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JayBachatero · Apr 03, 2008, 07:55 PM

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<h3>Outcome-Based Classification for Assessment of Thoracic Pedicular Screw Placement</h3>

Upendra, Bidre N. MS; Meena, Devkant MS; Chowdhury, Buddhadev MS; Ahmad, Abrar MS; Jayaswal, Arvind MS
From the Department of Orthopaedics, All India Institute of Medical Sciences, New Delhi, India.
Acknowledgment date: January 3, 2007. Revision date: July 1, 2007. Acceptance date: August 20, 2007.

The legal regulatory status of the device(s)/drug(s) that is/are the subject of this manuscript is not applicable in our country.
No funds were received in support of this work. No benefits in any form have been or will be received from a commercial party related directly or indirectly to the subject of this manuscript.
Address correspondence and reprint requests to Bidre Upendra, MS, Department of Orthopaedics, All India Institute of Medical Sciences, Ansari Nagar, New Delhi 110029, India; E-mail: upibn@yahoo.com

<h3>Abstract and Introduction</h3>

<h4>Abstract</h4>

 Study Design: Prospective cohort study.

 Objective: We propose a simple outcome-based classification for assessment of pedicle screw positions based on postoperative computed tomography scan. This bridges the gap between high rates of pedicle screw misplacement and minimal complications reported.

 Summary of Background Data: The main deterrent for the use of thoracic pedicular screws is the feared neurovascular complications due to screw misplacements. The literature shows that only a small fraction of the misplaced screws actually causes any complication, and some misplacements can be acceptable both in terms of safety and their biomechanical strength.

 Methods: Sixty patients with various spinal disorders were included in the study. The mean age was 29.6 years (range, 12-72 years). The patients were divided into 2 groups for assessment of pedicle screw placements using postoperative computed tomography scans: scoliosis group with 24 patients and the nonscoliosis group with 34 patients. Placements of screws were assessed using the outcome-based classification and the Rongming Xu criteria of screw placement.

 Results: A total of 341 screws were assessed from 60 patients with various spinal disorders (scoliosis and nonscoliosis groups). Using the Rongming Xu criteria, the overall screw misplacement in scoliosis group was 50.72% (68 of 138) and that in nonscoliosis group was 45.45% (80 of 176 screws). Assessment of these screws using the outcome-based classification showed a high percentage of acceptable screw placements (type 1) - 89.85% (124 of 138 screws) in the scoliosis group and 86.93% (153 of 176 screws) in the nonscoliosis group.

 Conclusion: The literature shows consensus over high rates of pedicle screw misplacement, but low clinical complications, in the hands of the best of spine surgeons. The concept of acceptable screw placements and the outcome classification makes the pedicle screw assessment results correlate better with the clinical outcome.



<h4>Introduction</h4>
Pedicular screw rod systems are the most widely used spinal stabilization systems worldwide today. The superior biomechanical strength of pedicular screws has yielded better fusion rates and has been shown to give better corrections in complex deformities.[1-3] Although used widely in various spinal disorders, pedicle screws are used with caution in the thoracic spine. The main deterrent for the use of thoracic pedicular screws is the feared neurovascular complications due to screw misplacements.[3-7] Screw misplacements in the thoracic spine range from 20% to 30% in the hands of the best of spine surgeons.[4,6,8,9] Pedicle fractures, screw breakage, and loosening range from 0.5% to 1% in most of the studies.[4,6,8] The neurovascular complications reported due to misplaced screws are between 0% and 1%.[4,10] Therefore, only a small fraction of the misplaced screws actually cause any complication. This makes one wonder if all misplacements of the pedicle screw are troublesome. Looking into further details, studies show that all neurologic deficits reported have medial pedicle perforation of more than 4 mm,[4,6,10] and no neurovascular complications have been reported with less than 2 mm medial perforation of the pedicle wall.[3,8,9] Volumetric intrusion studies by Polly et al[11] have shown that medial perforation of the pedicle by a screw should be more than 2 mm to be equivalent to the intrusion volume of a standard pedicle hook. Therefore, minimal perforations, up to 2 mm, into the canal can cause no potential complications. But do these misplacements lead to loss of pull-out strength? Reichle et al[12] in their study demonstrated that the strength of pedicle screws, which have no pure trabecular position and minimally perforate the pedicle wall, is as good as or slightly better than those having pure trabecular position. Therefore, it follows that some thoracic pedicle screw misplacements can be acceptable both in terms of safety and their biomechanical strength. In view of the above discussion, the following screw placements can be considered acceptable for thoracic pedicle screws (<a href="#FF1" class="ptLink">Figure 1</a>):

!!! FIGURE 1

<ul>
 <li>All screws completely within the pedicle medullary canal - ideal placement.</li>
 <li>All screws encroaching on any cortex of the pedicle but still within the pedicle.</li>
 <li>Screws with less than 2 mm penetration through any of the cortices.</li>
 <li>Screws contained within the pedicle rib unit (In-Out-In screws).[9]</li>
</ul>

In all the above situations, the pedicle screw trajectory beyond the pedicle and the screw tip should be contained within the body of the vertebra. In thoracic spine, the use of pedicle-rib unit for In-Out-In screws is an accepted technique with 70% to 80% of pull-out strength compared with that of conventional pedicle screws.[9]
With this background, we developed a simple outcome-based classification for assessment of thoracic pedicle screw positions based on postoperative computed tomography (CT) scans, which we have named as All India Institute of Medical Sciences (AIIMS) outcome-based classification after the Institute where the study was conducted.
Type I placements or acceptable placements: This includes all acceptable screw placements described above.
Type II placements or unacceptable placements: All screw placements other than type I, without any clinical neurovascular complications (<a href="#FF2" class="ptLink">Figure 2</a>).

!!! FIGURE 2

Type III (grievous placements): This includes all screw placements with clinical neurovascular complications with documented injury to the corresponding vital structures by the misplaced screw with CT or magnetic resonance imaging (<a href="#FF3" class="ptLink">Figures 3 and 4</a>).

!!! FIGURE 3

!!! FIGURE 4

As can be seen, the purpose of the outcome-based classification is to make the results of the assessment of thoracic pedicle screw placement correlate better with the clinically observed results. To evaluate the same, we undertook a study for the assessment of thoracic pedicle screw placements using the AIIMS outcome-based classification and the Rongming Xu et al criteria[5] with the help of postoperative CT scans.

<h3>Methods</h3>
The study was conducted between June 2001 and December 2002. Sixty patients with various spinal disorders were included in the study. The mean age was 29.6 years (range, 12-72 years), 26 of the patients were females and 34 were males. All underwent surgery with the use of pedicular screw-rod system by a common spine surgery team. Distribution of various spinal disorders among patients were as follows: Pott spine in 24, thoracolumbar fractures in 12, and surgery for scoliosis in 24. The patients were divided into 2 groups for assessment of thoracic pedicle screw placements: scoliosis group with 24 patients and the nonscoliosis group with 36 patients. All patients were subjected to routine preoperative CT scan for the assessment of pedicle anatomy.

