Try Tabatta Intervals: 8 repeats of 30s MAX effort followed by 90s rest (or active rest). This protocol allows for the same VO2 (and other) benefits as steady state cardio which is typically longer in duration. But the total of 4mins of actual work equals less orthopedic wear/tear and less of an adverse impact on the endocrine system.
One important concept to remember: Cardio is an expression of power which is best pursued only after the prerequisites of flexibility, stability (i.e. core), and strength have been met. To break the chain is to break the athlete. Thus, for anyone whose orthopedic condition is suspect, I would recommend choosing a non-weight bearing form of exercise to minimize impact loads if incorporating this type of training into your program. Swimming and cycling would be good choices.