The Spine Journal 14 (2014) 371–380
Technical Report
The medial cortical pedicle screw—a new technique for cervical pedicle screw placement with partial drilling of medial cortex Bijjawara Mahesh, MS, Bidre Upendra, MS*, Rajkumar Singh Mahan, MBBS Jain Institute of Spine Care And Research (JISAR), Bhagwan Mahaveer Jain Hospital, Vasanth Nagar, Bangalore 560052, India Received 5 January 2013; revised 21 August 2013; accepted 19 September 2013
Abstract
BACKGROUND CONTEXT: Studies on cadavers have shown that the appropriate insertion of cervical pedicle screw (C3–C7) should be done from a more lateral point and at a steeper angle in the axial plane, than that described by Abumi et al., to decrease the chances of lateral perforation. PURPOSE: We describe a new technique for cervical pedicle screw (CPS) placement (C3–C7) using high-speed pneumatic drill with partial drilling of medial cortex for decreasing the chances of lateral perforation. STUDY DESIGN: Description of new surgical technique with retrospective data analysis. PATIENT SAMPLE: Twenty-five patients undergoing cervical spine surgery with CPS instrumentation in the lower cervical spine (C3–C7) from April 2011 to October 2012 at our institute were included in the study. OUTCOME MEASURES: All patients were evaluated with computed tomography scans postoperatively for the assessment of pedicle screw placement. Pedicle screw perforations were graded with the following criteria: Grade I perforations having #50% of the screw outside the pedicle and Grade II perforations having O50% of the screw outside the pedicle. Clinical complications directly related to CPS placement were also recorded. METHODS: Twenty-five patients undergoing surgery with CPS instrumentation (C3–C7) at our institute between April 2011 and October 2012 were included in the study. Thirteen patients had cervical trauma, 10 had cervical spondylotic myelopathy, 1 had congenital cervico-dorsal scoliosis, and 1 was a patient with ankylosing spondylitis. Pedicle screw insertion was made according to the technique by Abumi et al., with the use of blunt pedicle probes in eight of these cases (Group I). In the other 17 cases (Group II), the pilot hole was made with the use of 2-mm diamond tipped burr, partially drilling the medial cortex and entering the vertebral body with the burr itself. RESULTS: A total of 131 CPSs (C3–C7) were inserted in 25 patients. In Group I, 43 pedicle screws were placed and 88 screws were placed in Group II, with partial drilling of medial cortex. Lateral perforations: in Group-I, more of lateral perforations were observed with 18.6% Grade I and 9.3% Grade II lateral perforations. In Group II, the lateral perforations were lower with 7.95% Grade I and 1.1% of Grade II lateral perforations. I Group-I, medial perforations were lower with 11.62% Grade I and 2.3% Grade II perforations. In Group II, the Grade I and Grade II medial perforations were 30.7% and 4.5%, respectively. The lateral perforations were more at C3, C4 levels, and the medial perforation was maximum at C5 level. No clinical neurovascular complications, directly related to screw placements, were seen in either of the groups postoperatively. CONCLUSIONS: With the use of the technique by Abumi et al., more than half of the cervical pedicle screw perforations described are lateral. Use of a blunt pedicle probe usually directs the surgeon toward the lateral cortex as the medial cortex is thicker and stronger. With the new medial cortical pedicle screw technique described, lateral perforations were low. However, surgeons attempting this technique should be aware of the increase in medial perforations experienced by
FDA device/drug status: Not approved for this indication (pedicle screw instrumentation in the lower cervical spine). Author disclosures: BM: Fellowship Support: Medtronic India (A, Paid directly to institution/employer). BU: Nothing to disclose. RSM: Nothing to disclose. 1529-9430/$ - see front matter Ó 2014 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.spinee.2013.09.033
The disclosure key can be found on the Table of Contents and at www. TheSpineJournalOnline.com. * Corresponding author. Jain Institute of Spine Care And Research (JISAR), Bhagwan Mahaveer Jain Hospital, Vasanth Nagar, Bangalore 560052, India. Tel.: þ91 9686803495. E-mail address: [email protected] (B. Upendra)
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the authors with the new technique. The study gives an additional option of technique to be considered by surgeons already using CPS placements in selected patients. Further evaluation for reproducibility of the medial cortical pedicle screw technique by other surgeons and testing of biomechanical strength of the screws is required. Ó 2014 Elsevier Inc. All rights reserved. Keywords:
Cervical pedicle screw; Blunt probe technique; Medial cortical pedicle screw technique; Medial cortical partial drilling; Lateral perforations
Introduction Cervical pedicle screws (CPSs) have been in use for rigid cervical stabilization since its description by Abumi et al. in 1994 [1]. However, it is still not used routinely due to the concerns regarding the screw misplacements and consequent neurovascular injury. Few studies on cadavers have pointed out the high incidence of pedicle screw misplacement in the cervical spine [2–4] (16.8%–87.5%) and have questioned the clinical use of the same when lateral mass screws are found to be sufficient in most situations for cervical spine stabilization. However, clinical results have shown that in selected patients, use of CPSs have minimal screw-related complications [5–7] and have highlighted the advantages of using CPSs due to their superior biomechanical strength [8,9]. At present, efforts are on to evolve a safe and reproducible technique for insertion of CPSs so that the clinical advantages of these screws can be extended to selected patients, requiring rigid stabilization, with minimal complications. The incidence of lateral perforations has been reported to be twice as common as compared with medial perforations [10]. This has been attributed to the fact that medial pedicle wall is thicker than the lateral wall especially from C3 to C5 [11]. Also, the consequences of a lateral perforation in the cervical spine are grievous [12]. There are reports of vertebral artery injury with lateral perforations, but injury to the cervical spinal cord is relatively rare with
marginal medial pedicle perforations [5,10]. Consequently, cadaveric and clinical studies are now concentrating on techniques to avoid lateral perforations by CPSs. Anatomical studies on cervical pedicle indicate that the appropriate insertion point for CPSs is more lateral and at a steeper angle in the axial plane, than that described by Abumi et al. (Fig. 1), which would help to decrease the chances of lateral perforation [13,14]. Clinical studies have found that the incidence of screw misplacements is higher in patients with cervical spondylotic myelopathy (CSM) than in trauma or rheumatoid patients [15]. This has been attributed to the difficulty in negotiating the sclerotic pedicles with the pedicle probe in CSM patients. Furthermore, there is considerable thrust and deformation of the cervical spine during insertion of the pedicle probe into the sclerotic pedicles encountered in spondylotic cervical spine. To decrease the lateral perforations and the force used to probe the sclerotic pedicles in patients with CSM, the authors started to partially drill the medial cortex of the pedicle with the entry point at the center of the lateral mass. The technique was found to be safe and reproducible by the authors and has been used for placement of pedicle screws in other pathologies as well. The purpose of this study was to describe the technique of medial cortical pedicle screw and evaluate the results of CPS placement with this new technique of partial drilling of the medial cortex, using high-speed pneumatic drill.
Methods Twenty-five patients undergoing surgery with CPS instrumentation (C3–C7) at our institute between April Table 1 Comparison of surgical techniques used for cervical pedicle screw insertion in this study
Technique
Fig. 1. The bold parallel lines show the true anatomical axis of the pedicle, and the dotted one shows the trajectory of pedicle screw placement using Abumi et al./blunt probe technique.
