A prosthesis is an artificial device that can aid or even replace an impaired or missing limb. The field of prosthetics has experienced a rich and storied evolution, dating back to Ancient Egyptian peg legs and hand hooks (circa 1500 BC). Though prosthetics have come a long way since, what remains is their impact.
The word “prosthesis” itself is rooted in the Greek words “pros” (in addition) and “tithenai” (to place). Prostheses are either placed externally or are implanted within the defective body part. Typically, a team of healthcare professionals chooses the “right” prosthesis and guides their patient through a rehabilitative phase of learning how to live with their new artificial device. Each patient’s reason for needing a prosthetic may vary, but it could involve:
- Birth defects
- Trauma (i.e. auto accidents, military combat)
- Circulatory problems from diabetes or atherosclerosis that culminate in an amputation
The process of building a customized prosthetic limb has become increasingly refined over the years. Tailoring a prosthetic device calls for careful consideration of the patient’s body measurements, muscle / bone remnant (if any), lifestyle and activity levels, and the overall level of impairment. Since prosthetic limbs are often left to assume the complex roles left by missing muscle and nerve, scientists are tasked with creating more innovative prosthetics.
New prostheses have become so advanced that we are now able to implant a tiny device into the brain of a paralyzed patient that sends thoughts and messages via email. It is no surprise that brain-computer interface (BCI) has become an area of interest for creators and patients, alike. A recent neuro study involving rhesus macaques inserted microelectrodes into the primary somatosensory cortex of their brain. The microelectrodes were able to record the activity pattern of their neurons and transmit to prosthetic sensors that guided the motion of the prosthetic hand. Studies with human subjects take a longer road, but with the announcement in May 2017 of a bionic hand that “sees and grabs” objects automatically it seems we are headed in a very positive direction.
Finely-tuned sensors are the new field of interest within prosthetics. Much of the research in this area has focused on using EEG signaling to perfect sensory feedback in BCI technology. For example, its goal is to not only send a signal from the brain to the robotic arm but also from the arm back to the brain.
A variety of materials can be used in the making of artificial limbs. Much of this depends on their destined function(s) and placement. Wood is the most popularly used material, globally, as it is accessible, light and resilient, and easily shaped. Aluminum alloy is used when lightness is desired, metals for reinforcement and control, and plastic is valued for its flexibility. Silicone and polyurethane are also commonly used.
When crafting prostheses that will be placed near a patient’s joints (i.e. knee), another consideration to make is the effect surface, shape, and geometry will have on the coefficient of friction of the artificial implant. This is an important element to account for because joint mobility (the space where two bones meet) relies on smooth, frictionless movement to protect our highly-innervated bones.
Powering the Artificial Limb
There are several ways to power an artificial limb. Most are powered by the residual muscle left on the affected limb, which can strengthen with physical therapy exercises involving springs, gears, locks and levers. A patient may also move an artificial limb by handling a series of switches. An even more advanced way is via myoelectric control; when muscles contract, they create electrical signals that are detected by small electrodes on the artificial device, which engage movement. Any one action may involve a sequence of controls. For example, in order to grip an object, a patient may need to contract a muscle and then use a switch to bend their elbow joint.
Examples of Prostheses
Prostheses come in all forms and are really customized to meet each patient’s needs. While neural prostheses may be indicated for patients with spinal injuries or paraplegia, others may need prosthetic knee systems or feet. We will discuss a few topical examples of prostheses.
Penile prostheses are indicated for patients with erectile dysfunction. The simplest type contains malleable rods that are implanted within the erection chambers to assist with penile erection. Newer options include hydraulic inflatable prosthesis that allow an erection that is more natural and easier to conceal.
Breast prostheses are commonly used by patients who have had mastectomies. Unlike penile prosthesis, breast options can be less technical. External breast prostheses can be as simple as using cotton or silicone gel padding in mastectomy bras. Transgender patients that are pre-hormonal are also known to create the illusion of breasts with skin fleshed padding/prosthesis. Likewise, women with asymmetric breasts or tubular breasts may incorporate teardrop prostheses into their wardrobe. Of course, a common type of “prosthesis” are the saline and silicone breast implants that are used for cosmetic enhancement.
An ocular prosthesis, or “artificial eye,” is a type of implant that is recommended following evisceration (the “jelly like” contents of the eye and cornea are removed) or enucleation (removal of the entire eye). Though it is traditionally referred to as a glass eye, ocular prostheses have a convex shape. Some reasons for why an eye may be removed is due to injury, glaucoma, severe eye infection, or eye tumors. Unfortunately, prosthetic eyes do not restore vision so they are mainly used to improve the appearance of the affected eye socket. They are generally made of hard plastic acrylic.