Introduction

Congenital absence of the lumbosacral articular facet joint is an uncommon anomaly [1–12]; 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].

We report a symptomatic case and the outcome of treatment.

Case report

A 39-year-old man presented with acute low back pain and right leg radiating pain. He had no history of trauma or other trigger event. 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. Hypoesthesia was noted in the right region innervated by L5 and S1 nerve roots. Neurologically, L5 and S1 nerve roots impairment was indicated. 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. 3). Multidetector three-dimensional computed tomography (3D-CT) confirmed the absence of S1 superior articular process (Fig. 4). Magnetic resonance (MR) images showed disc herniation on the right side at the L5-S1 intervertebral disc level (Fig. 5a). 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. 1
figure 1

Plain AP and lateral radiographs of the lumbar spine. Despite slight disc space narrowing at L5-S1, there was no clear abnormality

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Fig. 2
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Myelograms showing incomplete block at L5 pedicular level, especially on the right side (arrows)

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Fig. 3
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CT performed following myelography, showing absence of the right L5-S1 facet joint

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Fig. 4
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3D-CT showing complete absence of the S1 superior articular process (circle)

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Fig. 5
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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)

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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, no nerve root was found, and right L5 hemilaminectomy was additionally conducted.

We found that the L5 nerve root originated from the caudal level of the L5 pedicle and conjoined with the S1 nerve root (Fig. 6). 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. 6
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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

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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. 7). At present, our patient can walk with an ankle–foot orthosis without a cane. The patient was informed that data concerning the case would be submitted for publication, and he consented.

Fig. 7
figure 7

Sagittal MR images obtained 1 month after the surgery, showing two confluent (L5 and S1) rootlets (arrows)

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Discussion

Congenital absence of the lumbosacral articular facet joint is rare, with only 25 cases reported in the English-language literature [1–12]. In the past cases, the L5-S1 facet joint was mostly affected (80%). Rowe et al. found only one case of congenital absence of the inferior articular process of L5 in 1,539 specimens [11]. Roche et al. studied 4,200 skeletons and failed to find this anomaly [10].

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]. The facet defect is thought to originate from a failure of these ossification processes. However, the true etiology has not been clarified.

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 must surgical fusion of the vertebra be performed. Because of the small number of cases, treatment remains controversial.

Conjoined nerve roots are relatively common anomalies of the lumbosacral spine. Numerous cases have been reported in the literature [2, 9, 12, 14–26]. The incidence of lumbosacral conjoined nerve roots varies among reports. Kadish et al. found eight conjoined nerve roots in 100 cadavers (8% incidence) [22]. Recently, Artico et al. reported that 0.25% of 1,200 patients who underwent CT or MRI had conjoined nerve roots [2]. Kadish et al. also reported that conjoined L5 and S1 roots are the most frequent (52.2%) [22]. Postacchini et al. stated that lumbosacral nerve root anomalies were almost asymptomatic and sometimes presented with symptoms by disc herniation [9].

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 [24]. 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, 24–26]. Only three previous cases were similar to the present case (Table 1) [9, 12].

Table 1 Summary of previous cases of congenital absence of the lumbosacral articular facet joint associated with conjoined nerve root
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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.