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congenital absence of the lumbosacral articular facet joint is an uncommon anomaly [ 112 ] ; it is usually asymptomatic and sometimes presents with a concomitant nerve root anomaly . to our knowledge ,
only three cases of congenital absence of the lumbosacral articular facet joint and a concomitant conjoined nerve root have been reported in the literature [ 9 , 12 ] .
a 39-year - old man presented with acute low back pain and right leg radiating pain .
his symptoms had rapidly worsened until he was not able to walk because of right foot drop and severe leg pain .
he was diagnosed a lumbar disc herniation at l5-s , and was referred to our hospital for surgical treatment .
neurological examinations showed a positive straight - leg raising test at 30 on the right side . on manual muscle test ( mmt ) ,
muscle weakness was found in the right leg : tibialis anterior was 1/5 , extensor hallucis longus was 0/5 , right flexor hallucis longus was 3/5 , and triceps surae was 3/5 .
the right patella tendon reflex was normal and the right ankle jerk reflex was diminished .
these findings were unusual symptoms of single - level lumbar disc herniation at l5-s1 . from plain anteroposterior ( ap ) and lateral radiographs of the lumbar spine , it was difficult to detect some spinal abnormalities . slight disc space narrowing at l5-s1 was seen on the lateral view ( fig .  1 ) .
myelographs showed incomplete blockade at the l5 pedicular level , especially on the right side ( fig .  2 ) .
computed tomography ( ct ) following myelography showed absence of the right l5-s1 facet joint ( fig . 
multidetector three - dimensional computed tomography ( 3d - ct ) confirmed the absence of s1 superior articular process ( fig . 
magnetic resonance ( mr ) images showed disc herniation on the right side at the l5-s1 intervertebral disc level ( fig . 
axial mr t2-weighted images at the l5-s disc level showed disc herniation ( fig .  5b , left panel ) , which migrated cranially ( fig . 
5b , right panel ) . based on the above neurological and radiological investigations , we diagnosed the patient as having a migrating mass that compressed the l5 nerve root and disc level lumbar disc herniation that compressed the s1 nerve root , causing these symptoms.fig . 
1plain ap and lateral radiographs of the lumbar spine . despite slight disc space narrowing at l5-s1
2myelograms showing incomplete block at l5 pedicular level , especially on the right side ( arrows)fig . 
43d - ct showing complete absence of the s1 superior articular process ( circle)fig . 
b axial mr t2-weighted images showing the herniated nucleus pulposus ( hnp , left panel ) , which migrated cranially ( b , right panel ) plain ap and lateral radiographs of the lumbar spine . despite slight disc space narrowing at l5-s1 , there was no clear abnormality myelograms showing incomplete block at l5 pedicular level , especially on the right side ( arrows ) ct performed following myelography , showing absence of the right l5-s1 facet joint 3d - ct showing complete absence of the s1 superior articular process ( circle ) a mr sagittal images showing disc herniation at l5-s1 intervertebral disc level .
b axial mr t2-weighted images showing the herniated nucleus pulposus ( hnp , left panel ) , which migrated cranially ( b , right panel ) at first , to detect the l5 nerve root , we performed a fenestration at the right l4 - 5 using a microscope . at the l4 - 5 level
we found that the l5 nerve root originated from the caudal level of the l5 pedicle and conjoined with the s1 nerve root ( fig . 
the large disc herniation at l5-s1 migrated cranially and was found beneath the abnormal conjoined nerve roots .
after removal of the disc herniation and unroofing of the nerve root , no obvious mobility of the conjoined nerve root was obtained.fig . 
the absence of the s1 superior articular processes was obvious , and the l5 and s1 nerve roots were conjoined schema of anomalies in this case .
the absence of the s1 superior articular processes was obvious , and the l5 and s1 nerve roots were conjoined postoperatively , leg pain immediately disappeared and muscle weakness slightly improved 3  months later : mmt of the right tibialis anterior and extensor hallucis longus became 2/5 , and that of the right flexor hallucis longus and triceps surae became 5/5 . at follow - up
examination 1  month later , axial mr t2-weighted images from the s1 pedicle level showed the two confluent ( l5 and s1 ) rootlets ( fig . 
the patient was informed that data concerning the case would be submitted for publication , and he consented.fig . 
