Placement of Thoracic Pedicle Screws

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David W. Polly, Jr., Timothy R. Kuklo

Department of Orthopaedic Surgery and Rehabilitation, Walter Reed Army Medical Center, Washington, D.C., USA

Thoracic pedicle screws are experiencing a significant increase in utilization in North America, as well as in the rest of the developed world [2, 5, 20, 21, 30-34]. Consequently, there is a heightened interest in factors leading to successful use.

When placing pedicle screws in the thoracic spine, the surgeon primarily uses his or her knowledge of general thoracic pedicle anatomy along with a preoperative plan founded on sound biomechanical principles for the initial approach. Preoperative patient-specific imaging studies aid the surgeon in adapting general anatomy knowledge to patient-specific anatomy. These include a detailed study of the preoperative radiographs for pedicle position and size, in addition to possible computerized tomography (CT) and magnetic resonance (MR) imaging, which can further detail the pertinent anatomy. In concert with a thorough understanding of this anatomy, tactile feedback, however, remains the primary means of confirmation for successful screw placement.

More recent screw placement guidance tools include conventional fluoroscopy (fig. 1), two-dimensional (2-D) fluoroscopy (fig. 2), computer-aided image guidance (fig. 3), intraoperative advanced imaging (such as CT or MR), physiological monitoring guidance and emerging three-dimensional (3-D)

The opinions or assertions contained herein are the private views of the authors and are not to be construed as official or as reflecting the views of the United States Army or the Department of Defense. The authors are employees of the United States government. This work was prepared as part of their official duties and as such, there is no copyright to be transferred.

Fluoro guidance

Fig. 1. This is conventional image guidance with use of intraoperative fluoroscopy. This image demonstrates the typical 22° sagittal inclination of the thoracic pedicle when utilizing the anatomic axis.

2-D guidance

Fig. 2. This example of 2-D image guidance demonstrates two planar images allowing the surgeon to navigate simultaneously in these planes. Advantages of this include familiarity of the display (fluoroscopy) and the ability to update the images intraoperatively as needed.

3-D image guidance

3-D image guidance

Spine Screw Trajectory
Fig. 3. This example of 3-D image guidance demonstrates the utility of multiplanar navigation. It allows for simultaneous sagittal, coronal and axial planar navigation. This allows for true optimization of pedicle screw fit, fill and trajectory.

fluoroscopic guidance. Screw tract and screw placement confirmation is ultimately achieved by palpation, imaging, physiological monitoring or a combination of all of these techniques. Successful screw placement can be defined in a number of ways [2, 11]. With successful surgical outcome as the primary goal, successful screw placement may be a necessary, but not sufficient requirement.

Pedicle Anatomy

Applied thoracic pedicle anatomy has been well studied, and normative tables and graphs have been developed [7, 8, 14, 16, 22, 24, 35-38, 40, 41]. Currently, there are primarily two accepted screw trajectories - the straight-ahead trajectory (initially popularized by Roy-Camille, then Suk and Lenke), and the anatomic trajectory (utilized by Harms and others, the term initially suggested by Polly) [31].

Advantages of the straight-ahead trajectory include permitting the use of a fixed head (monoaxial) screw, as well as offering increased insertional torque and pull-out strength when compared to the anatomic trajectory [17, 19]. The major disadvantage is that this trajectory may require the screw to traverse a narrower portion of the pedicle isthmus to remain fully contained. The anatomic trajectory (directed along the sagittal pedicle axis - a 22° inclination from dorsal rostral to ventral caudal) permits the surgeon to navigate a larger portion of the pedicle isthmus and place a longer screw within the bone. It may also provide a 'toenail' effect, which typically provides the advantage of not sustaining direct in-line pull-out forces to the screw. Any potential benefit in construct performance, as opposed to individual screw performance, is unknown.

Pedicle Screw Tract Placement

A number of techniques have been used to successfully navigate the pedicle. Initial techniques utilized tactile feedback along with knowledge of the pedicle anatomy to place screws. The 'cancellous feel', familiar to surgeons experienced in lumbar pedicle screw placement, can also be used for thoracic screw placement. Since cortical bone has a distinctly different feel, a progressive cancellous resistance to the advancing pedicle probe is highly reassuring. Likewise, a sudden change in this feel is a cause for reassessment.

The neurocentral junction, however, is a predictable point where a distinct change in the 'feel' occurs. This is because the physeal growth plate is distinctly more dense than the cancellous bone of the pedicle and the vertebral body. Breeching this physeal scar has a similar feel to violating a cortical margin and requires a certain level of experience (or anatomic confirmation) to differentiate acceptable from unacceptable pedicle tract navigation.

