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Radiographic Evaluation of the Reliability of Neck Anatomic Structures as Anterior Cervical Surgical Landmarks
Original Article
Radiographic Evaluation of the Reliability of Neck Anatomic Structures as Anterior Cervical Surgical Landmarks Jia-Ming Liu1, Liu-Xue Du1, Xu Xiong2, Xuan-Yin Chen1, Yang Zhou1, Xin-Hua Long1, Shan-Hu Huang1, Zhi-Li Liu1
BACKGROUND: Accurate location of the skin incision is helpful to decrease the technical difficulty and save the operative time in anterior cervical spine surgery. Spine surgeons usually use the traditional neck anatomic structures (the hyoid bone, thyroid cartilage, and cricoid cartilage) as landmarks during the surgery. However, the reliability of these landmarks has not been validated in actual practice.
-
OBJECTIVE: To find out which landmark is the most accurate for identifying the cervical levels in anterior cervical spine surgery.
-
METHODS: The lateral flexion and extension radiographs of cervical spine in standing position from 30 consecutive patients from January 2015 to February 2015 were obtained. The cervical vertebral bodies from C2 to C7 were divided equally into 2 segments. The cervical segments corresponding to each of the surface landmarks were recorded on the flexion and extension radiographs, respectively, and the displacement of corresponding cervical segments from the flexion to extension radiographs for each landmark was calculated.
-
RESULTS: Based on the measurements, the main corresponding cervical levels for the mandibular angle were C2 on both of the flexion and extension films, for the hyoid bone were the C3eC4 interspace on flexion film and C3 on extension film, for the thyroid cartilage C5 on both of flexion and extension films, and for the cricoid cartilage C6 on flexion film and C5eC6 interspace on extension film, respectively. The ratios of displacement within 2 segments
-
Key words Cervical - Landmarks - Mandibular angle - Spine -
Abbreviations and Acronyms ACSS: Anterior cervical spine surgery
from flexion to extension were 83.3% (25/30) for mandibular angle, 56.7% (17/30) for hyoid bone, 66.7% (20/30) for thyroid cartilage, and 56.7% (17/30) for cricoid cartilage, respectively. The mean displacement from flexion to extension films were significantly less than 2 cervical segments for the mandibular angle but greater than 2 segments for the other landmarks. Significant differences were found between mandibular angle and the other 3 landmarks for the displacement from flexion to extension. CONCLUSIONS: The angle of mandible was found to be the most accurate landmark for identifying the cervical level, which corresponded to C2 and C2eC3 disc space. The hyoid bone, thyroid cartilage, and cricoid cartilage were not reliable to predict the cervical levels.
-
INTRODUCTION
A
nterior cervical spine surgery (ACSS) is a useful procedure for treating different cervical spinal disorders, such as cervical disc herniation, ossification of the posterior longitudinal ligament, and cervical vertebral fracture.1,2 Since Cloward3 and Smith and Robinson4 reported the use of transverse skin incision in the cervical anterior approach, this incision became popular because of good cosmesis for patients. Accurate location of the skin incision in ACSS is important to protect neurovascular and visceral structures of the neck and to achieve adequate exposure for cervical decompression and fusion. Improper position of the skin incision may increase technical difficulty, operative time, and the risk of esophageal injury due
To whom correspondence should be addressed: Zhi-Li Liu, M.D. [E-mail: [email protected]] Jia-Ming Liu and Liu-Xue Du contributed equally to this work and they are considered as co-first authors. Citation: World Neurosurg. (2017) 103:133-137. http://dx.doi.org/10.1016/j.wneu.2017.03.129 Journal homepage: www.WORLDNEUROSURGERY.org Available online: www.sciencedirect.com
From the 1Department of Orthopedic Surgery, the First Affiliated Hospital of Nanchang University, Nanchang; and 2Department of Orthopedic Surgery, the 94th Hospital of Chinese People’s Liberation Army, Nanchang, P. R. China
WORLD NEUROSURGERY 103: 133-137, JULY 2017
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NECK ANATOMIC LANDMARKS FOR ACSS
