Choose an answer, then open any option to work its reasoning.
MSK · Infectious Disease
MSK Infections: Septic Arthritis, Osteomyelitis, and the Destruction Clock
Fever plus a hot joint plus limited range of motion is septic until proven otherwise, and the clock starts the moment the patient walks in: cartilage is destroyed in hours to days. Learn the fluid, the organisms by host, and the order that matters: aspirate, stain, culture, empiric antibiotics, then the operating room when the joint or the fascia demands it.
What this page makes you able to do
- Read a synovial fluid panel and decide septic versus crystal disease
- Sequence the destruction clock: tap, stain, culture, antibiotics, drainage
- Name the organism by host: joint, bone, hardware, heel, and bursa
- Triage the red limb and the aching spine before tissue is lost
- Dr. Fatima Ali, DOPsychiatry residentPrimary reviewer
Last reviewed
The pearl
Hot joint + fever + limited range of motion is septic until proven otherwise, and the proof lives in the fluid, not the film: WBC above 50,000 with more than 90% neutrophils, gram stain, and culture. The aspirate is the first move, never something to schedule around imaging.
Prove it
Opening question
Answer before you read anything, then keep the order of operations in mind through every section.
A 58-year-old man comes to the emergency department because of 2 days of right knee pain and swelling after a minor scrape while gardening. He reports fever and chills. Temperature is 38.6 C, heart rate is 104/min. Examination shows a warm, swollen right knee with painful, limited range of motion and a moderate effusion. Laboratory studies show a peripheral WBC count of 14,000/mm3.Which of the following is the most appropriate next step?
- Why this is rightFever plus a hot, swollen joint with limited range of motion is septic until proven otherwise, and the proof lives in the fluid. The tap is diagnostic and therapeutic: cell count, gram stain, and culture come first, and empiric IV antibiotics follow immediately, never waiting for the results. Rule: aspirate today, empiric antibiotics now, and never let imaging or observation delay the tap.
- Why this failsAntibiotics before the tap can sterilize the fluid and wipe out the gram stain and culture that name the organism. The sequence is tap, stain, culture, then antibiotics: minutes apart, in that order. Rule: the tap comes first because the culture only works once.
- Why this failsMRI can see the effusion and the bone, but it takes time and adds nothing the needle cannot answer faster. Imaging maps osteomyelitis or a deep abscess later; it never delays the tap of a hot joint. Rule: imaging answers tomorrow's question, the aspirate answers today's.
- Why this failsA febrile patient with a hot joint is not a musculoskeletal sprain; observation hands the infection two days of cartilage. Fever on top of a hot joint is the septic-until-proven-otherwise package. Rule: ice and NSAIDs are how a septic joint is lost.
Work the reasoning
Septic arthritis is the one hot joint that destroys cartilage in hours to days, and its early signs are identical to crystal disease and trauma. The cost of waiting is the joint.
Purulent fluid runs a WBC above 50,000 with more than 90% neutrophils; the gram stain and culture name the organism. Crystals can coexist and never clear a septic joint.
After cultures, empiric IV antibiotics covering S. aureus (vancomycin plus ceftriaxone) for community adults, then drainage for the hip or for failure to improve.
The answer is A: aspirate the knee for cell count, gram stain, and culture, then start empiric antibiotics. The order is the treatment.
THE TICK AND THE JOINT
Lyme Arthritis: The Recurrent Knee That Culture Cannot Grow
A child with a third swollen knee this summer. The fluid is bland, the cultures stay negative, and the answer lives in two tiers of serology.
Borrelia burgdorferi arrives with erythema migrans in stage 1 and returns months later as arthritis, its most common late manifestation. The classic picture is a recurrent, monoarticular knee effusion in a child in an endemic area, often with a large effusion out of proportion to the pain. The joint fluid is inflammatory but not purulent, and the culture is always negative.
The test is two-tier serology: an ELISA first, and a Western blot to confirm a positive screen. IgM and IgG blots are read by the number of bands, not by feel. Treatment is doxycycline for 14 to 28 days (amoxicillin in young children); refractory arthritis after a full course gets a second look, not a third course by reflex. Untreated, the knee keeps recurring; treated, it settles.