<h4>Operative Technique</h4>
Intraoperative C-arm guidance was used in the placement of all screws. In thoracic spine, screw placement was done by free hand technique[13] or the In-Out-In technique described by Belmont et al.[9] In patients with scoliosis, we used the In-Out-In technique more frequently.
All patients were subjected to postoperative CT scans using Seimens IV generation CT scanner. To minimize radiation exposure, axial sections through pedicle screws were taken in the reformatted images. Thoracic pedicle screw misplacements were measured using a digital caliper (Mitutoyo company).
Placements of thoracic pedicle screws were assessed using the outcome-based classification and the Rongming Xu et al[5] criteria of screw placement. In Rongming Xu criteria, screws having pure trabacular positions only are acceptable with all other placements classified as grades of perforation or misplacements.
Grades of perforation as described by Rongming Xu:
Grade I: Minimal penetration of pedicle wall by the screw.
Grade II: Less than half of the diameter of the screw is outside the pedicle wall.
Grade III: More than half the diameter of the screw is outside the pedicle wall.

<h3>Results</h3>
A total of 314 thoracic pedicle screws were assessed from 60 patients with various spinal disorders. In the scoliosis group, there were 24 patients, 13 males and 11 females, with an average age of 14.5 years. All had adolescent idiopathic scoliosis with an average preoperative cobb angle of 740 (520-1020). A total of 138 thoracic pedicle screws were assessed in this group using postoperative CT scans. The results are shown in <a href="#TT1" class="ptLink">Table 1</a>. About 49.28% (68 of 138 screws) of the screws were completely within the endosteum of the pedicle, and 50.72% (70 of 138 screws) of screws had various grades of misplacement according to Rongming Xu criteria.[5] The grade III lateral perforations were the maximum due to the frequent use of the In-Out-In technique. Four (2.89%) screws had grade III medial perforations with the potential for neurovascular complications. The same sets of screws were also assessed using the outcome-based classification (<a href="#TT3" class="ptLink">Table 3</a>). The results showed that 89.85% (124 of 138 screws) screws had acceptable placements (type I) and 8.69% (12 of 138 screws) had unacceptable placements (type II). Two patients had transient neurologic deficit accounting for 1.44% (2 of 138 screws) type III or grievous placements.

!!! TABLE 1

!!! TABLE 3

In the nonscoliosis group, there were 36 patients with an average age of 32.1 years (15-72 years) and the male to female ratio was 1.8:1. A total of 176 screws were assessed using postoperative CT scans. The results are shown in <a href="#TT2" class="ptLink">Table 2</a>. Fifty percent (88 of 176 screws) of the screws were placed within the endosteum of the pedicle. The grade III lateral perforation was seen in 3.41% (6 screws) due to infrequent use of In-Out-In technique in the nonscoliotic spine. About 4.45% (8 of 176 screws) of screw placements were not amenable to assessment due to severe artifacts. Assessment of these screws using the outcome-based classification showed that 86.93% (153 of 176 screws) of the screws were acceptably placed (type I) and 8.52% (15 of 176 screws) had unacceptable placements (type II). There were no type III placements in the nonscoliosis group.

!!! TABLE 2

The combined results of screw placements in both the groups are shown in <a href="#TT3" class="ptLink">Tables 3 and 4</a>.

!!! TABLE 4

Apart from screw misplacements, we encountered 2 screw breakages (0.63%) and 1 pedicle fracture (0.32%). There were 2 patients with neurologic complications in the scoliosis group. One had monoparesis on the right side and another had paresthesias in right lower limb, both of which resolved completely in 6 weeks' duration. No attempt was made to remove the offending screws.

<h3>Discussion</h3>
The introduction of pedicle screw instrumentation is a milestone in the evolution of spinal instrumentation. The 3-column fixation of pedicle screws has been shown to be superior to all other posterior spinal fixation devices.[1,3] However, the use of pedicle screws in the thoracic spine was not encouraged until the pioneering studies by Suk et al,[14] who showed that consistently superior results can be achieved in terms of fixation and deformity correction with the use of thoracic pedicle screws with minimal complications. Today, surgeons across the world use pedicle screws in the thoracic spine for various spinal disorders, including scoliosis, to achieve better fixation and decrease the instrumentation levels. Nevertheless, the postoperative assessment of the pedicle screw placements, by well-known authors, shows that the percentage of misplaced screws in the thoracic spine is quite significant compared with the negligible complications observed. Liljenqvist et al[8] have reported 25% perforation rate in patients with scoliosis, Girtzbein and Robbin[6] reported 29.1% perforations, Vaccaro et al[15] reported that 41% of the 90 screws inserted in fresh cadavers had penetrated the pedicle cortex, and Weinstein et al[16] reported 21% perforations without any neurologic complications. Suk et al[4] reported 27% lateral and 6% medial perforations with 1 transient paresthesia. These studies attribute the low neurologic complications observed to the fact that most of the medial perforations were within the safe zone of 4 mm described by Girtzbein and Robbin.[6] Yalsin and Guven[10] have stated in their article that since neurologic problems are only described with screws perforating the pedicle medially by at least 4 mm, this question has great relevance concerning the definition of pedicular screw malposition. Youkilis et al[17] have mentioned about the structurally significant violations of pedicle screws as 02 mm perforations. In other words, there is a growing tendency to accept up to 2 mm pedicle wall perforations in the light of clinical experience of low complications. This is precisely what the outcome-based classification brings to the forefront. The acceptability of screw malposition depends not only on clinical safety but also on its biomechanical strength. In the light of the biomechanical study by Reichle et al[12] and volumetric intrusion studies by Polly et al,[11] screws with perforations up to 2 mm have been taken as acceptable placements. The screw placements with more than 2 mm perforations have high chances to cause clinical complications and also diminish in their biomechanical strength. Hence, these are placed under type II or unacceptable placements. The advantage of creating the type III placements is to make the results show the clinically observed complications directly (<a href="#TT3" class="ptLink">Tables 3 and 4</a>). Therefore, thoracic screw misplacements can be defined as screw placements causing or having the potential to cause neurovascular complications and/or have inferior biomechanical strength due to their mal-position. Any screw misplacement consistently found to have no clinical complication and having reasonable biomechanical strength can be labeled as acceptable placement. The classic example is the In-Out-In screws that have 070% the strength of the intrapedicular screws[9] and is found to cause no clinical complications. The acceptability criteria for the screws is an issue that is open to debate, and can further be modified with more information from appropriate clinical or biomechanical studies.
The results of this study, using the Rongming Xu et al[5] assessment criteria, show the gross discrepancy between the minimal clinical complications observed and the high rate of screw misplacements (<a href="#TT3" class="ptLink">Tables 3 and 4</a>). The results of the outcome classification for screw placement assessment show a high percentage of acceptable screw placements (<a href="#TT3" class="ptLink">Tables 3 and 4</a>). This correlates better with the low rate of clinical complications observed. The neurologic complications observed in our study, 2 of 24 patients with scoliosis with transient deficit, appears to give a relatively high percentage (8.33%) of neurovascular complications. However, only selected patients affording both pre- and postoperative CT scans and willing to participate were included in this study, making this a nonconsecutive series.
The other major advantage of the outcome classification is that it can bring about uniformity in the results of pedicle screw assessments, when used as a common classification system. Assessment of pedicle screws has been done with various classification systems, with most of the authors stressing the fact that perforations that are 02 mm can be acceptable.[18,19] In a recent meta-analysis[20] on pedicle screw placements, the authors have stated that The identification of 35 different pedicle screw placement assessment methods in our database leads us to the conclusion that a standardized assessment method is needed. The outcome classification, with the concept of acceptable placements, can be a common acceptable classification for expressing the results of screw placements, as we can differentiate between the benign marginal misplacements (acceptable) and dangerous placements (unacceptable). Therefore, the outcome classification can be an answer to the problem of numerous classification grades used for the assessment of pedicle screw placement.

