Abstract
The present work illustrates the dynamization of an orthopaedic plate for internal fracture fixation which is thought to shorten healing times and enhance the quality of the new formed bone. The dynamization is performed wirelessly thanks to a magnetic coupling. The paper shows the peculiarities of the design and manufacturing of this system: it involves two components, sliding with respect to each other with an uncertain coefficient of friction, and with a specific compounded geometry; there are stringent limits on component size, and on the required activation energy. Finally, the device belongs to medical devices and, as such, it must comply with the respective regulation (EU 2017/745, ASTM F382). The design of the dynamizable fracture fixation plate has required verifying the dynamic of the unlocking mechanism through the development of a parametric multibody model which has allowed us to fix the main design variables. As a second step, the fatigue strength of the device and the static strength of the whole bone-plate system was evaluated by finite element analysis. Both analyses have contributed to defining the final optimized geometry and the constitutive materials of the plate; finally, the respective working process was set up and its performance was tested experimentally on a reference fractured femur. As a result of these tests, the flexural stiffness of the bone-plate system resulted equal to 370 N/mm, while a maximum bending moment equal to 75.3 kN·mm can be withstood without plate failure. On the whole, the performance of this dynamic plate was proved to be equal or superior to those measured for static plates already on the market, with excellent clinical results. At the same time, pre-clinical tests will be an interesting step of the future research, for which more prototypes are now being produced.
Highlights
The incidence of lower limb fractures has been estimated to reach about 300,000 cases per year in the United States alone
Fracture fixation follows well-established procedures finalised to immobilise the respective position of bone fragments [3]
Dynamic fixation [6] is gaining consensus among orthopaedics, as an alternative technique where only torsional and flexural loads are bypassed by the bone synthesis device; by contrast, axial loads, produced by body weight or muscle contraction, are left to the contacting bone fragments [7]
Summary
The incidence of lower limb fractures has been estimated to reach about 300,000 cases per year in the United States alone. Dynamic fixation [6] is gaining consensus among orthopaedics, as an alternative technique where only torsional and flexural loads are bypassed by the bone synthesis device; by contrast, axial loads, produced by body weight or muscle contraction, are left to the contacting bone fragments [7]. This technique has very specific indications (sufficiently stable fractures); it deserves greater consideration due to its benefits which can be identified in a faster rehabilitation as well as an improved new-bone quality [8]. First prototypes were designed and realised according to an approved company process flow in accordance with ISO 13485:2016 [14]
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