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Dynamic close-mouth view radiograph method for the diagnosis of lateral dynamic instability of the atlantoaxial joint
Clinical Imaging 34 (2010) 375 – 378
Dynamic close-mouth view radiograph method for the diagnosis of lateral dynamic instability of the atlantoaxial joint Susumu Fujiwara a , Daisaku Tokunaga b,⁎, Ryo Oda b , Shogo Toyama b , Kan Imai b , Atushi Doi a , Toshikazu Kubo b a
Department of Radiological Technology, Kyoto Takeda Hospital, Minamikoromoda-cho, Nishi-shichijo, Shimogyo-ku, Kyoto 600-8884, Japan b Department of Orthopaedics, Graduate School of Medical Science, Kyoto Prefectural University of Medicine, Kawaramachi-Hirokoji, Kamigyo-ku, Kyoto 602-8566, Japan Received 3 July 2009; accepted 15 August 2009
Abstract The purpose of the current study is to establish the dynamic close-mouth view radiograph method. Seven healthy volunteers were enrolled. Dynamic open-mouth and close-mouth view radiographs were obtained. The odontoid lateral mass interval and the dynamic atlantodental lateral shift (ADLS) were measured. The ADLS was 9.4±5.3% by the close-mouth view, showing a significantly greater percentage than that of 5.3±4.0% by the open-mouth view. The dynamic close-mouth view can be useful for diagnosis of atlantoaxial lateral instability. © 2010 Elsevier Inc. All rights reserved. Keywords: Cervical spine; Open-mouth view; Close-mouth view; Atlantoaxial subluxation; Instability
1. Introduction The incidence of cervical involvement in patients with rheumatoid arthritis (RA) is very high, ranging from 23% to 86% in several reports [1–7]. Anterior atlantoaxial subluxation (aAAS) is the most common type of cervical spine involvement. It is very important to diagnose aAAS early since the progression of spinal cord compression may result in either a neurological deficit or even sudden death [1–3,8,9]. Lateral view radiographs of the cervical spine (during flexion and extension) are usually utilized for the diagnosis of aAAS. Static open-mouth view radiographs are often used to diagnose the collapse of the lateral mass of atlas(C1) and
axis(C2), thus, overriding the C1/C2 joint, that is, established static lateral displacement of the atlantoaxial joint [10]. Atlantoaxial involvement in RA is associated with anteroposterior sagittal instability, as well as lateral coronal instability. Dynamic open-mouth view radiographs were used to diagnose dynamic lateral instability of atlantoaxial joint [11]. However, the diagnosis of the atlantoaxial area using dynamic open-mouth view radiographs is sometimes difficult if the patient has a limited ability to open their mouth due to temporomandibular joint destruction or overlapping of the teeth, atlas, and axis. The purpose of current study is to establish the dynamic close-mouth view radiograph method for the diagnosis of lateral dynamic instability of the atlantoaxial joint. 2. Materials and methods
⁎ Corresponding author. Department of Orthopaedics, Graduate School of Medical Science, Kyoto Prefectural University of Medicine, Kawaramachi-Hirokoji, Kamigyo-ku, Kyoto, 602-8566, Japan. Tel.: +81 75 251 5549; fax: +81 75 251 5841. E-mail address: [email protected] (D. Tokunaga). 0899-7071/$ – see front matter © 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.clinimag.2009.08.027
Seven healthy volunteers were enrolled (all men; age range, 26–39 years; mean, 32 years). This study was approved by the local research ethics committee at Kyoto Takeda Hospital, Kyoto, Japan.
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2.1. Dynamic open-mouth view radiograph The patient was placed in a sitting position with their mouth open as wide as possible and the jaw tilted upward. In this position, the teeth overlap the occipital bone, making a line between the nasal spine and internal acoustic opening vertical to the cassette. The X-ray beam line was positioned at the center of the maxillary dental arcade and the mandibular dental arcade and vertical to the cassette. The patient was asked to bend his neck to the right and left as far as possible, keeping the center of mouth facing forward for the dynamic open-mouth view (Fig. 1). The radiograph was taken with 80 kV,·160 mA,·80 ms. 2.2. Dynamic close-mouth view radiograph For the close-mouth view radiograph, the patient was placed in a sitting position, with their mouth closed and the jaw down to overlap the nasal spine and occipital bone. The center of beam line was set 5 mm under the nasal spine and vertical to the cassette (Fig. 2). To obtain the dynamic close-mouth view, similar to that of the dynamic openmouth view, we asked the patient to bend his neck to the right and left as much as possible, keeping the center of the mouth facing forward (Fig. 3). The radiograph was taken with 83 kV,·160 mA,·80 ms.
