Proximal junctional failure (PJF) is a major issue in the field of spinal revision surgery. Lesions at the proximal junctional level, particularly in the thoracic region, are challenging to surgically address due to anatomical limitations compared to the lumbar region. In the thoracic area, the spinal cord is vulnerable to damage during retraction, and accessing lesions in the anterior part of the spinal cord is difficult. Consequently, only simple posterior decompression is often performed. Conventional open surgery accesses the disc through anterior approaches such as transthoracic or lateral methods. However, these approaches carry a high risk of iatrogenic injury and postoperative complications in the thoracic region due to anatomical obstacles such as the lungs and large vessels. The posterolateral approach can reduce the risk of pulmonary complications, but due to the narrow space, costotransversectomy is often required [1]. Unilateral biportal endoscopy (UBE) can be an alternative to overcome this narrow surgical field. The development of UBE techniques has diversified access routes during spinal surgery. In this technique, the viewing and working portals are separated, allowing free movement through the working portal while maintaining a stable field of view through the viewing portal. These advantages have gradually expanded the scope of UBE, enabling better outcomes not only through simple decompression using endoscopy but also by combining it with open surgery through various access routes.

In this case report, we present a case where thoracic interbody cage insertion was performed using UBE in combination with open surgery during a reoperation of the thoracic spine.

A 77-year-old female patient underwent fusion surgery at the L5–S1 level 20 years ago at another hospital. Ten years after surgery, the patient developed adjacent segment disease and underwent fusion surgery from L1 to L4 with the L5–S1 screw removed. Afterwards, 3 months before coming to our hospital, the patient underwent extension fusion surgery up to T11 at another hospital due to PJF. The patient also underwent additional posterior decompression surgery at T10–11 and interspinous device insertion due to worsening symptoms after surgery. However, persistent gait instability and increased bilateral knee jerks occurred. In addition, left hip flexion and knee extension strength decreased to grade 3, and the patient visited our hospital (Figure 1A). Magnetic resonance imaging examination performed at the hospital showed compression of the cord due to disc herniation in the front of the cord at the T10–11 level (Figure 1C). Accordingly, interbody fusion was performed at the T10–11 level using a UBE technique, the surgery was converted to open, additional decompression was performed, posterior fixation was extended to T8, and posterior fusion was performed (Figures 1B, D, E, and 2). The patient's motor weakness and gait instability improved 3 months after surgery, and no specific complications occurred at 6-month follow-up.

Because this study reported a single case and did not affect the course of the patient's disease, ethical approval was not required.

Because the thoracic spine has anatomically different characteristics from the lumbar spine, it is important to understand this during surgery. In the thoracic spine, unlike the lumbar spine, there is a costovertebral space with the rib head as the lateral border, the superior articular process (SAP) as the medial border, the nerve root as the upper border, and the trasverse process as the lower border (Figure 3). Rib heads may be present when approaching the disc space. It may be an obstacle to the surgical approach, but because it exists above the pleura, it acts as a shield to prevent damage to the parietal pleura and iatrogenic hydrothorax. The parietal pleura lies medial to the rib and is covered by the innermost intercostal muscle, endothoracic fascia, and extrapleural fat. Therefore, if the rib head is not completely resected, surgery can be performed safely from damage to the pleura.

The rib articulates with the upper and lower vertebrae from T2 to T9 in the thoracic vertebra and articulates in the form of a demifacet. At the lower thoracic level, the costovertebral joint is located relatively lower. From T10 to T12, a costal facet independently articulates with one vertebral body (Figure 4) [2]. Therefore, when preparing a disc, the rib head can be normally felt on the lateral side at the upper and middle thoracic levels. If you use this as the outer border and perform surgery by resection of the SAP toward the medial side, you can relatively safely remove the disc and insert the cage without damaging the pleura. From T10 to T12, the rib head may not be at the disc level, but it can be felt because the rib head is slightly larger than the costovertebral joint, and the area obscured by the rib head is smaller than that of the upper thoracic vertebrae, so a relatively wide space can be obtained. Since the size and location of the rib head vary depending on the individual, it must be checked through computed tomography before surgery to determine the space.

