Bone & Joint Infection & Tumors (Clinical)

Microbiology and routes of spread

S. aureus first. Staphylococcus aureus causes most osteomyelitis and most non-gonococcal septic arthritis at every age, so empiric therapy always covers it. Host clues point to the classic alternatives.

Routes of spread. In children, blood-borne bacteria lodge in the metaphysis of long bones, where sluggish flow through looping capillaries lets them settle; sluggish flow plus bacteremia seeds bone. Adults more often develop hematogenous vertebral osteomyelitis or contiguous and direct spread from surgery, an ulcer or trauma.

Diagnosing and treating osteomyelitis

Imaging and labs. MRI is the most sensitive and specific early test, showing marrow edema within days. Plain films are often obtained first but lag 1–2 weeks before showing lytic lucency and periosteal reaction, so a normal film does not exclude early infection. ESR and CRP are elevated and are trended to track response. Blood cultures sometimes identify the organism in hematogenous disease, but bone biopsy with culture is the gold standard for naming it.

Treatment. Give prolonged, typically 4–6 weeks, culture-directed antibiotics. Chronic osteomyelitis, with dead bone (sequestrum) walled off by reactive new bone (involucrum), needs surgical debridement, because antibiotics alone will not sterilize necrotic bone. A long-standing draining sinus tract can undergo malignant change to squamous cell carcinoma.

Diabetic foot. In a diabetic foot ulcer, a positive probe-to-bone test, a large ulcer (area over 2 cm²) or an ESR over 70 mm/h makes osteomyelitis likely. Infection is often polymicrobial and superficial swab cultures are unreliable, so confirm with MRI and, ideally, bone biopsy with culture before committing to weeks of therapy.

Vertebral osteomyelitis. It presents with focal back pain and point tenderness, fever and elevated ESR and CRP, classically in an IV drug user and most often from S. aureus. Obtain MRI of the spine and blood cultures, with CT-guided biopsy if cultures are negative. Watch for an epidural abscess: a new neurologic deficit is a surgical emergency.

Septic arthritis and prosthetic joint infection

Hot swollen joint. An acute monoarthritis (the knee is most common) that is red, hot and swollen, with the patient refusing to move it, is septic until proven otherwise. Non-gonococcal disease is usually S. aureus. In young, sexually active adults, disseminated gonococcal infection causes a triad of migratory polyarthralgia, tenosynovitis and pustular skin lesions, or a purulent monoarthritis; joint cultures are often negative, so send NAAT from urethral, cervical, pharyngeal and rectal sites, and treat with ceftriaxone.

Tap before antibiotics. Arthrocentesis comes first so that antibiotics do not blind the culture. Septic fluid typically has a WBC greater than 50,000/µL with over 90% neutrophils, low glucose and a positive Gram stain or culture. Crystals do not exclude infection, because gout and septic arthritis can coexist.

Drain it now. Septic arthritis is a joint-destroying emergency: arrange urgent drainage (arthroscopic, open arthrotomy or serial needle aspiration) plus IV antibiotics, because delayed drainage erodes cartilage within days. Empiric therapy follows the Gram stain: vancomycin for gram-positive cocci, a third-generation cephalosporin for gram-negative organisms, and both when the stain is negative.

Prosthetic joint infection. Early infection (under 3 months after surgery) is often from virulent S. aureus and presents acutely. Late, indolent infection with pain and implant loosening, often without fever, points to coagulase-negative staphylococci (S. epidermidis) or Cutibacterium living in biofilm; Cutibacterium is classic after shoulder arthroplasty. Debridement with implant retention (DAIR) is an option only for a stable implant with infection within about 30 days of surgery or acute hematogenous infection with under 3 weeks of symptoms. Otherwise cure usually requires hardware removal, classically a two-stage revision: remove the implant, place an antibiotic spacer and treat, then reimplant.

Biofilm on hardware is why chronic prosthetic infection rarely clears with antibiotics alone.

Bone tumors

Benign or malignant. Benign lesions are well defined with a sclerotic margin and are often painless or incidental, such as an osteochondroma. Malignant lesions are ill defined, break through the cortex, provoke aggressive periosteal reaction (Codman triangle, sunburst) and cause rest and night pain. Osteoid osteoma is the benign exception with night pain, which melts with NSAIDs. Evaluate with plain X-ray first, then MRI for local extent, then biopsy.

Malignant pearls. Osteosarcoma is the most common primary malignant bone tumor of adolescents, metastasizes to the lungs, and is treated with chemotherapy plus limb-sparing surgery. Ewing sarcoma can cause fever and a raised ESR, mimicking osteomyelitis.

Pathologic fracture

Recognition. A fracture through abnormal bone is pathologic; clues are preceding rest or night pain, a mass, or a break from low-energy trauma. In adults, metastatic disease is the most common cause, from breast, prostate, lung, kidney and thyroid primaries. Prostate metastases are classically blastic, whereas kidney and thyroid metastases are lytic.

Multiple myeloma. Myeloma produces punched-out lytic lesions with hypercalcemia. Because the lesions are purely lytic, a bone scan can miss them, so use a skeletal survey or low-dose whole-body CT. In children, a unicameral bone cyst of the proximal humerus is the classic benign cause of pathologic fracture.