to excessive retraction during the surgery.5-7 Spine surgeons usually use fluoroscopy or navigation to confirm the location of the target cervical segment and help to make the incision; however, fluoroscopy or navigation used in the surgery would increase the radiation, which is harmful to patients. During the past decade, several studies have reported the use of surface neck landmarks, such as hyoid bone, thyroid cartilage, and cricoid cartilage, to identify cervical spinal levels based on the study of preoperative radiographs.8-10 However, the reliability of those techniques was controversial, because the intraoperative positions of cervical spine were different from preoperative ones.11,12 Thus, it is unknown which landmark is reliable for predicting cervical levels no matter what position of the cervical spine; few studies reported about this. The purpose of this study is to determine which is the most accurate landmark to identify the cervical levels and provide information for making incision during ACSS. MATERIALS AND METHODS Patient Selection A study was performed on 30 consecutive patients (15 men and 15 women) who underwent cervical spine radiographic tests in the outpatient clinic because of cervical diseases from January 2015 to February 2015 at our hospital. The study protocol was given ethical approval by the human research ethics committee of the hospital. Patients meeting the following criteria were included in this study: 1) age between 20 to 40 years; 2) underwent both cervical flexion and extension radiographs in standing position, without severe cervical degeneration; 3) no cervical vertebral body and disc destruction. Patients with cervical kyphosis, previous cervical surgery, congenital spinal anomalies, cervical tumor, and tuberculosis were excluded from this study. Radiographic Measurement All lateral cervical spine radiographs of the included patients were obtained. The anterior landmarks and the corresponding cervical spinal levels were displayed clearly on the films. For the measurement, several lines were drawn on the films as follows: first, each of the cervical spinal vertebral body was equally divided into 2 segments from C2 to C7, including the upper and lower segments, and each of the cervical interspaces was defined as one segment (Figure 1). Second, a line connecting the tip of the odontoid process and the center of C7 vertebra was drawn as the sagittal vertical axis of cervical spine. Third, a horizontal line was set perpendicular to the sagittal vertical axis from the middle of each surface landmark to the cervical spine on both of the flexion and extension radiographs (Figure 2). The surface landmarks included in this study consisted of the angle of mandible, hyoid bone, thyroid cartilage, and cricoid cartilage. According to the horizontal line, the cervical segments corresponding to each of the surface landmark were recorded on both of the flexion and extension radiographs, and the displacement from the flexion to the extension radiographs for each landmark was calculated. If the displacement was greater than 2 segments (1 vertebra), it was determined that this landmark was not reliable to predict the cervical spinal levels. On the contrary, if the
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Figure 1. Cervical vertebral bodies from C2 to C7 were equally divided into 2 segments, and cervical interspace was defined as one segment for the measurement.
displacement was less than 2 segments, the landmark was determined to be reliable for identifying the cervical spinal levels. Statistical Analysis All data were presented as mean standard deviation in this study. All statistics were conducted with SPSS 17.0 software (SPSS Inc., Chicago, Illinois, USA). The data were analyzed with the Student’s t test for continuous variables and c2 test for categorical variables. Values of P < 0.05 were considered to be statistically significant. RESULTS The mean age of the patients was 32 years. Based on the measurement, the cervical spinal levels corresponding to the position of the surface landmarks on the lateral flexion and extension radiographs are displayed in Table 1. For the angle of the mandible, most of the corresponding spinal levels were C2 on both of the flexion and extension films. The majority of cervical spinal levels corresponding to the position of hyoid bone were C3eC4 interspace on flexion film and C3 on extension film. For the thyroid cartilage, the corresponding spinal levels on both of flexion and extension films were C5. The reference level for
WORLD NEUROSURGERY, http://dx.doi.org/10.1016/j.wneu.2017.03.129
ORIGINAL ARTICLE JIA-MING LIU ET AL.
NECK ANATOMIC LANDMARKS FOR ACSS
Figure 2. The sagittal vertical axis of cervical spine. (A) The corresponding cervical level of mandibular
angle on the flexion film. (B) the corresponding cervical level of mandibular angle on the extension film.
