New rod-plate anterior instrumentation for thoracolumbar/lumbar scoliosis: biomechanical evaluation compared with dual-rod and single-rod with structural interbody support

Range of motion after thoracolumbar corpectomy: evaluation of analogous constructs with a novel low-profile anterior dual-rod system and a traditional dual-rod system

STUDY DESIGN

An in vitro biomechanical study.

OBJECTIVES

To compare the biomechanical stability of traditional and low-profile thorocolumbar anterior instrumentation after a corpectomy with cross-connectors. Dual-rod anterior thoracolumbar lateral plates (ATLP) have been used clinically to stabilize the thorocolumbar spine.

METHODS

The stability of a low-profile dual-rod system (LP DRS) and a traditional dual-rod system (DRS) was compared using a calf spine model. Two groups of seven specimens were tested intact and then in the following order: (1) ATLP with two cross-connectors and spacer; (2) ATLP with one cross-connector and spacer; (3) ATLP with spacer. Data were normalized to intact (100 %) and statistical analysis was used to determine between-group significances.

RESULTS

Both constructs reduced motion compared to intact in flexion-extension and lateral bending. Axial rotation motion became unstable after the corpectomy and motion was greater than intact, even with two cross-connectors with both systems. Relative to their respective intact groups, LP DRS significantly reduced motion compared to analogous DRS in flexion-extension. The addition of cross-connectors reduced motion in all loading modes.

CONCLUSIONS

The LP DRS provides 7.5 mm of reduced height with similar biomechanical performance. The reduced height may be beneficiary by reduced irritation and impingement on adjacent structures.

Biomechanical evaluation of an interfacet joint decompression and stabilization system

A majority of the middle-aged population exhibit cervical spondylosis that may require decompression and fusion of the affected level. Minimally invasive cervical fusion is an attractive option for decreasing operative time, morbidity, and mortality rates. A novel interfacet joint spacer (DTRAX facet screw system, Providence Medical) promises minimally invasive deployment resulting in decompression of the neuroforamen and interfacet fusion. The present study investigates the effectiveness of the device in minimizing intervertebral motion to promote fusion, decompression of the nerve root during bending activity, and performance of the implant to adhere to anatomy during repeated bending loads. We observed flexion, extension, lateral bending, and axial rotation resonant overshoot mode (ROM) in cadaver models of c-spine treated with the interfacet joint spacer (FJ spacer) as stand-alone and supplementing anterior plating. The FJ spacer was deployed bilaterally at single levels. Specimens were placed at the limit of ROM in flexion, extension, axial bending, and lateral bending. 3D images of the foramen were taken and postprocessed to quantify changes in foraminal area. Stand-alone spacer specimens were subjected to 30,000 cycles at 2 Hz of nonsimultaneous flexion-extension and lateral bending under compressive load and X-ray imaged at regular cycle intervals for quantitative measurements of device loosening. The stand-alone FJ spacer increased specimen stiffness in all directions except extension. 86% of all deployments resulted in some level of foraminal distraction. The rate of effective distraction was maintained in flexed, extended, and axially rotated postures. Two specimens demonstrated no detectable implant loosening (<0.25 mm). Three showed unilateral subclinical loosening (0.4 mm maximum), and one had subclinical loosening bilaterally (0.5 mm maximum). Results of our study are comparable to previous investigations into the stiffness of other stand-alone minimally invasive technologies. The FJ spacer system effectively increased stiffness of the affected level comparable to predicate systems. Results of this study indicate the FJ spacer increases foraminal area in the cervical spine, and decompression is maintained during bending activities. Clinical studies will be necessary to determine whether the magnitude of decompression observed in this cadaveric study will effectively treat cervical radiculopathy; however, results of this study, taken in context of successful decompression treatments in the lumbar spine, are promising for the continued development of this product. Results of this biomechanical study are encouraging for the continued investigation of this device in animal and clinical trials, as they suggest the device is well fixated and mechanically competent.

Comparison of allograft bone and titanium cages for vertebral body replacement in the thoracolumbar spine: a biomechanical study

BACKGROUND

When an anterior approach to repair a burst fracture is indicated, several devices can be used to restore spinal stability (eg, bone graft, free-standing titanium cage, and expandable titanium cage).

OBJECTIVE

We compare the biomechanical stability and prices of each of these systems.

MATERIALS AND METHODS

Eight fresh human cadaver T11 through L3 vertebral specimens were harvested and cleaned of soft tissues. T11-T12 and L2-L3 were fixed by screws. The fixed ends were then set in automotive body filler (Bondo). The prepared specimens were tested in the Biaxial Instron tester (8874, Norwood, MA) after a sequence of the following: intact, after the creation of an anterior corpectomy at L1, and after insertion of both of the 2 different titanium cages and the fibular graft. A titanium screw-and-plate anterolateral system was used to secure the construct (VANTAGE, Medtronic Sofamor Danek, Memphis, TN). The conditions of displacement testing were as follows: rotation (+/- 3.5 degrees ), flexion and extension, and left and right bending (+/- 3.5 mm). For each mode of testing, the stiffness was calculated.

