SONK              vs                          Atraumatic AVN


>55                                              Often mid 30's

MFC                                              Multiple areas

99% unilateral                               80% bilateral

Knee only                                      60-90% other joint

Juxta-articular                                Epiphysis / diaphysis / metaphysis


Primary Spontaneous Osteonecrosis Knee (SONK)




Usually healthy woman age 60+ years

- sudden onset of severe knee pain with normal Xray




Almost always MFC (SONK is a MONK)

- there are case reports of LFC SONK


99% unilateral


Exquisite local tenderness

- may be effusion


SONK of tibial plateau less common 




Yamamoto et al JBJS Am 200

- histological study

- evidence of microtrauma / insufficiency fracture

- initial event

- postulated that osteonecrosis is then secondary event around lesion


Primary vascular osteonecrosis may be much more rare




Initially normal


Later develop

- subchondral lucent line / crescent Sign

- flattening of condyle

- patchy sclerosis

- can have rapid collapse into varus with development degenerative changes


Spontaneous Osteonecrosis of the Knee


Bone Scan


Normal x-ray & painful knee in 60 year old think AVN

- consider bone scan

- probably superceded by MRI



- focal increase in uptake on one side of joint 

- if tibia and femur more likely OA




May be normal in early stages



-  low signal areas in subchondral region 


SONK MRI Sagittal



- low signal

- surrounding high intensity signal secondary to oedema




Staging Insall


Stage 1 

- normal x-ray with positive bone scan / MRI


Stage 2 

- subtle flattening of weight bearing portion of condyle


Stage 3 

- typical lesion

- radiolucent area with sclerotic halo


Stage 4 

- halo thickened with subchondral collapse


SONK Xray Stage 4


Stage 5 

- degenerative change

- varus or valgus angulation



Arthroscopic findings


Localised area of flattened cartilage

- discoloured

- eventually demarcates

- develop flap of cartilage over necrotic bone


The articular sequestrum becomes partially separated as hinged flap

- may separate completely

- cartilage defect becomes filled with necrotic debris and fibrocartilage

- develop OA




Non Operative Management




Many will resolve spontaneously

- especially small lesions

- best prognosis if chondral surface intact


Yates et al Knee 2007

- followed up 20 patients diagnosed on MRI

- average resolution of symptoms and lesion over 6 months




Decrease impact exercises

Consider unloading brace

Analgesia / NSAID's

Consider bisphosphonates

Follow for 6 - 12 months with repeated MRI looking for resolution / progression


Operative Management


Intact chondral surface / Stage 1 lesion


Decompression / Percutaneous Drilling



- failure non operative treatment > 6/12


Forst et al Arch Orthop Trauma Surg 1998

- 16 patients with average age 60

- percutaneous drilling with 3 mm drill

- instant resolution of pain

- cannot prevent progression of disease if chondral flattening present


Chondral Defect




Akgun et al Arthroscopy 2005

- debridement of chondral defect and microfracture

- 26 patients average age 48

- 71% could participate in strenous exercise with minimal exertion

- in the remainder the ON progressed on MRI





- unload MFC

- younger high demand patient

- combine with microfracture / osteochondral grafting


Osteochondral grafting


Tanaka et al Knee 2009

- 6 patients average age 50

- stage III and IV

- good results in knee scores at 2 years




Good option as disease is unicompartmental


Langdown et al Acta Orthop 2005

- 29 knees treated with Oxford UKA

- good outcomes and no implant failures at average 5 years




Secondary osteonecrosis


Knee AVN 1Knee AVN 2



- Steroid Therapy (90%)

- Alcohol


- Sickle Cell Disease

- Diver's / Caisson's

- marrow proliferative disorder

- chemotherapy




Gradual onset of pain

- lateral condyle in 60%

- younger patients, mid 30's




Bilateral in 50%

- 70% have other joints involved




More extensive involvement through knee


Operative Options



- failure non operative treatment

- continued pain


Percutaneous Drilling / Decompression


Marulanda et al JBJS Br 2006

- percutaneous drilling in 61 knees with secondary ON

- successfull in all 24 knees with small lesions

- successful in 32/37 (86%) knees with large lesions