A patient presented with a 3-year history of chronic low back pain with daily symptoms, exacerbated by standing and ambulation. The pain radiated bilaterally to the lower extremities and was associated with numbness over the plantar aspects of both feet. Pain severity was reported as 9/10 on the visual analogue scale (VAS). The patient-reported prolonged sitting during work hours. Physical examination revealed tenderness over the bilateral lower lumbar paraspinal regions and hamstring tightness, more pronounced on the right side. Motor strength was preserved (grade 5/5) in all lower extremity muscle groups. Six months prior to presentation, the patient underwent targeted injections, including an L5–S1 facet joint block directed at the pseudoarticulation, as well as bilateral transforaminal epidural steroid injections. These interventions resulted in approximately 70% symptom relief lasting for 4 months. This combined diagnostic and therapeutic response supported the involvement of both the pseudoarticulation and L5 nerve root compression in symptom generation, thereby guiding our surgical strategy. Conservative treatment, including physical therapy and home-based exercises, provided only partial improvement. However, symptoms progressively worsened over the preceding month, limiting ambulation to less than 5 minutes.

Plain radiographs demonstrated a congenital block vertebra at L4–L5. At L5–S1, pseudoarticulation was observed between the right L5 transverse process and the sacral ala, consistent with a Castellvi type II anomaly on the right and type I on the left. Magnetic resonance imaging and computed tomography (CT) revealed bilateral far-out compression of the L5 nerve roots associated with pseudoarticulation (Figure 1). Three-dimensional CT reconstruction confirmed a Castellvi type IV LSTV with bilateral extraforaminal pathology (Figure 2).

The patient was positioned prone, and 2 pins were inserted into the left iliac crest to secure the navigation reference frame. A navigation system (Brainlab AG, Germany) was integrated with a mobile 2-dimensional/3-dimensional (2D/3D) C-arm (CIARTIC Move, Siemens Healthineers AG, Germany) to provide real-time intraoperative guidance. The L5–S1 disc level and pedicles were identified and marked on the skin. A navigated probe was first used to guide the skin incision, ensuring an accurate trajectory from the outset. The exact incision position may vary slightly between sides depending on patient anatomy; in general, the right-sided entry point is placed slightly more caudal and/or lateral to avoid a high iliac crest, whereas the left side typically permits a more direct trajectory. In addition, for a right-handed surgeon, the working portal on the right side is positioned slightly closer to the disc level to facilitate ergonomic access to the disc and foraminal region. Two 1-cm incisions were created lateral to the pedicles. Under navigation guidance, a probe was advanced toward the triangulation zone between the isthmus and transverse process of L5. A UBE approach was utilized with a 30° endoscope [4] (Figure 3). Intraoperatively, navigation was utilized continuously rather than for repeated docking: a navigated probe guided the initial trajectory, confirmed key anatomical landmarks before bony work, and verified the adequacy of the bony window after decompression, complementing direct endoscopic visualization. Although a 0° endoscope is more commonly employed in UBE procedures, a 30° endoscope was selected here to provide a wider field of view and deeper visualization in the paraspinal approach; the angled lens can be rotated medially to facilitate access to the intraforaminal region, enhancing surgical reach within the foramen. Its use should be considered selectively, based on the anatomical requirements of the case and the surgeon's experience.

A structured workflow was employed to achieve safe, reproducible outcomes during far-out decompression. The procedure begins with a paraspinal approach and initial docking at the L5 isthmus, followed by identification of key anatomical landmarks, including the transverse process and sacral ala. Foraminotomy is then performed to enlarge the neural foramen and improve access to the extraforaminal zone. Subsequent release of soft tissue structures, particularly the foraminal and iliolumbar ligaments, facilitates adequate exposure. Adhesiolysis and careful mobilization of the nerve root are carried out to relieve tension and improve visualization. When pseudarthrosis is present, it is assessed and resected as necessary; resection (pseudoarthrectomy) is indicated primarily in Jenkins type I and II cases, in which the pseudoarticulation is confirmed as the principal pain generator by a positive diagnostic block. Throughout these steps, navigation is used continuously rather than for repeated docking, confirming the medial and lateral bony margins and the adequacy of the bony window in addition to direct endoscopic visualization, until complete decompression is verified.

The right side demonstrated foraminal stenosis combined with pseudarthrosis. Initial docking was performed at the L5 isthmus (Figure 3). The transverse process margins were identified, followed by dissection of the pseudarthrosis capsule. Foraminotomy was performed using a 4.0-mm burr to resect the sacral ala. The foraminal ligament was released, and adhesiolysis was carried out to mobilize the exiting L5 nerve root. Navigation confirmed the anatomical location. Pseudarthrosis resection was performed between the inferior L5 transverse process and sacral ala using a diamond burr and Kerrison rongeur. The lateral and medial margins were confirmed with navigation to ensure complete resection. Final visualization demonstrated full decompression and free mobilization of the nerve root.