Fig. 2. Close-mouth view. Upper line: OM line (orbitomeatal baseline). Middle line: line between the nasal spine and the occipital bone. Lower line: beam line.
2.3. Evaluation of radiographs All the radiographies were sent to a DICOM server and interpreted and measured with a PACS viewer (Hitachi, Tokyo, Japan). The odontoid lateral mass interval (OLMI) described by Hohl et al. was measured in open and closed mouth positions radiographs in the neck neutral position. The OLMI is the interspace from the odontoid to the corresponding point on the medial aspect of the articular mass of the axis. The dynamic atlantodental lateral shift (ADLS) described by Taniguchi et al. [11] was measured for the evaluation of lateral shift of the atlantoaxial joint. The distance between the bilateral inner distal edges of the lateral masses of the atlas (a) was measured for the measurement of the ADLS. The central axis of the odontoid (CO) and the distance between the CO and the inner distal edge of the right lateral mass of the atlas (b) were measured. The distance b was divided by the distance a and displayed as a percentage ratio (100×b/a%). The difference of the scores for the
maximum right and left neck bending was calculated as the ADLS {(b1/a1−b2/a2)×100} (Fig. 4). 2.4. Statistics The scores of ADLS and OLMI were compared for the open- and close-mouth views. The statistical significance was calculated using the paired t test. 3. Results The OLMI in the cervical neutral position was 3.5±0.8 mm (mean±S.D.) by the open-mouth view while 4.0±0.7 mm by the close-mouth view, showing significant difference between the open-mouth view and closemouth view (Pb.05) (Fig. 5). The ADLS was 9.4±5.3% by the close-mouth view, showing a significantly greater
Fig. 1. Dynamic open-mouth view.
S. Fujiwara et al. / Clinical Imaging 34 (2010) 375–378
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Fig. 3. Dynamic close-mouth view.
percentage than that of 5.3±4.0% by the open-mouth view (Pb.01) (Fig. 6).
4. Discussion An open-mouth view radiograph has been commonly used since Jacobs [12] reported the usefulness of evaluating the atlantoaxial joint in 1938. Several studies have reported radiographic atlantoaxial asymmetry by open-mouth view, usually following cervical spine trauma [13,14]. Hohl and Baker [15] studied the lateral asymmetry of atlantoaxial joints using open-mouth views. They measured the interspace from the odontoid to the corresponding point on the medial aspect of the articular mass of the axis (odontoid lateral mass interspace: OLMI) and the lateral distance from the superior lateral margin of the articular mass of the axis to the corresponding inferior lateral margin of the atlas. Taniguchi et al. [11] evaluated the lateral instability of the atlantoaxial joint in RA, using dynamic open-mouth view radiographs, and reported that the ADLS in the RA group averaged 14.8%, and this was significantly greater than in the control group, in which it averaged 6.1%. The ADLS averaged 20.6% in the RA subgroup with aAAS and 12.7% in the RA subgroup without aAAS. They hypothesized that evaluating the dynamic lateral cervical instability can be useful for the early diagnosis of atlantoaxial lesions in RA. However, it is sometimes difficult to obtain a dynamic open-mouth view in elderly patients or in rheumatoid
patients, when such patients cannot open their mouths sufficiently due to a dysfunction of the temporomandibular joint. Under this condition, the upper teeth overlap with the atlantoaxial joint in the open-mouth view, making it impossible to measure the ADLS. The dynamic closemouth view was therefore developed to resolve this problem. The atlantoaxial area can be radiographed by this method, not through the oral but through the maxilla, so that the teeth do not overlap with the target zone. Moreover, it is easier to maintain a neck position that is more tolerable for the patients during examination using the close-mouth view than the open-mouth view. The results of current study demonstrate that the measurement of the OLMI was easily obtained by the close-mouth view, showing similar data with that obtained via the open-mouth view. Furthermore, the ADLS was able to measure dynamic views during right and left bending of the neck. Atlantodental lateral shift showed a significantly higher percentage in the dynamic close-mouth view than that of the dynamic open-mouth view. The difference may be because in the positioning of the dynamic close-mouth view, the neck is more flexed in comparison to the position of the dynamic open-mouth view. Relative flexion of C1/C2 makes the odontoid process sit closer to the transverse ligament, where lateral motion of the odontoid process is emphasized. Therefore, the dynamic close-mouth view can be more useful for diagnosis of atlantoaxial lateral instability in the presence of mechanical deficiency, the transverse ligament (i.e., RA). The influence of temporomandibular joint should also be considered.
