The future of permanent, fully integrated prosthetic limbs and bionic implants

Despite many incredible advances, the functionality of prosthetic limbs remains limited. There's no mystery why any kind of arm or leg that you strap on to the soft exterior surfaces of your body -- and remove just as handily -- will always remain a foreign contrivance with only mortal power. In order to wield any artificial limb with full strength and confidence we are going to need to plug it in properly, so that it becomes a real part of our musculoskeletal system. Researchers at the Royal National Orthopedic hospital have now created an implant that does just that by interfacing a leg prosthesis directly to your endoskeleton.

Bighorn sheep ram their heads together with impact forces exceeding 3400 newtons. Imagine if these guys, or perhaps giant elk, had to torque each other about on antlers held in place only by a cup and harness. Nobody would get the girl. Prosthetic designers know this and have finally begun to do what has to be done. A technique known as osseointegration was initially developed(Opens in a new window) [PDF] by various researchers, primarily to bond titanium implants to bone in the arm. The grand view is that once bone-implant continuity is achieved, the groundwork is there for overlying muscularization, sensory investment, and nervous motor control to be extended to the new machine-organ.

The realization that artificial arms strong enough to walk on are not the major design point has led to the leg becoming the new driver for widespread realization of the technology. The hugely successful Flex-foot, made famous by double-amputee Oscar Pistorius, demonstrates that the material construction of the implant itself is not the limiting factor in design or performance. Properly securing a Flex-foot that is required to absorb and deliver Olympic forces requires several hours of assembly and fitting. It no doubt is also unbearable to wear it longer term, even when not under load. Now infamous for other reasons, Oscar (pictured right) did not have the ideal implant on hand when the time came to stand up to an intruder.

The prosthetic leg recently implanted in medical trials by the Royal National researchers was developed by Stanmore Implants(Opens in a new window). It calls its device the ITAP (Intraosseous Transcutaneous Amputation Prosthesis). The inspiration for it came from a curious paper published some time ago in Journal of Anatomy(Opens in a new window) titled "Nature's answer to breaching the skin barrier." It describes the innovations used by mammals to create a strong and antiseptic bone-to-skin interface -- in other words, antlers. The researchers dissected the subcutaneous antler bone of red deer -- 20 of them actually -- and they found that they have highly porous geometry. This enables the surrounding soft tissue layers to grow directly into the bone where it can be stabilized.

Even the strongest soft-to-hard interface will eventually be compromised if it is not impervious to bacteria and viruses. As we know, skin breaches, even in the dry places like under your nails, are uniquely susceptible to infection. Interfaces that are moist, such as the gums or eyes, require extra accommodations and immune surveillance to keep them secure. By mimicking the antler construction, researchers were able to design implants that can form a tight seal with the surface and deeper level tissue and therefore keep infection out.

Stanmore Implants' main line of business is making products for internal fixation of bone that has been compromised by injury or cancer. Its experience in designed devices that incorporate HA to control bone growth makes it well-poised for the trans-skeletal (transhuman?) device market. In addition to the new ITAP implants, it has also developed an intriguing space-age method for elongating bone. The movie below shows how its "extendible" prostheses implanted into long bones works. It uses an integral 12000:1 reduction drive that is electromagnetically lengthened by the remote force of an external rotating magnetic field, without the need for additional surgery or anesthetic.

On the edge of permanent bionic implants

In its initial incarnation, direct-to-bone devices will be able to automatically detach when overloaded, much like a ski boot binding. A catastrophic or unknown failure mode, as might occur during a fall, is thereby removed from the equation by designing in a controllable weak point. It also makes for a convenient way to swap in different kinds of implants. The acid test of for any implant is that the wearer completely incorporate it into their body schema; meaning that they would probably have some kind of a phantom-limb experience(Opens in a new window) in its absence. If you remove your implant every night before bed, and need to look to determine whether you have a golf club or cross bow mounted currently on end of it, you are probably not there yet. The total package hinted to above complete with muscle and nerve is more likely what will be needed before designers make permanent osseointegrated limbs, and those may still be a long time coming.

That's not to say we won't eventually see other biological reindeer games used to help implants take hold. For instance, stem cells or even simple growth factors like bone morphogenic protein can play a role in augmenting regeneration. Recently it was discovered that humans have some capacity to regrow new ribs. In one patient, CT scans revealed that eight centimeters of missing bone and one centimeter of missing cartilage grew back in six months. Using stem cells from an area around the rib cartilage known as the perichondrium, it may be possible to generate new bone for limbs(Opens in a new window).

Real antler growth incorporates both endochondral and intramembranous ossification. These are two modes of bone growth that everybody learned about in biology which occur during fetal development. In deer, antler growth and shedding of the velvet layer are linked to various changes or surges in specific testosterone pathways. Hormones are not the whole story, and the many other regulators of the process await discovery. Knowledge of how other species create unique bony devices should also not go unnoticed. It was recently suggested, for example, that the "tooth" of the narwhal is not really a tooth at all but rather an antler that has been redirected to the front of the head.

Facial topology is a frequent plaything of evolution. The same kind of genetic, or even extragenetic, rejiggering that the cetacean has used to convert nostril to blowhole may or may not be relevant to antlers and teeth. It seems clear though at this point, that as far as extending the capabilities of any hardware that is added external to the body, equal effort must also be made preparing the internal to accept it. For most folks the first trans-skeletal implants may not be fancy limbs, but rather simple attachment points for external electronic gear.

Google Glass wearers may initially want just a simple tapped hole or threaded stud, but likely will be asking for more. Eventually we could see people leaving the tattoo parlor with brand new biocompatible studs covered by a thin layer of protective velvety skin. Those in the know would realize the velvet would soon be shed after taking a brief course of androgen supplements.