The classic definition of a Pes Planus is one of a foot with a low medial (longitudinal) arch, often called “your instep”, an everted rearfoot (heel rolled in), and an abducted, dorsiflexed midfoot.
There are two types of pes planus: a rigid type and a flexible type. Both can have a significant impact on the functioning of the foot and lower limb, and while they can look very similar, each requires a different approach to management.
The rigid type is likely due to tarsal coalition or a congenital vertical talus. A tarsal coalition occurs when two bones in the rearfoot (under the ankle) become joined by a bridge (called a “bar”) formed from bone, cartilage or fibrous connective tissue. A congenital verticle talus is a rare birth defect. This is often called “a true flat foot” as it will be flat even when not weightbearing.
A flexible pes planus may not produce any symptoms at all in some people. In others, a flexible pes planus may lead to pain and/or limited function. This condition is very common and affects approximately 23% of adults. This is often called a “functional flat foot”, as the person may well have an easily identifiable, even normal, medial arch in a non-weightbearing position.
The causes of a flexible pes planus are controversial. In 1977, Root, et al, proposed that the cause was altered rearfoot kinematics, especially rearfoot eversion. Williams III, et al (2001) proposed that altered foot and ankle kinematics as a cause, such as increase joint moments and abnormal loading forces. Hunt and Smith (2004) proposed altered physical function, such as altered muscle activation. Buld, et al (2015, 2018) proposed altered plantar pressures (pressure acting on the bottom of the foot) during gait, at least when compared to “normal” controls.
The consequences of pes planus include lower limb pain, fatigue, Achilles tendinopathy, osteoarthritis, hip pain, knee pain, HAV (bunions), Plantarfasciopathy, Hallux Limitis, and Hallux Rigidis.
The question of whether to treat or not is important. Generally, treatment should be offered if a pes planus foot is leading to pain and/or is limiting what the person can do. Treatment is only helpful if the underlying causes are diagnosed correctly and are part of the treatment plan. Orthoses are often prescribed as the principle form of treatment, although, depending on the underlying biomechanical issues, therapeutic exercises may also be given. Surgery may also be considered, and in severe cases, may be the most appropriate option, especially in cases of rigid pes planus.