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Dept of Orthopedics
Med College, Calicut

 

 

 

 

 

 

ORIGINAL ARTICLE

Pedicle Screw Placement Using Computer Assisted Navigation

Vikram Chatrath, Vinod Agrawal, Suryaprakash Nagariya, Pritesh Vyas

Lilavati Hospital, Mumbai, India.

Address for Correspondence:  

E- Mail: vchatrath@yahoo.com

Abstract

Level of Evidence : IV Case Series

Aim: This study was done to see the accuracy of placing lumbar pedicle screw fixation done using computer assisted navigation.
Methods: Ten cases were of lumbar spine stabilization were operated using computer assisted navigation surgery using CT based navigation system. A total of 44 pedicle screws were implanted in ten patients. All cases underwent post operative CT scans and x-rays to assess any breach in the pedicle wall.
Results: We found a ninety seven percent accuracy rate in placement of pedicle screws with regards to wall breach, length and diameter of screws. Out of 44 pedicle screw insertions, 1 resulted in perforation. There were no intraoperative or postoperative complications such as occurrence of neurological symptoms, vascular injuries or wound infections. The average time for registration and screw placement (4 screws) for one motion segment was 44.6 (40-70) minutes. Our registration achieved a mean error of 0.92 (0.7-1.4) mm.
Conclusions: Computer assisted navigation helps in accurate placement of screws. It also helps in placing thicker and longer screws with increased surgical confidence. It would be very useful specially if there are associated deformities, scoliosis, revision surgeries or for thoracic spine where the pedicles are small and margin for error minimal. The average time for registration and screw placement is longer than usual because of the initial learning curve for this technique.

J.Orthopaedics 2008;5(3)e12

 
Introduction:

Pedicle screw fixation of the lumbar spine is a standard procedure. There is still concern over the accurate placement of pedicle screws 2, 7, 10, 14 with some authors describing as high as 40% incidence of misplacement. Fluroscopy guided placement of pedicle screws also increases the radiation exposure 26, 27, 29 to both the surgical team and the patient.

All pedicle screw systems share the risk of damage to adjacent neural structures as a result of improper screw placement. A computer-assisted system allowing real-time intraoperative image localization has been used as an adjunctive tool for instrumentation of lumbar spine posteriorly. This system helps in accurate placement of pedicle screws and can be implemented using either a pre-operative CT Scan or CT free-computer assisted fluroscopy guided system.

Assessment of operative placement of pedicle screws is typically performed using skiagrams in the posteroanterior (PA) and lateral projections. It is generally believed that CT imaging is more accurate than conventional radiography in determining pedicle screw location, particularly for studying medial and lateral pedicle perforation 4, 5, 19, 20, 31, 33.  The purpose of this study is to assess position of pedicle screw and pedicle perforation using CT Scans and standard radiography post-operatively in surgeries performed under computer assisted navigation.

Material and Methods :

 Level of evidence IV

Eleven patients underwent lumbar spine stabilization using CT based computer assisted navigation system (Table 1). Of the eleven patients, 1 died due to unrelated causes three months after the surgery, so ten patients were available for follow-up. The mean age of the patients was 49.5 (39-62) years. All the patients were operated by a single surgeon and were informed pre-operatively about the use of navigation system. Approval was obtained by the ethics committee of the hospital.

Computerized tomography scans of the spine were obtained pre-operatively in 1-mm slices, and the data were imported into the computer workstations - Kolibri System (BrainLab, Inc., Munich Germany). The indications for the surgery were Spondylolisthesis with lumbar canal stenosis (4), prolapsed intervertebral disc with degenerative disc (3), spinal metastasis (1) and degenerative spondylolysthesis (2).  

A total of 44 pedicle screws were implanted in ten patients. The length and diameter of the screws was noted. A reference frame was attached to the spinous process of each vertebra and virtual image of vertebra was obtained (Fig -1) and the registration was performed using the standard techniques 1, 8, 11.

The accuracy of the pedicle screw placement was evaluated postoperatively by anteroposterior and lateral radiography. All patients underwent CT scan 3 months after the surgery. All scans were obtained with a Siemens Sensation 16 Multislice CT (Siemens Medical Systems Germany), using 3 × 3 mm axial imaging, matrix 520x520, field of view to fit, mA 300, 120 kVp. Windowing was done to optimize visualization of the screws within the pedicles (window width 2300 with a center of 600).

We used the system developed by Learch et al 20 to evaluate the position of pedicle screws and any perforations caused. On the PA radiograph, the position of the tip of the pedicle screw within the vertebral body was determined by dividing the body into four equal vertical quadrants with the center at the spinous process. Quadrant 2 on the PA radiograph refers to those cases where the screws are correctly placed and Quadrant 0 refers to screw tip placed lateral to the vertebral body.  On the lateral radiographs the pedicle was divided into 3 equal horizontal zones labeled 1 through 3 from superior to inferior. All correctly placed screws showed the central portion of the shaft within zone 2 of the pedicle on both the PA and the lateral view (Fig-2).

On the CT scan, lumbar pedicles are imaged on 6 sequential sections. All correct screw placements were positioned in the midportion of the pedicle, in either the third or the fourth axial section through the pedicle (Fig -3).

