What Is Bilateral Pes Planus? Causes and Treatment

Bilateral pes planus is the medical term for flat feet affecting both feet, where the inner (medial) longitudinal arch sits lower than normal or touches the ground entirely during standing. It is one of the most common foot conditions worldwide, and while many people with bilateral flat feet never experience a single symptom, others develop persistent pain, difficulty walking, and problems that ripple up through the knees, hips, and lower back. The condition can be something you are born with, something that develops gradually in adulthood, or something in between, and these different origins matter because they shape which treatments actually help.

What the Condition Actually Looks Like

The medial longitudinal arch is the curve along the inside of your foot that normally lifts off the ground when you stand. In pes planus, that arch height is reduced or lost entirely.¹ When both feet are affected, clinicians call it bilateral. In practice, bilateral flat feet are far more common than unilateral (one-sided) flat feet, because most of the underlying causes act on both feet at once.

Flat feet come in two broad categories. Flexible flat feet look flat when you stand on them but show a visible arch when you sit down, point your toes, or stand on tiptoe. Rigid flat feet remain flat regardless of position, which usually signals a structural problem in the bones or joints. The distinction matters because flexible flat feet are far more common, generally milder, and more responsive to conservative treatment, while rigid flat feet are more likely to need imaging and, in some cases, surgery.

Why Most Children Have Flat Feet and Why That Is Usually Fine

Almost all infants have flat feet. A fat pad fills the arch area, and the ligaments and bones are still developing. The arch rises on its own over the first decade of life in the vast majority of children, without any intervention.² No shoe insert, special shoe, or exercise has been shown to create an arch in a child’s foot where one wasn’t developing naturally. This is a point that can save parents a lot of money and anxiety: a pediatrician diagnosing bilateral pes planus in a toddler is usually describing a stage of development, not a disease.

The exception is a flexible flat foot paired with a tight Achilles tendon. That combination can cause pain and functional problems in older children and teenagers, and it does warrant closer follow-up and sometimes treatment.² Rigid flat feet in a child also raise a red flag, because they may point to a congenital bone abnormality like tarsal coalition, where two bones in the hindfoot that should move independently are fused together. Tarsal coalition frequently affects both feet.³

Causes of Bilateral Pes Planus in Adults

When flat feet develop or worsen during adulthood, the most common culprit is posterior tibial tendon dysfunction. The posterior tibial tendon runs along the inside of your ankle and helps hold up the arch. Over time, this tendon can weaken, stretch, or tear, and once it stops doing its job, the arch gradually collapses.⁴ The condition tends to be progressive, meaning it worsens if left untreated, and it often affects both feet because the same risk factors (age, weight, chronic strain) apply to both sides.⁵

Beyond tendon problems, several other factors contribute to bilateral flat feet in adults:

Obesity: Extra body weight loads the arch continuously, accelerating tendon fatigue and ligament stretch.
Aging: Tendons and ligaments lose elasticity over decades, which is why flat feet become more common with age even in people who had normal arches earlier in life.
Joint hypermobility: People whose joints are naturally looser often have lower arches because the ligaments that support the arch allow more movement than usual.
Systemic conditions: Rheumatoid arthritis, diabetes, and certain neurological conditions can damage the soft tissues or joints that maintain the arch.
Prolonged heavy loading: Occupations that involve hours of standing while carrying heavy equipment have been linked to higher rates of flat feet. A study of army soldiers found a prevalence of about 15%, with the weight of equipment carried showing a significant relationship with pes planus development.⁶

When Flat Feet Start Causing Problems

Plenty of people with bilateral pes planus walk, run, and exercise without any trouble. The condition becomes clinically meaningful when it produces symptoms. The most common complaint is aching pain along the inside of the foot and ankle, particularly after prolonged standing or walking. Some people also develop pain in the heel, along the outside of the ankle (from the foot rolling inward), or in the ball of the foot.