Fig. 4. Measurement of ADLS.
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Fig. 5. OLMI in the dynamic open- vs. close-mouth view. Fig. 6. Atlantodental lateral shift in the dynamic open- vs. close-mouth view.
A limitation of current method is that atlantoaxial visualization might be less clear using the close-mouth view in comparison to the open-mouth view, because the radiation is beamed through the maxilla. This method is therefore not recommended for the diagnosis of trauma, such as a small odontoid fracture. Further study is necessary to evaluate the usefulness of the dynamic close-mouth view method in various pathological conditions and for making an early diagnosis of rheumatoid cervical problems. 5. Conclusion The results suggest that the dynamic close-mouth view can be useful for diagnosis of atlantoaxial lateral instability. References [1] Neva MH, Kaarela K, Kauppi M. Prevalence of radiological changes in the cervical spine—a cross sectional study after 20 years from presentation of rheumatoid arthritis. J Rheumatol 2000;27:90–3. [2] Halla JT, Hardin JG. The spectrum of atlantoaxial facet joint involvement in rheumatoid arthritis. Arthritis Rheum 1990;33: 325–9. [3] Oda T, Fujiwara K, Yonenobu K, Azuma B, Ochi T. Natural course of cervical spine lesions in rheumatoid arthritis. Spine 1995;20: 1128–35. [4] Kauppi M, Sakaguchi M, Konttinen YT, Hamalainen M, Hakala M. Pathogenetic mechanism and prevalence of the stable atlantoaxial subluxation in rheumatoid arthritis. J Rheumatol 1996;23:831–4.
[5] Pisitkun P, Pattarowas C, Siriwongpairat P, Totemchokchyakarn K, Nantiruj K, Janwityanujit S. Reappraisal of cervical spine subluxation in Thai patients with rheumatoid arthritis. Clin Rheumatol 2004;23: 14–8. [6] Neva MH, Hakkinen A, Makinen H, Hannonen P, Kauppi M, Sokka T. High prevalence of asymptomatic cervical spine subluxation in patients with rheumatoid arthritis waiting for orthopaedic surgery. Ann Rheum Dis 2006;65:884–8. [7] Nguyen HV, Ludwig SC, Silber J, Gelb DE, Anderson PA, Frank L, Vaccaro AR. Rheumatoid arthritis of the cervical spine. Spine J 2004; 4:329–34. [8] Kim DH, Hilibrand. Rheumatoid arthritis in the cervical spine. J Am Acad Orthop Surg 2005;13:463–74. [9] Fujiwara K, Yonenobu K, Ochi T. Natural history of upper cervical lesions in rheumatoid arthritis. J Spinal Disord 1997;10:275–81. [10] Ellis GL. Imaging of the atlas (C1) and axis (C2). Emerg Med Clin North Am 1991;9:719–32. [11] Taniguchi D, Tokunaga D, Hase H, Mikami Y, Hojo T, Ikeda T, Oda R, Takatori R, Imai K, Kida Y, Otakara E, Ito H, Nishimura T, Kubo T. Evaluation of lateral instability of the atlanto-axial joint in rheumatoid arthritis using dynamic open-mouth view radiographs. Clin Rheumatol 2008;27:851–7. [12] Jacobs L. Roentgenography of the second cervical vertebra by Ottonello's method. Radiology 1938;31:412–3. [13] Ajmal M, O'Rourke SK. Odontoid lateral mass interval (OLMI) asymmetry and rotary subluxation: a retrospective study in cervical spine injury. J Surg Orthop Adv 2005;14:23–6. [14] Harty JA, Lenehan B, O'Rourke SK. Odontoid lateral mass asymmetry: do we over-investigate? Emerg Med J 2005;22:625–7. [15] Hohl M, Baker HR. The atlanto-axial joint. Roentgenographic and anatomical study of normal and abnormal motion. J Bone Joint Surg Am 1964;46:1739–52.