Group I Blunt probe (Abumi et al.) technique
Decortication 3–4 mm burr Entry point Lateral to center of lateral mass; 1–2 mm below superior articular margin Pilot hole 2-mm blunt pedicle probe preparation Direction 30–40 deg. medially; cranio-caudal direction-by lateral fluoroscopy
Group-II Medial cortical pedicle screw technique 3–4 mm burr Center of lateral mass; 1–2 mm below superior articular margin 2-mm diamond tipped burr (Midas) 30–35 deg. medially; cranio-caudal direction-by lateral fluoroscopy
P , sum; CS, congenital scoliosis. M, male; F, female; AS, ankylosing spondylitis; TR, trauma; CSM, cervical spondylotic myelopathy; Bolded values represent the sum of all the screws inserted at each level.
2 1 2
C3 C4 C5 C6 C7
0 0 1 2 2
2 1 2 0 0
0 0 0 2 2
2 1 1 2 0
0 0 1 1 0
0
2 2 2 2 2
6 6 11 10 10 43
2 1 1 2 0
0 2 2 2 2
2 1 1 1 2
2 0 0 0 0
0 0 2 2 0
0 0 0 2 2
2 1 2 0 0
0 0 2 2 2
2 2 1 1 2
2 1 2 0 0
2 0 0 0 0
0 2 1 2 0
2 2 2 0 0
0 2 2 2 2
0 0 2 2 0
0 0 2 2 0
0 0 0 2 2
16 14 22 22 14 88 0 2 2 0 2 Level
Fig. 3. Schematic representation showing the screw trajectory with Abumi/blunt probe technique (dotted rectangle) and the 2-mm diamond tipped burr showing the entry point and direction for the medial cortical pedicle screw technique.
Medial cortical pedicle screw technique
Pedicle screw insertion—Abumi et al.’s technique with blunt pedicle probes; eight cases (3 trauma, 3
Table 2 The distribution of cervical pedicle screws according to level and technique of placement
Group I: Blunt probe technique
Blunt probe technique
2011 and October 2012 were included in the study. Thirteen patients had cervical trauma, 10 had CSM, 1 had congenital cervico-dorsal scoliosis, and 1 was a patient with ankylosing spondylitis. Pedicle screw insertion was made according to the technique by Abumi et al., with the use of blunt pedicle probes in eight of these cases (Group I). In the other 17 cases (Group II), the pilot hole was made with the use of 2-mm diamond tipped burr, partially drilling the medial cortex and entering the vertebral body with the burr itself.
54/M 24/M 59/M 36/M 51/M 9/M 43/F 43/M 65/M 79/M 47/M 30/M 80/M 46/F 68/M 27/M 65/M 75/M 29/M 23/M 78/M 60/F 35/F 30/M 20/M P P AS TR CSM TR CSM CS TR CSM CSM CSM CSM TR TR TR CSM TR CSM CSM TR TR CSM TR TR TR TR
Fig. 2. Shows the entry point for pedicle screw insertion in the Abumi technique (black dot-lateral) and the medial cortical pedicle screw technique (transparent dot-medial). The larger dotted oval shows the initial decortication done with 3 to 4 mm cutting burr.
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Age/sex Diagnosis
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B. Mahesh et al. / The Spine Journal 14 (2014) 371–380 Table 3 Lateral screw perforations with blunt probe technique
Fig. 4. Grade I lateral screw perforation with blunt probe technique.
CSM, 1 cervico-dorsal scoliosis, and 1 ankylosing spondylitis)—43 screws. Group II: Medial cortical pedicle screw technique. Pilot hole with 2-mm diamond tipped burr, partially drilling the medial cortex; 17 cases (10 trauma, 7 CSM)—88 screws. Surgical technique A midline skin incision was made, and the posterior elements of the cervical vertebra to be instrumented were exposed subperiosteally. The exposure was carried out far laterally to expose the lateral border of the lateral mass and extended cranially and caudally as required. In the initial cases (Group I), the entry point was chosen as described by Abumi et al., at a point slightly lateral to the center of the lateral mass and in line with the lateral sulcus on the lateral border of the lateral mass. Decortication of the lateral mass was done with 3 to 4mm burr and cancellous area exposed. The cancellous core of the cervical pedicle was identified and a 2-mm blunt pedicle probe was gradually negotiated into the pedicle at an angle on 30 to 40 medially. The cranio-caudal angulation was determined with
Fig. 5. Grade II lateral screw perforation with blunt probe technique.
Level
Grade I (%)
Grade II (%)
C3 C4 C5 C6 C7 Total
2 2 2 2 0 8
2 0 0 2 0 4
(33.33) (33.33) (18.18) (20) (0) (18.60)
(33.33) (0) (0) (20) (0) (9.30)
lateral c-arm guidance. Once the probe reached 15 to 20 mm deep into the pedicle and body, the pilot hole was probed with ball tipped probe to examine for any breech in the pilot hole. Appropriate length screws were inserted after tapping the pilot hole (Table 1). In the later cases (Group II), the authors shifted the entry point slightly medial to that described by Abumi et al., at the center of the lateral mass and 1 to 2 mm below the articular margin of the superior articular process (corresponding to the lateral sulcus) (Fig. 2). After decortication of the lateral mass with a 3 to 4 mm burr, a 2 mm diamond tipped burr was chosen to make the pilot hole for pedicle screw insertion. The burr was directed about 30 to 35 medially, and the cranio-caudal direction was decided by lateral fluoroscopy (Fig. 3). The high-speed burr was introduced into the cancellous core of the pedicle with intentional pressure on the medial cortex at the point of entry into the pedicle. This aided in partially drilling the medial wall and decreased the chances of lateral perforation of the pilot hole. The aim was to use the medial cortical wall as a guide to enter the vertebral body and not perforating it. A ball tipped probe was used to ensure that the walls of the pilot hole were intact in all the directions. Since the screws partially go through the medial cortex of the pedicle, they are called as medial cortical pedicle screws. The pilot hole was tapped and standard polyaxial lateral mass screws (longer lengths) were used as pedicle screws in all the cases. Screws of 3.5 mm were used for C3, C4, and C5 pedicles, and 4.0-mm
Fig. 6. Grade I lateral screw perforation with medial cortical pedicle screw technique.
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Table 4 Lateral screw perforations with medial cortical pedicle screw technique Level
Grade I (%)
Grade II (%)
C3 C4 C5 C6 C7 Total
3 2 2 0 0 7
0 1 0 0 0 1
(18.75) (14.20) (9.09) (0.00) (0.00) (7.95)
(0) (7.14) (0) (0) (0) (1.1)
screws were used for C6 and C7 pedicles. All the screws in the study were inserted by either of the first two authors. All patients were evaluated with computed tomography (CT) scans postoperatively for assessment of pedicle screw placement. Pedicle screw perforations were graded with the following criteria [10]: Grade I perforations having #50% of the screw outside the pedicle and Grade II perforations having O50% of the screw outside the pedicle [10]. Clinical complications directly related to CPS placement were also recorded. Results A total of 131 CPSs (C3–C7) were inserted in 25 patients (Table 2). In Group I, 43 pedicle screws were placed and 88 screws were placed in Group II, with partial drilling of medial cortex. Lateral perforations: in Group I, more of lateral perforations were observed (Figs. 4 and 5) with 18.6% of Grade I and 9.30% of Grade II lateral perforations (Table 3). In Group II, the lateral perforations were lower (Figs. 6, 13, and 14) with 7.95% of Grade I and 1.1% of Grade II lateral perforations (Table 4). Medial perforations: In Group I, medial perforations were lower with 11.62% of Grade I and 2.3% of Grade II perforations (Figs. 7 and 8; Table 5). In Group II, the Grade I and Grade II medial perforations were 30.7% and 4.5%, respectively (Figs. 9 and 10; Table 6). In situation where there was difficulty in assessment of grade of perforation in the axial sections, the authors found that the corresponding coronal slices of the CT scans were helpful in increasing the accuracy for assessing screw perforations, as shown in Figs. 11 and 12.