RESULTS

The stiffness data, when statistically analyzed by repeated-measures analysis of variance (at P = .05 and power > 0.9), indicated no significant differences among these devices.

CONCLUSION

On the basis of this biomechanical study, the stiffness of the fibular graft was similar to that of the other metallic devices in this cadaver model.

[Anterior scoliosis surgery. State of the art and a comparison with posterior techniques]

For more than 2 decades ventral derotation spondylodesis (Zielke VDS) as a major improvement over Dwyer instrumentation (DI) was the gold standard of instrumented curve correction and stabilization from the anterior approach. As the first available system it enables a true three-dimensional curve correction. A disadvantage is the low internal stabilization capability with a need for long-term external stabilization by means of cast and brace treatment postoperatively. Meanwhile with the development of modern single and dual solid rod systems these disadvantages can be avoided completely. Video-assisted (thoracoscopic) anterior scoliosis surgery accounts for less than 2% of anteriorly treated scoliosis cases, mainly due to a long operating time and significant learning curve.From the posterior approach the Cotrel-Dubousset instrumentation (CDI) as a polysegmentally attached posterior hook threaded dual rod system used to be state of the art for a long time, since it eliminated the disadvantages of Harrington instrumentation (HI) in terms of only one-dimensional correction and low stabilization capabilities. However even with CDI effective derotation was impossible. In posterior scoliosis surgery there is a strong trend away from hook systems towards transpedicular segmentally fixed dual rod systems not only in the lumbar spine but also in the thoracic area. Advantages of these newer techniques are shorter fusion, improved correction, and less loss of correction over time.Advantages of modern anterior instrumentation systems in comparison to posterior transpedicular instrumented dual rod systems are less blood loss, better derotation, slightly shorter fusion levels, and a better influence on sagittal plane control or improvement especially for hypokyphotic thoracic scoliosis cases. Our data also document a superior spontaneous correction of the lumbar curve after selective anterior instrumented correction (Lenke 1B+C), although other studies could not find significant differences. In our experience the neurological risk of anterior instrumented correction is also lower than that of posterior scoliosis surgery, although the morbidity and mortality data of the Scoliosis Research Society could not prove that anymore in recent years. A negative effect of anterior transthoracic scoliosis surgery in comparison to posterior surgery is a more negative effect on lung function, which improves slower after surgery and does not quite reach the levels of posterior surgery at follow-up. But new data on posterior segmental transpedicular correction and fusion also prove a lordosating effect with negative effect on lung function.

Novel dual-rod screw for thoracoscopic anterior instrumentation: biomechanical evaluation compared with single-rod and double-screw/double-rod anterior constructs

STUDY DESIGN

A novel dual-rod screw was designed to provide a second-rod augmentation at the critical apical/middle segments of the single-rod thoracoscopic anterior construct. Biomechanical testing was performed on pig thoracic spines instrumented with 7-segment anterior scoliosis constructs.

OBJECTIVES

To analyze the biomechanical performance of the new implant, and compare it to a single-rod and double-rod anterior constructs.

SUMMARY OF BACKGROUND DATA

Using single-rod thoracoscopic anterior instrumentation for thoracic scoliosis, the complications of rod breakage at apex, high rate of nonunion, and resultant loss of coronal and sagittal correction has been reported. Inadequate construct stiffness because of a smaller diameter single rod has been implicated as the cause of these complications.

METHODS

Twelve pig thoracic spines were instrumented over 7 segments with: (1) single-rod construct, (2) short second-rod augmentation at the apex of the single-rod construct, (3) long second-rod augmentation at middle segments of the single-rod construct, and (4) double-screw/double-rod anterior construct. The spines were tested in flexion-extension, left-right lateral bending, and torsion, using pure bending moments. Strain gauges attached to the primary single rod at the cephalad, middle, and caudal portions were used and the maximum tensile stress was recorded.

RESULTS

In the single-rod construct, the middle portion stress was 39% to 51% greater than the stress in the cephalad and caudal portions in flexion-extension (P < 0.05), and the cephalad portion stress was 39% to 65% greater than the stress in the middle and caudal portions in right lateral bending and torsion (P < 0.05). When a second rod was added at the apical/middle portion, the middle portion stress decreased from 50% to 72% in flexion-extension and right lateral bending (P < 0.05). In addition, the second rod decreased the primary single-rod stress at the cephalad portion by 48% (left torsion) and the caudal portion by 50% (flexion). Double-screw/double-rod construct significantly increases the construct stiffness in comparison with the single-rod construct. However, it did not add any construct stiffness at the critical apical segments when compared to the constructs in which the second rod augmented the single-rod constructs.

CONCLUSION

A novel dual-rod screw was designed to combine the standard single-rod construct with the addition of a second rod at the critical apical/middle segments and increase construct stiffness and stability. This implant may therefore prevent pseudarthrosis and rod breakage by enhancing construct stiffness.