Limbitless – Limbs of the Future

Humans have been losing limbs since the dawn of time, be it from an altercation with a sabre tooth tiger or a YouTube fail video. Historically, battle wounds incurred during major wars have been the primary cause of limb loss and amputation. These days, vascular diseases such as diabetes are the leading cause of limb loss through amputation, affecting both men and women throughout the community.

The loss of a limb is a life-changing event, that bears significant disability, not only affecting the individual but their family and the wider community as a whole. Throughout history, we have tried to overcome such disabilities by replacing limbs with prostheses. The oldest record of prosthesis use comes from ancient Hindu texts Rig Veda, describing the story of a Warrior-Queen fitted with an iron leg, allowing her to return to battle; first published between 3500 and 1800 B.C. The earliest physical evidence of a prosthesis comes from Circa 710 B.C; a big toe made of cartonnage (an ancient Egyptian form of paper maché) found on a female mummy in Egypt. The fact that the toe was included in the mummification indicates that the toe was more than a functional replacement for a lost toe, but may have been a spiritual endeavour to enter the afterlife “whole”. This toe highlights the importance of not only replacing the function of lost limbs but also providing the person with a feeling of completeness and self. So where do we stand today, as far as replacing the function and feeling of wholeness for those who have lost limbs?

Prostheses still represent the most widely applicable method for limb replacement globally. Furthermore, prosthetic technology has moved in leaps and bounds over the last decade, to now encompass a range of robotic upper and lower limb replacements capable of replicating many of the functions of the limbs they replace, including a sense of touch. The current limitation to the widespread adaption of this technology is the ability to integrate the robotic prosthesis with the biological tissues of the limb stump, particularly nerves that control the limb, to enable natural control of the prosthesis. America is currently the world leader in developing this novel technology, which is likely to become a widespread reality within the next decade or so.

Limb transplantation is now a reality that serves to replace an amputee’s limbs with that of a deceased donor. Over 100 hand and upper limb transplants have been performed globally, with excellent patient outcomes and high graft survival. However, as with any transplant, the risk of graft rejection is a distinct possibility and so lifelong immunosuppression is required. New technologies to reduce the incidence of rejection without the need for immunosuppression is a high priority for many nations but remains elusive due to the complexity of the immune system. One distant option is to completely remove the cells from a donor limb and repopulate the limb with the cells of the recipient; ultimately avoiding detection of the limb as foreign by the immune system. While this has been shown to work in mice, the number of cells required to repopulate a human limb (or part thereof) is significantly greater, which represents a significant hurdle to achieving applications in humans. None the less, limb transplantation is entirely possible, and likely to become more frequent as our ability to modulate the immune system of both the donor and the recipient advance.

Limb regeneration is one of the most desired methods for replacing lost limbs; however, it is also the least feasible. Several species have the ability to regenerate lost limbs; such as the Salamander and are thoroughly researched for their ability to do so. Unfortunately,
mammals lack the ability to regenerate. There is a long-standing debate amongst surgeons that babies operated on while still in utero do not form scars and hence have the capacity to regenerate, which is somehow lost as we develop. Nonetheless, there is some evidence that adult mammals can regenerate very small amounts of tissue, such as fingertips under the right conditions. What constitutes these ideal conditions are yet to be discovered. Electrical stimulation of amputated rodent limbs has shown an ability to enhance regeneration ever so slightly, but significantly. Just how electrical stimulation would affect the human ability to regenerate, is yet to be studied.

As these collective technologies advance, both independently and collaboratively, we are likely to be witness to many of these exciting advances in limb replacement. For the time being, prosthesis remains the most widely applicable technology for replacing limbs. As the new age of robotic prostheses dawns, complete with neural control and the ability to experience touch, we may ask the question, do we even need to replace our limbs with tissue, or can we improve the human condition past the limitations of this meat sack that houses and protects our brain?

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