Dynamic close-mouth view radiograph method for the diagnosis of lateral dynamic instability of the atlantoaxial joint Susumu Fujiwara a , Daisaku Tokunaga b,⁎, Ryo Oda b , Shogo Toyama b , Kan Imai b , Atushi Doi a , Toshikazu Kubo b a
Department of Radiological Technology, Kyoto Takeda Hospital, Minamikoromoda-cho, Nishi-shichijo, Shimogyo-ku, Kyoto 600-8884, Japan b Department of Orthopaedics, Graduate School of Medical Science, Kyoto Prefectural University of Medicine, Kawaramachi-Hirokoji, Kamigyo-ku, Kyoto 602-8566, Japan Received 3 July 2009; accepted 15 August 2009
Abstract The purpose of the current study is to establish the dynamic close-mouth view radiograph method. Seven healthy volunteers were enrolled. Dynamic open-mouth and close-mouth view radiographs were obtained. The odontoid lateral mass interval and the dynamic atlantodental lateral shift (ADLS) were measured. The ADLS was 9.4±5.3% by the close-mouth view, showing a significantly greater percentage than that of 5.3±4.0% by the open-mouth view. The dynamic close-mouth view can be useful for diagnosis of atlantoaxial lateral instability. © 2010 Elsevier Inc. All rights reserved. Keywords: Cervical spine; Open-mouth view; Close-mouth view; Atlantoaxial subluxation; Instability
1. Introduction The incidence of cervical involvement in patients with rheumatoid arthritis (RA) is very high, ranging from 23% to 86% in several reports [1–7]. Anterior atlantoaxial subluxation (aAAS) is the most common type of cervical spine involvement. It is very important to diagnose aAAS early since the progression of spinal cord compression may result in either a neurological deficit or even sudden death [1–3,8,9]. Lateral view radiographs of the cervical spine (during flexion and extension) are usually utilized for the diagnosis of aAAS. Static open-mouth view radiographs are often used to diagnose the collapse of the lateral mass of atlas(C1) and
axis(C2), thus, overriding the C1/C2 joint, that is, established static lateral displacement of the atlantoaxial joint [10]. Atlantoaxial involvement in RA is associated with anteroposterior sagittal instability, as well as lateral coronal instability. Dynamic open-mouth view radiographs were used to diagnose dynamic lateral instability of atlantoaxial joint [11]. However, the diagnosis of the atlantoaxial area using dynamic open-mouth view radiographs is sometimes difficult if the patient has a limited ability to open their mouth due to temporomandibular joint destruction or overlapping of the teeth, atlas, and axis. The purpose of current study is to establish the dynamic close-mouth view radiograph method for the diagnosis of lateral dynamic instability of the atlantoaxial joint. 2. Materials and methods
⁎ Corresponding author. Department of Orthopaedics, Graduate School of Medical Science, Kyoto Prefectural University of Medicine, Kawaramachi-Hirokoji, Kamigyo-ku, Kyoto, 602-8566, Japan. Tel.: +81 75 251 5549; fax: +81 75 251 5841. E-mail address: [email protected] (D. Tokunaga). 0899-7071/$ – see front matter © 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.clinimag.2009.08.027
Seven healthy volunteers were enrolled (all men; age range, 26–39 years; mean, 32 years). This study was approved by the local research ethics committee at Kyoto Takeda Hospital, Kyoto, Japan.
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S. Fujiwara et al. / Clinical Imaging 34 (2010) 375–378
2.1. Dynamic open-mouth view radiograph The patient was placed in a sitting position with their mouth open as wide as possible and the jaw tilted upward. In this position, the teeth overlap the occipital bone, making a line between the nasal spine and internal acoustic opening vertical to the cassette. The X-ray beam line was positioned at the center of the maxillary dental arcade and the mandibular dental arcade and vertical to the cassette. The patient was asked to bend his neck to the right and left as far as possible, keeping the center of mouth facing forward for the dynamic open-mouth view (Fig. 1). The radiograph was taken with 80 kV,·160 mA,·80 ms. 2.2. Dynamic close-mouth view radiograph For the close-mouth view radiograph, the patient was placed in a sitting position, with their mouth closed and the jaw down to overlap the nasal spine and occipital bone. The center of beam line was set 5 mm under the nasal spine and vertical to the cassette (Fig. 2). To obtain the dynamic close-mouth view, similar to that of the dynamic openmouth view, we asked the patient to bend his neck to the right and left as much as possible, keeping the center of the mouth facing forward (Fig. 3). The radiograph was taken with 83 kV,·160 mA,·80 ms.