Fig. 8. Grade II medial screw perforation with blunt probe technique.
The lateral perforations were more at C3, C4 levels, and the medial perforation was maximum at C5 level. Statistical analysis: descriptive and inferential statistical analysis has been carried out in the present study. Results on continuous measurements are presented as mean6standard deviation (Min-Max), and results on categorical measurements are presented in number (%). Student t test (two tailed, independent) has been used to find the significance of study parameters on continuous scale between two groups (intergroup analysis) on metric parameters. The statistical software, namely SAS 9.2 (SAS Institute, Cary, NC, USA), SPSS 15.0 (SPSS, Chicago, IL, USA), Stata 10.1 (Stata, College Station, TX, USA), MedCalc 9.0.1 (MedCalc, Ostend, Belgium), Systat 12.0 (Systat, Chicago, IL, USA), and R environment ver.2.11.1 (available at: http://www.r-project.org/), were used for the analysis of the data, and Microsoft word and Excel (Microsoft, Redmond, WA, USA) have been used to generate tables. Table 7 shows that the reduction in Grade I lateral perforations in Group II as compared with Group I has only suggestive statistical significance, whereas the reduction in Grade II lateral perforations in Group II was found to be statistically significant (p5.009). In medial perforations, Group II had higher Grade I perforations than Group I, which was statistically significant (p5.008), whereas the Grade II medial perforations did not show any statistically significant difference between the two groups. No clinical neuro-vascular complications directly related to the screw placements were seen in either of the groups postoperatively. However, two patients developed postoperative C5 palsy in the CSM group. Both the patients had long segment CPS-rod instrumentation with C3 to C6 decompressive laminectomy. There were no misplaced Table 5 Medial screw perforations with blunt probe technique
Fig. 7. Grade I medial screw perforation with blunt probe technique.
Level
Grade I (%)
Grade II (%)
C3 C4 C5 C6 C7 Total
1 0 3 1 0 5
0 0 1 0 0 1
(16.70) (0) (27.30) (10) (0) (11.62)
(0) (0) (9.09) (0) (0) (2.3)
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Fig. 9. Grade I medial screw perforation with medial cortical pedicle screw technique.
screws, and both the palsies improved partially to functional level in 8 to 10 weeks, as reported in the literature. Discussion The present study indicates that medial cortical pedicle screw technique has relatively lesser rate of lateral perforations (7.95% of Grade I and 1.1% of Grade II) than the conventional technique for CPS insertion. However, the technique has increased rate of medial perforations, most of them being Grade I perforations (30.7%). A recent multicentric study from Japan on complications of CPS placement has shown that, among the misplaced screws, 75% (57/76) had lateral pedicle perforations and only 25% (19/76) constituted medial perforations [10]. The architecture of the cervical pedicle is such that it is predisposed to lateral perforation while pilot hole preparation, tapping, or while insertion of the screw [11]. The use of a blunt pedicle probe usually directs the surgeon toward the lateral cortex as the medial cortex is thicker and stronger than the lateral. Furthermore, the screw trajectory as described by Abumi et al. is at a slightly lesser medial angulation than the actual anatomical axis of the cervical pedicle (Fig. 1), as described by cadaveric studies. This means the screws are aimed to be placed from the medial portion of the pedicle proximally to the lateral portion of the pedicle distally. This increases the chances of the
Fig. 10. Grade II medial screw perforation with medial cortical pedicle screw technique.
Level
Grade I (%)
Grade II (%)
C3 C4 C5 C6 C7 Total
5 3 7 9 3 27
0 0 2 1 1 4
(31.25) (21) (31.80) (40.90) (21.40) (30.70)
(0) (0) (9.09) (4.50) (7.10) (4.50)
proximal part of the medial cortex of the pedicle deflecting the larger diameter pedicle screw (3.5 mm) laterally (Figs. 4 and 15) even when the pilot hole is contained within the pedicle (prepared with 2 mm blunt probe). One of the ways to avoid this deflection is to make a far lateral entry and angulate the probe and screw more medially along the true anatomical axis of the cervical pedicle (Fig. 1). However, this requires additional stab incisions lateral to the paraspinal musculature to avoid the pedicle probe being pushed medially by the paraspinal musculature [16]. The difficulty becomes compounded in patients with CSM who usually have sclerotic pedicles. Literature [15] shows a very high incidence of screw perforations (37.5%) in spondylotic myelopathy patients. The central cancellous core is either absent or very minimal for the blunt probe to be negotiated through the pedicle and also it requires a lot of force to make way through these sclerotic pedicles. This in turn increases the deformation of the cervical spine and transmits undue forces to the already compromised cervical spinal cord. It was during such difficulty faced in negotiating the blunt probe into the sclerotic pedicle, that the authors used a 2-mm diamond tipped burr to make the pilot hole for pedicle screw insertion. Since the risk of lateral perforation was perceived to be higher, the authors deliberately used the drill with more pressure toward the medial side, partially drilling the medial cortex. This reduced the force required to make the pilot hole
Fig. 11. Coronal section along the center of vertebral foramen showing the extent of lateral breech by the screw. This breech was taken as a Grade I lateral perforation, as more than half of the screw diameter was not outside the lateral wall.
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Fig. 12. Coronal section along the pedicle isthmus showing the extent of medial breech by the screw. This was taken as a Grade I medial perforation, as less than half of the screw diameter was outside the medial wall.
and consequently decreased the deformation of the cervical spine and the force transmitted to the spinal cord. The results of the present study show that partial drilling of medial cortex can decrease the chances of lateral perforations. There were no Grade II lateral perforations seen with the medial cortical pedicle screw technique till recently when one of the patients was found to have a significant breech of lateral wall with Grade II lateral perforation at C4 level. But there were no clinical complications related to the screw misplacement. On careful recollection, the authors were reasonably sure that the screw pilot hole was well contained within the pedicle as examined by ball point probe; the authors had then enlarged the entry point with a 3-mm burr to facilitate the engagement of the initial screw threads (Fig. 13). The surgeons had not used the tap before screw insertion in this instance. As depicted in Fig. 13, the proximal enlargement of the pilot hole was made more to the medial side of the pilot hole, which deflected the screw laterally as it approached the medial cortex, resulting in Grade II lateral perforation. Therefore, it is advisable to tap the pilot hole past the pedicle before screw insertion. Figure 14 shows the coronal section along the vertebral canal of the same screw. There were significantly more medial perforations in the new technique, although most of them being Grade I perforations (30.7%) due to partial drilling of medial cortex.
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Fig. 13. This shows the lone Grade II lateral perforation with medial cortical pedicle screw technique on the left side with pilot holes and the proximal hole enlargement, which led to the lateral screw deflection.
