Biomechanical stability of five pedicle screw fixation systems in a human lumbar spine instability model

A biomechanical analysis of an instrumented spinal fixator under torsional loads

The effect of design features of an internal spinal fixator on the loading of its individual components is paramount to the understanding of the interaction between the fixator and the instrumented spine. Using a corpectomy injury model, a strain gauge instrumented spinal fixation device was employed to investigate the role of clamp tightening torque and the inclusion of transverse bars on the distribution of bending and torsional moments acting on the fixator under torsional loading. The increase in clamp torque from 5 to 10 Nm caused a marked decrease (40%) in torsional moments acting on the vertical rods, an increase of 24% in torsional moments acting on the screws and an increase of 44% in bending moments acting on the rods along the sagittal plane of the fixator. The inclusion of transverse elements significantly increased (132%) the bending moment acting on the rods and decreased (92%) the bending moments acting on the screws along the sagittal plane. The change in both design parameters significantly reduced the response hysteresis and decreased the asymmetry of loading. A theoretical model, developed to elucidate the load path mechanisms underlying this response, successfully predicted the external response of the fixator. This model suggested both design parameters would affect the internal force and moment distribution across the fixator and the relative role of each load response mechanism in effecting this response. The changes in load patterns across the fixator will influence both its ability to augment the process of spinal fusion and the long-term performance of its components.

A biomechanical comparison of facet screw fixation and pedicle screw fixation: effects of short-term and long-term repetitive cycling

STUDY DESIGN

A biomechanical study was conducted to assess the stabilization performance of transfacet pedicle screw fixation.

OBJECTIVE

To compare the biomechanical effects of short-term and long-term cyclic loading on lumbar motion segments instrumented with either a pedicle screw or a transfacet pedicle screw construct.

SUMMARY OF BACKGROUND DATA

Facet screw fixation is an alternative to pedicle screw fixation that permits the use of a minimally invasive strategy. It is not known whether facet screw fixation can provide stability equivalent to pedicle screw fixation during cyclical loading. Therefore, transfacet pedicle screw fixation and standard pedicle screw fixation techniques were compared biomechanically.

METHODS

Lumbar motion segments were tested under short-term and long-term cyclic loading conditions. For the short-term phase, specimens were tested intact for six cycles (to 400 N or 4 Nm) in compression, flexion, extension, lateral bending, and torsion. The specimens then were instrumented with bilateral semicircular interbody spacers and pedicle screw instrumentation or transfacet pedicle screws, and the testing sequence was repeated. For the long-term phase, 12 specimens were instrumented in a similar manner and loaded to 6 Nm of flexion bending for 180,000 cycles.

RESULTS

For the short-term phase, both fixation systems had significantly greater stiffness and reduced range of motion, as compared with the intact state. No differences were observed between the fixation systems except in flexion, wherein transfacet pedicle screw specimens were significantly stiffer than traditional pedicle screw specimens. For the long-term phase, the stiffness and range of motion did not significantly increase or decrease over repetitive cycling of the instrumented specimens. Furthermore, no significant difference between the fixation systems was observed.

CONCLUSIONS

The stability provided by both transfacet pedicle screw fixation and traditional pedicle screw fixation was not compromised after repetitive cycling. In this model, transfacet pedicle screw fixation appears equivalent biomechanically to traditional pedicle screw fixation.

Pedicle screw adjustments affect stability of thoracolumbar burst fracture

STUDY DESIGN

An in vitro biomechanical study of the stabilizing effect of pedicle screw instrumentation on experimental thoracolumbar burst fractures.

OBJECTIVES

To evaluate the effects of different adjustments applied by the pedicle screw fixation device on the stability of the spine-device construct.

SUMMARY OF BACKGROUND DATA

Pedicle screw devices are widely used to accomplish spinal reduction and provide stability to an injured spine. In previous biomechanical studies the stability of the spine-device constructs has been examined for many devices. However, no study has quantitatively assessed the associations between the device adjustments and the stability of the construct.

