Can Flat Feet and Bunions Lead to Hammertoes?
Yes, flat feet (pes planus) are directly associated with the development of hammertoes, and this relationship is well-established in the biomechanical literature. The evidence shows that pes planus foot posture significantly increases the odds of developing hammer toe deformities 1.
The Biomechanical Connection
Flat feet create abnormal mechanical forces that predispose to hammertoe formation through altered weight distribution and muscle imbalance. When the arch collapses in pes planus, the forefoot compensates by pronating excessively, which disrupts the normal balance between intrinsic and extrinsic foot muscles. This imbalance progressively pulls the toes into flexion at the proximal interphalangeal joint, creating the characteristic hammertoe deformity 2.
The Framingham Foot Study—a large population-based investigation—demonstrated that individuals with pes planus foot posture had significantly increased odds of having hammer toes compared to those with normal arch height 1. This association held even after adjusting for other risk factors, confirming the independent contribution of flat feet to hammertoe development.
The Bunion Connection
The relationship between bunions (hallux valgus) and flat feet is bidirectional and creates a cascade of forefoot deformities. Research shows that adolescents with bunions have an 8 to 24 times greater incidence of flatfeet than expected in the general population 3. This suggests that flat feet may predispose to bunion formation, or that both conditions share common biomechanical origins.
When bunions and flat feet coexist, the risk of secondary hammertoe deformities in the lesser toes increases substantially 4. The hallux valgus deformity shifts weight-bearing forces laterally onto the lesser metatarsals, while the flat foot architecture fails to provide adequate support. This combination creates excessive pressure and abnormal toe positioning that drives hammertoe formation.
The Vicious Cycle of Progressive Deformity
Once hammertoes develop in the setting of flat feet and bunions, a self-perpetuating cycle of worsening deformity begins 5. The hammertoe deformity makes the windlass mechanism (the natural tightening of the plantar fascia during toe dorsiflexion) less effective. This reduces the foot's ability to support the arch, which further flattens the foot and increases abnormal loading patterns. Ground reaction forces shift laterally through the metatarsals, increasing stress concentration and risk of additional deformities 5.
Clinical Recognition and Management
The IWGDF guidelines recognize hammertoes, hallux valgus, and pes planus as distinct foot deformities that alter normal foot biomechanics and increase pressure at vulnerable sites 6. While these guidelines focus on diabetic foot complications, they acknowledge the interconnected nature of these deformities.
Key Clinical Points:
- Look for the triad: When you see flat feet, actively examine for bunions and hammertoes, as they commonly coexist
- Assess foot function, not just posture: Pronated foot function (how the foot moves during gait) increases odds of both hallux valgus and overlapping toes beyond what static arch height alone predicts 1
- Recognize early: The progression from flat feet to hammertoes is gradual but relentless without intervention
Management Approach:
For patients with flat feet and bunions who have not yet developed hammertoes, or who have early flexible deformities:
- Prescribe custom-made insoles designed to support the arch and redistribute plantar pressure away from the metatarsal heads 6
- Recommend extra-depth footwear to accommodate any existing toe deformities and prevent progression 6
- Consider toe orthoses (silicone or semi-rigid devices) to maintain proper toe alignment 6
For established hammertoes in the setting of flat feet and bunions:
- Non-rigid hammertoes with callus or pre-ulcerative lesions: Consider digital flexor tendon tenotomy or orthotic interventions 6
- Rigid deformities: Surgical options including tendon transfers, joint arthrodesis, or arthroplasty may be necessary 2
Important Caveat
Patients undergoing simultaneous bunion and hammertoe correction experience significantly less improvement in pain-related outcomes compared to isolated bunion correction 4. This doesn't mean you should avoid treating both conditions when indicated, but it does mean you must set realistic expectations about postoperative pain and recovery when addressing multiple deformities together.
The evidence clearly establishes that flat feet create biomechanical conditions favoring hammertoe development, and when bunions are also present, this risk is amplified through altered weight distribution and compensatory mechanisms.