Biomechanics of the Medial Longitudinal Arch

The medial longitudinal arch of the foot is comprised of the talus/calcaneus, the navicular, first cuneiform and first metatarsal. These osseous connections form a total of three individual joints that span the length of the arch. This is supported inferiorly by a series of tendons as well as the plantar fascia. Superiorly, the arch is supported by the tendons of the anterior and posterior tibialis medially and the peroneus longus laterally. While the arch is primarily osseous (boney) components, the multiple soft tissue structures allow for much mobility of the joints.

In normal function, the medial longitudinal arch is designed to flatten (pronate) to act as a shock absorber. The tissues absorb the energy and then release it to help with forward propulsion during ambulation. This provides efficiency to our movement.

The medial longitudinal arch can become dysfunctional in two distinct ways: pes planus (flat foot) and pes cavus (high arches).

PES PLANUS

Flat footedness occurs when there is a significant decrease in tone to the musculoskeletal components of the arch. This decrease in tone also results in decreased strength and stability of the arch. Individuals with crushing type traumas, such as having their foot ran over or dropping a very heavy object on it, may have significant ligament laxity also contributing to the lack of appropriate arch height.

PES CAVUS

High arches tend to be the more prevalent of midfoot dysfunctions. They occur due to significant shortening of the soft tissues of the medial longitudinal arch. Often, this is a trained response to the types of shoes we wear. For years, shoe manufacturers have increased the amount of arch support, especially in running shoes. However, more is not always better. The increase in support leads to weakening of the muscles as they no longer have to work to support the arch. The other soft tissues, such as ligaments and fascia, will shorten because of the prolonged posturing due to the arch support.

BIOMECHANICAL FAULT

Newton’s Third Law of Motion states “For every action there is an equal and opposite reaction”. When the body loses this initial shock absorber, the stress must still be dissipated somewhere in the body. In some individuals, this dysfunction will show up in stress reactions or stress fractures of the tibia. In other individuals, the stress will be absorbed at a greater rate at the knee that can lead to deterioration of medial meniscus with subsequent osteoarthritis of the medial compartment of the knee. Others will absorb the impact in the hip leading to osteoarthritis while others will absorb it in the sacroiliac joint creating low back pain and sciatica.

TREATING MIDFOOT DYSFUNCTION

When addressing pes planus, custom orthotics can be utilized short term to help reform the arch. However, a more appropriate clinical approach is to utilize corrective exercises to strengthen the anterior and posterior tibialis to be able to support the navicular and cuneiform. The final stage of rehabilitation should emphasize single leg stance activities to simulate the biomechanical stresses of unilateral loading during gait.

When treating pex cavus, a different approach should be taken. Significant soft tissue mobilization to lengthen to shortened soft tissue will assist in returning the arch to its normal height. Depending on the duration of dysfunction, joint mobilizations may also be necessary to return normal joint mechanics. Patients should be instructed on therapeutic home exercises to maintain soft tissue lengths. However, the most important step in treating pes cavus is to discontinue use of arch supports. With more chronic cases, it may require a more gradual approach to wean the patient from support as a total stoppage may exacerbate symptoms.