in 1991 were among the first to understand the three-dimensional anatomy of the cervical pedicle and to propose that the cervical pedicle could tolerate pedicle screw placement. In their cadaveric study of 12 cervical spines, Panjabi et al. systematically demonstrated that both the width and height of the cervical pedicle was the greatest at C2 (C2 width and height were on the average 7.7 and 9.4 mm, respectively) (table 1). In addition, they demonstrated that the cross-sectional area of the C2 pedicle was the greatest of all the cervical pedicles. From C3-C7, the pedicle angles to the transverse plane ranged from an average of 9.2° below to 13.4° above the transverse plane. In the sagittal plane, there was a decrement of the pedicle angle from an average of 41.6° at C3 to 33.1° at C7.
Other cadaveric studies have also demonstrated the feasibility of placing screws within the cervical pedicle. An et al.  in 1991 utilized a cadaveric study to investigate the pedicle anatomy from C7-T2. In terms of C7 pedicle anatomy, they demonstrated that the medial angulation averaged 34° at C7, while the mediolateral and superoinferior outer pedicle diameters were on average 6.9 and 7.5 mm, respectively. The pedicle distances (from the entry point to the posterior vertebral body line) measured 9.1 mm on average. An et al. recommended that for pedicle screw placement, the entry point should be 1 mm inferior to the midpoint of the facet with a 25-30° medial angle.
Shin et al.  further defined the cervical pedicle anatomy by addressing cross-sectional variability of the cervical pedicles. They demonstrated that the medial pedicle walls are consistently thicker than the lateral pedicle walls and that there was a substantial variability in the composition and shape of the cervical pedicle cross section.
Both Xu et al.  and Ugur et al.  expanded on prior anatomical studies by evaluating the relationship between the cervical pedicles and the adjacent neural structures. These studies showed that there was no gap between the pedicle and the superior portion of the nerve root and between the pedicle and the thecal sac from C3 to C7. The average distances between the pedicle and the
inferior nerve root margins ranged from 1.4 to 1.6 mm. Consequently, both studies concluded that the risk of neurological injuries may be higher in screw penetration of the medial or superior cortex of the pedicle rather than in screw penetration of the inferior cortex of the pedicle (fig. 1).
Ludwig et al.  compared the accuracy of three different techniques for placing pedicle screws in cadaveric specimens. With screws placed based on morphometric data alone, 12.5% of the screws were placed entirely within the pedicle; 21.9% had a noncritical breach, and 65% had a critical breach ('critical': encroachment of vertebral artery, nerve root or spinal cord by the screw, 'noncritical': violation of the pedicle cortex without injury to surrounding vital structure). In the second technique (laminoforaminotomy), 45% of the screws were within the pedicle; 15.4% had a noncritical breach, and 39.6% had a critical breach. In the third technique (computer-assisted surgical guidance system), 76% of the screws were placed entirely within the pedicle; 13.4% had a noncritical breach, and 10.6% had a critical breach.
Once the feasibility of screw placement within the cervical pedicle was demonstrated, biomechanical studies were required to justify the use of cervical pedicle screw fixation in light of the technically challenging nature of pedicle screw placement. Kotani et al.  in 1994 compared the biomechanical stability of seven different cervical fixation methods, including transpedicular screw fixation. They demonstrated that the three-column fixation of the cervical spine using cervical pedicle screws offered increased stability over other posterior cervical fixation systems. Even when both the anterior and middle columns were compromised, the stability provided by cervical pedicle fixation was similar to combined anterior plate and posterior triple wiring in one-level fixation.
Additionally, Jones et al.  in 1997 demonstrated in cadaveric specimens that cervical pedicle screws had a significantly higher pullout strength than lateral mass screws. The load failure mean for cervical pedicle screws was 677N in contrast to 355 N for lateral mass screws.
Kowalski et al.  in 2000 further evaluated the pullout strengths of pedicle screws. They compared the 'standard' method of pedicle screw placement (decortication of the lateral mass and passage of a hand drill prior to tapping) to the Abumi insertion method (decortication of the entire lateral mass, which provides a direct view of the pedicle introitus). There was no significant difference in the mean pullout resistance between the Abumi (696 N) and standard (636.5 N) insertion techniques (p = 0.41).
These studies demonstrated that cervical pedicle screw constructs are biomechanically stronger than lateral mass screw or wire fixation systems.