<h3>Conclusion</h3>
Thoracic pedicle screw placement is a technically demanding procedure, but is an invaluable tool for spinal stabilization due to its superior biomechanical strength. The literature shows consensus over high rates of screw misplacement, but low clinical complications, even in the hands of the best of spine surgeons. The present study showed that even with high rates of marginal misplacements, the risk of clinical neurovascular complications is low. The concept of acceptable screw placements makes the thoracic pedicle screw assessment results correlate better with clinical outcome, as we can differentiate between the benign marginal misplacements (acceptable) and those with dangerous placements (unacceptable).

<h3>Key Points</h3>
<ul>
 <li>The main deterrent for the use of thoracic pedicular screws is the feared neurovascular complications due to screw misplacements.</li>
 <li>Only a small fraction of the misplaced screws actually causes any complication.</li>

 <li>Outcome-based classification for the assessment of pedicle screw positions using postoperative CT scan.</li>
 <li>The concept of acceptable screw placements and the outcome classification makes the pedicle screw assessment results correlate better with the clinical outcome.</li>
</ul>

<h3>References</h3>
<ol>
 <li>Liljenqvist U, Hackenberg L, Link T, et al. Pullout strength of pedicle screws versus pedicle and laminar hooks in the thoracic spine. Acta Orthop Belg 2001;67:157-63.</li>
 <li>Belmont PJ Jr, Klemme WR, Robinson M, et al. Accuracy of thoracic pedicle screws in patients with and without coronal plane spinal deformities. Spine 2002;27:1558-66.</li>
 <li>Liljenqvist U, Lepsien U, Hackenberg L, et al. Comparative analysis of pedicle screw and hook instrumentation in posterior correction and fusion of idiopathic thoracic scoliosis. Eur Spine J 2002;11:336-43.</li>
 <li>Suk SI, Kim WJ, Lee SM, et al. Thoracic pedicle screw fixation in spinal deformities: are they really safe? Spine 2001;26:2049-57.</li>
 <li>Xu R, Ebraheim NA, Ou Y, et al. Anatomic considerations of pedicle screw placement in the thoracic spine. Roy-Camille technique versus open-lamina technique. Spine 1998;23:1065-8.</li>
 <li>Gertzbein SD, Robbins SE. Accuracy of pedicular screw placement in vivo. Spine 1990;15:11-4.</li>
 <li>Heini P, Scholl E, Wyler D, et al. Fatal cardiac tamponade associated with posterior spinal instrumentation. A case report. Spine 1998;23:2226-30.</li>
 <li>Liljenqvist UR, Halm HF, Link TM. Pedicle screw instrumentation of the thoracic spine in idiopathic scoliosis. Spine 1997;22:2239-45.</li>
 <li>Belmont PJ Jr, Klemme WR, Dhawan A, et al. In vivo accuracy of thoracic pedicle screws. Spine 2001;26:2340-6.</li>
 <li>Yalcin S, Guven O. Reversible anterior cord syndrome due to penetration of the spinal canal by pedicular screws. Paraplegia 1995;33:423-5.</li>
 <li>Polly DW Jr, Potter BK, Kuklo T, et al. Volumetric spinal canal intrusion: a comparison between thoracic pedicle screws and thoracic hooks. Spine 2004;29:63-9.</li>
 <li>Reichle E, Morlock M, Sellenschloh K, et al. Definition of pedicle malposition. Primary stability and loosening characteristics of pedicle screws in relation to position: spongious anchoring, cortical anchoring, perforation and malposition. Orthopade 2002;31:402-5.</li>
 <li>Kim YJ, Lenke LG, Bridwell KH, et al. Free hand pedicle screw placement in the thoracic spine: is it safe? Spine 2004;29:333-42.</li>
 <li>Suk SI, Lee CK, Kim WJ, et al. Segmental pedicle screw fixation in the treatment of thoracic idiopathic scoliosis. Spine 1995;20:1399-405.</li>
 <li>Vaccaro AR, Rizzolo SJ, Balderston RA, et al. Placement of pedicle screws in the thoracic spine. Part II: an anatomical and radiographic assessment. J Bone Joint Surg Am 1995;77:1200-6.</li>
 <li>Weinstein JN, Spratt KF, Spengler D, et al. Spinal pedicle fixation: reliability and validity of roentgenogram-based assessment and surgical factors on successful screw placement. Spine 1988;13:1012-8.</li>
 <li>Youkilis AS, Quint DJ, McGillicuddy JE, et al. Stereotactic navigation for placement of pedicle screws in the thoracic spine. Neurosurgery 2001;48:771-8, discussion 778-9.</li>
 <li>Kuklo TR, Lenke LG, O0Brien MF, et al. Accuracy and efficacy of thoracic pedicle screws in curves more than 90 degrees. Spine 2005;30:222-6.</li>
 <li>Fisher CG, Sahajpal V, Keynan O, et al. Accuracy and safety of pedicle screw fixation in thoracic spine trauma. J Neurosurg Spine 2006;5:520-6.</li>
 <li>Kosmopoulos V, Schizas C. Pedicle screw placement accuracy: a meta-analysis. Spine 2007;32:E111-E120.</li>
</ol>

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Guest · Apr 04, 2008, 03:11 PM

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<h3>Outcome-Based Classification for Assessment of Thoracic Pedicular Screw Placement</h3>

Upendra, Bidre N. MS; Meena, Devkant MS; Chowdhury, Buddhadev MS; Ahmad, Abrar MS; Jayaswal, Arvind MS
From the Department of Orthopaedics, All India Institute of Medical Sciences, New Delhi, India.
Acknowledgment date: January 3, 2007. Revision date: July 1, 2007. Acceptance date: August 20, 2007.

The legal regulatory status of the device(s)/drug(s) that is/are the subject of this manuscript is not applicable in our country.
No funds were received in support of this work. No benefits in any form have been or will be received from a commercial party related directly or indirectly to the subject of this manuscript.
Address correspondence and reprint requests to Bidre Upendra, MS, Department of Orthopaedics, All India Institute of Medical Sciences, Ansari Nagar, New Delhi 110029, India; E-mail: upibn@yahoo.com

<h3>Abstract and Introduction</h3>

<h4>Abstract</h4>

 Study Design: Prospective cohort study.

 Objective: We propose a simple outcome-based classification for assessment of pedicle screw positions based on postoperative computed tomography scan. This bridges the gap between high rates of pedicle screw misplacement and minimal complications reported.

 Summary of Background Data: The main deterrent for the use of thoracic pedicular screws is the feared neurovascular complications due to screw misplacements. The literature shows that only a small fraction of the misplaced screws actually causes any complication, and some misplacements can be acceptable both in terms of safety and their biomechanical strength.

 Methods: Sixty patients with various spinal disorders were included in the study. The mean age was 29.6 years (range, 12-72 years). The patients were divided into 2 groups for assessment of pedicle screw placements using postoperative computed tomography scans: scoliosis group with 24 patients and the nonscoliosis group with 34 patients. Placements of screws were assessed using the outcome-based classification and the Rongming Xu criteria of screw placement.

 Results: A total of 341 screws were assessed from 60 patients with various spinal disorders (scoliosis and nonscoliosis groups). Using the Rongming Xu criteria, the overall screw misplacement in scoliosis group was 50.72% (68 of 138) and that in nonscoliosis group was 45.45% (80 of 176 screws). Assessment of these screws using the outcome-based classification showed a high percentage of acceptable screw placements (type 1) - 89.85% (124 of 138 screws) in the scoliosis group and 86.93% (153 of 176 screws) in the nonscoliosis group.

