The Subtalar Joint is the Second Ankle Joint

While we know many things about ankles, the average person does not know about the second ankle joint. The ankle plays a critical role in walking and is connected to the leg bone by the all-important TALUS. In this article, I will talk about the function of the second ankle joint. You will see why the ankle is such a feat of mechanics! The obvious reason for selecting footwear that protects the ankle, in general, is to stop foot and ankle sprains. Sprains will be the subject of one of my next articles. Having discussed the true ankle joint, we must now consider the importance of the second ankle joint. This is no less important than the first. Unlike the true ankle joint, the subtalar joint or STJ is more difficult to understand. 

In this article, I highlight the following and will cover the function of the ankle joint –

• the difference between the true ankle (known as either the talocrural or tibia-talar joint)
• describe its unique function with the use of videos
• explain more about why this joint is implicated in ankle sprains
• introduce the idea of a third ankle joint

Some of the links used contain more information than you might not want or need, so use the pictures to help you understand the principles. 

Single versus multiple joints

Understanding that joints move in different ways

For the purpose of this discussion, a hip and knee joint appear as single joints. Click the highlights to see movements in 3D. Both videos are under a minute long.

One thing we learned is that single joints can have movement limited to one dimension. In the case of the hip and shoulder joint, both have wide ranges of movement. Indeed, the ankle joint is one joint and, like the knee, has movement predominantly in one dimension, extension and flexion. However, there is a small amount of movement in other dimensions. The clinician who studies the musculoskeletal system must understand how much movement occurs in each dimension and which dimension yields the most important movement. Dimensions are also known as PLANES of MOTION (see diagram or click the link in blue). 

The First ankle joint: The ankle movements are extension and flexion. Upwards is called dorsiflexion, and downwards is plantar flexion. It moves in the sagittal plane. This short video shows the movements. The ankle bone, called the talus, is grasped between the long leg bones.  It is useful to read my article on the ankle joint first, which explains the importance of the ankle bone and talus.

The top surface of the ankle bone is curved (not shown), but the undersurface is much more complex (illustrated) and is called the subtalar joint. This is a multi-joint or multi-faceted joint where the word facet means a connecting surface. (Click here for more information on the subtalar joint – STJ).

The second ankle joint: The subtalar joint principally moves in the frontal plane but does use a little movement in the sagittal and transverse planes. The first ankle joint offers most sagittal plane movement, i.e., dorsiflexion and plantar flexion.

The left diagram shows the undersurface of the talus separated by a channel. The right diagram is the top surface of the heel bone. There are two facets on the left (talus) and three facets on the right (heel bone or calcaneus). This doesn’t make sense, as there should be three on each. NO. In fact, the forward facet of the talus shares 2 facets with the calcaneus. Looking at these surfaces, it is now difficult to figure out what movement is going to occur. The true ankle is easy, so why don’t all joints appear as simple?

The following depiction of movement for the sub-talar joint is useful illustrated by video on movement in 3D with computerised illustrations. 

The difference between the true ankle joint and the subtalar joint comes down to different movements. The top part of the ankle provides flexion (plantar flexion) and extension (dorsiflexion). The bottom part, or the subtalar joint, provides leaning movements. These movements are called inversion and eversion. In this video – anatomical movement demonstrated using a skeletal foot model the two movement is demonstrated.

Why is the ankle joint open to sprains?

The hip and shoulder joints have large movements called circumduction because they have a big, round, ball-like joint that is stable. The ball fits a deep socket, making it a stronger fit, and it is supported by powerful ligaments and muscles.

The second ankle joint allows the proper ankle to retain the stability of the big joints. Because the ankle cannot do everything like a hip and knee because of its design, having a second ankle joint helps. To work correctly, the foot must move in three directions, or we would limp. To achieve this, the ankle is made up of three joints. This means the second ankle (STJ) must make up inversion and eversion, tilting the ankle sideways. To complicate matters, it has those smaller movements as well. This adapts to the foot to contact uneven ground safer and better.

The all-round four-wheel drive of the body

 As we walk up and down a hill, as shown in the previous picture of the walker, the ankle and foot have to adapt to the changing shape of the ground. The subtalar joint has more inversion than eversion. The movement is designed so that the ankle and foot can tip over when the ground is uneven. To prevent too much tipping, we have ligaments that stop the ankle from going over. The only problem is that these ligaments are much weaker and thinner on the outside of the ankle and can be damaged. Generally, as long as the true ankle is not plantar flexed with maximum inversion, we can cope pretty well. Sensors in the ligaments act as righting reflexes, so we quickly adjust. If we lose these reflexes, the second ankle joint can tip over, tearing the ligaments, and then we have pain and swelling. 

To overcome the problem of ankle sprains, wearing higher walking boots can help. Not everyone can wear high boots, and as shown in the picture, the woman prefers stout walking shoes. Foot orthotics can help stabilise the foot if the ankle keeps turning over. The other sensible approach to walking is to use adjustable walking poles.

The subtalar joint is designed to have a higher inversion movement than eversion. As Dr Kirby describes in his 1 1/2-minute video demonstration (above), eversion is limited because of the design of the joint shape (facets).

What have we learned?

The ankle joint has three joints. Each joint combines to help the other produce good 3D movement between leg and foot. The foot must be content with different terrain, as seen in a four-wheel vehicle. 

•  The true ankle joint has been described as having one dimension of movement, plantar flexion and dorsiflexion, which are up-and-down movements. (Talo-crural or tibia-talar joint in the sagittal plane)
• The lower ankle joint is known as the subtalar joint. It has dimensional movements called inversion and eversion in the frontal plane. These are tipping movements, but the subtalar joint also has marginal movements of abduction and adduction in the transverse plane. Additionally, the subtalar joint has some sagittal plane up-and-down movement.
• The third-dimensional movement ensures the remaining movements in the foot combine to offer extra movement. This way, the foot can cope better with uneven ground while the hind foot remains stable. (Talo-navicular joint). The third joint will be the subject of my last article on the functional ankle. 

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Thanks for reading “The Second Ankle Joint” by David R. Tollafield

Published by Busypencilcase Reflective Communications Est. 2015
24 January 2021. Modified January 2025.