Meniscus injury is common in sports-active population, and partial or total meniscectomy is standard surgery for meniscal tear. Meniscus plays an important role in load transferring, shock absorbing, knee joint stabilizing and chondral protection. Plenty of studies indicate that meniscus-deficiency increases the risk of OA and cause long-term poor outcomes.

Spontaneous human meniscus regeneration is rarely reported and whether regenerated meniscus is chondral-protective and can prevent OA progression remain unknown. During our clinical practice, we have encountered many cases with complete meniscal regeneration under arthroscopy. In this study, we will include all the patients who receiving meniscectomy and ACL reconstruction and knee arthroscopy 2 years after primary surgery. Patients demographic characters will be reviewed. The resected meniscus in the primary surgery and biopsied regenerated meniscus will be analyzed by histological and immunohistochemical method and their ultrastructure will be observed by electron microscope. Patients will be followed at 2-, 5- and 10-year after the primary surgery and the cartilage degeneration and OA progression will be assessed.

Inclusion Criteria:

1. Patients intending to undergo single-knee ACL reconstruction at our Institute of Sports Medicine.
2. The age of 18 ~ 50 years old at the time of operation.
3. Both men and women.
4. BMI < 24.
5. Complete or partial meniscectomy was performed at the same time as ACL reconstruction.
6. Patients willing to undergo a second arthroscopic exploration 2 years postoperatively and to take the portal screws used to fix the ligaments during the initial surgery.
7. Patients with preserved meniscus video during ACL reconstruction.
8. The degree of knee osteoarthritis (KOA) at the time of operation was < Ⅲ of Kellgren Laurence level.

Exclusion Criteria:

1. Patients with severe knee cartilage injury.
2. Patients with KOA≥KLⅢ at the time of operation.
3. The patients cannot be followed up.
4. Patients who refused the second arthroscopy and the removal of the internal fixation portal screw at the time of surgery.
5. Patients with rheumatoid arthritis or other arthritis.
6. Patients with other systemic diseases who take oral anti-inflammatory analgesics for years.
7. Patients with multiple injections of drugs in the joint due to various reasons.

all the patients receving meniscectomy