METHODS

Five-vertebrae human cadaveric specimens with burst fracture at L1 vertebra were studied. Pedicle screw fixation device was attached to the T12 and L2 vertebrae. Five device adjustments (pure compression, pure distraction, pure extension, a combination distraction-extension, and neutral posture) were studied. Multidirectional flexibility test was performed when intact, after burst fracture, and after each device adjustment to document spinal stability.

RESULTS

The construct stability had a complex association to the device adjustment. For example, the maximum flexion and extension stabilities were achieved by pure compression and distraction-extension combination adjustments, respectively. Pure distraction and pure extension adjustments decreased the construct stability.

CONCLUSIONS

The device adjustments affected the spinal construct stability differently in different directions. Although pure compression provided the most stability in most directions, the combined distraction-extension adjustment may be more suitable considering the neural decompression also.

A study of stiffness protocol as exemplified by testing of a burst fracture model in sagittal plane

STUDY DESIGN

An in vitro biomechanical study of lumbar spine segments.

OBJECTIVE

To study the characteristics of the stiffness test protocol.

SUMMARY OF BACKGROUND DATA

In an in vitro study using a flexibility protocol, forces are applied and motions are measured; no center of rotation needs to be specified. In a study using a stiffness protocol, the forces are measured and the motions are applied. This does require the center of rotation to be specified. Many biomechanical studies of the spine are available, but there is lack of clarity concerning which of these two test protocols is appropriate to achieve a certain study goal.

METHODS

Five-vertebrae lumbar spine specimens with burst fractures in the middle vertebrae (L1) were used. Specially designed apparatus applied flexion and extension rotations around five centers of rotations located on anteroposterior line through the middle of L1. Maximum moment of 4 Nm was applied.

RESULTS

The authors found load-displacement curves, ranges of motion, and neutral zones obtained at the five centers of rotations to be markedly different. The center of rotation located at the posterior longitudinal ligament produced large range of motion and neutral zones in comparison to the centers of rotation located at the anterior longitudinal ligament and the spinous process tip (P<0.01).

CONCLUSIONS

The stiffness protocol requires that a center of rotation be specified. Shown here is the significant variability in the load-displacement curves, depending on the choice of the location of the center of rotation. Certain center of rotation locations may block the natural motions of the spine and may result in tissue damage.

Stability potential of spinal instrumentations in tumor vertebral body replacement surgery

STUDY DESIGN

The multidirectional stability potential of anterior, posterior, and combined instrumentations applied at L1-L3 was studied after L2 corpectomy and replacement with a carbon-fiber implant.

OBJECTIVES

To evaluate the biomechanical characteristics of short-segment anterior, posterior, and combined instrumentations in lumbar spine tumor vertebral body replacement surgery.

SUMMARY OF BACKGROUND DATA

The biomechanical properties of many different spinal instrumentations have been studied in various spinal injury models. Only a few studies, however, investigate the stabilization methods in spinal tumor vertebral body replacement surgery.

METHODS

Eight fresh frozen human cadaveric thoracolumbar spine specimens (T12-L4) were prepared for biomechanical testing. Pure moments (2.5 Nm, 5 Nm, and 7.5 Nm) of flexion-extension, left-right axial torsion, and left-right lateral bending were applied to the top vertebra in a flexibility machine, and the motions of the L1 vertebra with respect to L3 were recorded with an optoelectronic motion measurement system after reconditioning. The L2 vertebral body was resected and replaced by a carbon-fiber cage. Different fixation methods were applied to the L1 and L3 vertebrae. One anterior, two posterior, and two combined instrumentations were tested. Load-displacement curves were recorded and neutral zone and range of motion parameters were determined.