 

Results : 

Of the 44 pedicle screw insertions, 1 resulted in perforation. (Table 1). Thus the accuracy in placement of the screws was 97.7 %. The perforation was seen on the left lateral pedicle wall at L4 level, but the patient was asymptomatic post-operatively. No medial or inferior walls were penetrated in any case. There were no intraoperative or postoperative complications such as occurrence of neurological symptoms, vascular injuries or wound infections.

Registration was carried out 12 times. Registration at one level would usually suffice for a level above and below, except in 2 cases where the registration had to be repeated. The average time for registration and screw placement (4 screws) for one motion segment was 44.6 (40-70) minutes. Our registration achieved a mean error of 0.92 (0.7-1.4) mm (Fig 4). The diameter of screw placement was 6 mm (42), 7 mm (2). The length of the screws was 35 mm (5), 40 mm (33) and 45 mm (6).

Discussion :

Traditionally spinal instrumentation has been based on intraoperative localization of anatomic landmarks in concert with the use of fluroscopy. However, the transverse width and angle of the pedicle, have revealed critical and significant variability 8, 13, 18, 25, 34. Consequently, faulty placement of pedicle screw may cause perforation of the cortex and impingement on adjacent neural structures 22, 23. Pedicle screw malplacement rates of between 21.1 and 39.8% have been reported in clinical studies with conventional insertion techniques and an adequate postoperative CT assessment 2, 7, 19. Infect, in a comparison of different fixation devices, the highest incidence of symptomatic impingement occurs with pedicle screws, with nerve root injury or irritation occurring in a reported 3.2% of cases 30.

Medially, the pedicle cortex is separated from the dural sac by a thin layer of epidural fat, which is typically 2 mm in thickness 28. The nerve root of the corresponding vertebra passes inferiorly to the pedicle. Because of this, the safest placement of pedicle screws is within the cephalad portion of the pedicle. If the superior cortex is violated, there is surrounding fat separating the pedicle from the exiting nerve root of the vertebra above 10. The most common error is in the placement of the screw in the saggital plane leading to either medial or lateral wall perforation 7, 8. The choice of insertion in the axial direction is variable and surgeon dependent.

Fig.4

Fig.5

For pedicle screw placement in which minor deviation in the screw trajectory may result in a pedicle perforation, a CT-based image-guidance system provides the surgeon with a view of the pedicle and surrounding structures in multiple planes (Fig 5). For each pedicle, an ideal entry point and trajectory for screw placement may be chosen. An image-guidance system is able to track and provide updated information about the location of the probe to avoid injury to the spinal cord 3, 15, 16.  Real time imaging also helps to place longer and thicker screws without the fear of violating the anterior cortex and increased surgical confidence (Fig 6).

 

Fig.6

Not all cortex violations cause neurologic injury; minor violations of the cortex are not uncommon and may be asymptomatic. However this assumes significance if there are associated deformities, scoliosis, revision surgeries or for thoracic spine where the pedicles are small and margin for error minimal 6, 17, 21, 24, 32.  

Another interesting aspect is the use of navigation in teaching. Hoelzl et al 12 have evaluated the use of navigation in training of young spine surgeons and have found it to be very useful. The three-dimensional view offered by this system provides a better orientation and also makes it safer to allow placement of screws by a trainee under supervision. This helps to eliminate risk and makes the surgical experience practice- and patient-related. We found it very useful to teach the residents of our institute in the operating theatre.

 

Conventionally radiography has been used to asses the placement of pedicle screws post-operatively. Weinstein et al 31 evaluated the accuracy of pedicle screw placement using fluoroscopic guided placement of 124 screws into 8 cadaver thoracolumbar spines. An interobserver agreement rate of 74% between radiographic assessment and dissection and visual inspection of screws was obtained. In their study, the sensitivity of the radiographic evaluation for screw perforation was only 31%, whereas the specificity was 90%. Low sensitivity of radiographs for determining screw perforation is unacceptable because the majority of failed screw placements were classified as correctly placed on the basis of conventional radiographic assessment. The system developed by Learch et al 20 to evaluate the position of pedicle screws uses both post-operative CT scans and radiography for assessment of pedicle screw position.

The vital link between achieving a zero perforation rate is registration. This process helps us to correlate the pre-operative image with the surgical anatomy. The quality of the preoperative image needs to be defined more clearly by standardizing the CT scan images and the methodology across different surgical systems. This along with variability in registration by different surgeons needs to be addressed and standardized.

The average time for registration and screw placement (4 screws) for one motion segment was 44.6 (40-70). This is longer than usual because of several factors. Firstly, the initial cases involve a learning curve for this technique. Secondly the registration process increases surgical time. However with a CT based system it is usually possible to use a single level registration for a level above and below without compromising accuracy. The unfamiliarity of the scrub nurse and other support staff with the navigation system was also observed to be a contributory factor. In a developing country the initial cost of purchasing the navigation equipment is a deterrent. Moreover, the cost of pre-operative CT scan further inflicts financial constraints.

Conclusion: 

Computer assisted surgery appears to decrease the risk of pedicle wall perforation and provides the surgeon with a virtual track for screw insertion and helps to determine the correct entry point and trajectory. With further use and acceptance of this technique it would definitely be of much aid in difficult spine surgeries.

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This is a peer reviewed paper 

Please cite as : Vikram Chatrath : Pedicle Screw Placement Using Computer Assisted Navigation

J.Orthopaedics 2008;5(3)e12

URL: http://www.jortho.org/2008/5/3/e12

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