1 Introduction

Free-floating disc herniation is relatively rare in clinical practice, with most cases involving the nucleus pulposus located anterior to the vertebral canal.Migration of the nucleus pulposus to the dorsal side of the nerve root is extremely rare.with an estimated incidence of 0.27 %.¹ Such cases are often misdiagnosed as intraspinal tumors in clinical settings.²At the Minimally Invasive Spine Centre of our hospital, we recently admitted a patient presenting with a large nucleus pulposus at the L4/5 level that had migrated dorsally to the nerve root.The details of this case are presented below (see Figs. 1–12).

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Fig. 1 Preoperative T2-weighted MRI of the lumbar spine.

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Fig. 2 Preoperative T1-weighted MRI of the lumbar spine.

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Fig. 3 Preoperative lumbar spine MRI in the transverse plane.

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Fig. 4 Preoperative T2-weighted contrast-enhanced MRI of the lumbar spine.

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Fig. 5 Preoperative T1-weighted contrast-enhanced MRI of the lumbar spine.

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Fig. 6 Preoperative contrast-enhanced MRI of the lumbar spine in the transverse plane.

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Fig. 7 Intraoperative endoscopic image depicting the dorsal free fragment of nucleus pulposus adjacent to the L5 nerve.

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Fig. 8 A large nucleus pulposus was removed.

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Fig. 9 Postoperative T2-weighted MRI of the lumbar spine.

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Fig. 10 Postoperative T1-weighted MRI of the lumbar spine.

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Fig. 11 Postoperative lumbar spine MRI Improved terminology.

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Fig. 12 pathological examination: degenerative mucinous fibrocartilage tissue, consistent with nucleus pulposus characteristics.

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2 Case report

The patient, a 54-year-old male, was admitted to our department on September 18, 2020, with complaints of low back pain radiating to the left lower leg,which had progressively worsened over the past three months and significantly intensified in the last three days. The initial onset of symptoms occurred three months ago without any clear cause, with pain radiating to the anterolateral aspect of the left lower leg.Conservative treatments, including acupuncture, massage, traditional Chinese medicine, and oral anti-inflammatory and analgesic medications, did not yield significant improvement. three days prior to admission, the patient experienced worsening of symptoms following fatigue, prompting him to seek medical care at our hospital. Magnetic resonance imaging (MRI) of the lumbar spine revealed abnormal signals in the left posterolateral aspect of the dural sac at the L4/L5 segment. An enhanced MRI was recommended, which the patient subsequently obtained from an external facility.The enhanced MRI showed abnormal signal plaques in the same region. with initial consideration given to free nucleus pulposus fragments, though incomplete space occupation could not be ruled out.the patient was diagnosed with lumbar disc herniation and a possible spinal canal space-occupying lesion during the outpatient consultation and was admitted for further evaluation and treatment.

Physical examination:The patient exhibited significant tenderness upon percussion between the spinous processes at the L4/5 level. the left straight leg raise test was positive, as was the strengthening test. there was hypoesthesia over the lateral aspect of the left lower leg and the dorsum of the foot. muscle strength of the left extensor hallucis was grade 4, while the plantar flexor muscle strength was grade 5.on the right side, both extensor hallucis and plantar flexor muscle strengths were grade 5. the knee and Achilles tendon reflexes were symmetrical bilaterally. patellar and ankle clonus tests were negative bilaterally,and the Babinski sign was also negative on both sides.

Pre-admission examination: On September 17, 2020, a MRI of the lumbar spine performed at our hospital revealed a herniated intervertebral disc at the L4/5 segment with secondary spinal stenosis and an abnormal signal in the left posterolateral aspect of the dura mater. an enhanced MRI was recommended (Figures A, B, and C). On September 18, 2020,an enhanced MRI from an external facility showed an abnormal signal plaque in the left posterolateral aspect of the dural sac at the L4/5 segment. the initial differential diagnosis included free nucleus pulposus fragments, with incomplete exclusion of a space-occupying lesion (Figures D, E, and F). the patient was informed that the presence of a space-occupying lesion could not be definitively ruled out,though the primary consideration was a free nucleus pulposus fragment,It was recommended that the nucleus pulposus be removed using percutaneous transforaminal endoscopic discectomy (PTED).The patient was also informed that if an intraoperative assessment confirmed a space-occupying lesion requiring further intervention, a secondary open surgery might be necessary.after thorough communication, transforaminal endoscopic lumbar discectomy via an interlaminar approach at L4/5 was performed under general anesthesia on September 21, 2020.During the procedure, a large free fragment of the nucleus pulposus was identified on the dorsal side of the left L5 nerve root. The nucleus pulposus was successfully excised using nucleus pulposus forceps, and subsequent inspection revealed decompression of the left L5 nerve root (Figures G, H, and I).Postoperative MRI demonstrated resolution of L4/5 segment compression and disappearance of the abnormal signal plaque in the left posterolateral dura mater (Figures J, K, and L).Histopathological examination confirmed that the excised tissue was nucleus pulposus (Figure M).Following the surgery, the patient experienced significant relief from both low back pain and radiating pain in the left lower extremity.No recurrence of symptoms has been observed during follow-up,and the patient expressed satisfaction with the treatment outcome.