 Conclusion: The literature shows consensus over high rates of pedicle screw misplacement, but low clinical complications, in the hands of the best of spine surgeons. The concept of acceptable screw placements and the outcome classification makes the pedicle screw assessment results correlate better with the clinical outcome.



<h4>Introduction</h4>
Pedicular screw rod systems are the most widely used spinal stabilization systems worldwide today. The superior biomechanical strength of pedicular screws has yielded better fusion rates and has been shown to give better corrections in complex deformities.[1-3] Although used widely in various spinal disorders, pedicle screws are used with caution in the thoracic spine. The main deterrent for the use of thoracic pedicular screws is the feared neurovascular complications due to screw misplacements.[3-7] Screw misplacements in the thoracic spine range from 20% to 30% in the hands of the best of spine surgeons.[4,6,8,9] Pedicle fractures, screw breakage, and loosening range from 0.5% to 1% in most of the studies.[4,6,8] The neurovascular complications reported due to misplaced screws are between 0% and 1%.[4,10] Therefore, only a small fraction of the misplaced screws actually cause any complication. This makes one wonder if all misplacements of the pedicle screw are troublesome. Looking into further details, studies show that all neurologic deficits reported have medial pedicle perforation of more than 4 mm,[4,6,10] and no neurovascular complications have been reported with less than 2 mm medial perforation of the pedicle wall.[3,8,9] Volumetric intrusion studies by Polly et al[11] have shown that medial perforation of the pedicle by a screw should be more than 2 mm to be equivalent to the intrusion volume of a standard pedicle hook. Therefore, minimal perforations, up to 2 mm, into the canal can cause no potential complications. But do these misplacements lead to loss of pull-out strength? Reichle et al[12] in their study demonstrated that the strength of pedicle screws, which have no pure trabecular position and minimally perforate the pedicle wall, is as good as or slightly better than those having pure trabecular position. Therefore, it follows that some thoracic pedicle screw misplacements can be acceptable both in terms of safety and their biomechanical strength. In view of the above discussion, the following screw placements can be considered acceptable for thoracic pedicle screws (<a href="#FF1" class="ptLink">Figure 1</a>):

!!! FIGURE 1

<ul>
 <li>All screws completely within the pedicle medullary canal - ideal placement.</li>
 <li>All screws encroaching on any cortex of the pedicle but still within the pedicle.</li>
 <li>Screws with less than 2 mm penetration through any of the cortices.</li>
 <li>Screws contained within the pedicle rib unit (In-Out-In screws).[9]</li>
</ul>

In all the above situations, the pedicle screw trajectory beyond the pedicle and the screw tip should be contained within the body of the vertebra. In thoracic spine, the use of pedicle-rib unit for In-Out-In screws is an accepted technique with 70% to 80% of pull-out strength compared with that of conventional pedicle screws.[9]
With this background, we developed a simple outcome-based classification for assessment of thoracic pedicle screw positions based on postoperative computed tomography (CT) scans, which we have named as All India Institute of Medical Sciences (AIIMS) outcome-based classification after the Institute where the study was conducted.
Type I placements or acceptable placements: This includes all acceptable screw placements described above.
Type II placements or unacceptable placements: All screw placements other than type I, without any clinical neurovascular complications (<a href="#FF2" class="ptLink">Figure 2</a>).

!!! FIGURE 2

Type III (grievous placements): This includes all screw placements with clinical neurovascular complications with documented injury to the corresponding vital structures by the misplaced screw with CT or magnetic resonance imaging (<a href="#FF3" class="ptLink">Figures 3 and 4</a>).

!!! FIGURE 3

!!! FIGURE 4

As can be seen, the purpose of the outcome-based classification is to make the results of the assessment of thoracic pedicle screw placement correlate better with the clinically observed results. To evaluate the same, we undertook a study for the assessment of thoracic pedicle screw placements using the AIIMS outcome-based classification and the Rongming Xu et al criteria[5] with the help of postoperative CT scans.

<h3>Methods</h3>
The study was conducted between June 2001 and December 2002. Sixty patients with various spinal disorders were included in the study. The mean age was 29.6 years (range, 12-72 years), 26 of the patients were females and 34 were males. All underwent surgery with the use of pedicular screw-rod system by a common spine surgery team. Distribution of various spinal disorders among patients were as follows: Pott spine in 24, thoracolumbar fractures in 12, and surgery for scoliosis in 24. The patients were divided into 2 groups for assessment of thoracic pedicle screw placements: scoliosis group with 24 patients and the nonscoliosis group with 36 patients. All patients were subjected to routine preoperative CT scan for the assessment of pedicle anatomy.

<h4>Operative Technique</h4>
Intraoperative C-arm guidance was used in the placement of all screws. In thoracic spine, screw placement was done by free hand technique[13] or the In-Out-In technique described by Belmont et al.[9] In patients with scoliosis, we used the In-Out-In technique more frequently.
All patients were subjected to postoperative CT scans using Seimens IV generation CT scanner. To minimize radiation exposure, axial sections through pedicle screws were taken in the reformatted images. Thoracic pedicle screw misplacements were measured using a digital caliper (Mitutoyo company).
Placements of thoracic pedicle screws were assessed using the outcome-based classification and the Rongming Xu et al[5] criteria of screw placement. In Rongming Xu criteria, screws having pure trabacular positions only are acceptable with all other placements classified as grades of perforation or misplacements.
Grades of perforation as described by Rongming Xu:
Grade I: Minimal penetration of pedicle wall by the screw.
Grade II: Less than half of the diameter of the screw is outside the pedicle wall.
Grade III: More than half the diameter of the screw is outside the pedicle wall.

<h3>Results</h3>
A total of 314 thoracic pedicle screws were assessed from 60 patients with various spinal disorders. In the scoliosis group, there were 24 patients, 13 males and 11 females, with an average age of 14.5 years. All had adolescent idiopathic scoliosis with an average preoperative cobb angle of 740 (520-1020). A total of 138 thoracic pedicle screws were assessed in this group using postoperative CT scans. The results are shown in <a href="#TT1" class="ptLink">Table 1</a>. About 49.28% (68 of 138 screws) of the screws were completely within the endosteum of the pedicle, and 50.72% (70 of 138 screws) of screws had various grades of misplacement according to Rongming Xu criteria.[5] The grade III lateral perforations were the maximum due to the frequent use of the In-Out-In technique. Four (2.89%) screws had grade III medial perforations with the potential for neurovascular complications. The same sets of screws were also assessed using the outcome-based classification (<a href="#TT3" class="ptLink">Table 3</a>). The results showed that 89.85% (124 of 138 screws) screws had acceptable placements (type I) and 8.69% (12 of 138 screws) had unacceptable placements (type II). Two patients had transient neurologic deficit accounting for 1.44% (2 of 138 screws) type III or grievous placements.

!!! TABLE 1

!!! TABLE 3

In the nonscoliosis group, there were 36 patients with an average age of 32.1 years (15-72 years) and the male to female ratio was 1.8:1. A total of 176 screws were assessed using postoperative CT scans. The results are shown in <a href="#TT2" class="ptLink">Table 2</a>. Fifty percent (88 of 176 screws) of the screws were placed within the endosteum of the pedicle. The grade III lateral perforation was seen in 3.41% (6 screws) due to infrequent use of In-Out-In technique in the nonscoliotic spine. About 4.45% (8 of 176 screws) of screw placements were not amenable to assessment due to severe artifacts. Assessment of these screws using the outcome-based classification showed that 86.93% (153 of 176 screws) of the screws were acceptably placed (type I) and 8.52% (15 of 176 screws) had unacceptable placements (type II). There were no type III placements in the nonscoliosis group.

