OSSEOINTEGRATED ARTIFICIAL LIMBS
Osseointegrated Artificial Limbs with Fine-Motor Control and Sensory Feedback
Nolan Cummins
The University of Texas at Austin
OSSEOINTEGRATED ARTIFICIAL LIMBS
Abstract
The human body automatically activates dozens of muscles in perfect tandem to perform
seemingly basic functions, making it the dream of biomechanical engineers to mimic this
functionality with mechanical systems for use in prosthetics. Until 2014, however, when
Swedish researchers successfully combined an osseointegrated prosthetic with epimysially
implanted electrodes to create a working, controllable prosthetic, artificial limb technology had
remained stagnant due to the lack of development in neurotechnology, particularly regarding
converting muscle activation into readable digital signals. Although their research did not
involve the invention of new technology, their utilization of preexisting findings created a
breakthrough in artificial limb development which will enable further improvements in
mechanical prosthetic functionality, clinical viability of artificial limbs, and a better quality of
life for those missing limbs.
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Table of Contents
List of Figures ................................................................................................................................. 4
Osseointegration vs. Socket-Suspension ........................................................................................ 5
Surface vs. Epimysial Electrodes.................................................................................................... 6
Sensory Feedback ........................................................................................................................... 8
Conclusion ...................................................................................................................................... 9
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List of Figures
Figure 1. Toward neural control of artificial limbs (Ortiz-Catalan et al., 2014). ........................... 6
Figure 2. Improved prosthetic control in daily living activities (Ortiz-Catalan et al., 2014). ........ 7
Figure 3. Tactile perception via neurostimulation (Ortiz-Catalan et al., 2014). ............................. 9
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Osseointegrated Artificial Limbs with Fine-Motor Control and Sensory Feedback
Since the early 1960s, slow progress in myoelectric control interfaces, which measure
electrical signals from the muscles, have left advancements in artificial limb technology stunted
(Li, & Felländer-Tsai, 2021; Ortiz-Catalan et al., 2014). That was true, however, until 2014 when
Swedish researchers successfully implanted an osseointegrated prosthetic with epimysial
electrodes into a patient with a trans-humeral (above the elbow) amputation that returned basic
sensory feedback and fine-control over the prosthetic, effectively mimicking a natural limb
(Ortiz-Catalan et al., 2014). This combination of osseointegration, epimysial electrodes, and their
uses in fine-motor control and sensory feedback has made mechanical prosthetics a practical
route to recreating the motion and feeling of able limbs for amputees.
Osseointegration vs. Socket-Suspension
Traditionally, prosthetics are held in sockets against the skin on residual limbs (Figure
1A). On the other hand, osseointegration involves directly implanting an abutment into the bone
near the amputation, transferring the load of the limb to the bone (Ortiz-Catalan et al., 2014). The
prosthetic is then screwed onto the end of the abutment, allowing for a greater range of motion
compared to socket-suspended prosthetics (Figure 1B). Furthermore, while constant use of
socket-suspended prosthetics can cause skin irritation and ulcers due to friction between the
socket and the skin, a study conducted between 1999 and 2007 on 51 osseointegrated amputees
reported a 92% prosthetic-success rate, or a generalized proportion of how well the prosthetics
performed, over two-years with their implants (Brånemark et al., 2014; Reiber, 1994). This
indicates that, compared to conventional socket-suspended prosthetics, osseointegrated
prosthetics avoid problems with soft tissue irritation, increase the range of motion, and shift the
weight-bearing to a more natural position, all of which improve the quality of life for amputees.
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Figure 1. Toward neural control of artificial limbs (Ortiz-Catalan et al., 2014).
Surface vs. Epimysial Electrodes
A key component to artificial limbs is the ability to control the mechanical prosthetic.
Typically, this is done using surface electrodes (sEMG) placed on the skin which measure
myoelectric signals in hertz, or the electrical signals emitted when a muscle activates. However,
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this results in two main issues: tissue
between the muscle and the electrode
weakens the myoelectric signals and
unsecured positioning of the electrodes
results in a lessened range of motion and
myoelectric crosstalk (Ortiz-Catalan et al.,
2014). If, as in the former issue, the signal
is weak (Figure 2C), then activation
requires a stronger stimulus from the
patient, which reduces prosthetic accuracy
(Figures 2D & 2E). Similarly, regarding the
Figure 2. Improved prosthetic control in daily living activities
(Ortiz-Catalan et al., 2014).
latter, since the electrodes are adhered to the surface of the skin, when a patient raises their arm
above 80° normal to their chest, electrical signals from the shoulder muscles interfere with the
intended muscle targets, often known as myoelectric crosstalk, and render the prosthetic
incontrollable; a similar result occurs when reaching too far down (Ortiz-Catalan et al., 2014).