Fig. 2. Close-mouth view. Upper line: OM line (orbitomeatal baseline). Middle line: line between the nasal spine and the occipital bone. Lower line: beam line.
2.3. Evaluation of radiographs All the radiographies were sent to a DICOM server and interpreted and measured with a PACS viewer (Hitachi, Tokyo, Japan). The odontoid lateral mass interval (OLMI) described by Hohl et al. was measured in open and closed mouth positions radiographs in the neck neutral position. The OLMI is the interspace from the odontoid to the corresponding point on the medial aspect of the articular mass of the axis. The dynamic atlantodental lateral shift (ADLS) described by Taniguchi et al. [11] was measured for the evaluation of lateral shift of the atlantoaxial joint. The distance between the bilateral inner distal edges of the lateral masses of the atlas (a) was measured for the measurement of the ADLS. The central axis of the odontoid (CO) and the distance between the CO and the inner distal edge of the right lateral mass of the atlas (b) were measured. The distance b was divided by the distance a and displayed as a percentage ratio (100×b/a%). The difference of the scores for the
maximum right and left neck bending was calculated as the ADLS {(b1/a1−b2/a2)×100} (Fig. 4). 2.4. Statistics The scores of ADLS and OLMI were compared for the open- and close-mouth views. The statistical significance was calculated using the paired t test. 3. Results The OLMI in the cervical neutral position was 3.5±0.8 mm (mean±S.D.) by the open-mouth view while 4.0±0.7 mm by the close-mouth view, showing significant difference between the open-mouth view and closemouth view (Pb.05) (Fig. 5). The ADLS was 9.4±5.3% by the close-mouth view, showing a significantly greater
Fig. 1. Dynamic open-mouth view.
S. Fujiwara et al. / Clinical Imaging 34 (2010) 375–378
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Fig. 3. Dynamic close-mouth view.
percentage than that of 5.3±4.0% by the open-mouth view (Pb.01) (Fig. 6).
4. Discussion An open-mouth view radiograph has been commonly used since Jacobs [12] reported the usefulness of evaluating the atlantoaxial joint in 1938. Several studies have reported radiographic atlantoaxial asymmetry by open-mouth view, usually following cervical spine trauma [13,14]. Hohl and Baker [15] studied the lateral asymmetry of atlantoaxial joints using open-mouth views. They measured the interspace from the odontoid to the corresponding point on the medial aspect of the articular mass of the axis (odontoid lateral mass interspace: OLMI) and the lateral distance from the superior lateral margin of the articular mass of the axis to the corresponding inferior lateral margin of the atlas. Taniguchi et al. [11] evaluated the lateral instability of the atlantoaxial joint in RA, using dynamic open-mouth view radiographs, and reported that the ADLS in the RA group averaged 14.8%, and this was significantly greater than in the control group, in which it averaged 6.1%. The ADLS averaged 20.6% in the RA subgroup with aAAS and 12.7% in the RA subgroup without aAAS. They hypothesized that evaluating the dynamic lateral cervical instability can be useful for the early diagnosis of atlantoaxial lesions in RA. However, it is sometimes difficult to obtain a dynamic open-mouth view in elderly patients or in rheumatoid
patients, when such patients cannot open their mouths sufficiently due to a dysfunction of the temporomandibular joint. Under this condition, the upper teeth overlap with the atlantoaxial joint in the open-mouth view, making it impossible to measure the ADLS. The dynamic closemouth view was therefore developed to resolve this problem. The atlantoaxial area can be radiographed by this method, not through the oral but through the maxilla, so that the teeth do not overlap with the target zone. Moreover, it is easier to maintain a neck position that is more tolerable for the patients during examination using the close-mouth view than the open-mouth view. The results of current study demonstrate that the measurement of the OLMI was easily obtained by the close-mouth view, showing similar data with that obtained via the open-mouth view. Furthermore, the ADLS was able to measure dynamic views during right and left bending of the neck. Atlantodental lateral shift showed a significantly higher percentage in the dynamic close-mouth view than that of the dynamic open-mouth view. The difference may be because in the positioning of the dynamic close-mouth view, the neck is more flexed in comparison to the position of the dynamic open-mouth view. Relative flexion of C1/C2 makes the odontoid process sit closer to the transverse ligament, where lateral motion of the odontoid process is emphasized. Therefore, the dynamic close-mouth view can be more useful for diagnosis of atlantoaxial lateral instability in the presence of mechanical deficiency, the transverse ligament (i.e., RA). The influence of temporomandibular joint should also be considered.
