Evolution in Design of Integral Leg Prosthesis Leads to Improved Outcomes

The rate of infection among patients with transfemoral amputation who received percutaneous Integral Leg Prostheses through the process of osseointegration was significantly reduced during a 15-year period with the evolution of the device designs over time, according to a study published in the Journal of Rehabilitation Research and Development.

In the retrospective cohort study from January 1999 to December 2013, 69 patents with transfemoral amputation underwent osseointegrated skeletal docking of their artificial limb in Germany. All surgeries were performed by Horst H. Aschoff, MD, a surgeon at the Clinic for Plastic, Hand and Reconstructive Surgery at SANA Kliniken Lubeck in Lübeck, Germany and senior author of the study.

First study of its kind

The purpose of the study was “to analyze the results of the evolution of three separate device designs and two different surgical techniques,” according to James P. Beck, MD, adjunct professor in the Department of Orthopaedics at the University of Utah School of Medicine and Department of Veterans Affairs Salt Lake City Health Care System in Salt Lake City and a co-author of the study.

Beck noted this type of study could not have been conducted in the United States during this time interval.

“In Europe, they have a different system. They do not have quite as much regulation, so they started doing [osseointegration] a long time ago. In the United States, there have been some implants done – not this particular one, but different designs essentially on a humanitarian basis,” he said, noting the United States has been in the stage of FDA approval that required translational animal models prior to clinical application of implant devices.

The original device used the Spongiosa-Metal 2 from Orthodynamics GmbH in Lübeck, Germany to achieve osseointegration. Changes in device design and surgical procedure were all made by Aschoff over time as he became more familiar with the device and the procedure.

“The problem with all percutaneous devices has been the rate of infection at the skin/implant exit and that varies with implant designs,” Beck said. “In Dr. Aschoff’s initial device designs, [patients] had a fairly high rate of infection. Fortunately, the devices were modular in design so that the porous portion that was exposed to overlying soft tissue could be removed if necessary.”

As described in the study, design A (the original design) was comprised of two main components: the endomodule and the skin protruding “bridging connector,” the rod connecting the medullary implant to the prosthetic limb. Both components were made with a medical-grade cobalt chrome alloy. The bracket and distal surface of the shaft of the prosthesis were coated with a rough, textured surface intended to aid the attachment of the device to the skin.

“Initially [the creators of the device] thought this would allow the skin to grow into it, but instead it acted like a rasp and abraded the skin. And those became infected,” Beck said.

For design B, Aschoff removed the rough surface from the distal portion of the implant. The osseointegrating surface in the femoral canal remained the same. He also made the “bridging connector” smaller and more polished. The diameter of the bridging connector was reduced, but the length of the connector from implant to stoma stayed the same.

For design C — the most recent design — Aschoff removed the abrasive bracket and made the soft tissue channel wider than the bridging connector and decreased the depth. He also coated the connector and the bone-capping portion of the implant with a nonabrasive titanium niobium oxynitride ceramic.

“The ceramic is more easily cleaned. It allows drainage from the stoma,” Beck said.

Osseointegration is performed with two separate and different surgeries, conducted 6 weeks apart. In the first-stage surgery, Aschoff revised the residual limb through debulking and optimizing the soft tissues of the stump and after retrograde reaming of the femoral shaft, implanted the endomodule, capped the end of the implant and closed the skin wound. For patients with the final implant design, Aschoff also thinned the subcutaneous fat to a depth of 2 cm. The 6-week interval between surgical stages allowed the wound and soft tissues to heal and establish an improved blood supply and the endosteal bone to grow into the implant surface. During the second-stage surgery, Aschoff created a soft tissue stoma and attached the skin-perforating bridging connector to the endomodule. For patients receiving the final design, Aschoff created a larger stoma to allow drainage of tissue fluids.


Overcoming infection

Aschoff and colleagues categorized patients into two groups: group 1 patients (n=30) underwent osseointegration between 1999 and 2008 and received either design A or design B, while group 2 patients (n=39) underwent the procedure between 2009 and 2013 and received design C. A total of 242 operative procedures took place, including 69 two-stage surgical implantations and 104 interventions including stomal revision for infection and removal of modular components.