cricoid cartilage was C6 on the flexion film and C5eC6 interspace on the extension film. From the flexion to extension films, the cervical segment corresponding to each surface landmark was different
(Table 2). According to the measurement, the ratios of displacing within 2 segments from flexion to extension were 83.3% (25/30) for mandibular angle, 56.7% (17/30) for the hyoid bone, 66.7% (20/30) for the thyroid cartilage, and
Table 1. The Corresponding Levels of Mandibular Angle, Hyoid Bone, Thyroid Cartilage, and Cricoid Cartilage on the Flexion and Extension Lateral Radiographs Mandibular Angle Location
Hyoid Bone
Thyroid Cartilage
Cricoid Cartilage
Flexion (n)
Extension (n)
Flexion (n)
Extension (n)
Flexion (n)
Extension (n)
Flexion (n)
Extension (n)
C1
0
2
0
0
0
0
0
0
C2
19
24
0
0
0
0
0
0
C2eC3
6
4
0
7
0
0
0
0
C3
5
0
1
19
0
2
0
0
C3/4
0
0
10
3
0
2
0
0
C4
0
0
9
1
1
7
0
0
C4eC5
0
0
4
0
5
8
1
5
C5
0
0
1
0
18
10
4
1
C5eC6
0
0
0
0
2
1
7
10
C6
0
0
0
0
4
0
14
4
C6eC7
0
0
0
0
0
0
3
0
C7
0
0
0
0
1
0
1
0
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NECK ANATOMIC LANDMARKS FOR ACSS
Table 2. The Displacement of Mandibular Angle, Hyoid Bone, Thyroid Cartilage, and Cricoid Cartilage from the Flexion to Extension Radiographs Mandibular Angle (n)
Displacement No
Hyoid Bone (n)
Thyroid Cartilage (n)
Cricoid Cartilage (n)
1
1
2
0
One segment
14
9
12
8
Two segments
10
7
6
9
Three segments
5
7
5
10
Four segments
0
4
2
2
Five segments or greater
0
2
3
1
56.7% (17/30) for the cricoid cartilage, respectively. Compared with other surface landmarks, we found that the angle of mandible was more reliable in identifying the cervical spinal level. The mean displacement from flexion to extension films was 1.60 0.86 segments for mandibular angle, which was significantly less than 2 segments (P ¼ 0.02). On the contrary, the average displacement from flexion to extension reference to the positions of hyoid bone, thyroid cartilage, and cricoid cartilage was greater than 2 segments (P > 0.05) (Table 3). In addition, significantly statistical differences were found between the mandibular angle and the other 3 landmarks for the displacement from flexion to extension (P values were 0.024, 0.049, and 0.024, respectively). However, there were no significant differences among hyoid bone, thyroid cartilage, and carotid tubercle for the displacement (P ¼ 0.08). DISCUSSION ACSS has become a widely accepted procedure for the treatment of cervical pathology because of good clinical results and high fusion rate. Accurate skin incision is important for ACSS, and proper placement of skin incision will provide easy access to the operative window and minimize the risks associated with excessive retraction in the cervical spine. Many different kinds of surface landmarks are used for the localization of anterior cervical level, such as the hyoid bone, thyroid cartilage, and cricoid
Table 3. The Mean Displacement of Mandibular Angle, Hyoid Bone, Thyroid Cartilage, and Cricoid Cartilage from the Flexion to the Extension Films Displacement (Segments)
Testing Value (Segments)
P Value
1.60 0.86
2
0.02
Hyoid bone
2.40 1.48
2
0.15
Thyroid cartilage
2.17 1.68
2
0.59
Cricoid cartilage
2.33 1.15
2
0.16
Landmarks Mandibular angle
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cartilage. Previous studies have reported that the thyroid cartilage is located at the anterior of C4 and C5 body, the thyroid cartilage usually corresponds to the C4 body, C4eC5 interspace, and the C5 body, and the cricoid cartilage is commonly referenced to the C6 body.9,10 However, these corresponding relationships between surface landmarks and cervical levels were identified just based on the measurement of radiologic images in neutral position before surgery. The surgical posture for ACSS requires a patient’s head fall backwards and the shoulder subsided. Thus, the corresponding level to the landmark would change during the surgery.13 Therefore, we tried to find out which landmark is the most accurate for the identification of cervical spine level no matter what position of the spine. Auerbach et al.14 found that the mandibular angle was located at the level of C2eC3 disc space and suggested it was the most consistently palpable landmark. However, the results were concluded based on preoperative neutral standing films and intraoperative supine films and may be not reliable when the cervical spine changes its position from flexion to extension. In the present study, the flexion and extension lateral cervical spine radiographs in standing position were obtained for measurement. We used both of the flexion and extension films for the measurement because we tried to make clear which landmark was the most accurate for identifying the cervical level no matter what position of the spine was. For mandibular angle, 25 cases (83.3%) located at level of the C2 body and C2eC3 disc space on flexion films, and 24 cases (80.0%) located at the level of C2 body and C2eC3 disc space on extension films. The mean displacement from flexion to extension films for mandibular angle was 1.60 0.86 segments, which was less than those of other landmarks. Therefore, in our opinion, the angle of mandible was more reliable than the other landmarks for predicting the cervical level while position changing. In addition, it showed that significant differences were found between the mandibular angle and the other 3 landmarks for the mean displacement from flexion to extension, but no statistical differences were detected among the hyoid bone, thyroid cartilage, and carotid tubercle. Therefore, we believe that the mandibular angle is more accurate than the other landmarks (hyoid bone, thyroid cartilage, and cricoid cartilage) for identifying the cervical level. Lim et al.13 compared the position of the hyoid bone before and after anesthesia and found that the position of it changed on lateral cervical spine images. The results showed 73.4% of hyoid bone position had changed more than 1 cervical vertebral body. In line with the study from Lim et al., most of the landmarks in our study changed position more than 2 segments (1 vertebral body) from flexion to extension. It demonstrated that most of the hyoid bone was located anterior to the disc space of C3eC4 on flexion radiographs and C3 body on and extension radiographs. In addition, 43.3% of cases changed more than 2 segments (mean, 2.40 1.48) from flexion films to extension films for hyoid bone. Therefore, it suggested that the hyoid bone was not a reliable surface landmark for identifying the cervical level. Moreover, the ratios of displacement more than 2 segments for thyroid cartilage and cricoid cartilage were 40.0% and 43.3%, respectively. Thus, we believe that these 2 anatomic structures should not be trusted as landmarks during ACSS.
