Cervical pedicle screws: conventional versus computer-assisted placement of cannulated screws

A comparison of optical and electromagnetic computer-assisted navigation systems for fluoroscopic targeting

OBJECTIVES

Freehand targeting using fluoroscopic guidance is routine for placement of interlocking screws associated with intramedullary nailing and for insertion of screws for reconstruction of pelvic and acetabular injuries. New technologies that use fluoroscopy with the assistance of computer guidance have the potential to improve accuracy and reduce radiation exposure to patient and surgeon. We sought to compare 2 fluoroscopic navigation tracking technologies, optical and electromagnetic versus standard freehand fluoroscopic targeting in a standardized model.

INTERVENTION

Three experienced orthopaedic trauma surgeons placed 3.2-mm guide pins through test foam blocks that simulate cancellous bone. The entry site for each pin was within a circular (18-mm) entry zone. On the opposite surface of the test block (130-mm across), the target was a 1-mm-diameter radioopaque spherical ball marker. Each surgeon placed 10 pins using freehand targeting (control group) navigation using Medtronic iON StealthStation (Optical A), navigation using BrainLAB VectorVision (Optical B), or navigation using GE Medical Systems InstaTrak 3500 system (EM).

OUTCOME MEASUREMENTS

Data were collected for accuracy (the distance from the exit site of the guidewire to the target spherical ball marker), fluoroscopy time (seconds), and total number of individual fluoroscopy images taken.

RESULTS

The 2 optical systems and the electromagnetic system provided significantly improved accuracy compared to freehand technique. The average distance from the target was significantly (3.5 times) greater for controls (7.1 mm) than for each of the navigated systems (Optical A = 2.1 mm, Optical B = 1.9 mm EM = 2.4 mm; P < .05). Accuracy was similar for the 3 navigated systems, (P > 0.05). The ability to place guidewires in a 5-mm safe zone surrounding the target sphere was also significantly improved with the optical systems and the EM system (99% of wires in the safe zone) compared to controls (47% in the safe zone) (P < 0.002). Safe zone placement was similar among the 3 navigated systems (P > 0.05). Fluoroscopy time (seconds) and number of fluoroscopy images were similar among the three navigated groups (P > 0.05). Each of these parameters was significantly less when using the computer-guided systems than for freehand-unguided insertion (P < 0.01).

CONCLUSIONS

Both optical and electromagnetic computer-assisted guidance systems have the potential to improve accuracy and reduce radiation use for freehand fluoroscopic targeting in orthopaedic surgery.

Quantitative evaluation of the location of the vertebral artery in relation to the transverse foramen in the lower cervical spine

STUDY DESIGN

This study assessed the relationship of the vertebral artery and the transverse foramens in the lower cervical spine by computer tomographic angiography (CTA) measurements.

OBJECTIVE

To study the location and the size of the vertebral artery in relation to the transverse foramens in the lower cervical spine.

SUMMARY OF BACKGROUND DATA

Although many transverse foramen violations had been reported during pedicle screw insertion, the vertebral artery injury seems not as much as expected. The location and size of the vertebral artery in relation to the transverse foramen in the lower cervical spine is not very clear until now.

METHODS

Fifty patients were selected in our study group. Several parameters were measured on the CTA axial imagines of C3-C7, including the maximal width (W) and the maximal height (H) of transverse foramen, the diameter of vertebral artery (AL), the shortest distance between the vertebral artery and the border of transverse foramen medial (M), lateral (L), anterior (A) and posterior (P), the shortest distance between the vertebral artery and pedicle (h), the area of vertebral artery (AVA) and transverse foramen (AF), and the ratio between AVA and AF (AVA/AF).

RESULTS

W, H, and AF increase from C3-C6. M is shorter than L at all measurements. A is shorter than P at C3, C4, and C5, a little bit longer than P at C6 on the contrary. The shortest distance between vertebral artery and pedicle (h) vary from C3-C7, the shortest 0.5 +/- 0.2 mm at C3, the longest 7.3 +/- 2.7 mm at C7. AVA/AF decrease from C3-C6, 35.7 +/- 15.7% at C3 and 27.4 +/- 12.8% at C6.

CONCLUSION

Vertebral artery is in the medial part of transverse foramen. There is a relative "abundance space" for vertebral artery in transverse foramen.

A CT-free, intra-operative planning and navigation system for minimally invasive anterior spinal surgery - an accuracy study

OBJECTIVE

A comprehensive study was performed to evaluate the accuracy of a newly developed CT-free, intra-operative planning and navigation system for anterior spine surgery.

MATERIALS AND METHODS

Instruments and an image intensifier were tracked using the SurgiGATE navigation system. A laboratory study was performed on 27 plastic vertebrae. Fiducial markers were implanted in the vertebrae for accuracy evaluation purposes, and a dynamic reference base was placed on the vertebrae to establish a patient coordinate system (P-COS). Two fluoroscopic images were used for intra-operative planning. The graft bed plan was recorded in P-COS, followed by surgical formation of the graft bed, which was visualized. To evaluate the accuracy, the vertebrae were scanned with CT, and the markers were used to calculate an accurate paired-point registered transformation between the CT coordinate system and P-COS.

