20/04/2025
Medial Heel Skive Orthoses and Posterior Tibial Tendon Dysfunction
All patients with posterior tibial tendon dysfunction (PTTD) have medially deviated subtalar joint (STJ) axes. [My illustration below shows a frontal plane cross-section of a right foot with PTTD in a hiking boot standing on a custom foot orthosis with a medial heel skive.]
A medially deviated STJ axis simply means that the STJ axis spatial location is more medially translated and adducted than normal. Because of this abnormal medial deviation of the STJ axis, ground reaction force (GRF), when it acts on the plantar foot, will cause increased magnitudes of STJ pronation moments during weightbearing activities when compared to a foot with a normal STJ axis location.
In order to effectively treat significant cases of PTTD conservatively, a combination of very specific custom foot orthosis design modifications are necessary to achieve optimal treatment of this disabling condition. First of all, a foot orthosis with a deep heel cup and a medial heel skive is necessary to shift orthosis reaction force (ORF) more medial on the plantar heel. The medial heel skive, by shifting GRF more medial on the plantar heel, will increase the magnitude of STJ supination moment from GRF during weightbearing activities (Kirby KA: The medial heel skive technique: improving pronation control in foot orthoses. JAPMA, 82: 177-188, 1992).
https://pubmed.ncbi.nlm.nih.gov/1597827/
In addition, the orthosis should have a well-formed and stiff medial longitudinal arch to help shift GRF more medial on the plantar midfoot which will also increase the magnitude of STJ supination moment and decrease the magnitude of STJ pronation moment from the mechanical effects of GRF. Finally, a hiking boot or high top shoe can also add significant magnitudes of STJ moment by creating a force couple where the lateral heel of the boot heel counter creates a medially directed force vector on the lateral plantar calcaneus and the medial upper of the boot creates a laterally directed force vector on the medial malleolus.
The combination of these two oppositely directed force vectors from the boot create a powerful supination moment across the STJ axis that will act synergistically with a medial heel skive foot orthosis to increase the magnitude of STJ supination moment. As a result of this increase in STJ supination moment, the central nervous system (CNS) will recognize that the posterior tibial (PT) muscle no longer needs to be so active during gait to supinate the foot so, as a result, tension stress within the PT tendon will decrease during weightbearing activities.
As mentioned above, the most likely explanation for decreased PT tendon tension force is due to the CNS decreasing the efferent output to the PTl muscle when the medial heel skive orthosis and boot are being worn. In other words, when the CNS recognizes that it will not have to increase the contractile activity of the PT muscle since the foot orthosis is creating STJ supination moment that the PT muscle normally creates during gait, the CNS will then reduce contractile activity of the PT muscle and, as a result, PT muscle and tendon tension stress will be significantly decreased during gait.
One important point to remember is that foot orthoses cause change in STJ moments by alterations of GRF acting only on the plantar foot. This means that since the foot orthosis only acts on the plantar foot inferior to the STJ axis, it can generate STJ supination moments only by altering the GRF on the plantar foot. On the other hand, a high top boot or Arizona brace, both have the ability to exert direct mechanical effects both inferior and superior to the STJ axis. As a result of their ability to place corrective forces both inferior and superior to the STJ axis, high top boots and AFO -style braces have a very significant mechanical potential to generate additional STJ moments across the STJ axis.
The end result of these time-tested methods of using medial heel skive foot orthosis and hiking boots in the treatment of patients with PTTD is that they nearly always report that they can immediately ambulate with much less pain. In addition, over time, these patients can often avoid foot surgery and show significant healing from their PTTD when expertly designed custom foot orthoses and other conservative therapeutic measures are employed for their disabling condition (Kirby KA: Conservative treatment of posterior tibial dysfunction. Podiatry Management, 19:73-82, 2000).
https://podiatrym.com/cme/september200kirby.pdf
