retro - odontoid pseudotumor ( rop ) or pannus is a non - neoplastic mass at the craniovertebral junction,813 ) which can cause cord compression and neurological symptoms.28 ) in most of cases , rop is associated with atlantoaxial subluxation ( aas ) in patients with rheumatoid arthritis ( ra).8121314 ) however , rop has been rarely reported in patients without ra and ass.13 ) the exact mechanism of rop formation without aas and ra is unclear . here , we report a rare case of rop developed in patients without both ra and aas and suggest a possible mechanism with literature review . a 76-year - old woman presented with progressive gait disturbance and dysesthesia at posterior neck for 3 weeks . neurological examination revealed typical cervical myelopathy with clumsiness of both hands , gait disturbance and hypesthesia on both extremities . in laboratory study , serum rheumatoid factor and anti - a physical examination showed no signs of ra , such as polyarthritis of the hands , wrist , and feet and extraarticular disease manifestation . dynamic radiography of the cervical spine showed no evidence of aas , showing only 2 mm of atlantodental interval ( figure 1 ) . magnetic resonance image ( mri ) showed a retro - odontoid mass extending from posterior part of axis up to cranial base , and compressing the spinal cord . the mass was seen as an iso - intensity area on t1-weighted images ( wi ) , a mixture of low- and high - intensity on t2-wi , with absence of contrast enhancement after gadolinium administration . the t2-wi showed a high - intensity area in the intramedullary region of the upper cervical spinal cord ( figure 2 ) . computed tomography ( ct ) showed no calcification in the mass and severe spondylotic change with facet joint osteoarthritis at whole cervical spinal segments ( figure 3 ) . based on the radiographical findings , the differential diagnosis was rop . however , because the patient had no ra and aas , the tumorous lesions or disc herniation should be considered as a differential diagnosis . considering her old age and no aas , we planned mass removal and biopsy only via a minimal laminectomy . first , left lateral suboccipital craniotomy and hemilaminectomy of c1 were performed , but the mass was not exposed by extradural view . paramedian durotomy was performed and ventral dura also opened beside the spinal cord and rootlets . after opening of ventral dura , yellowish fluid was expelled and yellowish friable mass was easily removed by a ring curette . enough decompression was performed and ventral and dorsal dura was closed water - tightly . further c1 - 2 fixation was not performed . postoperatively , her symptoms were slightly improved , but again aggravated 2 months after surgery . dynamic x - rays showed newly - onset aas showing 5 mm of atlantodental interval . a follow - up mri showed the re - grown mass , but still spinal cord compression due to the mass and aas ( figure 4b ) . based on histopathological diagnosis of pannus , a posterior c1 lateral mass - c2 pedicle screw fixation was performed without further mass removal or laminectomy . from 3rd postoperative day twelve months after the operation , a follow - up mri showed complete resolution of the rop with c1 - 2 bony fusion and her symptoms completely improved ( figure 4c ) . rop is frequently associated with ra.38121314 ) in ra , the pannus formation is attributed to inflammation of the synovial membrane , resulting in overgrowth of the hyaline cartilage and peri - articular inflammation.4 ) consequentially , inflammation , ligament laxity , and bone erosion result in c1 - 2 joint laxity and subluxation . although uncommon , a rop can be developed due to aas in patients without ra.13 ) aas exerts mechanical stress and injury on the transverse ligament of the atlas , resulting in a reactive hypertrophied mass . therefore , aas may play an important role in the development of rop.61012 ) however , like our case , a rop unassociated with both aas and ra is extremely rare . only four cases of rop unassociated with both aas and ra have been previously reported ( table 1).271314 ) the exact mechanism of pannus formation is unclear . however , the previous reports suggested that loss of mobility at middle and lower cervical segment , and resultant excessive motion of the c1 - 2 segment transfer the mechanical stress to the atlantoaxial joint , resulting in pannus formation.1315 ) in the case , mobility below the c2 cervical spine was limited by spondylosis and facet fusion at c2 - 3 - 4 