!!! TABLE 2

The combined results of screw placements in both the groups are shown in <a href="#TT3" class="ptLink">Tables 3 and 4</a>.

!!! TABLE 4

Apart from screw misplacements, we encountered 2 screw breakages (0.63%) and 1 pedicle fracture (0.32%). There were 2 patients with neurologic complications in the scoliosis group. One had monoparesis on the right side and another had paresthesias in right lower limb, both of which resolved completely in 6 weeks' duration. No attempt was made to remove the offending screws.

<h3>Discussion</h3>
The introduction of pedicle screw instrumentation is a milestone in the evolution of spinal instrumentation. The 3-column fixation of pedicle screws has been shown to be superior to all other posterior spinal fixation devices.[1,3] However, the use of pedicle screws in the thoracic spine was not encouraged until the pioneering studies by Suk et al,[14] who showed that consistently superior results can be achieved in terms of fixation and deformity correction with the use of thoracic pedicle screws with minimal complications. Today, surgeons across the world use pedicle screws in the thoracic spine for various spinal disorders, including scoliosis, to achieve better fixation and decrease the instrumentation levels. Nevertheless, the postoperative assessment of the pedicle screw placements, by well-known authors, shows that the percentage of misplaced screws in the thoracic spine is quite significant compared with the negligible complications observed. Liljenqvist et al[8] have reported 25% perforation rate in patients with scoliosis, Girtzbein and Robbin[6] reported 29.1% perforations, Vaccaro et al[15] reported that 41% of the 90 screws inserted in fresh cadavers had penetrated the pedicle cortex, and Weinstein et al[16] reported 21% perforations without any neurologic complications. Suk et al[4] reported 27% lateral and 6% medial perforations with 1 transient paresthesia. These studies attribute the low neurologic complications observed to the fact that most of the medial perforations were within the safe zone of 4 mm described by Girtzbein and Robbin.[6] Yalsin and Guven[10] have stated in their article that since neurologic problems are only described with screws perforating the pedicle medially by at least 4 mm, this question has great relevance concerning the definition of pedicular screw malposition. Youkilis et al[17] have mentioned about the structurally significant violations of pedicle screws as 02 mm perforations. In other words, there is a growing tendency to accept up to 2 mm pedicle wall perforations in the light of clinical experience of low complications. This is precisely what the outcome-based classification brings to the forefront. The acceptability of screw malposition depends not only on clinical safety but also on its biomechanical strength. In the light of the biomechanical study by Reichle et al[12] and volumetric intrusion studies by Polly et al,[11] screws with perforations up to 2 mm have been taken as acceptable placements. The screw placements with more than 2 mm perforations have high chances to cause clinical complications and also diminish in their biomechanical strength. Hence, these are placed under type II or unacceptable placements. The advantage of creating the type III placements is to make the results show the clinically observed complications directly (<a href="#TT3" class="ptLink">Tables 3 and 4</a>). Therefore, thoracic screw misplacements can be defined as screw placements causing or having the potential to cause neurovascular complications and/or have inferior biomechanical strength due to their mal-position. Any screw misplacement consistently found to have no clinical complication and having reasonable biomechanical strength can be labeled as acceptable placement. The classic example is the In-Out-In screws that have 070% the strength of the intrapedicular screws[9] and is found to cause no clinical complications. The acceptability criteria for the screws is an issue that is open to debate, and can further be modified with more information from appropriate clinical or biomechanical studies.
The results of this study, using the Rongming Xu et al[5] assessment criteria, show the gross discrepancy between the minimal clinical complications observed and the high rate of screw misplacements (<a href="#TT3" class="ptLink">Tables 3 and 4</a>). The results of the outcome classification for screw placement assessment show a high percentage of acceptable screw placements (<a href="#TT3" class="ptLink">Tables 3 and 4</a>). This correlates better with the low rate of clinical complications observed. The neurologic complications observed in our study, 2 of 24 patients with scoliosis with transient deficit, appears to give a relatively high percentage (8.33%) of neurovascular complications. However, only selected patients affording both pre- and postoperative CT scans and willing to participate were included in this study, making this a nonconsecutive series.
The other major advantage of the outcome classification is that it can bring about uniformity in the results of pedicle screw assessments, when used as a common classification system. Assessment of pedicle screws has been done with various classification systems, with most of the authors stressing the fact that perforations that are 02 mm can be acceptable.[18,19] In a recent meta-analysis[20] on pedicle screw placements, the authors have stated that The identification of 35 different pedicle screw placement assessment methods in our database leads us to the conclusion that a standardized assessment method is needed. The outcome classification, with the concept of acceptable placements, can be a common acceptable classification for expressing the results of screw placements, as we can differentiate between the benign marginal misplacements (acceptable) and dangerous placements (unacceptable). Therefore, the outcome classification can be an answer to the problem of numerous classification grades used for the assessment of pedicle screw placement.

<h3>Conclusion</h3>
Thoracic pedicle screw placement is a technically demanding procedure, but is an invaluable tool for spinal stabilization due to its superior biomechanical strength. The literature shows consensus over high rates of screw misplacement, but low clinical complications, even in the hands of the best of spine surgeons. The present study showed that even with high rates of marginal misplacements, the risk of clinical neurovascular complications is low. The concept of acceptable screw placements makes the thoracic pedicle screw assessment results correlate better with clinical outcome, as we can differentiate between the benign marginal misplacements (acceptable) and those with dangerous placements (unacceptable).

<h3>Key Points</h3>
<ul>
 <li>The main deterrent for the use of thoracic pedicular screws is the feared neurovascular complications due to screw misplacements.</li>
 <li>Only a small fraction of the misplaced screws actually causes any complication.</li>

 <li>Outcome-based classification for the assessment of pedicle screw positions using postoperative CT scan.</li>
 <li>The concept of acceptable screw placements and the outcome classification makes the pedicle screw assessment results correlate better with the clinical outcome.</li>
</ul>