Partial breach of medial pedicle cortex by screw threads in our patients had no clinical consequences, may be due to the fact that dura or cervical roots are not in contact with medial cortex in cervical spine [17]. Anatomical studies have shown that the cervical roots exit the spinal canal at an angle of 45 anteriorly and 10 caudally, and the root is lodged in the lower part of the neural foramen [18,19]. This in effect means that the medial and inferior aspect of the cervical pedicle is not in contact with any vital structures with about 2 to 6 mm of safe zone described medial to the cervical pedicle [17]. However, it should not be considered that medial perforations are always benign; a Grade II medial perforation can be more catastrophic than a lateral perforation with spinal cord injury. It has to be emphasized that surgeons should not accept Grade I medial perforations as normal
Table 7 Comparison of mean screws in the two groups studied (mean6SD) Perforations Lateral perforations Lateral perforations Medial perforations Medial perforations Mean total screws
Grade Grade Grade Grade
I II I II
Group I
Group II
p Value
1.0060.93 0.5060.54 0.6360.74 0.1360.35 5.3862.26
0.4160.51 0.0660.24 1.5960.79 0.2460.44 5.1861.88
.050* .009y .008y .540 .819
SD, standard deviation. * Suggestive significance (p value: .05!p!.10). y Strongly significant (p value: p#.01).
Fig. 14. The coronal section of the computed tomography scan in Fig. 13 shows the residual vertebral canal space available with lateral perforation of the screw. Note a similar encroachment of the vertebral canal on the right side with uncovertebral osteophytes.
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Table 8 Clinical and cadaveric studies showing cervical pedicle screw perforation rates Results (%)
Study
Type of study
No. of patients/screws
Surgical technique
In
Abumi et al. (2000) [5]
Clinical
n5180 (669 screws)
93.3
Kotani et al. (2003) [20] Ludwig et al. (2000) [21]
Clinical Cadaveric study
n517
Kast et al. (2006) [22] Kimimura et al. (2000) [23]
Clinical Cadaveric study
n526
Free-handþlateral fluoro CT-based navigation CT-based navigation Free-handþlateral fluoro CT-based navigation CT-based navigation
Grade I/ non-critical breach
Grade II/ critical breach
6.7
99 52 60
1 30 28
0 18 12
70 77
21 13
9 10
CT, computed tomography.
and begin to aim at placing CPSs with Grade I perforation medially while screw insertion itself. This will then increase the chances of Grade II perforations significantly. The authors have experienced a significant rate of Grade I medial perforations having aimed at intrapedicular placement and after having verified the intact medial cortex before screw insertion. None of the screw placements in this study were made when the medial cortex was found to be breeched with the pilot hole made by 2-mm burr. Placement of screw, while having a breeched medial pedicle cortex with the pilot hole (2 mm), will usually lead to Grade II medial perforations after screw insertion (3.5 mm), with increased potential for spinal cord injury. Literature shows wide range of screw misplacement rate from 6.7% to 29% with the free-hand technique of CPS insertion [5,20]. This has prompted use of assisted techniques
for CPS insertion to reduce the rates of perforations and bring consistency in screw placement. With the use of navigation systems [21], relatively lesser perforation rates have been described. However, the results have not been reproducible consistently and there are studies showing more perforations with navigated screws than with the free-hand technique [20,22] (Table 8). In general, the cadaveric studies with or without navigation have shown more screw misplacements than clinical studies [23]. Furthermore, all the screw insertion techniques used so far have had more of lateral perforations than medial perforations [10]. In contrast, the present study shows more of medial perforations than lateral, with the new technique described. This becomes important with regard to clinical outcome, as there are reports of late cerebral infarctions due to laterally perforated CPS [12], although there were no immediate postoperative
Fig. 15. Axial computed tomography scan sections of screw placements using the blunt probe (Abumi et al.) technique. Note the tendency of the thick medial cortex to push the screw laterally.
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Fig. 16. Axial computed tomography scan sections of screw placements using the medial cortical pedicle screw technique. Note the effect of partial drilling of the thick medial cortex, which allows the screw to be placed more medially in the vertebral body and consequently avoiding screw perforation laterally.
clinical complications with screws breeching the vertebral foramen. The study is limited in the sense that the evaluation of pedicle screw placements were done only in the mediolateral plane, and the supero-inferior displacements were not assessed. Furthermore, all the screw placements were evaluated by one of the authors involved in the surgery, which could have led to bias in evaluation. In addition, the results of this study need to be considered in relation to the relatively small number of screw placements evaluated in a heterogeneous group of patients. In this regard, the reproducibility of this technique with its proposed safety needs to be evaluated with a larger multicentric study involving multiple surgeons and comparable patient groups. However, the purpose of this study was to share a new surgical technique for CPS insertion and encourage fellow surgeons to evaluate the technique for its reproducibility and effectiveness in preventing lateral perforations. In this regard, the two figures (Figs. 15 and 16) showing the CT scan axial sections of the screw placement by the
conventional technique and the medial cortical pedicle screw technique make the essence of the study. The study does not intend to propagate the use of CPSs in place where lateral mass screws are sufficiently giving good clinical results. The study gives an additional option of techniques to be considered by surgeons already using CPS placements in selected patients. Conclusion Cervical pedicle screw placement using medial cortical pedicle screw technique is relatively safe and decreases the incidence of lateral perforations as compared with the use of a blunt pedicle probe. However, surgeons attempting this technique should be aware of the increase in medial perforations experienced by the authors. Furthermore, the use of the high-speed burr decreases the force transmitted to the cervical spine while preparation of the pilot hole, which is highly desirable in patients with unstable cervical spine. Further evaluation for reproducibility of the medial
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cortical pedicle screw technique by other surgeons and testing of biomechanical strength of the screws is required. References [1] Abumi K, Ito H, Taneichi H, Kaneda K. Transpedicular screw fixations for traumatic lesions of the middle and lower cervical spine: description of the techniques and preliminary report. J Spinal Disord 1994;7:19–28. [2] Karaikovic EE, Yingsakmongkol W, Gaines RW. Accuracy of cervical pedicle screw placement using the funnel technique. Spine 2001;26:2456–62. [3] Ludwig SC, Kramer DL, Balderston RA, et al. Placement of pedicle screws in the human cadaveric cervical spine. Comparative accuracy of three techniques. Spine 2000;25:1655–67. [4] Miller RM, Ebraheim NA, Xu R, et al. Anatomic consideration of transpedicular screw placement in the cervical spine: an analysis of two approaches. Spine 1996;21:2317–22. [5] Abumi K, Shono Y, Ito M, et al. Complications of pedicle screw fixation in reconstructive surgery of the cervical spine. Spine 2000;25:962–9. [6] Yukawa Y, Kato F, Yoshihara H, et al. Cervical pedicle screw fixation for 100 cases of unstable cervical injuries using pedicle axis views by fluoroscopy. J Neurosurg Spine 2006;5:488–93. [7] Abumi K, Kaneda K. Pedicle screw fixation for nontraumatic lesions of the cervical spine. Spine 1997;22:1853–63. [8] Jones EL, Heller JG, Silcox DH, Hutton WC. Cervical pedicle screws versus lateral mass screws: anatomic feasibility and biomechanical comparison. Spine 1997;22:977–82. [9] Kotani Y, Cunningham BW, Abumi K, McAfee PC. Biomechanical analysis of cervical stabilization systems: an assessment of transpedicular screw fixation in the cervical spine. Spine 1994;19:2529–39. [10] Nakashima H, Yukawa Y, Imagama S, et al. Complications of cervical pedicle screw fixation for nontraumatic lesions: a multicenter study of 84 patients. J Neurosurg Spine 2012;16:238–47. [11] Panjabi MM, Shin EK, Chen NC, Wang JL. Internal morphology of human cervical pedicles. Spine 2000;25:1197–205.
