If you’ve noticed your arch flattening over the past year or two, or you’ve developed a dull ache on the inside of your ankle that gets worse with walking or standing, the tibialis posterior tendon is worth ruling in or out. This condition, known as tibialis posterior tendon dysfunction (PTTD) or posterior tibial tendonitis, is the most common cause of acquired flatfoot in adults, and it’s frequently missed in its early stages because it develops gradually rather than announcing itself with a single injury.
What the tendon actually does
The tibialis posterior tendon 
runs from the calf, behind the inside of the ankle bone, and attaches mainly onto the navicular bone in the arch. Its job is to support the arch and control the position of the hindfoot as you walk. Particularly during the point in your stride where your heel lifts and your foot needs to become a rigid lever to push off. When this tendon is working properly, it holds the arch up and locks the foot into a stable position for propulsion. When it isn’t, the arch is left unsupported at exactly the moment it needs the most control.
PTTD usually develops as a tendinopathy: a gradual failure of the tendon’s structure under repeated load, rather than a sudden tear.
Contributing factors include;
- Age-related degeneration
- Being overweight
- Inflammatory joint disease
- Diabetes
- Biomechanical factors
An already low arch or excessive pronation, which increase the mechanical demand on the tendon with every step. Once the tendon starts to lengthen and lose its supporting function, the arch drops further, which increases the load on the tendon again. It’s a cycle that tends to get worse rather than better if left alone.
How it presents, and how it progresses
Early on, patients typically describe pain and swelling along the inside of the ankle, worse after walking, standing for long periods, or climbing stairs. At this stage the arch shape may still look relatively normal, and the tendon is inflamed and painful rather than structurally failed.
As the condition progresses, 
the arch begins to visibly flatten, the heel rolls outward. And in more advanced cases, the front of the foot abducts away from the leg, sometimes described as the “too many toes” sign when viewed from behind.
Pain can also shift from the inside of the ankle to the outside, as the collapsed heel position causes impingement between the fibula and the calcaneus. A useful clinical marker throughout this progression is the single-leg heel raise: someone with an intact, functioning tendon can rise onto one leg without difficulty, while someone with significant PTTD often cannot.
This staged progression (commonly described using the Johnson and Strom classification) matters because it changes what we’re aiming for. In the earlier stages, the goal is to calm the tendon down and support it so it can recover.
In later stages, where the deformity has become fixed or the joints themselves have started to compensate. The goals shift toward stability and function, and the conversation about whether orthotic management alone will be sufficient becomes more honest.
When it needs more than a watch-and-wait approach
Most cases of PTTD are managed conservatively, but there are situations where I’d want to look further or involve other clinicians. Sudden, significant pain and swelling after a specific incident raises the possibility of an acute tear rather than a gradual dysfunction, and this is assessed differently. A rapidly progressing flatfoot, particularly with new numbness, marked deformity, or skin changes, warrants imaging and a broader work-up rather than a standard conservative plan. Inflammatory arthropathies can also present with tenosynovitis around this tendon, so a pattern of symmetrical joint involvement or unexplained systemic symptoms is something I’ll ask about directly.
How we approach assessment and treatment
Assessment starts with a proper biomechanical and gait evaluation, not just a look at the painful area in isolation. I want to understand how you load your foot through the gait cycle, what your footwear is doing for or against you, whether there’s a training or occupational load pattern driving the problem, and how flexible or fixed any arch change already is. Ultrasound imaging is useful where there’s diagnostic uncertainty or where the degree of tendon involvement will change the management plan, and it’s something I’ll discuss with you rather than order as a default.
Treatment in the earlier, more reversible stages generally combines three things:
- Offloading the tendon enough to let irritability settle
- Supporting the arch mechanically so the tendon isn’t doing all the work on its own
- Progressively loading the tendon back up through targeted strengthening once symptoms allow.
Foot orthoses have reasonable evidence behind them in stage I and II PTTD, and they work by reducing the amount of pronation moment the tendon has to resist with each step. Eccentric loading of the posterior tibial tendon, borrowing principles from Achilles tendinopathy rehabilitation, has shown promise for earlier-stage tendinopathy, though the evidence base here is smaller and less settled than for orthoses, so I treat it as one part of a plan rather than a guaranteed fix.
Footwear with a firm heel counter and good midfoot support matters more here than in most conditions I see, because a shoe that allows the midfoot to collapse works directly against everything else the treatment plan is trying to achieve.
Where the tendon and joints have become more structurally compromised, bracing or more rigid orthotic control may be needed to keep someone comfortable and mobile, and this is where I’ll have an honest conversation about whether a surgical opinion should be part of the discussion. I’d rather say that plainly than imply a boot and some exercises will resolve a stage III deformity.
The takeaway
PTTD is progressive by nature, which is exactly why early recognition matters. Ankle ache that’s dismissed as “just getting older” or general fatigue is a common way for this condition to be missed until the arch has already changed shape. If you’ve noticed a flattening arch, an ache on the inside of your ankle, or you’re finding heel raises harder on one side than the other, it’s worth getting assessed properly rather than waiting to see if it settles on its own.
If any of this sounds familiar, book in for a biomechanical assessment at RS Podiatry Clinic in Hampton-in-Arden, and we’ll work out exactly what’s happening and what the right plan looks like for you.