Flat feet, known medically as pes planus, is a common condition characterized by the collapse of the medial longitudinal arch, causing the entire sole or a significant portion of it to contact the ground. While 20% to 37% of the adult population may have pes planus without symptoms, this structural difference can cause pain in joints further up the leg. This article explores the biomechanical link between a collapsed arch and the development of pain in the knee joint.
Understanding Foot Structure and Biomechanics
The connection between the foot and the knee is understood through the kinetic chain, which describes how interconnected body segments influence one another during movement. The foot acts as the foundation, and structural changes there initiate a chain reaction traveling upward through the ankle, knee, and hip.
In a foot with a collapsed arch, the primary biomechanical fault is excessive pronation, the inward rolling motion of the foot during walking or running. While normal gait involves natural pronation to absorb shock, flat feet cause this inward rolling to be prolonged or exaggerated.
This excessive pronation at the ankle’s subtalar joint forces the tibia (shinbone) to internally rotate. The internal rotation of the tibia transfers rotational stress directly into the knee joint. This twisting of the lower leg relative to the femur destabilizes the knee’s mechanics, setting the stage for misalignment and the development of pain.
How Flat Feet Stress the Knee Joint
The internal rotation of the tibia, caused by the collapsed arch and excessive pronation, impacts the knee joint’s function in two primary ways. The first is the development of patellar tracking problems, often leading to Patellofemoral Pain Syndrome (PFPS), also known as “runner’s knee.”
The kneecap, or patella, is meant to glide smoothly within a groove in the femur when the knee bends and straightens. When the tibia internally rotates, the leg can drift into a valgus position, causing the knee to move inward toward the midline. This altered alignment increases the lateral pull on the patella, forcing it to track improperly outside its normal groove. The resulting friction and shearing forces lead to irritation and inflammation, causing pain around or behind the kneecap, especially during high-impact activities.
The second consequence is the altered distribution of load within the main knee joint. The inward collapse of the knee due to tibial rotation increases pressure on the inside, or medial, compartment of the knee. This chronic, abnormal loading pattern accelerates wear and tear on the cartilage and meniscus. Over time, this mechanical stress contributes to pain and potentially degenerative changes in the joint structures.
Corrective Measures and Treatment
Management of knee pain related to flat feet focuses on correcting the underlying biomechanical fault to reduce rotational stress on the joint. Orthotic devices are a common and effective intervention used to provide support to the collapsed arch and limit excessive pronation. Custom-made orthotics are designed specifically to the contour of an individual’s foot, offering superior support and alignment control compared to generic, over-the-counter arch supports.
These inserts function by mechanically supporting the medial longitudinal arch, which limits the foot’s tendency to roll inward during the gait cycle. By preventing overpronation, the orthotic indirectly reduces the pathological internal rotation of the tibia, which then allows the kneecap to track more correctly. This realignment helps to redistribute pressure evenly across the foot and up the leg, lowering the strain on the knee joint.
Physical therapy also plays a significant role by addressing muscle imbalances higher up the kinetic chain. Targeted exercises focus on strengthening the hip abductors and external rotators, such as the gluteal muscles. When these hip muscles are weak, they allow the femur to rotate internally, which compounds the rotational stress already coming from the flat foot. Strengthening these stabilizers helps to control the inward movement of the leg, providing better stability for the knee and reducing the overall rotational forces that lead to knee pain.