[12] Onishi E, Sekimoto Y, Fukumitsu R, et al. Cerebral infarction due to an embolism after cervical pedicle screw fixation. Spine 2010;35: E63–6. [13] Onibokun A, Khoo LT, Bistazzoni S, et al. Anatomical considerations for cervical pedicle screw insertion: the use of multiplanar computerized tomography measurements in 122 consecutive clinical cases. Spine J 2009;9:729–34. [14] Sakamoto T, Neo M, Nakamura T. Transpedicular screw placement evaluated by axial computed tomography of the cervical pedicle. Spine 2004;29:2510–5. [15] Uehara M, Takahashi J, Hirabayashi H, et al. Perforation rates of cervical pedicle screw insertion by disease and vertebral level. Open Orthop J 2010;4:142–6. [16] Reinhold M, Bach C, Audige L, et al. Comparison of two novel fluoroscopy-based stereotactic methods for cervical pedicle screw placement and review of the literature. Eur Spine J 2008;17: 564–75. [17] Huang D, Du K, Zeng S, et al. The security analysis of transpedicular screw fixation in the lower cervical spine and a case report. Spine 2011;36:E1702–8. [18] Daniels DL, Hyde JS, Kneeland J, et al. The cervical nerves and foramina: local-coil MRI imaging. AJNR Am J Neuroradiol 1986;7: 129–33. [19] Pech P, Daniels DL, Williams AL, Haughton VM. The cervical neural foramina: correlation of microtomy and CT anatomy. Radiology 1985;155:143–6. [20] Kast E, Mohr K, Richter HP, Borm W. Complications of transpedicular screw fixation in the cervical spine. Eur Spine J 2006;15:327–34. [21] Kotani Y, Abumi K, Ito M, Minami A. Improved accuracy of computerassisted cervical pedicle screw insertion. J Neurosurg 2003;99(3 Suppl): 257–63. [22] Ludwig SC, Kowalski JM, Edwards CC, Heller JG. Cervical pedicle screws: comparative accuracy of two insertion techniques. Spine 2000;25:2675–81. [23] Kamimura M, Ebara S, Itoh H, et al. Cervical pedicle screw insertion: assessment of safety and accuracy with computer-assisted image guidance. J Spinal Disord 2000;13:218–24.
Technical Report
The medial cortical pedicle screw—a new technique for cervical pedicle screw placement with partial drilling of medial cortex Bijjawara Mahesh, MS, Bidre Upendra, MS*, Rajkumar Singh Mahan, MBBS Jain Institute of Spine Care And Research (JISAR), Bhagwan Mahaveer Jain Hospital, Vasanth Nagar, Bangalore 560052, India Received 5 January 2013; revised 21 August 2013; accepted 19 September 2013
Abstract
BACKGROUND CONTEXT: Studies on cadavers have shown that the appropriate insertion of cervical pedicle screw (C3–C7) should be done from a more lateral point and at a steeper angle in the axial plane, than that described by Abumi et al., to decrease the chances of lateral perforation. PURPOSE: We describe a new technique for cervical pedicle screw (CPS) placement (C3–C7) using high-speed pneumatic drill with partial drilling of medial cortex for decreasing the chances of lateral perforation. STUDY DESIGN: Description of new surgical technique with retrospective data analysis. PATIENT SAMPLE: Twenty-five patients undergoing cervical spine surgery with CPS instrumentation in the lower cervical spine (C3–C7) from April 2011 to October 2012 at our institute were included in the study. OUTCOME MEASURES: All patients were evaluated with computed tomography scans postoperatively for the assessment of pedicle screw placement. Pedicle screw perforations were graded with the following criteria: Grade I perforations having #50% of the screw outside the pedicle and Grade II perforations having O50% of the screw outside the pedicle. Clinical complications directly related to CPS placement were also recorded. METHODS: Twenty-five patients undergoing surgery with CPS instrumentation (C3–C7) at our institute between April 2011 and October 2012 were included in the study. Thirteen patients had cervical trauma, 10 had cervical spondylotic myelopathy, 1 had congenital cervico-dorsal scoliosis, and 1 was a patient with ankylosing spondylitis. Pedicle screw insertion was made according to the technique by Abumi et al., with the use of blunt pedicle probes in eight of these cases (Group I). In the other 17 cases (Group II), the pilot hole was made with the use of 2-mm diamond tipped burr, partially drilling the medial cortex and entering the vertebral body with the burr itself. RESULTS: A total of 131 CPSs (C3–C7) were inserted in 25 patients. In Group I, 43 pedicle screws were placed and 88 screws were placed in Group II, with partial drilling of medial cortex. Lateral perforations: in Group-I, more of lateral perforations were observed with 18.6% Grade I and 9.3% Grade II lateral perforations. In Group II, the lateral perforations were lower with 7.95% Grade I and 1.1% of Grade II lateral perforations. I Group-I, medial perforations were lower with 11.62% Grade I and 2.3% Grade II perforations. In Group II, the Grade I and Grade II medial perforations were 30.7% and 4.5%, respectively. The lateral perforations were more at C3, C4 levels, and the medial perforation was maximum at C5 level. No clinical neurovascular complications, directly related to screw placements, were seen in either of the groups postoperatively. CONCLUSIONS: With the use of the technique by Abumi et al., more than half of the cervical pedicle screw perforations described are lateral. Use of a blunt pedicle probe usually directs the surgeon toward the lateral cortex as the medial cortex is thicker and stronger. With the new medial cortical pedicle screw technique described, lateral perforations were low. However, surgeons attempting this technique should be aware of the increase in medial perforations experienced by
FDA device/drug status: Not approved for this indication (pedicle screw instrumentation in the lower cervical spine). Author disclosures: BM: Fellowship Support: Medtronic India (A, Paid directly to institution/employer). BU: Nothing to disclose. RSM: Nothing to disclose. 1529-9430/$ - see front matter Ó 2014 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.spinee.2013.09.033
The disclosure key can be found on the Table of Contents and at www. TheSpineJournalOnline.com. * Corresponding author. Jain Institute of Spine Care And Research (JISAR), Bhagwan Mahaveer Jain Hospital, Vasanth Nagar, Bangalore 560052, India. Tel.: þ91 9686803495. E-mail address: [email protected] (B. Upendra)
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the authors with the new technique. The study gives an additional option of technique to be considered by surgeons already using CPS placements in selected patients. Further evaluation for reproducibility of the medial cortical pedicle screw technique by other surgeons and testing of biomechanical strength of the screws is required. Ó 2014 Elsevier Inc. All rights reserved. Keywords:
Cervical pedicle screw; Blunt probe technique; Medial cortical pedicle screw technique; Medial cortical partial drilling; Lateral perforations
Introduction Cervical pedicle screws (CPSs) have been in use for rigid cervical stabilization since its description by Abumi et al. in 1994 [1]. However, it is still not used routinely due to the concerns regarding the screw misplacements and consequent neurovascular injury. Few studies on cadavers have pointed out the high incidence of pedicle screw misplacement in the cervical spine [2–4] (16.8%–87.5%) and have questioned the clinical use of the same when lateral mass screws are found to be sufficient in most situations for cervical spine stabilization. However, clinical results have shown that in selected patients, use of CPSs have minimal screw-related complications [5–7] and have highlighted the advantages of using CPSs due to their superior biomechanical strength [8,9]. At present, efforts are on to evolve a safe and reproducible technique for insertion of CPSs so that the clinical advantages of these screws can be extended to selected patients, requiring rigid stabilization, with minimal complications. The incidence of lateral perforations has been reported to be twice as common as compared with medial perforations [10]. This has been attributed to the fact that medial pedicle wall is thicker than the lateral wall especially from C3 to C5 [11]. Also, the consequences of a lateral perforation in the cervical spine are grievous [12]. There are reports of vertebral artery injury with lateral perforations, but injury to the cervical spinal cord is relatively rare with
marginal medial pedicle perforations [5,10]. Consequently, cadaveric and clinical studies are now concentrating on techniques to avoid lateral perforations by CPSs. Anatomical studies on cervical pedicle indicate that the appropriate insertion point for CPSs is more lateral and at a steeper angle in the axial plane, than that described by Abumi et al. (Fig. 1), which would help to decrease the chances of lateral perforation [13,14]. Clinical studies have found that the incidence of screw misplacements is higher in patients with cervical spondylotic myelopathy (CSM) than in trauma or rheumatoid patients [15]. This has been attributed to the difficulty in negotiating the sclerotic pedicles with the pedicle probe in CSM patients. Furthermore, there is considerable thrust and deformation of the cervical spine during insertion of the pedicle probe into the sclerotic pedicles encountered in spondylotic cervical spine. To decrease the lateral perforations and the force used to probe the sclerotic pedicles in patients with CSM, the authors started to partially drill the medial cortex of the pedicle with the entry point at the center of the lateral mass. The technique was found to be safe and reproducible by the authors and has been used for placement of pedicle screws in other pathologies as well. The purpose of this study was to describe the technique of medial cortical pedicle screw and evaluate the results of CPS placement with this new technique of partial drilling of the medial cortex, using high-speed pneumatic drill.
