), University of Utah Orthopaedic Center, 590 Wakara Way, Room A0100, Salt Lake City, UT 84108. E-mail address for K.N. Bachus: kent.bachus@hsc.utah.edu

2 Department of Neurosurgery, University of Utah School of Medicine, 30N 1900E, Suite 3B409 SOM, Salt Lake City, UT 84132

3 Rocky Mountain Neurosurgical Alliance, 701 E Hampden Avenue #510, Englewood, CO 80110

Article Outline

Anterior cervical spine surgery was introduced in the late 1950s by Smith and Robinson1 and by Cloward2. The goals of this surgery include decompression of neural structures, reduction of deformity, immediate stability, and creation of a conducive environment for osseous fusion to occur. Recently, anterior plating systems have been used in the hope of improving outcomes following anterior cervical discectomy or corpectomy for degenerative, traumatic, and oncologic disorders3-5. Proponents of plating systems have cited numerous advantages, including earlier patient mobilization, cost-effectiveness, a decreased need for orthotics, a diminished rate of graft dislodgment and migration, superior fusion rates, immediate stabilization, and the prevention of spinal deformity6-13. These potential benefits are much more evident in patients who undergo multilevel surgery, such as corpectomy and reconstruction, than those who undergo single-level surgery14,15.

The designs of anterior cervical plates have evolved substantially. The initial plate designs required bicortical fixation, as the screws were not locked to the plate6. Constrained systems, in which the screw locked to the plate, enabled unicortical fixation and reduced the problem of screw back-out16. More recently, pseudarthrosis has been noted in association with static plates, which had been thought to bridge and unload the interface between a subsided graft and the end plate. Dynamic cervical plates were developed in response to this problem. Rotationally dynamic plates allow the screws to pivot or toggle yet continue to prevent screw back-out. Translational dynamic plates not only allow the screws to pivot but also to slide vertically in the plate.

Graft subsidence is common during healing after anterior cervical fusion surgery17,18. Dynamic plates, in theory, allow continued contact between the graft and the end plate after graft subsidence has occurred, thus improving the chance of obtaining a fusion by maintaining a compressive load on the graft19,20. Subsidence and its effects on the stiffness of a plating system have received little attention from both clinical and biomechanical standpoints.

The goal of this study was to document the effects of the use of different anterior cervical plates (a static construct, a rotationally dynamic system, and a translationally dynamic system) following a single-level corpectomy and placement of a full-length strut graft and the effects following 10% graft subsidence.

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Materials and Methods

Cadaver Specimens

Fresh-frozen and thawed human cervical spine specimens (C2-T1) used in this study were free of gross osseous abnormalities as assessed visually and with use of anteroposterior and lateral radiographs. The relative bone quality of each of the donors was measured with use of a dual x-ray absorptiometry system (General Electric Medical Systems, Madison, Wisconsin). Specimens with either a pathologic defect or dual-energy x-ray absorptiometry values of greater than one standard deviation below the pool of specimens were not included in this study. Following these screening techniques, a total of twenty-one specimens were obtained from fifteen male and six female donors who had been an average (and standard deviation) of 62 ± 11 years (range, forty-six to eighty-two years) at the time of death.

The specimens were kept frozen at –20°C in sealed plastic bags (S-1987 4 Mil Poly Tubing; Uline, Duluth, Georgia) and were thawed in a refrigeration system for twelve hours prior to testing. On the day of testing, the specimens were prepared by removing all remaining skin and most of the paraspinal cervical musculature; care was taken to keep all ligaments, joint capsules, osseous components, and intervertebral discs intact. To supplement the potting fixation, three drywall screws were inserted radially into the vertebral bodies and lateral masses of C2 and T1. These end vertebrae were then placed into 4-cm-deep polyvinyl chloride potting fixtures and were embedded to the middle of the vertebra in a two-part filler compound (Bondo body filler; Bondo, Atlanta, Georgia).

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Mechanical Testing of the Corpectomy Defect

Nondestructive range-of-motion tests of the harvested spines were conducted on a pneumatically controlled custom spine simulator21-25. This device produces immediate feedback on controlled flexion-extension, lateral bending, and axial rotation moments through both the top and bottom mounts of the specimen, as well as axial compression of the multisegmented specimen. Two six-degrees-of-freedom load-cells, one mounted at each end of the specimen, provided direct feedback for the moment-producing pneumatic actuators on both ends of the specimen. Sinusoidal moments of ±2.5 Nm were sequentially applied in flexion-extension, lateral bending, and axial rotation. By applying moments in one plane while actively controlling the other planes at zero moment, the spine simulator's computer-controlled pneumatic actuators produce a nearly pure moment, neutralizing any off-axis loads. A 20-N axial compressive load was maintained throughout testing. Displacements of the vertebral bodies under load were measured with an optoelectronic motion measurement system (Optotrak 3020; Northern Digital, Waterloo, Ontario, Canada). The system uses infrared light-emitting-diode marking flags, rigidly attached to each vertebra above and below the construct. The specimens were loaded for five cycles in flexion-extension, lateral bending, and axial rotation with range-of-motion data collected from the fifth cycle. Throughout testing, the specimens were kept moist by a layer of white petroleum jelly (Vaseline; Chesebrough-Ponds, Greenwich, Connecticut) and periodic sprays of 0.9% saline solution.