levels . in our case , however , in the other reports , a rop developed in patients with mild spondylosis or without spondylosis in subaxial cervical spine.61011 ) therefore , only loss of mobility associated with subaxial cervical spondylosis is difficult to completely explain the mechanism of rop formation in patients without aas . reviewing the reported cases and our case , we found two common points in the patients . all patients were more than 70 years old and most had moderate or severe osteoarthritic change at atlantoaxial joint . thus , we speculated that moderate or severe osteoarthritis of c1 - 2 itself causes a partial tear or degradation of the transverse ligament . resultant vicious cycle of repair and injury may lead to progressive rop . however , unlike ra , the rop in the elderly may be not associated with overt aas . instead , we suggest that stiffness due to severe osteoarthritis of lateral atlantoaxial joints may prevent the overt aas , offsetting the effect of loose transverse ligament . nevertheless , we still believe that c1 - 2 instability play a key role in development or progression of rop because most of rop associated with aas and often resolve spontaneously after c1 - 2 fusion.5678121314 ) in the elderly with osteoarthritis , even though c1 - 2 instability is not " overt aas " on the radiographs , " micro - instability " may be exist and affect the progression of rop . surgical treatment of rop can be classified into two modalities : 1 ) direct excision of the mass and 2 ) posterior decompression and fusion aiming spontaneous tumor regression.114 ) direct excision of the mass can be performed by the transoral approach and epidural or transdural approach by posterior laminectomy.1911 ) however , the transoral approach is vulnerable to cerebrospinal fluid leakage , postoperative infection and postoperative instability.911 ) the epidural approach may force retraction of the nerve root and spinal cord and have hemorrhagic risk.11 ) a posterior transdural approach may decrease the distance needed to reach the rop and does not require retraction of the spinal cord and nerve root . however , if the mass located in the midline of cervical axis or presented with huge size , the posterior transdural approach is limited used for radical resection due to risk of injuries . in our case , differential diagnosis of tumorous lesion or disc herniation should be necessary due to absence of aas and ra.613 ) therefore , in the first operation , we performed direct mass debulking and biopsy by transdural approach with minimal hemilaminectomy . however , the direct mass excision and hemilaminectomy can unexpectedly new - onset aas associated with additional ligament injury , resulting in re - growth of rop and neurological aggravation . recently , posterior decompression and fusion may be considered as first choice of treatment for rop with aas.68121314 ) in our case , c1 lateral mass - c2 pedicle screw fixation without further laminectomy or mass removal was performed and the rop regressed completely . goel and dange4 ) reported the immediate postoperative regression of rop after lateral mass reconstruction in the patient with ra . we report a case of rop without aas and ra , successfully treated with c1 - 2 fixation alone and achieved the resolution of rop and good neurological outcome .
we present a case of retro - odontoid pseudotumor ( rop ) without rheumatoid arthritis ( ra ) or atlantoaxial subluxation ( aas ) . 
 a 76-year - old woman presented with paresthesia and weakness of both lower and upper extremities . 
 she had no laboratory or physical findings of ra . 
 dynamic x - ray showed no aas and magnetic resonance imaging ( mri ) showed a retro - odontoid mass compressing the spinal cord . 
 transdural mass debulking and biopsy were performed via minimal left suboccipital craniectomy and c1 hemilaminectomy . 
 two months after surgery , her symptoms were aggravated . 
 follow - up mri visualized mass re - growth and spinal cord compression due to the mass and aas . 
 posterior c1 - 2 fixation was performed without further decompression . 
 twelve months after posterior fixation , no symptoms were apparent and follow - up mri showed complete resolution of the rop with c1 - 2 bony fusion . 
 the rop with c1 - 2 instability might be completely resolved only c1 - 2 fusion without decompression . 
 furthermore , we speculated that osteoarthritis of c1 - 2 itself causes a partial tear or degradation of the transverse ligament , that induced formation of rop .