WORLD NEUROSURGERY, http://dx.doi.org/10.1016/j.wneu.2017.03.129
ORIGINAL ARTICLE JIA-MING LIU ET AL.
NECK ANATOMIC LANDMARKS FOR ACSS
Although this study demonstrates satisfactory results about the use of the mandibular angle as a reliable landmark for determining the location of cervical levels, some limitations are presented, including small samples and without other landmarks in the comparative analysis. Patients with different age, body mass index, and height were not analyzed in this study. In addition, the data in this study were measured from healthy patients without severe cervical spine degeneration. The conclusions from the present study may not fully applicable to those patients with severe degenerative cervical spine. The accuracy of determining cervical segment by this technique still requires clinical validation, and
REFERENCES 1. Suk KS, Lee SH, Park SY, Kim HS, Moon SH, Lee HM. Clinical outcome and changes of foraminal dimension in patients with foraminal stenosis after ACDF. J Spinal Disord Tech. 2015;28:E449-E453. 2. Nandyala SV, Marquez-Lara A, Fineberg SJ, Singh K. Comparison of revision surgeries for one- to two-level cervical TDR and ACDF from 2002 to 2011. Spine J. 2014;14:2841-2846. 3. Cloward RB. The anterior approach for removal of ruptured cervical disks. J Neurosurg Spine. 1958;6: 496-511. 4. Smith GW, Robinson RA. The treatment of certain cervical-spine disorders by anterior removal of the intervertebral disc and interbody fusion. J Bone Joint Surg Am. 1958;40-A:607-624. 5. Song KJ, Choi BW, Kim HY, Jeon TS, Chang H. Efficacy of postoperative radiograph for evaluating the prevertebral soft tissue swelling after anterior cervical discectomy and fusion. Clin Orthop Surg. 2012;4:77-82.
further study with a large sample and differently aged population is necessary to verify the results of this study. In conclusion, based on the measurement, the mandibular angle was found to be the most accurate landmark for identifying the cervical level, which corresponded to C2 and C2eC3 disc space. The hyoid bone, thyroid cartilage, and cricoid cartilage were not reliable to predict the cervical level when performing the ACSS. However, further study with a large sample is necessary to verify the results of this study. Surgeons should pay attention the spinal degeneration when determining the location of target cervical level.
6. Olsson EC, Jobson M, Lim MR. Risk factors for persistent dysphagia after anterior cervical spine surgery. Orthopedics. 2015;38:e319-e323. 7. Acosta-Mérida MA, Medina-Velázquez R, Marchena-Gómez J, Alonso-Gómez J, Mhaildli H. Dysphagia after cervical spine fusion caused by migration of the prosthetic material. Cir Esp. 2015; 93:537-539. 8. Oh CH, Ji GY, Yoon SH, Hyun D, Choi CG, Lim HK, et al. Surface landmarks do not correspond to exact levels of the cervical spine: references according to the sex, age and height. Korean J Spine. 2014;11:178-182. 9. Tran DQ, Dugani S, Finlayson RJ. A randomized comparison between ultrasound-guided and landmark-based superficial cervical plexus block. Reg Anesth Pain Med. 2010;35:539-543.
