Flat Feet - Pes Planus

An estimated 20–30% of the general population have an arch that simply never develops in one or both feet [1]. Flat feet (also called pes planus or fallen arches) is a postural deformity in which the arches of the foot collapse, with the entire sole of the foot coming into complete or near-complete contact with the ground. 

There is a functional relationship between the structure of the arch of the foot and the biomechanics of the lower leg. The arch provides an elastic, springy connection between the forefoot and the hind foot. This association safeguards so that a majority of the forces incurred during weight bearing of the foot can be dissipated before the force reaches the long bones of the leg and thigh.

In pes planus, the head of the talus bone is displaced medially and distal from the navicular. As a result, the Plantar calcaneonavicular ligament (spring ligament) and the tendon of the tibialis posterior muscle are stretched, so much so that the individual with pes planus loses the function of the medial longitudinal arch (MLA).

If the MLA is absent or nonfunctional in both the seated and standing positions, the individual has “rigid” flatfoot. If the MLA is present and functional while the individual is sitting or standing up on their toes, but this arch disappears when assuming a foot-flat stance, the individual has “supple” flatfoot. This latter condition can be correctable with well-fitting arch supports.[2]

Symptoms
The most common symptom of flat feet is pain in the feet. This can occur as a result of strained muscles and connecting ligaments.
Abnormal stresses on the knee and hip may result in pain in these joints. These stresses are likely if the ankles turn inward.
Pain most commonly affects the following parts of the body:

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inside ankle, alongside possible swelling

• arch of the foot
• calf
• knee
• hip
• lower back
• lower legs

Treatment
Some people with flat feet may automatically align their limbs in a way that prevents symptoms. People who are not experiencing symptoms do not usually require treatment.
If flat feet are causing pain, then supportive, well-fitted shoes can help. Extra-wide-fitting shoes can provide relief.
Fitted insoles and orthotics or custom-designed arch supports may relieve pressure on the arch and reduce pain if the feet roll too far inward. However, these products only treat the symptoms and do not provide long-lasting benefits.
People with posterior tibial tendonitis might also benefit from inserting a wedge into their footwear along the inside edge of the orthotic. This should relieve some of the load that the body places on the tendon tissue.
Wearing an ankle brace may also be beneficial until the inflammation reduces.
Doctors may advise some people to rest until their symptoms improve and to avoid activities that might aggravate the foot or feet.
A person with arthritis or a ruptured tendon might find that a combination of an insole and pain relievers can minimize their symptoms. If these do not work, surgery may be necessary.
Some bones do not develop properly in childhood, which can result in flat feet from birth continuing into adulthood. In these rare cases, surgical intervention may be necessary to separate fused bones.
When obesity is the cause of flat feet, losing weight might improve the symptoms. [3]

​References

1. "Common foot and ankle conditions". physiciansfootcare.com. Retrieved 2019-02-23.
2. Franco, Abby Herzog (1987). "Pes Cavus and Pes Planus Analyses and Treatment". Physical Therapy. 67 (5): 688–94. PMID 3575426.​
3. https://www.webmd.com/pain-management/what-are-fallen-arches

https://www.medicalnewstoday.com/articles/168608.php