OPLL is a multifactorial condition caused by ectopic hyperostosis, calcification and ossification of the posterior longitudinal ligament and commonly presents with myelopathy and radiculopathy. However, the symptom of myelopathy in T-OPLL are more severe than cervical OPLL due to the narrow canal space, tenuous blood supply, and inability to withstand much compression. Conservative treatment is inefficacious for the symptomatic T-OPLL. Hence, surgical intervention is suggested according to the severity of clinical symptoms.
At present, the common surgical decompression procedures for the treatment of T-OPLL include anterior decompression[2, 11, 12, 13] via transthoracic approach, extrapleural anterolateral approach, and posterior decompression[1, 2, 14] via laminoplasty or laminectomy and fusion. Although the anterior approach could directly reach the ossified posterior longitudinal ligament and achieve targeted decompression of the ventral side of the dural sac, this approach is traumatic and surgical related complications including atelectasis, intercostal neuralgia et al are more severe[15]. The extrapleural anterolateral approach could penetrate deep into the anatomical space, which is also technically difficult and requires a long operation time, but it is better than other lateral and posterior approaches in exposing the ventral side of the spinal cord. Therefore, the posterior approach may be more safe and common but also demands higher requirements of the surgeons for the decompression of the ventral side of dural sac[16]. Above all, the conventional surgical procedures are more traumatic and have longer postoperative recovery duration and more surgical related complications.
Recently, several reports indicated that the surgical procedures via the above approaches using thoracoscopy or expandable channels could achieve thoroughly decompression and satisfactory clinical results for the treatment of thoracic spinal stenosis[17, 18, 19]. As the most representative minimally invasive spinal technique in recent years, PESS is based on targeted decompression, which is an important part of the stepwise strategy between conservative treatment and conventional interventions, and has advantages of a smaller skin incision, less trauma, reduced intraoperative blood loss, and fast postoperative functional recovery. A number of studies have shown that PESS with skillful manipulation of endoscopy and endoscopic instruments could achieve the same clinical outcomes as traditional surgeries for the treatment of cervical and lumbar degenerative diseases[20, 21]. In PESS, the whole procedure was performed under continuous fluid flow with a 0.9% saline solution, and hydraulic pressure could reduce most intraoperative bleeding and provide a clear, magnified and tridimensional endoscopic view of the targeted decompression area and neural structures. Moreover, post-procedural epidural adhesion may be a common complication in laminoplasty or laminectomy. The symptomatic postoperative epidural adhesion was not found in the 15 cases in our study because of the above advantage of the water-mediated procedure. Due to the narrow canal space and tenuous blood supply, the thoracic spinal cord may be more prone to being injured. Under local anesthesia, patients were conscious and could communicate to surgeons timely throughout PESS, which facilitated neurological function monitoring and contributed to surgical instrument adjustment during OPLL extirpation and reduced anesthesia-related complications and secondary spinal injury in elderly patients. Moreover, PESS under local anesthesia can provide patients who are intolerant to general anesthesia with opportunities for OPLL extirpation surgery.
With the continuous development of minimally invasive spinal surgery, PESS has been widely used for spinal degenerative diseases and has achieved good clinical outcomes[6, 9, 22, 23, 24]. Choi et al[25]reported the good clinical outcomes of PESS via transforaminal approach for the soft lateral or central thoracic disc herniation. Wagner et al[26]successfully performed the PESS via transforaminal approach for the thoracic disc herniation at T8/T9 level. In a recent study of 55 cases underwent uniportal decompression via interlaminar, extraforaminal or transthoracic retropleural approach for thoracic disc herniation and stenosis, Ruetten et al[27] reported the satisfactory clinical outcomes of PESS at 18 months follow-up. Above studies indicated that PESS via the transforaminal approach could achieve the targeted decompression of the thoracic spinal cord.
During PESS, part of the superior facet joint, the medial part of the pedicle and minor portion of the posterior vertebral wall were resected to fully expose the ossified posterior longitudinal ligament and dural sac, and resection of the above structures reduced the operative trauma and damage to the stability of the posterior spinal column compared with conventional posterior open surgeries including laminoplasty or laminectomy and fusion. Several studies[5, 22, 24] indicated that the risk of surgically induced instability was minimized via resecting a portion of the facet joint and medial part of the pedicle during the treatment of PESS for lumbar recess stenosis. Moreover, in our previous study[7, 8] on PESS for cervical disc herniation, postoperative bone healing was found in both the medial part of pedicle and the trench of the posterior margin of the vertebral wall with CT examinations at the 6-month follow-up.
In our study, the PESS for OPLL extirpation was performed under local and intravenous combined anesthesia via transforaminal approach, all procedures were successfully completed and the clinical outcomes at 32 months follow-up were good. During the procedure, dexmedetomidine was administrated with initial bolus injection at 0.5 mg/kg and continuous pumping at 0.1–0.5 mg/(kg*h) for relieving pain and improving the patient’s intraoperative tolerance. Dexmedetomidine, as a highly selective alpha-2 agonist, has been safely and widely used for various diagnostic and therapeutic procedures to facilitate patients’ comfort[28, 29]. Under local anesthesia, patients were conscious and could communicate to surgeons timely throughout the PESS, which facilitated neurological function monitoring and contributed to surgical instrument adjustment during OPLL extirpation. During the insertion of the guide rod, sliding the guide rod from the outside margin of the lamina and ventral part of the superior facet joint into the foramina using our “sliding technique” is recommended due to the potential of iatrogenic injury to the kidney or lung. When using the trephine cannula for foraminotomy, a thin layer of “eggshell” cortical bone is suggested to be left in order to avoid iatrogenic injury to the spinal cord. In our procedure, the medial part of the pedicle and a minor portion of the posterior vertebral wall were drilled via the “trench approach” to create a groove for endoscopic manipulation and fully expose the basal part of the ossified posterior longitudinal ligament and minimize the interference with neural structures.