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:

Usefulness of dynamic stabilisation with mobile percutaneous pedicle screw for thoracic vertebral fractures in diffuse idiopathic skeletal hyperostosis

We report a case of vertebral fracture with diffuse idiopathic skeletal hyperostosis (DISH) who underwent posterior dynamic stabilisation using mobile percutaneous pedicle screws (PPS) with 1 above-1 below and obtained good bone fusion. A 76-year-old man experienced severe low back pain after he fell backward 1 m off a stepladder during work. A 12th thoracic vertebral fracture with DISH was observed. As the fractured part was unstable due to a three-column injury, and the conservative treatment of resting was not successful, posterior dynamic stabilisation with a mobile PPS between T11-L1 was performed the 38th day after injury. Immediately after surgery, a fracture gap was observed, but 5 months later, vertebral body height was shortened by about 4 mm, and good bone fusion was observed without loosening of the screw. The mobile PPS flexibly adapts to spinal plasticity and may be useful for bone union in vertebral fractures associated with DISH.

A computational biomechanical investigation of posterior dynamic instrumentation: combination of dynamic rod and hinged (dynamic) screw

Currently, rigid fixation systems are the gold standard for degenerative disk disease treatment. Dynamic fixation systems have been proposed as alternatives for the treatment of a variety of spinal disorders. These systems address the main drawbacks of traditional rigid fixation systems, such as adjacent segment degeneration and instrumentation failure. Pedicle-screw-based dynamic stabilization (PDS) is one type of these alternative systems. The aim of this study was to simulate the biomechanical effect of a novel posterior dynamic stabilization system, which is comprised of dynamic (hinged) screws interconnected with a coiled, spring-based dynamic rod (DSDR), and compare it to semirigid (DSRR and RSRR) and rigid stabilization (RSRR) systems. A validated finite element (FE) model of L1-S1 was used to quantify the biomechanical parameters of the spine, such as range of motion, intradiskal pressure, stresses and facet loads after single-level instrumentation with different posterior stabilization systems. The results obtained from in vitro experimental intact and instrumented spines were used to validate the FE model, and the validated model was then used to compare the biomechanical effects of different fixation and stabilization constructs with intact under a hybrid loading protocol. The segmental motion at L4-L5 increased by 9.5% and 16.3% in flexion and left rotation, respectively, in DSDR with respect to the intact spine, whereas it was reduced by 6.4% and 10.9% in extension and left-bending loads, respectively. After instrumentation-induced intradiskal pressure at adjacent segments, L3-L4 and L5-S1 became less than the intact in dynamic rod constructs (DSDR and RSDR) except in the RSDR model in extension where the motion was higher than intact by 9.7% at L3-L4 and 11.3% at L5-S1. The facet loads were insignificant, not exceeding 12N in any of the instrumented cases in flexion. In extension, the facet load in DSDR case was similar to that in intact spine. The dynamic rod constructions (DSDR and RSDR) led to a lesser peak stress at screws compared with rigid rod constructions (DSRR and RSRR) in all loading cases. A dynamic construct consisting of a dynamic rod and a dynamic screw did protect the adjacent level from excessive motion.

Pedicle-Screw-Based Dynamic Systems and Degenerative Lumbar Diseases: Biomechanical and Clinical Experiences of Dynamic Fusion with Isobar TTL

Dynamic systems in the lumbar spine are believed to reduce main fusion drawbacks such as pseudarthrosis, bone rarefaction, and mechanical failure. Compared to fusion achieved with rigid constructs, biomechanical studies underlined some advantages of dynamic instrumentation including increased load sharing between the instrumentation and interbody bone graft and stresses reduction at bone-to-screw interface. These advantages may result in increased fusion rates, limitation of bone rarefaction, and reduction of mechanical complications with the ultimate objective to reduce reoperations rates. However published clinical evidence for dynamic systems remains limited. In addition to providing biomechanical evaluation of a pedicle-screw-based dynamic system, the present study offers a long-term (average 10.2 years) insight view of the clinical outcomes of 18 patients treated by fusion with dynamic systems for degenerative lumbar spine diseases. The findings outline significant and stable symptoms relief, absence of implant-related complications, no revision surgery, and few adjacent segment degenerative changes. In spite of sample limitations, this is the first long-term report of outcomes of dynamic fusion that opens an interesting perspective for clinical outcomes of dynamic systems that need to be explored at larger scale.

