AVN Knee

  • Can be Idiopathic or Secondary

SONK

  • Spontaneous Osteonecrosis Knee
  • Aetiology
  • Microtrauma
  • Primary vascular ischaemia

Pathogenesis/ Pathology

  • Necrosis
  • Inflammation/ Revascularisation
  • Repair
  • Remodelling
  • ± secondary Osteoarthritis
  • Initially localized depression of condylar surface
  • Then articular sequestrum becomes partially separated as hinged flap
  • May separate completely
  • Result is crater with fibrocartilage base

Clinical Features

  • Usually healthy woman age 60+ years
  • Sudden onset of severe knee pain
  • Almost always medial femoral condyle
  • Exquisite local tenderness
  • May be effusion
  • Other side rarely involved
  • SONK of tibial plateau less common
  • Otherwise same as MFC
  • May resolve spontaneously » allow course of non-op Treatment

Radiology

X-ray

  • Initially normal
  • Later develop
  • Subchondral lucent line
  • Crescent Sign
  • Flattening of condyle
  • Patchy sclerosis
  • Can have rapid collapse into varus

Bone Scan

  • Investigation of choice
  • If have normal x-ray & painful knee in 60yr old think AVN & consider bone scan as may have degenerative meniscal tear at a/scope but don't get better
  • Focal ↑ in uptake
  • One side of joint
  • If both sides involved then more likely Osteoarthritis

MRI

  • Discrete well defined low signal areas in subchondral region on T1

Classification

  • Modified Ficat & Arlet for Knee
  • Stage I
  • Normal plain X-ray
  • Stage II
  • Cystic or sclerotic lesion but normal bony contours
  • Stage III
  • Cresent sign or subchondral collapse
  • Stage IV
  • Narrowing of joint space with secondary changes
  • Insall
  • Stage1
  • Normal x-ray
  • Bone scan +ve
  • Stage2
  • Subtle flattening of weight-bearing portion of condyle
  • Stage3
  • Typical lesion with radiolucent area surrounded by sclerotic halo proximal & laterally
  • Stage4
  • Subchondral collapse
  • Stage 5
  • Degenerative change & varus or valgus angulation
  • Extent determined on AP film
  • Width of lesion > 50% of condyle » poor prognosis & tend to deteriorate rapidly

Management

  • Options are
  • Initially watch to see if resolves
  • A/S debridement & drilling
  • Retrograde or Antegrade
  • Bone graft or Osteochondral graft
  • Valgizing HTO
  • TKR if > 65yo
  • See treatment algorithm below

Secondary AVN

  • Much less common
  • Pathogenesis
  • Same sequence of events as SONK
  • Caused by
  • Steroid Therapy
  • Alcohol
  • Diver's
  • Marrow Proliferative Disorder
  • Sickle Cell Disease
  • SLE

Clinical Features

  • Gradual onset of pain
  • Lateral condyle in 60%
  • Bilateral in 50%
  • Is the natural history less predictable than hip AVN?

Radiology

  • As for SONK

Management

  • No standard management as uncommon
  • Options as for SONK
  • SONK vs Atraumatic AVN
  • SONK Atraumatic AVN
  • > 55yrs Often mid 30's
  • MFC Multiple areas
  • 99% unilateral 80% bilateral
  • Knee only 60-90% other joint
  • Juxta-articular Epiphyseal/ diaphyseal/ metaphyseal
  • Treatment Results
  • AVN may spontaneously resolve » allow nonoperative treatment first
  • Arthroscope doesn't appear to alter natural Hx
  • Core Decompression
  • Some success in stages 1 & 2
  • Site of lesion not predictive of outcome
  • Size of juxta-articular lesion
  • Large lesion fared worse than medium or small both for tibia & femur
  • But figures not recorded
  • Role not as defined as AVN of hip
  • Allograft experimental
  • HTO
  • Usually in younger high demand patient
  • Koshino
  • 37pts with 5yr follow-up
  • Only 1pt with TKR
  • Best when initially varus & lesion pre-drilled or grafted
  • Arthroplasty
  • Unicompartmental reasonable in SONK as virtually always medial comp only
  • Marmor CORR 1993
  • 89% good or excellent in 34 knees
  • TKR
  • Results not as good as for Osteoarthritis
  • 85% success at 5yrs
  • Mont, Hungerford et al Sept 2000 TKR successful in 60%
  • 71% = 34 of 48 TKR successful at 9 years
  • CONCLUSION
  • INITALLY WATCH AS MAY RESOLVE
  • CORE DECOMPRESSION WORTHWHILE WITH GOOD OUTCOME
  • TKR POOR RESULTS NOT UNLIKE AVN OF HIP
  • ALGORITHM
  • Stages I-III
    • Non-Op 1st
  • Persistent symptoms + no progression
  • Core decompression
  • Persistent symptoms + X-ray progression » TKR
  • Initial presentation at Stage IV »TKR
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