LDH is one of the most common diseases seen in department of orthopaedics4. Most herniation sites are located at L4/5 and L5/S15. Percutaneous endoscopic transforaminal discectomy (PETD) has been widely used in L4-5 LDH over the past several years. Unlike L4-5, L5-S1 has its own anatomical characteristics6-7, such as a high iliac crest and a large L5 transverse process, which limit the transforaminal access to the L5-S1 disc space. PEID is a minimally invasive technique that can directly reach the spinal canal via the interlaminar space, which avoids factors such as a high iliac crest and transverse process variation and has obvious advantages in the treatment of L5-S1 LDH. PEID is one of the most sophisticated operative procedures for the treatment of L5-S1 LDH. Ruetten8 et al. were the first to perform intervertebral disc discectomy and decompression by creating an intervertebral foramen in the vertebral canal between the upper and lower vertebral discs. Compared with open discectomy, PEID offers the advantages of small incision size, short hospital stay, less blood loss, less destruction of surrounding tissues, and faster functional recovery9.
For PETD, transforaminal access involves passing endoscopes and instruments through a cannula placed safely in the Kambin’s safe zone to avoid nerve injury10. However, PEID beginners should first realize there is no such “safe zone”. The procedure from the initial punctures to the final placement of working cannel is the most difficult and critical part of the surgery11. Also, it is the most likely part to injure nerve. Sometimes, the working channel was unexpectedly inserted into the spinal canal through the widened interlaminar space, which could caused dural tear or nerve root injury. There is a concern in PEID with the possibility of cauda equina injury or durotomy because of excessive neural retraction and manipulation of neural tissue, especially for an inexperienced surgeon.
In order to reduce the risk of nerve injure, many spine surgeons had tried a lot of ways and it worked really well. Ruetten12 suggsted that ligamentum flavum was firstly opened with endoscopic scissors and forceps, then the opening was enlarged with working tube. After identification of epidural fat and neural tissue, the spinal endoscope was safely advanced into the spinal canal. Choi13 recommended that a dilator was inserted under fluoroscopy to reach the intervertebral space and the use of dilators from small to large in diameter to prevent nerve injury. Nicoletti14 advocated the incision of ligamentum flavum and placement of working catheter should under the supervision of complete endoscopy, which can ensure safety at any step of endoscopic procedures. Coi et al15 suggested that the interlaminar approach was safe for reaching the axillary space. For a axillary herniation, the nerve root was easily identified using a nerve dissector along the lateral recess under endoscopy. Because the nerve root travels horizontally at the L5/S1 level, it is safe to manage the axillary lesion. The oblique distal opening of the sheath should be toward the midline of the spinal canal to effectively expose the lesion and protect the nerve root and dura mater. Sufficient attention should be paid to intraoperative protection of the dura mater. In order to prevent cerebrospinal fluid leakage, care should be taken not to tear the dura during rotation. In this study, we tried to stand on the opposite side of disc protrusion to perform PEID. We found axillary herniated discs could be removed easily when surgeon stood on the opposite side of the disc protrusion. A working cannula was introduced into the epidural space at an oblique angle to the horizontal plane, which can directly access the axillary herniated disc and remove the disc fragment with minimal manipulation of the neural structure. The skin entry point of paraspinal approach is medial and the angle to the skin is less steeper compared with angle of the conventional approach, which can effectively reduce the risk of dura sac injury.
Excellent outcomes were observed after surgery. We found an improvement of VAS, ODI and JOA, which were significantly different from those in preoperation, suggesting that PEID would be beneficial for L5-S1 disc herniations to relieve pain. The modified MacNab criteria were calculated at the last follow-up.The excellent and good portion in MacNab accounted for 90%, which could also explain the good results. In our study, we report a recurrence rate of 5%, which is similar with the previous reports16 . No patient had cauda equina syndrome, nerve root injury, cerebrospinal fluid leakage, infections, or major blood vessel at the last follow-up. Leg pain persisted postoperatively in only one case. Three patients complained of transient lower-extremity dysesthesia after surgery, which disappeared gradually after 3 month without any intervention. Stretch injury of the nerve root during operation may be a main factor. Researchers17 compared the surgical outcomes of the interlaminar and the transforaminal approaches and found that postoperative lower-extremity dysesthesia occurred mainly during the interlaminar approach. Suess18 found that dysesthesia generally occurred during the early postoperative period, and resection of the lesion step-by-step under direct vision can reduce the risk of potential nerve root injury. We believe nerve root traction caused by the rotation of the working cannula may result in lower-extremity dysesthesia during the early postoperative period.
We are aware of several limitations to this study. First, the study was performed as an observational study, no control group has been included for comparison. Second, the number of patients was small, a larger number of cases was advisable.