RESULTS

Using the new SPO module, accurate planning and navigation of a resection of the vertebral body is possible using two fluoroscopic images. The overall mean error between the planned resection volume and the actual resection was 0.98 mm. In addition, the module can serve as an educational tool for training spine surgeons.

CONCLUSIONS

The new fluoroscopy-based system can be used safely for accurate performance of anterior resection during spondylodesis. New methods for safe and accurate registration during anterior spine surgery need to be developed.

Pedicle screw placement accuracy: a meta-analysis

STUDY DESIGN

A meta-analysis of the published literature was conducted specifically looking at accuracy and the postoperative methods used for the assessment of pedicle screw placement in the human spine.

OBJECTIVES

This study specifically aimed to identify postoperative methods used for pedicle screw placement assessment, including the most common method, and to report cumulative pedicle screw placement study statistics from synthesis of the published literature.

SUMMARY OF BACKGROUND DATA

Safety concerns have driven specific interests in the accuracy and precision of pedicle screw placement. A large variation in reported accuracy may exist partly due to the lack of a standardized evaluation method and/or the lack of consensus to what, or in which range, is pedicle screw placement accuracy considered satisfactory.

METHODS

A MEDLINE search was executed covering the span from 1966 until 2006, and references from identified papers were reviewed. An extensive database was constructed for synthesis of the identified studies. Subgroups and descriptive statistics were determined based on the type of population, in vivo or cadaveric, and separated based on whether the assistance of navigation was employed.

RESULTS

In total, we report on 130 studies resulting in 37,337 total pedicle screws implanted, of which 34,107 (91.3%) were identified as accurately placed for the combined in vivo and cadaveric populations. The most common assessment method identified pedicle screw violations simply as either present or absent. Overall, the median placement accuracy for the in vivo assisted navigation subgroup (95.2%) was higher than that of the subgroup without the use of navigation (90.3%).

CONCLUSIONS

Navigation does indeed provide a higher accuracy in the placement of pedicle screws for most of the subgroups presented. However, an exception is found at the thoracic levels for both the in vivo and cadaveric populations, where no advantage in the use of navigation was found.

Efficacy of computer-assisted pedicle screw insertion for cervical instability in RA patients

We evaluated the efficacy of the computer-assisted cervical pedicle screw insertion, compared with those inserted without the help of the system on the cervix of rheumatoid arthritis (RA) patients. Eighty-six cervical pedicle screws were inserted without the help of the system. Of the 86, 59 screws were in non-RA patients with degenerative spine, and 27 were in RA patients. The accuracy of the screw insertions was evaluated by a CT-based method. Then, 25 screws were inserted with the system into the cervical spines between C2 and C6 in RA patients (Navigation group). The efficacy of the system was assessed by the CT-based method, compared with 27 screws inserted without the system (Conventional group). The screws in RA patients tended to be more deviated than those in non-RA patients. In Conventional group, four screws (15%) were placed far laterally, and two (7%), far medially. In contrast, no screw was placed far laterally or medially in Navigation group (P < 0.05).

Real-time tracking of vertebral body movement with implantable reference microsensors

OBJECTIVE

In the spine, navigation techniques serve mainly to control and accurately target insertion of implants. The main source of error is that the spine is not a rigid organ, but rather a chain of semiflexible movement segments. Any intraoperative manipulation of the patient alters the geometry and volumetry as compared to the 3D volume model created from the image data. Thus, the objective of the study was to implement the theoretical principle of microsensor referencing in a model experiment and to clarify which anatomical structures are suitable for intermittent implantation of positional sensors, as illustrated with cervical vertebral bodies.

MATERIALS AND METHODS

Laboratory tests were conducted using 70 models of human cervical vertebral bodies. The first experiment investigated whether arbitrary movements of vertebral bodies can be tracked with the positional information from the implanted microsensors. The accuracy of this movement monitoring was determined quantitatively on the basis of positional error measurement. In the second experiment, different ventral and dorsal surgical operations were simulated on five models of the cervical spine. Quantifiable measurement values such as the spatial extension of the intervertebral space and the relative positions of the planes of the upper plates were determined.

RESULTS

With respect to the differing anatomy of the individual vertebral bodies of the cervical spine, the sensors could be placed securely with a 5x2 mm drill. The registration error (RE) was determined as a root mean square error. The mean value was 0.9425 mm (range: 0.57-1.2 mm; median: 0.9400 mm; SD: 0.1903 mm). The precision of the movement monitoring of the vertebral body was investigated along its three main axes. The error tolerance between post-interventional 3D reconstruction and direct measurement on the model did not exceed 1.3 mm in the distance measurements or 2.5 degrees in the angular measurements. The tomograms on the system monitor could be updated in close to real time on the basis of the positional information from the reference sensor.

CONCLUSIONS

Motion sensors implanted into the vertebral bodies communicated any change in position to the navigation system in close to real time, thus enabling the preoperative image data set to be updated. The experiments described could ultimately show that continuous real-time visualization of individual vertebral body movements along the movement axes (flexion-extension, tilting and rotation) is possible with high accuracy using implantable microsensors. A future application of such microsensors might be the integration of robot systems into spinal microsurgery.