spontaneous osteonecrosis of the knee

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Definition

A condition of unknown aetiology resulting in an area of osteonecrosis of the weight bearing surface of the medial femoral condyle (MFC).

 

Epidemiology

  • Females more common than males.
  • Patient > 65 years.
  • Medial > Lateral 20:1.

 

Aetiology

  • Unknown
  • ? MFC because of end-artery supply (terminal branch of middle geniculate)

 

Presentation

  • Sudden onset of sever knee pain
    • Pain on weight bearing
    • Mild rest pain
    • Significant night pain

 

Examination

  • Small effusion
  • Good (near normal) ROM
  • Localised tenderness
  • Pain on loading medial compartment

 

Radiology

Xray

  • May be normal à do bone scan
  • May show typical area of AVN

 

Bone scan

  • Focally hot

 

MRI

  • Focal region of Low T1
  • T2 – central area of low T2 surrounded by ­ signal (oedema)

 

Staging

  1. Stage 1 – Incipient
    • Pain + Normal Xray
    • Positive bone scan or MRI
  1. Stage 2 – Avascular
    • Xray – radiolucent oval on MFC with middle articular flattening
  1. Stage 3 – Collapse
    • Subchondral collapse with a calcified subchondral plate and sclerotic halo
  1. Stage 4 – Degenerative
    • Shallow, concave articular surface with secondary OA changes

 

Natural History

  1. Stage 1 and 2 à may recover spontaneously
    • There is no evidence that any intervention changes the natural history
  1. Stage 3 and 4 à Progression
    • Inevitable progression to OA

 

Size

  • Size of the lesion is also prognostic
    • Good prognosis if lesion is:

i.     < 3.5cm2

ii.     < 0.4 the diameter of the MFC

 

Differential diagnosis

1.     OCD

2.     Osteonecrosis like syndrome

3.     Medial compartment OA

4.     Degenerate medial meniscus

5.     Secondary osteonecrosis of the MFC

    • Responds well to drilling and decompression

6.     Stress fracture

7.     Other

 

Treatment

  1. Stage 1 and 2 – Conservative
    • NWB – does aid prognosis
    • Symptomatic
    • Endocrinologist - bisphosphonates
  1. Stage 3 and 4 – Arthroplasty

Osteonecrosis like syndrome of the knee

 

  • Intra-osseous lesion situated on the articular margin of the femoral or tibial condyle
  • Commonest site is medial TIBIAL condyle
    • Sudden onset of pain
    • Tenderness at margin of tibial condyle
    • Xray shows no features of OA or osteonecrosis

Treatment

  1. Supportive – pain settles over 6-9 months
    • Analgesics
    • Walking aids
    • Reassurance