Osteonecrosis of the Knee

Osteonecrosis of the knee is death of bone and marrow in the femoral condyles or tibial plateau from an interrupted blood supply. Three forms exist: spontaneous osteonecrosis of the knee (SONK), now often regarded as a subchondral insufficiency fracture, secondary osteonecrosis (from corticosteroids, alcohol, sickle cell disease, systemic lupus erythematosus, renal transplantation, chemotherapy, radiation and decompression sickness), and post-arthroscopic osteonecrosis (after meniscectomy or laser/radiofrequency treatment).

SONK typically affects women over age 55 (the female to male ratio is about 3 to 1), with sudden onset of medial knee pain without major trauma. It most often involves the medial femoral condyle, less often the medial tibial plateau or the lateral condyle. Secondary osteonecrosis affects younger patients, is often bilateral, multifocal, and involves the femoral condyles and the tibial plateau.

The Ahlbäck classification grades radiographs: grade I normal or minimal flattening, grade II flattening of the weight bearing surface, grade III depression with a halo, grade IV depression with erosion of the tibial plateau, and grade V arthritis. Complications are subchondral collapse, secondary arthritis, chronic pain and need for arthroplasty.

Pathophysiology.

Microfracture of weakened subchondral bone (osteoporosis, meniscal root tear with extrusion or abnormal loads) leads to marrow oedema, ischaemia and local necrosis. In secondary forms, fat emboli, vascular occlusion, increased intraosseous pressure and cellular toxicity cause necrosis. Collapse of the subchondral plate follows within months of onset, and the prognosis depends on the lesion size (over 40% of the condyle width, or area over 5 cm2, predicts collapse).

Sudden severe medial knee pain, often at night, made worse by weight bearing, with a limp, local tenderness over the medial femoral condyle or tibial plateau, a small effusion and reduced flexion. Pain may be present at rest and wakes the patient. Examine the joint line, effusion, range, alignment (varus), menisci and ligaments.

In secondary disease, ask about corticosteroids, alcohol, sickle cell disease, lupus, transplantation, chemotherapy and bilateral symptoms.

• Radiographs: normal in the first weeks; later, flattening and a subchondral radiolucent crescent in the condyle, followed by sclerosis and collapse. Use weight bearing anteroposterior, lateral and skyline views.
• MRI is the most sensitive and specific: bone marrow oedema, a low signal line (subchondral crescent) and a geographic area of necrosis. A double line sign suggests established necrosis. Assess the menisci (root tears) and cartilage.
• Bone scan shows increased uptake early; CT shows subchondral collapse.
• Blood tests: full blood count, C-reactive protein, erythrocyte sedimentation rate, calcium, 25-hydroxyvitamin D, lipids, haemoglobin electrophoresis, and autoantibodies as indicated.
• Bone density (DXA) in SONK.

Differential diagnosis.

• Medial meniscal root tear and subchondral insufficiency fracture.
• Transient bone marrow oedema and osteoarthritis.
• Stress fracture and pes anserine bursitis.
• Septic arthritis, tumour and complex regional pain syndrome.

Early stage (Ahlbäck I and II; small lesions).

• Protected weight bearing with crutches for 4 to 8 weeks, then progressive loading.
• Analgesia (paracetamol, short NSAID course), and a knee brace or unloader brace for varus.
• Bisphosphonate: alendronate 70 mg orally once weekly (off label for this use; the evidence is limited) for 6 to 12 months, taken fasting with a full glass of water and kept upright for 30 minutes.
• Alendronate safety: check calcium, 25-hydroxyvitamin D and creatinine; avoid with eGFR under 35 mL/min/1.73 m2, uncorrected hypocalcaemia or oesophageal stricture.
• Iloprost infusion is used in some centres for bone marrow oedema.
• Treat osteoporosis and vitamin D deficiency. Stop alcohol, and review corticosteroids.

Intermediate stage.

• Core decompression, with or without bone graft or bone marrow cell injection, for pre-collapse lesions in younger patients.
• Subchondral fixation or injection (calcium phosphate) for insufficiency fractures; arthroscopic treatment of meniscal root tears.
• High tibial osteotomy (opening or closing wedge) to offload the medial compartment in patients under age 60 with varus and preserved lateral compartment.
• Osteochondral allograft transplantation for focal lesions in selected young patients.

Advanced stage (collapse, arthritis).

• Unicompartmental knee arthroplasty for isolated medial lesions (good results in SONK).
• Total knee arthroplasty for large or multicompartment lesions or secondary osteonecrosis (outcomes are less predictable in steroid induced cases).

Operative antibiotic prophylaxis. Cefazolin 2 g intravenously (3 g when weight is 120 kg or over) within 60 minutes before incision as a single dose (repeat after 4 hours or blood loss over 1500 mL).

With severe penicillin allergy, use vancomycin 15 mg/kg intravenously starting 90 to 120 minutes before incision as a single dose, with levels and creatinine checked in prolonged cases.

Thromboprophylaxis. Enoxaparin 40 mg subcutaneously once daily, started 6 to 12 hours after surgery and continued for 10 to 14 days after arthroplasty or osteotomy (up to 28 to 35 days after total knee arthroplasty in high risk patients).

Dose adjustments: 20 mg once daily at creatinine clearance under 30 mL/min; adjust by anti-Xa level in obesity and renal impairment. Check the platelet count on days 5 to 14 for heparin induced thrombocytopenia. Use mechanical prophylaxis until then.

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 days (maximum 1200 mg daily); avoid with eGFR under 30 mL/min/1.73 m2, peptic ulcer, heart failure or anticoagulant use.
• 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 function at 6 weeks, 3 months, 6 months and 12 months. Radiographs every 3 months for the first year to detect collapse, and MRI at 3 to 6 months. Bone density and vitamin D in SONK. Contralateral knee and other joints (hips, shoulders) in secondary disease.

Referral and escalation. Refer suspected osteonecrosis to orthopaedics within 2 weeks for MRI and a treatment plan. Urgent referral for a hot swollen joint, collapse with mechanical block, or severe unremitting pain.

Special groups.

• Corticosteroid treated and sickle cell patients: screen other large joints; lesions are often bilateral and multifocal.
• Younger adults: preserve the joint with decompression or osteotomy.
• Postmenopausal women: SONK; treat bone health.
• Kidney impairment: avoid NSAIDs and bisphosphonates at eGFR under 35 mL/min/1.73 m2, and reduce morphine doses.