A strain on the brain – walking with a prosthetic knee

In a unique study from the School of Health and Welfare, it is now possible to see how walking with a prosthetic knee affects the cognitive functioning of a person. Saffran Möller, a qualified physiotherapist and PhD, has been using a new type of instrument, called fNIRS (functional near-infrared spectroscopy) that can measure the brain activity of a person whilst they use their prosthetic.

Traditional methods for testing how well a patient is able to walk with a prosthesis and at the same time pay attention to potential hazards (such as uneven surfaces or passing traffic) around them is done using a so-called ‘dual task testing’. A patient is required to walk while simultaneously performing a cognitive task, such as counting backwards or subtracting by sevens. Performance of the gait (they way they walk) and the cognitive task is then evaluated.

Functional near-infrared spectroscopy technology can measure the brain activity of a person whilst they walk using their prosthetic.(photo: Dayfotografi)

Now, thanks to fNIRS technology, scientists can perform these tests whilst actually seeing what is happening at a neurological level. Participants are given specially designed headgear that contains infrared sensors connected to a computer that is worn in a rucksack on their back.

“This is the first time that a mobile system has been used to measure what is happening in the brain when a person is walking with a prosthetic knee. Previously it has been almost impossible to measure this – MRI scans require a person to lie still and so haven’t been suitable for what we want to see,” says Saffran Möller.

There are many types of knee mechanisms, but they generally fall into two types: mechanical and computerised (see fact box). Studies have shown that people who walk with a computerised knee joint can walk faster with less risk of falling, and that they don’t have to focus on walking as much. This is the kind of feedback that Saffran Möller, as a physiotherapist, has received from her own patients, one of whom is Camilla Åberg who lives in Jönköping:

A prosthetic with a computerised knee joint. (Copyright Össur 2018)

“Wearing a computerised prosthetic knee means I can think of other things and enjoy my surroundings whilst I’m walking. I hardly ever fall,” she explains.

Verbal feedback from patients is important, but for more people to receive computerised prosthetic knees, then proof is needed to show that they provide a better overall functionality and quality of life.

“By comparing those wearing computerised and non-computerised prosthetics, we could show that a computerised prosthetic knee relieves the strain on brain activity. This is just one way that they can improve a person’s quality of life, but it’s important when you consider what the long-term effects of repetitive cognitive strain could be,” explains Saffran Möller.

There are currently 5000-5500 people in Sweden with amputations of a lower extremity (SwedAmp 2017), and about a third of those have undergone amputations on or above the knee. Computerised prosthetic knees cost on average 8 times more than a mechanical prosthetic.

“Computerised prosthetics are expensive. Guidelines for prescribing computerised prosthetic knees generally focus on how physically active a person is,” explains Saffran Möller. “In the surveys I carried out as part of my research, participants older than 66 years had not been prescribed computerised prosthetic knees. I feel that we shouldn’t really be looking at age, we should be looking at functionality and need – and this includes the need for good balance, to feel confident and safe when walking.”

Computerised knees – how they work

Prosthetic knees are designed to mimic the bending (flexion) and swinging (extension) of the anatomical knee as a person walks. The knee also needs to lock to give stability when the person is standing or supporting their body, and to secure the swinging forward when a person takes the next step.

Prosthetic knees using either a manual lock, a mechanical friction brake or hydraulic mechanism to control the brake and lock. Computerised knees have in addition an adaptive microprocessor system that continuously adjusts the mechanical properties of the knee to adapt to every individual need in varying situiations (for example, for altering speed or compensating for a stumble).

Saffran Möller