Direct Skeletal Prosthetic Attachment
by Zach Harvey CPO
As a prosthetist, I strive for the perfect socket fit and design. However, even when I’m at my best and my patients keep coming back in for adjustments, as I want them to, the comfort level and wear time is merely a product of residual limb presentation, patient motivation, my skills, and the technology that’s available. Our combined effort leads to an optimal outcome, as good as we can get. Adjustable sockets and interfaces are continuing to develop (see my April, 2016 blog ), but what if “as good as it gets” could become “much better?” What if we could bypass the socket altogether?
Transcutaneous (through the skin) Osseointegration (OI) refers to the firm anchoring of a surgical implant (as in dentistry or in bone surgery) by the growth of bone around it. Archeological evidence of OI dates back as far as 4000 years ago from China. Human remains were found with carved bamboo teeth pounded through the gums into bone. In the early 1950’s, Swedish professor Per Ingvar Branemark began conducting experiments on the legs of rabbits using titanium implants and discovered that bone grew directly into the implant, making it difficult to remove. This unintended discovery later led to stronger implant designs. In 1965, the first humans received dental implants directly attached to bone. Osseointegration is now commonplace in dental/craniofacial reconstruction, bone anchored hearing implants, and joint replacements.
So, why not try OI with an amputated limb? After all, prosthetic sockets are hot, sweaty, and range from uncomfortable to painful. Plus, the residual limb moves around inside the socket. Sockets can be difficult to don and sometimes don’t stay on very well. Volume fluctuations can easily change the fit of the socket, causing a risk of skin breakdown. High socket trim lines are sometimes necessary, but may restrict joint range of motion. Wouldn’t it make sense to directly connect the prosthesis to the skeleton? The short answer is yes, and yes it’s being done. That said, there are risks associated with the procedure. However, before we go into the risks, it’s beneficial to have a basic understanding of the history of this procedure as it relates to amputated limbs.
In 1990, Per Ingvar Branemark’s son, Rickard Branemark, MD, PhD, performed the first transcutaneous OI on a person with transfemoral amputation. Starting in 1995, this procedure started to be used clinically in Sweden, Germany, and the United Kingdom. Each country’s implant design was slightly different, but they incorporated a similar two stage procedure in which a fixture was surgically placed inside the intramedullary rod of the femur and then the skin closed to allow healing. An abutment for use with a prosthesis is later attached through the skin to the fixture. A fail-safe mechanism is used in between the abutment and the prosthetic knee in order to protect the fixture from the risk of failure. This mechanism would break away before the implant in the case of a fall. Fast forward to today, and OI is being performed in a few more countries throughout the world for various amputation levels, including arms, thumbs, and fingers. The procedure has been somewhat slow to adopt in the United States due to long rehabilitation times and studies reporting high infection rates, some requiring implant removal. However, newer implant designs could address this problem and there are a few centers within the United States that are starting to perform OI surgeries. The Veterans Administration in Salt Lake City is currently conducting a study with 10 veteran amputees and carefully measuring outcome data1. This will lead to 300 additional surgeries to include other surgical centers across the nation within the next 3-4 years.
One benefit of OI is osseoperception, which is the feeling and proprioception (spatial awareness) gained through OI. This feeling is helpful, taking away cognitive demand and reliance on eyesight when walking, in the case of a lower limb amputee. This is also thought to attribute to reduction of phantom limb pain. Osseoperception is not present with a prostheses that incorporates a socket and it’s obvious why this is the case when we look at how much movement occurs with the bone and soft tissue in relation to the socket. This movement also translates into increased energy expenditure and an increased need in compensatory gait mechanics. In other words, it’s harder to walk symmetrically and it takes more effort. For both lower and upper extremity prosthesis wearers, there is decreased range of motion and more effort required due to movement of the residual limb inside the socket. Pistoning (up and down movement) causes wasted energy as well as friction against the skin. With OI, wherever the bone moves, the prosthesis follows. Research has shown an increase in walking speed, quality of life, function (K-level), prosthetic wear time, and mobility following OI.
As mentioned above, however, there are some risks associated with OI. In a study out of Sweden2 published in 2009, 100 patients with transfemoral OI surgery had about a 30% infection rate with 68 patients still using the implant. Similar infection rates were reported out of Germany. A standardized 18-month rehabilitation protocol, called OPRA, was developed in 1999. This prolonged timeframe is certainly a drawback to the procedure, although the timeframe has now become more customized. An Australian group3 has published results of an accelerated rehabilitation protocol of 4+ month following surgery with good overall results, but with 27/50 patients experiencing adverse effects such as infections and fractures due to falls. Some of the goals of the Salt Lake City VA study are to assess bone remodeling over time, and to reduce the number of surgeries and amount of rehabilitation time. Traditionally, patients who are too active may not be good candidates for OI because high impact activities, such as running, are not permitted. With better implant designs and better understanding of the strength at the implant at the molecular level, we will be able to predict how much impact a device is capable of withstanding. Hopefully, this new understanding will lead to the abiity for individuals with OI to engage in high impact activities.
At Presbyterian St Luke hospital, where I work, Dr. Ronald Hugate, MD will be performing the first percutaneous OI one-stage procedure on two patients with his unique design. As an engineer and orthopedic oncologist, Dr. Hugate has spent a number of years designing and testing this method in partnership with universities, veterinarians, and a company called Zimmer Biomet. The design incorporates trabecular metal with porosity designed for bone, muscle, and skin to integrate with, thus sealing off the abutment. This could be a very big advancement with regard to infection risk. One patient who will be having this one-stage OI done has bilateral transfemoral/knee disarticulation amputations, and will be receiving the implant on his shorter side. The other patient, Jace, has a unilateral transfemoral amputation. I’ve known Jace for almost 10 years and worked with him periodically at Walter Reed trying every kind of socket design to keep up with his active lifestyle. He deployed back to combat in intelligence after amputation and has since then retired from the military. It’s exciting to know that this procedure may help both patients become more mobile.
Osseointegration is not likely to replace prosthetic sockets overnight. Even as methods and technology advance, not everyone will be a candidate. For example, someone with past infection risk, poor vascularity, or poor bone density would likely not be a candidate for OI. I see OI as a tool to advance mobility and quality of life, especially beneficial for those people with amputations who are suffering from few prosthetic options or struggle with getting a comfortable fit. Examples are very short residual limbs, large amounts of soft tissue mass, fingers and thumbs. With stronger implant designs and techniques to reduce the risk of infection, I see this as a viable option for a select patient population. I’m excited to see how this progresses and am optimistic about the future!