What is the differential diagnosis and how to test a 41-year-old female (F) with knee pain and limping after a traumatic injury to the back of the knee, who reports a sensation of her kneecap moving side to side?

Differential Diagnosis and Examination for Traumatic Knee Pain with Patellar Instability

This patient's description of the kneecap "moving side to side" after posterior knee trauma strongly suggests patellar subluxation or dislocation, which should be your primary diagnostic consideration. 1

Most Likely Diagnoses

Patellar Dislocation/Subluxation (Primary Concern)

• Transient lateral patellar dislocation causes acute swelling with the sensation of kneecap instability that this patient describes 1
• Patellar dislocations cause immediate swelling with focal patellar tenderness 1
• This injury creates characteristic bone contusion patterns visible on MRI if imaging is needed 1

Other Important Considerations After Posterior Knee Trauma

• Posterior cruciate ligament (PCL) rupture - direct blow to anterior tibia or posterior knee can cause PCL injury 2
• Posterolateral corner injury - associated with ACL ruptures in 19.7% of cases and causes significant morbidity if missed 1
• Meniscal tear - can occur with twisting or direct trauma, particularly in patients under 40 years 3
• Infrapatellar bursitis - direct trauma to back of knee could cause deep infrapatellar bursal inflammation 4
• Tibial plateau fracture - though less likely given the mechanism, should be excluded 2, 1

Essential Physical Examination Maneuvers

For Patellar Instability (Your Primary Focus)

• Patellar apprehension test: With knee at 20-30 degrees flexion, apply lateral pressure to the medial patella while observing for patient apprehension or guarding - positive test indicates patellar instability 5, 6
• Patellar glide test: Assess lateral and medial patellar mobility with knee extended - excessive lateral translation (>50% of patellar width) suggests instability 6
• Inspect for focal patellar tenderness, particularly medial retinaculum tenderness from stretching during dislocation 1
• Check for joint effusion - presence suggests intra-articular pathology 4

For PCL Injury (Given Posterior Trauma Mechanism)

• Posterior drawer test: With knee flexed to 90 degrees and foot stabilized, apply posterior force to proximal tibia - excessive posterior translation indicates PCL rupture 2, 5
• Tibial sag test: With knee flexed to 90 degrees, observe from lateral view for posterior tibial displacement relative to femur 5

For Meniscal Pathology

• McMurray test: Concurrent knee rotation (internal or external) with extension has 61% sensitivity and 84% specificity for meniscal tears 3
• Joint line tenderness: Palpation along medial and lateral joint lines has 83% sensitivity and 83% specificity for meniscal tears 3

For Ligamentous Stability

• Lachman test: At 20-30 degrees knee flexion, has 84% sensitivity and 96% specificity for ACL tears when performed 4-5 days post-injury 2
• Valgus and varus stress testing: Assess collateral ligament integrity 5

Critical Red Flags to Assess

• Ability to bear weight: Inability requires radiographs per Ottawa criteria to exclude fracture 4, 6
• Ability to flex to 90 degrees: Inability mandates radiographic evaluation 4
• Gross deformity or palpable mass: Requires immediate radiographs 1
• Neurovascular status: Check distal pulses and sensation - vascular injury occurs in 30% of posterior knee dislocations 1

Diagnostic Testing Approach

Initial Imaging

• Plain radiographs first if patient cannot bear weight, cannot flex to 90 degrees, or has bony tenderness 4, 6
• Cross-table lateral view can detect lipohemarthrosis (fat-fluid level) indicating intra-articular fracture 1
• Segond avulsion fractures on lateral tibial rim suggest ACL and anterolateral ligament injuries 1

Advanced Imaging When Indicated

• MRI is the definitive test for soft tissue injuries including meniscal tears (96% sensitivity, 97% specificity), ligamentous injuries, and bone contusions 2, 1
• MRI should be considered for joint effusion >10 mm on lateral radiograph in patients <40 years 1
• Ultrasound is preferred for suspected bursitis to confirm diagnosis and distinguish from cellulitis 4

Common Pitfalls to Avoid

• Missing posterolateral corner injuries that accompany ACL tears, leading to persistent instability 1
• Assuming negative radiographs exclude significant injury - occult fractures, bone contusions, and soft tissue injuries require MRI if clinical suspicion persists 1
• Overlooking patellar dislocation when patient describes kneecap instability - this is a clinical diagnosis that may reduce spontaneously before examination 1
• Delaying vascular assessment in posterior knee trauma - femoro-tibial knee dislocation has vascular injury in 30% of posterior dislocations 1