HY lecture notes:
Septic arthritis is becoming increasingly HY on Step 1, and is exceedingly HY on FM, IM, surgery, and peds NBME forms – essentially just on the 2CK in general.
On the USMLE, for septic arthritis questions, there are three main points you need to know:
- The answer always = “aspiration of the knee joint” or “arthrocentesis” before antibiotics. We’re looking for elevated leukocytes + will do a culture of the joint aspirate.
- You do NOT need to have fever. There is an FM NBME Q where the temp is 99F. In this Q, however, the rest of the presentation is hardcore obvious, with the afebrile state being the odd factor out. Pretty much every time a student sees this Q they always say, “but how can this be septic joint when they don’t have a fever?” And I have to say, yeah I know, it’s weird. Apparently you don’t have to have a fever, but as you can see, the rest of the vignette is obvious for septic joint.
- Biggest risk factor for septic arthritis = abnormal joint architecture.
The USMLE will give vignettes of septic arthritis in all of following patient groups:
- Prosthetic joints –> you can’t be more abnormal than having a prosthetic joint, so these patients have the greatest risk.
- Rheumatoid arthritis (RA), osteoarthritis (OA), and juvenile rheumatoid arthritis (JRA; Still disease).
- Recent intense exercise in otherwise young, healthy patients (the implication being microtrauma causing the abnormal architecture –> e.g., 16F had a kickboxing workout yesterday, or she went hiking for 8 hours yesterday; a 17M had a soccer tournament last weekend.
- Recent joint trauma –> e.g., 16F who injured her knee in a car accident.
USMLE likes to give you a vignette of RA, where they’ll say 32F has sore wrists/hands + ulnar deviation + hot, red, painful knee, where your initial thought might be, “well she clearly has RA, so couldn’t her knee presentation just be part of an RA flare?” It could be, yes, but we still have to rule out septic arthritis with an arthrocentesis. Patients with RA absolutely are at increased risk of septic arthritis.
For osteoarthritis, they’ll say 55M + BMI of 40 + red, hot, painful knee –> answer = “arthrocentesis.” Fairly simple. You just say, “okay, well the patient is overweight, which is the biggest risk factor for osteoarthritis, and he has a red, hot, painful knee, so it’s definitely septic arthritis they’re getting at.”
I’ve also seen a question on either an FM or IM form where they said a young-ish woman who’s 6’2″ (correct, 6’2″) with a BMI of ~30 had a red, hot, painful knee. Same deal –> OA –> “joint aspiration.” The trickier part being here, “well, she’s actually on the younger side, which is less common for OA patients, as most OA patients are >50, but she’s tall” –> patients who are “big and tall” are at increased risk at a younger age –> e.g., a guy who’s 250 lbs and 6’4″ with “bad knees.”
I bolded the intense exercise and recent joint trauma points above because not only are these presentations exceedingly HY for septic arthritis, but it’s also rare that students make the connection between the two. Once again, 16F who has a red, hot, sore knee + she had a kickboxing workout yesterday –> answer = arthrocentesis.
Septic joints are also seen on the peds shelf –> 6M with recurrent joint flares (so you’re immediately thinking JRA) + today he has a fever of 101F + a hot, red, painful knee –> answer = joint aspiration. Same deal –> abnormal joint architecture as the risk factor.
The latter contrasts with toxic synovitis (aka transient synovitis), which is a different peds diagnosis –> usually a viral infection in kids age 3-8 causing inflammation of the hip joint. Kids are usually afebrile and will have no Hx of joint trauma or JRA. The kid can bear weight and the pain improves throughout the course of the day. A hip x-ray is done early as toxic synovitis is a diagnosis of exclusion. You will not get asked about arthocentesis regarding toxic synovitis. They merely want you to know the Dx based on recent URTI in a kid who now has hip pain.