On the left side, only foraminal decompression was required. Docking was performed at the L5 isthmus, followed by identification of key bony landmarks (Figure 3). The foraminal ligament and the iliolumbar ligament were released. Adhesiolysis was performed, and complete decompression of the exiting nerve root was confirmed.

On postoperative day 1, the patient-reported immediate improvement in both back and leg pain, with a VAS score of 5/10, further improving to 1/10 at the 3-month follow-up. Functional outcomes showed parallel improvement, with the Oswestry Disability Index decreasing from 80.0 preoperatively to 48.9 on postoperative day 1 and 31.1 at 3 months. Postoperative radiographs demonstrated complete resection of the right-sided pseudarthrosis with preservation of the L5–S1 facet joint. Postoperative CT and magnetic resonance imaging further confirmed adequate decompression and preservation of the facet joint (Supplementary Video Clip 1). At 3-month follow-up, motor strength was grade 5/5 in all muscle groups, with no sensory deficits or limitations in walking and daily activities.

In this study, we review far-out syndrome, including its clinical relevance and underlying anatomy. We discuss the biomechanical mechanisms of L5 nerve root compression, the diagnostic challenges associated with extraforaminal pathology, and current management strategies. Finally, we present a concise surgical workflow for far-out decompression.

Bertolotti syndrome refers to chronic low back pain attributed to LSTV. LSTV is a common congenital anomaly characterized by enlargement of the L5 transverse process, which may form a pseudoarticulation or complete osseous fusion with the sacrum, resulting in an abnormal biomechanical connection. Despite being asymptomatic in many cases, LSTVs have a reported prevalence of 4%–35%, with an average of 12.3% on plain radiographs. The condition is commonly observed in patients younger than 30 years presenting with chronic low back pain, in whom degenerative changes are typically minimal.

The pain in Bertolotti syndrome is multifactorial and arises from several distinct mechanisms: arthritic changes – degeneration of the Castellvi type II pseudoarticulation can provoke inflammation and nociceptive pain. Adjacent segment degeneration – increased rigidity at L5–S1 shifts biomechanical stress to the L4–L5 segment, promoting hypermobility, disc degeneration, herniation, and facet arthrosis. Contralateral facet arthrosis – unilateral LSTV alters load distribution, resulting in progressive contralateral facet arthrosis at L5–S1. Nerve root impingement – depending on the dominant pathology, neural compression may involve the L5 nerve root directly or extraforaminally, or the L4 nerve root due to adjacent segment degeneration at L4–5.

Bertolotti’s syndrome is frequently overlooked and misdiagnosed as “sacroiliac joint dysfunction” or general “mechanical low back pain.” On average, patients suffer from symptoms for 41.4 months before reaching a correct diagnosis. This delay results in a significant patient burden, including failed treatments and chronic psychological distress. Research indicates that Bertolotti syndrome patients often report worse mental and physical health scores (Patient-Reported Outcomes Measurement Information System) compared to those with standard lumbosacral radiculopathy.

The management path follows a structured progression based on symptoms. Conservative care: initial treatment for all patients involves nonsteroidal anti-inflammatory drugs and specialized physical therapy focusing on core and gluteal strengthening to stabilize the hypermobile levels. Diagnostic injections: the gold standard and most critical diagnostic step. A fluoroscopic-guided injection of local anesthetic into the pseudoarticulation must provide greater than 50%–75% temporary relief to confirm the joint as the pain source.

Surgical intervention is reserved for those who fail more than 6 months of conservative care and have a mandatory positive diagnostic block. Resection (pseudoarthrectomy): resection is particularly suitable for Jenkins type I and II cases, in which the pseudoarticulation itself is the primary pain generator. Minimally invasive techniques, including microscopic and endoscopic approaches, can be considered viable options, offering favorable clinical outcomes. Fusion: often a more definitive biomechanical solution for Castellvi type 3 and 4 cases or those with underlying instability.

The patient presented with a 3-year history of chronic low back pain with daily symptoms, aggravated by standing and ambulation. The pain radiated bilaterally to the lower extremities and was accompanied by numbness over both plantar aspects of the feet. Pain intensity was rated as 9/10. The patient-reported prolonged sitting during work hours. On physical examination, there was tenderness over the bilateral lower lumbar paraspinal regions and hamstring tightness, more pronounced on the right side. Motor strength was normal (5/5) in all lower extremity muscle groups. Six months prior to presentation, the patient underwent diagnostic facet joint block at L5–S1 and bilateral L5–S1 transforaminal epidural steroid injections, resulting in approximately 70% improvement in both axial back pain and radicular symptoms for 4 months. Previous conservative management, including physical therapy with lumbar traction and home-based stretching exercises, provided partial symptom relief. Over the past month, the patient’s symptoms progressively worsened, limiting ambulation to less than 5 minutes. The pain intensity increased again to 9/10.

