Abstract: Objective   To explore the application value of the ratio of the tibial tubercle-trochlear groove(TT-TG) distance to the tibial plateau maximum axis(TPMA) in patellar instability. 
Methods  A total of 45 patients with patellar instability as well as healthy people were collected and analyzed by MRI. The TT-TG distance, TPMA, the radio of the TT-TG distance to TPMA(TT-TG/TPMA ratio) were compared between the two groups. 
Results  The TT-TG distance was(12.133±3.226) mm in healthy control group and(18.638±3.228) mm in patellar instability group, and the difference between the two groups was statistically significant(P＜0.05). The TPMA between two groups was(71.933±8.751) mm in healthy control group, and(68.442±6.091) mm in patellar instability group, and the difference between the two groups was statistically significant(P＜0.05). The TT-TG/TPMA ratio was(0.167±0.031) in healthy control group, which was smaller than that(0.263±0.040) of patellar instability group(P＜0.05). There were 14 cases in the healthy control group whose TT-TG distance were from 15 to 20 mm, of which 1 case had TT-TG/TPMA ratio＞0.23, accounting for 1.5%(1/66) of the total.There were 27 cases in patellar group whose TT-TG distance were from 15 to 20 mm, of which 2 cases had TT-TG/TPMA ratio< 0.23, accounting for 4.4%(2/45) of the total. There were 52 cases with TT-TG distance less than 15 mm, of which 0 case had TT-TG/TPMA ratio＞0.23 in healthy control group. There were 6 cases with TT-TG distance less than 15 mm， of which 3 cases had TT-TG/TPMA ratio＜0.23, accounting for 6.7%(3/45) of the total. 
Conclusion  The TT-TG/TPMA ratio of knee joint is a reliable method to evaluate patellar instability, which reduces the false positive rate of the patellar instability only by TT-TG distance. The TT-TG/TPMA ratio＞0.23 indicates patellar instability.

Key words: patella, tibia, therapeutic uses