After the range-of-motion tests of the intact specimens were completed, a single-level corpectomy was performed at C5 in a standard fashion. The C4-C5 and C5-C6 discs were removed first, with the anterior anulus incised with a number-15 blade and then rongeurs and curets were used to remove all disc material and cartilaginous end plates. The C5 corpectomy was performed from one uncovertebral joint to the other with rongeurs. Following the corpectomy and end-plate preparation, a full-length interbody spacer was inserted (Fig. 1). This device was custom-made from an acetal resin plunger (Delrin; DuPont, Wilmington, Delaware) that was housed in an aluminum cylindrical cup containing a subminiature compression load-cell (model 13; Honeywell Sensotec, Columbus, Ohio). The interbody spacer served two purposes. First, it maintained the space height between C4 and C6 in a way that was similar to a standard interbody graft used clinically. Second, it provided a load-cell to measure the axial loading being delivered to the interbody spacer.

Seven specimens were used for each of three plate designs (Fig. 2). All plates were of the same screw-to-screw length and were applied to the C4 and C6 vertebral bodies. All three plate designs and screws were from Medtronic Sofamor Danek (Memphis, Tennessee). The static plate was the ATLANTIS anterior cervical plate system with fixed-angle screws (4.0 × 15-mm cancellous-bone screws). The rotationally dynamic plate was also the ATLANTIS system but with variable-angle screws (4.0 × 15-mm cancellous-bone screws). The translationally dynamic plate was the PREMIER plate system with use of self-tapping screws (4.0 × 15-mm cancellous-bone screws).

The constructs were sequentially loaded axially to 0, 15, 30, 45, 60, and 90 n. The percentage of load-sharing for each construct was obtained by dividing the force measured through the interbody spacer (n) by the total force applied (N) at each interval, and multiplying by 100 (n/N × 100). The load-sharing values for each applied load within one specimen were averaged to provide the load-sharing for the specimen for a specific plate construct. Nondestructive range-of-motion testing of the full-length constructs was performed as described above for the harvested specimens. Finally, to simulate subsidence, a shim (part of the interbody spacer) was removed leaving the spacer 10% short of its original height, and load-sharing and range-of-motion testing were repeated.

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Statistical Analysis

The means and standard deviations were calculated for the load-sharing data for the full-length interbody spacer and the specimens with 10% subsidence of the interbody spacer. The means and standard deviations of the range of motion in flexion-extension, lateral bending, and axial rotation were calculated for the intact specimen, full-length interbody spacer, and the specimens with 10% shortening of the interbody spacer. Significance was measured with use of Student t tests and an alpha of ≤0.05.

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Results

Load Sharing

With a full-length interbody spacer in place, there was no significant difference in the ability of any of the three constructs to share load through the grafts (Table I, Fig. 3). However, when the interbody spacer was shortened by 10% to simulate subsidence, the static plate constructs lost nearly 70% of their load-sharing capabilities and were not able to share as much of the load through the interbody spacer as were either of the dynamic plate constructs (p ≤ 0.001). The two dynamic plate constructs were not significantly different in this regard.

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Range of Motion

Within each testing direction (flexion-extension, lateral bending, and axial rotation), no significant differences were found in range of motion among the three plating constructs with the full-length interbody spacer (Figs. 4, 5, and 6).

When the interbody spacer was shortened by 10% to simulate subsidence, the cervical spines fixed with a static plate had an increase of approximately 60% (p < 0.05) in flexion-extension range of motion (Fig. 4). There was no significant change in the range of motion for the specimens with the static plate in lateral bending (Fig. 5) and in axial rotation (Fig. 6). No significant differences were noted between the spines fixed with the two types of dynamic plates in any direction (Figs. 4, 5, and 6).

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Discussion

Anterior cervical plating systems have evolved dramatically over the last decade. Early models, such as the Caspar system, had unrestricted screw systems that caused early screw-loosening failures26. This required bicortical screw purchase within the vertebral body, placing the neural structures at risk of injury. Since that time, screw-locking mechanisms have become a standard feature on all current plating systems. The use of plating systems has become widespread when performing anterior cervical reconstructions. Each new system claims to incorporate the latest biomechanical advantage, which will supposedly lead to improved clinical outcomes. However, there are conflicting reports as to whether the added cost of hardware and added operating time yield superior results, particularly in single-level fusions. Wang et al.27 treated eighty patients with a single-level discectomy; forty-four of them had supplementation with a plating system and thirty-six did not. Although the application of the plate was found to be safe, it did not decrease the risk of pseudarthrosis or improve the clinical outcome according to the criteria of Odom et al.28.