With the development of UBE technique, endoscopy is being used in various areas of spine surgery. Previous studies on UBE technique focused on minimizing normal body tissue damage. This study focused on surgery in narrow spaces with the advantage of being able to use various corridors, making it easier to perform surgeries such as thoracic cage insertion that were difficult to perform in conventional open surgery.

Because the thoracic level is the spinal cord level, unlike the lumbar level, cord injury may occur during retraction of the dural sac, so there are limitations to the conventional posterior approach. As a result, various approaches have been developed. The anterior approach for the treatment of thoracic disc lesions was first described by Crafoord et al. [4] Since then, many studies have reported surgical treatment using the anterior approach. Approaching the thoracic disc using the anterior approach can be said to be a relatively safe method when considering the spinal cord, but the risk increases in patients with chest lesions or respiratory problems. Additionally, respiratory complications may occur after surgery. The incidence of respiratory complications in thoracic disc surgery has been reported to be 7% for the transthoracic approach, 5% for the lateral approach, and 0% for the posterolateral approach [1]. In addition, there is a problem that it takes additional time because the position needs to be changed for posterior fixation.

Several previous studies have reported on microscopy assisted thoracic interbody fusion through a posterolateral approach [5,6]. However, in the case of the thoracic spine, when using a microscopic approach through a conventional open incision, the movement of the instrument is not free due to the narrow surgical field, which has technical limitations, so it has not been widely used in actual clinical practice. Additionally, there were cases where costotransversectomy was needed to access the disc space due to the presence of the rib head. Using UBE offers a less invasive alternative. If there is a 10-mm space where the cage can be inserted, as confirmed with an endoscopy camera, the rib can remain intact or only be partially ground down, allowing for disc removal and cage insertion. This method has the advantage of the rib head protecting the pleura during the procedure, potentially enhancing safety. Thus, UBE could represent a viable and safer option for thoracic interbody fusion in certain cases. Generally, a minimum width of 10 mm is required for cage insertion, but at levels above T9, sufficient space may not be available due to the rib head, so partial resection of the rib head is required. It is thought that future cadaveric or radiological studies will be needed to determine the size of costovertebral space at each level according to gender.

UBE is a surgical procedure that maintains visibility by using continuous water flow. This flow helps clear away debris, ensuring a clear field of vision, and is thought to reduce infection risks through constant irrigation. It is recommended that water pressure should not exceed 30 mmHg during UBE [7]. Particular care must be taken with the thoracic spinal cord, as it is vulnerable to damage from pressure, so it is crucial to maintain the appropriate pressure and ensure sufficient outflow. Prolonged surgeries could lead to nerve damage due to increased water pressure, so the duration of the procedure should be kept within limits. Therefore, an experienced surgeon with a well-structured surgical plan is essential. However, no studies have yet determined the cutoff time or hydraulic pressure thresholds for neurological complications, and further research in this area is required. In thoracic surgery, if the instrument is inserted too deeply in the wrong direction, there is a possibility that water may enter the pleural space. Therefore, an accurate understanding of the anatomical structure of the thoracic spine before surgery is essential. During surgery, landmarks must be identified under fluoroscopic guidance. If water enters the pleura, chest drain or aspiration may be necessary.

To our knowledge, this is the first report of endoscopy-assisted thoracic cage insertion combined with open surgery. This combined surgery shows that UBE is not simply a minimal invasive technique, but can be used to diversify the surgical corridor and overcome existing technical limitations.

UBE for thoracic interbody cage insertion represents a promising advancement in spinal surgery techniques. This approach enhances surgical precision while minimizing the risk of pulmonary complications compared to traditional methods. By combining UBE with open surgery, we demonstrated its effectiveness in overcoming technical challenges and improving safety, particularly in complex thoracic spine revision cases.