Fig. 4. Measurement of ADLS.
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Fig. 5. OLMI in the dynamic open- vs. close-mouth view. Fig. 6. Atlantodental lateral shift in the dynamic open- vs. close-mouth view.
A limitation of current method is that atlantoaxial visualization might be less clear using the close-mouth view in comparison to the open-mouth view, because the radiation is beamed through the maxilla. This method is therefore not recommended for the diagnosis of trauma, such as a small odontoid fracture. Further study is necessary to evaluate the usefulness of the dynamic close-mouth view method in various pathological conditions and for making an early diagnosis of rheumatoid cervical problems. 5. Conclusion The results suggest that the dynamic close-mouth view can be useful for diagnosis of atlantoaxial lateral instability. References [1] Neva MH, Kaarela K, Kauppi M. Prevalence of radiological changes in the cervical spine—a cross sectional study after 20 years from presentation of rheumatoid arthritis. J Rheumatol 2000;27:90–3. [2] Halla JT, Hardin JG. The spectrum of atlantoaxial facet joint involvement in rheumatoid arthritis. Arthritis Rheum 1990;33: 325–9. [3] Oda T, Fujiwara K, Yonenobu K, Azuma B, Ochi T. Natural course of cervical spine lesions in rheumatoid arthritis. Spine 1995;20: 1128–35. [4] Kauppi M, Sakaguchi M, Konttinen YT, Hamalainen M, Hakala M. Pathogenetic mechanism and prevalence of the stable atlantoaxial subluxation in rheumatoid arthritis. J Rheumatol 1996;23:831–4.
[5] Pisitkun P, Pattarowas C, Siriwongpairat P, Totemchokchyakarn K, Nantiruj K, Janwityanujit S. Reappraisal of cervical spine subluxation in Thai patients with rheumatoid arthritis. Clin Rheumatol 2004;23: 14–8. [6] Neva MH, Hakkinen A, Makinen H, Hannonen P, Kauppi M, Sokka T. High prevalence of asymptomatic cervical spine subluxation in patients with rheumatoid arthritis waiting for orthopaedic surgery. Ann Rheum Dis 2006;65:884–8. [7] Nguyen HV, Ludwig SC, Silber J, Gelb DE, Anderson PA, Frank L, Vaccaro AR. Rheumatoid arthritis of the cervical spine. Spine J 2004; 4:329–34. [8] Kim DH, Hilibrand. Rheumatoid arthritis in the cervical spine. J Am Acad Orthop Surg 2005;13:463–74. [9] Fujiwara K, Yonenobu K, Ochi T. Natural history of upper cervical lesions in rheumatoid arthritis. J Spinal Disord 1997;10:275–81. [10] Ellis GL. Imaging of the atlas (C1) and axis (C2). Emerg Med Clin North Am 1991;9:719–32. [11] Taniguchi D, Tokunaga D, Hase H, Mikami Y, Hojo T, Ikeda T, Oda R, Takatori R, Imai K, Kida Y, Otakara E, Ito H, Nishimura T, Kubo T. Evaluation of lateral instability of the atlanto-axial joint in rheumatoid arthritis using dynamic open-mouth view radiographs. Clin Rheumatol 2008;27:851–7. [12] Jacobs L. Roentgenography of the second cervical vertebra by Ottonello's method. Radiology 1938;31:412–3. [13] Ajmal M, O'Rourke SK. Odontoid lateral mass interval (OLMI) asymmetry and rotary subluxation: a retrospective study in cervical spine injury. J Surg Orthop Adv 2005;14:23–6. [14] Harty JA, Lenehan B, O'Rourke SK. Odontoid lateral mass asymmetry: do we over-investigate? Emerg Med J 2005;22:625–7. [15] Hohl M, Baker HR. The atlanto-axial joint. Roentgenographic and anatomical study of normal and abnormal motion. J Bone Joint Surg Am 1964;46:1739–52.