Skip to content
Explore atlas
PH-MS09

Osteomyelitis and septic arthritis

How pyogenic bacteria reach bone and joints, why childhood bone infection starts in the metaphysis, how trapped pus kills bone to leave a sequestrum and chronic infection, and how enzymes destroy a septic joint within days.

  • Free preview
  • 6 steps · 10 questions
  • 3D normal anatomy

By the end, you can

  • Describe the three routes by which bacteria reach bone and the organisms typical of each group of patients.
  • Explain why acute haematogenous osteomyelitis starts in the metaphysis in children and how its spread changes with age.
  • Relate pus under pressure in rigid bone to the sequestrum, involucrum, cloacae, sinus tracts and Brodie abscess.
  • Explain why vertebral osteomyelitis destroys a disc and the two vertebrae beside it, and what an epidural abscess can do.
  • Explain how septic arthritis destroys cartilage within days, and contrast Staphylococcus aureus with gonococcal arthritis.
  • Recognise the classic appearances, tests and organisms examiners ask about, and explain how antibiotics, drainage and debridement act on the mechanism.

What it is

Osteomyelitis is infection of bone and its marrow; septic arthritis is infection of a joint space. This lesson covers non-specific (pyogenic) infection, mostly bacterial, with neutrophils and pus, as opposed to the granulomatous (“specific”) infections such as tuberculosis. Osteomyelitis divides by route (through the blood, from nearby tissue or by direct inoculation) and by course (acute, subacute or chronic).

5 types to tell apart
Acute haematogenous osteomyelitis
Bacteria carried in the blood settle in bone, usually a single organism. Mostly children, in the metaphyses of long bones, and in adults the vertebrae.
Contiguous-focus osteomyelitis
Infection spreads into bone from infected tissue beside it: a diabetic foot ulcer, a deep pressure ulcer over the sacrum or hip, an infected tooth or sinus. Often several organisms; the commonest form overall.
Post-traumatic and post-operative osteomyelitis
Bacteria inoculated directly into bone through an open fracture or at surgery, often growing as a biofilm on dead bone fragments or an implant.
Chronic osteomyelitis
Persistent infection around dead bone, with sinus tracts and flares over months to years. It follows incompletely treated acute infection or begins insidiously.
Septic arthritis
Pyogenic infection of a joint, reached through the blood, from osteomyelitis next to it or by direct penetration. In adults it divides into gonococcal and non-gonococcal arthritis, the latter mostly S. aureus.

Normal function

A growing long bone and its blood supply

A long bone has a shaft (diaphysis) of compact bone around the marrow cavity and two ends (epiphyses) of spongy bone. Between each end and the shaft lies the metaphysis, the flared region next to the growth plate (physis), a disc of cartilage that lengthens the bone until it fuses in early adulthood. The fastest-growing metaphyses are round the knee: the lower end of the and the upper end of the .

Blood reaches the marrow through a nutrient artery that enters the shaft and branches towards each end. Beneath the growth plate its smallest branches turn back in tight loops and open into wide, slow-flowing venous sinusoids. The surface is covered by periosteum, a fibrous sleeve whose vessels supply the outer part of the cortex; in children it is thick and only loosely attached, except at the growth plate.

Synovial joints are enclosed by a capsule lined with vascular synovium, which makes the fluid that feeds the articular cartilage, itself without vessels. At the hip, and to a lesser extent the shoulder, elbow and ankle, the capsule attaches beyond the growth plate, so part of the metaphysis lies inside the joint.

Point to it on the model