Dysbaric Osteonecrosis

Dysbaric osteonecrosis is ischaemic bone death caused by nitrogen bubbles during decompression in divers and compressed air workers. It affects the humeral and femoral heads and the lower femur and upper tibia, and can destroy joints.

Exposure. Commercial, military and sport divers, caisson and tunnel workers. Risk rises with depth, long exposure, repeated dives, rapid ascent and past decompression sickness.

Epidemiology. Prevalence varies widely: from under 5% in recreational divers to 15 to 25% or more in compressed air workers and deep commercial divers.

Lesions. Juxta articular lesions of the head and neck risk subchondral collapse. Shaft lesions in the metaphysis and diaphysis are usually silent.

Latency. Radiographic change can appear months to years after exposure and may lag the first MRI change by months or years.

Pathophysiology.

Bubbles form in fatty marrow and compress sinusoids, and intravascular bubbles activate clotting and inflammation. Marrow pressure rises and blood supply falls, causing bone infarction.

In weight bearing subchondral bone, collapse of necrotic bone produces joint incongruity and secondary arthritis.

• Often asymptomatic, found on screening images.
• Joint pain at the shoulder, hip or knee, worse with use, with later restricted movement and limp.
• Collapse causes sudden worsening pain and mechanical symptoms.

Red flags.

• Rapid onset of severe joint pain or inability to bear weight (collapse).
• Fever, joint swelling and redness (septic arthritis).

• Dive and exposure history, past decompression sickness, steroid and alcohol use.
• Radiographs of shoulders, hips and knees: patchy sclerosis, lucencies, subchondral crescent sign, collapse.
• MRI is the most sensitive test: band like lesion with double line sign. Use it for symptoms with normal radiographs.
• Baseline radiographs and periodic screening for compressed air workers and saturation divers.
• Blood tests for risk factors (alcohol, sickle haemoglobin, lipids, lupus, thrombophilia) when atypical.

Differential diagnosis. Osteonecrosis from steroids, alcohol, sickle cell disease or trauma, bone infarct from other causes, osteoarthritis, osteochondritis dissecans, rotator cuff disease, stress fracture, transient osteoporosis.

• Asymptomatic shaft lesions: observation and exposure review; do not stop work for these alone.
• Lesions of the head or neck: orthopaedic referral; protected weight bearing with crutches; core decompression for pre-collapse hip lesions; joint replacement for collapse or advanced arthritis.
• Paracetamol 1 g orally every 6 hours as needed, maximum 4 g in 24 hours (3 g if under 50 kg, chronic alcohol excess or liver disease), for the shortest period that controls pain. Reduce dose in hepatic impairment; extend dosing interval when eGFR is under 30.
• Ibuprofen 400 mg orally three times daily with food for up to 7 days, maximum 2400 mg daily. Monitor blood pressure, renal function and gastrointestinal symptoms. Avoid if eGFR is under 30, in active peptic ulcer, heart failure or the third trimester; add gastroprotection in older adults.
• Hyperbaric oxygen and bisphosphonates have no proven benefit.
• Diving fitness is decided by a diving physician: divers with juxta articular lesions are generally not cleared for further deep or long exposure.

Monitoring. Radiograph or MRI at 6 to 12 months for known lesions, sooner with new pain.

Referral and escalation. Orthopaedic and diving medicine specialists at diagnosis; occupational physician for fitness and compensation.