These problems led scientists to look toward epimysial electrodes (eEMG), or electrodes placed
directly on the muscles themselves (Figure 1C) as an alternative solution.
Epimyisal electrode implantation had been around for decades but lost popularity to
difficulties at achieving long-term success. An early attempt in 1977 at implementing epimysial
electrodes with an artificial limb failed due to an infection in the percutaneous interface; the
leads to the electrodes penetrated through the skin and remained unsecured and unreliable (OrtizCatalan et al., 2014). Swedish researchers in 2014, however, worked around this issue by
inserting the leads through the bone, circumnavigating the skin entirely which reduces the
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possibility of infection (Figure 1B). This breakthrough allowed scientists to quantitatively
measure the advantages of using epimysial electrodes over surface electrodes (Figure 2). In sum,
epimysial electrodes are more accurate in measuring myoelectric signals (Figure 1C), which
allows for finer-motor control of mechanical prosthetics (Figures 2D & 2E), such as “handling
smaller or more delicate objects, [like] eggs,” and can remain in place for a near-permanent
amount of time without interference, myoelectric crosstalk, or electrode displacement due to
their embedment within and throughout the bone (Ortiz-Catalan et al., 2014, p. 3).
Sensory Feedback
One of the most challenging problems within the prosthetics field is “artificially
providing somatosensory information through neurostimulation,” or mimicking the ability to feel
in an artificial limb by sending electrical signals into the existing nerves (Ortiz-Catalan et al.,
2014, p. 4). Prior to the breakthrough by Swedish researchers, no long-term studies over six
weeks had been conducted on replicable methods to reproduce tactile perception in prosthetics
(Dhillon & Horch, 2005; Dhillon et. al, 2004; Horch et al., 2011; Ortiz-Catalan et al., 2014;
Raspopovic et al., 2014). However, aforementioned researchers managed to chronically
reproduce, or after eleven months, “repeatedly similar tactile perception [through] direct
electrical stimulation of the peripheral nerves” based on three factors: quality, or how accurately
the patient “was able to discriminate individual pulses,” magnitude, or how precise a stimuli the
patient could recognize, and localized projection, or where the stimuli were perceived on the
phantom limb (Figure 3A) (Ortiz-Catalan et al., 2014, p. 4).
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Figure 3. Tactile perception via neurostimulation (Ortiz-Catalan et al., 2014).
To conduct the study, alongside their osseointegrated prosthetic, the patient had three
electrodes distributed on the ulnar nerve (Figure 3B) that delivered a single, increasing pulsating
current until the patient reported perception, otherwise known as the stimulation threshold
(Ortiz-Catalan et al., 2014). The researchers discovered that, depending on which electrode the
signal was sent through, the patient reported stimulation at different stimulation thresholds
(Figure 3C) resulting in varying projections (Figure 3A). On the other hand, the results on
quality, where the patient was able to distinguish pulses up to 8 to 10 Hz, and magnitude, where
the patient described the smallest perception as “the tip of a pin,” coincided with existing studies
on “intraneural micro-stimulation,” or stimulating somatosenses through electrical signals or
other micro-stimuli (Ochoa & Torebjörk, 1983, p. 4). In sum, although it is unclear whether this
technology can be utilized to return complete or near-complete mimicking of somatosenses in
missing limbs, the results show that direct neurostimulation is a viable option for implementing
sensory feedback in artificial limbs.
Conclusion
In today’s rapidly developing era of microtechnology, the breakthrough by Swedish
researchers exposed the greatest fallacy of cutting-edge research. Prior to their discovery, the
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field of mechanical prosthetics—artificial limbs especially—had been stagnant for the past halfcentury. This is because, while both osseointegration and the notion of implanting electrodes
directly on the muscles have been around for decades, it takes enormous time and effort to attain
conclusive results in neurotechnological research. Their success was drawn from the fact that,
alongside new developments in neurostimulation research, the Swedish researchers used
concrete, preexisting findings in osseointegration and epimysial implantation technology in a
way never thought of before, making artificial limbs a medically viable alternative to typical
prosthetics and opening the path toward further developments in the mechanical prosthetics field.
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