Hi Maxs Mum, really glad that you had such a good and straightforward recovery. I agree about strengthening exercises - I have to follow the consultant's advice and wear a protective brace at present while it heals, but long term I want to do the exercises and strengthen it so it can once more do what it's supposed to do, not permanently wear orthotics.
chiro: with respect, there's a difference between an acute injury and a chronic degenerative condition; most discussions of tibialis posterior tendon dysfunction are referring to the chronic degenerative condition of the tendon (e.g. as described in Kohls-Gatzoulis et al. (2004) BMJ 329, p1328). I have an acute tendon overload injury with a partial tear, but six weeks after the accident I was able to do a single-leg heel raise, with good hindfoot inversion, I've not got acquired flat foot, no "too many toes" sign or anything like that. To my knowledge there is nothing wrong with my posterior tibial MUSCLE (well, except for some inevitable disuse atrophy which should be reversible once I'm allowed to stop wearing the brace and do some exercises).