7sagittal mr images obtained 1  month after the surgery , showing two confluent ( l5 and s1 ) rootlets ( arrows ) sagittal mr images obtained 1  month after the surgery , showing two confluent ( l5 and s1 ) rootlets ( arrows )
congenital absence of the lumbosacral articular facet joint is rare , with only 25 cases reported in the english - language literature [ 112 ] . in the past cases ,
rowe et  al . found only one case of congenital absence of the inferior articular process of l5 in 1,539 specimens .
the etiology of this anomaly is generally accepted as follows : there are three primary ossification centers in each vertebra : one center in the vertebral body and two in the neural arches [ 5 , 6 , 8 , 11 , 13 ] .
although congenital absence of the lumbosacral articular facet joint seems to cause spinal instability , in only 3 of 25 cases was spinal fusion performed [ 5 , 7 , 13 ] .
most of the symptomatic patients were treated with conservative therapy and became asymptomatic . only in severe cases
numerous cases have been reported in the literature [ 2 , 9 , 12 , 1426 ] .
reported that 0.25% of 1,200 patients who underwent ct or mri had conjoined nerve roots .
kadish et  al . also reported that conjoined l5 and s1 roots are the most frequent ( 52.2% ) .
stated that lumbosacral nerve root anomalies were almost asymptomatic and sometimes presented with symptoms by disc herniation .
the present case showed both anomalies , i.e. , complete absence of the superior articular process of s1 and conjoined l5 and s1 nerve roots , which were symptomatically compressed by a large disc herniation .
osseous anomalies in the lumbosacral spine associated with conjoined nerve roots such as this case have been rarely reported .
however , rask stated that the presence of congenital osseous defects of the lumbosacral spine should indicate presence of nerve root anomalies . to our knowledge
, there have been 11 cases , including defect or anomaly of the vertebral arch , spondylosis , spondylolisthesis , and bifid sacrum , presenting with conjoined nerve roots [ 9 , 11 , 12 , 15 , 17 , 2426 ] .
only three previous cases were similar to the present case ( table  1 ) [ 9 , 12].table  1summary of previous cases of congenital absence of the lumbosacral articular facet joint associated with conjoined nerve rootcaseauthorssex , age ( years)symptomslocation of anomaliesconjoined nerve rootspresence of hnp1postacchini et  al .
unknownlbpl5-s1l5 & s1present caseyoshioka et al.male , 39lbp , leg pain , palsyl5-s1l5 & s1+hnp herniated nucleus pulposus , lbp low back pain summary of previous cases of congenital absence of the lumbosacral articular facet joint associated with conjoined nerve root hnp herniated nucleus pulposus , lbp low back pain in summary , we report a quite rare case of congenital absence of lumbosacral articular facet and conjoined nerve root .
surgeons should be aware of this nerve root anomaly when a patient shows an unusual clinical presentation and/or congenital osseous anomaly .
we report a rare case of congenital absence of the l5-s1 facet joint , which was associated with a conjoined nerve root . 
 combination of these two anomalies has been quite rarely reported in the literature . 
 a 39-year - old man presented with acute low back pain and right leg radiating pain . muscle weakness and sensory disturbance of the right leg were also apparent in the region innervated by l5 and s1 nerve roots . 
 preoperative multidetector three - dimensional computed tomography ( 3d - ct ) showed complete absence of the right s1 superior articular process . 
 magnetic resonance ( mr ) images showed lumbar disc herniation at right l5-s1 level that migrated cranially . 
 intraoperative findings revealed that the right l5 nerve root and s1 nerve root were conjoined , and the conjoined nerve root was compressed by l5-s1 disc herniation , which led to impairment of the conjoined nerve root by a single - level lumbar disc herniation . 
 after removal of the disc herniation , his right leg pain immediately subsided , however muscle weakness and sensory disturbance persisted . 
 surgeons should be aware of this nerve root anomaly when examining a patient who shows an unusual clinical presentation and/or congenital osseous anomaly .