Surgical Pedicle Navigation Techniques

Proposed techniques for pedicle navigation include the 'freehand' technique as well described by Lenke et al. [20]. Similarly, drilling has been utilized by many surgeons while the use of a small diameter drill followed by placement of k-wires with conventional radiographic confirmation has been popularized by Suk et al. [32]. The 'funnel technique' of breeching the dorsal cortex with a burr and then navigating the pedicle with a small size curette (such as a 2-0 Cloward curette) has been popularized by Gaines [10], and espoused by others for cervical pedicle navigation (find the hole, do not make the hole). Nonetheless, each of these techniques requires tactile feedback as the primary means of confirmation.

This tactile feedback can be supplemented with a number of adjunctive technologies [1]. Conventional fluoroscopy has been widely used, with a high level of accuracy [2]. It has the advantage of being a real time evaluation in a familiar format. Disadvantages include the presence of the fluoroscopic unit at the operative field, the conventional difficulties associated with planar radiography (visualizing the upper thoracic spine, penetrating large body mass patients, surgeon impairment from lead gowns) and exposure to both the patient and the operative team. A skilled technologist is also required.

2-D image guidance (such as FluoroNav, Medtronic Sofamor Danek, Memphis, Tenn., USA) acquires intraoperative images through a tracking arc attached to the patient and displays the images on a computer screen to assist the surgeon to navigate the pedicle [27, 28] (fig. 2). The major advantage of this system is that it permits the surgeon to navigate in multiple conventional fluo-roscopic planes simultaneously. This concept, the use of conventional imaging linked in multiple planes, is a comfortable concept for many surgeons due to the familiarity of the radiographic images. The disadvantages include the requirement for a sophisticated unit that requires an additional user familiar with the technical operation of the system.

3-D image guidance, or frameless stereotactic image guidance, utilizes preoperative axial imaging studies (most typically CT) with intraoperative registration to correlate segmental anatomy and display multiplanar images for navigation [4] (fig. 3). The most significant advantage of this technique is the display of axial images during the navigation process. This is perhaps the most critical anatomic information required by the surgeon during pedicle navigation. Disadvantages include the need for the sophisticated unit (and subsequent cost), obtaining the preoperative axial imaging studies (and additional cost), transferring the information to the unit and then obtaining appropriate intraoperative segmental registration of acceptable precision. There is a significant time investment necessary in the early use of the technology.

Physiological intraoperative guidance is based on stimulating a pedicle probe during screw tract navigation, thus looking for a neural response from either a nerve root or the spinal cord. This technique has been extensively researched in the lumbar spine where the nerve root level myotomes provide good discrimination. In the thoracic spine, this has proven to be more difficult. Monitoring from the rectus abdominis appears to give reasonable evoked potentials for T6-T12 [29]. To date, this technique has proven to be less reliable rostral to T6. Newer technologies may allow real time monitoring of nerve root proximity, as well as directional information.

Future technological developments may include sophisticated haptic feedback that can discriminate between bone, nerve and blood vessels. Ideally, this technology should permit differentiation between cortical and cancellous bone, thus providing an excellent adjunct in pedicle navigation. The merging of multiple modalities could further enhance the margin of safety provided for navigating these narrow access corridors.

Evaluation of the Pedicle Tract

Classically, evaluation of the pedicle tract is done by direct bony palpation. This includes 'five-wall' confirmation, or palpation of the medial, lateral, superior, inferior and anterior walls. However, the predictive value of this maneuver varies between surgeons, and is not foolproof [18]. If a breech is detected, there is a high positive predictive value; however, if no breech is detected, the negative predictive value is not as good. An 'experience' factor is also present and appears to play a significant role.

Confirmation of a bony pedicle breach requires further analysis by the surgeon to determine whether or not a screw can still be placed safely. One technique, described by Dvorak et al. [6], places a pedicle screw along a trajectory lateral to the pedicle, but within the confines of the pedicle-rib complex, and ultimately into the vertebral body. This has subsequently been termed the 'in-out-in' technique or the 'pedicle-rib' technique. O'Brien et al. [23] have demonstrated the anatomy of this corridor. So simply having a small lateral cortical breach is of no particular clinical consequence. It also appears that a small medial breach can be tolerated [2, 11]. Further, Polly et al. [26] analyzed volumetric spinal canal intrusion from medially positioned screws in a computer model and found that a screw must have over a 2-mm medial breach to have the same volumetric spinal canal intrusion as a perfectly positioned pediatric laminar hook. However, in spinal deformity, a medial breach on the concave apex would not be tolerated, and any change in neurological monitoring is always a cause for significant concern [25].

