Sacrococcygeal pilonidal sinus is a chronic sinus inflammation of the sacrococcygeal region. The pathogenesis of the disease is based on the theory that the disease is a congenital disease and the sinus tract is a skin inclusion formed by the residual medullary canal or the developmental malformation of the sacrococcygeal suture. ② Another theory suggests that the disease originates from an acquired origin, based on the fact that the disease is more common in adolescent, hirsute buttocks, and obese males. Arydakis proposed a three-factor theory of etiology: skin damage, loose hairs, and internal aspiration. Morbidity characteristics: ① The morbidity rate is about 26/100,000. The incidence is low in the national population, but the high rate of misdiagnosis and recurrence (especially in patients with recurrent or multiple surgeries) make the diagnosis difficult. Diagnosis: ① Signs and clinical manifestations: the most important diagnostic sign is an acute or chronic abscess in the sacrococcygeal region or the presence of a sinus tract with repeated overflowing secretions, sometimes accompanied by the growth of hairs from the sinus opening that are not connected to the surrounding skin. The disease can only be felt as thickening or hard nodules of the caudal skin before infection occurs, or there are recurrent small abscesses in the sacrococcygeal area, which are often misdiagnosed as boils and sebaceous cysts. If a sacrococcygeal abscess or sinus tract is present, it is easily confused with a presacral teratoma or presacral cyst, but is most likely to be misdiagnosed as an anal fistula. The presence of hair at the sinus orifice is a distinctive feature, but the literature reports that the positive rate of hair in the sinus is about 40%-50%, so there is still a high rate of misdiagnosis based only on the presence or absence of hair at the sinus orifice to diagnose the sinus. The hypoechoic canal of the anal fistula extends toward the anus and is very close to or reaches the rectal cavity of the anal canal, whereas the longitudinal depth of the foci of the hairy sinus is shallower than that of the anal fistula, and the end of the sinus canal is farther from the anal canal and the overall direction of the sinus canal tends to be cranial. Using MRI to differentiate the hairy sinus from the anal fistula, the diagnostic accuracy of the hairy sinus is 86%, and the positive predictive value is 100%, so MR should be considered a valuable diagnostic tool. Treatment: Control of infection is the basis of treatment of the hairy sinus, and if infection is suspected, multiple ultrasound examinations or punctures should be performed to clarify. There are many treatment options for this disease, including non-surgical and surgical. For patients without infection or in the quiescent phase, non-surgical treatment methods (using caustic sclerosing agents injected into the sinus tract to destroy the sinus tract endothelium and close the sinus tract and the cystic cavity) were once widely used because they had few complications and could be repeated many times, but the recurrence rate is high and is now almost replaced by surgery. The most effective means is currently considered to be surgical, with specific methods such as post-excisional one-stage suture, post-excisional drainage, and post-excisional open wound sub-stage suture. One-stage suture after excision is 0% to 22%, open secondary suture after excision is 0% to 22%, and open treatment is 7% to 24%. Currently, it is considered that one-stage surgical excision followed by suture is a better method, and the recurrence rate in this study was 7.7% with one-stage surgical excision followed by suture. For those with complex sinus tracts, a Karydakis incision can be used, which eliminates the gluteal sulcus while removing the sinus tract intact and can effectively prevent recurrence of the hidden hair sinus, and is currently considered – an ideal surgical excision method. To summarize the advantages of this approach: simple surgical method; short healing time; soft tissue between the scar and the sacrum, which can tolerate damage; low recurrence rate, etc. Prognosis: Sinus carcinoma is relatively rare, with about 50 cases recorded in the literature, mostly well-differentiated squamous cell carcinoma, easily metastasized through lymph nodes, and it has been reported in the literature that the 5-year survival rate reaches 51% without lymph node metastasis, but if there is inguinal lymph node metastasis, its 5-year survival rate drops to 14.5%. Therefore, early diagnosis and complete removal of sinus tracts become very important in the diagnosis and treatment of the hidden hair sinus. Note: Asymmetric excision with intradermal suture (Karydakis procedure): A longitudinal off-center shuttle incision is made to free the subcutaneous hyperplastic granulation tissue up to the sacral fascia, and the lower part of the wound, especially near the gluteal sulcus, should be separated to a depth of about 4-125 px, with complete excision of the diseased tissue, the extent of which depends on the amount of hyperplastic granulation tissue. Negative pressure suction was placed on the deep part of the wound and the subcutaneous tissue was closed with absorbable sutures, and the skin was closed with polypropylene sutures for intradermal sutures. The wound was closed with the midline drawn to one side, approximately 1.5-50 px off, and treated with 3rd generation cephalosporin and metronidazole for 48 h. If microbial growth was present, antibiotic therapy was maintained for at least 5 days. Negative pressure drainage should be placed for 2-3 days. After asymmetric resection with intradermal sutures, the wound scar is pulled to one side from the midline, flattening the gluteal sulcus and/or the posterior median sulcus. Thus the local suction that could be generated is eliminated. At the same time, intradermal suturing is used to avoid the recurrence of early hair attachment caused by the traditional interrupted suturing method, which causes the suture needle to pierce the skin several times. The procedure is uncomplicated and the complication and recurrence rate is very low (0.9%).