Your right knee feels better because you are no longer using it to   compensate for the left knee, which has not been properly rehabbed.

Most  doctors will tell you the #1 cause of knee pain in this country is  overweight. Add pounds, add pain. They are only  built to take so much  pounding, and running triples the G-forces of  standing, which in your  case would amount to 60 pounds added to each footstrike,  not just 20. A  few pounds really matter, and just losing 10 over the  last year has made  a world of difference for me. Mileage, however, is  not the way to lose  it in your case.

I  believe running is in your future -  not arthritis - but you'll have to  make some changes. First change is  the composition of your food while  recovering from your injury: Less  carbs,  of course, and taper them down toward the end of the day with  only  lean protein after 6 or 7pm (ex: whey protein). Start your day with   only the carbs you intend to burn. Make them more complex as the day  wears on until evening when you cut them almost completely. Carbs at  night interfere with the production of critical hormones that will  help  you heal. Proteins will help you synthesize the sleep hormones you  need  to rest.

You can  still run for 20 minutes a day three  times a week? Super. That's all  you'll need for the next part of the  plan. First thing in the morning,  even before breakfast, is the time for  your 20 minute workout,  consisting of a brief 2-3 minute warm-up  followed by up to eight  30-second repeats at near maximum intensity  (what you think you could  do 8 times), separated by 90 second recovery  jogs over the same  distance back to where you ran from (out &  backs). Don't run it for  distance, but for time and intensity only. Take  a brief 2-3 minute cooldown jog and have your breakfast, though you  should consider lowering the breakfast carbs on your workout days. You  heard me right - this will make you burn fat like crazy.

Meanwhile,   there is your knee to consider. You survived the fall. It's not as bad   as it used to be. This is a great time to dig in and make it feel  worse!  Find that tender spot below the joint you've been nursing and  work it  hard with your fingers to get the junk out. This may take weeks  of  application but it will eventually lessen or eliminate the pain.  You are  going to direct healing inflammation to that spot. Next, let's  handle  the trick knee.

Weakness  and apparent instability of  the knee are often due to an imbalance in  the pulling force of the  component muscles of the Quadriceps.  Specifically, it is likely the  Vastus Medialis, or inner quad, needs some work - not exercise, but a  targeted rub-down. Straighten your leg  so the knee locks and observe the  heads of your quad muscles as they  rise just above the knee. You want to  find the fat middle of the one  toward the inside, which should be a few  inches above the knee cap and  over. Note that spot and relax the knee.

Using your  thumbs in a circular motion, find sensitive  or rubbery spots in that  part of the muscle and increase the pressure.  When you find the worst  knots hold the pressure for a few seconds,  release and repeat. Now bend  the knee and see the difference. The pain  below your knee may still be  there, but that is to be expected as your  quad anchors at that spot.  Relax the quad and you will release the  pressure. If stiffness  persists, use the above method to check the  longitudinal center of the  top thigh and the lateral side of the thigh  until these muscles relax.  They have been splinting your knee for months  after the accident, and  you must retrain them to relax.

Arthritis  is common in  cultures where diet is poor, as it often is here in the  States.  Contrary to popular belief, a moderate amount of running  actually  improves joint health in most people. Muscle imbalance and  tension is  common where people spend a lot of time in chairs as we do  here,  eating, driving or whatever. Stay gently active except for your   fat-burning workouts, and eat to live well, rather than simply eating   well. Do it right and it will cut your cravings and improve your rest.

Stay on this limited workout/diet and rehab regimen for a few weeks and  see how you feel. It may take longer, but you'll lose nothing but pounds  in the interim. I'll bet the ranch you'll soon feel like running again,  but ease back into it when you do.

Carbs are OK at certain times, but it's the timing that is most        important. I used to think right after a workout was the best time, as        some sports supplement manufacturers suggest, but it depends on  your       goal. Carbs have value during active competition, when muscle   glycogen runs low and blood sugar levels need to be restored as soon  as  possible, say, during a marathon.    The problem for you with carbs   after your workout is  two-fold:  your  body   will not need to burn  off  stored fat, and the  production of   growth   hormone will be  curtailed  as carbs are being  metabolized.

