The pathogenesis of sacral Tarlov cysts (TCs) is still unclear. In this study, histological techniques were used to clarify the anatomical membranous layers of TCs and further explore the pathogenesis of them.Although many approaches have been used to treat TCs, there is no consensus on the optimal treatment. Microsurgery is now increasingly recommended as the preferred treatment with the best long-term outcomes.However, some authors have proposed the opposite view because current microsurgical techniques fail to completely close the ostium between the cyst and subarachnoid space.Consequently, could lead to leakage of cerebrospinal fluid, pseudomeningocele , or a high frequency of cysts recurrence, which are the main reasons for surgical failure and also the biggest scruple when microsurgery is chosen. Herein, we present a new method of cyst separation and ostium closure, and evaluate its clinical reliability and effectiveness for surgical treatment of Tarlov cysts through the prospective study.

1. Part of the intact cyst wall was removed under the microscope and fixed in 10% formalin solution. After dehydration and paraffin embedding, tissue sections (thickness: 6μm) were prepared and dewaxed to water. Sirius red stain drops were stained for 1 hour. The staining liquid on the surface of the slices was removed after a little flushing with water. The slices were dehydrated and transparent, sealed with neutral gum, and the membranous layers of the capsule walls were observed under a microscope.It has been observed in previous surgeries that all cysts were incompressible and remained filled after opening the distal wall of the cyst to release cerebrospinal fluid（CSF). These findings lent support for the proposed valve mechanism as the etiology of the entity becoming symptomatic. In this study, the cyst was observed in the following way: the dural sac was opened to expose the ostium to observe whether the cystic fluid would reverse outflow and shrink the cyst; In this way, the existence of a "one-way valve" can be further verified.
2. Modified microsurgical method is used to treat patients who met the inclusion criteria. To evaluate the surgical effect, we record and statistically compare the patients'outcomes before surgery and at the postoperative follow-up, which include the following: 1) Demographic data: sex, age, height, weight, body mass index (BMI), etc.; 2) clinical manifestations: symptoms and their features, complications, and long-term recovery; 3) Imaging findings: maximum diameter, location, shape, number, intracapsular nerve root condition, and postoperative outcome of TCs. 4) Surgical characteristics: operative process and time, intraoperative blood loss, assistive technologies, etc.; 5) Neurologic assessment using the following tools: The visual analogue scale (VAS), Scoring System for the Clinical Evaluation of Patients with Spinal Processes (hereinafter referred to as SCPS), Oswestry Disability Index (ODI), Japanese Orthopedic Association Scores 29 (JOA), and modified evaluation criteria for the efficacy of lumbar function (MacNab).All patients are required to return for MRI review and neurofunctional assessment at 3 and 12 months postoperatively, followed by annual examinations thereafter.

Inclusion Criteria:

• Conform to the surgical indications and undergo modified microscopic surgery in our department;
• Undergo regular review and long-term clinical effect evaluation;
• Agree to participate in this study.

Exclusion Criteria:

• Ineligible for modified microscopic surgery or unwilling to undergo it;
• Failure to cooperate with follow-up work or lost to follow-up；
• Did not agree to participate in the study.

All IPD that underlie results in a publication

From the completion of data collection to 1 month after the publication of the article