The results showed only patients in group 1 needed reoperations or revisions due to infection. Among group 1 patients, 77% (n=23) underwent surgical interventions secondary to infection. Statistical analysis showed a significant difference in infection and surgical intervention (P < 0.001 for both) between the groups. At the conclusion of the study, the infection rate following implantation of design C devices remained at 0%, an absolute risk reduction of 97%.

Aschoff has since performed the procedure on an additional 31 patients, almost all of whom have remained infection-free, according to Beck. A forthcoming publication will include the overall results of all 100 patients.

The researchers hypothesize the reduced risk of infection is due to the nonabrasive ceramic coating applied to the bone-capping and stomal exit portion of the implant in design C. The coating is believed to have prevented the abrasion between the soft tissue of the inner lining and the bone-capping portion of the implant seen in the first two design iterations, according to the study.

Beck said Aschoff advised patients with design C implants to cleanse the exit surface twice daily with a hand-held shower using mild soap and a soft shaving brush, and to avoid the use of antibiotics.

Promising alternative

Beck said osseointegration offers a number of advantages as an alternative to a traditional socket suspension prosthesis, described in the study as an “exoprosthetic attachment.”

“Skeletal docking of prosthetic limbs obviates the complications of sockets,” he said. “These well-known complications include limited wear time, skin abrasion, infection and discomfort because of heat, stump swelling and poor and difficult fitting in the multiple limb amputee with short residual limbs. All of these difficulties are eliminated with this type of suspension directly through the skeleton.”

Beck said the procedure is particularly beneficial for patients with short residual limbs, a group he noted includes many veterans. If the residual limb is too short to attach a socket suspension system, then many transfemoral amputees can become wheelchair-bound, he said.

Osseointegration also can make bilateral amputees more comfortable, as “they can walk and sit without the two sockets bumping into each other particularly in the groin.”

Some patients may still need to use a cane or other walking device, Beck said, but “it lets them wear the limb all day long, it lets them be more active [and] it lets them be up walking if they have lost both of their legs.”

Although its benefits have not yet been quantified by Aschoff’s study, osseoperception can be another advantage of osseointegration.

“Bone has nerves that help us understand the position the bone is in in space and whether we are bearing weight,” Beck said.

The direct attachment of the device to the bone through osseointegration allows the patient to experience tactile sensation.

“When the terrain changes they can feel the difference,” Beck said.

The main setback for widespread use of the device has been the rate of infection, Beck said. About 50% of the original devices caused an infection in patients after implantation.

“Infection was the main risk associated with osseointegration,” Beck said. “Aschoff has gone a long way to prevent that now. It took 15 years and it took three device designs, but I think the evidence is extremely strong.”


Next steps

Beck and his colleagues at Department of Veterans Affairs Salt Lake City Health Care System plan to continue this research by finding out why new devices, such as Aschoff’s, have a markedly reduced infection rate.

“We think [the limb] is not becoming infected because it is easier to clear bacteria from the stoma and they do not have a good place to attach to, also the bone end is capped so [the bacteria] have difficulty entering the inside of the bone … but we have not been able to scientifically prove it,” he said.

In a separate study using a similar device, Beck and colleagues plan to analyze the colonizing bacteria that normally reside at the stoma but do not cause infection. This microbiome study is intended to determine whether the bacteria around the stoma are commensal, or “friendly,” bacteria.

“You can culture only about 3% of bacteria that are in any location,” he said. “Using polymerase chain reaction, we can analyze what bacterial DNAs are at the stoma and identify most of the bacteria present but unseen by culture… So, over time, we can measure the microbial ecology of the limb and stoma and hopefully see if there is a way to optimize good bacteria at the stoma; those that would keep the bad bacterial pathogens out.”

Beck said Aschoff’s results could help move osseointegration toward an FDA approval more quickly.

“This is the first English [language] publication of his work. I think this will make it a lot more likely that this device will be of use in the United States, especially when we add the remaining [results] from 2013 to 2016 for the additional 31 patients he has done,” Beck said.

He added, “In the next 2 years, various European and American systems of skeletal docking of artificial limbs are going to become more and more visible in the United States.” – by Amanda Alexander

Disclosures: The researchers report no relevant financial disclosures.

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