Yes, we are all twisted: Part 2
Last time we spoke of the 3 major versional changes in the normal course of development of he lower extremity and their importance (if you missed out or forgot, click here). In this post, lets look at the talar neck.
The talus is to the foot, as the lunate is to the hand. It is the only bone that has the entire weight of the body passing through it before being distributed to the foot. It’s motion during pronation should be flexion, adduction and eversion, and in supination: extension, abduction and inversion.
At birth, the angle between the talar neck and talar dome is 30 degrees adduction. This reduces to 18-20 degrees in the adult (see above). During this reduction of angle, the talar head also everts or “twists” laterally (ie promotes pronation), which helps to correct the supination and adducted position of the forefoot in adults present in infants (Saffarian 2011).
Abnormal talar loading and “untwisting” in development (see bottom right picture) has been linked to formation of a Rothbart foot type, also known as metatarsus primus elavatus (Rothbart 2003, 2009,2010. 2012). The 1st metatarsal is elevated and inverted with respect tot eh rest of the foot, with it behaving much like a fore foot varus.
Talar torsion (sometimes called subtalar version) results when there is a 10 degree or greater change in the final position of the talar head. This can cause an adducted position of the forefoot, often mistakenly called “forefoot adductus’, which actually only applies to the metatarsals, and not at all to the talus.
An adducted forefoot provides challenges to gait with many possible compensations. As discussed last time, there are at least 3 reasons we need to understand torsions and versions:
1. They will often alter the progression angle (forgot about progression angles? click here). In talar adduction, there will often be a decreased progression angle of the foot.
2. They affect available ranges of motion of the limb. We remember that the lower leg needs to internally rotate the requisite 4-6 degrees from initial contact to midstance, If it is already fully internally rotated, that range of motion must be created elsewhere. This may result in external rotation of the affected lower limb, excessive pronation through the deformity (if possible), or rolling off the lateral aspect of the foot.
3. They often can effect the coronal plane orientation of the lower limb. In talar torsion, the head of the talus often does not “untwist” appropriately resulting in a functional forefoot varus, with excessive forefoot pronation occurring at terminal stance and pre swing.
There you have it in a nutshell. Talar tosion: Present in 8% of the population (Bleck 1982) and coming to your clinic or a shoe store near you!
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