Adult acquired flat foot – also called posterior tibial tendon dysfunction (PTTD) – is a condition distinctly different from the flat feet most people are familiar with. Structural flat feet are present from childhood and the person has always had a low or absent arch. Adult acquired flat foot develops in a person who previously had a normal arch – the arch collapses progressively in adulthood as the posterior tibial tendon, the primary dynamic supporter of the medial arch, fails.
This distinction matters enormously for treatment. The insole requirements for adult acquired flat foot are more demanding than for simple flat feet, and recognising the condition early – before it progresses to a stage requiring surgery – is one of the most important things a person with this condition can do.
For the related condition of simple flat feet, see our flat feet insoles guide. For the complete picture of how orthotic insoles work, read the complete guide.
What Is the Posterior Tibial Tendon and Why Does Its Failure Cause Arch Collapse?
The posterior tibial muscle originates in the deep posterior compartment of the calf and its tendon travels behind the medial malleolus (inner ankle bone) and fans out to attach to the navicular bone, the cuneiform bones, and the bases of the metatarsals on the inner and plantar surface of the foot. Its function is to:
- Invert the foot (turn the sole inward) and plantarflex the ankle – producing the push-off motion in gait.
- Support and elevate the medial arch dynamically during weight-bearing – particularly during the single-leg stance phase of gait when the entire body weight is on one foot.
- Lock the midfoot joints during push-off – stiffening the midfoot into a rigid lever for efficient propulsion.
When the posterior tibial tendon fails – through progressive inflammation, degeneration, and ultimately rupture – all three of these functions are lost or reduced. The medial arch loses its primary dynamic support. The foot progressively collapses into a pronated, everted, flat position with each step and eventually at rest.
What Causes Posterior Tibial Tendon Dysfunction?
The exact cause is multifactorial, but the most important risk factors are:
- Obesity: Increased body weight proportionally increases the load on the posterior tibial tendon. Studies consistently show that PTTD is significantly more common in obese individuals.
- Age and tendon degeneration: The posterior tibial tendon, like all tendons, undergoes degenerative changes with age that reduce its tensile strength and repair capacity. The natural biology of ageing makes the tendon progressively more vulnerable to the loads of daily activity.
- Female sex: PTTD affects women significantly more commonly than men – studies report a female-to-male ratio of approximately 3 to 5:1. The reasons are not fully understood but may include hormonal effects on tendon properties, anatomical differences in foot structure, and footwear patterns.
- Hypertension and diabetes: Both conditions affect the microvascular supply to tendon tissue, reducing the tendon’s ability to repair micro-damage from daily activity. In India, where both conditions are prevalent and often coexist, PTTD risk is elevated.
- Inflammatory conditions: Rheumatoid arthritis and other inflammatory conditions directly attack tendon tissue and are associated with elevated PTTD risk. See our arthritis insoles guide for the related information.
- Previous ankle injury: A previous ankle sprain or fracture that altered the mechanics of the hindfoot can predispose to posterior tibial tendon dysfunction.
- Occupation and activity: Occupations requiring prolonged standing and walking increase cumulative tendon loading and PTTD risk. See our long standing hours guide.
What Are the Stages of Adult Acquired Flat Foot?
Johnson and Strom’s classification describes four stages:
Stage I – Tendinopathy Without Deformity
The tendon is inflamed and locally painful but still intact and functional. The arch height is preserved. Clinically, the patient can perform a single-leg heel rise (standing on tiptoe on one foot) with pain but intact function.
Symptoms: pain and swelling along the course of the tendon, from behind the medial malleolus to the navicular. Stiffness and aching that is worse after prolonged activity and better with rest.
Treatment: Orthotic insoles and activity modification can prevent progression. This is the most important stage to identify – stopping progression here avoids all the more complex later-stage treatments.
Stage II – Flexible Deformity
The tendon is degenerated and elongated but not completely ruptured. The arch has collapsed and the heel has drifted into valgus (outward tilt), but the deformity is still flexible – it can be passively corrected. The single-leg heel rise is weak or impossible.
The “too many toes” sign is positive – when viewed from behind, more toes are visible on the outer side of the affected foot than the unaffected side, because the forefoot has abducted.
Treatment: Orthotic insoles with substantial medial arch support, heel valgus correction, and ankle bracing are the primary conservative treatments. Physiotherapy to strengthen the posterior tibial muscle is essential. This stage may be managed conservatively for months to years.
Stage III – Rigid Deformity
The deformity is rigid – it cannot be passively corrected. The bones and joints of the hindfoot are fixed in the pronated, everted, flat position. Surgical reconstruction is usually required at this stage – osteotomies and tendon transfers.
Orthotic insoles can reduce symptoms and slow further progression at Stage III but cannot correct the deformity.
Stage IV – Deformity Affecting the Ankle
The rigid hindfoot deformity produces valgus stress on the ankle joint, producing ankle osteoarthritis. Surgery is required.
How Do Insoles Treat Adult Acquired Flat Foot?
At Stage I and II – the stages where orthotic management can genuinely change the outcome – insoles must provide more than standard arch support:
- Substantial medial arch support: The PP shell must be firm enough to substitute for the failing tendon’s dynamic support function. The arch support must contact the medial arch through the full range of weight-bearing loading.
- Medial heel wedge: A slight elevation of the inner heel edge corrects the heel’s valgus tilt, placing the calcaneus in a more neutral position. This reduces the tensile load on the failing tendon and reduces the progression of the deformity.
- Midfoot stabilisation: Full-length construction that stabilises the midfoot joints that become unstable as the tendon fails.
- MCR top layer: The sustained comfort required for the extended wearing of orthotic insoles in PTTD management is best provided by MCR’s therapeutic cushioning properties.
Browse Insoleace adult acquired flat foot insoles. For related information see flat feet insoles, arthritis insoles, and long standing hours insoles.
FAQ
What is the difference between flat feet and adult acquired flat foot? Structural flat feet are present from childhood – the person has always had a low arch. Adult acquired flat foot develops in adulthood in a person who previously had a normal arch, due to progressive failure of the posterior tibial tendon.
Can adult acquired flat foot be treated without surgery? At Stages I and II, yes. Orthotic insoles, ankle bracing, physiotherapy, and activity modification can manage PTTD effectively and prevent progression. Stage III and IV typically require surgery.
What insoles are best for adult acquired flat foot? Insoles with substantial medial arch support, heel valgus correction, and MCR cushioning. Browse at insoleace.com/adult-acquired-flat-foot.
Are women more likely to develop adult acquired flat foot? Yes. Women develop PTTD at three to five times the rate of men. The condition most commonly presents in women over 40.