Introduction
The technique of callus distraction for the treatment of bone defects started at the beginning of the nineteenth century [1]. Despite significant improvements, the treatment of large bone defects remains challenging in many ways. Callus distraction can be accomplished either through extramedullary systems (EMS) such as the Ilizarov method or through totally implantable intramedullary distraction systems (IMS) [2-5]. External fixators in EMS are poorly accepted by patients, frequently resulting in pain, stiffness, irritation and pin track infections [6]. The overall clinical utility for EMS is poor [7]. Even though IMS avoid the problem of pin track infections and are preferred in maintaining quality of life, they are not widely used as the existing models are still limited in terms of function and control [8]. It is known that a series of biological and mechanical elements, most of them not fully understood, are involved in the distraction process. Essential information on variables such as the best velocity of distraction, the traction forces involved in the transport process and whether or not the position of the osteotomy is of relevance is lacking [9].
In order to improve on existing models and develop a novel callus distraction systems (CDS), basic biomechanical knowledge regarding traction forces involved in bone segment transport (BST) is needed. A better understanding may help reduce frequent complications such as muscle contractures, axial deviation and traction injury to vessels and nerves but would also allow modifying the treatment regimen. There are little data on forces in callus distraction systems. Previous studies measured distraction forces either in complicated or in using inaccurate systems ignoring frictional force; many were conducted in animal experiments having less relevance for human callus distraction [10-14]. Whether the predominant part of the force is generated by the viscoelasticity of the soft tissues or by the callus itself, is uncertain.
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The overall force required for BST consists of several different load components [9, 15, 16]. All adherent structures of the transporting bone segment such as tendons, fasciae and muscles generate a force due to the distraction of the soft tissues. Another component of the overall traction force is directly related to the callus and the new forming bone tissue of the regenerate. The tissue reproduction in callus distraction is stimulated by traction, and the process is known as distraction osteogenesis [17]. A percentage of the force can be attributed to the displacement of the tissue blocking the bone defect and some is directly related to the measuring system itself. Precise knowledge on the applied forces involved in the treatment process is fundamental. The data are diverse in the literature on the overall force required for callus distraction but also the distribution of forces. This study aims to measure the mechanical forces applied by the soft tissues in BST over the whole period of distraction using a novel intramedullary CDS.
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