Osteomyelitis and septic arthritis

Written by the X-Ray Atlas editorial team from the sources listed below · Medical review pending · Updated 24 October 2026 · Level: Intermediate

Bone and joint infections are emergencies that the plain radiograph is poor at diagnosing early. Osteomyelitis (infection of bone) usually shows nothing on X-ray for the first 10–14 days, and septic arthritis (infection of a joint) may show only an effusion. Waiting for the X-ray to "turn positive" means waiting for bone destruction. So the X-ray has three jobs here: provide a baseline, look for other causes such as fracture or tumour, and detect established or chronic infection. Diagnosis rests on clinical assessment, blood tests, joint aspiration and, for bone, MRI.

1. Septic arthritis

A joint infection can destroy cartilage within days. The typical patient has a hot, swollen, painful joint with restricted movement, often with fever, though older, immunosuppressed and very young patients may not mount a fever. Risk factors include existing joint disease (such as rheumatoid arthritis), a prosthetic joint, diabetes, immunosuppression and injecting drug use.

X‑ray findings

• Early: often normal, or showing only soft-tissue swelling and an effusion (raised fat pads at the elbow, a distended suprapatellar pouch at the knee, widening of the medial joint space in a child's hip).
• Periarticular osteopenia from hyperaemia.
• Later: rapid, uniform loss of joint space and erosions on both sides of the joint, progressing to destruction and eventually fusion.
• Gas in the joint or soft tissues is uncommon and suggests gas-forming organisms or a fistula.

Crystals in the fluid do not exclude infection; the two can coexist. See gout and crystal arthritis.

2. Osteomyelitis: the X‑ray timeline

Approximate time	X-ray
First days	Soft-tissue swelling and loss of the normal fat planes next to the bone; the bone itself looks normal.
About 10–14 days	Enough bone loss to show ill-defined lucency, often in the metaphysis, with periosteal reaction.
Weeks	Progressive lytic destruction, which can be moth-eaten or permeative, with lamellated periosteal new bone.
Months (untreated or chronic)	Sclerosis, thickened bone, and the chronic features below.

MRI is the test of choice for early osteomyelitis: it shows bone marrow oedema, abscesses and soft-tissue extent within days of onset. Nuclear medicine and CT have roles in specific situations, for example where MRI is unavailable or contraindicated, or to look for sequestra.

3. Chronic osteomyelitis: sequestrum, involucrum, cloaca

• Sequestrum: a fragment of dead bone, cut off from its blood supply, which appears denser than the surrounding bone. It harbours bacteria and often needs surgical removal.
• Involucrum: a thick shell of new living bone laid down around the dead bone by the periosteum.
• Cloaca: a hole in the involucrum through which pus drains, sometimes leading to a sinus track to the skin.
• Thickened, sclerotic, irregular bone with lucent areas.

Long-standing draining sinuses rarely undergo malignant change (squamous cell carcinoma). A new destructive area in chronic osteomyelitis needs further assessment.

4. Infection in children

• In children, osteomyelitis is usually haematogenous (spread through the blood), and typically starts in the metaphysis of long bones around the knee, where blood flow is slow in looping vessels.
• In infants, vessels cross the physis, so infection can spread into the epiphysis and joint, causing combined osteomyelitis and septic arthritis.
• The thick periosteum in children lifts easily, so subperiosteal abscesses and prominent periosteal reaction are common.
• The irritable or painful hip in a child needs septic arthritis excluded on clinical and laboratory grounds; the X-ray is usually normal. See the paediatric hip.
• A toddler who refuses to walk with a normal X-ray may have early infection, a toddler's fracture or another cause; follow-up and further imaging may be needed.

5. Brodie abscess

A Brodie abscess is a form of subacute osteomyelitis: a walled-off abscess, often in the tibial metaphysis of a child or young adult, with relatively mild symptoms.

• A well-defined lucency with a sclerotic rim, sometimes with a lucent channel extending towards the growth plate.
• On MRI a "penumbra sign" (a rim of slightly brighter tissue lining the abscess on T1 images) supports the diagnosis.
• It can mimic an osteoid osteoma or other benign lesions (see bone tumours).

6. The diabetic foot and Charcot joints

In people with diabetes and neuropathy, foot ulcers can extend to bone. Infection under an ulcer that probes to bone is common.

• X-ray signs of osteomyelitis: cortical erosion and destruction under an ulcer, periosteal reaction, and soft-tissue gas or swelling. Serial films showing progressive destruction are more convincing than a single image.
• The plain film is a useful first test, but MRI is more sensitive and helps to plan surgery.

Charcot (neuropathic) joint

Loss of protective sensation leads to repeated unnoticed injury and joint collapse. The classic description is the "6 Ds":

• Distension (effusion)
• Density increased (sclerosis)
• Debris (bone fragments)
• Disorganisation
• Dislocation
• Destruction

Causes include diabetes (typically the midfoot and tarsometatarsal joints), tabes dorsalis (knees), syringomyelia (shoulder and elbow), leprosy and alcohol. An acute Charcot foot can look normal on X-ray, while being red, hot and swollen, and is easily confused with infection or gout. Separating Charcot from osteomyelitis is difficult on plain films; MRI and specialist foot team assessment are needed. Midfoot disorganisation can also mimic a Lisfranc injury.

7. Pitfalls and mimics

• A normal X-ray does not exclude osteomyelitis or septic arthritis. Act on the clinical picture.
• Ewing sarcoma mimics osteomyelitis: both cause permeative destruction, lamellated periosteal reaction, fever and raised inflammatory markers. Persistent concern needs MRI and specialist review.
• Stress fractures produce periosteal new bone and sclerosis without infection.
• Gout and CPPD cause hot joints and erosions; aspiration separates them.
• Post-operative changes and loosening around metalwork can mimic or mask infection; compare with previous films.
• Soft-tissue gas in a rapidly spreading infection suggests necrotising soft-tissue infection, which is a surgical emergency, but its absence on X-ray does not exclude it.

Red flags: act now

What to focus on, by role

Role	Priorities
Medical student	The 10–14 day lag, sequestrum and involucrum, and why aspiration comes before imaging in septic arthritis.
Nurse / practitioner	Recognise and escalate the hot joint and the septic child; regular foot checks in diabetes.
Junior doctor	Do not be reassured by a normal film; aspirate, culture and refer, and request MRI for suspected osteomyelitis.
Radiographer	Good soft-tissue detail, and views that show the whole affected bone and the nearby joint.
Advanced / FRCR	Osteomyelitis versus Ewing sarcoma, Charcot versus infection, and post-operative infection patterns.

Practise on bone and joint X-rays →

References

1. Radiology Masterclass. Musculoskeletal X-ray tutorials. radiologymasterclass.co.uk
2. Radiopaedia. Osteomyelitis. radiopaedia.org
3. National Institute for Health and Care Excellence. Diabetic foot problems: prevention and management (NG19). nice.org.uk
4. British Orthopaedic Association. BOAST guidance. boa.ac.uk
5. Royal College of Radiologists. iRefer: making the best use of clinical radiology. irefer.org.uk

For education only. Not a substitute for a formal report, senior review or your local policy. Never use this page to make decisions about a real patient.