Methods Twenty-five patients undergoing surgery with CPS instrumentation (C3–C7) at our institute between April Table 1 Comparison of surgical techniques used for cervical pedicle screw insertion in this study
Technique
Fig. 1. The bold parallel lines show the true anatomical axis of the pedicle, and the dotted one shows the trajectory of pedicle screw placement using Abumi et al./blunt probe technique.
Group I Blunt probe (Abumi et al.) technique
Decortication 3–4 mm burr Entry point Lateral to center of lateral mass; 1–2 mm below superior articular margin Pilot hole 2-mm blunt pedicle probe preparation Direction 30–40 deg. medially; cranio-caudal direction-by lateral fluoroscopy
Group-II Medial cortical pedicle screw technique 3–4 mm burr Center of lateral mass; 1–2 mm below superior articular margin 2-mm diamond tipped burr (Midas) 30–35 deg. medially; cranio-caudal direction-by lateral fluoroscopy
P , sum; CS, congenital scoliosis. M, male; F, female; AS, ankylosing spondylitis; TR, trauma; CSM, cervical spondylotic myelopathy; Bolded values represent the sum of all the screws inserted at each level.
2 1 2
C3 C4 C5 C6 C7
0 0 1 2 2
2 1 2 0 0
0 0 0 2 2
2 1 1 2 0
0 0 1 1 0
0
2 2 2 2 2
6 6 11 10 10 43
2 1 1 2 0
0 2 2 2 2
2 1 1 1 2
2 0 0 0 0
0 0 2 2 0
0 0 0 2 2
2 1 2 0 0
0 0 2 2 2
2 2 1 1 2
2 1 2 0 0
2 0 0 0 0
0 2 1 2 0
2 2 2 0 0
0 2 2 2 2
0 0 2 2 0
0 0 2 2 0
0 0 0 2 2
16 14 22 22 14 88 0 2 2 0 2 Level
Fig. 3. Schematic representation showing the screw trajectory with Abumi/blunt probe technique (dotted rectangle) and the 2-mm diamond tipped burr showing the entry point and direction for the medial cortical pedicle screw technique.
Medial cortical pedicle screw technique
Pedicle screw insertion—Abumi et al.’s technique with blunt pedicle probes; eight cases (3 trauma, 3
Table 2 The distribution of cervical pedicle screws according to level and technique of placement
Group I: Blunt probe technique
Blunt probe technique
2011 and October 2012 were included in the study. Thirteen patients had cervical trauma, 10 had CSM, 1 had congenital cervico-dorsal scoliosis, and 1 was a patient with ankylosing spondylitis. Pedicle screw insertion was made according to the technique by Abumi et al., with the use of blunt pedicle probes in eight of these cases (Group I). In the other 17 cases (Group II), the pilot hole was made with the use of 2-mm diamond tipped burr, partially drilling the medial cortex and entering the vertebral body with the burr itself.
54/M 24/M 59/M 36/M 51/M 9/M 43/F 43/M 65/M 79/M 47/M 30/M 80/M 46/F 68/M 27/M 65/M 75/M 29/M 23/M 78/M 60/F 35/F 30/M 20/M P P AS TR CSM TR CSM CS TR CSM CSM CSM CSM TR TR TR CSM TR CSM CSM TR TR CSM TR TR TR TR
Fig. 2. Shows the entry point for pedicle screw insertion in the Abumi technique (black dot-lateral) and the medial cortical pedicle screw technique (transparent dot-medial). The larger dotted oval shows the initial decortication done with 3 to 4 mm cutting burr.
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Age/sex Diagnosis
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B. Mahesh et al. / The Spine Journal 14 (2014) 371–380 Table 3 Lateral screw perforations with blunt probe technique
Fig. 4. Grade I lateral screw perforation with blunt probe technique.
CSM, 1 cervico-dorsal scoliosis, and 1 ankylosing spondylitis)—43 screws. Group II: Medial cortical pedicle screw technique. Pilot hole with 2-mm diamond tipped burr, partially drilling the medial cortex; 17 cases (10 trauma, 7 CSM)—88 screws. Surgical technique A midline skin incision was made, and the posterior elements of the cervical vertebra to be instrumented were exposed subperiosteally. The exposure was carried out far laterally to expose the lateral border of the lateral mass and extended cranially and caudally as required. In the initial cases (Group I), the entry point was chosen as described by Abumi et al., at a point slightly lateral to the center of the lateral mass and in line with the lateral sulcus on the lateral border of the lateral mass. Decortication of the lateral mass was done with 3 to 4mm burr and cancellous area exposed. The cancellous core of the cervical pedicle was identified and a 2-mm blunt pedicle probe was gradually negotiated into the pedicle at an angle on 30 to 40 medially. The cranio-caudal angulation was determined with
Fig. 5. Grade II lateral screw perforation with blunt probe technique.
Level
Grade I (%)
Grade II (%)
C3 C4 C5 C6 C7 Total
2 2 2 2 0 8
2 0 0 2 0 4
(33.33) (33.33) (18.18) (20) (0) (18.60)
(33.33) (0) (0) (20) (0) (9.30)
lateral c-arm guidance. Once the probe reached 15 to 20 mm deep into the pedicle and body, the pilot hole was probed with ball tipped probe to examine for any breech in the pilot hole. Appropriate length screws were inserted after tapping the pilot hole (Table 1). In the later cases (Group II), the authors shifted the entry point slightly medial to that described by Abumi et al., at the center of the lateral mass and 1 to 2 mm below the articular margin of the superior articular process (corresponding to the lateral sulcus) (Fig. 2). After decortication of the lateral mass with a 3 to 4 mm burr, a 2 mm diamond tipped burr was chosen to make the pilot hole for pedicle screw insertion. The burr was directed about 30 to 35 medially, and the cranio-caudal direction was decided by lateral fluoroscopy (Fig. 3). The high-speed burr was introduced into the cancellous core of the pedicle with intentional pressure on the medial cortex at the point of entry into the pedicle. This aided in partially drilling the medial wall and decreased the chances of lateral perforation of the pilot hole. The aim was to use the medial cortical wall as a guide to enter the vertebral body and not perforating it. A ball tipped probe was used to ensure that the walls of the pilot hole were intact in all the directions. Since the screws partially go through the medial cortex of the pedicle, they are called as medial cortical pedicle screws. The pilot hole was tapped and standard polyaxial lateral mass screws (longer lengths) were used as pedicle screws in all the cases. Screws of 3.5 mm were used for C3, C4, and C5 pedicles, and 4.0-mm
Fig. 6. Grade I lateral screw perforation with medial cortical pedicle screw technique.