In a controlled cohort study, Kaiser et al.10 compared the outcomes of one or two-level anterior cervical discectomies in 540 patients who were managed without plating (289 patients) or with plating (251 patients). The fusion rates for both the patients with single-level and those with double-level instrumentation were significantly better than the rate for the patients managed without instrumentation, and the patients managed with instrumentation had fewer graft-related complications. Others have suggested that the increased costs of instrumentation for patients are offset by the benefits of earlier mobilization29.

Subsidence is a process that occurs as the graft is being incorporated, and it results in a decrease in the height of the graft or penetration of the graft through the end plate into the vertebral body. The amount of subsidence appears to be dependent on several factors, including construct length18, the size of graft used17, and the type of graft (allograft or autograft)30-32. To accommodate subsidence, plates have been designed to allow for shrinkage of the construct, maintaining load and contact at the host bone-graft interface19.

Our previously published work with use of a noncadaveric polyethylene model showed that both the static and the dynamic plating systems shared the loads with a full-length interbody spacer. However, with the interbody spacer shortened to simulate graft subsidence, the dynamic constructs maintained load-sharing, whereas the static plates shielded the graft from compressive forces19. These findings were replicated in the current study, with use of a human cadaver model. With a full-length interbody spacer, the load-sharing percentages with the static construct (fixed-angle ATLANTIS), the rotationally dynamic system (variable-angle ATLANTIS), and the translationally dynamic system (PREMIER) were 60%, 68%, and 58%, respectively. When the interbody grafts were shortened by 10%, they carried slightly less load, although the difference was not significant; the rotationally dynamic and the translationally dynamic constructs carried 57% and 51%, respectively, of the applied load compared with a significantly decreased load-sharing of 17% for the static constructs. There was no corresponding increase in range of motion for the dynamic plates that would suggest a loss of construct stability. The static plate had an increased range of motion in flexion-extension with the shortened interbody spacer compared with that for both dynamic systems.

Using a bovine thoracic cadaveric model, Rapoff et al.33,34 calculated the graft load-sharing values for the PREMIER, ZEPHIR (Medtronic Sofamor Danek), and CSLP (cervical spine locking plate) systems (Synthes, West Chester, Pennsylvania) and found them to be 77%, 68%, and 41%, respectively. To our knowledge, our reports are the only ones in which graft load-sharing was calculated when subsidence was simulated.

A major concern for spine surgeons is the effect of dynamic plating on stability and clinical outcome, both in the immediate postoperative period and the long term. For one or two-level degenerative disease, the type of plating system used is primarily dependent on surgeon preference as the existing data have supported static or dynamic systems equally10,35,36. However, for more extensive spondylotic decompression, oncologic resections, and traumatic injuries, some surgeons have advocated the rigid, static contructs37. We know of no study, either clinical or biomechanical, that supports one system over another. Further studies with an instability model, fatigue testing, and follow-up of clinical outcomes will help to clarify the advantages and disadvantages of static and dynamic cervical plate designs.

The present in vitro study on cadaver specimens has limitations. The relationship of load-sharing and range of motion in the laboratory to the clinical goal of fusion, while suggestive, is not proven. Also, the clinical fusion rate is not directly associated with a particular stiffness value or decrease in motion.

In summary, this study provides an improved understanding of the immediate performance of anterior cervical fusion surgery with plate fixation. In facing the choice between the use of a static plate, a rotationally dynamic plate, or a translationally dynamic plate, this study suggests that there is little difference in the limitation of motion imparted by these plates when there is good contact between the interbody graft and the end plate with use of a full-length graft. The static plate provides no greater stiffness in the stable corpectomy reconstruction. However, if subsidence between the interbody graft and end plate occurs, a static plate bridges the reconstruction, decreasing the load on the graft and limiting the ability of the graft to participate in the construct stability. The result is increased motion at the fusion site, which may lead to an increased rate of pseudarthrosis. Both the rotationally dynamic plates and translationally dynamic plates allow continued contact between the interbody strut and the end plate, even in the face of 10% subsidence. This allows the graft to continue to participate in the construct stiffness, and the limitation on motion is maintained, potentially leading to an increased rate of fusion. ▪

In support of their research for or preparation of this manuscript, one or more of the authors received grants or outside funding from Medtronic. None of the authors received payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity. A commercial entity (Medtronic) paid or directed, or agreed to pay or direct, benefits to a research fund, foundation, educational institution, or other charitable or nonprofit organization with which the authors are affiliated or associated.

Investigation performed at the Department of Orthopaedics and the Orthopaedic Research Laboratory, University of Utah Orthopaedic Center, Salt Lake City, Utah

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