Which imaging modalities are recommended for evaluating knee pain in primary care, and when should I refer for further investigation?

For evaluating knee pain in primary care, imaging modalities are generally not routinely recommended for diagnosis or management of osteoarthritis (OA) unless specific criteria are met ⁴.

• Diagnosis of Osteoarthritis: OA can typically be diagnosed clinically without imaging in individuals aged 45 or over who have activity-related joint pain and morning stiffness lasting no longer than 30 minutes ⁴. Imaging should not be routinely used for diagnosis unless there are atypical features or signs suggesting an alternative or additional diagnosis ⁴.
• Management of Osteoarthritis: Imaging is not routinely used for follow-up or to guide non-surgical management of OA ⁴.
• Sprains and Strains: Magnetic Resonance Imaging (MRI) is considered the gold standard for imaging ligamentous and intra-articular structures of the knee ³. However, its routine use for acute injuries is limited by cost and availability ³. MRI should be reserved for cases of persistent symptoms in severe sprains or chronic instability to rule out osteochondral defects, missed syndesmotic injuries, or bony injuries not identified on X-ray ³.
• Specialised Assessment: Appropriately focused imaging for knee pain is in most cases better performed as part of further specialised assessment ¹. Direct access to MRI may be available depending on local arrangements and criteria ¹.

You should refer a patient for further investigation under the following circumstances:

• Urgent Referral (to be seen within 2 weeks):
  • If a tumour is suspected ¹,⁵.
  • If persistent synovitis of undetermined cause is suspected, especially if small joints of the hand or feet are affected, more than one joint is affected, or there has been a delay of 3 months or longer between symptom onset and seeking medical help ¹. Refer urgently to a rheumatologist to assess for inflammatory polyarthritis ¹.
  • If a child or young person presents with suspected juvenile idiopathic arthritis, refer urgently to a paediatric rheumatologist and discuss immediate management with the on-call rheumatologist ¹.
  • If osteonecrosis of the knee is suspected, refer urgently to an orthopaedic surgeon ¹.
• Immediate Assessment:
  • If patellar dislocation occurs in a person with recurrent dislocation and is associated with moderate or severe swelling ¹.
  • If Henoch-Schönlein purpura is suspected ¹.
  • If a child presents with a limp ¹.
• Referral for Further Assessment and/or Advice (to an integrated musculoskeletal service or orthopaedic surgeon):
  • Persisting knee pain or other symptoms for more than six weeks ¹.
  • A suspected degenerative meniscal tear ¹.
  • Symptomatic bipartite patella ¹.
  • Fat pad impingement/inflammation ¹.
  • Plicae syndrome ¹.
  • Osteochondritis dissecans ¹.
  • Recurrent patellar dislocation/subluxation ¹.
  • Patellofemoral pain syndrome, particularly if not improved after 6 weeks of conservative management ¹.
  • Iliotibial band syndrome ¹.
• Other Specific Referrals:
  • If complex regional pain syndrome is suspected, refer to an appropriate specialist for confirmation of diagnosis, to rule out ongoing pathology, and for symptom control and rehabilitation ¹.
  • If the diagnosis is uncertain, consider referring to an appropriate specialist (e.g., rheumatologist, orthopaedic surgeon, sports physician, or other musculoskeletal specialist) ¹.
  • For osteoarthritis, consider referral to an orthopaedic surgeon if non-surgical management is unsuitable or ineffective after 3 months, especially if symptoms substantially impact the person's quality of life, there is diagnostic uncertainty or atypical features, or a sudden worsening of symptoms ². The decision to refer for joint surgery should not be based on age, sex, smoking status, comorbidities, or BMI ²,⁴.