Introduction
The acutely swollen knee is a frequent complaint in healthcare settings, from primary care clinics to emergency rooms. Knee joint, being a major weight-bearing structure, is particularly vulnerable to injuries and various pathologies. Unlike the hip and ankle, the knee’s stability relies less on bony congruity, making it more susceptible to injury [1]. A systematic approach, starting with a comprehensive patient history and physical examination, is crucial for accurate diagnosis and effective management. This initial assessment guides further investigations and determines the need for specialist referral.
Aims of this Review
This review is designed to provide healthcare professionals in primary care and emergency departments with a structured approach to evaluating and managing patients presenting with an acutely swollen knee. It integrates evidence from a PubMed literature search using keywords such as ‘swollen knee’, ‘knee swelling’, ‘knee effusion’, and ‘knee haemarthrosis’, combined with clinical expertise.
Anatomical Considerations in Knee Swelling
Understanding the knee’s anatomy is fundamental to diagnosing the cause of acute swelling (Figure 1). Thinking anatomically helps categorize potential pathologies and narrow down the differential diagnosis.
Figure 1. Anatomical Causes of Acute Knee Swelling
Anatomical Diagram of Knee Joint Highlighting Structures Involved in Swelling
Pain as a Diagnostic Clue
Pain is a significant symptom in acute knee swelling, and a detailed pain history is essential. Key aspects of pain assessment include:
- Pain Location: A significant majority of patients (around 69%) can pinpoint the exact location of their knee pain [2]. This localization is valuable for identifying the affected anatomical structures.
- Aggravating Factors: Understanding what triggers or worsens the pain is crucial. For instance, pain exacerbated by twisting or turning movements may suggest a meniscal tear or ligamentous instability, particularly cruciate ligament injuries.
Etiology of Knee Pain: Considering Different Activities
Pain related to specific activities can provide further diagnostic insights. Pain when ascending or descending stairs often points towards patellofemoral joint pathology. This is due to increased pressure on the patella when the knee is flexed under load [3]. Patellofemoral disorders are more prevalent in women [4].
Night pain is a red flag symptom. While degenerative arthritis is a common cause of night pain in adults [5], in children and young adults, it may indicate more serious conditions like infection or neoplasm. Persistent night pain in older patients with normal knee X-rays should also raise suspicion for underlying pathology.
Localizing Knee Pain for Differential Diagnosis
Referred pain from the spine or hip must always be considered. However, patient’s description of knee pain location is often diagnostically helpful [6]. Patients commonly describe pain as medial, lateral, posterior, anterior, or generalized “all around” the knee. Correlating pain location with broad categories of knee pathology, further refined by patient age, helps establish a diagnostic framework (Table 1). While generalizations, this approach assists in initial differential diagnosis.
Table 1. Diagnostic Framework for Knee Pain Etiology by Location and Age
| Age (years) | 9–15 | 15–30