Symptomatic flat feet can be genuinely disabling. Research comparing people with painful pes planus to controls has found substantially higher pain and disability scores, with nearly half of the symptomatic group showing signs of altered pain processing, suggesting that long-standing foot pain can change the way the nervous system handles pain signals.⁷ That finding underscores why waiting too long to treat painful flat feet is not ideal: the longer the pain persists, the harder it can become to treat.

Flat feet also affect the rest of your body’s alignment. When the arch collapses, the ankle tilts inward, and that shift can push the knee inward as well. This increased knee valgus, or knock-knee tendency, has been documented in people with flexible flat feet and may raise the risk of knee pain and other lower-limb problems over time.⁸

How Flat Feet Are Diagnosed

A clinician can usually identify flat feet by watching you stand and walk. The wet footprint test, where you step on a surface with a wet foot, gives a quick visual of how much arch contact you have. A flexible flat foot will show an arch reappearing when you rise onto your toes; a rigid one will not.

When imaging is needed, weight-bearing X-rays of the foot are the standard. Two angles dominate the diagnostic conversation. Meary’s angle measures the alignment between the talus (the bone on top of the foot) and the first metatarsal (the long bone leading to the big toe). In a normally aligned foot, these two bones form roughly a straight line. A diagnosis of flat foot is typically made when this angle exceeds about 4 degrees.⁹ The calcaneal pitch, which measures how steeply the heel bone sits relative to the ground, is the other key measurement; a lower pitch indicates a flatter foot.

More detailed imaging, like weight-bearing CT scans, can map the three-dimensional shifts in the bones. Research using this technology has found that flatfoot involves changes across multiple planes, not just the obvious arch drop visible from the side. The hindfoot bones rotate and shift in ways that simple X-rays can miss.¹⁰ For most patients, standard X-rays are plenty. Three-dimensional imaging is reserved for surgical planning or cases where the diagnosis is unclear.

Conservative Treatment That Actually Works

The first line of treatment for symptomatic bilateral pes planus is almost always non-surgical, and for most people it is the only treatment they will need. The three main tools are foot orthoses (insoles), targeted exercises, and a combination of both.

Foot Orthoses

Custom and prefabricated arch-support insoles are the most widely prescribed intervention. A meta-analysis of flatfoot management found that insole use improved calcaneal pitch, a key radiographic marker of arch alignment, and reduced center-of-pressure displacement during walking, meaning the foot moved more efficiently.¹¹ Both custom-made and off-the-shelf orthoses have been studied, with both showing improvements in how the foot handles pressure and distributes load during gait.¹²

Custom orthoses are molded to your individual foot shape and tend to cost considerably more. The evidence so far does not consistently show them outperforming well-chosen prefabricated insoles for mild to moderate flat feet. If you are deciding between a $400 custom device and a $40 over-the-counter one, trying the cheaper option first is reasonable for most people.

Short Foot Exercises

Short foot exercises involve actively trying to shorten the foot by pulling the ball of the foot toward the heel without curling the toes. The motion engages the small intrinsic muscles on the bottom of the foot that support the arch from below. Research has confirmed that these exercises improve arch height and reduce pain.¹³ Interestingly, that same meta-analysis on flatfoot management found that exercise alone produced the greatest improvement in navicular drop (how much the arch bone sinks), even outperforming insoles on that specific measurement.¹¹

Combining Insoles and Exercise

The best results appear when you use both tools together. A randomized trial tested six weeks of short foot exercises (three sets of ten repetitions daily) combined with wearing insoles for eight hours a day, comparing that to insoles alone. The combination group showed greater improvement in pain, function, and foot pressure distribution than insoles by themselves.¹⁴ The takeaway is practical: an insole provides passive support, but adding active muscle training helps your foot build its own scaffolding. If a clinician hands you insoles and sends you on your way without mentioning exercises, it is worth asking about them.

When Surgery Becomes the Conversation

Surgery is reserved for people whose symptoms remain significant after months of conservative care, or for those with rigid deformities that non-surgical treatment cannot address. There is no single “flat foot surgery.” Instead, surgeons choose from a menu of procedures depending on the severity, flexibility, and location of the deformity.

