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Hinged-Dynamic Posterior Device Permits Greater Loads on the Graft and Similar Stability as Compared with Its Equivalent Rigid Device: A Three-Dimensional Finite Element Assessment

Vijay K. Goel, PhD
Regina J. Konz, MS
H. T. Chang, MS
Nicole M. Grosland, PhD
Leon J. Grobler, MD
K. D. Chesmel

ABSTRACT

A three-dimensional finite element model was developed to compare the load sharing capabilities of the hinged (or dynamic) versus rigid states of a posterior lumbar spinal fixation device. The objective was to show that a dynamic state of a fixation system enables more load to be transferred through the graft (anterior column) as compared with an alternative rigid state, without compromising the stability imparted to the stabilized segment. The stability aspects of the rigid and dynamic pedicle screw-rod/plate fixation devices have been reported. Load sharing data of dynamic devices, as compared with rigid devices, are sparse. A three-dimensional nonlinear, experimentally validated, midsagittal symmetric finite element model of a L3-L4 motion segment was used for analysis. The intact model was modified to simulate a facetectomy and stabilization using both the hinged and rigid devices. The nucleus of the intact model was replaced to simulate a graft of cancellous bone, cortical bone, or titanium. The hinged device allowed for greater axial displacement while it maintained rotational stability similar to that of the rigid device. As the stiffness of the nucleus was increased from cancellous bone to titanium, the hinged device allowed a greater load through the nucleus compared with the rigid device predictions. However, the difference in load through the nucleus was minimal for the cortical and titanium grafts. The load transferred through the nucleus was greater for the hinged device, while it imparted rotational stability similar to the rigid device. Thus, a hinged screw-rod device offers a more favorable environment for enhanced fusion of the graft. Finite element models of a destabilized lumbar motion segment supplemented by a hinged or rigid posterior fixation device predicted that the hinged design permits greater load through the nucleus without any decrease in the rotational stability. Such systems are expected to enhance the fusion process.

Keywords:Posterior fixation device (hinged and rigid), biomechanics, load sharing, finite element modeling

Stabilization techniques have evolved to include a wide variety of internal fixation devices, including posterior pedicle screw rod/plate fixation devices to enhance biological fusion. These devices provide an effective way for correcting and stabilizing the spine, leading to an increased rate and degree of bony fusion.^1-8 The rigidity across a stabilized segment of the spine provided by a posterior fixation device and its ability to load share with the fusion mass are considered essential for fusion or bone healing to occur.^1,6,8-12 If the load transferred through the fusion can be increased without sacrificing its ability to impart stability to the decompressed segment, a more favorable environment for fusion may be created. In recent years, several such devices have appeared on the market. In vitro cadaveric studies including load-displacement experiments provide a means to delineate the stability afforded by the various stabilization systems (ie, dynamic vs. rigid system).¹³ However, it is not practical to estimate the loads through the bone graft by using an experimental approach. Accordingly, this study uses a finite element analysis to compare the load sharing ability of a rigid versus a hinged pedicle screw-rod system. The Segmental Spine Correction System (SSCS) pedicle hinged screw-rod system (Osteotech, Inc., Eatontown, NJ 07724) was used for this study (Figure 1 ). The hinged screw allows ± 15° of movement about the axis A-A (Figure 2 ). As the hinge mechanism contacts the screw shaft, it is halted at its maximum range of motion, after which it acts in a manner similar to the rigid screw. This ±15° motion is what allows for the (theoretical) increase in load sharing capability of the device as compared with its equivalent SSCS rigid-screw system. Because the device designs were identical, except for the hinge aspect, direct comparisons could easily be made and inferences drawn about the biomechanical validity of any results determined in the study.

Materials and Methods

A three-dimensional nonlinear, experimentally validated finite element model of an intact L3-L4 motion segment was used for the analysis.^14,15 The capsular ligaments and part of the facets of the intact model were removed to simulate a facetectomy. From this model, two additional models were created to simulate stabilization across the L3-L4 segment for both the rigid-screw and hinged-screw devices. This was accomplished using 6-mm rigid and hinged pedicle screw-rod devices, including a cross bar at the L3-L4 disc interspace (Figure 3 ). The hinge part of the screw was modeled as a revolute joint between the screws and the hinges. The +15° and -15° constraints were defined by the contact elements between the nodes on the stoppers of the hinges and the surface of the screws in the model. In the model, the hinge axis was oriented perpendicular to the pedicular axis and was in the transverse plane. The initial hinge position was presumed to be 0°. The nucleus in these models was replaced with a graft (termed as nucleus) representing cancellous bone, cortical bone, or titanium (Table 1 ). The annulus was left intact in this simulation. Height of the graft equaled the disc/nucleus height. The interface between the graft/nucleus and the end-plate was simulated using gap elements so that the interface could transmit only compressive loads. This simulates the situation immediately after surgery and prior to the fusion of the graft.