From     your  original  post it appears you  want to burn fat and heal an old      injury, so  carbs would interfere  with both after a workout. Quickly      absorbed  proteins, on the other  hand, would insure the body does not      need to  cannibalize muscle in  order to find an energy source in  lieu of      carbs. The closer to your  workout, the more important  these things    are.   The rest of the day  and on non-workout days,  low-glycemic  (slower     absorbed) carbs like  whole oatmeal are a good  way to  restock your   main   energy source,  that is until the last  meal of the  day, which   should once   again lean  towards proteins and  away from  carbs and fats.   The reasons   are  similar, because fats  and carbs  will interfere with   your sleep and    hormone production,  while  proteins will enhance this   process,    particularly for the  sleep  hormone melatonin.

You   are    wise to take care  when  selecting grain products. The endosperm  of  a    plant is meant  to  provide the raw material for seed germination,   and  it   is a very   concentrated energy source that has an explosive   effect  on   blood   sugar levels when ground into flour and ingested in   the  form of     breads, cakes, and processed cereals. Whole grains have to    be broken     down to release their carbs, therefor are lower-glycemic    and less   likely   to trigger fat storage. They also take more energy to      digest. It is   easy to get too much of a good thing with grains,      though, so restrict   their use after lunch. Ditto for white potato      products and white rice, while brown rice and sweet   potatoes  (ironically) are lower-glycemic.. if     you've got to eat starches   at  all. Watch yourself closely, and you     will see how many simple-carb    choices we are faced with each day that     are entirely unnecessary.    Dressings, sauces, buns, pastas... the     people who eat them usually  grow   fat. Eventually, when your  metabolism    has been restored, you  will be   able to eat some of these  things    without gaining weight,  as I and many   runners can now, but  the change    does not happen  overnight.

It   will,  however, happen   much  faster if  you follow the workout I   suggested.  The beauty of the    short,  intense workouts is that you will   get the  maximum 24-hr fat-burning  benefit  with   minimum wear & tear, and maximum   healing   potential for your   knee.  While I wouldn't call these measured     workouts sprints, they  may  seem  that way at first. The idea is to   finish   the eighth repeat   feeling  like you could do one more just as   fast. In   other words, try   to stay  under control.

Regarding   exercises  for  the   quads, in  many cases dysfunctional, unbalanced   quads are behind   knee   pain, as I  suspect may be true in your case   after the accident    trauma  (see  diagram from Gray's). A muscle is   not a monolithic  structure, and  can be  relaxed in   some spots while   being hypertonic  (tight) in  others. Your  goal should be   to find  the  tight spots and  iron them  out, without  messing with the   rest  of  the muscle. When you  have  restored the muscle  to its normal,     relaxed state, your knee  pain  should diminish and  strengthening     exercises will be productive.  The  exercises can make the  muscle worse     otherwise, which is why  people  often have to stop  exercising to   heal.   You can't go wrong by  relaxing  the tight muscle  first, before   you begin   retraining.

http://upload.wikimedia.org/wikipedia/commons/thumb/1/14/Rectus_femoris.png/250px-Rectus_femoris.png

The     above rendering shows the area, just below the knee, where   the   muscles of the quad and Sartorius muscle insert and intersect. Of   the   five individual muscles at this junction (Vastus Intermedius not   shown   because it is under Rectus Femoris, the one most likely to   produce pain   under the knee is V. Medialis, on the right. The spot   where the name  tag  is drawn should be your most productive   spot to rub. You  may  want to check and see how this area feels when   you attempt to sit   "indian style," activating the Sartorius (far right),   just to see if it is   involved.

On  running shoes, make  sure any support  shoe   does  not restrict your   natural movement, or  you may find as others  on   this  forum have,  that  more things start  to hurt that didn't hurt   before   you switched  shoes.  If you must  err, err on the side of too   little   support  rather than too  much. A  recommendation for control  shoes  is  not  a  ringing endorsement  of  your running style, so there  may be some     biomechanical issues to   address. Some stores offer  running analysis,     and I think you should   find out what your style of  running is, so  you    can make the  necessary  changes before something  else begins to  hurt.  Changes may  include a customized orthotic for your  shoes, which  when  properly  designed, can be more important than the  shoe itself.  This is more productive than trying to force yourself to run  a  particular way,  because your running style tends to reflect the   structure of your foot  (which you can work with), as well as the knee  and hip, which you  can't do much about (you can do some things, but  that's another story).