takeuchi et al.1 explained that the transverse ligament becomes degenerated and hypertrophic because of chronic mechanical stress by atlantoaxial subluxation .
then , a part of the ligament develops reactive granulation tissue with a small vessel formation .
finally , rupture of these small vessels cause repeated episodes of microbleeding , resulting in formation of a cyst
. direct excision of the cysts and fixation has commonly been performed for the surgical treatment of retro - odontoid cystic mass associated with chronic atlantoaxial subluxation.12345 however , when the cyst is located in the ventral side of spinal cord , direct resection is technically dangerous . here ,
we present a case of a retro - odontoid cyst associated with chronic atlantoaxial subluxation and located in the ventral side of spinal cord .
the cystic mass disappeared after posterior c1 and partial c2 laminectomy and c1-c2 pedicle screws fixation without resection of the retro - odontoid cyst .
a 64 year old woman reported experiencing a sudden onset of neck pain , hand and foot paresthesia when she woke early in the morning .
she was sent to a local hospital , where she was treated conservatively with external stabilization of the neck and bed rest . at 2 weeks later
x - ray showed that the atlantoaxial instability [ figure 1a - c ] . computed tomography revealed an abnormality of the odontoid process [ figure 1d and e ] .
magnetic resonance imaging detected an oval retro - odontoid cystic mass , which compressed the spinal cord .
the mass showed uniform low intensity on t1-weighted image [ figure 1f ] and uniform high signal intensity on t2-weighted image [ figure 1 g ] .
( a - c ) preoperative open - mouth anteroposterior radiograph and dynamic flexion and extension radiograph showing the atlantoaxial instability .
( d and e ) preoperative computed tomography revealed an abnormality of the odontoid process .
( g ) the mass showed uniform high signal intensity on t2-weighted image the surgery was carried out with the patient positioned prone with the head slightly flexed and a midline incision was made .
subsequently , paraspinal muscles were peeled to expose the bilateral laminae of c1 and c2 .
the laminectomy of c1 and partial c2 were performed , and then the neck was slightly extended to reduce the atlanto - axial joint .
finally , an autogenous iliac crest graft was implanted in to c1 and c2 bilateral joints to facilitate fusion .
x - ray [ figure 2a and b ] , computed tomography [ figure 2c ] and magnetic resonance imaging [ figure 2d ] were taken on the day of surgery . at 24 months after surgery ,
the retro - odontoid cystic mass was no longer present [ figure 2e ] , and the patient remained symptom free and returned to independent daily living . magnetic resonance imaging scans at 24 months after surgery showed that atlantodental interval was wider , compared to the day of surgery .
( a and b ) postoperative open mouth anteroposterior radiograph and lateral radiograph showing the rigid fixation between c1-c2 and atlantoaxial joint had been reset .
( d ) postoperative sagittal t2-weighted magnetic resonance image showed that the mass still compressed the spinal cord .
( e ) sagittal t2-weighted magnetic resonance image taken 24 months after surgery showed that the mass was no longer present
an atlantoaxial subluxation associated with a non tumorous lesion in the posterior region of the odontoid process had been described as a retro - odontoid cyst or retro - odontoid pseudotumor in the initial report by sze et al.6 it usually occurs in the elderly , without clear association to any antecedent trauma .
most patients are between 60- and 80-years - old , with both sexes being equally affected .
all of these patients are seronegative for rheumatoid arthritis or other inflammatory arthropathies.7 however , the etiology of the cysts remains controversial . in the postulated view on its pathomechanism , preexisting atlantoaxial instability
was presumed to cause repeated tear and subsequent hypertrophy of the transverse ligament , thus leading to the formation of the pseudotumor.8 takeuchi et al.1 explained that the transverse ligament of axis became degenerated and hypertrophic because of chronic mechanical stress by atlantoaxial subluxation .
then , a part of the ligament developed reactive granulation tissue with small vessel formation .
finally , rupture of these small vessels caused repeated episodes of microbleeding , resulting in formation of a cyst .
optimal treatment for a retro - odontoid cystic mass associated with chronic atlantoaxial subluxation has not been established .
. transoral or anterolateral decompression is theoretically the optimal option.2345 takeuchi et al.1 reported a case that they incised the dura to reach the mass and resect through the posterior approach .
however , the anterior and posterior surgical excision involve a lot of complex technical problems and requires more operating time and blood loses , higher risks and complications and should be performed only by an experienced surgical team .
it is also reported that posterior fixation of such degenerative tissue would reduce repetitive mechanical stimulation , thereby inhibiting stress on the spinal cord and reducing the inflammation which had produced the degenerative lesions.910 similar to degenerative cysts , retro - odontoid hypertrophy of soft tissue mass in a non rheumatoid patient with atlantoaxial subluxation has been reported to decrease after posterior fixation.1112131415 nevertheless , the surgical procedure is appropriate for no neurologic symptoms and the small cystic mass . for patients with large cystic mass leading to severe canal stenosis ,
the atlanto - axial joint reduction is prone to trauma to the spinal cord , which might cause spinal damage or even paraplegia .
we selected posterior c1 and partial c2 laminectomy and c1-c2 pedicle screw fixation without resection of the retro - odontoid cyst in the patient because the cyst was located in the ventral side of spinal cord and the neurological deficit was less obvious .
the posterior arch of the atlas and partial laminae of c2 were resected , providing the available space for the spinal cord drift . as far as possible , avoid damaging the nervous system .
c1-c2 pedicle screws fixation might provide immediate and long term stability of the cervical spine , and is helpful in preventing an increase in the size and also to shrink the cyst . during the 24 months
followup period , the cyst disappeared completely , and the patient remained symptom free and returned to independent daily living .
these findings suggest that posterior laminectomy and fixation without resection of the retro - odontoid cyst is relatively simple and safe , and the results are satisfactory when the large cyst was located in the ventral side of spinal cord and the neurological deficit was less obvious .
however , if the neurological deficit is progressive and severe , then resection of the retro - odontoid cysts should be selected because it could take time for the pseudotumor to disappear .
 retro - odontoid cysts associated with chronic atlantoaxial subluxation are extremely rare . 
 this article describes a case of retro - odontoid cystic mass associated with chronic atlantoaxial subluxation and its management with posterior c1 and partial c2 laminectomy and c1-c2 pedicle screw fixation without resection of the retro - odontoid cyst . 
 a 64-year - old woman experienced a sudden onset of neck pain , hand and foot paresthesia . 
 atlantoaxial instability associated with a retro - odontoid cystic mass was found in the imaging . 
 the patient underwent posterior c1 and partial c2 laminectomy and c1-c2 pedicle screws fixation without resection of the retro - odontoid cyst . during the 24 months followup period , the cyst disappeared completely and the patient remained symptom free and returned to independent daily living . 
 these findings suggest that posterior laminectomy and fixation without resection of the retro - odontoid cyst is relatively simple and safe and the results are satisfactory . 
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