Knee implant becoming loose soon after TKR

Hi. I have has bilateral knee replacement in the past year and half. Now my first implant has become loose and my surgeon says all he has to do is to change the plastic spacer to a thicker one. My concern is that doing so will result in one leg becoming slightly longer than the other one and result in posture problems and imbalances. Has anybody had such an experience? What has been the outcome? Thanks