<h3>References</h3>
<ol>
 <li>Liljenqvist U, Hackenberg L, Link T, et al. Pullout strength of pedicle screws versus pedicle and laminar hooks in the thoracic spine. Acta Orthop Belg 2001;67:157-63.</li>
 <li>Belmont PJ Jr, Klemme WR, Robinson M, et al. Accuracy of thoracic pedicle screws in patients with and without coronal plane spinal deformities. Spine 2002;27:1558-66.</li>
 <li>Liljenqvist U, Lepsien U, Hackenberg L, et al. Comparative analysis of pedicle screw and hook instrumentation in posterior correction and fusion of idiopathic thoracic scoliosis. Eur Spine J 2002;11:336-43.</li>
 <li>Suk SI, Kim WJ, Lee SM, et al. Thoracic pedicle screw fixation in spinal deformities: are they really safe? Spine 2001;26:2049-57.</li>
 <li>Xu R, Ebraheim NA, Ou Y, et al. Anatomic considerations of pedicle screw placement in the thoracic spine. Roy-Camille technique versus open-lamina technique. Spine 1998;23:1065-8.</li>
 <li>Gertzbein SD, Robbins SE. Accuracy of pedicular screw placement in vivo. Spine 1990;15:11-4.</li>
 <li>Heini P, Scholl E, Wyler D, et al. Fatal cardiac tamponade associated with posterior spinal instrumentation. A case report. Spine 1998;23:2226-30.</li>
 <li>Liljenqvist UR, Halm HF, Link TM. Pedicle screw instrumentation of the thoracic spine in idiopathic scoliosis. Spine 1997;22:2239-45.</li>
 <li>Belmont PJ Jr, Klemme WR, Dhawan A, et al. In vivo accuracy of thoracic pedicle screws. Spine 2001;26:2340-6.</li>
 <li>Yalcin S, Guven O. Reversible anterior cord syndrome due to penetration of the spinal canal by pedicular screws. Paraplegia 1995;33:423-5.</li>
 <li>Polly DW Jr, Potter BK, Kuklo T, et al. Volumetric spinal canal intrusion: a comparison between thoracic pedicle screws and thoracic hooks. Spine 2004;29:63-9.</li>
 <li>Reichle E, Morlock M, Sellenschloh K, et al. Definition of pedicle malposition. Primary stability and loosening characteristics of pedicle screws in relation to position: spongious anchoring, cortical anchoring, perforation and malposition. Orthopade 2002;31:402-5.</li>
 <li>Kim YJ, Lenke LG, Bridwell KH, et al. Free hand pedicle screw placement in the thoracic spine: is it safe? Spine 2004;29:333-42.</li>
 <li>Suk SI, Lee CK, Kim WJ, et al. Segmental pedicle screw fixation in the treatment of thoracic idiopathic scoliosis. Spine 1995;20:1399-405.</li>
 <li>Vaccaro AR, Rizzolo SJ, Balderston RA, et al. Placement of pedicle screws in the thoracic spine. Part II: an anatomical and radiographic assessment. J Bone Joint Surg Am 1995;77:1200-6.</li>
 <li>Weinstein JN, Spratt KF, Spengler D, et al. Spinal pedicle fixation: reliability and validity of roentgenogram-based assessment and surgical factors on successful screw placement. Spine 1988;13:1012-8.</li>
 <li>Youkilis AS, Quint DJ, McGillicuddy JE, et al. Stereotactic navigation for placement of pedicle screws in the thoracic spine. Neurosurgery 2001;48:771-8, discussion 778-9.</li>
 <li>Kuklo TR, Lenke LG, O0Brien MF, et al. Accuracy and efficacy of thoracic pedicle screws in curves more than 90 degrees. Spine 2005;30:222-6.</li>
 <li>Fisher CG, Sahajpal V, Keynan O, et al. Accuracy and safety of pedicle screw fixation in thoracic spine trauma. J Neurosurg Spine 2006;5:520-6.</li>
 <li>Kosmopoulos V, Schizas C. Pedicle screw placement accuracy: a meta-analysis. Spine 2007;32:E111-E120.</li>
</ol>

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<h3>Outcome-Based Classification for Assessment of Thoracic Pedicular Screw Placement</h3>

Upendra, Bidre N. MS; Meena, Devkant MS; Chowdhury, Buddhadev MS; Ahmad, Abrar MS; Jayaswal, Arvind MS
From the Department of Orthopaedics, All India Institute of Medical Sciences, New Delhi, India.
Acknowledgment date: January 3, 2007. Revision date: July 1, 2007. Acceptance date: August 20, 2007.

The legal regulatory status of the device(s)/drug(s) that is/are the subject of this manuscript is not applicable in our country.
No funds were received in support of this work. No benefits in any form have been or will be received from a commercial party related directly or indirectly to the subject of this manuscript.
Address correspondence and reprint requests to Bidre Upendra, MS, Department of Orthopaedics, All India Institute of Medical Sciences, Ansari Nagar, New Delhi 110029, India; E-mail: upibn@yahoo.com

<h3>Abstract and Introduction</h3>

<h4>Abstract</h4>

 Study Design: Prospective cohort study.

 Objective: We propose a simple outcome-based classification for assessment of pedicle screw positions based on postoperative computed tomography scan. This bridges the gap between high rates of pedicle screw misplacement and minimal complications reported.

 Summary of Background Data: The main deterrent for the use of thoracic pedicular screws is the feared neurovascular complications due to screw misplacements. The literature shows that only a small fraction of the misplaced screws actually causes any complication, and some misplacements can be acceptable both in terms of safety and their biomechanical strength.

 Methods: Sixty patients with various spinal disorders were included in the study. The mean age was 29.6 years (range, 12-72 years). The patients were divided into 2 groups for assessment of pedicle screw placements using postoperative computed tomography scans: scoliosis group with 24 patients and the nonscoliosis group with 34 patients. Placements of screws were assessed using the outcome-based classification and the Rongming Xu criteria of screw placement.

 Results: A total of 341 screws were assessed from 60 patients with various spinal disorders (scoliosis and nonscoliosis groups). Using the Rongming Xu criteria, the overall screw misplacement in scoliosis group was 50.72% (68 of 138) and that in nonscoliosis group was 45.45% (80 of 176 screws). Assessment of these screws using the outcome-based classification showed a high percentage of acceptable screw placements (type 1) - 89.85% (124 of 138 screws) in the scoliosis group and 86.93% (153 of 176 screws) in the nonscoliosis group.

 Conclusion: The literature shows consensus over high rates of pedicle screw misplacement, but low clinical complications, in the hands of the best of spine surgeons. The concept of acceptable screw placements and the outcome classification makes the pedicle screw assessment results correlate better with the clinical outcome.



<h4>Introduction</h4>
Pedicular screw rod systems are the most widely used spinal stabilization systems worldwide today. The superior biomechanical strength of pedicular screws has yielded better fusion rates and has been shown to give better corrections in complex deformities.[1-3] Although used widely in various spinal disorders, pedicle screws are used with caution in the thoracic spine. The main deterrent for the use of thoracic pedicular screws is the feared neurovascular complications due to screw misplacements.[3-7] Screw misplacements in the thoracic spine range from 20% to 30% in the hands of the best of spine surgeons.[4,6,8,9] Pedicle fractures, screw breakage, and loosening range from 0.5% to 1% in most of the studies.[4,6,8] The neurovascular complications reported due to misplaced screws are between 0% and 1%.[4,10] Therefore, only a small fraction of the misplaced screws actually cause any complication. This makes one wonder if all misplacements of the pedicle screw are troublesome. Looking into further details, studies show that all neurologic deficits reported have medial pedicle perforation of more than 4 mm,[4,6,10] and no neurovascular complications have been reported with less than 2 mm medial perforation of the pedicle wall.[3,8,9] Volumetric intrusion studies by Polly et al[11] have shown that medial perforation of the pedicle by a screw should be more than 2 mm to be equivalent to the intrusion volume of a standard pedicle hook. Therefore, minimal perforations, up to 2 mm, into the canal can cause no potential complications. But do these misplacements lead to loss of pull-out strength? Reichle et al[12] in their study demonstrated that the strength of pedicle screws, which have no pure trabecular position and minimally perforate the pedicle wall, is as good as or slightly better than those having pure trabecular position. Therefore, it follows that some thoracic pedicle screw misplacements can be acceptable both in terms of safety and their biomechanical strength. In view of the above discussion, the following screw placements can be considered acceptable for thoracic pedicle screws (<a href="#FF1" class="ptLink">Figure 1</a>):

!!! FIGURE 1

<ul>
 <li>All screws completely within the pedicle medullary canal - ideal placement.</li>
 <li>All screws encroaching on any cortex of the pedicle but still within the pedicle.</li>
 <li>Screws with less than 2 mm penetration through any of the cortices.</li>
 <li>Screws contained within the pedicle rib unit (In-Out-In screws).[9]</li>
</ul>

In all the above situations, the pedicle screw trajectory beyond the pedicle and the screw tip should be contained within the body of the vertebra. In thoracic spine, the use of pedicle-rib unit for In-Out-In screws is an accepted technique with 70% to 80% of pull-out strength compared with that of conventional pedicle screws.[9]
With this background, we developed a simple outcome-based classification for assessment of thoracic pedicle screw positions based on postoperative computed tomography (CT) scans, which we have named as All India Institute of Medical Sciences (AIIMS) outcome-based classification after the Institute where the study was conducted.
Type I placements or acceptable placements: This includes all acceptable screw placements described above.
Type II placements or unacceptable placements: All screw placements other than type I, without any clinical neurovascular complications (<a href="#FF2" class="ptLink">Figure 2</a>).