Calcaneal Osteotomy and Soft Tissue Reconstruction

Calcaneal osteotomy involves cutting and shifting the heel bone to improve alignment. It is a joint-sparing procedure, meaning it does not fuse any joints, and is commonly used for moderate flat foot deformity.¹⁵ Often, the osteotomy is paired with soft tissue work on the medial side of the foot: tightening or transferring tendons, repairing the spring ligament, and reinforcing the structures that normally hold the arch up. For adult-acquired flat foot caused by posterior tibial tendon dysfunction, lengthening the lateral column of the foot through calcaneal osteotomy combined with medial soft tissue reconstruction has shown significant improvements in alignment.¹⁶

In children with severe symptomatic flexible flat feet that have not improved with growth and conservative measures, osteotomies combined with soft tissue procedures have also produced favorable outcomes, both in terms of X-ray measurements and functional recovery.¹⁷

Subtalar Arthroereisis

Subtalar arthroereisis is a less invasive option that involves placing a small implant into the sinus tarsi, a natural space between the talus and calcaneus bones. The implant blocks excessive inward rolling of the foot, effectively propping up the arch mechanically. It is used most frequently in children and adolescents but also has applications in adults.¹⁸ A meta-analysis of outcomes found that the procedure reliably corrects the radiographic abnormalities associated with pes planus and reduces pain in appropriately selected patients.¹⁹ Recovery tends to be faster than with osteotomy, and the implant can sometimes be removed later if needed.

Joint Fusion

For advanced, rigid flat foot deformity, particularly when arthritis has developed in the hindfoot joints, fusion (arthrodesis) may be the only option that reliably eliminates pain. Fusion permanently locks one or more joints in a corrected position. The trade-off is reduced foot motion, which changes how you walk and can shift stress to adjacent joints. This is generally a last-resort procedure for severe cases.

The Biomechanical Ripple Effect

Your foot is the foundation of a kinetic chain that runs up through the ankle, knee, hip, and spine. When both arches collapse, the alignment shifts are symmetrical, which means the consequences are often felt in both knees and sometimes in the lower back. The increased dynamic knee valgus associated with flat feet is not just a theoretical concern: it has been implicated in patellofemoral pain (pain around the kneecap), shin splints, and Achilles tendon problems.⁸

This chain reaction is why clinicians treating chronic knee pain or recurrent shin splints will sometimes examine the feet first. A pair of arch-support insoles combined with strengthening exercises may address what looks like a knee problem by fixing what is actually a foot problem. The reverse is also true: hip weakness can worsen the inward knee collapse that flat feet initiate, so a comprehensive approach that includes hip and core strengthening often produces better results than treating the foot in isolation.

The Evolutionary Angle

The human medial arch is not just a structural feature; it is an energy-recycling mechanism. Research into the biomechanics of bipedal locomotion has shown that the arch’s ability to deform under load and then spring back during push-off helps the ankle maintain an efficient upright posture while walking.²⁰ This recoil mechanism likely played a role in driving the evolution of the longitudinal arch after our lineage diverged from ancestors that lacked this feature. The practical implication is that a collapsed arch does not just change your foot’s shape; it compromises one of the foot’s built-in energy-recovery systems, making walking less efficient and more tiring over long distances.

How Flat Feet Affect Day-to-Day Life

Beyond the biomechanics, bilateral pes planus has a measurable impact on quality of life. Research using validated foot-health questionnaires has found that flat feet reduce foot-specific quality of life and foot function scores. People with flat feet rate their foot health lower and report more difficulty with everyday activities involving their feet.²¹ The same study found that flat feet did not significantly alter broader physical or mental health dimensions, which suggests the impact is concentrated in foot-related tasks rather than overall wellbeing. That distinction matters: having flat feet does not make you globally less healthy, but it can make specific activities, such as long walks, standing-intensive work, or recreational sports, harder and less enjoyable.

