QUAD F FOOT TYPE
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At initial contact, the calcaneus strikes the ground in a slightly inverted alignment relative to the
floor. 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 extremely unstable. Up until this point,
this foot functions exactly like the D Quad foot-type. Unlike the
D quad foot-type though, the forefoot does not lower to the ground in a
normal plantigrade position. In the F Quad foot-type, instead of
being plantigrade, the forefoot is lowered to the ground in a varus
state of alignment. Since the lateral column of the foot is
grossly unstable, the 5th ray is driven dorsally due to ground reaction
forces.
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Normally in gait, co-contraction of the peroneals and the gastrocsoleus should create a pronatory twist of the forefoot, and re-supination of the subtalar joint. This is known as the midtarsal joint locking mechanism. This mechanism provides the stability necessary to lock up the midtarsal joint in preparation for propulsion. Unfortunately, in the F Quad foot-type, there is a persisting state of severe subtalar joint pronation that prohibits this from happening. The midtarsal joint remains unlocked and mobile, while the overall height of the arch drops significantly in a vertical direction. We call this motion midfoot sag, and it is the primary form of compensation for this foot-type (just like the D Quad foot-type). This arch collapses most severely of all foot-types, and usually ends up flat, or nearly flat, to the ground. Unlike the D Quad foot-type, this F Quad foot-type also has a forefoot varus deformity. In this case, midfoot sag alone will not be enough to load the medial column of the foot. As a result, the arch will start to shift horizontally (parallel to the floor) in a medial direction at the level of the oblique midtarsal joint. This form of compensation is referred to as midfoot shelfing. It is a major form of compensation whenever a moderate to large forefoot varus deformity is present, leading to a splaying positive toe sign (forefoot abduction). This combined midfoot sag and shelfing results in gross instability of the midtarsal joint. This allows the rearfoot to evert into a valgus heel to floor alignment, hence the term pes planovalgus. Note that in many cases, this eversion motion is not completely occurring at the subtalar joint alone, as is commonly believed. Often, once the subtalar joint has reached its end range of motion, the entire rearfoot (including the tibia, talus and calcaneus) move together as one functional unit, continuing to pronate about the longitudinal midtarsal joint axis. It is also important to note that along with all this foot pronation, there is increased tibial/fibular internal rotation creating further disadvantages for this foot. First, the normal Q-angle of the knee is compromised when the tibia is excessively internally rotated, leading to patellafemoral mal-alignment and pain. Secondly, the posterior tibialis must work very hard in an attempt (or should we say failing attempt) to decelerate or limit rearfoot pronation, eventually 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.
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As the heel lifts off the ground, initiating the propulsive phase of gait, the lateral column should remain straight and stable. Instead, like the D Quad foot-type, you may notice that the F Quad foot-type similarly “bends backwards” at the level of the midtarsal joint. What you are actually seeing is dorsiflexion and abduction of the forefoot about the oblique midtarsal joint axis, a further sign that the midfoot is still unlocked. During midstance, even with all the midfoot sag and shelfing, it is unlikely that the medial forefoot was able to adequately load due to the presence of the large forefoot varus deformity. Thus, upon entering propulsion, this foot-type still needs to find an alternative or additional compensatory mechanism to complete medial forefoot loading. Further compensation for this foot-type is a medial heel whip that occurs as the heel rises from the ground. The heel whip maneuver requires a pivot point. Since the 5th ray was driven dorsally during contact phase, weight is now borne more proximally on the base of the 5th metatarsal. As the heel rises, the foot pivots around the base of the 5th metatarsal, transferring weight from the lateral to the medial side of the foot. This medial heel whip creates heavy shearing and rotatory forces at the base of the 5th metatarsal, leading to skin thickening in this area. Peroneal functioning is significantly impaired due to midtarsal joint instability. This leads to hyper mobility of the1st ray allowing it to be driven dorsally by ground reaction forces. Weight then transfers to the 2nd metatarsal, driving it dorsally, and then to the 3rd metatarsal and so on, until there is complete reversal of the transverse metatarsal arch. Final propulsion is off the 2nd and 3rd metatarsal heads rather than the 1st metatarsal, leading to the development of a rather large and diffuse callus formation over the central metatarsal heads. We fondly refer to this heavy callus formation of the central metatarsal heads as a “Cyclops callus.” |
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For more information, contact us at info@whatsmyfoottype.com |