knee without improvement…

the case.

a 64 year old type-II diabetic presents to ED at 3am with ongoing severe knee pain & fevers.

She has been on high-dose non-steroidals for the past 8-9 days for a presumed diagnosis of ‘gout’ by her general practitioner with minimal improvement in her symptoms and was seen in your department 48 hours earlier for ongoing pain. During this visit she underwent left knee arthrocentesis which failed to show crystals. Gram stain & subsequent cultures were negative.

On examination today, she is febrile to 39*C and tachycardic at 120 per minute. Her left knee appears unremarkable on inspection [no erythema, effusion, oedema etc], with the exception of a moderately sized ‘dimple’ on the medial aspect of her distal thigh. She explains that this is a result of an injury sustained in childhood.

As you are further examine her knee’s range of motion, you grab her distal femur which feels unexpectedly ‘swollen’. She winces in pain as this happens….

[DDET You decide to review her knee xray taken 48 hours ago…]

Knee XR 02 Knee XR 01

Abnormality of the distal femur with bony deformity, periosteal reaction & sclerosis.

Given this unexpected finding you arrange dedicated femoral views…


[DDET Here are todays formal femur x-rays…]

Femur XR 01 Femur XR 02

Obvious deformity of the distal third of the femoral shaft with sclerosis, periosteal reaction &
a suggestion of central cyst formation or necrosis.

Differential diagnoses include;

  • Primary bony malignancy
  • Old fracture
  • Osteomyelitis
  • Metabolic: Paget’s.


[DDET What is your next move…?]

  • Her full blood count reveals a white-cell count of 17,000.
  • CRP has climbed to 440.
  • With the ongoing fever, she is commenced on empiric flucloxacillin !
  • She is admitted under Orthopaedics with arrangements for advanced imaging….

[DDET Here is her CT….]

Femoral Osteomyelitis CT


[DDET An MRI on day 2 of admission…]

Femoral Osteomyelitis MRI

Femoral Osteomyelitis MRI#2



[DDET The diagnosis…]


An infection of bone by resulting in bony changes & destruction.

  • ~80% contiguous spread from adjacent infection or direct inoculum post-trauma/injury.
  • ~20% haematogenous spread
    • Children – typically long bones.
    • Adults – typically the spine.


  • Bacteraemia
  • Elderly
  • Post-operative [± retained orthopaedic hardware]
  • Previous trauma
  • Diabetes or vascular insufficiency
  • IV drug use
  • Sickle cell disease
  • Migrant from developing country
  • Bites [both human + animal]


  • S. aureus [most common, 80-90% of all cases]
  • E. coli.
  • Pseudomonas spp.
  • Klebsiella.
  • Salmonella – assoc. with Sickle cell disease.
  • M. tuberculosis – migrants, immunosuppressed.



  • Blood cultures may identify the culprit organism
  • Bone biopsy: required if blood cultures negative.
  • In chronic infection; sinus cultures may be misleading.
  • Consider alternate diagnosis if cultures are persistently negative, however continue anti-staphylococcal therapy during further investigation.


  • ESR > 70mm/hr in a diabetic = Sn 83-92% for osteomyelitis [from one reference].



  • Superior to both MRI + plain x-ray is depicting bony margins & identifying sequestrum/involucrum.


  • Currently the imaging modality of choice.
  • Most sensitive (95%) + specific.
  • Able to identify soft-tissue/joint complications.


  • Ultrasound – allows easy drainage of soft-tissue collections, but little direct benefit in cases of osteomyelitis.
  • Nuclear medicine – including Gallium & Technetium studies.
  • PET scan.


  • Charcot joint
  • Primary bone neoplasm.
    • Osteosarcoma
    • Ewing sarcoma
    • Lymphoma
    • Multiple myeloma
  • Bony metastases


Empiric therapy.

  • Flucloxacillin 2 grams [or 50mg/kg in children] IV q6h.
    • Cephazolin [same dosage] for penicillin hypersensitivity.
    • Vancomycin for immediate penicillin-hypersensitivity/anaphylaxis [see dose below]

Specific therapy/populations.

  • MRSA.
    • Vancomycin 1.5 gram IV BD [30mg/kg in children]. Adjust for renal function.
  • Gram-negatives, including patients with indwelling lines.
    • Cefotaxime 2g IV q8h [or 50mg/kg in children] or
    • Ceftriaxone 2g IV daily [or 50mg/kg in children] or
    • Tazocin [Piperacillin/Tazobactam] 4.5g IV q8h. Adjust for renal function.
  • Diabetics.
    • Tazocin [Piperacillin/Tazobactam] 4.5g IV q8h. Adjust for renal function.
  • Sickle-cell disease.
    • Ciprofloxacin 400mg IV ± Vancomycin
  • Bites [human or animal]
    • Tazocin [Piperacillin/Tazobactam] 4.5g IV q8h. Adjust for renal function.

Duration of therapy.

  • Acute osteomyelitis.
    • Neonates: 4 weeks (IV) total.
    • Children: 3 days (minimum IV), 4 weeks (total, minimum).
    • Adults: 4 weeks (IV), 6 weeks (total).
  • Chronic osteomyelitis.
    • Children + adults: months !!
    • Adults require at least two weeks IV.


  • Analgesia
  • DVT prophylaxis
  • Detection of complications.
    • Pathological fracture
    • Secondary amyloidosis or sarcoma
    • Sinus formation


[DDET Case follow-up…]

  • The above images were diagnostic of chronic osteomyelitis, thought to be secondary to her childhood injury.
  • Blood cultures positive.
    • Methicillin-sensitive staph. aureus.
  • After a 7 days as an in-patient, she is discharged home with ongoing IV antibiotics continuing with community nursing.


[DDET References]

  1. Rosenʼs Emergency Medicine. Concepts and Clinical Approach. 7th Edition
  2. Tintinalli’s Emergency Medicine: A Comprehensive Study Guide. 7th Edition.
  3. Osteomyelitis – Therapeutic Guidelines (eTG) via CIAP.
  4. Bone & Joint Infections – Paul Young’s Intensive Care Mind Maps 
  5. Osteomyelitis – Radiopaedia.org 


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