Read the pattern and name the organism.
THE HOT JOINT
The Hot Joint: Septic Until Proven Otherwise
One hot, swollen joint with limited range of motion is an infection until the fluid says otherwise. Learn the organisms by host, then read the fluid panel the way the boards grade it.
Septic arthritis is the one hot joint that destroys cartilage in hours to days, and its early presentation is identical to gout, pseudogout, and trauma. That is why the rule is absolute: fever plus a hot, swollen joint plus limited range of motion is septic until proven otherwise, and the proof lives in the fluid. The aspirate answers three questions at once: cell count and neutrophil percentage, gram stain, and culture. Purulent fluid runs a WBC above 50,000 with more than 90% neutrophils; crystals can coexist with infection, so a positive crystal reading never cancels the culture.
The organism follows the host, and boards hand you the host in the first sentence. Staphylococcus aureus is the default in native and prosthetic joints alike. Neisseria gonorrhoeae owns the young, sexually active adult with the triad of dermatitis, arthritis, and tenosynovitis, and its joint culture is frequently negative. Borrelia burgdorferi gives the monoarticular knee in an endemic area with a large effusion out of proportion to pain. Pseudomonas aeruginosa lives in the IV drug user with a hot sternoclavicular joint. Salmonella joins S. aureus in sickle cell disease. The neonate with a floppy limb gets S. aureus or group B streptococcus, and under age four Kingella kingae sneaks into the differential for osteomyelitis and septic arthritis.
Read the fluid panel and commit to the next step.
A 58-year-old man with 2 days of a hot, swollen knee has fever and chills. Arthrocentesis returns cloudy fluid with a WBC count of 78,000/mm3, 94% neutrophils, gram positive cocci in clusters, and no crystals. Which of the following is the most appropriate next step?
A. Start empiric IV antibiotics and plan joint drainage. WBC above 50,000 with more than 90% neutrophils and gram positive cocci in clusters is septic arthritis with S. aureus until culture says otherwise. Empiric IV antibiotics start immediately after cultures are drawn, and drainage is part of the treatment. B. Colchicine. No crystals were seen, and colchicine does not treat infection. C. Repeat aspiration. The count already answers the question; repeating it hands the infection two more days of cartilage. D. NSAID. A febrile patient with purulent fluid needs antibiotics and drainage, not anti-inflammatories. Rule: purulent fluid plus cocci equals antibiotics plus drainage; nothing about this joint can wait.
THE DESTRUCTION CLOCK
The Destruction Clock: Sequence Before the Cartilage Dies
Every MSK infection has an order of operations, and the order is the treatment. The hip is the case that makes the clock real.
The hip is the dangerous joint because of anatomy, not attitude. The femoral head is intracapsular, its blood supply is fragile, and pus under pressure can infarct the epiphysis. The bedside sign is pseudoparalysis: the child stops moving the limb entirely, unlike the limper who still uses it. The child holds the hip flexed and externally rotated because that is where the capsule is most capacious and the pressure lowest. A child with a high fever, a WBC of 16,000, an ESR above 40, and refusal to bear weight has a septic hip until the tap says otherwise, and the hip is drained surgically, not watched.
The sequence is the same for every deep infection: aspirate, stain, culture, empiric antibiotics, then drainage. Each step buys the tissue behind it time, and skipping ahead or waiting for imaging is how a joint or an epiphysis is lost.
Put the destruction clock in order.
THE BONE LAYERS
The Bone Layers: Osteomyelitis From Metaphysis to Sequestrum
Acute hematogenous osteomyelitis lives in the metaphysis of a child. Chronic disease lives in the dead bone it leaves behind: sequestrum, involucrum, and Brodie abscess.
Acute hematogenous osteomyelitis seeds the metaphysis, where sluggish sinusoidal flow near the growth plate lets bacteria settle. The classic child has fever and bone pain at the distal femur or proximal tibia and refuses to bear weight. S. aureus dominates; Kingella kingae is the under-age-four consideration, and Salmonella joins S. aureus in sickle cell disease. MRI is the best test: marrow edema shows early, while plain films lag 7 to 10 days, so a normal radiograph never rules out early osteomyelitis. Blood cultures plus imaging make the diagnosis; bone biopsy confirms when it is uncertain.
