Relationship Between Pes Planus and Ankle Instability
Pes planus (flatfoot) is biomechanically linked to ankle instability through altered foot mechanics that increase stress on lateral ankle ligaments and compromise proprioceptive function, creating a cycle where flatfoot deformity predisposes to ankle sprains and recurrent instability.
Biomechanical Connection
The relationship between pes planus and ankle instability operates through several interconnected mechanisms:
Ligamentous stress cascade: Flatfoot deformity involves hindfoot eversion, forefoot abduction, and loss of the medial longitudinal arch, which alters the mechanical axis of the lower extremity 1. This malalignment increases tensile forces on the lateral ankle stabilizing ligaments (anterior talofibular, calcaneofibular, and posterior talofibular ligaments) during weight-bearing activities 2.
Posterior tibial tendon dysfunction: The most common cause of acquired pes planus in adults is posterior tibial tendon dysfunction, which creates a vicious cycle where tendon weakness leads to progressive arch collapse, further increasing abnormal plantar pressures and perpetuating tissue damage 3, 4. This biomechanical instability at the hindfoot directly compromises ankle joint stability.
Achilles tendon contribution: Achilles tendon contracture or overpull significantly worsens pes planus severity, particularly increasing arch depression and forefoot abduction 1. This creates additional mechanical disadvantage for the ankle stabilizers and increases injury risk during high-impact activities 3.
Clinical Implications for Ankle Instability
Increased sprain risk: Individuals with pes planus demonstrate altered loading patterns and compromised proprioception, which are established risk factors for lateral ankle sprains 2. The combination of structural malalignment and impaired neuromuscular control creates vulnerability to inversion injuries.
Chronic ankle instability development: Approximately 40% of individuals who sustain a lateral ankle sprain develop chronic ankle instability (CAI), characterized by persistent pain, swelling, and recurrent sprains 2. Pre-existing pes planus may contribute to this progression by maintaining abnormal biomechanics that prevent complete functional recovery.
Recurrent injury pattern: A history of previous ankle sprain is a moderate risk factor for subsequent ankle injuries 2. When combined with uncorrected pes planus deformity, this creates a particularly high-risk scenario for recurrent instability.
Management Algorithm
Initial Assessment (Days 0-5)
Acute injury evaluation: For patients presenting with acute ankle injury and known or suspected pes planus, apply the Ottawa Ankle and Foot Rules to exclude fracture and determine need for immediate radiography 2.
Delayed examination: Re-examine 3-5 days post-injury when pain and swelling have improved to better assess ligamentous integrity and underlying foot structure 2.
Imaging for chronic instability: MRI demonstrates 97% diagnostic accuracy for anterior talofibular ligament injury and can identify associated pathology including tenosynovitis, tendon injury, and osteochondral lesions that may complicate pes planus 2.
Acute Phase Management (First 2-4 Weeks)
PRICE protocol: Implement Protection, Rest, Ice, Compression, and Elevation as the foundation of acute ankle sprain treatment 2.
NSAIDs with caution: Use NSAIDs to reduce pain and swelling, but recognize they may suppress natural healing processes and should not be used indiscriminately 2. Topical NSAIDs reduce tendon pain while eliminating gastrointestinal hemorrhage risk associated with systemic NSAIDs 5.
Functional support over immobilization: Use semirigid or lace-up ankle supports rather than elastic bandages or prolonged immobilization, as functional rehabilitation is superior for recovery 2.
Subacute and Rehabilitation Phase (Weeks 2-12)
Supervised exercise-based programs: Prioritize exercise therapy over passive modalities, as it stimulates recovery of functional joint stability 2. Include coordination, balance training, and proprioceptive neuromuscular facilitation exercises, which have demonstrated effectiveness in preventing recurrent ankle sprains 2.
Manual therapy adjunct: Combine joint mobilization with exercise therapy for enhanced efficacy, as this combination improves ankle dorsiflexion range of motion and reduces pain better than exercise alone 2.
Address underlying pes planus: Prescribe properly fitted footwear with arch-support orthotic inserts or custom orthotics to reduce excessive ankle eversion and normalize foot pressure distribution patterns 5. This addresses the biomechanical root cause contributing to instability.
Long-Term Management and Prevention
Orthotic intervention: For patients with flexible pes planus and recurrent ankle instability, foot orthoses provide moderate-level evidence for improving physical function and reducing abnormal rearfoot kinematics 6. Custom or prefabricated arch-support devices should provide ≥30% reduction in peak plantar pressure 5.
Footwear modifications: Ensure shoes have adequate depth, width, and length with good flexibility, flat heel, heel support, and laces or straps to prevent excessive foot movement 5. This is particularly critical for patients with both pes planus and ankle instability.
Prophylactic bracing: Use semirigid or lace-up ankle supports to decrease risk of recurrent ankle injury, especially in patients with history of recurrent sprains and underlying pes planus 2.
Activity modification: For high-impact activities (running, jumping sports, military rucking), temporary reduction in weight-bearing activities may be necessary to allow tissue healing when both conditions are symptomatic 3.
Stretching and strengthening: Implement regular calf-muscle and anterior compartment stretching exercises to reduce tension on affected structures 5. If tendinopathy develops secondary to the combined pathology, eccentric strength training promotes tendon healing 5.
Surgical Considerations
Reserve for refractory cases: Surgery should be reserved for patients with chronic instability who have not responded to comprehensive exercise-based physiotherapy programs addressing both ankle instability and pes planus biomechanics 2.
Individual decision-making: While functional treatment is preferred for most patients (60-70% respond well to non-surgical management), professional athletes or individuals with severe combined deformity may benefit from earlier surgical intervention 2.
Timing considerations: For pediatric pes planus contributing to ankle instability, simple procedures like arthroereisis or osteotomy in young children may prevent need for arthrodesis in adolescence or adulthood as the foot becomes rigid 7.
Common Pitfalls to Avoid
Failing to address biomechanical factors: Treating ankle instability without correcting underlying pes planus deformity perpetuates the cycle of recurrent injury 5. Both conditions must be addressed simultaneously for optimal outcomes.
Inadequate footwear modification: Continuing to use tight, poorly fitted shoes or shoes without adequate arch support will maintain abnormal biomechanics and prevent resolution of symptoms 5.
Premature return to activity: Returning to high-impact activities before achieving adequate proprioceptive recovery and biomechanical correction increases risk of recurrent ankle sprains 3.
Over-reliance on passive modalities: Ultrasound, laser therapy, electrotherapy, and shortwave therapy show no significant effect on pain, edema, function, or return to play for ankle sprains 2. Focus resources on active rehabilitation and orthotic management instead.