What are the differences between patellar dislocation and knee dislocation regarding mechanism, typical age/sex distribution, clinical findings, and acute management?

Patellar Dislocation vs Knee Dislocation: Key Differences

Patellar dislocation and knee dislocation are fundamentally different injuries with distinct mechanisms, demographics, clinical presentations, and management strategies—confusing them can lead to catastrophic delays in limb-threatening vascular injury recognition.

---

Mechanism of Injury

Patellar Dislocation

• Non-contact twisting injury during sports or physical activity, with the knee in flexion and valgus, causing lateral displacement of the patella out of the trochlear groove 1
• Occurs in 93% of cases through indirect force with valgus and external rotation of the tibia relative to the femur 2, 1
• Requires pre-existing anatomical predisposing factors (trochlear dysplasia, patella alta, increased TT-TG distance) because dislocation is unlikely in a knee with normal patellofemoral anatomy 2, 1

Knee Dislocation

• High-energy trauma such as motor vehicle collisions, falls from height, or severe sports injuries causing complete disruption of multiple ligamentous structures 3
• Results in displacement of the tibia relative to the femur with disruption of at least 3 of the 4 major knee ligaments (ACL, PCL, MCL, LCL) 3
• Frequently spontaneously reduces before presentation, making diagnosis challenging 3

---

Age and Sex Distribution

Patellar Dislocation

• Adolescents and young adults, with two-thirds occurring in patients less than 20 years old 1
• More common in young active females participating in sports 2, 1
• Younger age at first dislocation correlates with increased severity of predisposing anatomical factors 2

Knee Dislocation

• No specific age predilection, though mechanism varies by age (high-energy trauma in younger patients, lower-energy falls in elderly) 3
• Occurs across all age groups depending on trauma exposure 3

---

Clinical Findings

Patellar Dislocation

• Acute knee pain with hemarthrosis (joint effusion) after twisting injury 4, 1
• Spontaneous reduction occurs frequently, making diagnosis clinically unsuspected in 50% of cases 4
• Medial patellar tenderness at the site of medial patellofemoral ligament (MPFL) rupture, which occurs in 94-100% of first-time dislocations 5
• Positive apprehension test with lateral patellar glide exceeding 50% of patellar width 1
• No vascular compromise—pulses remain intact 4, 1
• May have visible lateral displacement if not yet reduced 1

Knee Dislocation

• Gross deformity if not spontaneously reduced, or history of obvious deformity that self-corrected 3
• Vascular injury in approximately 30% of posterior dislocations—absent pulses, pallor, cool extremity, delayed capillary refill 3, 6
• Neurological deficits from peroneal nerve (foot drop, dorsal foot numbness) or tibial nerve injury (inability to plantar-flex, sole numbness) 3, 6
• Massive hemarthrosis with multiligamentous laxity on examination 3
• Limb-threatening emergency requiring immediate vascular assessment 3, 6

---

Acute Management

Patellar Dislocation

Initial Assessment:

• Obtain anteroposterior and lateral radiographs (lateral with knee at 25-30° flexion) to identify osteochondral fractures and assess alignment 7, 1
• Add patellofemoral (sunrise) view to evaluate patellar position and fractures 7

Imaging Protocol:

• MRI without contrast is indicated after radiographs to assess MPFL injury, osteochondral fractures, kissing contusions (medial patella and lateral femoral condyle), and predisposing anatomical factors 8, 4, 1
• MRI reveals characteristic findings even when clinical diagnosis is unsuspected 4

Treatment Decision Algorithm:

1. Conservative management (immobilization, rehabilitation) for first-time dislocation without osteochondral fracture, free bodies, or persistent lateral subluxation 1, 9
2. Surgical intervention indicated for:
   • Osteochondral fracture or free bodies on MRI 1, 9
   • Substantial disruption of medial stabilizers with persistent lateral subluxation 9
   • Recurrent dislocation after failed conservative treatment 2, 5
   • Multiple severe predisposing anatomical factors in young patients 2

Surgical Options:

• MPFL reconstruction is the primary procedure for recurrent instability, as MPFL ruptures in almost all dislocations 2, 5
• Tibial tuberosity osteotomy or trochleoplasty as adjunct procedures for severe anatomical abnormalities 2

---

Knee Dislocation

Immediate Life-Threatening Assessment:

• Check vascular status immediately—palpate pulses, assess capillary refill, skin temperature, and color 3, 6
• Document neurological function before and after any manipulation (peroneal and tibial nerve testing) 3, 6

Reduction and Immobilization:

• Perform closed reduction emergently if dislocation is still present 6
• Immobilize in 15-20° of flexion using a removable splint (not rigid cast) to permit ongoing neurovascular monitoring 6
• Never immobilize in full extension—increases neurovascular tension 6

Imaging Protocol:

1. Anteroposterior and lateral radiographs after reduction to confirm alignment and identify fractures 3, 6
2. CTA of lower extremity immediately if ankle-brachial index <0.9 or any signs of vascular compromise (absent pulses, pallor, cool extremity) 3, 6
   • CTA is less invasive than conventional angiography with comparable accuracy for popliteal artery injury detection 3, 6
3. MRI without contrast after initial radiographs to evaluate multiligamentous injury, meniscal tears, and occult fractures 3
4. CT without contrast for better characterization of complex fractures (tibial plateau) identified on radiographs 3

Disposition:

• Urgent surgical intervention if vascular injury confirmed—arterial repair takes priority for limb salvage 3
• Urgent orthopedic follow-up within 24-48 hours for definitive ligamentous assessment even if vascular status normal 6
• Time to revascularization is critical—delays worsen limb salvage outcomes 3

---

Critical Pitfalls to Avoid

For Patellar Dislocation

• Do not assume spontaneous reduction means minor injury—MPFL rupture occurs in nearly all cases and MRI findings are often significant despite clinical underestimation 4, 5
• Do not order MRI as initial imaging—radiographs must precede MRI to exclude fracture 8
• Do not use IV contrast MRI or MR arthrography for acute patellar dislocation—non-contrast MRI is sufficient 8

For Knee Dislocation

• Do not delay vascular imaging if any concern for arterial injury exists—popliteal artery disruption occurs in 7.5-30% of cases and time to revascularization determines limb viability 3
• Do not miss spontaneously reduced knee dislocation—history of gross deformity that self-corrected requires full dislocation workup including vascular assessment 3
• Do not use rigid casts immediately after reduction—they prevent serial neurovascular examinations 6
• Do not assume normal pulses exclude vascular injury—obtain CTA if mechanism suggests dislocation even with palpable pulses 3