12. Lee JH, Lee JH, Lee HS, Lee DY, Lee DO. The efficacy of carotid tubercle as an anatomical landmark for identification of cervical spinal level in the anterior cervical surgery: comparison with preoperative Carm fluoroscopy. Clin Orthop Surg. 2013;5:129-133. 13. Lim JH, Wang SI, Kim DY, Song KJ, Kim TG, Lee KB. Positional change of hyoid bone after anesthesia in anterior surgery of upper cervical spine. Spine J. 2014;14:1890-1894. 14. Auerbach J, Weidner Z, Pill S, Mehta S, Chin K. The mandibular angle as a landmark for identification of cervical spinal level. Spine. 2009;34:1006-1011. Conflict of interest statement: This work was supported by the Department of Science and Technology Program Funds of Jiangxi Province, P.R. China (No. 20162BCB22022, 20162BCB23057, 20121BBG70037). Received 18 December 2016; accepted 27 March 2017
10. Lemaire V, Jacquemin G, Nelissen X, Heymans O. Tip of the greater horn of the hyoid bone: a landmark for cervical surgery. Surg Radiol Anat. 2005;27:33-36.
Citation: World Neurosurg. (2017) 103:133-137. http://dx.doi.org/10.1016/j.wneu.2017.03.129
11. Westbury JR. Mandible and hyoid bone movements during speech. J Speech Hear Res. 1988;31:405-416.
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137
Radiographic Evaluation of the Reliability of Neck Anatomic Structures as Anterior Cervical Surgical Landmarks Jia-Ming Liu1, Liu-Xue Du1, Xu Xiong2, Xuan-Yin Chen1, Yang Zhou1, Xin-Hua Long1, Shan-Hu Huang1, Zhi-Li Liu1
BACKGROUND: Accurate location of the skin incision is helpful to decrease the technical difficulty and save the operative time in anterior cervical spine surgery. Spine surgeons usually use the traditional neck anatomic structures (the hyoid bone, thyroid cartilage, and cricoid cartilage) as landmarks during the surgery. However, the reliability of these landmarks has not been validated in actual practice.
-
OBJECTIVE: To find out which landmark is the most accurate for identifying the cervical levels in anterior cervical spine surgery.
-
METHODS: The lateral flexion and extension radiographs of cervical spine in standing position from 30 consecutive patients from January 2015 to February 2015 were obtained. The cervical vertebral bodies from C2 to C7 were divided equally into 2 segments. The cervical segments corresponding to each of the surface landmarks were recorded on the flexion and extension radiographs, respectively, and the displacement of corresponding cervical segments from the flexion to extension radiographs for each landmark was calculated.
-
RESULTS: Based on the measurements, the main corresponding cervical levels for the mandibular angle were C2 on both of the flexion and extension films, for the hyoid bone were the C3eC4 interspace on flexion film and C3 on extension film, for the thyroid cartilage C5 on both of flexion and extension films, and for the cricoid cartilage C6 on flexion film and C5eC6 interspace on extension film, respectively. The ratios of displacement within 2 segments
-
Key words Cervical - Landmarks - Mandibular angle - Spine -
Abbreviations and Acronyms ACSS: Anterior cervical spine surgery
from flexion to extension were 83.3% (25/30) for mandibular angle, 56.7% (17/30) for hyoid bone, 66.7% (20/30) for thyroid cartilage, and 56.7% (17/30) for cricoid cartilage, respectively. The mean displacement from flexion to extension films were significantly less than 2 cervical segments for the mandibular angle but greater than 2 segments for the other landmarks. Significant differences were found between mandibular angle and the other 3 landmarks for the displacement from flexion to extension. CONCLUSIONS: The angle of mandible was found to be the most accurate landmark for identifying the cervical level, which corresponded to C2 and C2eC3 disc space. The hyoid bone, thyroid cartilage, and cricoid cartilage were not reliable to predict the cervical levels.
-
INTRODUCTION
A
nterior cervical spine surgery (ACSS) is a useful procedure for treating different cervical spinal disorders, such as cervical disc herniation, ossification of the posterior longitudinal ligament, and cervical vertebral fracture.1,2 Since Cloward3 and Smith and Robinson4 reported the use of transverse skin incision in the cervical anterior approach, this incision became popular because of good cosmesis for patients. Accurate location of the skin incision in ACSS is important to protect neurovascular and visceral structures of the neck and to achieve adequate exposure for cervical decompression and fusion. Improper position of the skin incision may increase technical difficulty, operative time, and the risk of esophageal injury due
To whom correspondence should be addressed: Zhi-Li Liu, M.D. [E-mail: [email protected]] Jia-Ming Liu and Liu-Xue Du contributed equally to this work and they are considered as co-first authors. Citation: World Neurosurg. (2017) 103:133-137. http://dx.doi.org/10.1016/j.wneu.2017.03.129 Journal homepage: www.WORLDNEUROSURGERY.org Available online: www.sciencedirect.com
From the 1Department of Orthopedic Surgery, the First Affiliated Hospital of Nanchang University, Nanchang; and 2Department of Orthopedic Surgery, the 94th Hospital of Chinese People’s Liberation Army, Nanchang, P. R. China
WORLD NEUROSURGERY 103: 133-137, JULY 2017
1878-8750/$ - see front matter ª 2017 Elsevier Inc. All rights reserved.