Shoe shopping becomes its own minor ordeal. Shoes with minimal arch support or narrow toe boxes tend to aggravate symptoms. People with flat feet generally do better in shoes with firmer midsoles, structured arch support, and a slight heel-to-toe drop. Motion-control running shoes, which limit the inward rolling of the foot, are specifically designed for this foot type. Flip-flops and completely flat shoes are among the worst choices for symptomatic flat feet, and even stylish options like ballet flats or unsupported canvas sneakers can make a long day painful.

Common Misconceptions Worth Correcting

One persistent myth is that flat feet disqualify you from military service or athletic activity. While severe symptomatic flat feet have historically been a concern in military screening, asymptomatic flat feet are no longer an automatic disqualifier in most armed forces. Many elite athletes, including professional runners, have flat feet and perform at the highest levels.

Another misconception is that children need corrective shoes or arch inserts to “fix” flat feet. As discussed earlier, the arch develops naturally in most children, and no external device has been shown to speed that process along.² Intervention in children is warranted only when there is pain, a tight Achilles tendon, a rigid deformity, or functional limitation. Treating a normal developmental variant as a problem can lead to unnecessary expense and parental worry.

A third misconception is that orthotics are a permanent fix. Insoles support the arch while you are wearing them, but they do not permanently reshape the foot. If you stop wearing them, your arch returns to wherever it was before. This is why combining orthotics with strengthening exercises matters: the exercises build muscular support that persists even when the insoles come off.

1
Journal of Pediatric Orthopaedics. Is a keystone Bone Anomaly the Main Cause of Flatfoot (Pes Planus)?
2
PubMed Central. Flexible flatfoot in children and adolescents
3
PubMed Central. Complete bony coalition of the talus and navicular: decades of discomfort
4
PubMed Central. Posterior tibial tendon dysfunction: an overlooked cause of foot deformity
5
PubMed Central. Posterior Tibial Tendon Dysfunction: An Overview
6
Journal of Bashir Institute of Health Sciences. Prevalence of Pes Planus Among Army Soldiers of Rawalpindi and Islamabad Due to Prolong Standing and Heavy Equipment Utility
7
ScienceDirect / The Journal of Foot and Ankle Surgery. Central sensitization and pain phenotypes in symptomatic pes planus: A multicenter cross-sectional study
8
Journal of Clinical Physiotherapy Research. The Effects of a Comprehensive Exercise Program combined with Foot Orthoses on Foot Alignment and Dynamic Knee Valgus Kinematics in Individuals with Flexible Flatfoot: A Randomized Controlled Trial
9
Scientific Reports. Deep learning-based automated angle measurement for flatfoot diagnosis in weight-bearing lateral radiographs
10
PubMed Central. Morphological changes in flatfoot: a 3D analysis using weight-bearing CT scans
11
American Journal of Physical Medicine & Rehabilitation. Optimizing Flatfoot Management With Foot Orthoses: A Systemic Review and Meta-analysis
12
PubMed Central. Evidence for foot orthoses for adults with flatfoot: a systematic review
13
PubMed Central. Short foot exercises for flatfoot therapy: Status and prospects
14
PubMed Central. The combined effect of short foot exercises and orthosis in symptomatic flexible flatfoot: a randomized controlled trial
15
PubMed Central. Calcaneus osteotomy
16
PubMed. Lengthening of the lateral column and reconstruction of the medial soft tissue for treatment of acquired flatfoot deformity associated with insufficiency of the posterior tibial tendon
17
PubMed Central. Osteotomies combined with soft tissue procedures for symptomatic flexible flatfoot deformity in children
18
PubMed Central. The role of arthroereisis of the subtalar joint for flatfoot in children and adults
19
PubMed. The outcomes of subtalar arthroereisis in pes planus: a systemic review and meta-analysis
20
PubMed Central. Mobility of the human foot’s medial arch helps enable upright bipedal locomotion
21
PubMed Central. Flat Foot in a Random Population and its Impact on Quality of Life and Functionality