The above answer is perfect and I also like to add to it that knee joints bears the weight of the body, the joints are obviously adversely suffered because of overweight causing overfriction between the joints. The secondary reasons may be absence / less amount of synovial fuid.
Apart from this, you may also consult with Endocrinologist to check whether hormones (thyroid) may be the cause for the overweight.
As regards the alternative mode of treatment, the same considers the following points:
(i) Focus on your dietary habits that help reduce your weight. This can be rectified by any Diet Specialist / Nutritionist.
(ii) Yoga Therapy - Consult Qualified Yoga Therapist / Yoga Instructor
(iii) Naturopathy - Consult Qualified Naturopath. The Naturopathy serves through Mud therapy, Diet Therapy, Hydrotherapy, Enema, etc.
To let me prescribe in terms of diet therapy, it would be convenient for me after going through case history and daily diet routine including your past disease [chronic if any], etc.
So far as yoga therapy is concerned, you may do the following on empty stomach with the precautions (never overstrain, never go beyond pain instead under pain while exercise and never focus on instant results; rather focus on breath during exercise):
(i) Warm up for 20 minutes at stationary cycling without overstrain and slow speed for 3 days + Shavasana
(ii) Same Warm up for 25 minutes for next 3 days at stationary cycling. This would develop cardiorespiratory stamina and burn up fat without over strain. In summer, if you do not suffer from high b.p., take nimbu pani [with some salt] [normal temperature and not cold] after workout. That should be sipped and not gulped. + Shavasana
(iii) Same warm up for 25 minutes + Tadasana [ 5 minutes ] for next 5 days. + Shavasana
(iv) the above (iii) work out (20 minutes) + tadasana (5 minutes) + Katichakrasana ( 3 minutes) + Griva Shaktivikasak (5 minutes) + Shavasana for next 4 days.
I hope the above asanas needs no explanation here..You may better search on internet or feel free to write to me for further explanations on them.
The above schedule is for 15 days. You may try without any overstrain and without reaching beyond pain and let me know so as to let you know further.