A Case of Spontaneous Knee Pain in a Middle Aged Patient

A 67-year-old female presented to her GP with a sudden, non-traumatic onset of right knee pain, associated with difficulty weight-bearing. She had previously been fit and well, with no history of knee symptoms or prior knee surgery. There were no symptoms affecting other systems, and she appeared systemically well at presentation.

On Examination:
The patient was unable to weight bear in the clinic. The extensor mechanism was intact, with good range of motion and a stable knee joint. Despite this, she exhibited marked tenderness diffusely around the knee, with no external signs of trauma.

Examination of the hips and spine was normal, with no evidence of referred pain or proximal pathology.

• Initial AP and lateral radiographs of the right knee were obtained.

• An MRI scan was subsequently performed, including coronal and sagittal T2-weighted sequences.

• Pain management with appropriate analgesia

• Off-loading of the limb, with non–weight bearing as tolerated

• Use of a range-of-motion (ROM) brace to support the knee while allowing controlled movement

• Early physiotherapy and guided rehabilitation exercises to maintain mobility and prevent stiffness

• Surgery is generally not indicated, as most cases respond well to conservative management

Spontaneous osteonecrosis of the knee (SONK) most commonly affects the subchondral bone of the medial femoral condyle, which bears the highest load during weight-bearing activities. Involvement of the lateral femoral condyle or tibial plateau is far less frequent. SONK is characterised by crescent-shaped subchondral lesions, typically presenting with sudden, severe knee pain, swelling, functional limitation, and a risk of progressive joint deterioration.
It predominantly affects individuals over the age of 55, suggesting a relationship with postmenopausal osteoporosis and underlying bone insufficiency.

Aetiology and Pathogenesis

The exact cause of SONK remains uncertain due to its multifactorial nature, but several theories have been proposed:

• Subchondral insufficiency fractures in osteoporotic bone leading to compromised vascularity

• Venous congestion resulting in impaired subchondral circulation

• Localized trauma or repetitive microtrauma, even in the absence of a clear injury
  Although the clinical presentation is often nonspecific, these mechanisms contribute to the development of subchondral collapse and secondary osteoarthritis if untreated.

Diagnosis

Diagnosis relies on a combination of clinical assessment and advanced imaging:

• MRI is the gold standard, detecting early marrow oedema and defining lesion size, location, and stage.

• Plain radiographs may be normal in the early phase, but in later stages can show:

  • Subchondral sclerosis

  • Crescent sign or collapse

  • Flattening of the femoral condyle

  • Progressive joint space narrowing
    Diagnostic work-up should also include investigations to exclude alternative causes of acute knee pain, such as inflammatory arthropathy or infection.

Management

Management of SONK is individualised, depending on the size of the lesion, disease stage, symptom severity, and the patient’s overall health.

Nonoperative Management (Early / Small Lesions)

• Analgesia and NSAIDs

• Bisphosphonates to reduce bone resorption

• Off-loading or non–weight bearing

• ROM bracing

• Physiotherapy and early mobilisation
  Small lesions often stabilise or regress with conservative treatment.

Operative Management (Advanced / Larger Lesions or Failed Conservative Care)

When symptoms persist or imaging shows progression, surgical options include:

• Arthroscopic debridement

• Core decompression

• Microfracture

• Osteochondral grafting (autograft or allograft)
  In post-collapse disease, total knee arthroplasty becomes an appropriate and definitive solution.

• The diagnosis of SONK requires a high index of suspicion.

• Careful, attentive history-taking is essential – the patient’s description of pain and function often provides the key clue.

• If symptoms do not correlate with the initial X-rays, always consider further imaging or investigations (e.g. MRI).

• A “normal” knee X-ray does not necessarily mean a normal knee – occult pathology such as SONK may still be present.