One of the interesting things about being old (45+, sorry to tell you that) and active and is that we develop conditions that we then get to read about.
That's typical. But if you're old and active and a physician there’s a twist. You get to compare the medical textbooks (and web references to your personal experience, and because of the old-part this reading is further informed by a finely tuned bullshit detector.
The bullshit detector is first developed in medical school. No, it’s not when we learn that following the exam preparation advice of professors is suicide — that’s the betrayal and pit-of-knives detector. For me it was the illuminating moment when I realized my 1986 renal physiology professors really had no idea how the kidney really worked. In their hearts they knew this, but there were exams to write and textbooks to teach to — so they faked it.
Later we run into seemingly erudite residents and medical students who we sooner or later realize are just spinning during medical rounds. Attendings varied in their response, I think some were hapless while others found it amusing. Or not amusing.
Much later, sometime after the first few years of practice, we realize that most journal articles are rather like those residents. (More recently reproducibility studies have made this rather more apparent.) We begin to spot the handwaving in textbooks — and to treasure the few that are relatively honest about ignorance.
Which brings me to my the pain below my left patella (knee cap). It could be related to the patellar tendon, to the “fluid-filled sacs” (bursa) that are usually said to be  under and around various tendons and neighboring bones, or the knee joint (cartilage/arthritis).
The cause is important to treatment. There’s nothing much to do for arthritis except rest and general muscle strengthening. Patellar tendonosis is treated with knee extension exercise starting at 90 degrees, but something (I’ll get to that) called “patellar-femoral syndrome” is treated with knee extension exercise starting close to full extension. So it’s good to differentiate those two.
The differentiation turns out to be relatively simple. If pain hurts coming down stairs (down > up) it’s likely “patella-femoral syndrome”; you won’t be able to do resisted extension at 90 degrees of flexion but you’ll be fine doing it at at 10 degrees of flexion. Also, “patello-femoral syndrome” is much more common than patellar tendonpathy. A related characteristic is that discomfort is maximal between 15 and 35 degrees (stair descent) — so I have no discomfort extended or in a deep squat.
The medical knowledge/bullshit detector bit comes with reading about "patello-femoral syndrome”. As far back as 1990 one of our texts, “Practical Orthopedics” by Lonnie Mercier, refreshingly admitted that this might as well be called something like “sore knee syndrome”. It’s probably a bunch of things involving some degree of irritation of the bursae and tendons (patellar, iliotibial) beneath and below the patella along the course of the patellar tendon. We used to think it had something to do with the cartilage below the patella, but as far back as 1990 Mercier’s text suggested that “chondromalacia patellae" was relatively infrequent, not clearly related to symptoms, and ought to be carved out as a separate diagnosis.
Reviewing a 2007 AAFP article it looks like nothing fundamental has changed . So points to Mercier.
I wasn’t able to find a persuasive evidence-based treatment program for “sore knee syndrome”; I liked Dr Lee Cohen’s PDF for its guide to resuming exercise . Basically I’m avoiding what hurts (flexing knee on stair descent), doing what doesn’t hurt (high rep, low pressure cycling and swimming), doing near-full-extension quad cybex-style weight. I'll increase extension range as the knee improves, and I’ll try some of Dr. Cohen’s routines. I don’t like NSAIDs because of repeated studies showing they delay tendon healing, so not doing those.
Once I can do squats with 50 lbs or so (very light) and run a mile or so without discomfort I’ll go back to CrossFit...
- fn -
 human anatomy is more variable than the diagrams suggests
 I don’t particularly recommend that article btw. It reminded me a bit of 1986 renal physiology.
 Low intensity mountain and road biking feels fine, so I’m doing that. I’m “on leave” from CrossFit until I can do squats without pain or swelling — one of the great things about contract-free CrossFit at our gym is they’ll stop fees if we’re out for 2 weeks or more.
Update 9/12/2015: Occurred to me that I should change to flats on my mountain bike until the knee is done healing (it’s improving well). Normally SPD cleats allow a lot of lateral mention, but sometimes my mountain pedals get jammed with sand. That’s a formula for worsening my knee problem. So flats for now (which, these days, work pretty well anyway).
Update 11/24/2015: I learned a few more things, which really ought to be in a textbook.
My own knee did get better after a few weeks of CrossFit abstention, a bit less mountain biking (week off), and quad strengthening. Subsequently, however, I found CrossFit didn’t bother it much at all, but heavy mountain biking could be annoying. Since mountain biking season ended I have been mostly doing CrossFit and ice hockey — and it is now better. I ended up thinking the mountain biking was probably the greater aggravating factor.
Ahh, but there’s a twist as well. I have since learned that what mountain biking and stairway descent have in common is 15 degrees of flexion, which is when the patella is most in contact with the femur. At greater degrees of flexion strong quadriceps pull the patellar undersurface away from the femur — which is why my strong quadriceps limited the pain. Alas, the clinical presentation cannot distinguish inflammatory arthritis (idiopathic, as in OA involving primarily the synovium), from psoriatic arthritis (less idiopathic, with tendon involvement). In my case other clinical findings point to more chronic conditions - psoriatic arthritis or idiopathic inflammatory arthritis (aka, erosive osteoarthritis). So I’ll write a bit more about that sort of thing over time.