Following insertion, screw purchase is routinely evaluated. If the screw is loose (poor purchase), then it is not acceptably placed, whereas good purchase does not necessarily confirm adequate placement. Again, screw removal and direct palpation may assist the surgeon in identifying a breach. Similarly, elec-trophysiological monitoring can suggest a medial cortical violation, especially from T6-T12, but it is less reliable at detecting lateral or anterior breaches. Intraoperative imaging (fluoro or plain film) can also be used to assess placement, yet interpretation of the images can be challenging. Several checks have been found to be helpful. First, with multiple level fusions, one can appreciate the progressive orientation of the screws on the PA view, and a break in the progression will alert the surgeon to a potentially misplaced screw. Further, the pedicle can usually be identified, and the screw should appear to be tracking

Case example

T11 T12

Fig. 4. a-l This is an example of a right thoracic left lumbar progressive scoliotic deformity in a skeletally immature individual (curve pattern Lenke 3CN, King curve type double major). Because of the curve magnitude (>50°), and her immaturity, she met operative indications. The lumbar modifier (Lenke C) and the thoracolumbar junctional kyphosis mandated fusion of both curves. The preoperative coronal and sagittal radiographs demonstrate the deformity, the postoperative radiographs demonstrate the excellent correction in the coronal and sagittal planes while preserving the motion of the intervertebral disks from L3 to the sacrum. The sequential CT images demonstrate the axial views of the screw position at each level T5 through L3. The physeal scar (or neurocentral growth plate) is well visualized on the right at T10 and T11.

Fig. 4. a-l This is an example of a right thoracic left lumbar progressive scoliotic deformity in a skeletally immature individual (curve pattern Lenke 3CN, King curve type double major). Because of the curve magnitude (>50°), and her immaturity, she met operative indications. The lumbar modifier (Lenke C) and the thoracolumbar junctional kyphosis mandated fusion of both curves. The preoperative coronal and sagittal radiographs demonstrate the deformity, the postoperative radiographs demonstrate the excellent correction in the coronal and sagittal planes while preserving the motion of the intervertebral disks from L3 to the sacrum. The sequential CT images demonstrate the axial views of the screw position at each level T5 through L3. The physeal scar (or neurocentral growth plate) is well visualized on the right at T10 and T11.

through the pedicle projection. Multiple views, especially in spinal deformity, can be of further help. To date, the accuracy of planar radiography to determine pedicular screw placement in the thoracic spine has not been validated. Currently, CT scanning is considered the gold standard, although it is more realistically a worst case analysis since there is some magnification artifact even with titanium implants [3, 9, 12, 13, 39]. Intraoperative CT scanners have been utilized at some institutions. Obviously, this requires a significant infrastructure and operating room modifications. For particularly high-risk screw placement, one strategy might involve a planned intraoperative transport to a scanner within the institution. Again, this requires significant coordination and increased operative time. We have found that the routine use of postoperative CT scans in patients who have undergone placement of thoracic pedicle screws has added an extra margin of safety and an invaluable educational opportunity for the operating surgeons to critically assess their technique (fig. 4a-l).

Successful Screw Placement

Successful screw placement can be defined in a number of ways. The most obvious criteria would be the presence or absence of neurological or visceral compromise as a result of the screw placement [15]. By all definitions, presence of these conditions would be considered an unsuccessful screw placement. The second criterion may be screw purchase. In other words, does the screw have enough purchase to hold against the applied loads? However, even if placed with anatomic precision, placement can still be unsuccessful if there is inadequate bone stock. Third, did the screws successfully achieve the goal of instrumentation, i.e. was the deformity corrected or was the spine stabilized sufficiently to achieve the long-term biological solution of fusion?


Individual decisions and techniques, including the use of neuronaviga-tional aids, are premised to successfully and safely achieve the goals of surgical treatment. Placement of thoracic pedicle screws is only one component of the surgical process. However, for the surgery to be a success, the screws must be placed safely and effectively. This can be achieved through a variety of techniques using one of several accepted strategies. The surgeon must have enough resources available to accomplish the task. Given the variety of experience, skill and patient-specific anatomy, different resources will be required.


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Department of Orthopaedic Surgery and Rehabilitation

Walter Reed Army Medical Center, Washington, DC 20307 (USA)

Tel. +1 202 782 5851, Fax +1 202 782 4365, 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 107-127

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