Dear shep96,
very sorry to hear about your knee and understand that it is quite a traumatic event, especially for a sporty person who have been spared for any previous serious injuries. I believe it makes the "shock" a bit harder to take on board.
First of all, apply these principles.
Ice it, several times a day. I usually suggest my patients to wrap a cold, moist towel around an icepack. You can keep the moist towel in the fridge, so that it is easily available and you won't need to soak it every time you are about to ice it. I go for sessions of 20 minutes on, 20 minutes off, and 20 minutes on again, 3-4-5 times a day, but especially within the first 48 hours. The icing is generally for easing pain, decrease the bleeding and to hinder excessive edema. You are beyond the 48 hrs time-frame, but it might do you good to decrease the swelling since you are telling that it is not that painful anymore, especially if you feel the swelling is warm. I also suggest this procedure if my patients easily swell up after exercise.
Try to put some compression around it. Elastic bandage is good for this. This is to give some support to the knee, but mainly to control the swelling. Not too tight, not too loosely... Keep it on the whole day, but take it off for a few minutes and rewrap a few times a day. Also elevate the knee whenever possible. Elevation is simply to limit the gravitational forces acting on the swelling. Excessive swelling is believed to compress the blood vessels, and you want to avoid this because the blood vessels carry nutrition and agents that is important for the healing process. Excessive swelling will also often be painful as it might compress sensitive nerve endings and complicate movement due to increased internal pressure/edema in the joint.
In general, with such traumatic events, all research points towards early mobilization being of short- and longterm benefit for your rehab. With that being said, everything within pain limits! Your body starts the healing processes within a minute after such injuries and there is ongoing discussion of how much we should interfere with these natural healing processes through such advices that I give now. Some advice early and active use of non-steroideal anti-inflammatory drugs (such as Ibuprofen/Naproxen), but there is emerging good evidence that this actually delays healing processes to such an extent that the long-term rehab end results are actually poorer than for those not taking them post-operatively. So I'd advice, if you can do without, let them be. I'd try Paracetamol for simple pain control, before going on to the "stronger" meds in any case.
Back to the early mobilization issues, since it's still early after your injury, I'd advice 1-3 weeks of "carefulness" depending on your degree of injury (especially since you feel the knee being unstable and gives way sometimes), but MOVE the knee WITHIN pain limits several times a day, even a few minutes a time does a lot of good. Not too much, because then it might disturb any collagen fiber network that is being laid down, but enough to get those fibers functionally aligned - it'll give you a better and stronger scar tissue. A very general exercise I often give for early knee and ankle injuries is to sit on the tip of a chair and slide your injured foot geeeently back and forth. Use your hip muscle actively to adjust how much weight the knee joint will be taking. If it swells up right after the exercise, you've been doing too much - use ice. There are heaps of other exercises you probably could do (move your ankle a lot to keep your calf muscles active, tighten your gluts while sitting and so on), but these better be given by a professional who have assessed you first.
There are several things that COULD have happened to your knee - good that you are up for a MRI-scan as all I am about to say are PURE speculations.
To me it sounds like you might had two traumas. The one that made you sit down at first, and the second when you tried to get going again. If you hear a pop, its often likely that you have torn a ligament. I assume you were running straight forward since you don't mention any twisting motion. If there were no sudden stop or direct tackle involved, and you say your knee went one way, your body the other way, I again assume that your right knee went to the left and your body to the right (knees want to go "inwards"). This mechanism of injury suggest a tear of the medial collateral ligament (MCL) that supports the "inside" of your knee and also undoubtedly being the ligament most often to go. It is quite typical that a full MCL tear is very painful initially, but after a few minutes as you say, is suddenly considerably less painful. Being in less pain, you wanted to get going again, but because your medial knee support was gone (due to the torn ligament), the knee slipped again. That you still feel its unstable and sometimes gives way supports this hypothesis. I would be very surprised if your MRI-scan shows an intact medial collateral ligament.
The next part is where I get a bit confused. You say that you felt the knee-cap of your right knee go left, and popped back again when you took the weight off it. Usually the knee-cap luxates laterally, if I remember correctly (so in your case with right knee, the knee-cap "should" go right). On the other hand, people with knee-cap luxations/subluxations usually have a good concept of that "it was definitely the knee-cap moving" so I'm not doubting your perception of which direction it went. Supporting evidence from your information pointing towards a subluxated knee-cap is that it went back when you took weight off it (you probably straightened out the knee in this movement, and this often makes subluxated knee-caps go back). The location of your swelling suggests injury to the medial patellar retinaculum and/or the medial patellofemoral ligament. Cruciate ligaments or meniscus injuries would NOT give swelling ABOVE the knee-cap as you tell of, not alone at least. So if it was a subluxation of the knee-cap, the medial patellar retinaculum is very likely to get sprained too, potentially causing the swelling above the knee-cap you talk of. Subluxed knee-caps will also limit bending significantly as in your case. The locking feeling when you almost manage to straighten it fully, could be due to maltracking of the patella...
So in short, my guess is that you tore the medial collateral ligament in your "first" trauma and possibly subluxed the knee-cap in your second trauma.
A major injury (involving MCL, medial meniscus and anterior cruciate ligament) would give quite a hefty swelling within one hour, which is not what I understand happened in your case. So I'd guess the anterior cruciate ligament is intact, also because there was no direct hit or forceful stop. Since there was no twisting, I'd guess the medial meniscus is quite intact too, but keeping in mind that the MCL partially attaches to the meniscus, it's difficult to totally rule out a small medial meniscus sprain yet.
Your long term abilities after rehab depends a lot on what type of activity you want to get back too, and a lot on the quality that is put in from you and those rehab'ing you. I've made A LOT OF speculations now, based on VERY limited information, so please DO TAKE THIS WITH A PINCH OF SALT!! I got a bit involved as I started writing, so please forgive me for not being able to compress my comment at this point...
I do however, hope you are able to extract some useful information from the above, and wish you good luck onwards. If its not too much to ask, it would be interesting to hear the outcome of your MRI-scan as well.
Kind regards,
Sigurd Mikkelsen