For nearly 40 years, SynboneVR has developed synthetic bones used in biomechanical research as a way to offset some of the cost and risk of using biological bone. Biomechanical engineers continue to rely on these synthetic bones as standards for comparison in their own orthopedic design work. However, there is no prior research that summarizes the mechanical properties of these synthetic bones in order to understand their general performance, or how well they represent biological bone.
“Many researchers use these artificial bones in their research, but there is little information about how well they represent real human bones,” said Radovan Zdero, professor of biomechanical engineering at Western University in London, Ont.
To investigate this more thoroughly, Zdero and his research partners—Aleksandar Djuricic and Emil H. Schemitsch of the Orthopedic Biomechanics Lab at Victoria Hospital in London, Ont.—launched a literature search to delve deeper into the mechanical properties of synthetic bones. Their search was successful in finding relevant data on ballistics, dentistry, orthopedics, and materials science testing that used samples of SynboneVR bone as surrogates.
The study was published in the ASME Journal of Biomechanical Engineering.
“Our review of this data, pros and cons, and future work recommendations will hopefully assist biomechanical engineers and physicists who use these synthetic bones as they develop experimental testing regimes and computational models,” Zdero said.
SynboneVR has been creating synthetic bone, as well as bones in basic anatomical shapes, for medical education for decades. These bones are made from polyurethane of various densities and architectures to simulate hard “cortical” and spongy “cancellous” bone.
“The first challenge was to find all the relevant articles we wanted to survey and present in a very practical and useful way to potential readers,” Zdero said.
Research papers were only included if they characterized mechanical properties of intact synthetic bones that were previously unreported, validated mechanical properties of intact or implanted synthetic bones against biological bone, and optimized mechanical properties of intact or implanted synthetic bones to make them more realistic by varying their geometry or material.
The team was interested in three characteristics of the synthetic bone. One was a comparison of tissue-level mechanical properties of synthetic versus human bone. Another was the organ-level mechanical properties of implanted synthetic versus human bones. And the team tracked screw pullout results for synthetic versus biological bone.
Zdero, Djuricic, and Schemitsch conducted transverse and longitudinal tests to evaluate elastic modulus, plastic yield stress, ultimate stress, and tortional stress (where appropriate). Metal screw insertion and pullout tests were also conducted, measuring peak torque at stripping, peak axial force at stripping, and peak force at pullout. Ballistics tests were done by propelling bullets against synthetic shapes.
The researchers also compared the performance of synthetic bone according to types of bone shapes, including mandible, humerus, ulna, pelvis, femur, tibia, and calcaneus.
A Challenging Undertaking
Comparing mechanical results between synthetic bone and biological bone was extremely difficult to do because of the differences in test set-ups, loading protocols, bone preparation techniques, investigation goals, and reported results that were used in the older studies.
“It was surprising that some of these artificial bone models are very poor representations of real bone, while others are excellent representations of real bone,” Zdero said. “In addition, we found that the mechanical properties of some of these artificial bone models have not been studied at all nor validated.”
This variability would have been minimized if the researchers had used standard protocols relevant to biomechanical studies from organizations like the American Society for Testing and Materials and International Organization for Standardization. The differences in research approaches made it difficult to derive meaningful conclusions from comparing synthetic bones to biological bones.
“Reasons for this may be the large variation in the geometry and properties of biological bones, lack of consistent reporting about the quality of biological bones, and different ways researchers prepared, mechanically tested, or computationally analyzed synthetic or biological bones,” Zdero said.
A clear result from the data analysis showed that synthetic bones almost always had a much smaller coefficient of variation versus biological bones. This shows that the manufacturing methods used to create synthetic bone had high consistency, while simultaneously showing the high inter-specimen variation common among biological bones.
“Consequently, this attribute of these synthetic bones is compelling to most investigators who wish to reduce confounding variables that may affect statistical comparisons between groups,” Zdereo said.
The researchers thought more biomechanical properties should have been tested in the past synthetic bone studies that they had evaluated. To add value and reliability to future research, Zdero recommends that more tissue-level properties such as tensile and shear modulus and strength, fracture toughness, friction coefficient, fatigue life, and hardness be evaluated.
Few of the papers optimized the mechanical performance of synthetic bones by altering their geometric or material parameters. In general, no particular geometric or material variable can change synthetic bone performance to perfectly match biological bone.
“However, successful optimization usually involves adjusting geometric and material parameters at the same time,” Zdero said. “In this regard, experimental studies plus finite element models could be design tools for concurrently investigating the effect of numerous variables.”
Zdero and his colleagues are the first researchers to compile and interpret the artificial bone properties provided by SynboneVR products. “Our work has outlined specific practical ways to improve the mechanical properties of these artificial bones for future researchers,” Zdero said. “We also hope that the SynboneVR company will improve on the mechanical properties of their artificial bones and that biomedical engineers will perform validation studies on those artificial bones that have yet to be evaluated.”
Mark Crawford is a technology writer in Corrales, N.M.
