The Biomechanical Optimisation of AFOs - why is it important?

Ava

Welcome to the show, everyone! I'm Ava, and joining me is David. Today, we are discussing the biomechanics of lower-limb orthotic design, specifically focusing on a seminal 2010 paper by Elaine Owen from the Child Development Centre in Bangor, Wales. David, let's start with a number that totally shifts how we think about clinical gait analysis: ten to twelve degrees. That is the forward incline of the shank during midstance in non-pathological gait.

David (2)

Ten to twelve degrees. That is a significant departure from what most of us believed was happening at the shank. I mean, the myth is that the shank, or the tibia, should be completely vertical at midstance. Zero degrees. Holding the shank vertically can disrupt the segment alignment needed for an efficient upright posture.

Ava

It really does. With the shank and thigh vertical, maintaining balance comfortably generally requires anterior pelvic tilt, hip flexion and forward trunk lean. Some people can remain more upright, but this shifts the ground reaction force towards the extreme posterior of the base of support

David (2)

Precisely. You are fighting gravity because your joint centres are stacked directly over the ankle joint. To keep that ground reaction force vector in the middle of the foot, you have to lean the trunk forward like a folding pocketknife. People with pathology may instead adopt a forward trunk lean or combine a reclined thigh with a forward trunk lean, thereby imposing hip and knee flexion. This SVA is therefore an important contributor to stable, upright and potentially more energy-efficient gait.

Ava

Exactly. And Elaine Owen shows that this incline is what enables "trunk glide." That is a term coined originally by Jacqueline Perry. As the soleus restrains the forward movement of the shank, the shank becomes relatively stationary, and momentum carries the thigh, pelvis and trunk forward, helping to extend the knee

David (2)

That is beautiful engineering. It is passive energy transfer. By slowing the shank, you use momentum to extend the knee and hip without requiring a massive active muscle contraction. Owen describes this as facilitating the ballistic movement of the thigh, pelvis and trunk. The inclined shank also shortens the effective stance limb and lowers the vertical excursion of the centre of mass when it is near its highest point. Owen suggests that this may contribute to energy conservation.

Ava

Yes! And that brings us to another major myth: that maximum knee extension occurs at midstance. It doesn't. In a non-pathological gait, the knee is actually slightly flexed during midstance. Maximum knee extension doesn't happen until forty percent of the gait cycle, which is in terminal stance, followed by maximum hip extension at fifty percent of the gait cycle.

David (2)

Forty and fifty percent. That timing is crucial because, during terminal stance, the ankle is relatively rigid in dorsiflexion, exhibiting what the paper describes as quasi-stiffness. It is that rigid ankle that allows the ground reaction force vector to align anterior to the knee and posterior to the hip. This supports stabilising knee and hip extension moments and facilitates the ‘Big V’, with associated opportunities to stretch muscle groups including the hip flexors and gastrocnemius.

Ava

That "Big V" is essentially therapy while walking because of that natural stretching. But to achieve it in a patient with pathology, we have to look at the device itself. Owen stresses that we shouldn't just talk about an ankle-foot orthosis, or AFO. We must refer to it as an AFOFC: an Ankle-Foot Orthosis Footwear Combination. The shoe is not just covering the orthosis; it is an active mechanical component of the system.

David (2)

That distinction is paramount. We must look at the entire ground interface. If you change the heel-to-sole differential or the pitch of the shoe, you change the SVA. And this is the great clinical insight of Owen's paper: the Angle of the Ankle inside the AFO and the Shank to Vertical Angle of the overall system are completely independent.

Ava

That is a massive point. It's often assumed that if you want a twelve-degree SVA, you have to set the ankle joint of the AFO to twelve degrees of dorsiflexion. But that's not true. You can set the ankle of the AFO to a neutral plantigrade position, or even in plantarflexion, and still achieve a ten to twelve-degree SVA by altering the heel height or adding wedges to the shoe.

David (2)

This is crucial for patients with tight gastrocnemius muscles. If a child has structural contractures and cannot tolerate a ninety-degree ankle, you can cast them in a plantarflexed AFO, say at five or ten degrees plantar flexed, but then tune the shoe's heel-sole differential to tilt the entire system forward to get the optimum SVA. You accommodate the tight muscle inside the orthosis while maintaining ideal alignment relative to gravity.

Ava

Exactly. And Owen's research on seventy-four children using one hundred and twelve AFOFCs demonstrated this. The optimally tuned SVAs ranged from 7 to 15 degrees, with a mean of 11.36 degrees. No child had an optimal SVA between 0 and 6 degrees, and the resulting alignments were more inclined than the traditional 0-to-7-degree setting. They required a significant incline to stabilise the knee and transition the ground reaction force vector. These results relate specifically to the population of independently ambulant children studied, although Owen proposes 10 to 12 degrees as a useful starting point for tuning

David (2)

Now, to control these forces, the paper outlines a clear clinical algorithm. If a patient has abnormal shank kinematics, Owen strongly recommends a fixed ankle AFO over a hinged one. Hinges are often inadequate because if you cannot control the ankle, you cannot control the shank, which means you cannot control the knee or hip moments.

Ava

Fixing the ankle restricts the anatomical ankle rockers, and in some designs, the MTP joints may also be constrained. Appropriate rocker functions must therefore be simulated through the footwear.

David (2)

Right, especially the third rocker. If a patient has a crouched gait with an excessively inclined shank, you have to use what Owen calls a "Point Loading Rocker," or PLR. This is a highly rigid sole with an acute rocker angle and a specific toe spring. In the sample of 23 legs, the mean PLR length was 78% of footwear length, with a range from 70 to 88%. Owen recommends starting at approximately 80 to 85% and adjusting the position through gait analysis. The mean toe-spring angle was 33 degrees, with a range from 18 to 50 degrees.

Ava

The PLR is used to help control excessive forward shank inclination and optimise ground reaction force alignment through tuning.

David (2)

It is pure vector management. Through careful design and tuning, the aim is to produce shank kinematics and ground reaction force alignment that are as close to normal gait as possible, while improving stability and gait efficiency. It really shows that we cannot treat the orthosis and the shoe as separate entities. They are a single, integrated mechanical system. For anyone wanting to dive into the full clinical algorithms and evidence, we highly recommend reading the full paper 'the importance of being earnest about shank and thigh kinematics, especially when using ankle-foot orthoses' by Elaine Owen, published in Prosthetics and Orthotics International, 2010. It is an important paper to help understand the impact of alignment when using AFOs.

Ava

We hope you have enjoyed this discussion, about AFOFC alignment, and it's got you interested in reading the paper and learning more. We look forward to discussing more prosthetics and orthotics research in the next podcast. See you soon.