Bone, Joint & Device Infections

Educational study digest pitched at a new ID fellow. Clinical recommendations trace to the cited guidelines and trials; the enrichment box is reviewer synthesis of additional evidence, each claim cited. Not medical advice, and not a substitute for the primary guidelines.

◆The pressure ulcer that probes to bone — how do you prove it, and whom do you treat?

A stage IV sacral pressure ulcer that tracks down to bone raises osteomyelitis, but two questions have to be separated: how to prove it, and whether to treat it at all. Proof does not come from a superficial wound swab — that grows the polymicrobial flora colonizing the ulcer surface, not the organism in the bone. The definitive diagnosis is a bone biopsy, sent for both histopathology and microbiology: histology establishes the inflammatory and necrotic bone changes while culture names the pathogen and its susceptibilities so therapy can be targeted.

The harder and more heavily tested question is selection. Only patients who have a plan for definitive coverage of the wound should be treated for the underlying osteomyelitis. A decubitus ulcer left open over the sacrum continuously recontaminates the bone beneath it; antibiotics given without a strategy to offload pressure and achieve durable soft-tissue coverage — debridement, flap closure — treat the culture result rather than the patient and simply select resistance. Put concretely: pursue the bone biopsy and commit to a prolonged antimicrobial course when there is a surgical and wound-care plan to close the defect; absent that plan, the honest move is wound care and comfort-directed management, not a course of antibiotics that cannot succeed.

◆A prosthetic joint fails — what does the timing tell you, and how is PJI defined?

When a prosthetic joint fails, the interval since implantation is the single most useful clue to the organism. The classification: early PJI is <3 months from surgery and is typically caused by virulent pathogens (Staphylococcus aureus, gram-negative rods) seeded at the time of implantation; delayed PJI runs 3–24 months and is typically caused by indolent, low-virulence organisms — coagulase-negative staphylococci, Cutibacterium — that present as loosening or chronic pain rather than sepsis; and late PJI is >24 months out and again tends to be virulent, usually reflecting hematogenous seeding of the joint from a distant focus. The practical payoff is that a hot, acutely painful joint a few weeks after arthroplasty and an indolently loosening joint at a year are pointing at different organisms before a single culture returns.

Diagnosis is a composite rather than a single test, and the framework the teaching anchors to is the IDSA prosthetic-joint-infection guideline. PJI is definite when a sinus tract communicates with the prosthesis, when purulence surrounds the implant, when acute inflammation is present on histopathology of periprosthetic tissue, or when the same organism is recovered from two or more separate periprosthetic cultures. Serum inflammatory markers (ESR and CRP) and a joint aspirate showing an elevated synovial leukocyte count with neutrophil predominance are supportive and drive the decision to sample. The reason two matching cultures matter so much is precisely the delayed, low-virulence group: one coagulase-negative staph could be a skin contaminant, but the same organism in two samples is the pathogen.

◆DAIR, one-stage, or two-stage — how does the antibiotic plan track the surgery?

Antimicrobial management in PJI is inseparable from the surgical strategy, because the drugs do a different job depending on whether hardware stays or goes. In debridement, antibiotics, and implant retention (DAIR) — used for early or acute-hematogenous infection with a stable implant — the biofilm is left in place, so a biofilm-active regimen is essential. For staphylococcal PJI managed with DAIR, that means a targeted IV agent plus rifampin 300–450 mg BID for 2–6 weeks, then a step-down to a targeted oral agent plus rifampin to complete a total of 3 months for a hip (and for shoulder, elbow, and ankle) or 6 months for a knee. The preferred oral companions to rifampin are the fluoroquinolones — ciprofloxacin or levofloxacin — with co-trimoxazole, doxycycline or minocycline, cephalexin, and dicloxacillin as secondary options. If rifampin cannot be used (allergy or intolerance), the regimen shifts to 4–6 weeks of IV therapy without it; non-staphylococcal PJI is likewise treated with 4–6 weeks of IV or a highly bioavailable oral agent.

The exchange operations differ in how the implant-free interval is handled. One-stage exchange removes and reimplants in a single operation and is managed medically like DAIR. Two-stage exchange removes the implant, places a spacer, and delays reimplantation: here the patient gets 4–6 weeks of targeted IV or highly bioavailable oral therapy (extended to 6 weeks for virulent pathogens such as S. aureus), rifampin is not routinely used, and an antibiotic-free window precedes reimplantation — ESR and CRP are rechecked, and synovial fluid is cultured after roughly two weeks off antibiotics to confirm the infection is cleared before the new prosthesis goes in. Chronic suppressive therapy is considered when the hardware is retained and durable cure is unlikely.

◆Why rifampin for staphylococcal hardware — and what does adding it cost?