www.WORLDNEUROSURGERY.org
133
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NECK ANATOMIC LANDMARKS FOR ACSS
to excessive retraction during the surgery.5-7 Spine surgeons usually use fluoroscopy or navigation to confirm the location of the target cervical segment and help to make the incision; however, fluoroscopy or navigation used in the surgery would increase the radiation, which is harmful to patients. During the past decade, several studies have reported the use of surface neck landmarks, such as hyoid bone, thyroid cartilage, and cricoid cartilage, to identify cervical spinal levels based on the study of preoperative radiographs.8-10 However, the reliability of those techniques was controversial, because the intraoperative positions of cervical spine were different from preoperative ones.11,12 Thus, it is unknown which landmark is reliable for predicting cervical levels no matter what position of the cervical spine; few studies reported about this. The purpose of this study is to determine which is the most accurate landmark to identify the cervical levels and provide information for making incision during ACSS. MATERIALS AND METHODS Patient Selection A study was performed on 30 consecutive patients (15 men and 15 women) who underwent cervical spine radiographic tests in the outpatient clinic because of cervical diseases from January 2015 to February 2015 at our hospital. The study protocol was given ethical approval by the human research ethics committee of the hospital. Patients meeting the following criteria were included in this study: 1) age between 20 to 40 years; 2) underwent both cervical flexion and extension radiographs in standing position, without severe cervical degeneration; 3) no cervical vertebral body and disc destruction. Patients with cervical kyphosis, previous cervical surgery, congenital spinal anomalies, cervical tumor, and tuberculosis were excluded from this study. Radiographic Measurement All lateral cervical spine radiographs of the included patients were obtained. The anterior landmarks and the corresponding cervical spinal levels were displayed clearly on the films. For the measurement, several lines were drawn on the films as follows: first, each of the cervical spinal vertebral body was equally divided into 2 segments from C2 to C7, including the upper and lower segments, and each of the cervical interspaces was defined as one segment (Figure 1). Second, a line connecting the tip of the odontoid process and the center of C7 vertebra was drawn as the sagittal vertical axis of cervical spine. Third, a horizontal line was set perpendicular to the sagittal vertical axis from the middle of each surface landmark to the cervical spine on both of the flexion and extension radiographs (Figure 2). The surface landmarks included in this study consisted of the angle of mandible, hyoid bone, thyroid cartilage, and cricoid cartilage. According to the horizontal line, the cervical segments corresponding to each of the surface landmark were recorded on both of the flexion and extension radiographs, and the displacement from the flexion to the extension radiographs for each landmark was calculated. If the displacement was greater than 2 segments (1 vertebra), it was determined that this landmark was not reliable to predict the cervical spinal levels. On the contrary, if the
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Figure 1. Cervical vertebral bodies from C2 to C7 were equally divided into 2 segments, and cervical interspace was defined as one segment for the measurement.
displacement was less than 2 segments, the landmark was determined to be reliable for identifying the cervical spinal levels. Statistical Analysis All data were presented as mean standard deviation in this study. All statistics were conducted with SPSS 17.0 software (SPSS Inc., Chicago, Illinois, USA). The data were analyzed with the Student’s t test for continuous variables and c2 test for categorical variables. Values of P < 0.05 were considered to be statistically significant. RESULTS The mean age of the patients was 32 years. Based on the measurement, the cervical spinal levels corresponding to the position of the surface landmarks on the lateral flexion and extension radiographs are displayed in Table 1. For the angle of the mandible, most of the corresponding spinal levels were C2 on both of the flexion and extension films. The majority of cervical spinal levels corresponding to the position of hyoid bone were C3eC4 interspace on flexion film and C3 on extension film. For the thyroid cartilage, the corresponding spinal levels on both of flexion and extension films were C5. The reference level for
WORLD NEUROSURGERY, http://dx.doi.org/10.1016/j.wneu.2017.03.129
ORIGINAL ARTICLE JIA-MING LIU ET AL.
