What is the differential diagnosis for a patient presenting with a laterally shifted patella?

Differential Diagnosis for Laterally Shifted Patella

A laterally shifted patella most commonly results from patellofemoral instability due to extensor mechanism imbalance, component malrotation in post-arthroplasty patients, or acute traumatic dislocation with medial patellofemoral ligament (MPFL) disruption.

Post-Total Knee Arthroplasty Context

If the patient has undergone total knee arthroplasty (TKA), the differential diagnosis includes:

Patellofemoral Instability (Most Common)

• Internal malrotation of femoral and/or tibial components is the leading cause, occurring in 1-12% of TKA patients 1
• Excessive combined internal rotation of components is directly proportional to the severity of patellofemoral complications 1
• Component malposition affects patellar tracking and alignment 1

Extensor Mechanism Imbalance

• Excessive tightness of the lateral retinaculum causing lateral pull 1
• Valgus alignment of the extensor retinaculum 1
• Imbalance in soft tissue tension around the patella 1

Patellar Component Issues

• Component loosening or wear (uncommon but requires revision when present) 1
• Failure of patellar resurfacing component, potentially leading to metal-on-metal contact 1
• Patellar remodeling when patella has not been resurfaced 1

Patellar Fracture

• Occurs in up to 5.2% of TKA patients, usually within first few postoperative years 1
• Transverse fractures are associated with patellar maltracking 1
• Many are asymptomatic, emphasizing the importance of radiographic identification 1

Osteonecrosis and Impingement

• Patellar osteonecrosis as a complication of TKA 1
• Component impingement affecting patellar tracking 1

Native Knee (Non-Arthroplasty) Context

Acute Traumatic Dislocation

• Most common in young active patients under 20 years old 2, 3
• Non-contact knee sprain in flexion and valgus accounts for 93% of cases 3
• MPFL is almost always injured, most frequently at its femoral attachment 3
• Lateral displacement tears medial stabilizing structures 2

Anatomic Risk Factors

• Trochlear dysplasia predisposing to lateral patellar instability 3
• Increased tibial tubercle-trochlear groove distance (TT-TG) 3
• Foot pronation or pes planus contributing to malalignment 4
• Patella alta or other anatomic variants 1

Congenital Dislocation

• Very rare condition where patella is dislocated laterally and cannot be manually repositioned 5
• May be associated with other anomalies such as posterior cruciate ligament aplasia 5

Chronic Subluxation

• Lateral shift typically exceeds 31% of patellar width with tilt angle around 32 degrees 6
• Worsens with quadriceps contraction (mean increase of 27.7% in affected knees vs 14% in normal) 6
• May produce pain and instability in young people 6

Diagnostic Approach

Initial Imaging

• Standard radiographs (AP, lateral, and patellofemoral/axial views) are the first-line imaging modality 1, 4
• Axial radiographs demonstrate the degree of patellar tilt or subluxation 1
• Weight-bearing axial radiographs better assess patellofemoral kinematics 1
• Full-length standing radiographs evaluate overall limb alignment 1

Advanced Imaging When Indicated

• CT with metal artifact reduction is most commonly used for measuring component rotation in TKA patients 1
• CT provides detailed assessment of axial malrotation and bony anatomy for surgical planning 4
• MRI gives information on cartilage status, capsulo-ligamentous injuries, and MPFL integrity 3
• MRI is mandatory to rule out osteochondral fracture or free bodies requiring surgical treatment 3

Critical Clinical Pearls

Common pitfall: In post-TKA patients, do not assume patellar complications are always symptomatic—many patellar fractures are asymptomatic, making routine radiographic surveillance essential 1.

Key distinction: Lateral patellar glide exceeding 50% of patellar width is considered abnormal and may induce apprehension on examination 3.

Treatment implications: In native knees with first-time traumatic dislocation, initial nonoperative management is recommended unless there is osteochondral fracture, substantial MPFL disruption, persistent lateral subluxation with normal contralateral alignment, or failure to improve with rehabilitation 2.