Rifampin earns its central place in staphylococcal device infection for two pharmacologic reasons: it penetrates osteoblasts and it penetrates biofilm, retaining antimicrobial activity in both compartments where staphylococci hide from other drugs, and it has broad activity against the gram-positive organisms that dominate device and diabetic-foot osteomyelitis. That biofilm reach is exactly why it is added to DAIR and one-stage regimens but omitted once the hardware is gone (two-stage exchange). Its adoption reflects this: by some estimates 56–100% of European practitioners choose oral therapy with adjunctive rifampin for osteomyelitis.

The non-negotiable rule is that rifampin has a low barrier to resistance and must never be used as monotherapy — given alone, resistant staphylococci emerge quickly, which is why every regimen above pairs it with a partner drug. The other cost is a formidable interaction profile: rifampin is a potent CYP3A4 inducer that lowers serum concentrations of a long list of co-medications. Some pairings are contraindicated outright; many others simply require anticipatory dose adjustment. The interactions most relevant on an ID service are the antiretrovirals and azoles, and they set up the HIV scenario below.

Two more practical notes the teaching flags: rifampin causes orange discoloration of urine and secretions and can cause hepatitis, and intermittent or interrupted dosing can provoke a flu-like syndrome — an argument for steady, uninterrupted administration once it is started.

◆Prosthetic-valve endocarditis with an inconclusive echo — cardiac-gated CT or PET/CT?

When a patient with a prosthetic valve has suspected endocarditis but the echocardiogram is inconclusive, two advanced modalities extend the workup, and they answer different questions. Cardiac-gated CT is the anatomic study: it detects perivalvular complications — abscess, pseudoaneurysm, and fistula — and structural valve dysfunction such as abnormal prosthetic-leaflet mobility and dehiscence. Its characteristic weakness is motion artifact, which ECG-gating is specifically designed to reduce. PET/CT is the metabolic study, recommended when echocardiography is inconclusive for prosthetic-valve endocarditis.

The performance figures are the tested nugget. For prosthetic-valve endocarditis, PET/CT runs roughly 86% sensitivity and 84% specificity; for native-valve endocarditis it is only about 31% sensitive against 98% specific — useful to rule in prosthetic disease, far too insensitive to exclude native-valve disease. Two error modes are worth memorizing: false positives arise with a recently implanted valve (post-surgical inflammation), vasculitis, and plaque or tumor, while false negatives follow prior antibiotic therapy that has already quieted the metabolic signal. One discriminator rides along with any endocarditis discussion: among streptococci, S. pyogenes and S. pneumoniae are the two species at low risk of causing endocarditis — a helpful negative when a vignette tries to pin an endocarditis picture on either.

◆Spinal hardware infection in a patient with HIV — how do you run rifampin with ART?

The device-infection principles collide with antiretroviral pharmacology in a board-favorite scenario: a patient living with HIV who develops a spinal epidural abscess and hardware infection after cervical discectomy-and-fusion. The organisms are the ones expected on spinal hardware — Pseudomonas aeruginosa, Staphylococcus epidermidis, and Staphylococcus lugdunensis (the last a coagulase-negative staph that behaves with S. aureus-like aggression). Empiric therapy anchors on cefepime to cover the Pseudomonas, and the biofilm logic of the earlier sections argues for adding rifampin for the staphylococcal component on the hardware.

That “± rifampin” is where HIV changes the calculus. Rifampin is a potent enzyme inducer, and this patient’s integrase-based regimen (bictegravir/emtricitabine/tenofovir alafenamide) is exactly the kind of ART whose levels rifampin drives down — recall that integrase inhibitors and protease inhibitors sit on rifampin’s contraindicated list. Co-administering a rifamycin with ART risks subtherapeutic antiretroviral levels and virologic escape, so the interaction must be managed, not ignored: substitute the far weaker inducer rifabutin, or restructure the ART. The classic teaching case is TB/HIV coinfection, where both rifampin and ART are non-negotiable; the maneuver there is often a twice-daily INSTI (for example, dolutegravir 50 mg BID) paired with a tenofovir (TDF)-based NRTI backbone, which accommodates rifampin while keeping the regimen potent. The full rifamycin–ART grid — dolutegravir dose-doubling, rifabutin substitution for protease inhibitors and rilpivirine, and the absolute contraindication with long-acting cabotegravir/rilpivirine — is laid out in the companion HIV, ART & Opportunistic Infections digest.

Sources: IDSA Diagnosis and Management of Prosthetic Joint Infection (Osmon DR et al., Clin Infect Dis 2013) — PJI classification, diagnostic criteria, and DAIR / one-stage / two-stage antimicrobial regimens; reimplantation microbiology after two-stage exchange (Melendez DP et al., J Clin Microbiol 2016); OVIVA — Oral versus Intravenous Antibiotics for Bone and Joint Infection (N Engl J Med 2019; doi:10.1056/NEJMoa1710926; PMID 30699315). Prosthetic-valve endocarditis imaging performance (cardiac-gated CT, PET/CT) and rifampin pharmacology and drug-interaction figures are reproduced as presented in the reviewed board-review and case-conference teaching.