Although both PEID and PETD could achieve good clinical efficacy in the treatment of L5-S1 CLDH, our study showed that PEID group had shorter operative time and fewer intraoperative fluoroscopy times compared with the PETD group.
Compared with children, CLDH is more common in adults. Children usually are treated by conservative treatments, while adults fail to. Calcified disc herniation adheres extensively to nerve roots and the dural sac, which not only increases the difficulty of removal by PELD, but also enlarges the risk of nerve root injury and dural sac tear. Some previous studies showed that PELD was applied to treat CLDH [10,11,12,13,14]. Dabo et al.  showed that 30 patients of CLDH were treated by PEID with a trephine instrument, but 16 patients had postoperative dysesthesia 3 months after operation. Yu et al.  reported that the symptoms of 25 CLDH patients treated by PETD were relieved, but 7 patients had postoperative dysesthesia and 1 patient had recurrence of herniation.
In our study, postoperative outcomes demonstrated that the symptoms of all patients were significantly relieved by PEID or PETD. For the treatment of CLDH, we used an ultrasonic osteotome to remove the calcified disc. Ultrasonic osteotome had selective osteotomy properties and retains the adjacent soft tissue [17, 18]. Compared with grinding drills, ultrasonic osteotome was considered to be safe, accurate and effective for the removal of bone tissue . Calcified intervertebral disc, nerve roots and dural sac should be carefully separated and exposed during the procedure. Care should be taken when pulling the nerve root and dural sac. Part of the soft herniated intervertebral disc should be taken out to create a large enough safe space. Small pieces of calcified intervertebral disc could be removed directly. Large calcified disc could be first divided into small pieces with an ultrasonic osteotome and removed in turn. The above measures can reduce the risk of the injury of nerve root and dural sac. Both PEID and PETD may be safer and more effective surgical methods for olderly, obviously frail or economically difficult CLDH patients.
Compared with PEID, PETD has some deficiencies in the treatment of L5-S1 CLDH. PETD is a relatively complex procedure with a long learning curve. Multiple punctures and adequate foraminoplasty are needed, especially in the L5-S1 level with foraminal stenosis and a high iliac crest, which not only increases the operative time and fluoroscopy times, but also enlarges the risk of the exiting nerve root injury. Moreover, surgeons and patients are also exposed to much radiation . The working channel is not flexible enough during the procedure of PETD, which makes it difficult to sufficient decompression of the herniated calcified disc. Nie et al.  found that the operative time and fluoroscopy times of PEID were significantly shorter than those of PETD. The results were similar to those of the present study.
In our research, no significant difference was observed in the complication rate between the PEID and PETD groups, which may be due to the small number of cases or short follow-up time. In the PEID group, 1 patient had a dural sac tear but no cerebrospinal fluid leak, and 1 patient had postoperative dysesthesia. In addition, 1 patient had postoperative dysesthesia, and 1 patient had residue of herniation in the PETD group. According to our experience, repeated punctures and foraminoplasty may cause the nerve root injury, which may lead to postoperative dysesthesia. The tight adhesion between calcified intervertebral disc and surrounding nerve tissues may result in nerve root injury, dural sac tear and even cerebrospinal fluid leakage. Residue of herniation may be caused by the poor position of the working channel due to the obstruction of the high iliac crest, resulting in insufficient decompression.
There were some limitations in our research. First, this was a retrospective study. Moreover, the sample size is small and the follow-up period is short. Further studies with multicenter, large sample and long-term follow-up will be conducted in the future.