I am planning for arthroscopic knee surgery for medial meniscus tear.i have heard they will trim of torn miniscus, isn't it reason for early arthritis?

Recent study: There was a recent study done in europe comparing surgery for meniscal tears and a sham operation where no meniscus was trimmed. This was a very well controlled study and there was no difference in outcome. There is some feeling that some of these surgeries for meniscal tears are not necessary.

No: Having surgery does not cause arthritis in and of itself. Injuring your knee/tearing the meniscus may lead to arthritis whether you have surgery or not. Once the meniscus is torn it (the torn part) is not functional. The reason for surgery is to eliminate pain and/or mechanical symptoms (locking, catching). You may develop arthritis in future from injury, not the surgery itself.