Abstract: BACKGROUND: Current osteoarthritis modeling methods include anterior cruciate ligament transection (ACLT) and ACLT combined with medial meniscal anterior horn resection. ACLT requires excessive postoperative exercise, which is time and labor-intensive. Complete removal of anterior horn of the medial meniscus can cause collateral damage and increase variability in modeling outcomes, requiring higher surgical skills from the surgeon.
OBJECTIVE: To modify and simplify the traditional method to create animal osteoarthritis model and compare osteoarthritis symptoms of different modeling methods under a low-load exercise environment. 
METHODS: Forty-eight Sprague-Dawley rats were randomly assigned in four groups (n=12 per group): sham operation (complete exposure of the knee cavity of the left hind limb followed by suturing the joint cavity and skin), ACLT, ACLT+anterior horn resection (removal of the anterior horn of the medial meniscus) and ACLT+anterior horn tear (anterior horn tear of the medial meniscus). At 4 weeks after modeling, the rats were euthanized and their knee specimens were collected for gross observation, X-ray and CT scans, pathological observation, and PCR detection.
RESULTS AND CONCLUSION: Gross observation: Mild meniscal wear was observed in the ACLT group. In the ACLT+anterior horn tear group, severe wear of the lateral condyle articular surface, mild wear of the medial condyle articular surface, severe meniscal wear, and full wear of the medial meniscus were observed. The ACLT+resection group showed severe wear of the lateral condyle articular surface, mild wear of the medial condyle articular surface, absence of the anterior horn of the medial meniscus, and meniscus wear area > 50%. Imaging examinations showed no significant difference among the four groups. However, the anterior tibial translocation sign was observed in the three operation groups and the anterior horn of the medial meniscus was missing in the ACLT+anterior horn resection group. Histopathological section observation: Hematoxylin-eosin, toluidine blue, and Sirius red staining showed smooth joint surfaces in the sham operation group and ACLT group; cartilage damage and matrix degradation were evident in the ACLT+anterior horn tear and ACLT+anterior horn transection groups, with less cartilage damage and matrix degradation in the ACLT+anterior horn tear group. PCR results showed higher mRNA expressions of interleukin 1β, interleukin 6, interleukin 8, tumor necrosis factor α, matrix metalloproteinase 1 and matrix metalloproteinase 3 and lower mRNA expressions of aggrecan in the ACLT+anterior horn tear group and ACLT+anterior horn resection group than in the sham operation group and ACLT group (P < 0.05). The mRNA expressions of interleukin 6, matrix metalloproteinase 1, and matrix metalloproteinase 3 were higher in the ACLT + anterior horn resection group than in the ACLT +anterior horn tear group (P < 0.05). To conclude, ACLT alone is less likely to induce osteoarthritis with obvious cartilage wear. ACLT combined with anterior horn resection or tear of the medial meniscus can induce obvious symptoms of osteoarthritis and achieve similar modeling effects. 

Key words: osteoarthritis, anterior cruciate ligament transection, medial meniscus instability, post-traumatic osteoarthritis, experimental modeling