!!! FIGURE 2

Type III (grievous placements): This includes all screw placements with clinical neurovascular complications with documented injury to the corresponding vital structures by the misplaced screw with CT or magnetic resonance imaging (<a href="#FF3" class="ptLink">Figures 3 and 4</a>).

!!! FIGURE 3

!!! FIGURE 4

As can be seen, the purpose of the outcome-based classification is to make the results of the assessment of thoracic pedicle screw placement correlate better with the clinically observed results. To evaluate the same, we undertook a study for the assessment of thoracic pedicle screw placements using the AIIMS outcome-based classification and the Rongming Xu et al criteria[5] with the help of postoperative CT scans.

<h3>Methods</h3>
The study was conducted between June 2001 and December 2002. Sixty patients with various spinal disorders were included in the study. The mean age was 29.6 years (range, 12-72 years), 26 of the patients were females and 34 were males. All underwent surgery with the use of pedicular screw-rod system by a common spine surgery team. Distribution of various spinal disorders among patients were as follows: Pott spine in 24, thoracolumbar fractures in 12, and surgery for scoliosis in 24. The patients were divided into 2 groups for assessment of thoracic pedicle screw placements: scoliosis group with 24 patients and the nonscoliosis group with 36 patients. All patients were subjected to routine preoperative CT scan for the assessment of pedicle anatomy.

<h4>Operative Technique</h4>
Intraoperative C-arm guidance was used in the placement of all screws. In thoracic spine, screw placement was done by free hand technique[13] or the In-Out-In technique described by Belmont et al.[9] In patients with scoliosis, we used the In-Out-In technique more frequently.
All patients were subjected to postoperative CT scans using Seimens IV generation CT scanner. To minimize radiation exposure, axial sections through pedicle screws were taken in the reformatted images. Thoracic pedicle screw misplacements were measured using a digital caliper (Mitutoyo company).
Placements of thoracic pedicle screws were assessed using the outcome-based classification and the Rongming Xu et al[5] criteria of screw placement. In Rongming Xu criteria, screws having pure trabacular positions only are acceptable with all other placements classified as grades of perforation or misplacements.
Grades of perforation as described by Rongming Xu:
Grade I: Minimal penetration of pedicle wall by the screw.
Grade II: Less than half of the diameter of the screw is outside the pedicle wall.
Grade III: More than half the diameter of the screw is outside the pedicle wall.

<h3>Results</h3>
A total of 314 thoracic pedicle screws were assessed from 60 patients with various spinal disorders. In the scoliosis group, there were 24 patients, 13 males and 11 females, with an average age of 14.5 years. All had adolescent idiopathic scoliosis with an average preoperative cobb angle of 740 (520-1020). A total of 138 thoracic pedicle screws were assessed in this group using postoperative CT scans. The results are shown in <a href="#TT1" class="ptLink">Table 1</a>. About 49.28% (68 of 138 screws) of the screws were completely within the endosteum of the pedicle, and 50.72% (70 of 138 screws) of screws had various grades of misplacement according to Rongming Xu criteria.[5] The grade III lateral perforations were the maximum due to the frequent use of the In-Out-In technique. Four (2.89%) screws had grade III medial perforations with the potential for neurovascular complications. The same sets of screws were also assessed using the outcome-based classification (<a href="#TT3" class="ptLink">Table 3</a>). The results showed that 89.85% (124 of 138 screws) screws had acceptable placements (type I) and 8.69% (12 of 138 screws) had unacceptable placements (type II). Two patients had transient neurologic deficit accounting for 1.44% (2 of 138 screws) type III or grievous placements.

!!! TABLE 1

!!! TABLE 3

In the nonscoliosis group, there were 36 patients with an average age of 32.1 years (15-72 years) and the male to female ratio was 1.8:1. A total of 176 screws were assessed using postoperative CT scans. The results are shown in <a href="#TT2" class="ptLink">Table 2</a>. Fifty percent (88 of 176 screws) of the screws were placed within the endosteum of the pedicle. The grade III lateral perforation was seen in 3.41% (6 screws) due to infrequent use of In-Out-In technique in the nonscoliotic spine. About 4.45% (8 of 176 screws) of screw placements were not amenable to assessment due to severe artifacts. Assessment of these screws using the outcome-based classification showed that 86.93% (153 of 176 screws) of the screws were acceptably placed (type I) and 8.52% (15 of 176 screws) had unacceptable placements (type II). There were no type III placements in the nonscoliosis group.

!!! TABLE 2

The combined results of screw placements in both the groups are shown in <a href="#TT3" class="ptLink">Tables 3 and 4</a>.

!!! TABLE 4

Apart from screw misplacements, we encountered 2 screw breakages (0.63%) and 1 pedicle fracture (0.32%). There were 2 patients with neurologic complications in the scoliosis group. One had monoparesis on the right side and another had paresthesias in right lower limb, both of which resolved completely in 6 weeks' duration. No attempt was made to remove the offending screws.

