Transient Osteoporosis of the Hip

Transient osteoporosis of the hip (also called bone marrow oedema syndrome of the hip) is a self-limiting condition of sudden hip pain with bone marrow oedema of the femoral head (sometimes the neck), without trauma or other identified cause, and with spontaneous resolution over 3 to 12 months. It is considered by some to be a reversible form of the early stage of osteonecrosis.

It occurs most often in men aged 30 to 55 years (the left hip is more often affected) and in women in the third trimester of pregnancy (the left hip is more often affected). It can migrate to other joints (regional migratory osteoporosis), including the opposite hip, knee and ankle, and recur.

The cause is unknown. Proposed factors are microtrauma, vascular compromise, transient ischaemia, reflex sympathetic dystrophy, hormonal changes, and low bone density. Complications are femoral neck stress fracture or subchondral fracture (up to 5 to 10%, particularly in pregnancy), progression to osteonecrosis, prolonged disability, and recurrence at the same or another site.

Pathophysiology.

Raised marrow pressure, oedema and increased bone turnover produce local osteopenia and pain. Radiographs show osteopenia only 4 to 8 weeks after onset, while MRI shows marrow oedema within 48 hours. The oedema resolves without residual necrosis in most cases.

Sudden onset of groin, anterior thigh or buttock pain that increases with weight bearing and is relieved by rest, with an antalgic limp, over weeks. Pain at night may be present. No history of significant trauma, fever, or infection. Range of motion is restricted and painful at the extremes (internal rotation and flexion), with a normal examination otherwise.

Ask about pregnancy and lactation, alcohol, corticosteroids, metabolic bone disease, smoking, previous fractures and similar pain in other joints.

• Radiographs: normal initially; later, patchy osteopenia of the femoral head and neck with preserved joint space and no sclerosis, no collapse and no crescent sign.
• MRI is the key investigation: diffuse bone marrow oedema (low signal on T1, high signal on fluid sensitive sequences) in the femoral head extending into the neck and intertrochanteric region, with a joint effusion. There is no double line sign or focal necrotic area. Look for a subchondral fracture line.
• Bone scan: increased uptake in the femoral head. DXA shows reduced bone density.
• Blood tests: full blood count, C-reactive protein, erythrocyte sedimentation rate, calcium, phosphate, alkaline phosphatase, 25-hydroxyvitamin D, parathyroid hormone and thyroid function (all usually normal or showing metabolic bone risk factors).
• Pregnancy: MRI without gadolinium.

Differential diagnosis.

• Osteonecrosis of the femoral head and femoral neck stress fracture.
• Septic arthritis and osteomyelitis.
• Subchondral insufficiency fracture and labral tear.
• Tumour, osteoid osteoma and inflammatory arthritis.

The condition is self-limiting; treatment relieves pain and protects the femoral neck.

Conservative treatment.

• Protected weight bearing with crutches for 4 to 8 weeks (or until pain settles), then gradual return. Avoid prolonged bed rest, which increases bone loss.
• Analgesia (paracetamol with a short NSAID course), and physiotherapy to maintain range and strength.
• Correct vitamin D deficiency and calcium intake: colecalciferol 800 to 2000 IU orally daily with dietary calcium to a total of 1000 to 1200 mg daily (maximum total 2000 mg daily); correct vitamin D deficiency first.
• Treat contributing factors (alcohol, smoking, corticosteroids).

Drug treatment for severe or prolonged pain.

• Bisphosphonate (off label for this condition; not in pregnancy or lactation): alendronate 70 mg orally once weekly for 3 to 6 months, taken fasting with a full glass of water and kept upright for 30 minutes (check calcium, 25-hydroxyvitamin D and creatinine; avoid with eGFR under 35 mL/min/1.73 m2, uncorrected hypocalcaemia or oesophageal stricture), or zoledronic acid 5 mg intravenously over at least 15 minutes as a single dose when oral therapy is not tolerated and eGFR is 35 mL/min/1.73 m2 or over (check creatinine and calcium before the dose; give vitamin D first; warn about acute phase reaction).
• Iloprost 20 micrograms intravenously daily for 5 days over 6 hours for each infusion (off label; available in some centres; monitor blood pressure and heart rate; avoid with unstable angina, severe heart failure or bleeding risk). Teriparatide is used in selected patients (off label).

Pregnancy. Protected weight bearing, calcium and vitamin D, and delivery planning (caesarean section may be needed if a subchondral fracture is present). Bisphosphonates are avoided until after pregnancy and lactation.

Surgery. Core decompression for persistent pain (over 6 to 12 months) or progression to osteonecrosis. Fixation of a femoral neck stress fracture (cannulated screws), or protected weight bearing for undisplaced lesions.

Analgesia.

• Paracetamol 1 g orally every 6 hours as needed for up to 5 to 7 days (maximum 4 g daily; 3 g daily when weight is under 50 kg, with chronic alcohol use or hepatic impairment).
• Ibuprofen 400 mg orally every 8 hours with food for up to 5 to 7 days (maximum 1200 mg daily); avoid with eGFR under 30 mL/min/1.73 m2, peptic ulcer, heart failure or anticoagulant use; avoid in the third trimester of pregnancy.
• Oral morphine 5 to 10 mg every 4 hours as needed for severe pain for up to 3 days (start 2.5 to 5 mg with age over 75 or eGFR under 30 mL/min/1.73 m2; avoid exceeding 60 mg in 24 hours without senior review); monitor sedation and respiration and prescribe a laxative.

Monitoring. Pain and weight bearing at 2 weeks, 6 weeks and 3 months. MRI at 3 months (or earlier for increasing pain) to confirm resolution, and to detect fracture or necrosis. Radiographs at 6 weeks and 3 months. Contralateral hip and other joints if pain develops.

Referral and escalation. Refer suspected transient osteoporosis to orthopaedics for MRI. Urgent referral for an inability to bear weight, a suspected femoral neck fracture on MRI or radiographs, fever, or deterioration. Obstetric input in pregnancy.

Special groups.

• Pregnancy: mostly the third trimester, usually the left hip, with a risk of femoral neck fracture; protected weight bearing and delivery planning.
• Middle aged men: look for metabolic causes and alcohol.
• Children: rare; consider Perthes disease and transient synovitis.
• Kidney impairment: avoid ibuprofen and bisphosphonates at eGFR under 35 mL/min/1.73 m2, and reduce morphine doses.