The F QUAD Foot-Type
is a commonly referred to as a Pes Planovalgus foot deformity because of its very poor alignment to the floor. This is a true "flat foot." The condition occurs when a Compensated Rearfoot Varus is coupled with a large Flexible Forefoot Varus (also called Forefoot Supinatus).
The F QUAD foot-type is the most hypermobile or flexible of the foot-types. This hypermobility leads to great instability throughout the foot and ankle, and can be prevalent throughout the body. These feet look "very flat" at an early age, and can only worsen into adulthood. This severe instability also makes it difficult to develop and maintain core strength throughout the legs and trunk. Muscles need to work "overtime" to do the same job as someone with better functioning feet, drastically increasing energy expenditure. This foot-type causes a lot of damage to the forefoot during propulsion. In addition to transverse metatarsal arch reversal, don't be surprised to see hammertoes, hallux abductovalgus deformity, functional hallux limitus, and painful corns and calluses.
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As the heel lifts off the ground, the 1st metatarsal head should be fully loaded. The 1st metatarsal is our "kickstand" for our foot. This did not occur because pronation persisted too far into midstance and the foot is trying to "lock up" too late. In fact, in this case, abnormal pronation continues well into propulsion. If the forefoot varus is too large and the medial column has not loaded successfully, the heel may whip medially as the foot pivots around the 5th metatarsal base in order to complete loading of the 1st metatarsal. This medial heel whip creates heavy shearing over the base of the 5th metatarsal, leading to skin thickening or callus formation. Because of the severe unstable nature of this foot type, you may notice the foot "bending backwards" at heel rise. This can be seen by looking at the lateral side of the midfoot. This lateral column should be straight and stable, but notice how it is bending backward and sagging. This is a sign of significant midfoot instability. Because of severe instability and abnormal pronation through propulsion, the 1st metatarsal may be driven dorsally by the floor reaction forces, resulting in abnormal pressure and callusing across the 2nd and 3rd metatarsal heads. We fondly refer to this heavy callus formation of the central metatarsal heads as a "Cyclops callus." Final propulsion is off the 2nd and 3rd metatarsal heads rather than the 1st metatarsal.
As weight moves forward over the foot, the arch and midtarsal joint have completely unlocked. Ideally, the midtarsal joint should only partially unlock to allow for shock absorption and help the foot adapt to uneven terrain. In this case, the arch drops vertically as the primary compensation. However, if the forefoot varus is large enough, the arch/midtarsal joint will start to bend horizontally (parallel to the floor) on the medial side of foot as well. This also may actually allow the subtalar joint to evert beyond vertical to the floor, hence the term "pes planovalgus." This medial/horizontal breakdown of the oblique midtarsal joint is referred to as "shelfing." This coincides with a splaying positive toe sign (forefoot abduction) due to the flexible nature of the foot type. This midtarsal joint pronation is abnormal at this point in the gait cycle. The foot should begin to start to lock up at this point in order to prepare for propulsion. It is also important to note the excessive internal tibial/fibular rotation that has occurred. Observe the medial side of the foot and ankle and note how it is bulging inward towards the other leg, giving it a stretched out appearance. This is known as posterior tibial tendon dysfunction.
At initial contact, the calcaneus strikes the ground in a slightly inverted alignment relative to the ground. Since the rearfoot varus is primarily compensated, the subtalar joint immediately pronates, bringing the calcaneal alignment vertical to the floor and unlocking the mid-tarsal joint, allowing the arch to drop completely to the floor. Some subtalar joint pronation is normal during this phase in order to dissipate forces and adapt to uneven terrain. In this case though, the pronation is excessive, leading to hypermobility of the midtarsal joint. The 5th metatarsal may be subjected to excessive pressure because of the forefoot varus alignment. Limb position for this foot-type is generally mild toe-out.