What is the differential diagnosis for a patient with acute onset of right knee pain, popping, and knee slipping, considering possible ligamentous injuries such as anterior cruciate ligament (ACL) sprain or tear, or meniscal tear?

Differential Diagnosis for Acute Right Knee Pain with Popping and Knee Slipping

The most likely diagnoses are ACL tear, meniscal tear, or patellar dislocation/subluxation, with ACL injury being the primary concern given the acute popping and instability symptoms. 1

Primary Diagnostic Considerations

ACL Tear (Most Likely)

• Acute popping sensation at the time of injury is highly characteristic of ACL rupture, often accompanied by immediate knee instability and the sensation of the knee "giving way" or slipping 1
• The Lachman test is the most sensitive clinical examination for ACL rupture, achieving 84% sensitivity and 96% specificity when performed 4-5 days post-injury with the knee flexed at 20-30 degrees 1
• ACL tears frequently occur with associated meniscal injuries—approximately 46% of acute ACL tears have concomitant meniscal damage 2

Meniscal Tear

• Can occur following acute trauma with twisting injury in patients younger than 40 years, or as a degenerative condition in those aged 40 years or older 3
• The McMurray test (concurrent knee rotation and extension) has 61% sensitivity and 84% specificity for meniscal tears 3
• Joint line tenderness is highly sensitive (75-83%) but not specific (27-83%) for meniscal injury 3, 4
• The bounce test, McMurray's test, and Apley's grind test aid in diagnosis 5

Patellar Dislocation/Subluxation

• Presents with acute onset of pain, popping sensation, and knee instability 6
• Focal patellar tenderness and joint effusion are key findings 6
• Additional patellofemoral radiographic view should be obtained to evaluate for patellar fractures and/or subluxation or dislocation 6

Secondary Considerations

Collateral Ligament Injury

• Valgus and varus stress testing provides assessment of medial and collateral ligaments 5
• Often occurs in combination with ACL injuries—54% of ACL tears with collateral ligament involvement have associated meniscal injuries 2

Patellar Tendon Rupture

• Can result from acute twisting injuries with rotational forces applied to the knee while weight-bearing 7
• Characterized by hemarthrosis and inability to extend the knee 7

Tibial Plateau Fracture

• Must be excluded in the setting of significant trauma 6
• Radiographs are the initial imaging modality when Ottawa knee rule criteria are positive 6

Diagnostic Algorithm

Initial Clinical Assessment

• Perform the Lachman test ideally 4-5 days post-injury to allow swelling to subside, as excessive swelling and pain can limit examination accuracy up to 48 hours after injury 1, 5
• Assess for joint line tenderness (meniscal injury), patellar tenderness and effusion (patellar pathology), and perform McMurray test if meniscal tear suspected 3, 4
• Always examine the uninjured knee first for comparison 5

Imaging Strategy

• Obtain knee radiographs (anteroposterior and lateral views minimum) if any Ottawa knee rule criteria are positive: age >55 years, tenderness at the head of the fibula or patella, inability to bear weight for 4 steps, or inability to flex the knee to 90 degrees 6, 4
• Add patellofemoral view if patellar dislocation/subluxation is suspected 6
• If radiographs are negative but clinical suspicion for ACL tear or meniscal injury remains high, proceed directly to MRI 1, 8
• MRI is the gold standard with 92-98.3% diagnostic accuracy for ACL tears and 96% sensitivity/97% specificity for meniscal tears 1, 8
• MRI identifies critical associated injuries including meniscal tears, bone contusions, and posterolateral corner injuries that affect surgical planning 1

Critical Pitfalls to Avoid

• Do not rely solely on initial examination within 48 hours of injury—reexamine at 3-5 days post-injury to distinguish partial tears from complete ligament ruptures, as acute swelling limits examination accuracy 5
• Do not assume a single isolated injury—nearly half of acute ACL tears have associated meniscal damage requiring thorough evaluation 2
• Do not order MRI as initial imaging—radiographs must be obtained first to exclude fracture when Ottawa knee rule criteria are positive 6, 1
• Knee effusion >10 mm on lateral radiograph is associated with higher incidence of internal derangement and warrants MRI even with negative radiographs 8