Contiguous disease walks in from the outside. The diabetic foot ulcer with probe-to-bone is osteomyelitis until proven otherwise, usually polymicrobial, and MRI plans the debridement. Chronic osteomyelitis leaves the structures boards love: the sequestrum is dead bone that separated from living cortex, antibiotics cannot penetrate it, and it must be excised; the involucrum is the periosteal new-bone shell around it and is preserved; the Brodie abscess is a chronic metaphyseal cavity walled by granulation tissue. Antibiotics alone cannot cure a sequestrum; the dead bone has to come out.
Tap each site and read why it seeds.
Tap a site.
THE FLESH EATER
The Flesh Eater: Necrotizing Fasciitis Is an OR Problem
Pain out of proportion, skin that looks better than the patient feels, and a fascia that dies in hours. This is the MSK infection where the answer is never watch and rarely scan first.
Necrotizing fasciitis is a surgical emergency because the infection tracks along fascial planes faster than antibiotics can act. Type II is the classic community form: Streptococcus pyogenes (group A strep) alone or with S. aureus in an otherwise healthy patient after a minor wound. Type I is polymicrobial (gram positives, gram negatives, and anaerobes) in the diabetic, postoperative, or immunocompromised host, and Fournier gangrene is the same disease in the perineum. Gas gangrene from Clostridium perfringens is myonecrosis, not fasciitis, but both belong in the operating room now.
The exam is the diagnosis: pain out of proportion to the wound, rapidly spreading erythema, then bullae, duskiness, and skin necrosis over a limb that can look deceptively intact. Fever, tachycardia, and an elevated CK complete the picture. The LRINEC score (CRP, WBC, hemoglobin, sodium, creatinine, glucose) was built to help, but a patient who looks too well for the pain does not wait for a score. When necrotizing fasciitis is suspected on exam, the OR is the next stop; imaging is obtained only when it will not delay surgery, because hours of delay cost tissue and life. Debridement is both diagnostic and therapeutic, and the patient comes back to the OR for repeat debridement at 24 to 48 hours.
Hot red limb: which door does this patient walk through?
A patient arrives with a hot, red limb. Which door fits which presentation?
THE SPINE AND THE COUGH
The Spine and the Cough: Pott Disease vs Pyogenic Spondylitis
Back pain plus fever plus night sweats is a spine infection until proven otherwise. Tuberculosis picks the thoracic spine and builds a kyphus; pyogenic disease is faster and angrier.
Pott disease (tuberculous spondylitis) is the slow burn: Mycobacterium tuberculosis seeds the anterior vertebral body, and weeks to months of insidious back pain, fever, night sweats, and weight loss follow. Collapse of the affected bodies creates the sharp kyphotic gibbus, and a paravertebral abscess can track down the psoas. It favors the thoracic spine, and the young patient from a TB-endemic region with thoracic back pain and a gibbus is TB until proven otherwise. Diagnosis is imaging plus culture or nucleic acid amplification of the lesion, with a look for pulmonary TB; treatment is multi-drug anti-tuberculous therapy (RIPE) for months, with surgery for abscess, instability, or neurologic deficit.
Pyogenic vertebral osteomyelitis is the fast fire: acute to subacute severe back pain with high fever in older adults, IV drug users, and diabetics. It destroys the endplate and narrows the disc space on MRI, usually from S. aureus, and an epidural abscess is a surgical urgency: back pain plus fever plus neurologic signs. Blood cultures and MRI make the diagnosis; the disc space is biopsied when blood cultures are negative. Treatment is IV antibiotics for weeks, with drainage or decompression for epidural abscess or neurologic compromise.
Flip between the two spinal infections.
HARDWARE, HEEL, AND ELBOW
Hardware, Heel, and Elbow: PJI, Puncture Wounds, and Septic Bursitis
The joint replacement, the sneaker nail, and the bursa over the pointy parts: three infections with different organisms, different depths, and different timelines.
