Accuracy of Fluoroscopically-Assisted Laser Targeting of the Cadaveric Thoracic and Lumbar Spine to Place Transpedicular Vertebral Screws

Purpose:

A simplified method of targeting a coaxial view of the vertebral pedicle that reduces x-ray exposure to the patient and operating room staff was developed. We investigated the accuracy of this device to place vertebral screws through the thoracic and lumbar pedicles.

Materials and Methods:

The dual radiation targeting system (DRTS™), a laser targeting system for fluoroscopically guided procedures, was developed to provide accurate surface point of entry and angle of approach to radiographic landmarks. The system identifies a path of x-ray radiation from the x-ray source to the image intensifier, positions a laser beam on that line and provides a target symbol visible on the TV monitor. After fluoroscopic cross hair target localization of the coaxial view of the pedicle, x-ray radiation is turned off, and the laser beam guides the instruments and screw through the center of the pedicle.

Nine cadaver thoracal-lumbar spines were removed and mounted on a support that allowed three plane rotation and positioning of the specimen. Three surgeons inexperienced in the technique of pedicle screw placement targeted and instrumented 184 pedicles between L5 and T5. A total of 83 lumbar pedicles and 101 thoracic pedicles underwent 5mm screw placement. The specimens were then divided in half in the midline sagittal plane and all soft tissue was removed for inspection of each pedicle and vertebral body. The amount and location of any screw penetration was measured.

Results:

There were three screws that penetrated the pedicle, an error rate of 1.6%. No screw was more than 1mm outside the pedicle. These involved a screw at L4 that was 1mm through the lateral pedicle cortex, a screw at L5 that was 1 mm though the medial cortex, and a small medial cortical crack at T9 using a 5mm screw through a pedicle that measured 4mm. Five other screws, four in the lumbar spine and one in the thoracic spine, (error rate 2.7%) were directed too far laterally and penetrated the lateral vertebral body. The total rate of poorly directed or penetrated screws was 4.6% (2% for the thoracic spine and 7% for the lumbar spine).

Discussion:

Our pedicle penetration rate of 1.6% by inexperienced surgeons compares very favorably to much higher pedicle penetration rates of up to 41% by experienced surgeons when no imaging was utilized (Vaccaro 1995). Adverse neurologic effects would not be expected with this small amount of penetration. We noticed a tendency towards excessive lateral angulation of the screws, which emphasizes the importance of understanding the pedicle anatomy and obtaining pre-operative CT imaging. These encouraging results will need to be confirmed in clinical studies of patients.

Conclusion:

We believe that the DRTS™ system can assist both the experienced and inexperienced surgeon achieve safe and accurate trans-pedicular vertebral screw placement with minimum radiation exposure. The device appears to be particularly safe and accurate for the placement of thoracic pedicle screws.

Vaccaro AR, Rizzolo SJ, Balderston RA, Allardyce TJ, Garfin SR, Dolinskas C, An HS. Placement of pedicle screws in the thoracic spine. Part II. An anatomical and radiographic assessment. J Bone Joint Surg 1995; 77A: 1200-1206.

(This abstract was submitted to the 33rd annual meeting of the Scoliosis Research Society, Sept 16-20, 1998.)