In 1989 Roy-Camille  described the technique and the indication for the placement of a transpars screw at C2 for Hangman's fractures. For C2, he recommended drilling approximately 15° in the medial direction and 35° in the superior direction.
Abumi et al.  in 1994 was the first to report placement of pedicle screws in the subaxial spine. Thirteen patients with fractures/dislocation of the middle and lower cervical spine underwent transpedicular screw fixation. The angle of the cervical pedicle screws of Abumi et al. ranged from 25 to 45° medial to the midline in the transverse plane. All patients had solid fusion without loss of correction at an average of 22 months' follow-up. Despite three cortical breaches of the 52 screws that were placed, no neurological or vascular complications were observed. This study demonstrated that safe and successful placement of cervical pedicle screws was possible.
Abumi and Kaneda  further utilized pedicle screw fixation for nontrau-matic lesions of the cervical spine. They analyzed the clinical results in 45 patients and demonstrated that the solid fusion was obtained in all patients except 8 patients who did not receive bone graft. There was one case of transient radiculopathy.
Abumi et al.  in 2000 followed this study with another large retrospective study analyzing the complications associated with pedicle screw fixation of the cervical spine. Seven hundred twelve screws were inserted into the cervical pedicles, and the locations of 669 screws were radiologi-cally evaluated in 180 consecutive patients. Forty-five screws (6.7%) were found to penetrate the pedicle, and 2 of the 45 screws caused a postoperative radiculopathy. Abumi et al. concluded that the incidence of clinically significant complications caused by cervical pedicle screw insertion was extremely low.
Since then other studies have reported successful placement of pedicle screws in the cervical spine. Albert et al.  demonstrated successful use of C7 pedicle screws in 21 patients. Pedicle screws were placed after direct palpation of the pedicle with a right angle nerve hook after laminoforamino-tomy at C7. There were no neurological complications related to pedicle screw placement and no failures of fixation or complications at 1-year follow-up.
More recently, Harms and Melcher  in 2001, Mummaneni et al.  in 2002, and Fiore et al.  in 2002 demonstrated a novel technique of atlantoax-ial stabilization using lateral mass fixation at C1 and C2 pars screw fixation with minipolyaxial screws and rods. No neural or vascular damage related to this technique was observed in these studies. The early clinical and radiologic follow-up data indicated solid fusion in all patients.
For posterior cervical pedicle screw (and lateral mass screw) fixation, we prefer to use a polyaxial screw-rod system (VERTEX, Medtronic Sofamor Danek, Memphis, Tenn., USA). This system is more versatile than standard lateral mass plating systems and allows for more varied screw entry points and screw angles because the screw placement is not dependent on the predetermined plate entry holes.
We recommend a screw entry point 3 mm superior and 3 mm lateral to the C2/3 facet joint. We drill approximately 15° in the medial direction and 35° in the superior direction with direct visualization and palpation of the medial and superior aspect of the C2 pars with a Penfield 4 to decrease the chance of cortex violation (fig. 2-4). We use a handheld drill to create the screw pathway. We then tap and place a polyaxial VERTEX screw.
C2 Pedicle screw
C2 Pedicle screw
Fig. 2. A C2 pars screw (A) has a higher risk of vertebral artery injury than a C2 pedicle screw (B) because the vertebral artery runs occasionally through the inferior portion of the pars of C2. SAP = Superior articulating process; IAP = inferior articulating process.
Fig. 3. The trajectory of a C2 pedicle screw (A) and the trajectory of a C2 pars screw (B) are shown.
Fig. 4. Lateral cervical x-ray shows C1 lateral mass screws and C2 pars screws.
The entry point for C2 pedicle screws is 1-2 mm superior and 1-2 mm more lateral than that of the C2 pars screw. We expose and palpate the medial portion of the C2 pars to guide our medial trajectory, which is approximately 25°
(this is more medial angulation than the pars screw as the entry point is more lateral). We also angle approximately 25° in the superior direction. We use fluoroscopy or image guidance to help with screw trajectory.
Since the vertebral artery occasionally runs within the inferior pars of C2, the entry point of the C2 pedicle screw is safer than the entry point of the C2 pars screw because the C2 pedicle screw entry point is more superior than the entry point of the C2 pars screw.