Prosthetic joint infection follows the timeline. Early PJI (weeks to months after arthroplasty) is inoculated at surgery, with S. epidermidis and S. aureus causing pain, wound drainage, and stiffness. Late PJI (months to years) is hematogenous seeding of a previously well joint after a bacteremia, with S. aureus the most common invader. Bacteria form biofilm on hardware, so antibiotics alone rarely cure, and the workup ends in aspiration: ESR, CRP, and joint fluid for cell count and culture before any antibiotic. Established infection is managed surgically, with debridement and implant retention only in select early cases, and one- or two-stage exchange for chronic infection.
Puncture wound osteochondritis is the sneaker story: the child steps on a nail through the moist foam of the sneaker, and Pseudomonas aeruginosa seeds the bone and cartilage beneath the puncture. Days later the foot is red, swollen, and deeply tender. Treatment is surgical debridement of the tract plus antipseudomonal antibiotics, and the sneaker is the clue that separates Pseudomonas from routine skin flora.
Septic bursitis lives over the pointy parts: olecranon in the elbow-leaner, prepatellar in the knee-kneeler. S. aureus is the organism, usually through a break in the skin, and the exam distinguishes it from septic arthritis in one move: the joint's range of motion is preserved. Aspirate the bursa for cell count, gram stain, and culture, then treat with oral antistaphylococcal antibiotics; refractory cases get incision and drainage.
Tick every scenario that changes your empiric regimen off the native-joint default.
Checked 0
Open each regimen and hold the duration facts.
Empiric vancomycin plus ceftriaxone after cultures, then organism-directed therapy; total duration is typically 2 to 4 weeks of IV antibiotics with prompt drainage of the hip or a joint that fails to improve.
Debridement of dead bone plus 4 to 6 weeks of pathogen-directed IV (or highly bioavailable oral) therapy, counted from the last debridement. A sequestrum that stays in place never clears.
Broad-spectrum empiric IV therapy (vancomycin plus piperacillin-tazobactam or a carbapenem) with clindamycin for group A strep toxin suppression, alongside immediate and repeat debridement every 24 to 48 hours.
Multi-drug anti-tuberculous therapy with RIPE (rifampin, isoniazid, pyrazinamide, ethambutol) for months, typically 6 to 12 months; surgery is reserved for abscess, instability, or neurologic deficit.
Puncture osteochondritis: debridement plus an antipseudomonal beta-lactam or a fluoroquinolone. Chronic PJI: cultures before antibiotics, then surgical exchange (one- or two-stage) with organism-directed therapy after the explant cultures return.
Aspiration for cell count, gram stain, and culture, then oral antistaphylococcal therapy such as dicloxacillin or cephalexin; add MRSA coverage for a resistant host and escalate to incision and drainage when the bursa does not respond.
THE TRIAGE DISCRIMINATOR
The Triage Discriminator
Every hot joint, red limb, or aching spine on boards is one of these doors, and the door decides how fast you move.
Run the doors in order. Hot joint with fever and limited range: aspirate, then empiric antibiotics, then drainage when the hip or a failing joint demands it. Child who will not bear weight: the Kocher criteria (fever, non-weight-bearing, ESR, WBC) predict the septic hip, and three or more criteria means the hip is aspirated and drained. Red limb with pain out of proportion: the OR, not the scanner. Aching thoracic spine with night sweats: Pott disease until proven otherwise, and the workup ends in culture or nucleic acid amplification. Prosthetic joint that suddenly hurts: aspirate before antibiotics. Sneaker nail: Pseudomonas. Swollen pointy part with a preserved joint: the bursa, treated with aspiration and oral antistaphylococcal therapy.
Tap each letter of the Kocher criteria.
FEW C
FFever above 38.5 C
EESR above 40 mm/h
WWBC above 12,000/mm3
CCan't walk: refusal to bear weight (pseudoparalysis)
Name the organism from the host, one clue at a time.
Prove it
Walkthrough: race the destruction clock
Original practice scenarios, one at a time. Choose an answer, then open any option to work its reasoning.
Clinical walkthrough