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Table 4 Lateral screw perforations with medial cortical pedicle screw technique Level
Grade I (%)
Grade II (%)
C3 C4 C5 C6 C7 Total
3 2 2 0 0 7
0 1 0 0 0 1
(18.75) (14.20) (9.09) (0.00) (0.00) (7.95)
(0) (7.14) (0) (0) (0) (1.1)
screws were used for C6 and C7 pedicles. All the screws in the study were inserted by either of the first two authors. All patients were evaluated with computed tomography (CT) scans postoperatively for assessment of pedicle screw placement. Pedicle screw perforations were graded with the following criteria [10]: Grade I perforations having #50% of the screw outside the pedicle and Grade II perforations having O50% of the screw outside the pedicle [10]. Clinical complications directly related to CPS placement were also recorded. Results A total of 131 CPSs (C3–C7) were inserted in 25 patients (Table 2). In Group I, 43 pedicle screws were placed and 88 screws were placed in Group II, with partial drilling of medial cortex. Lateral perforations: in Group I, more of lateral perforations were observed (Figs. 4 and 5) with 18.6% of Grade I and 9.30% of Grade II lateral perforations (Table 3). In Group II, the lateral perforations were lower (Figs. 6, 13, and 14) with 7.95% of Grade I and 1.1% of Grade II lateral perforations (Table 4). Medial perforations: In Group I, medial perforations were lower with 11.62% of Grade I and 2.3% of Grade II perforations (Figs. 7 and 8; Table 5). In Group II, the Grade I and Grade II medial perforations were 30.7% and 4.5%, respectively (Figs. 9 and 10; Table 6). In situation where there was difficulty in assessment of grade of perforation in the axial sections, the authors found that the corresponding coronal slices of the CT scans were helpful in increasing the accuracy for assessing screw perforations, as shown in Figs. 11 and 12.
Fig. 8. Grade II medial screw perforation with blunt probe technique.
The lateral perforations were more at C3, C4 levels, and the medial perforation was maximum at C5 level. Statistical analysis: descriptive and inferential statistical analysis has been carried out in the present study. Results on continuous measurements are presented as mean6standard deviation (Min-Max), and results on categorical measurements are presented in number (%). Student t test (two tailed, independent) has been used to find the significance of study parameters on continuous scale between two groups (intergroup analysis) on metric parameters. The statistical software, namely SAS 9.2 (SAS Institute, Cary, NC, USA), SPSS 15.0 (SPSS, Chicago, IL, USA), Stata 10.1 (Stata, College Station, TX, USA), MedCalc 9.0.1 (MedCalc, Ostend, Belgium), Systat 12.0 (Systat, Chicago, IL, USA), and R environment ver.2.11.1 (available at: http://www.r-project.org/), were used for the analysis of the data, and Microsoft word and Excel (Microsoft, Redmond, WA, USA) have been used to generate tables. Table 7 shows that the reduction in Grade I lateral perforations in Group II as compared with Group I has only suggestive statistical significance, whereas the reduction in Grade II lateral perforations in Group II was found to be statistically significant (p5.009). In medial perforations, Group II had higher Grade I perforations than Group I, which was statistically significant (p5.008), whereas the Grade II medial perforations did not show any statistically significant difference between the two groups. No clinical neuro-vascular complications directly related to the screw placements were seen in either of the groups postoperatively. However, two patients developed postoperative C5 palsy in the CSM group. Both the patients had long segment CPS-rod instrumentation with C3 to C6 decompressive laminectomy. There were no misplaced Table 5 Medial screw perforations with blunt probe technique
Fig. 7. Grade I medial screw perforation with blunt probe technique.
Level
Grade I (%)
Grade II (%)
C3 C4 C5 C6 C7 Total
1 0 3 1 0 5
0 0 1 0 0 1
(16.70) (0) (27.30) (10) (0) (11.62)
(0) (0) (9.09) (0) (0) (2.3)
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Fig. 9. Grade I medial screw perforation with medial cortical pedicle screw technique.
screws, and both the palsies improved partially to functional level in 8 to 10 weeks, as reported in the literature. Discussion The present study indicates that medial cortical pedicle screw technique has relatively lesser rate of lateral perforations (7.95% of Grade I and 1.1% of Grade II) than the conventional technique for CPS insertion. However, the technique has increased rate of medial perforations, most of them being Grade I perforations (30.7%). A recent multicentric study from Japan on complications of CPS placement has shown that, among the misplaced screws, 75% (57/76) had lateral pedicle perforations and only 25% (19/76) constituted medial perforations [10]. The architecture of the cervical pedicle is such that it is predisposed to lateral perforation while pilot hole preparation, tapping, or while insertion of the screw [11]. The use of a blunt pedicle probe usually directs the surgeon toward the lateral cortex as the medial cortex is thicker and stronger than the lateral. Furthermore, the screw trajectory as described by Abumi et al. is at a slightly lesser medial angulation than the actual anatomical axis of the cervical pedicle (Fig. 1), as described by cadaveric studies. This means the screws are aimed to be placed from the medial portion of the pedicle proximally to the lateral portion of the pedicle distally. This increases the chances of the
Fig. 10. Grade II medial screw perforation with medial cortical pedicle screw technique.
Level
Grade I (%)
Grade II (%)
C3 C4 C5 C6 C7 Total
5 3 7 9 3 27
0 0 2 1 1 4
(31.25) (21) (31.80) (40.90) (21.40) (30.70)
(0) (0) (9.09) (4.50) (7.10) (4.50)
proximal part of the medial cortex of the pedicle deflecting the larger diameter pedicle screw (3.5 mm) laterally (Figs. 4 and 15) even when the pilot hole is contained within the pedicle (prepared with 2 mm blunt probe). One of the ways to avoid this deflection is to make a far lateral entry and angulate the probe and screw more medially along the true anatomical axis of the cervical pedicle (Fig. 1). However, this requires additional stab incisions lateral to the paraspinal musculature to avoid the pedicle probe being pushed medially by the paraspinal musculature [16]. The difficulty becomes compounded in patients with CSM who usually have sclerotic pedicles. Literature [15] shows a very high incidence of screw perforations (37.5%) in spondylotic myelopathy patients. The central cancellous core is either absent or very minimal for the blunt probe to be negotiated through the pedicle and also it requires a lot of force to make way through these sclerotic pedicles. This in turn increases the deformation of the cervical spine and transmits undue forces to the already compromised cervical spinal cord. It was during such difficulty faced in negotiating the blunt probe into the sclerotic pedicle, that the authors used a 2-mm diamond tipped burr to make the pilot hole for pedicle screw insertion. Since the risk of lateral perforation was perceived to be higher, the authors deliberately used the drill with more pressure toward the medial side, partially drilling the medial cortex. This reduced the force required to make the pilot hole
Fig. 11. Coronal section along the center of vertebral foramen showing the extent of lateral breech by the screw. This breech was taken as a Grade I lateral perforation, as more than half of the screw diameter was not outside the lateral wall.
