I do not want to do a complete write up on posterior tibial tendon dysfunction (PTTD) as that information is readily available elsewhere (for eg, see Podiatry Arena’s threads or the Foot Health Forum’s patient questions on it), but cover highlight some of the recent thoughts and ideas (we cover this in the Boot Camps).
Using our supination resistance device, it became very clear that all those with posterior tibial tendon dysfunction all had very high supination resistance (they usually about twice the normal or average values). Just try and supinate a foot with posterior tibial tendon dysfunction by place two fingers under the medial plantar side of the talonavicular joint. You probably can’t supinate them and if you can, you usually need two hands to do so.
This high supination force needed raise several things:
1. If this is how hard you have to push to supinate the foot, imagine how hard the posterior tibial tendon has to work over the lifetime of that patient. No wonder they get PTTD, as the muscles literally just gives up and can no longer supinate the foot. And then the progressive pronated foot with pain develops
2. If this is how hard you have to push, the that is how hard a foot orthotic has to push. No softer orthotic is going to come close to resisting that magnitude of forces. No wonder anything less that an extremely rigid orthotic generally does not work.
3. Our previous work, showed that the supination resistance force is about one third explained by the transverse plane position of the subtalar joint axis; so clinically we almost always see a medial axis in these people.
This last point raises an interesting question. If the axis is more medial, then the posterior tibial tendon has a very short lever arm to the axis (so the muscle has to work hard to supinate the foot) and the peroneal tendons will have a relatively longer lever arm to the more medial subtalar joint axis. So when we do our inversion/eversion or pronation/supination muscle strength testing in these people, the inversion/supination muscles are at a mechanical disadvantage due to their shorter lever arm to the medial subtalar joint axis and the eversion/pronation muscles are at a mechanical advantage due to their longer lever arm to the medial subtalar joint axis.
So the final point, is the posterior tibial muscle actually weaker in these people with posterior tibial tendon dysfunction or is it actually very strong, but just has poor lever arm to supinate the foot and give the appearance that it is weak?
I also blogged about this problem in runnes.