The models were fixed at the base, and axial compressive loads up to 800 N were applied to the superior end plate of the L3 vertebral body. It was evenly distributed across the end plate. The output was processed to determine axial displacement, flexion/extension rotation, and the loads transmitted through the graft material (nucleus) and the rods.

Results

In the axial compression mode, the hinged device allowed for greater axial displacement, while it maintained angular stability similar to that of the rigid device (Table 2 ). The load transmitted through the nucleus increased with the use of hinged device as compared with the rigid device (Table 3 ). The amount of increase in the load through the nucleus was minimal for the titanium-graft case as compared with the cortical graft case (Table 4 ). Because load ratios were similar regardless of the load magnitude, only results for the 800 N are reported.

Discussion

Two biomechanical issues are important in assessing the role of spinal devices in promoting successful fusion. First is the ability of a device to immobilize the injured segment for the fusion to occur, at least in the initial phase of the healing process. Second is the device's ability to share the load with the fusion mass, thereby enhancing the fusion process. A reduction in the rigidity of the device can increase the amount of load shared by the fusion mass. However, such an approach will decrease the ability of the device to immobilize the decompressed segment in comparison with the rigid system. Patient-specific optimal rigidity of a device is not known. However, to be safe, the increase in load through the fusion mass should be achieved without an appreciable decrease in the ability of the device to provide stability. The hinge and rigid configurations afforded by the posterior system used in this study are both capable of achieving these objectives. The results of our in vitro investigation have shown that both designs provided similar stability across a severely destabilized spinal segment.13 The present study was undertaken to test the hypothesis that the hinged screw design could also lead to increased load through the nucleus as compared with the rigid screw system.

The present finite element study has predicted the changes in load sharing due to alterations in the design of the device from a rigid to hinged system. Furthermore, the model predicted similar rotational stability for the two designs. Thus, the present study complements our in vitro investigation.¹⁵ Under the conditions simulated in this model, the predicted results clearly show that the hinged system permits a greater load through the graft material used to replace the nucleus, as compared with the equivalent rigid screw design. Although the percent relative increase in load due to hinge screws is smaller, hinge screws permit greater axial displacements, as compared with rigid screws (Table 1 ). This increased axial displacement due to the hinge design of the screw has the potential to compensate for the settling of the bone graft during the initial phase of healing. Thus, the hinge design will ensure load transfer through the graft despite bone graft settling, as compared with the rigid design. The hinged screw-rod device, therefore, offers a design more likely to promote the fusion process by creating a more favorable environment. Additional studies are essential to address the issues of asymmetric placement of hinge screws and load sharing aspects in flexion, extension, lateral bending, and torsional modes to gain further insight into the mechanics of using hinge screws as opposed to conventional system.

Acknowledgements

This work was supported in part by Osteotech, Inc., Whitaker Foundation and the University of Iowa Spine Foundation Research Fund. The work was undertaken at the University of Iowa.

VIJAY K. GOEL, PhD, is Professor and Chair of the Department of Bioengineering, The University of Toledo, Toledo, OH. Vijay K. Goel, PhD, Department of Bioengineering, 5051C Nitschke Hall, The University of Toledo, Toledo, OH 43606; (419) 530-8035; fax: (419) 530-8076; E-mail: vgoel@eng.utoledo.edu

REGINA J. KONZ, MS, is with the Iowa Spine Research Center, Departments of Biomedical Engineering and Orthopaedics, University of Iowa, Iowa City IA.

H. T. CHANG, MS, is with the Iowa Spine Research Center, Departments of Biomedical Engineering and Orthopaedics, University of Iowa, Iowa City IA.

NICOLE M. GROSLAND, PhD, is with the Iowa Spine Research Center, Departments of Biomedical Engineering and Orthopaedics, University of Iowa, Iowa City IA.

LEON J. GROBLER, MD, is with the Iowa Spine Research Center, Departments of Biomedical Engineering and Orthopaedics, University of Iowa, Iowa City IA.

K. D. CHESMEL is with Osteotech, Inc., Eatontown, NJ.

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