3 Discussion

Due to prolonged exposure, herniated nucleus pulposus tissue may become eroded or encapsulated by inflammatory granulation tissue, resulting in difficulty identifying the free nucleus pulposus. this makes it challenging to radiographically distinguish it from spinal canal tumors, potentially leading to misdiagnosis and inappropriate treatment, with serious consequences.³ a literature review⁴ summarizes the causes of misdiagnosing a free nucleus pulposus as a spinal canal tumor as follows: (1) a free nucleus pulposus may cause clinical symptoms similar to those of an intraspinal tumor; (2) similar to lumbar disc herniation, a free nucleus pulposus most frequently occurs at L4/5 and L5/S1; (3) unlike the normally avascular nucleus pulposus surrounded by the annulus fibrosus, a free nucleus pulposus outside the dural sac can become vascularized, resulting in varying degrees of enhancement in MRI contrast imaging, which can contribute to misdiagnosis. Some researchers have used discography to differentiate between a nucleus pulposus and a spinal canal tumor.⁵ other researchers⁶ suggest that MRI images of spinal canal tumors are generally round, oval, or dumbbell-shaped, with clear boundaries. most enhanced MRI scans show increased enhancement due to the tumor's abundant blood supply, which produces signals inconsistent with the surrounding intervertebral disc tissue. In the present case, preoperative enhanced MRI of the lumbar spine revealed an abnormal signal plaque on the left posterolateral side of the dural sac at the L4/5 segment, consistent with the appearance of a free nucleus pulposus.postoperative pathological diagnosis confirmed this finding.

Symptoms caused by a free nucleus pulposus are typically severe and often necessitate early surgical decompression.^1,6,8 however, there is currently no consensus regarding the optimal surgical approach. some studies^4,7,8 have reported satisfactory clinical outcomes using total laminectomy for treating a free nucleus pulposus located dorsally to the dura.for patients with a free nucleus pulposus in zones 1 and 4, resection of the articular process and part of the lamina is often required, which can potentially lead to lumbar instability. Previously, some researchers^9–12 suggested that free nucleus pulposus in zones 2 and 3 could be treated using intervertebral foraminoscopy, whereas zones 1 and 4 required open surgery.as the percutaneous transforaminal endoscopic discectomy (PTED) technique has matured, its indications have expanded, making it suitable for treating most cases of free nucleus pulposus. for cases involving a free nucleus pulposus located posterior to the dura mater, percutaneous endoscopic interlaminar discectomy (PEID) can be used. comparisons of the PTED technique with laminotomy and decompression surgery have indicated that PTED can effectively remove extradurally located disc tissue under local anesthesia in a safer, minimally invasive manner, with clinical outcomes comparable to those of traditional laminotomy and decompression.^11,12

In this case, the patient's free nucleus pulposus was located dorsally to the L5 nerve root. the percutaneous endoscopic interlaminar discectomy (PEID) technique was employed to remove a portion of the ligamentum flavum, allowing for rapid identification of the free nucleus pulposus on the dorsal side of the nerve root. the nucleus pulposus was then successfully excised using nucleus pulposus forceps. postoperatively, the patient's radicular symptoms were completely resolved, with minimal surgical trauma and a swift recovery.

CRediT authorship contribution statement

Xiao-Long Zhu: Conceptualization, data collection, work design, Writing – original draft. Yong-Jun Hua: Conceptualization, manuscript approval and supervised the study.

Informed consent

Patient were informed that their data would be submitted for publication and provided consent.

Ethical statement

Publication of this case report was approved by the Clinical Research Ethics Committee of Hangzhou Fuyang Hospital of Orthopedics of Traditional Chinese Medicine.

Consent for publication

Written informed consent was obtained from the patient for publication of this case report.

Funding

This article was funded by Hangzhou Medicine and Health Science and Technology Project Fund, Project No. B20220070.