Rheuma Case: 52-year-old Man with Chronic Right Knee Pain

00:01 Alright, today we're gonna talk about the most common cause of joint pain, osteoarthritis.

00:07 Here's a case.

00:09 A 52-year old man presents with acute-on-chronic right knee pain.

00:14 Now he has intermittently had knee pain for about 5 years, worse after activity, with morning stiffness for up to 15 minutes everyday.

00:22 This past Saturday, he spent much of the day helping his son move into his college dorm and the next morning, he reports that his right knee is swollen and painful.

00:32 Denies any fevers or chills, no other involved joints, his pain is worst at the medial aspect of the knee.

00:39 Initial vitals, completely unremarkable and on gross inspection we see symmetric, bony enlargement of the knees.

00:48 On range of motion testing, his right knee is limited to about 90 degrees of flexion.

00:53 It's limited by pain and stiffness and you do note some crepitus on exam.

00:58 On palpation of the right knee, we find a tender medial joint line, no pain over the pes anserine bursa, no increased warmth and a small effusion is present.

01:10 Alright, so let's highlight some of the important features of this case.

01:14 It sounds like this is an acute on chronic issue.

01:17 He's had pain for 5 years but something seems to have made it worse over the past day or so.

01:22 The pattern of joint involvement is also important.

01:25 It sounds like it's asymmetric, it's mostly involving just one knee and of course it's monoarticular.

01:32 Next stop in terms of joint inflammation, well, on the one hand we have swelling and we have a painful joint which tends to make us think of joint inflammation or probably end up of having to get an arthrocentesis to see how inflamed this joint really is.

01:46 And then next stop, systemic involvement.

01:49 Well, review of systems is completely unrevealing.

01:51 We're not getting any fevers or any evidence of any other systemic illness.

01:55 So, I think we could say there is no systemic involvement.

01:57 So, with that history in mind, what is the most likely diagnosis? Let's start with pseudogout.

02:03 Pseudogout as you may recall is also CPPD, calcium pyrophosphate deposition and it is a relatively common type of crystalline arthropathy.

02:12 Most importantly, the most common joint affected in CPPD is the knees.

02:18 Now, we'd wanna ask about some other common risk factors for pseudogout like hyperparathyroidism, hemochromatosis, I'd like to think of that as iron-hyper-iron and then hypomag or hypophosphatemia.

02:32 Of course, only in arthrocentesis is really gonna help us to include or exclude that diagnosis.

02:38 We'll keep a question mark there for now.

02:40 And then, osteoarthritis.

02:42 As I've said at the beginning of this talk, it's the most likely diagnosis 'cause it's the most common cause of pain.

02:48 Typically you can have an acute-on-chronic picture, knee involvement is very common and even this description of bony enlargement of the bilateral knees leads us towards that diagnosis.

02:59 But what about this effusion issue? Can that happen with osteoarthritis? I think we'll have to leave a question mark in that box for now.

03:08 Next stop, rheumatoid arthritis.

03:10 Now, we need to consider rheumatoid arthritis though there are some things that are gonna conflict with that diagnosis.

03:17 First of all, it's monoarticular.

03:19 We tend to expect to have symmetric findings, you expect to see more stiffness, he's saying he's only stiff for 15 minutes in the morning, we'd wanna see stiffness for 30 minutes to nearly an hour and also we don't have any systemic symptoms at all.

03:32 The effusion, however, would be pretty typical with rheumatoid arthritis, so, I think we'll just keep that on the list for now as well.

03:40 Next stop is patellar tendinopathy.

03:43 Now remember, patellar tendinopathy is also known as "jumper's knee".

03:47 It's basically inflammation, swelling and even potentially thickening of the tendon that attaches the distal patella to the tibial tuberosity as shown in this image here.

03:58 It's typically an overuse injury most common in athletes.

04:02 It would typically cause anterior knee pain rather than medial knee pain and it would be pretty unusual in a middle-aged non-athlete.

04:11 So, I think it's safe for us to take that one off the list.

04:15 Next stop, pes anserine bursitis.

04:18 Now, there are several bursa, it can get kind of complicated that surround the knee.

04:23 Including, shown here, the suprapatella, the prepatellar, the infrapatellar bursa.

04:29 In addition, not depicted here because of the plane of this image, is the pes anserine bursa which lies just distal to the medial joint line of the knee on both sides.

04:41 The bursa in general serve as soft buffers between tendons and hard bony surfaces, protecting tendons from wear and tear.

04:50 When they become inflamed from overuse, occasionally from immune mediated inflammation and more rarely from infection, the bursa can become exquisitely painful, swollen and warm to the touch.

05:03 Now as I've said, the pes anserine bursa is medially located, which is where our patient is experiencing pain, but again, it's about 2 cm distal from the medial joint line, distal to the medial tibial plateau.

05:17 So really a good physical exam should be able to distinguish between tenderness at the medial joint line versus tenderness distal to that at the pes anserine bursa and we were given a pretty thorough exam.

05:28 So I think we can safely rule out pes anserine bursitis as well.

05:33 So, we're left with pseudogout, osteoarthritis and rheumatoid arthritis.

05:39 Looks like we're gonna need an orthocentesis to tease these apart.

05:42 Okay, so here's our results.

05:45 No crystals, that's reassuring.

05:47 Cell count is 1,300 nucleated cells and gram stain is negative.

05:52 Culture is no growth in case we're ever considering aseptic arthritis.

05:57 Let's look at that cell count and try and put that into perspective.

06:01 So, here's a nice schematic showing how to interpret the synovial fluid cell count.

06:07 It looks like our cell count of 1,300 is gonna put us into the non-inflammatory column here on the far left of the spectrum which supports osteoarthritis after all.

06:17 Here's the key point.

06:19 While it's unusual to have acute synovitis with an effusion like osteoarthritis, it definitely happens and it can certainly look at first like an inflammatory arthritis.

06:29 But the key clues here are a relatively cool effusion which is how it was described in our HPI compared with the hot effusion you might see with septic arthritis or gout.

06:39 Also this patient had no systemic symptoms which would lead us away with something like rheumatoid arthritis.

06:45 And lastly, this cell count of less than 2,000 is gonna definitely push us into a non-inflammatory category.

06:51 Remember, things like gout are typically gonna have 20 to 80,000 cells and in septic arthritis is certainly gonna be more than 50,000 cells potentially as high as 100,000 cells.

07:03 Alright, final point.

07:05 Bland synovial fluid with less than 2,000 cells, think osteoarthritis.