Unexpected high-impact loading scenarios, such as a fall, pose a risk of possible bone fracture in patients with osseointegrated prostheses. Recent study results published in Clinical Biomechanics demonstrated how interface loads depend on amputation height in normal gait and falling, which may lead to improved prosthesis safety to protect against bending moments.
“In previous studies we found that severe loads act on osseointegrated prosthesis interfaces during falling. From an engineering perspective it was obvious that the amputation height plays an important role in this matter,” Michael Schwarze, Dipl. –Ing., of the Laboratory for Biomechanics and Biomaterials at the Hannover Medical School, Hannover, Germany, told O&P Business News. “Therefore, we wanted to quantify the amount of influence of the amputation height on forces and moments. In the long term, this is important to determine to which loads in a specific patient a safety element needs to be specified. But for this final decision we need to investigate the influence of amputation height on the strength of the remaining bone.”
Forces and moments
Schwarz and colleagues determined forces and moments at four amputation heights using a skeletal model of a transfemoral amputee using kinematic and kinetic data from an able-bodied participant who performed normal gait and mimicked three falling scenarios of amputees. Researchers selected two forward falling scenarios, one from a standing position and one during gait, and a single backward-falling scenario. Each task was repeated at least six times with distinct force plate strikes and all falling scenarios were self-induced by the participant.
Study results showed resultant forces were not affected by amputation height in the three falling scenarios, while ground reaction forces were higher in all scenarios compared with peak resultant forces at the prosthesis fixation, independent of amputation height. Ground reaction forces were also about 5% higher compared with resultant forces at the lowest amputation height during gait, according to study results.
For the two forward-falling scenarios, researchers found resultant moment increased as amputation height increased. Researchers also found the initial peak moment was greatest for the upper amputation height in the backward-falling scenario. However, the moment for the remainder of the motion was greatest for the lower amputation height.
“It should be noted that bending moments of level walking are of similar magnitudes as those during a forward falling scenario, for a long residual limb. This might indicate a general reduced risk of fracture for long amputation lengths, when one assumes that ultimate bending strength is not reduced proportionally to the length of the residual limb,” the researchers wrote. “Future work is required to determine the relationship between amputation height and maximum bending strength, as well as the influence of patient anthropometry. With such information, final conclusions will be possible relating to the factor of safety for certain amputation levels.”
One complication with osseointegration is the risk of bone fracture after unexpected high-impact loading scenarios, such as a fall. Previous studies have shown utilizing safety devices, such as the Rotasafe (Rota Val, Rielasingen, Germany) and the Axor (Integrum AB, Molndal, Sweden), help protect against torsional overloads and bending moments. However, according to the researchers, the load limits of the safety devices must be appropriate for each patient and, since amputation height varies from patient to patient, implant interface loads remain unknown.
“Devices protecting solely against torsional overload need not account for amputation height. However, devices protecting against bending moments, such as the Axor, should have their release limits set based on the amputation height of each individual patient,” the researchers wrote. “Additional research is required before this personalization can occur, both to determine the ultimate bending strength at a variety of amputation heights, and to determine the precise influences of body mass and height on bending moments at the implant interface.” — by Casey Tingle
Disclosure: Schwarze has no relevant financial disclosures.