gms | German Medical Science

Standard pedicle screw fixation involves open exposures and extensive muscle dissection. During the last years different less invasive spinal procedures have been designed in order to minimize pain, morbidity, and disability. Furthermore, image-guided systems reduced the rate of misplaced srews as well as of associated nerve injuries. However, there were only minimal efforts in developing a technique for percutaneous dorsal instrumentation. We report on our first experimental and clinical results of a novel device for percutaneous posterior fixation of the thoracolumbar spine.

For our development we used an electromagnetic tracking system equipped with the software ACCISS II (SchaererMayfield). The instruments and implantats are made of titanium in order to not disturb the magnetic field. All instruments as well as the multiaxial pedicle screws are cannulated allowing their guidance by a navigated titanium wire, which is implanted at the beginning of the procedure. The insertion of the rod is guided by the navigator using the possibility to track the multiaxial screw heads and the axis of the rod simultanously. For this purpose, we developed a special rod-insertion device, which requires minimal access and no muscle dissection. Because this device is tracked by the navigator, no visual feedback is necessary. The dynamic reference frame is fixed percutaneously on the spinous process, and it enables the computer to perform image-to-patient registration automatically.

Mechanical tests showed a higher rigidity of our implants compared to leading commercially available products. The operating table as well as the intraoperative mobile CT system had no influence on the accuracy of the electromagnetic tracking system. Usual operating instruments, however, cause a reproducable translation error of up to 5 mm by interferring the magnetic field of the localization system. The accuracy tests of the navigation system and the instruments performed on a phantom showed translational errors of 0.3 to 0.6 mm and angulation errors of 0.5 grade. During first evaluation, 10 patients with various disorders of the thoracolumbar spine have been operated using the open technique and image guidance. 46 screws were implanted and there occurred no misplacemts and no complications related to the implants.

Percutaneous pedicle screw placement has been described previously. However, current systems have only limited abilities for image guidance, and the insertion of the rods is limited to one vertebral level. Further steps of developement will include a prospective clinical trial and the refining of the navigation software and the surgical instruments.