Violation of the medial pedicle wall is unlikely with a C2 pedicle screw because the bone here is cortical and quite strong. We prefer to use the tap from the VERTEX set to enter and create a screw pathway because the tap is less likely to create a cortical wall breach than is the drill.
The entry point of C3-C6 pedicle screws is slightly lateral to the center of the facet and close to the posterior margin of the superior articular surface. After decorticating the lateral mass, the pedicle can then be probed to validate screw trajectory. The tap from the VERTEX (Medtronic Sofamor Danek) instrumentation set is particularly good for this maneuver because it is delicate with fine cutting edges and has a tendency to be 'sucked down the pedicle' (fig. 5-8).
Laminotomies are not routinely performed to identify the medial aspect of the C3-C6 pedicle unless prior facetectomy or laminotomy has been performed for decompressive purposes. Image guidance with Stealth is helpful for appropriate screw placement. Based on measurements from preoperative CT images, the intended angle of the pedicle screw (usually 30-40° medial to the midline in the transverse plane) can be determined, and this is confirmed intraopera-tively with image guidance.
For C7 and T1 pedicle screw placement, we prefer to create lamino-foraminotomies at C6-C7 and at C7-T1, respectively. This window allows for direct palpation of the medial, superior, and inferior walls of the pedicle with a right angle nerve hook (fig. 8).
We use a 2-mm burr to establish an entry point that is based on the direct palpation of the medial wall of the pedicle. We then use the tap from the VERTEX instrumentation system to cut and tap the pedicle. The VERTEX tap has a tendency to be 'sucked down' the pedicle with minimal downward force.
If the pedicle is sclerotic, then a drill with an automatic stop at 18 mm can be used instead. It is especially important to sound the pedicle through the laminoforaminotomy when using the drill as perforations are more likely with this instrument than with the VERTEX tap.
If a cortical perforation is made at C7 or T1, the safest place to perforate the pedicle is directly lateral to it since the vertebral artery is not present at C7 or T1. The important structures are the thecal sac medially and the nerve roots superior and inferior to the pedicle. The lateral cortex of the pedicle is a relatively 'safe zone'. In fact, at T1, a lateral cortical penetration may be performed and a longer screw used to fixate the end of the screw into the costotransverse junction, which can increase the pullout strength of the screw .
Safe placement of cervical pedicle screws requires knowledge of the three-dimensional anatomical structure of the pedicles and entry points, and the pedicles' relationship to neural and vascular structures. Given the wide variability of cervical pedicle dimensions and questionable reliability of topographical surface anatomy, greater reliance on visualization/palpation of pedicle and/or image guidance is preferred.
Ludwig et al.  in 2000 investigated comparative accuracy of three different techniques of pedicle screw placement in cadaveric specimens. They compared screw placement using morphometric data versus laminoforamino-tomy versus image guidance. They showed that the computer-assisted, image-guided placement had the lowest error rate.
It is our preference to use image guidance for pedicle screw placement from C3-C6. For C2, C7, and T1 it has been our experience that anatomic landmarks and medial pedicle wall palpation with a Penfield 4 are adequate for accurate screw placement.
The purpose of posterior cervical fixation is to provide adequate fixation to resist deforming forces until solid bony fusion occurs. To this end, lateral mass plating has been widely utilized. The procedure has been shown to be relatively easy and complications are minimal. Unfortunately, there are circumstances, such as when lateral masses are eroded or not available, that require an alternative fixation technique. Under these circumstances, cervical pedicle screws may be a viable alternative for fixation.
The biomechanical advantages of cervical pedicle screws have been demonstrated. Nevertheless, current data seems to indicate that unless one is intimately familiar with cervical pedicle anatomy and is well versed in cervical pedicle screw placement, this fixation option should be performed sparingly.
Use of image guidance for accurate cervical pedicle screw placement seems promising, and we have successfully utilized the Stealth system for this purpose.
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Jay Y. Chun, MD, PhD UCSF, Department of Neurosurgery 505 Parnassus Ave., M-779, Box 0112 San Francisco, CA 94143 (USA)
Tel. +1 415 753 1772, E-Mail [email protected]
Haid RW Jr, Subach BR, Rodts GE Jr (eds): Advances in Spinal Stabilization. Prog Neurol Surg. Basel, Karger, 2003, vol 16, pp 165-175
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