NECK ANATOMIC LANDMARKS FOR ACSS
Figure 2. The sagittal vertical axis of cervical spine. (A) The corresponding cervical level of mandibular
angle on the flexion film. (B) the corresponding cervical level of mandibular angle on the extension film.
cricoid cartilage was C6 on the flexion film and C5eC6 interspace on the extension film. From the flexion to extension films, the cervical segment corresponding to each surface landmark was different
(Table 2). According to the measurement, the ratios of displacing within 2 segments from flexion to extension were 83.3% (25/30) for mandibular angle, 56.7% (17/30) for the hyoid bone, 66.7% (20/30) for the thyroid cartilage, and
Table 1. The Corresponding Levels of Mandibular Angle, Hyoid Bone, Thyroid Cartilage, and Cricoid Cartilage on the Flexion and Extension Lateral Radiographs Mandibular Angle Location
Hyoid Bone
Thyroid Cartilage
Cricoid Cartilage
Flexion (n)
Extension (n)
Flexion (n)
Extension (n)
Flexion (n)
Extension (n)
Flexion (n)
Extension (n)
C1
0
2
0
0
0
0
0
0
C2
19
24
0
0
0
0
0
0
C2eC3
6
4
0
7
0
0
0
0
C3
5
0
1
19
0
2
0
0
C3/4
0
0
10
3
0
2
0
0
C4
0
0
9
1
1
7
0
0
C4eC5
0
0
4
0
5
8
1
5
C5
0
0
1
0
18
10
4
1
C5eC6
0
0
0
0
2
1
7
10
C6
0
0
0
0
4
0
14
4
C6eC7
0
0
0
0
0
0
3
0
C7
0
0
0
0
1
0
1
0
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Table 2. The Displacement of Mandibular Angle, Hyoid Bone, Thyroid Cartilage, and Cricoid Cartilage from the Flexion to Extension Radiographs Mandibular Angle (n)
Displacement No
Hyoid Bone (n)
Thyroid Cartilage (n)
Cricoid Cartilage (n)
1
1
2
0
One segment
14
9
12
8
Two segments
10
7
6
9
Three segments
5
7
5
10
Four segments
0
4
2
2
Five segments or greater
0
2
3
1
56.7% (17/30) for the cricoid cartilage, respectively. Compared with other surface landmarks, we found that the angle of mandible was more reliable in identifying the cervical spinal level. The mean displacement from flexion to extension films was 1.60 0.86 segments for mandibular angle, which was significantly less than 2 segments (P ¼ 0.02). On the contrary, the average displacement from flexion to extension reference to the positions of hyoid bone, thyroid cartilage, and cricoid cartilage was greater than 2 segments (P > 0.05) (Table 3). In addition, significantly statistical differences were found between the mandibular angle and the other 3 landmarks for the displacement from flexion to extension (P values were 0.024, 0.049, and 0.024, respectively). However, there were no significant differences among hyoid bone, thyroid cartilage, and carotid tubercle for the displacement (P ¼ 0.08). DISCUSSION ACSS has become a widely accepted procedure for the treatment of cervical pathology because of good clinical results and high fusion rate. Accurate skin incision is important for ACSS, and proper placement of skin incision will provide easy access to the operative window and minimize the risks associated with excessive retraction in the cervical spine. Many different kinds of surface landmarks are used for the localization of anterior cervical level, such as the hyoid bone, thyroid cartilage, and cricoid
Table 3. The Mean Displacement of Mandibular Angle, Hyoid Bone, Thyroid Cartilage, and Cricoid Cartilage from the Flexion to the Extension Films Displacement (Segments)
Testing Value (Segments)
P Value
1.60 0.86
2
0.02
Hyoid bone
2.40 1.48
2
0.15
Thyroid cartilage
2.17 1.68
2
0.59
Cricoid cartilage
2.33 1.15
2
0.16
Landmarks Mandibular angle
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cartilage. Previous studies have reported that the thyroid cartilage is located at the anterior of C4 and C5 body, the thyroid cartilage usually corresponds to the C4 body, C4eC5 interspace, and the C5 body, and the cricoid cartilage is commonly referenced to the C6 body.9,10 However, these corresponding relationships between surface landmarks and cervical levels were identified just based on the measurement of radiologic images in neutral position before surgery. The surgical posture for ACSS requires a patient’s head fall backwards and the shoulder subsided. Thus, the corresponding level to the landmark would change during the surgery.13 Therefore, we tried to find out which landmark is the most accurate for the identification of cervical spine level no matter what position of the spine. Auerbach et al.14 found that the mandibular angle was located at the level of C2eC3 disc space and suggested it was the most consistently palpable landmark. However, the results were concluded based on preoperative neutral standing films and intraoperative supine films and may be not reliable when the cervical spine changes its position from flexion to extension. In the present study, the flexion and extension lateral cervical spine radiographs in standing position were obtained for measurement. We used both of the flexion and extension films for the measurement because