I am going to type this really slow:
Posterior tibial tendon dysfunction is serious.
Posterior tibial tendon dysfunction is NOT the same as posterior tibial tendonitis.
I try to pull out what little hair I have left when I come across a post on a running forum from a runner wanting advice on posterior tibial tendon dysfunction (PTTD) and the responses assume that it is posterior tibial tendonitis. They are not the same thing. I have anonymously joined a number of forums to berate posters about their advice and how dangerous it is. It is a perfect example of why you should not be getting medical advice online!
PTTD is a condition of degeneration in the muscle-tendon complex of the posterior tibial muscle and tendon. Essentially what happens is the complex just gives up on its job of supporting the arch of the foot and supinating the foot, so a progressive flat foot develops (usually called adult acquired flat foot). Medial foot and ankle pain develops and it will continue to get progressively worse. This is not a very common problem in runners (posterior tibial tendonitis is way more common), but when it does happen it has to be taken very seriously. There are a number of stages of PTTD and it has to be stopped at Stages 1 or 2, otherwise a surgical reconstruction of the rearfoot will invariably be needed if it progresses beyond that (and you probably will not ever be running again). PTTD can develop into a very disabling condition if it is not dealt with properly and promptly.
Even though PTTD is rare in runners, it is becoming more common and there are more and more reports of it and more people asking advice in running forums about it (assuming they were given the right diagnosis that they really do have PTTD and did not get confused with tendonitis). Forefoot & midfoot striking and minimalism will increase the risk for PTTD or make it worse, as the posterior tibial muscle works harder when forefoot striking. So you can imagine the “anger” that I feel when I see advice being given to either start minimalism or keep going with it. This may be the right approach for other injuries, but NOT this one.
Whenever I have seen a runner with PTTD, what running form they are using is the last thing I will be thinking about. They need to stop running for 6 months! That is how serious this is. They need to be counseled as to the seriousness and consequences of this problem. I would often suggest that they consider never running again (the risk of recurrence is high) and take up another fitness activity. The eversion moments in the rearfoot causing the muscle to work so hard have to be countered. I would make foot orthotics out of steel for these people if I could. That is the kind of force that they need to counter those joint moments that are causing this.
If you have developed PTTD, please take it seriously, please make sure the diagnosis is right (ie its not tendonitis), and do not follow the advice on online forums, no matter how well meaning it is.