<h3>Discussion</h3>
The introduction of pedicle screw instrumentation is a milestone in the evolution of spinal instrumentation. The 3-column fixation of pedicle screws has been shown to be superior to all other posterior spinal fixation devices.[1,3] However, the use of pedicle screws in the thoracic spine was not encouraged until the pioneering studies by Suk et al,[14] who showed that consistently superior results can be achieved in terms of fixation and deformity correction with the use of thoracic pedicle screws with minimal complications. Today, surgeons across the world use pedicle screws in the thoracic spine for various spinal disorders, including scoliosis, to achieve better fixation and decrease the instrumentation levels. Nevertheless, the postoperative assessment of the pedicle screw placements, by well-known authors, shows that the percentage of misplaced screws in the thoracic spine is quite significant compared with the negligible complications observed. Liljenqvist et al[8] have reported 25% perforation rate in patients with scoliosis, Girtzbein and Robbin[6] reported 29.1% perforations, Vaccaro et al[15] reported that 41% of the 90 screws inserted in fresh cadavers had penetrated the pedicle cortex, and Weinstein et al[16] reported 21% perforations without any neurologic complications. Suk et al[4] reported 27% lateral and 6% medial perforations with 1 transient paresthesia. These studies attribute the low neurologic complications observed to the fact that most of the medial perforations were within the safe zone of 4 mm described by Girtzbein and Robbin.[6] Yalsin and Guven[10] have stated in their article that since neurologic problems are only described with screws perforating the pedicle medially by at least 4 mm, this question has great relevance concerning the definition of pedicular screw malposition. Youkilis et al[17] have mentioned about the structurally significant violations of pedicle screws as 02 mm perforations. In other words, there is a growing tendency to accept up to 2 mm pedicle wall perforations in the light of clinical experience of low complications. This is precisely what the outcome-based classification brings to the forefront. The acceptability of screw malposition depends not only on clinical safety but also on its biomechanical strength. In the light of the biomechanical study by Reichle et al[12] and volumetric intrusion studies by Polly et al,[11] screws with perforations up to 2 mm have been taken as acceptable placements. The screw placements with more than 2 mm perforations have high chances to cause clinical complications and also diminish in their biomechanical strength. Hence, these are placed under type II or unacceptable placements. The advantage of creating the type III placements is to make the results show the clinically observed complications directly (<a href="#TT3" class="ptLink">Tables 3 and 4</a>). Therefore, thoracic screw misplacements can be defined as screw placements causing or having the potential to cause neurovascular complications and/or have inferior biomechanical strength due to their mal-position. Any screw misplacement consistently found to have no clinical complication and having reasonable biomechanical strength can be labeled as acceptable placement. The classic example is the In-Out-In screws that have 070% the strength of the intrapedicular screws[9] and is found to cause no clinical complications. The acceptability criteria for the screws is an issue that is open to debate, and can further be modified with more information from appropriate clinical or biomechanical studies.
The results of this study, using the Rongming Xu et al[5] assessment criteria, show the gross discrepancy between the minimal clinical complications observed and the high rate of screw misplacements (<a href="#TT3" class="ptLink">Tables 3 and 4</a>). The results of the outcome classification for screw placement assessment show a high percentage of acceptable screw placements (<a href="#TT3" class="ptLink">Tables 3 and 4</a>). This correlates better with the low rate of clinical complications observed. The neurologic complications observed in our study, 2 of 24 patients with scoliosis with transient deficit, appears to give a relatively high percentage (8.33%) of neurovascular complications. However, only selected patients affording both pre- and postoperative CT scans and willing to participate were included in this study, making this a nonconsecutive series.
The other major advantage of the outcome classification is that it can bring about uniformity in the results of pedicle screw assessments, when used as a common classification system. Assessment of pedicle screws has been done with various classification systems, with most of the authors stressing the fact that perforations that are 02 mm can be acceptable.[18,19] In a recent meta-analysis[20] on pedicle screw placements, the authors have stated that The identification of 35 different pedicle screw placement assessment methods in our database leads us to the conclusion that a standardized assessment method is needed. The outcome classification, with the concept of acceptable placements, can be a common acceptable classification for expressing the results of screw placements, as we can differentiate between the benign marginal misplacements (acceptable) and dangerous placements (unacceptable). Therefore, the outcome classification can be an answer to the problem of numerous classification grades used for the assessment of pedicle screw placement.

<h3>Conclusion</h3>
Thoracic pedicle screw placement is a technically demanding procedure, but is an invaluable tool for spinal stabilization due to its superior biomechanical strength. The literature shows consensus over high rates of screw misplacement, but low clinical complications, even in the hands of the best of spine surgeons. The present study showed that even with high rates of marginal misplacements, the risk of clinical neurovascular complications is low. The concept of acceptable screw placements makes the thoracic pedicle screw assessment results correlate better with clinical outcome, as we can differentiate between the benign marginal misplacements (acceptable) and those with dangerous placements (unacceptable).

<h3>Key Points</h3>
<ul>
 <li>The main deterrent for the use of thoracic pedicular screws is the feared neurovascular complications due to screw misplacements.</li>
 <li>Only a small fraction of the misplaced screws actually causes any complication.</li>

 <li>Outcome-based classification for the assessment of pedicle screw positions using postoperative CT scan.</li>
 <li>The concept of acceptable screw placements and the outcome classification makes the pedicle screw assessment results correlate better with the clinical outcome.</li>
</ul>

<h3>References</h3>
<ol>
 <li>Liljenqvist U, Hackenberg L, Link T, et al. Pullout strength of pedicle screws versus pedicle and laminar hooks in the thoracic spine. Acta Orthop Belg 2001;67:157-63.</li>
 <li>Belmont PJ Jr, Klemme WR, Robinson M, et al. Accuracy of thoracic pedicle screws in patients with and without coronal plane spinal deformities. Spine 2002;27:1558-66.</li>
 <li>Liljenqvist U, Lepsien U, Hackenberg L, et al. Comparative analysis of pedicle screw and hook instrumentation in posterior correction and fusion of idiopathic thoracic scoliosis. Eur Spine J 2002;11:336-43.</li>
 <li>Suk SI, Kim WJ, Lee SM, et al. Thoracic pedicle screw fixation in spinal deformities: are they really safe? Spine 2001;26:2049-57.</li>
 <li>Xu R, Ebraheim NA, Ou Y, et al. Anatomic considerations of pedicle screw placement in the thoracic spine. Roy-Camille technique versus open-lamina technique. Spine 1998;23:1065-8.</li>
 <li>Gertzbein SD, Robbins SE. Accuracy of pedicular screw placement in vivo. Spine 1990;15:11-4.</li>
 <li>Heini P, Scholl E, Wyler D, et al. Fatal cardiac tamponade associated with posterior spinal instrumentation. A case report. Spine 1998;23:2226-30.</li>
 <li>Liljenqvist UR, Halm HF, Link TM. Pedicle screw instrumentation of the thoracic spine in idiopathic scoliosis. Spine 1997;22:2239-45.</li>
 <li>Belmont PJ Jr, Klemme WR, Dhawan A, et al. In vivo accuracy of thoracic pedicle screws. Spine 2001;26:2340-6.</li>
 <li>Yalcin S, Guven O. Reversible anterior cord syndrome due to penetration of the spinal canal by pedicular screws. Paraplegia 1995;33:423-5.</li>
 <li>Polly DW Jr, Potter BK, Kuklo T, et al. Volumetric spinal canal intrusion: a comparison between thoracic pedicle screws and thoracic hooks. Spine 2004;29:63-9.</li>
 <li>Reichle E, Morlock M, Sellenschloh K, et al. Definition of pedicle malposition. Primary stability and loosening characteristics of pedicle screws in relation to position: spongious anchoring, cortical anchoring, perforation and malposition. Orthopade 2002;31:402-5.</li>
 <li>Kim YJ, Lenke LG, Bridwell KH, et al. Free hand pedicle screw placement in the thoracic spine: is it safe? Spine 2004;29:333-42.</li>
 <li>Suk SI, Lee CK, Kim WJ, et al. Segmental pedicle screw fixation in the treatment of thoracic idiopathic scoliosis. Spine 1995;20:1399-405.</li>
 <li>Vaccaro AR, Rizzolo SJ, Balderston RA, et al. Placement of pedicle screws in the thoracic spine. Part II: an anatomical and radiographic assessment. J Bone Joint Surg Am 1995;77:1200-6.</li>
 <li>Weinstein JN, Spratt KF, Spengler D, et al. Spinal pedicle fixation: reliability and validity of roentgenogram-based assessment and surgical factors on successful screw placement. Spine 1988;13:1012-8.</li>
 <li>Youkilis AS, Quint DJ, McGillicuddy JE, et al. Stereotactic navigation for placement of pedicle screws in the thoracic spine. Neurosurgery 2001;48:771-8, discussion 778-9.</li>
 <li>Kuklo TR, Lenke LG, O0Brien MF, et al. Accuracy and efficacy of thoracic pedicle screws in curves more than 90 degrees. Spine 2005;30:222-6.</li>
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