The D QUAD Foot-Type is a moderately over-pronated foot-type. This foot-type occurs when a Compensated Rearfoot Varus exists with a normal or neutral forefoot alignment.
The Quad D foot-type is congenitally a partially unstable foot and is often diagnosed in children as developmental flat foot. Make no mistake, if you think that this child will "out-grow the deformity," just ask Mom and Dad and their older siblings to take off their shoes and socks. If family members demonstrate similar foot characteristics, chances are that this child is not going to develop an arch. During gait, this foot begins to pronate at the subtalar joint in contact phase, and continues to pronate throughout midstance. In propulsion, the 1st ray will plantarflex to load the medial column of the foot and allow the foot to re-supinate.
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As the heel lifts off the ground, initiating propulsive period of gait, you may notice the foot "bending backwards" at the level of the midtarsal joint, further evidencing the severe unstable nature of this foot type. 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 into propulsion, the 1st metatarsal gets driven up dorsally, causing abnormal pressure across the 2nd and 3rd metatarsal heads and associated transverse metatarsal arch reversal. This leads to callus formation of the central metatarsal heads. Final propulsion is off the 2nd and 3rd metatarsal heads rather than the 1st metatarsal.
the 1st metatarsal head should be fully loaded. This did not occur because pronation persisted too far into midstance and the foot is trying to "lock up" too late!
As weight moves forward over the foot in midstance, the midtarsal joint remains abnormally unlocked. Thus the foot is in a state of persisting mobility and the overall height of the arch is allowed to drop in a vertical direction. The drop in arch height is sometimes referred to as navicular drop, however we alternatively refer to this motion as "midfoot sag," more clearly differentiating it from medial shelfing (i.e. medial or horizontal displacement of the arch that evident in the E and F foot types). Normally in gait co-contraction of the gastrocsoleus and the peroneals should create a pronatory twist of the forefoot to facilitate resupination of the subtalar joint and relocking of the midtarsal joint in preparation for propulsion. Unfortunately for this foot, the persisting state of subtalar joint pronation prohibits this from happening. Additionally, along with all this pronation and midfoot sag, there is increased tibial internal rotation creating additional disadvantages for this foot. First of all, the normal Q-angle of the knee is compromised leading to patellar femoral pain. Secondly, the posterior tibialis must work very hard in a failing attempt to decelerate or limit foot pronation, leading to posterior tibial tendon dysfunction. To observe the strain on the posterior tibial tendon, look at 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.
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 at heel strike causing the calcaneus to evert to a vertical position, which in turn unlocks the midtarsal joint. Ideally, the midtarsal joint should only partially unlock to allow for shock absorption and help the foot adapt to uneven terrain during contact period. In this foot type however, the foot is a little too unstable. One more important observation is that during gait the foot progression angle is fairly linear or mildly toe-out at most.