Knee: Spontaneous & Idiopathic Conditions

A round-up of knee conditions with no traumatic or inflammatory trigger — traction apophysitis, osteochondral necrosis, and spontaneous osteonecrosis. See also Foot: Spontaneous & Idiopathic Conditions (Köhler and Freiberg disease — the same family of condition, elsewhere in the foot) and Achilles Disorders (Sever disease).

Osgood-Schlatter disorder

A traction apophysitis from repetitive traction of the patellar tendon on the tibial tubercle in early adolescence. See also knee pain (approach).

• Demographics: age 10–14; boys 3× more than girls; bilateral in ~1/3; running/kicking/jumping sports.
• History: localised tibial-tubercle pain during/after activity ± a visible lump; aggravated by kneeling and descending stairs.
• Examination: pain on resisted knee extension; palpable tender lump over the tibial tuberosity.
• Investigations: X-ray shows apophyseal widening ± fragmentation — mainly to exclude tumour/fracture.
• Management:
  • conservative (usually) — resolves over 6–18 months (average 12)
  • ice, analgesia
  • rest from aggravating sport, then graded return
  • gentle quadriceps stretching
  • do NOT inject corticosteroid or immobilise in a cast
  • surgery considered only if a persistent ossicle remains after apophyseal fusion (~age 18).

Osteochondritis dissecans

A segment of articular cartilage and subchondral bone (classically of the femoral condyle) undergoes necrosis and can separate to form an intra-articular loose body.

Juvenile form:

• Demographics: boys aged 10–15.
• History: activity-related, poorly localised pain; recurrent effusions; locking if a loose body forms.
• Investigations: X-ray classically shows sclerosis of the lateral aspect of the medial femoral condyle; may progress to a loose body (MRI stages lesion stability).
• Management: specialist referral. Stable lesions before physeal fusion: restricted weight-bearing ± bracing — good healing potential. Unstable lesions/loose bodies: arthroscopy ± fixation of the fragment.

Adult form: results from subchondral cysts of OA fracturing into the joint. Poorer prognosis (physes closed, little healing potential); consider knee arthroplasty if age >60.

Osteonecrosis of the knee

Falls into two distinct categories — spontaneous (SPONK/SONK) and secondary. For hip osteonecrosis (subcapital fracture, SCFE/Perthes sequelae, corticosteroids, SLE, sickle cell), see other hip disorders.

Spontaneous osteonecrosis of the knee (SPONK):

Cause unknown; affects the epiphysis of the medial femoral condyle; most cases resolve with conservative treatment.

• Demographics: women more than men; age >55.
• History: sudden onset of severe knee pain with a normal X-ray; usually unilateral (~99%).
• Examination: effusion; tenderness over the medial femoral condyle; limited ROM secondary to pain.
• Investigations: X-ray usually normal initially. MRI is most useful and differentiates SPONK from secondary osteonecrosis — not Medicare- covered for patients >50 on this indication alone.
• Management: NSAIDs/analgesia, protected weight-bearing, physiotherapy for quad strengthening; arthroplasty if symptoms fail to resolve conservatively.