Objective To compare the knee function in patients with intraoperative medial collateral ligament(MCL)injury treated with or with?out increased prosthetic constraint. Methods The records of 19 cases who encountered with iatrogenic injury to the MCL during total knee arthro?plasty(TKA)between January 2005 and December 2010 were retrospectively reviewed. Eight patients(LCCK group)were treated with increased prosthetic constraint;the remaining 11 patients(LPS group)received increased prosthetic constraint between January 2005 and December 2010 served as controls. The MCL was repaired after TKA. The complications were observed after operation. Knee society scores(KSS)subjective knee scores were used to assess the knee function. No patient was lost for follow?up. The mean follow?up was 5 years. Results Until last follow?up(60 months),The KSS subjective score was 87.4 for LCCK group compared with 93.3 for the LPS group. No revisions for knee instability were per?formed in the 11 patients treated with non?prosthetic constraint;however,2 patients treated with increased prosthetic constraint were revised due to joint loosening. Conclusion The MCL tear should be repaired during TKA;the type of the prosthesis should not be increased when MCL injury is recognized during TKA.