Plain lumbosacral radiographs (anteroposterior and lateral views) demonstrate a congenital block vertebra at the L4–L5 level. At L5–S1, there is pseudoarticulation between the right L5 transverse process and the sacral ala, consistent with a Castellvi type II anomaly on the right side and Castellvi type I on the left. Magnetic resonance imaging of the lumbosacral spine demonstrates pseudoarticulation (orange arrow). The blue arrow indicates right-sided far-out compression, while the red arrow shows left-sided far-out compression. CT scan of the lumbosacral spine demonstrates pseudoarticulation (orange arrow). The blue arrow indicates right-sided far-out compression, while the red arrow shows left-sided far-out compression. Three-dimensional CT reconstruction demonstrates overall far-out pathology consistent with a Castellvi type IV LSTV.

Two pins of the navigation reference frame were inserted into the left iliac crest to establish rigid fixation for the navigation system. A navigation platform (Brainlab AG) was utilized and integrated with the CIARTIC Move (Siemens Healthineers AG), a self-driving 2D/3D mobile C-arm system, to enable intraoperative image acquisition and real-time navigated guidance. The surgical incisions were planned by first marking the L5–S1 disc level and identifying the pedicles of L5 and S1 on the skin surface. A 1-cm skin incision was then created lateral to each pedicle. Under navigation guidance, a probe was introduced through the incision and directed toward the triangulation zone at the isthmus and transverse process of L5 to achieve accurate targeting.

The workflow for far-out decompression surgery begins with a paraspinal approach to achieve adequate exposure and decompression of the exiting nerve root. Initial docking is performed at the isthmus, followed by foraminoplasty to widen the neural foramen. The compressive pathology, including the iliolumbar ligament, is carefully identified and addressed. Resection of the pseudarthrosis is then performed. Subsequently, docking is repositioned at the lower edge of the L5 transverse process and the iliac crest to optimize access. The medial and lateral borders of pseudarthrosis are clearly identified to ensure complete and adequate decompression.

The primary right-sided pathology consisted of foraminal compression associated with pseudarthrosis; therefore, we aimed to demonstrate UBE treatment addressing both foraminal decompression and pseudarthrosis resection. The endoscopic view, obtained using a 30° angled scope, demonstrates initial docking at the L5 isthmus, confirmed with a navigated probe. The superior and inferior margins of the L5 transverse process were subsequently identified. Careful dissection of the pseudarthrosis capsule was then performed to fully expose the pseudoarticulation. Foraminoplasty was initiated using a 4.0-mm oval cutting burr (CONMED Oval Bur, REF: H9101) to resect the sacral ala. Bony removal was continued until the endoscope could be advanced deeply toward the superior articular process. An angled curette was used to dissect and release the foraminal ligament. All fibrous adhesions were lysed to achieve free mobilization of the nerve root. Navigation was used to confirm the position at the shoulder of the right L5 exiting nerve root. Initial resection of the pseudarthrosis was performed. The pseudoarticulation was resected between the inferior edge of the L5 transverse process and the sacral ala. A 3-mm diamond burr was then used for controlled bony refinement, followed by Kerrison rongeur to complete the resection and decompression. Anterior capsule was resected after identifying the lateral margin of the pseudarthrosis. The lateral margin of the pseudarthrosis resection was first confirmed using a navigated probe followed by medial boundary verification, then complete resection of the pseudarthrosis. The final endoscopic view demonstrated full decompression of the exiting nerve root with complete resection of the pseudarthrosis and adequate neural mobilization.

On the left side, the primary pathology involved isolated foraminal stenosis; therefore, only foraminal decompression was performed. The procedure began with docking at the L5 isthmus, and the position was confirmed using image-guided navigation. The inferior margin of the L5 transverse process was then identified, followed by localization of the superior border of the sacral ala to establish the bony landmarks for foraminoplasty. The foraminal ligament was dissected to expose the exiting nerve root. Adhesiolysis was performed to mobilize the nerve root, including release of the iliolumbar ligament. An angled probe was then used to dissect the remaining fibrotic tissue. Finally, the shoulder and axillary zones of the nerve root were identified and confirmed using a navigated probe.

Postoperative radiographs demonstrate complete resection of the right-sided pseudarthrosis, while the L5–S1 facet joint remains intact.

On postoperative day 1, the patient-reported immediate improvement in both back and leg pain, with a VAS score of 5/10. At 3-month follow-up, motor strength was grade 5 in all muscle groups, with no sensory deficits. The patient had no limitation in walking or daily activities.

Navigation-assisted UBE enables precise and safe decompression in far-out syndrome, particularly in complex lumbosacral anatomy, while preserving spinal stability.