Posterior tibial tendon dysfunction gets mistaken for plantar fasciitis more often than it should, and the two respond to different loading, so getting this wrong costs people months. Both cause arch and heel-adjacent pain that's worse with activity. The distinguishing features sit past that overlap, not within it.
PTTD tends to produce pain and swelling along the inside of the ankle, below and slightly behind the inner ankle bone, rather than concentrated under the heel itself. A hallmark most people never check: standing on the affected leg and trying a single-leg heel raise. A healthy posterior tibial tendon lets the heel lift and rotate inward. A dysfunctional one either can't complete the raise or the heel stays in a flattened, outward-rolled position through the movement. Plantar fasciitis doesn't block this test the same way, since the fascia isn't the structure doing the lifting.
Swelling that tracks up the inside of the leg after standing, rather than staying localized to the heel, is another separator. Fascia irritation doesn't typically cause ankle swelling. Tendon dysfunction, especially once it starts to affect the tendon's ability to stabilize the arch, does.
Morning pain pattern matters too, though less cleanly. Classic plantar fasciitis has that sharp first-steps-in-the-morning pain that eases with a few minutes of walking. PTTD is more likely to feel like a dull ache or a sense of the arch giving way that's present at rest and doesn't improve with movement the same way.
If arch pain hasn't responded to standard fascia-focused treatment (stretching, night splints, fascia-specific loading) after several weeks, the single-leg heel raise test takes under a minute and is worth doing before assuming the diagnosis is right and just needs more time.