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Fig. 12. Coronal section along the pedicle isthmus showing the extent of medial breech by the screw. This was taken as a Grade I medial perforation, as less than half of the screw diameter was outside the medial wall.
and consequently decreased the deformation of the cervical spine and the force transmitted to the spinal cord. The results of the present study show that partial drilling of medial cortex can decrease the chances of lateral perforations. There were no Grade II lateral perforations seen with the medial cortical pedicle screw technique till recently when one of the patients was found to have a significant breech of lateral wall with Grade II lateral perforation at C4 level. But there were no clinical complications related to the screw misplacement. On careful recollection, the authors were reasonably sure that the screw pilot hole was well contained within the pedicle as examined by ball point probe; the authors had then enlarged the entry point with a 3-mm burr to facilitate the engagement of the initial screw threads (Fig. 13). The surgeons had not used the tap before screw insertion in this instance. As depicted in Fig. 13, the proximal enlargement of the pilot hole was made more to the medial side of the pilot hole, which deflected the screw laterally as it approached the medial cortex, resulting in Grade II lateral perforation. Therefore, it is advisable to tap the pilot hole past the pedicle before screw insertion. Figure 14 shows the coronal section along the vertebral canal of the same screw. There were significantly more medial perforations in the new technique, although most of them being Grade I perforations (30.7%) due to partial drilling of medial cortex.
377
Fig. 13. This shows the lone Grade II lateral perforation with medial cortical pedicle screw technique on the left side with pilot holes and the proximal hole enlargement, which led to the lateral screw deflection.
Partial breach of medial pedicle cortex by screw threads in our patients had no clinical consequences, may be due to the fact that dura or cervical roots are not in contact with medial cortex in cervical spine [17]. Anatomical studies have shown that the cervical roots exit the spinal canal at an angle of 45 anteriorly and 10 caudally, and the root is lodged in the lower part of the neural foramen [18,19]. This in effect means that the medial and inferior aspect of the cervical pedicle is not in contact with any vital structures with about 2 to 6 mm of safe zone described medial to the cervical pedicle [17]. However, it should not be considered that medial perforations are always benign; a Grade II medial perforation can be more catastrophic than a lateral perforation with spinal cord injury. It has to be emphasized that surgeons should not accept Grade I medial perforations as normal
Table 7 Comparison of mean screws in the two groups studied (mean6SD) Perforations Lateral perforations Lateral perforations Medial perforations Medial perforations Mean total screws
Grade Grade Grade Grade
I II I II
Group I
Group II
p Value
1.0060.93 0.5060.54 0.6360.74 0.1360.35 5.3862.26
0.4160.51 0.0660.24 1.5960.79 0.2460.44 5.1861.88
.050* .009y .008y .540 .819
SD, standard deviation. * Suggestive significance (p value: .05!p!.10). y Strongly significant (p value: p#.01).
Fig. 14. The coronal section of the computed tomography scan in Fig. 13 shows the residual vertebral canal space available with lateral perforation of the screw. Note a similar encroachment of the vertebral canal on the right side with uncovertebral osteophytes.
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Table 8 Clinical and cadaveric studies showing cervical pedicle screw perforation rates Results (%)
Study
Type of study
No. of patients/screws
Surgical technique
In
Abumi et al. (2000) [5]
Clinical
n5180 (669 screws)
93.3
Kotani et al. (2003) [20] Ludwig et al. (2000) [21]
Clinical Cadaveric study
n517
Kast et al. (2006) [22] Kimimura et al. (2000) [23]
Clinical Cadaveric study
n526
Free-handþlateral fluoro CT-based navigation CT-based navigation Free-handþlateral fluoro CT-based navigation CT-based navigation
Grade I/ non-critical breach
Grade II/ critical breach
6.7
99 52 60
1 30 28
0 18 12
70 77
21 13
9 10
CT, computed tomography.
and begin to aim at placing CPSs with Grade I perforation medially while screw insertion itself. This will then increase the chances of Grade II perforations significantly. The authors have experienced a significant rate of Grade I medial perforations having aimed at intrapedicular placement and after having verified the intact medial cortex before screw insertion. None of the screw placements in this study were made when the medial cortex was found to be breeched with the pilot hole made by 2-mm burr. Placement of screw, while having a breeched medial pedicle cortex with the pilot hole (2 mm), will usually lead to Grade II medial perforations after screw insertion (3.5 mm), with increased potential for spinal cord injury. Literature shows wide range of screw misplacement rate from 6.7% to 29% with the free-hand technique of CPS insertion [5,20]. This has prompted use of assisted techniques
for CPS insertion to reduce the rates of perforations and bring consistency in screw placement. With the use of navigation systems [21], relatively lesser perforation rates have been described. However, the results have not been reproducible consistently and there are studies showing more perforations with navigated screws than with the free-hand technique [20,22] (Table 8). In general, the cadaveric studies with or without navigation have shown more screw misplacements than clinical studies [23]. Furthermore, all the screw insertion techniques used so far have had more of lateral perforations than medial perforations [10]. In contrast, the present study shows more of medial perforations than lateral, with the new technique described. This becomes important with regard to clinical outcome, as there are reports of late cerebral infarctions due to laterally perforated CPS [12], although there were no immediate postoperative
Fig. 15. Axial computed tomography scan sections of screw placements using the blunt probe (Abumi et al.) technique. Note the tendency of the thick medial cortex to push the screw laterally.
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Fig. 16. Axial computed tomography scan sections of screw placements using the medial cortical pedicle screw technique. Note the effect of partial drilling of the thick medial cortex, which allows the screw to be placed more medially in the vertebral body and consequently avoiding screw perforation laterally.
clinical complications with screws breeching the vertebral foramen. The study is limited in the sense that the evaluation of pedicle screw placements were done only in the mediolateral plane, and the supero-inferior displacements were not assessed. Furthermore, all the screw placements were evaluated by one of the authors involved in the surgery, which could have led to bias in evaluation. In addition, the results of this study need to be considered in relation to the relatively small number of screw placements evaluated in a heterogeneous group of patients. In this regard, the reproducibility of this technique with its proposed safety needs to be evaluated with a larger multicentric study involving multiple surgeons and comparable patient groups. However, the purpose of this study was to share a new surgical technique for CPS insertion and encourage fellow surgeons to evaluate the technique for its reproducibility and effectiveness in preventing lateral perforations. In this regard, the two figures (Figs. 15 and 16) showing the CT scan axial sections of the screw placement by the
conventional technique and the medial cortical pedicle screw technique make the essence of the study. The study does not intend to propagate the use of CPSs in place where lateral mass screws are sufficiently giving good clinical results. The study gives an additional option of techniques to be considered by surgeons already using CPS placements in selected patients. Conclusion Cervical pedicle screw placement using medial cortical pedicle screw technique is relatively safe and decreases the incidence of lateral perforations as compared with the use of a blunt pedicle probe. However, surgeons attempting this technique should be aware of the increase in medial perforations experienced by the authors. Furthermore, the use of the high-speed burr decreases the force transmitted to the cervical spine while preparation of the pilot hole, which is highly desirable in patients with unstable cervical spine. Further evaluation for reproducibility of the medial
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