Muscle Atrophy in the Quadriceps

I have always had great muscular quadriceps.  I had a knee injury last year and now I have arthritis in my right knee.  I went through therapy a year ago, but my right quadricep does not look like my left quadricep.  My question is:  How do I build the muscles back up in my quadriceps (as a matter of fact, my entire thigh area)?