When it comes to treating hand and finger fractures, early rehabilitation is the key to avoiding joint stiffness and loss of mobility. Therefore, it is crucial that fracture fixation implants are strong enough to maintain alignment of the bone fractures during controlled flexing exercises. But how strong is strong enough? What are the loads exerted on a fractured bone during these exercises? There are papers in the literature that aim to externally measure the forces required to do every day exercises, such as opening a jar. But it is difficult to predict the actual bending load that the fixation implant experiences during a rehabilitation exercise. This challenge is addressed in the latest paper from the BoneFix consortium.
Led by Dr. Peter Schwarzenberg at AO Research Institute Davos (ARI), with contributions from the RegionH, KTH and BMB teams, the paper describes an experiment to determine the bending force exerted on an osteosynthesis of the proximal phalanx during a simple fingertip-to-palm exercise. Fractures with gaps were made in the proximal phalanges of human cadaver specimens, which were then fixated by plastic implants. The tendons of the phalanges were then pulled, causing the finger to flex and the plastic implant to bend. Flags attached to the implant allowed the bending to be monitored with a motion-tracking camera. Since the mechanical properties of the plastic implant were known, the value of the force could be determined from the extent of the bending using finite element (FE) models of the implants. The paper revealed that the average bending moment experienced by the implant during the fingertip to palm exercise was 6.78 ± 1.62 Nmm.
In addition, the team fixated proximal phalanx fractures, both with a gap and without, in the hand specimens with the same composite and screws technique used in the BoneFix project. The fixations were then subjected to the same fingertip-to-palm exercise. In all cases, the composite based fixations survived the exercise, even those maintaining a fracture with a gap. This suggested that the fixations could withstand the forces required for this simple rehabilitation exercise. The composite approach has several advantages over traditional metal plating, since the composite has been shown to not induce soft-tissue adhesions and the customizability and in situ curing of the composite does not restrict the positioning of the screws in the bone fragments or the shape of the implant; granting surgeons unparalleled freedom in the treatment of bone fractures.
“This study highlighted the multidisciplinary nature of the BoneFix team,” explained Peter, “with engineers, clinicians, and chemists working together, allowing us to investigate clinically relevant biomechanical questions with this novel platform. Additionally, the methods developed here can extend beyond the BoneFix project and help answer similar implant loading questions in different areas of the body. “
The paper, titled “Determination of the internal loads experienced by proximal phalanx fracture fixations during rehabilitation exercises” was published in Frontiers in Bioengineering and Biotechnology, and can be found at https://www.frontiersin.org/journals/bioengineering-and-biotechnology/articles/10.3389/fbioe.2024.1388399/full. The data from the paper is also available in an open access data repository at https://zenodo.org/records/10677536.
An example of the finite element model of a fractured proximal phalanx fixated with the plastic implant, which shows how the implant bent during the fingertip-to-palm exercise. The use of such models allowed the bending force exerted on the implant to be determined.
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