Knee pain that is severe, persists at night or at rest, or is associated with bone pain at other sites

• Urgently investigate or refer children with knee pain that is severe, persists at night or at rest, or is associated with bone pain at other sites — a bone tumour should be suspected.

Knee pain associated with systemic symptoms

• Urgently investigate or refer children with knee pain associated with systemic symptoms [University of Michigan Health System, 2005]. Causes include [Davidson et al, 2008]:
• Septic arthritis — presents with severe pain, swelling, warmth, and marked reluctance to move the affected joint. It is usually associated with systemic symptoms.
• Bone tumours — are a rare cause of knee pain. They may be associated with weight loss or general malaise [Australian Acute Musculoskeletal Pain Guidelines Group, 2003b].
• Juvenile idiopathic arthritis — may affect the knee joint alone or with other joints presenting with acute pain, swelling, and erythema. Arthritis may occasionally occur with, or be preceded by, fever, rash, lymphadenopathy, or hepatosplenomegaly.

Knee pain associated with an abnormal examination of the hip

• Urgently investigate or refer children with knee pain referred from the hip. Conditions that may cause referred pain include [Davidson et al, 2008]:
• Slipped proximal femoral epiphysis. This condition typically presents during the adolescent growth spurt, most frequently in obese children. When there is sudden displacement of the epiphysis, the child presents with sudden onset of severe hip pain with the leg held in external rotation. Gradual displacement of the epiphysis may cause only mild discomfort of the hip or only referred knee pain.
• Transient synovitis. Characteristically, this presents as transient hip pain in a systemically well child younger than of 10 years of age. In up to 70% of children, it occurs with, or follows, upper respiratory tract infection.
• Perthes disease. This is caused by avascular necrosis of the femoral head epiphysis due to a disturbance of its blood supply. It is much more common in boys and most often presents between 4 and 9 years of age with a persistent irritable hip.

Knee pain associated with injury

• A serious traumatic knee injury is likely when knee pain starts suddenly and is associated with abnormal physical signs [University of Michigan Health System, 2005]. Consider referral for specialist assessment of the cause. Causes include [Gholve et al, 2007; Davidson et al, 2008]:
• Meniscal injuries.
• Collateral and cruciate ligament injuries.
• Stress fractures of the patella.
• Tibial tuberosity fracture.
• Prepatellar and infrapatellar bursitis.

Knee pain not associated with trauma or systemic symptoms

• Osteochondritis dissecans is caused by avascular necrosis of subchondral bone of an articular surface and commonly affects the knee joint [Houghton, 2007]. If suspected, refer for specialist assessment.
• It presents in adolescents, with an insidious onset of activity-related pain and swelling. Locking of the knee may occur owing to a loose body within the knee.
• Examination may reveal focal bony tenderness, an effusion, or evidence of a loose body.
• Sinding–Larsen–Johansson syndrome has the same cause as Osgood–Schlatter's disease, except that the lower pole of the patella is affected instead of the tibial tuberosity [Houghton, 2007].
• It presents with knee pain that is typically exacerbated by activity and relieved by rest.
• Pain over the lower pole of the patella is provoked by knee extension against resistance.
• Patellofemoral pain syndrome is thought to be caused by abnormal tracking of the patella within the trochlear groove [Houghton, 2007]. Abnormal tracking of the patella may be associated with abnormal alignment of the bony structures or by quadriceps muscle weakness or imbalance.
• It most commonly presents during the adolescent growth spurt with diffuse pain around the patellofemoral joint. Usually both knees are affected, with one side worse than the other.
• It is confirmed by pain or crepitus with the patella grind test. This involves the person lying supine while the examiner pushes the patella distally in the trochlear groove. The person then tightens their quadriceps against patella resistance.