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).
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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