Posterior Transpedicular Dynamic Stabilization versus Total Disc Replacement in the Treatment of Lumbar Painful Degenerative Disc Disease: A Comparison of Clinical Results

Study Design. Prospective clinical study. Objective. This study compares the clinical results of anterior lumbar total disc replacement and posterior transpedicular dynamic stabilization in the treatment of degenerative disc disease. Summary and Background Data. Over the last two decades, both techniques have emerged as alternative treatment options to fusion surgery. Methods. This study was conducted between 2004 and 2010 with a total of 50 patients (25 in each group). The mean age of the patients in total disc prosthesis group was 37,32 years. The mean age of the patients in posterior dynamic transpedicular stabilization was 43,08. Clinical (VAS and Oswestry) and radiological evaluations (lumbar lordosis and segmental lordosis angles) of the patients were carried out prior to the operation and 3, 12, and 24 months after the operation. We compared the average duration of surgery, blood loss during the surgery and the length of hospital stay of both groups. Results. Both techniques offered significant improvements in clinical parameters. There was no significant change in radiologic evaluations after the surgery for both techniques. Conclusion. Both dynamic systems provided spine stability. However, the posterior dynamic system had a slight advantage over anterior disc prosthesis because of its convenient application and fewer possible complications.

Dynamic lumbar pedicle screw-rod stabilization: in vitro biomechanical comparison with standard rigid pedicle screw-rod stabilization

OBJECT

It is unclear how the biomechanics of dynamic posterior lumbar stabilization systems and traditional rigid pedicle screw-rod systems differ. This study examined the biomechanical response of a hinged-dynamic pedicle screw compared with a standard rigid screw used in a 1-level pedicle screw-rod construct.

METHODS

Unembalmed human cadaveric L3-S1 segments were tested intact, after L4-5 discectomy, after rigid pedicle screw-rod fixation, and after dynamic pedicle screw-rod fixation. Specimens were loaded using pure moments to induce flexion, extension, lateral bending, and axial rotation while recording motion optoelectronically. Specimens were then loaded in physiological flexion-extension while applying 400 N of compression. Moment and force across instrumentation were recorded from pairs of strain gauges mounted on the interconnecting rods.

RESULTS

The hinged-dynamic screws allowed an average of 160% greater range of motion during flexion, extension, lateral bending, and axial rotation than standard rigid screws (p < 0.03) but 30% less motion than normal. When using standard screws, bending moments and axial loads on the rods were greater than the bending moments and axial loads on the rods when using dynamic screws during most loading modes (p < 0.05). The axis of rotation shifted significantly posteriorly more than 10 mm from its normal position with both devices.

CONCLUSIONS

In a 1-level pedicle screw-rod construct, hinged-dynamic screws allowed a quantity of motion that was substantially closer to normal motion than that allowed by rigid pedicle screws. Both systems altered kinematics similarly. Less load was borne by the hinged screw construct, indicating that the hinged-dynamic screws allow less stress shielding than standard rigid screws.

Biomechanical evaluation of a dynamic pedicle screw fixation device

BACKGROUND

Recent innovations in dynamic devices have promised a reduction in stress shielding, protection of adjacent segment degeneration, and decreased implant failure. However, there have been few studies comparing the biomechanical properties of a rigid device in comparison to a dynamic posterior fixation device. The purpose of this study was to compare the immediate stability of a new dynamic pedicle screw fixation device with an equivalent rigid device.

METHODS

Six thoracolumbar cadaver spines (T10-L4) were fixed in a biomechanical testing frame. Pure moments of 10Nm were loaded in six directions: flexion, extension, right and left lateral bending, and right and left axial rotation. For each spine, four different stages were tested: intact, destabilization of the middle segment, fixation with the dynamic device, and fixation with the rigid device. Ranges of motion were measured using stereophotogrammetry. The specimens with each device were then subjected to flexion-compression loading for five cycles on a MTS 858 Universal Testing Machine. The average stiffness of the last three cycles was recorded.

FINDINGS

Both dynamic and rigid devices were found to provide stability for the injured segment in flexion-extension and lateral bending. In axial rotation, the devices could restore the stability to levels similar to those in an intact spine. Results also indicated a slight increase in range of motion in flexion-extension and significant reduction in stiffness of flexion-compression with the dynamic device (P < 0.01), in comparison to the rigid device.

INTERPRETATION

The dynamic device offers a system that may alter favorably the movement and load transmission of a spinal motion segment without sacrificing construct stability.