we tried to make clear which landmark was the most accurate for identifying the cervical level no matter what position of the spine was. For mandibular angle, 25 cases (83.3%) located at level of the C2 body and C2eC3 disc space on flexion films, and 24 cases (80.0%) located at the level of C2 body and C2eC3 disc space on extension films. The mean displacement from flexion to extension films for mandibular angle was 1.60 0.86 segments, which was less than those of other landmarks. Therefore, in our opinion, the angle of mandible was more reliable than the other landmarks for predicting the cervical level while position changing. In addition, it showed that significant differences were found between the mandibular angle and the other 3 landmarks for the mean displacement from flexion to extension, but no statistical differences were detected among the hyoid bone, thyroid cartilage, and carotid tubercle. Therefore, we believe that the mandibular angle is more accurate than the other landmarks (hyoid bone, thyroid cartilage, and cricoid cartilage) for identifying the cervical level. Lim et al.13 compared the position of the hyoid bone before and after anesthesia and found that the position of it changed on lateral cervical spine images. The results showed 73.4% of hyoid bone position had changed more than 1 cervical vertebral body. In line with the study from Lim et al., most of the landmarks in our study changed position more than 2 segments (1 vertebral body) from flexion to extension. It demonstrated that most of the hyoid bone was located anterior to the disc space of C3eC4 on flexion radiographs and C3 body on and extension radiographs. In addition, 43.3% of cases changed more than 2 segments (mean, 2.40 1.48) from flexion films to extension films for hyoid bone. Therefore, it suggested that the hyoid bone was not a reliable surface landmark for identifying the cervical level. Moreover, the ratios of displacement more than 2 segments for thyroid cartilage and cricoid cartilage were 40.0% and 43.3%, respectively. Thus, we believe that these 2 anatomic structures should not be trusted as landmarks during ACSS.
WORLD NEUROSURGERY, http://dx.doi.org/10.1016/j.wneu.2017.03.129
ORIGINAL ARTICLE JIA-MING LIU ET AL.
NECK ANATOMIC LANDMARKS FOR ACSS
Although this study demonstrates satisfactory results about the use of the mandibular angle as a reliable landmark for determining the location of cervical levels, some limitations are presented, including small samples and without other landmarks in the comparative analysis. Patients with different age, body mass index, and height were not analyzed in this study. In addition, the data in this study were measured from healthy patients without severe cervical spine degeneration. The conclusions from the present study may not fully applicable to those patients with severe degenerative cervical spine. The accuracy of determining cervical segment by this technique still requires clinical validation, and
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further study with a large sample and differently aged population is necessary to verify the results of this study. In conclusion, based on the measurement, the mandibular angle was found to be the most accurate landmark for identifying the cervical level, which corresponded to C2 and C2eC3 disc space. The hyoid bone, thyroid cartilage, and cricoid cartilage were not reliable to predict the cervical level when performing the ACSS. However, further study with a large sample is necessary to verify the results of this study. Surgeons should pay attention the spinal degeneration when determining the location of target cervical level.
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12. Lee JH, Lee JH, Lee HS, Lee DY, Lee DO. The efficacy of carotid tubercle as an anatomical landmark for identification of cervical spinal level in the anterior cervical surgery: comparison with preoperative Carm fluoroscopy. Clin Orthop Surg. 2013;5:129-133. 13. Lim JH, Wang SI, Kim DY, Song KJ, Kim TG, Lee KB. Positional change of hyoid bone after anesthesia in anterior surgery of upper cervical spine. Spine J. 2014;14:1890-1894. 14. Auerbach J, Weidner Z, Pill S, Mehta S, Chin K. The mandibular angle as a landmark for identification of cervical spinal level. Spine. 2009;34:1006-1011. Conflict of interest statement: This work was supported by the Department of Science and Technology Program Funds of Jiangxi Province, P.R. China (No. 20162BCB22022, 20162BCB23057, 20121BBG70037). Received 18 December 2016; accepted 27 March 2017
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Citation: World Neurosurg. (2017) 103:133-137. http://dx.doi.org/10.1016/j.wneu.2017.03.129
11. Westbury JR. Mandible and hyoid bone movements during speech. J Speech Hear Res. 1988;31:405-416.
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