How does flat foot (pes planus) cause foot pain that radiates to the knee?

How Flat Foot Causes Foot and Knee Pain

Flat foot (pes planus) causes foot pain that can radiate to the knee primarily through biomechanical alterations that increase stress on supporting structures and create abnormal movement patterns throughout the lower extremity kinetic chain.

Biomechanical Mechanism of Pain

Primary Foot Pain Mechanisms

• Excessive Pronation: When the medial longitudinal arch collapses, it leads to:

  • Increased plantar pressures in the midfoot during weight-bearing activities 1
  • Overstretching of supporting ligaments, particularly the spring ligament and plantar fascia 2
  • Abnormal stress on the posterior tibial tendon, which is a primary arch supporter 3

• Structural Imbalance:

  • The falling of the medial longitudinal arch causes eversion of the hindfoot and abduction of the loaded forefoot 4
  • This creates abnormal pressure distribution across the foot, leading to localized areas of increased stress 2
  • Repetitive biomechanical stress on these areas during walking and standing causes inflammation and pain 1

Knee Pain Connection

• Altered Kinetic Chain:

  • Flat foot deformity disrupts normal lower limb biomechanics, creating a chain reaction that extends upward 4
  • Excessive foot pronation causes internal rotation of the tibia, which affects knee alignment 1
  • This altered alignment increases stress on knee structures, particularly the patellofemoral joint 1

• Compensatory Mechanisms:

  • The body attempts to compensate for the abnormal foot position, leading to:
    • Changes in gait pattern and muscle activation
    • Increased load on knee joint structures
    • Development of patellofemoral pain as knee mechanics become compromised 1

Clinical Manifestations

Foot Symptoms

• Midfoot pain during weight-bearing activities
• Pain that worsens with prolonged standing or walking
• Fatigue in the arch area
• Swelling along the inside of the ankle
• Difficulty with certain footwear

Knee Symptoms

• Pain typically localized to the anterior knee (patellofemoral region)
• Pain that worsens with activities like climbing stairs, squatting, or prolonged sitting
• Sensation of knee instability or "giving way"
• Occasional clicking or popping sensations

Assessment Approach

• Objective Evaluation:

  • Assess hip and knee strength using hand-held dynamometry
  • Observe movement patterns during functional tasks like single-leg squat
  • Evaluate foot biomechanics during gait 1
  • Check for tissue tolerance to load and presence of effusion

• Functional Assessment:

  • Observe how subjective symptoms manifest during functional tasks
  • Look for fear avoidance behaviors during movement 1
  • Evaluate the impact on daily activities and quality of life

Management Considerations

Conservative Approaches

• Orthotic Intervention:

  • Prefabricated or custom-made foot orthoses to support the arch and redistribute pressure 4
  • Orthoses should be prescribed when patients respond favorably to treatment direction tests 1

• Exercise Therapy:

  • Knee-targeted exercise therapy for quadriceps strengthening
  • Hip-focused exercises for patients with poor tolerance to loaded knee flexion 1
  • Calf-muscle and hamstring stretching exercises 5

• Offloading Strategies:

  • Non-removable knee-high offloading devices for severe cases 1
  • Removable below-knee walkers for cases requiring frequent monitoring 1
  • Appropriate footwear with adequate depth and width 5

Advanced Interventions

• For persistent symptoms despite conservative management:
  • Movement/running retraining for task-specific biomechanical issues 1
  • Manual therapy for cases with high symptom severity 1
  • Taping techniques to improve patellofemoral alignment

Common Pitfalls in Management

1. Focusing only on the foot: Treating only the foot symptoms without addressing the knee involvement often leads to incomplete resolution.

2. Overlooking the importance of proper footwear: Inadequate footwear can exacerbate symptoms despite other appropriate interventions.

3. Insufficient duration of conservative treatment: A minimum of 6 weeks of consistent intervention should be attempted before considering more invasive approaches 1.

4. Neglecting weight management: Excess body weight increases arch-flattening effects and can undermine treatment efforts 2.

5. Failing to distinguish between flexible and rigid flat foot: Rigid flat foot may require subspecialist referral and different management approaches 6.

By understanding the biomechanical link between flat foot and knee pain, clinicians can implement comprehensive treatment strategies that address both the primary foot deformity and the secondary knee symptoms, improving overall outcomes for patients with this common condition.