RESULTS

The anterior instrumentation provided less potential stability than the posterior and combined instrumentations in all motion directions. The anterior instrumentation, after vertebral body replacement, showed greater motion than the intact spine, especially in axial torsion (range of motion, 10.3 degrees vs 5.5 degrees; neutral zone, 2.9 degrees vs. 0.7 degrees; P < 0.05). Posterior instrumentation provided greater rigidity than the anterior instrumentation, especially in flexion-extension (range of motion, 2.1 degrees vs. 12.6 degrees; neutral zone, 0.6 degrees vs. 6.1 degrees; P < 0.05). The combined instrumentation provided superior rigidity in all directions compared with all other instrumentations.

CONCLUSIONS

Posterior and combined instrumentations provided greater rigidity than anterior instrumentation. Anterior instrumentation should not be used alone in vertebral body replacement.

Load-displacement properties of the thoracolumbar calf spine: experimental results and comparison to known human data

The availability of human cadaveric spine specimens for in vitro tests is limited and the risk of infection is now of vital concern. As an alternative or supplement, calf spines have been used as models for human spines, in particular to evaluate spinal implants. However, neither qualitative nor quantitative biomechanical data on calf spines are available for comparison with data on human specimens. The purpose of this study was to determine the fundamental biomechanical properties of calf spines and to compare them with existing data from human specimens. Range of motion, neutral zone, and stiffness properties of thoracolumbar calf spines (T6-L6) were determined under pure moment loading in flexion and extension, axial left/right rotation and right/left lateral bending. Biomechanical similarities were observed between the calf and reported human data, most notably in axial rotation and lateral bending. Range of motion in the lumbar spine in flexion and extension was somewhat less in the calf than that typically reported for the human, though still within the range. These results suggest that the calf spine can be considered on a limited basis as a model for the human spine in certain in vitro tests.

New means in spinal pedicle hook fixation. A biomechanical evaluation

Pedicle hooks which are used as an anchorage for posterior spinal instrumentation may be subjected to considerable three-dimensional forces. In order to achieve stronger attachment to the implantation site, hooks using screws for additional fixation have been developed. The failure loads and mechanisms of three such devices have been experimentally determined on human thoracic vertebrae: the Universal Spine System (USS) pedicle hook with one screw, a prototype pedicle hook with two screws and the Cotrel-Dubousset (CD) pedicle hook with screw. The USS hooks use 3.2-mm self-tapping fixation screws which pass into the pedicle, whereas the CD hook is stabilised with a 3-mm set screw pressing against the superior part of the facet joint. A clinically established 5-mm pedicle screw was tested for comparison. A matched pair experimental design was implemented to evaluate these implants in constrained (series I) and rotationally unconstrained (series II) posterior pull-out tests. In the constrained tests the pedicle screw was the strongest implant, with an average pull-out force of 1650 N (SD 623 N). The prototype hook was comparable, with an average failure load of 1530 N (SD 414 N). The average pull-out force of the USS hook with one screw was 910 N (SD 243 N), not significantly different to the CD hook's average failure load of 740 N (SD 189 N). The result of the unconstrained tests were similar, with the prototype hook being the strongest device (average 1617 N, SD 652 N). However, in this series the difference in failure load between the USS hook with one screw and the CD hook was significant. Average failure loads of 792 N (SD 184 N) for the USS hook and 464 N (SD 279 N) for the CD hook were measured. A pedicular fracture in the plane of the fixation screw was the most common failure mode for USS hooks.(ABSTRACT TRUNCATED AT 250 WORDS)

A universal spine tester for in vitro experiments with muscle force simulation

We report a new apparatus to determine the quasistatic, three-dimensional, load-displacement characteristics of spines including muscle forces. The loading frame can be adapted to mono- and polysegmental specimens from the lumbar or cervical spine as well as to entire spines. Three force and three moment components can be applied in either direction individually or in combination with no constraint on the resulting motion; the loads can be applied at user-chosen rates of application and release with continuous recording of displacements, so as to study either creep or relaxation. The loads and displacement-measuring devices are computer-controlled. Thus, this testing device provides a tool for many kinds of stability tests and for basic research of spine biomechanics. A first experiment shows that the application of muscle forces significantly affects the load-deformation characteristics and intradiscal pressure.